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DMT04 – SOLAR POWERED STIRLING ENGINE

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Executive Summary

The objective of the Solar Powered Stirling Engine project was to design, make and test a solar

collector that would sustain an existing Stirling engine while automatically tracking the sun

throughout operation. This product would be proof of concept that solar radiation is a viable

method of obtaining mechanical work.

DMT04 used a Fresnel lens to focus solar radiation onto the hot end of the Stirling engine. A frame

with an adjustable pan and tilt was designed to maintain the Stirling engine at the focal point of

the Fresnel lens. A digital control system was built into the assembly to allow the continuous

tracking of the sun throughout the day.

The test results obtained by DMT04 are proof of a successful completion of the project. The

control system, with an accuracy of ±10 mm, was capable of ensuring continuous operation of the

Stirling engine, provided solar intensity exceeded a minimum of 300 W/m2. Furthermore, the

project was completed within budget, at a cost of £588.62.

This report will detail the progress of the project throughout its design, manufacture and testing

phases. A detailed discussion will be included which addresses the key results and limitations of

the project. Team DMT04 also suggests possible future developments of the project.


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Group Comment

The team consisted of four team members, all with different areas of responsibility. The team

member and their respective contributions to the project are outlined in this section.

Andrew Tan

Andrew was the designated Project Manager. His role included organising meetings internally

within the group as well as with supervisors; it was his responsibility to oversee that minutes were

written. In addition, Andrew was in charge of the control system which involved sourcing

adequate components, writing the solar tracker code and troubleshooting the system when in

service. Further, Andrew was the creator of the official website of the project and product.

Marcus Ulmefors

Marcus was in charge of the mechanical design. This role entailed verifying design decisions and

their compatibility with the assembly. Material selection for the mechanical design and the

procurement of these parts were also parts of the role. During the testing phase Marcus was

responsible for safety documentation and communication on the group's behalf in order to ensure

safe and efficient testing conditions.

Charles Peurois

Charles was the responsible for coordinating manufacture tasks and ensuring that all parts were

produced on time and to tolerance. He was also in charge of the power transmission design and

together with Andrew its integration with the control system. Charles was also responsible for

industrial contacts and contributed to the wealth of the website.

Maira Bana

Maira Bana was in charge of documentation and reporting. This role involved overseeing that

meeting minutes and reporting was of high quality and always backed up safely. Her role also

entailed delegating sections of report writing to each of the group members and overseeing the

compilation of the individual contributions to form coherent reports. Maira was also responsible

for correctness of drawings, bill of materials and costing.


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Contents 1. Introduction .............................................................................................................................................. 1

2. Background ............................................................................................................................................... 2

2.1. Stirling Engine ..................................................................................................................................... 2

2.2. Solar Power ......................................................................................................................................... 2

3. Objectives .................................................................................................................................................. 4

4. Conceptual Design .................................................................................................................................... 5

4.1. Brainstorm .......................................................................................................................................... 5

4.2. Concepts ............................................................................................................................................. 7

4.3. Concept Evaluation ............................................................................................................................. 8

4.4. Control System .................................................................................................................................... 9

5. Design Phase ........................................................................................................................................... 11

5.1. Experiments Conducted .................................................................................................................... 11

5.2. Structural and Mechanical Design .................................................................................................... 12

5.2.1. Assembly ....................................................................................................................................... 12

5.2.2. Joining Side A-frames .................................................................................................................... 12

5.2.3. Frame Support ............................................................................................................................... 13

5.2.4. Mounting of Upper Pulley ............................................................................................................. 13

5.2.5. Stirling Engine Mount .................................................................................................................... 14

5.2.6. Constant Width ............................................................................................................................. 15

5.2.7. Fixing Legs to Rotating Base Plate ................................................................................................. 15

5.2.8. Base Plate Assembly ...................................................................................................................... 15

5.2.9. Lens Clamps ................................................................................................................................... 16

5.2.10. Upper Motor Plates ................................................................................................................. 16

5.2.11. Material Selection .................................................................................................................... 17

5.3. Control System Design ...................................................................................................................... 18

5.3.1. Circuit Construction ....................................................................................................................... 19

5.3.2. Tracking Procedure ....................................................................................................................... 23

5.3.3. Control System Process ................................................................................................................. 23

5.4. Transmission Design ......................................................................................................................... 26

5.4.1. Initial Torque Calculations ............................................................................................................. 26

5.4.2. Motor Type Selection .................................................................................................................... 28

5.4.3. Motor Characteristics .................................................................................................................... 28

5.4.4. Motor Transmission System .......................................................................................................... 30

5.4.5. Details on the Belt Drive Transmission ......................................................................................... 31

5.4.6. Extension Shaft .............................................................................................................................. 32

5.4.7. Power Supply ................................................................................................................................. 33

6. Manufacture ........................................................................................................................................... 34

6.1. Design Amendments ......................................................................................................................... 34

6.1.1. Stiffening Plate .............................................................................................................................. 34


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6.1.2. Rotation of Stirling Engine ............................................................................................................. 34

6.1.3. Shielding Box ................................................................................................................................. 35

6.1.4. Larger Step on Shaft ...................................................................................................................... 36

6.1.5. Rubber on Large Pulleys ................................................................................................................ 36

6.1.6. Wooden Mounts for Castors ......................................................................................................... 36

6.1.7. Second Positioning Hole in Lower Motor Plate............................................................................. 37

6.1.8. Control System Amendments ....................................................................................................... 37

6.2. Manufacturing Processes .................................................................................................................. 38

6.2.1. Frames ........................................................................................................................................... 38

6.2.2. Wooden Parts ................................................................................................................................ 39

6.2.3. Metal Parts .................................................................................................................................... 40

6.2.4. Fasteners ....................................................................................................................................... 41

6.2.5. Control System .............................................................................................................................. 41

6.2.6. Painting.......................................................................................................................................... 43

6.3. General Assembly ............................................................................................................................. 44

7. Testing ..................................................................................................................................................... 46

7.1. Performance Tests ............................................................................................................................ 46

7.1.1. No Load Speed with Lens Partially Covered .................................................................................. 46

7.1.2. Control System Update Frequency ............................................................................................... 46

7.1.3. Transient Speed Test ..................................................................................................................... 47

7.1.4. Continuous Tracking ...................................................................................................................... 48

7.1.5. Tracking System Accuracy ............................................................................................................. 48

7.2. Design Tests ...................................................................................................................................... 49

7.2.1. Time of Assembly .......................................................................................................................... 49

7.2.2. Compact Design............................................................................................................................. 49

7.2.3. Portability ...................................................................................................................................... 50

8. Expenses .................................................................................................................................................. 51

9. Discussion ................................................................................................................................................ 52

9.1. Assessment of Fulfilment of PDS Criteria ......................................................................................... 52

9.2. Management ..................................................................................................................................... 53

9.3. Results and Improvements ............................................................................................................... 56

9.4. Mass Production ............................................................................................................................... 57

9.5. Sustainability Considerations ............................................................................................................ 58

10. Conclusion ............................................................................................................................................... 59

11. Future Developments ............................................................................................................................. 60

12. References............................................................................................................................................... 61

13. Acknowledgements ................................................................................................................................. 62

Appendix A: Calculations – Stress Analysis of Frame Bars ............................................................................. 63

Appendix B: Calculations – Battery Life .......................................................................................................... 65

Appendix C: Calculations – Control ................................................................................................................. 66


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Appendix D: Calculations – Transmission ....................................................................................................... 67

Appendix E: Control Program ......................................................................................................................... 70

Appendix F: Control Flowchart ....................................................................................................................... 72

Individual Critiques ......................................................................................................................................... 73


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1. Introduction

Emissions of CO2 have been a major concern in our society for decades. In 2010, global carbon

dioxide emissions reached a record high of 30.6 Gt1, which corresponds to an increase of 1.6 Gt over

emissions in 2009. The International Energy Agency (IEA) predicted that in order to avoid extreme

climate changes the annual CO2 emission level should not exceed 32 Gt by 2020. Moreover, the

Department of Energy and Climate Change targets a 25%2 increase in the fraction of renewables in

the United Kingdom’s electricity makeup between the years 2009 and 2020. At present, a vast

amount of research is being conducted in search of efficient and cost effective means of producing

energy from renewable sources. Solar energy is one of the most promising renewable sources of

energy with the average solar energy received on earth estimated to be 10,0003 greater than the

human consumption today.

In response to the current situation, this Design, Make and Test project investigates the combination

of solar power and Stirling engine technologies. The DMT04 team was given an existing Stirling

engine, designed and manufactured by students in the academic year 2009-2010 and had for mission:

"To design, make and test a solar collector capable of sustaining an existing Stirling engine through

concentrated solar power. The system should track the sun throughout the day without human input."

This report outlines the objectives, design progress and end product performance of the project. The

conceptual designs that were formulated as a response to the design brief will be presented and the

choice of the preferred concept will be justified. The detailed design of the individual components

and the assembly will also be presented. A document containing a full set of engineering drawing and

a complete bill of materials is submitted along with this report.

The manufacture of the product will be described as well as the design amendments that arose from

knowledge obtained in the manufacture and assembly processes. The final sections of the report will

outline the methods and results of the experiments conducted in order to measure and quantify the

success of the project. Tests include robustness of control system, the ability of the system to sustain

the Stirling engine and the adherence of the final design to the initial design requirements.

1 International Energy Agency (IEA). http://www.iea.org/index_info.asp?id=1959 [Accessed 02/06/2011] 2 Department of Energy and Climate Change. www.news.bbc.co.uk/1/hi/sci/tech/8150919.stm [Accessed 20/05/2011] 3 Space Future. http://www.spacefuture.com/power/introduction.shtml [Accessed 22/05/2011]

http://www.iea.org/index_info.asp?id=1959

http://www.news.bbc.co.uk/1/hi/sci/tech/8150919.stm

http://www.spacefuture.com/power/introduction.shtml


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2. Background (derived from Plan Report)

In preparation for this project, the group conducted research into Stirling engines and solar power.

This section delivers the brief account of the group’s findings that was outlined in the Project Plan

Report.

2.1. Stirling Engine

The first Stirling engine was developed in 1816 by Robert Stirling, but never really reached the stage

of mass production4. Although the technology offers many advantages such as high thermal

efficiency, low operating noise and the ability to be powered by a variety of heat sources, Stirling

engines generally have low specific power, work better at constant speed and can be quite costly due

to the need for heat exchangers. The three latter reasons explain why these engines are not

extensively used. There are four kinds of Stirling engine namely Alpha (Two cylinder), Beta (Single

cylinder), Gamma (Ross Yoke) and combined engines. The principles of operation are the same but

the cylinder organisation varies. In this project, an engine based on the gamma design was used.

Figure 2.1 shows the general arrangement of the engine.

Figure 2.1: Gamma Type Stirling Engine

2.2. Solar Power

As explained, Stirling engines can be powered by various sources of heat. Indeed, Stirling engines

work when exposed to a temperature differential between their hot and cold sides. Therefore, green

and combustion free heat sources can be applied to the hot side of these engines to convert heat to

mechanical work. Solar power is one of these heat sources. More precisely, concentrated solar power

can be used to focus a large amount of solar radiation onto a much smaller area. This technology,

4 Wikipedia. http://en.wikipedia.org/wiki/Stirling_engine [Accessed 07/05/2011]

http://en.wikipedia.org/wiki/Stirling_engine


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being a renewable energy, offers many advantages. However, its price and its exclusivity to highly

solar exposed regions do not make it as appealing as the alternatives, and explain why it is not used

to a great extent. Nevertheless, due to the increasing need for renewable energies, and with regard

to a study by Greenpeace International, the European Solar Thermal Electricity Association and the

International Energy Agency's SolarPACES group, the global investment in concentrated solar power

could rise from 2 billion Euros in 2009 to 92.5 billion Euros by 20505. Also, thanks to the development

of new materials, namely silver polymer sheet, solar collectors could become much cheaper than the

current glass-based versions. This would translate in a price reduction of 30%6.

Figure 2.2: Solar Collector Powering a Stirling Engine7

All these facts give an idea of the potential of this technology, and this is why the project will study

how solar collectors can be associated with Stirling engines.

5 Guardian. http://www.guardian.co.uk/environment/2009/may/26/solarpower-renewableenergy [Accessed 18/11/2010] 6 Wikipedia. http://en.wikipedia.org/wiki/Concentrated_solar_power [Accessed 18/11/2010] 7 Solar Central. http://solarcentral.org/ [Accessed 19/11/2010]


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3. Objectives

The brief of the Solar Powered Stirling Engine projects was to design, make and test a solar collector

that would work to sustain an existing Stirling engine by the concentration of solar radiation. Aspects

of the product design were specified, and the team were then able to lay out their objectives for the

product. The degree to which the product achieves the desired objectives would give an indication of

the success of the project.

1. The solar collector must sustain the existing Stirling engine at solar intensity of at least 600

W/m2. A manual kick-start may be required.

2. A solar tracking system should be implemented, and must work without human input.

3. The solar collector should be of a sturdy construction so as it withstand windy conditions.


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4. Conceptual Design (derived from Progress Report)

The material in this section was derived from the Project Progress Report and involved identifying the

design requirements, as laid out in the product design specification, and developing concepts that are

in keeping with them. The team employed ideation techniques – conducting a brainstorming session,

sketching and evaluating conceptual designs.

4.1. Brainstorm

The team brainstorm session began with defining the key functions of the product such that the

design would meet the requirements. The “post-it” method, depicted in fig.4.1, allowed for rapid

idea generation with regards to each function, as displayed in fig.4.2 – fig.4.5.

Figure 4.1: DMT04 Post-its Brainstorm

The first design feature to consider was the system by which the solar radiation could be used to

power the Stirling engine, and this would involve directing the sunlight.

Figure 4.2: Idea Generation for a System to Direct Sunlight onto Stirling Engine


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In order to track the sun, the solar collector must consist of a method for locating the position of the

sun. Figure 4.3 shows the results of the Post-its brainstorm regarding this aspect.

Figure 4.3: Idea Generation for a System to Locate the Position of the Sun

The team established that the hot end of the Stirling engine would have to be situated at the focal

point of the solar collector, which would be tracking the sun. This required generation of ideas on

how to hold the Stirling engine with respect to the moving collector.

Figure 4.4: Idea Generation for a Method of Holding the Stirling Engine with respect to the Solar Collector


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As the sun follows its path the solar collector will have to follow it such that the incidence of the

radiation is perpendicular. Figure 4.5 identifies the ideas drawn up in the brainstorm for the method

of solar tracking.

Figure 4.5: Idea Generation for the Shape/Mechanism for a Solar Tracking Device

4.2. Concepts

Three conceptual designs were drawn up as a result of the ideas generated in the brainstorm. Figure

4.6 shows the three sketched concepts that were evaluated before the final selection. It was

established that in tracking the sun, the solar collector would have to move in two orientations.

Concept 1 consists of a mirrored dish as the solar collector, with the Stirling engine attached by an

adjustable arm. The motion of the dish would be driven by a geared motor in one orientation, and a

hydraulic arm in the other. The photoresistors are arranged such that the left and right resistors are

the sensors in the pan direction, controlled by the motor, and the top and bottom resistors control

the tilt by the hydraulic arm.

Concept 2 is similar to concept 1 with regards to the transmission, but employs a Fresnel lens instead

of the dish. The frame is designed to incorporate the solar collector and proposed method of

transmission.


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Concept 3 also makes use of a Fresnel lens, held in a pyramidal-shaped frame. Both drive orientations

are controlled by geared/belt-driving motors, with the same photoresistor set-up as concept 2. The

Stirling engine is held stationary with respect to the lens. Both motors are mounted on the rotating

base, with the battery pack and control system.

Figure 4.6: Sketches of Conceptual Designs – (a) Concept 1 (b) Concept 2 (c) Concept 3

4.3. Concept Evaluation

Concept 1 employs a mirrored dish as the solar collector. It was established that the geometry of a

parabolic dish would define the focal distance – the more open the dish, the shorter the focal length.

The shape of the dish would have to be shallow enough to receive the incident solar radiation, but

deep enough to define a focal point in close proximity. This is a challenging aspect to concept 1,

which employs an extended, adjustable arm that holds the Stirling engine at the focal point. The

second issue with the design is that the mirrored finish of the dish should be free of defects in order

(a) (b)

(c)


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to optimise the focus of radiation – this would call for a large curved mirror, which would be

expensive – an aluminium foil coating would present imperfections. The merits of concept 1 include

the tripod frame, which would be simple to implement, but would have to support a great load.

Driving the dish with a hydraulic arm is an interesting idea, which would require the team to conduct

research into the methods of implementing such a system. Rotating the base plate with a DC or

stepper motor is a sound idea, but the toothed base plate would probably be impractical to

manufacture. An alternative driving mechanism would be considered in the progression of the design

if concept 1 or 2 were to be developed. Similarly, a method of holding the motor on the rotating plate

should be recommended so as to avoid tangling.

Concept 2 consists of a Fresnel lens that will focus the radiation onto the engine. It was expected that

the accuracy of a well-manufactured lens would be similar to that of a perfectly finished mirrored

dish, but probably easier to source. The transmission methods shown as similar to those employed in

concept 1 – a hydraulic arm controlling motion in one direction, and a rotating plate in the other. The

team had deemed it suitable to contain the solar collector and Stirling engine in the same frame, so

as to be stationary with respect to one another. Furthermore, this design positions the Stirling engine

behind the solar collector with respect to the sun, allowing the incoming radiation to reach the hot

end unhindered.

Concept 3 was also drawn up with the intent of using a Fresnel lens. The benefits of Fresnel lenses

had been identified in the Project Plan Report – the team noted that Fresnel lenses lose little energy

by absorption compared with conventional lenses, are lighter and have a shorter focal length making

them practical for the task at hand. A merit of this concept is the frame, which is a well-defined

structure that would prove sturdy in operation. As with the other concepts, the Stirling engine is fixed

with respect to the lens. Concept 3 makes use of a motor, which may drive the upper frame by

gearing, belts or chains. This idea was considered more feasible than implementing a hydraulic arm.

The base plate is rotated by a geared motor, mounted on the plate itself. One issue with the

proposed design is the tripod base, which would have to be designed in careful consideration of the

centre of gravity of the contraption.

4.4. Control System

It was decided that a control system would be necessary to track the movement of the sun and

instruct the motors which way to turn. Two different types of control systems were considered –

digital or analogue. The characteristics of each system considered by the team whilst choosing

between the two systems are illustrated in table 4.1.


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Table 4.1: Digital and Analogue Comparison

Parameter Digital Analogue

Use of Chip Involves the use of a chip and analogue to digital convertors to interpret data.

Does not involve the use of a chip and therefore requires no analogue to digital conversion.

Data Interpretation Data is collector by sensors and interpreted by a program written into the chip.

Data is interpreted by a series of circuit components between sensors and transducers.

Circuit complexity Relatively simple circuit. Relatively complex circuit.

Ease of troubleshooting

Troubleshooting is manageable as it simply involves making changes to the control program.

Troubleshooting is difficult as it involves making changes to the electrical circuit, and removing or inserting new components.

Cost Potentially higher because of the chip. Potentially lower.

Eventually, despite the potential higher cost of the digital system, it was decided upon to be the

easiest type of control system to implement. This was later confirmed by Dr. Rodriguez y Baena

during a meeting with him.


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5. Design Phase (derived from Progress Report)

Section 5 has been derived from the Project Progress Report and outlines the design that was agreed

upon and brought to manufacture. Three key sections of the project will be presented – the

mechanical design, the control system and the transmission system. Section 5.1 provides brief

descriptions of the experiments that allowed the team to detail several design specifications.

5.1. Experiments Conducted

Three experiments were conducted in order to source a Fresnel lens of suitable performance, and

determine the overall dimensions of the solar tracker structure.

Stirling Engine Experiment

Once a decision had been made to utilise a Fresnel lens to collect and focus the solar radiation, the

required size needed to be determined. Crucially, a lens of insufficient size would not be able to

concentrate enough energy to successfully run the Stirling engine, so a comparison with supplier data

would be conducted.

Using a Bunsen burner (Usbeck, canister model 1430, nozzle model 1420) it was possible to start the

engine after the hot end had been exposed to the open flame for 85 seconds at an ambient

temperature of 13° C. The same burner was used to boil 12 oz (355 ml) of water in approximately 6

minutes. The supplier claimed that the Fresnel lens would boil 12 oz of water in 70 seconds. Although

UK sunlight would be less intense than the American conditions used by the supplier, it was agreed

that the proposed 49" spot lens could be purchased confidently.

Focal Point Experiment

The technical specifications for the Fresnel lens provided an estimate of the lens focal length of 29"

(737 mm). The exact position needed to be known since this would determine the positioning of the

Stirling engine with respect to the lens and therefore the overall frame dimensions. An experiment

was conducted positioning the lens in front of a whiteboard, pointing a perpendicular laser pointer at

different locations on the lens and tracing the focused points on the board. The experiment was

repeated at several distances from the board and the best fit was taken as the focal point. The focal

length observed conducting this experiment was 726 mm. Confirming that the two numbers agreed

within acceptable range, the result was accepted. As presented in Section 6.1.2, it was found that the

focal length data provided by the supplier was incorrect; and so were the experimental methods

employed. As a consequence the focal length had to be revised and changed to 910 mm with

implications to the final design.


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Fresnel Lens Experiment

A test was undertaken in order to confirm the ability of the lens to power the Stirling engine. The hot

end of the Stirling engine was positioned at the estimated focal point and allowed to heat up. After a

few minutes the temperature difference was large enough to sustain continuous operation for more

than 60 seconds. The solar radiation at the time was approximately 300 W/m2. This result confirmed

that the lens would be sufficiently large to meet the target of sustaining the engine at solar radiation

intensity of 600 W/m2.

5.2. Structural and Mechanical Design

This section of the report outlines key structural and mechanical design features of the product and

the assembly at the point where manufacture was to commence.

5.2.1. Assembly

Figure 5.1 presents the assembly before the start of manufacture. This assembly contextualises the

detail design discussions in the forthcoming sections. Key dimensions for appreciating length scales:

Lens frame is and wooden base plates are diameter.

Figure 5.1: Assembly of Design Prior to Manufacture

5.2.2. Joining Side A-frames

The entire structure was initially going to be welded to provide good strength, making mild steel the

material of choice. In further design development it was suggested that the side A-frames be welded

but that the A-frames then be joined together by a temporary means, thus resulting in a design

solution with ease of portability, storage, manufacture and assembly. In the final design, the


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horizontal joining bars would slide onto pegs and this fitting would then be secured with M10 nuts

and bolts as demonstrated in fig.5.2.

(a) (b)

Figure 5.2: (a) Disassembled A-frame and Joining Bar (b) Joining bar Assembled with A-frame and Secured with Nut & Bolt

5.2.3. Frame Support

The initial concept for supporting the frame was based on joining the legs to either a hexagonal or

round bar as shown in fig.5.3(a). The shaft would sit in a bearing positioned in the bore. In the revised

design both these solutions were discarded due to high material cost and difficulty in aligning the two

leg assemblies and therefore the two shafts. The proposed solution was to manufacture a brass bush

which would support the shaft. The bush would be fastened with screws to a steel sheet that in turn

would be welded to the leg frame, as depicted in fig.5.3(b). In the final design, the bush was made

from bronze by virtue of being soft and easier to source directly at College.

(a) (b)

Figure 5.3: (a) Earlier Concept Consisting of Round Bearing Housing with Ball Bearing (b) Final Design Using Bronze Bush to Support Frame Structure

5.2.4. Mounting of Upper Pulley

In the initial frame transmission concept the pulley would drive the shaft that in turn would transmit

the torque to the frame, as demonstrated in fig.5.4(a). Upon review this idea was discarded, and it

was decided that the pulley would be fastened to the mid-bar of the A-frame directly as in fig.5.4(b),

benefitting from a larger diameter than the shaft, and thus lower force at a given transmitted torque.

As described in sec.5.2.3, the ball bearing was replaced with a bronze bush to facilitate manufacture

and shaft alignment. A bronze bush with a clearance fit would be satisfactory given the low number


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of revolutions in service. A step in the shaft was introduced in order to maintain clearance between

stationary and moving parts.

(a) (b)

Figure 5.4: (a) First Concept of Bearing Housing and Power Transmission from Pulley to Upper Frame (b) Current design of frame support

5.2.5. Stirling Engine Mount

Three concepts were proposed for supporting and positioning the Stirling engine with respect to the

lens. Since the manufacturing and assembly would possibly result in inaccuracies it was important

that the design would allow for movement of the Stirling engine in three dimensions, should the focal

point not coincide with the position of the Stirling engine hot end.

The first concept involved a steel plate fixed to the lower rear steel bar joining the two side A-frames.

The Stirling engine could be repositioned on the plate, raised in height by adding sheet material and

clamped down when the correct position is found.

The second concept involved an additional bar on each of the side A-frames. A plate would be

fastened and held in place on two locations per side by bolts. The bolts would slide up and down

along manufactured slots to reposition the plate and the Stirling engine could be relocated on the

plate.

The third concept involved connecting the joining bars with studding, as shown in fig.5.5. Slots would

be machined along the length of the joining bars allowing for the studding to slide in one dimension.

Lindapter flange clamps would clamp the Stirling engine base plate. The flange clamps would then be

moved along the axis of the studding to reposition the height of the engine and be locked into

desired position with nuts. Movement in the third dimension would be achieved by clamping at a

different location of the Stirling engine base plate.

The third concept was finally agreed upon due to its low cost, straight forward manufacturing and

assembly. In a later revision it was suggested that U-shaped clamps be added to prevent splaying of

the joining bars upon tightening of the nuts.


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Figure 5.5: Stirling Engine Mount.

5.2.6. Constant Width

In the conceptual design a "chopped pyramid" shaped frame structure was suggested. This would

save material and facilitate clamping of the Stirling engine. However, a design with constant width

was finally opted for – the horizontal rear joining bars of the upper frame would be of the same

length as those at the lens end. To begin with, joining bars at right angles would be easier; and

secondly, rotating the structure would be much easier if the side A-frames were assembled in

parallel.

5.2.7. Fixing Legs to Rotating Base Plate

Fastening the legs securely to the base plate would be important to ensure construction stability. At

first, the design featured a horizontal steel sheet welded to the underside of the legs. This steel sheet

would then be screwed into the base plate. This design was upon review changed to triangular

bracings that would support horizontal loads. In the final design, as presented in fig.5.6, flanges were

added to the bracings to secure the legs safely to the wood while providing improved stability.

Figure 5.6: Bracings Securing Legs to Rotating Base Plate.

5.2.8. Base Plate Assembly

The top base plate would be rotated by one of the motors, providing the panning mechanism.

Concentricity was first to be achieved by a wooden shaft, which was then replaced by a steel shaft

supported by a ball bearing. The ball bearing was subsequently discarded in favour of the bronze


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bush bearing. Fixed castor wheels were desired to avoid swivel and difficulty of moving the structure

when changing direction often, which would be common in the tracking system. Due to an error in

the supplier item description, swivel castors were delivered which indeed caused unsatisfactory

operation of the panning mechanism. This was solved by applying strong epoxy glue at the swivel

mechanism and thereby removing the swivel effect and ensuring good rotation.

(a) (b)

Figure 5.7: (a) Load Supported by Castor Wheels, Top Base Plate Positioned by Shaft and Bush (b) Square Steel Sheets Positioning Shaft and Preventing Rotation

The stepped steel shaft would be positioned in a 4 mm thick aluminium plate with a 2 mm counter

bore, preventing the shaft from sliding out of position.

5.2.9. Lens Clamps

The Fresnel lens was supplied with a wooden frame which would be attached to the steel structure of

the upper frame. The initial concept was a simple L-shaped clamp that would be attached to the steel

frame and the wooden lens frame using self tapping screws. The L-clamps were subsequently

replaced with S-shaped clamps, shown in fig.5.8, in order to allow for mounting of an LDR. By

positioning the LDR 5 cm behind the wooden frame a shadow would be cast upon it if the lens was

misaligned with respect to the sun, facilitating accurate tracking.

Figure 5.8: Clamping Lens Frame to Steel Structure with S-clamp

5.2.10. Upper Motor Plates

The upper motor was attached to the horizontal bar of the leg structure. To ensure that the motor

pulley would be aligned with the larger wooden pulley it was decided that the motor be positioned

further away from the frame. This was achieved by inserting an aluminium bar between the


17

horizontal bar of the leg assembly and the motor plate. The aluminium spacer was manufactured by

cutting and milling an aluminium bar to thickness 10 mm and length 185 mm.

Figure 5.9: Upper Motor Plate.

5.2.11. Material Selection

The legs and upper frame (A-frames and bars) were all made of mild steel square tube. This decision

took into account several criteria. To begin with, the material had to be sufficiently strong. It would

also need to be available in suitable size so as to allow for the joining method described in sec.5.2.2.

Further, it was decided that the best joining method for the A-frames would be welding. As a

consequence, mild steel was chosen. The thicker square tube was of dimensions

and the thinner of dimensions . The theoretical clearance between

the bars, , would be:

When mounting the Fresnel lens and Stirling engine and taking the weight of the bars into account,

two points of interest arose in terms of bending induced stresses: (i) the diagonal bar joining the lens

side with the Stirling engine side and (ii) the mid-bar which rests on the shaft about which the tilt

mechanism works. The second moment of area, , was calculated:

(Eqn.5.1)

for the thinner bars and identically for the thicker bars. Adding the loads for the diagonal bars the

total bending moment was estimated to . The bending stress was thus calculated:

(Eqn.5.2)

This is well within the yield strength of mild steel, giving a safety factor of approximately .

Adding the loads for the mid-bar, a maximum bending moment of was expected. In

addition, a 12 mm hole was to be drill through all which would result in loss of cross section (and a

reduction in second moment of area). Further, stress concentration would be present at the hole.

The stress concentration factor was conservatively estimated to . The bending stress at the

hole was calculated:

(Eqn.5.3)


18

which would result in a safety factor of approximately 4 (refer to Appendix A for detailed

calculations).It is noted that the safety factors are high for this application but reducing the weight

further to optimise performance would result in more different steel thicknesses being required and

thus a higher cost of the project.

5.3. Control System Design

Having decided that the control system would be constructed with the use of a digital microcontroller

unit, team DMT04 set out to design a control system that could effectively detect the motion of the

sun and move the solar collector correspondingly. It was decided early on that in order to track the

sun effectively, it would be necessary to use a closed loop control system with feedback like the one

illustrated in fig.5.10.

Figure 5.10: Generic Closed Loop Control System Flowchart with Feedback

As can be seen in fig.5.10, a closed loop control system involves recording a measurable output

parameter and feeding it back into the controller input. The controller then interprets the fed back

data and outputs a logic signal through power amplifiers to actuators, which aim to adjust the output

parameter. Once the output has been modified by a suitable process, it is measured by the sensors

again and fed back to the controller allowing the loop to repeat itself until the desired output is

reached.

With respect to the project at hand, the desired output is the appropriate alignment of the solar

collector with respect to the sun. This will be deemed to have occurred when the Fresnel lens is

effectively focussing light onto the hot end of the Stirling engine. The control system would therefore

require a mechanism for determining the position of the sun and moving the frame to face it.


19

5.3.1. Circuit Construction

Figure 5.11 is a schematic detailing the construction of the control circuit. It is labelled in order to

identify the different components of the control circuit.

Figure 5.11: Completed Circuit Diagram

Examination of fig.5.11 reveals that the completed control circuit contains all of the necessary

components required of a closed loop control system as described in fig.5.10. The rest of this section

sets out to describe the decisions made between the conceptualisation of the control system to its

finalisation at the design freeze, and will elaborate on a few of the additional characteristics of the

control system.

Controller

The first component chosen was the controller. This is the component labelled in orange in both

fig.5.10 and fig.5.11. After conducting sufficient research, the team decided to use the Arduino UNO

Microcontroller, the important specifications of which are shown in table 5.1.

Table 5.1: Arduino MCU Specifications

Parameter Value

Processor ATmega328

Operating Voltage 5V

Recommended Input Voltage 7-12V

Digital I/O Pins 14 (6 of which provide PWM output)

Analogue Input Pins 6

DC Current per I/O Pin 40mA

Using the Arduino UNO presented the team with several benefits. Firstly, it is a comparatively cheap

MCU which could be purchased at a discounted price from Farnell – a supplier with which Imperial


20

College has an open account. It also uses an integrated development environment software (IDE) that

could be downloaded from the manufacturer’s website free of charge. The chip contains a built in

analogue to digital convertor (ADC), allowing the team to easily use it as a voltmeter able to read

input voltages and later interpret them. Provided a suitable adapter is used, it can be powered for

about 28 days by 6AA 1300 mAh batteries (Refer to Appendix B for Battery Life Calculations). It has a

5 V power output terminal which can be used to power a potential divider circuit. Finally, it has six

digital pins capable of delivering a pulse width modulated output, which is very useful for converting

a digital signal into an analogue form, and could later be used for the controlling the speed of the

motors.

Sensor

The sensor is responsible for determining the position of the sun and is highlighted in red in fig.5.10

and fig.5.11. It was decided that this could be done by placing four light sensitive sensors at extreme

ends of the lens as shown in fig.5.12. The sensors would then work in pairs, with one pair comprising

the sensors on the top and bottom, and the other pair on the left and right of the lens. This would

allow the Arduino to make a comparison between the light intensities at the top and bottom, and

between the left and right of the lens. The control system would then move the frame until the

sensors on the left and right, and top and bottom, sensed equal light intensity.

Figure 5.12: Placement of 4 Light Sensitive Transducers

In order for these sensors to be of use in the control system, it would be necessary for variations in

light intensity to produce some measurable change in the characteristics of the sensor. Two suitable

sensors were identified – the light dependent resistor (LDR) and the solar cell.

The solar cell is an active photovoltaic transducer that produces a voltage under solar radiation. The

voltage generated varies in a directly proportional manner to the light intensity incident on it. The

voltage can then be interpreted by the Arduino’s built-in ADC, converting the voltage into an integer

value between 1 and 1024 that can be interpreted by the chip.

The LDR is a passive transducer that decreases in resistance with increasing solar radiation. Unlike a

solar cell, its resistance varies in an exponential manner with respect to solar radiation. Since the LDR

does not produce any voltage of its own, it would have to be used in conjunction with an external

voltage source in a potential divider circuit. Eventually, the LDR was selected over the solar cell


21

because a suitable circuit comprising solar cells was found to cost approximately 10 times as much as

a suitable LDR circuit. Ultimately, the cost of employing solar cells outweighed the possible benefits.

Eventually, the potential divider circuit in fig.5.11 was designed. The letters next to each LDR in the

circuit diagram identify the position of each LDR (Top, Bottom, Left or Right). At each pin connected

to the Arduino, in higher light intensity, the voltage measured by the Arduino is approximately 0.5 V.

During lower light intensity, the voltage measured by the Arduino is approximately 5.0 V. So as not to

be limited by the non-linear nature of the LDRs, the team developed a setup to allow each LDR to

only detect light when it is coming from a certain direction. Both LDRs in each pair will sense light

simultaneously only when the solar collector is directly aligned with the sun.

Power Amplification

As will be described in sec.5.4.3, the motors chosen to drive the frames are 2 DC motors with a rated

current of approximately 8 A. It was evident that the small Arduino with a maximum output current

of 40 mA was not going to power both motors on its own. Therefore a power amplifier would be

required to interpret a logic voltage received from the Arduino and supply power to the motors. This

component is labelled in purple in both fig.5.10 and fig.5.11. Amplification would have to occur in

conjunction with a 12 V motorcycle battery, elaborated upon in sec.5.4.7.

Some of the components considered for use as power amplifiers were operational amplifiers, bipolar

junction transistors, and MOSFETs and H bridges. However, a consultation session with Dr. Rodriguez

y Baena revealed that the most ideal component to use for signal amplification was a motor driver.

Motor drivers are components designed specifically to interpret a signal in order to power a high

power motor and incorporate the usage of an H bridge.

The motor driver that the team eventually decided to use was the Sabertooth 2X10A Motor Driver. It

is a dual carriage motor driver that would allow the control of 2 DC Motors whilst allowing each

motor to draw 10 A of current continuously. The important characteristics of the Sabertooth 2X10A

are illustrated in table 5.2.

Table 5.2: Sabertooth 2X10A Specifications

Parameter Value

Input Voltage 6-24V

Output Current Per Channel 10A

Peak Output Current Per Channel 15A

The Sabertooth 2X10A has a specific mode of operation and would work in conjunction with the MCU

board. Each motor would be controlled based on the logic voltage, supplied by the MCU, and


22

measured at its corresponding input channel. A 2.5 V logic voltage keeps the motion stationary. A 0 V

logic voltage turns the DC motor with full power in the forward direction, and a 5 V logic voltage

turns the motor with full power in the backward direction, subject to the power source used. The

Sabertooth 2X10A also has the added benefit of having a 5 V power output terminal which can be

used to supply power to low current devices. This output terminal was initially chosen to be used to

power the potential divider circuit shown in red in fig.5.11, but was eventually decided against in

favour of using the output terminals from the Arduino, as the Arduino was revealed to offer a cleaner

voltage signal.

Delay

A delay mechanism was incorporated into the control system that would allow the motor driver to be

shut down when it was not in use. This was done in conjunction with the relay circuit labelled in blue

in fig.5.11. The motivation for the delay was to avoid unnecessary continuous tracking and thus

achieve a reduction in power consumption.

The relay circuit shown in fig.5.11 is centred on a normally open, single pole single throw, 12 V relay.

It would allow pin 13 on the Arduino board to turn on and off the motor driver as necessary. A

suitable transistor was chosen and calculations were performed to determine a suitable value for the

resistor in the circuit (Refer to Appendix C for Control Calculations).

Shutdown

The control system was designed to be completely shut down with the flick of an external rocker

switch. This external rocker switch simply shuts off the power supply to the Arduino MCU board. For

simplicity, this is the only human input that would ever be necessary with respect to the control

system and the system was designed to begin functioning the moment that the switch is turned on.

Special care was taken to ensure that no power is transmitted to any of the circuit components once

the rocker switch is turned off. For example, a normally open relay was chosen for use in the relay

circuit. This would cause the motor driver to be turned off when the Arduino is supplying the relay

circuit a LOW logic voltage through pin 13. Conversely, the use of a normally closed relay would

switch on the motor drivers as soon as the rocker switch is turned off and the Arduino stops

supplying any voltage to the relay. The situation is made worse considering that the logic voltage for

both motors would be 0 V causing the motors to continuously turn in a backward direction, putting

the people nearby at risk of injury.


23

Safety

Considerations were made to ensure smooth functioning of the control system as well as to protect

the control system components. Diodes were inserted in locations where surges in current were

expected in order to protect the NPN transistor in the relay circuit as shown in fig.5.11. These diodes

were inserted after a control system circuit review with Mr. Asanka Munasinghe.

In order to protect the very expensive motor driver, power connectors were used to connect the 12 V

battery to the motor driver. The reason for this is that power connectors can help to prevent the end

user from connecting the battery to the motor driver incorrectly which would definitely destroy the

motor driver. Using power connectors have the added benefit of enabling the battery to be removed

easily for charging.

5.3.2. Tracking Procedure

The effectiveness of the tracking system is largely dependent on the program written into the

Arduino board. The program would interpret the data fed into it from the four light dependent

resistors and controls the motors. In designing the tracking procedure, the team decided to first use

the Arduino to compare the voltages across the top and bottom LDR pair, and then operate the tilt

mechanism, rotating the upper frame. The Arduino would then compare the voltages across the left

and right LDR pair, and then operate the pan mechanism, rotating the upper base plate.

5.3.3. Control System Process

Throughout the progress of the project, the control system process underwent several iterations.

However, it was finally modified to exhibit the following desirable characteristics:

Ensure low battery consumption

Allow safe operation

Easy troubleshooting

Allow oscillation about desired position to account for initial overshoot

Avoid infinite loops

Eventually, a two stage control system was designed with a primary stage, and a secondary stage

which consists of the pan and tilt sub-processes. It was written to consist of two stages to enable

users of the program to understand the program more simply than if it were presented as a single,

large program. These stages are described by the flowcharts shown in the proceeding sections of the

report. The final control system was written with the Arduino IDE (Refer to Appendix E for Control

Program).

Primary Stage

The primary stage occurs according to the flowchart shown in fig.5.13.


24

Figure 5.13: Primary Stage Flowchart

As can be seen from fig.5.13, the first stage of the control program involves defining the variables and

the I/O pins on the MCU board. The I/O pins are defined according to the circuit shown in fig.5.11.

This program incorporates the usage of loop counters n1 and n2 to avoid infinite loops. A delay of 5

minutes has been incorporated into each cycle. This is to conserve the battery life and prevent


25

overheating of the system components. Overheating could prove especially dangerous since the

control system components are mounted on wood.

A comparison of this flow chart with the circuit diagram shown in fig.5.11 shows that the motor

driver has been set to maintain both motors at zero velocity before power is actually transmitted to

the motors. The blue rectangles on the flow chart activate the “pan and tilt” sub-processes that

constitute the next stage of the control program. Furthermore, the part of the flowchart within the

red box can be repeated as many times as necessary if greater accuracy is required.

Pan and Tilt Stage

The primary aim of this stage is to conduct an initial comparison between the two pairs of LDRs (the

top and bottom for tilt, and left and right for pan) and output a logic voltage to inform the motor

drivers of the type of motion to impart to the DC motors. This stage has been split up into the tilt and

pan sub-processes with the tilt sub-process occurring before the pan sub-process. A flowchart

describing the tilt sub-process is displayed in fig.5.14.

Figure 5.14: Tilt Sub-Process Flowchart

As can be seen from observing the flowchart in fig.5.14 and the circuit diagram in fig.5.11, this stage

involves comparing the light intensities on the top (pin 0) and bottom (pin 1) of the lens. Should the

difference between the voltages across the pair of LDRs be less than the tolerance voltage, x, as

defined in the primary stage of the control process, the tilt sub-process will end immediately, leading

on to the pan sub-process (Refer to Appendix F for Pan Sub-Process Flowchart). This process would


26

also be terminated if the loop counter, n1, incorporated to avoid infinite loops, exceeds a value of

200, indicating that the tilt sub-processes has repeated 200 times. This corresponds to approximately

10 seconds for which the motor will be turned on. These conditions are written into the control

system program by use of a two-condition while loop.

One feature that is observed in this stage is the tolerance value, x, in order to allow for certain

inaccuracies of the sensors. In the event that the voltage detected by pin 0 is larger than that of pin 1

by a value greater than the tolerance voltage, the Arduino would recognise that the light intensity is

greater below the lens, causing the motor to move down. Conversely, if the voltage detected by pin 0

is lower than that of pin 1, the Arduino would cause the motor to move up. It can also be seen that

the program allows for oscillation about the desired point in order to account for overshoot.

5.4. Transmission Design

Once the concept was selected and an initial design developed, research was carried out to select

motors and transmission systems for both axes of rotation.

5.4.1. Initial Torque Calculations

Knowing the dimensions and masses of the lens and Stirling engine, it was possible to calculate the

torques required to move the frames around both axes of motion. The assumptions made for these

calculations are given in table 5.3. The mass, length, distances to axes of rotation of the frame bars,

base plates, control system components, motors and transmission system components were

approximated according to initial concepts in order to calculate their moments of inertia (Refer to

Appendix D for Component Layout). The moments of inertia of the Stirling engine for both axes of

rotation were estimated using the measured mass and dimensions of the engine and considering it as

a circular lamina.

Table 5.3: Assumptions Made for Torque Calculation

Engine Lens Assembly

Mass (kg) 3.00 Mass (kg) 5.50 Focal length (m) 0.74

Length (m) 0.48 Length (m) 0.26 Wind speed (m/s) 8.00

Radius of rotation (m) 0.10 Incident angle (deg) 0 - 90 Width of the lens (m) 0.76

Moment of inertia (kg.m2) 0.70 Moment of inertia (kg.m2) 0.85 Height of the lens (m) 1.02

Acceleration (rad/s2) 0.63

Density of air (kg/m3) 1.29

Friction torque base plate (Nm) 4

Friction torque upper frame (Nm) 2

The moments of inertia of the mechanical structure followed directly from the geometry and

material choices and were calculated assuming rectangular bodies for the steel bars and circular body


27

for the base plate. They were estimated to be 0.52 kg m² and 2.78 kg m² for the vertical and

horizontal rotations respectively.

Once the moments of inertia had been estimated and assumptions on the atmospheric conditions

had been made, both the wind induced torques and inertial torques could be estimated. Firstly, the

wind force was calculated:

eq. 5.4

where the ρ is the density, v is the wind speed, h is the height of the lens, w is the width of the lens

and θ is the wind incident angle.

The associated moment is then given by:

eq. 5.5

where d is the moment arm and is the angular position.

Then, the inertial moment is simply calculated from:

eq. 5.6

where is the angular acceleration.

A frictional torque was added to each axis of motion. The frictional torque of the pan mechanism

(horizontal) was approximated as being 7 Nm due to the larger friction associated with the castor

wheels. The frictional torque of the tilt mechanism (vertical), on the other hand, was calculated to be

around 3 Nm.

The sensitivity of the assembly moment of inertia to changes in the dimensions and mass was found

to be relatively low. The inertia torque revealed to be of the same order as the wind-induced torques

and frictional torques. Also, the maximum torque requirements were found to be when the incident

wind angles are 45°. The results of these initial calculations were plotted in fig 5.15.

Figure 5.15: Drive Torque Required versus Wind Incident Angle to the Lens

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0 20 40 60 80

Torq

ue

Re

qu

ire

d f

or

Ro

tati

on

(N

m)

Wind Incident Angle to the Lens (degrees)

Base Plate

Upper Frame


28

According to initial calculations and fig.5.15, approximate maximum torques of 11 Nm and 8 Nm

would be needed to drive the base plate and the upper frame respectively.

5.4.2. Motor Type Selection

Three different types of motors were considered for providing the necessary torque. These were

stepper motors, servomotors and DC permanent magnet motors. After some research and discussing

of the most important issues with Dr. Rodriguez y Baena, the pros and cons of the three types of

motors were summarised in table 5.4.

Table 5.4: Motor Selection Characteristics

Stepper Motors Servomotors DC Permanent Magnet

Motors

Pros Accurate

Simple transmission

Accurate

Easy to use

Affordable

Easy to use

Cons Complex system

Expensive Very expensive

Less accurate

Need a high gear ratio

The price of stepper motors and servomotors were found to be very high for this type of application.

From this research it resulted that the DC permanent magnet motors were the most practical and

cost effective solution. Since the cost of the motors and of the associated control system was a

crucial aspect, DC Permanent Magnet Motors were selected.

5.4.3. Motor Characteristics

Following the torque calculations and motor selection, some research was carried out in order to find

affordable high torque transmissions systems. The different solutions were analysed using an Issue

Based Information System (IBIS) method, shown in fig.5.16. This method enabled the team to

generate new ideas as well as see the pros and cons of each idea. This was of great help during the

preliminary design phase.


29

Figure 5.16: IBIS for the Transmission System

The option of having a low power motor coupled to a contact wheel transmission system was

dropped after careful review. Although being quite appealing from an expenses point of view

(expected £65 for two transmission systems), this transmission system alone would have been

difficult to implement. The need for a spring loaded motor mounting and an extremely high gear ratio

would have generated other sorts of potentially high costs. Moreover, according to the output torque

requirements and to the gear ratio achievable by using a friction drive, typical cheap low power

motors have torque/speed characteristics that would require further gearing reduction. Indeed,

typically low power motors have torque in the order of 0.01 Nm and with the size of the upper base

plate given (1220 mm diameter), an achievable gear ratio could have been 100 which only increases

the torque to 1 Nm. This is below the expected required torque of 7 Nm and in order to utilise such a

motor, a gearbox in addition to the friction drive would have to be used. The costs of the

transmission system would thus increase. Also, there would have been a high level of uncertainty on

the achievable contact surface friction forces, and as a consequence, the torque transmittable

through the wheel. Another uncertain yet important characteristic was the back-drivable torque. Non


30

back-drivability was considered as a key aspect of the transmission system since wind or unbalance in

the frames may otherwise cause the frames to constantly rotate and lose track of the sun. It was

crucial to ensure compatibility between the motors and the motor driver, and therefore to use

similar DC motors for both axes of rotation. However, a friction drive transmission would have been

suitable for the pan mechanism, through the base plate, only. For all these reasons, the idea of using

a low torque motor coupled to a friction wheel transmission was abandoned.

The possible solution of designing and manufacturing or buying a multiple stage gearbox used in

conjunction with a low power motor was not analysed in great detail. This was because, for low

production volumes, the costs associated with manufacturing or buying an independent multiple

stages gearbox is considerably higher than the other solutions considered.

High torque motors were also investigated. High torque motors based on the design of windshield

wiper motors were sourced and seen as practical and easy to use. The large volumes of production

make them very affordable. Other important points were that these motors are manufactured in-line

with worm gearboxes which, not only give good output torque, but also provide very high back-

drivable torque. The required characteristics of the motors made the motor selection process

relatively straightforward. These characteristics, defined thanks to initial torque calculations and

available hardware for the control system, were:

Maximise torque: > 2 Nm

Minimise price: <£30

Minimise speed: <70 rpm

Minimise rated current: <10 A

Practical rated voltage: 12 V

Maximise back-drivable torque: >10 Nm

Minimise power: <30 W

Using these requirements, the best motor was found to be the AME 214 model from AM Equipment,

an American supplier, shown in fig.5.17.

Figure 5.17: AME 214 Motor

5.4.4. Motor Transmission System

Initially, the chain and sprocket transmission system was adopted. This was due to its appealing

reliability, sturdiness and practicality to implement. However, this solution was found to be too


31

expensive. For that reason, it was rejected upon closer review. Table 5.5 shows the costing analysis

for the sprocket and chain system.

Table 5.5: Costing Analysis for the Sprocket and Chain Transmission

Item Supplier Price (Ea in GBP)

Quantity Sub-Total (in GBP)

¼” Pitch Sprocket (S) – 8

HPC Gears

6.84 2 13.68

¼” Pitch Sprocket (S) – 60

HPC Gears

23.38 2 46.76

¼” Pitch Chain (S) – 600 mm

HPC Gears

17.76 2 45.52 (10 delivery)

Set screws, keys ME

Stores 10 1 10

Total 115.96

Belt drives were more appealing because of their ease of implementation and low cost. Also, precise

force and friction calculations could be carried out. Indeed, it is clear, from the IBIS analysis on

fig.5.16 that belt drives offer the best trade-off in terms of cost and reliability while being easily

implementable. This system was therefore selected for the final design.

5.4.5. Details on the Belt Drive Transmission

For the belt drive system, a gear ratio of three five was selected because it offered the best cost and

force trade-off (Refer to Appendix D Figure D4 for Force-Price Comparison). The use of a timing belt

transmission system in conjunction with the AME 214 worm gear motor was supported by the

project’s supervisors.

From the motor driver specifications, the maximum current that will pass through the motors will be

10 A. According to the motor specifications (Refer to Appendix D Figure D3) it is possible to read the

corresponding maximum torque obtainable from the motor. This torque, corresponding to 6 Nm, was

used in the belt transmission calculations. All data used were taken from the HPC Gears catalogue. It

was agreed that since HPC Gears have an account with the Mechanical Engineering Department of

Imperial College and are reliable suppliers, the pulley and belt systems would be ordered from them.

Initial force analysis for the belt revealed that 13 mm wide belts were too weak in terms of tooth

shear strength. The calculations were then carried out for a 19 mm wide belt, the second available

choice from HPC. The details of the calculations for a timing belt of 3/8” pitch, 19 mm width are given

in table 5.6


32

Table 5.6: Force Calculations for the Pulley/Belt Transmission System

Belt Tensile Strength

Formula Used Obtained Value

[5.7]

where T is the torque, d1/2 is the pitch circle diameter,

µ is the friction factor between the wood and the belt

and α is the angle of contact

F = 485 N; which is below the maximum tensile

force (Fmax = 1260 N) applicable to the belt.

Tooth Shear Strength

Formula Used Obtained Value

[5.8]

where Mspez is the specific torque, Z1 is the number of teeth in the small pulley, Ze is the number of teeth in mesh and b is the width of the belt.

Mmax = 7.7 Nm; which is above the maximum

torque (T = 6 Nm) obtainable from the motor.

Eq. 5.7 and 5.8 come from the belt manufacturer and were deemed to be applicable. Eq 5.8 was

modified to account for the fact that there are no teeth of the large pulleys. These pulleys were

treated as being purely friction driven.

From this analysis, the 3/8” pitch, 19 mm wide timing belt was found to be appropriate for this

application. The larger pulley of the transmission system would be made out of wood because

manufacturing of the pulley could be performed in the College workshops and also because the

material cost was considerably decreased.

5.4.6. Extension Shaft

Due to the short length of the motor shaft (13 mm) and the large width of the small pulley (25.4 mm),

an extension shaft had to be manufactured. In order to induce minimal unwanted moment about the

motor shaft, the extension shaft was designed with minimum length. Both the extension shaft and

the pulley were attached with grub screws. Figure 5.18 shows a solid model of the extension shaft.

Figure 5.18: CAD Model of the Extension Shaft


33

5.4.7. Power Supply

The power drawn by the motors is delivered by a battery. It was calculated that a 12 V, 9 Ah

motorcycle battery would be capable of powering the motors for about 7.8 days (Refer to Appendix B

for Battery Life Calculations). The battery was purchased from Ebay because the batteries as well as

the shipping costs were low compared to other suppliers.

The following procedure was used to obtain an estimation of the battery life of the system.

Assumptions made throughout battery life calculations:

8 hours of sunlight of intensity above 600W/m2 on a given average sunny day.

Rated current of the motors of 8 A

Rated current of the Arduino of 32mA for the 6AA 1300 mAh batteries

The motors are on for 1.5 seconds every 5 minutes

Power consumption:

Running time coefficient:

Running time:

Battery life:

.

Therefore the battery life of the transmission system was calculated to be approximately 7.8 days.


34

6. Manufacture

The following section outlines the design amendments and manufacturing processes employed

during the manufacture phase of the project.

6.1. Design Amendments

Shortly after the Progress Report submission the design was frozen and no further changes were

planned. However, with the advent of the manufacturing process, and more importantly that of

assembly, it became evident that several design amendments were necessary to achieve improved

performance of the product.

6.1.1. Stiffening Plate

When tension was applied to the belt connecting the upper pulleys, the plate holding the motor

became subject to a bending moment. This caused the plate to bend and the motor pulley to lose

alignment. This was solved by cutting a steel plate, bending it to an L-shape, and bolting it tightly

onto the motor plate, as shown in fig.6.1. The flanges of the new stiffening plate would now rest on

the leg assembly, holding the motor plate in position even with high tension applied.

Figure 6.1: Stiffening Plate added to Motor Plate

6.1.2. Rotation of Stirling Engine

When testing the frame assembly with the Stirling engine and Fresnel lens mounted it was found that

the focal length depended on which side of the lens was facing the sun. It was also observed that the

concentration of sunlight was not equally efficient in both orientations. The focal length of the less

efficient orientation coincided with the results obtained in the laser experiment (737 mm) and the

product specification produced by the supplier8. The focal length observed when flipping the lens was

920 mm, but the improved concentration of sunlight resulted in much more efficient heating of the

Stirling engine. There were now two strategies available. The original design could be kept which

would ensure that the focal point would be located on the Stirling engine, but it would be less

efficient. Alternatively the lens could be flipped, allowing for enhanced heating at the focal point.

However, the latter option would require a design change to position the Stirling engine further away

8 Green Power Science. http://greenpowerscience.com/ [Accessed 12/12/2010]


35

from the lens. It was decided that the better option was to modify the design so that a better final

result could be achieved. The Stirling engine was rotated 90 degrees and pushed backwards in its

mounts and the slots in the rear bar were extended to accommodate the Stirling engine in its new

position as shown in fig.6.2(b).

(a) (b) Figure 6.2: (a) Original Position of Stirling Engine (b) Rotated Stirling Engine

6.1.3. Shielding Box

Moving the Stirling engine further away from the lens meant it was no longer housed within the

frame structure. As a result, the criterion of user protection from burns was no longer met. To

address this increased risk of injury a shielding box was manufactured from aluminium alloy sheet.

The component fills two important functions. To begin with, it houses the Stirling engine hot end so

that a user cannot accidentally burn his or her hand when being close to the product. Secondly, the

orifice in the plate is aligned so that sunlight will only pass through the plate when the tracker is

correctly positioned. If the lens is not facing the sun correctly, the light will fall onto the aluminium

sheet. This is advantageous because it impedes the light from hitting the Stirling engine cold end,

causing the temperature difference to decrease and thus slowing down the Stirling engine. Fig 6.3

shows the installed shielding box housing the Stirling engine hot end and protecting users from

incoming concentrated solar energy.

Fig 6.3: Installed Shielding box


36

6.1.4. Larger Step on Shaft

After assembly of the frame it became evident that the S-clamp holding the LDR on the side opposite

the tilt motor would hit the legs upon rotation of the frame. It was decided that the best way to

account for the S-clamp would be to manufacture a shaft with a new step length of 16 mm,

compared to the previous 4 mm.

6.1.5. Rubber on Large Pulleys

When testing the upper and lower motors it was observed that there was some slippage on the large

wooden pulleys, particularly on the lower pulley since it drives a greater moment of inertia and

requires a higher torque. The use of timing belts on the smooth surface of the large wooden pulleys

resulted in a loss of effective contact area. Rather than cutting grooves to match the belt teeth it was

decided that a rubber bicycle tube be acquired, cut to adequate pieces and fastened with epoxy glue

and staples to the groove surface of the large wooden pulleys, as demonstrated in fig.6.4.

Figure 6.4: Adding Rubber to Large Pulleys

6.1.6. Wooden Mounts for Castors

Having assembled the two wooden base plates and the large bottom pulley, it was discovered that

the top base plate bent more than anticipated due to the heavy load exerted by the entire assembly.

As a consequence, the centre of the plate rubbed against the top surface of the wooden pulley when

turning. This was solved by cutting 5 mm thick wooden spacing plates which were added between

the top base plate and the individual castors.

(a) (b)

Figure 6.5: (a) Original Castor Assembly (b) Castor Assembly with Wooden Mount


37

6.1.7. Second Positioning Hole in Lower Motor Plate

The studding used to add tension to the lower belt was observed to cause rotation of the lower

motor plate when high tension was reached. As a consequence, alignment was lost and the

transmission would not be satisfactory. To avoid the bending effect it was decided that a second

positioning screw be added.

6.1.8. Control System Amendments

Pulse Width Modulation to Control Motor Speed

Before the team became familiar with the motor driver, it was unsure as to how effectively pulse

width modulation would be capable of controlling the speed of the motors. Whereas initially it had

been proposed to simply use a digital output of either HIGH or LOW to cause the motors to rotate in

either the anticlockwise or clockwise direction, experimentation during the manufacture phase

allowed modification of the control system program to allow the motors to turn at slower speeds, in

order to consume less power, and allow the LDRs more time to respond. It was also discovered that

the motor controlling the pan mechanism required a higher voltage to allow successful rotation than

the tilt mechanism motor. The control system program was modified to reflect these changes.

Potential Divider Circuit Resistors

Upon testing of the control system circuit, it was found that the 22kΩ resistors were too high for the

potential divider circuit as the light dependent resistors were found to exhibit a resistance of

approximately 50Ω under the light and 160Ω under the shadow. This meant that the voltage received

by the Arduino MCU under light and dark conditions were roughly the same. The situation was made

worse by the resolution of the Arduino MCU of approximately 4mV. The 22kΩ resistors were

therefore replaced with 68Ω resistors giving a voltage range of approximately 2.4-3.5V.

Multiple Loops

During calibration, it was found that a single pan and tilt sub-process per cycle was insufficient for

accurate tracking of the sun. Therefore, the control system program was modified to allow the

system to conduct a second pan and tilt sub-process within the same cycle, by looping the part of the

flowchart within the red rectangle on fig.5.13. This proved successful and allowed for much greater

accuracy in tracking.


38

6.2. Manufacturing Processes

6.2.1. Frames

Upper Frame

The solar collector structure incorporates two main parts which are both composed of two welded

frames. The upper frame is made of two A-shaped welded frames joined together by removable

horizontal bars. On the other hand, the lower frame comprises two welded leg frames which support

the upper frame on each side and the motor positioned on the right. Welding of the side frames was

found to offer the best trade off in terms of sturdiness and convenience. It enabled a rigid structure

while allowing the user to flat pack the product. It was crucial to cut the bars to the correct length

and angles because welding can, due to the relatively rough positioning of the structure and to

thermal effects, introduce severe deformation of the frames. To do so, all the steel bars were first

neatly cut at one end, the correct length was measured using engineering rulers or Vernier callipers

were possible. Once the lengths of the bars were marked, a mill was used to obtain a sufficiently nice

finish and to achieve the correct angle. The latter process can be seen on fig 6.6a. Welding was

carried out in two phases. First the A-frames were welded without the side plugs. Once this was done

the plugs were introduced and welded to both frames. This welding procedure was very important

because it facilitated access to crucial corners during the first phase and enabled to minimise the

effect of thermal residual strain on the position of the plugs. Fig 6.6b shows one A-frame ready for

the first welding frame.

(a) (b)

Fig 6.6: (a) Frame bar being milled to the correct size and angle and (b) A-Frame ready to be welded

Once the plugs were welded to each A-frame, an angle grinder and files were used to remove some

material on the outside of the plugs and inside of the horizontal bars until slight clearance fits were

reached. The completed upper frame could then be assembled.


39

Leg Assemblies

It was decided to define the position of the triangular bearing mounts once the welding of the legs

was complete. At first only one bearing mount was welded to a leg assembly. Once this was done the

other plate could be welded to the other leg frame making sure that the holes for the bush bearings

on the two mounts were at the same vertical level. This was procedure was chosen to ensure that the

effects of thermal residual strains and stresses were minimised, and thus, that the tilt axis of motion

was horizontal. As can be seen on fig 6.7a, one leg assembly possesses the bearing mount while the

other does not. The correct position of the remaining mount was then marked and the leg assembly

was sent to welding.

Once the lower frame was manufactured, the two frames were put together to perform a preliminary

check of the quality design and assembly. This is shown on fig 6.7b.

(a) (b)

Fig 6.7: (a) Legs after the first welding stage and (b) Preliminary check of the frames assembly

6.2.2. Wooden Parts

The final product contains multiple wooden parts. These were manufactured in the IDEAs Lab. The

most important wooden components are base plates and the pulleys. The processes that were

carried out are presented in the following section.

Pulleys

The pulleys were first cut to their approximate shape using a band saw and a sander. Since it was

difficult to obtain very good concentricity using these two machines only, the pulleys were fitted onto

a wood lathe and centred with a M12 bolt which was placed in the centre hole of each pulley. The

choice of using this process was reinforced by the need to make grooves for the belts. Indeed the

grooves are relatively deep and narrow which made any other processes, like sanding, difficult to

carry out. A low feed rate was used when turning the pulleys to avoid the propagation of any crack.

All other features like counter-bore bolts were realised using a pillar drill.

Base Plates

The size of the base plates restrains the choice of manufacturing processes because it is difficult to

find machines which can deal with components of diameter larger than 1 m. For that reason the


40

plates were manufactured using hand tools. An foot plywood sheet was cut in half before

marking and cutting circles of the correct diameter. A jigsaw was used during this process. All holes

were made using power drills and the slots for the tensioning system of the lower motor were made

by drilling two holes at the extremities and sawing in between these holes with a jigsaw. The 4 mm

square groove on the lower base plate (seen on fig.6.8a) was made using a router as shown fig.6.8b.

For safety reasons, the router was operated by Mr Gordon Addy, supervisor of the IDEAs workshop.

(a) (b)

Fig 6.8: (a) Base Plate Square Groove Feature and (b) Router Used During Manufacture

Other Wooden Parts

There are other wooden parts such as the lower tensioning block, upper motor spacer and the castor

mounts which were manufactured using a band saw, a pillar drill and a sander. Indeed, these

machines offered enough accuracy and the advantage of simple and quick manufacturing.

6.2.3. Metal Parts

Sheet Metal

The final design contains a number of sheet metal parts which were manufactured in the Student

Teaching Workshop. The parts were made of either 1 mm thick aluminium or 2 mm thick mild steel

and were cut to the required dimensions using a guillotine and, for some features, a band saw. A

metal folding machine was then used to introduce the required angles. These machines were found

to be the simplest, least dangerous and fastest way to manufacture these components. The

tolerances to be met as defined by the drawings were also found to be achievable using these

machines. Figure 6.9 shows the guillotine and bending machine used.


41

(a) (b)

Fig 6.9: (a) Guillotine used to cut sheet metal and (b) Bending machine used to bend sheet metal

Metal Blocks

Some components were too thick to be manufactured with the guillotine. These were namely, the

Stirling engine support plate, the aluminium plate under the base plate and the aluminium spacer for

the upper motor. These parts were principally produced using mills and pillar drills. The shaft plate –

under the lower base plate – needed, nevertheless, to be put in a four jaw chuck in order to

manufacture the counter bore.

6.2.4. Fasteners

The choice of fasteners followed directly from the design of the final product. The correct sizes were

picked. It was decided however for aesthetic reasons that socket head bolts would be preferred to

hexagon head bolts. Only the upper frame assembly comprises hexagon head bolts. These bolts

generally used for structural application were chosen because socket head bolts were not available in

the dimension needed.

Fastener choice for wooden parts was slightly different. For parts which sustain large forces the use

of a bolt on one side and a nut on the other was preferred. An example is the lower transmission

system with the lower motor plate and lower tensioning block fixed on the plywood plate by bolts

and nuts. However, most other parts were either fixed using self-tapping screws or threaded bolts

with the adequate wood inserts. The frequency of disassembling was a crucial factor for the fastener

choice. For example, the castors, which are not disassembled very often, were fastened with self-

tapping screws. On the other hand, the lens or the legs which are removed relatively often for safety

and transport were fixed with M4 bolts and corresponding M4 inserts tapped in the wood which have

a longer longevity than the wood alone.

6.2.5. Control System

The manufacture of the control system required the assembly of the circuit in accordance with the

circuit diagram in sec.5.3.1, and the writing of the control system program to match the flowcharts

displayed in sec.5.3.3. This involved attaching the components onto prototyping circuit board (PCB)


42

using a soldering iron and some solder. This was a fairly time consuming process and somewhat

dangerous as it was necessary to deal with molten metal. Since high temperatures of up to 400 C

were reached, soldering had to be done quickly so as not to damage any of the electrical components

purchased. Fig.6.10. shows the potential divider circuit and how the components have been soldered

onto the PCB.

Figure 6.10: Front (left) and Back (right) of Potential Divider Circuit

Once the components had been successfully soldered onto PCB, heat shrink or electrical tape was

applied around individual wires in order to protect the electrical components from external contact,

and to prevent adjacent wires from coming into contact with each other. Silicon was then applied

around key circuit components in order to protect them from contact with water. In order to further

protect the electrical components from external influence, a control system box was designed to

provide an external housing to the Arduino MCU, batteries, motor driver and relay circuit.

Several measures were taken to ensure quick assembly and disassembly. Firstly, in order to prevent

the wires from coming loose from the Arduino circuit board, they were first soldered onto a right

angle male header row, which could then be securely plugged into the corresponding female header

rows on the MCU board. Deans and bullet connectors were also used to connect wires together.

Fig.6.11. contains an image of the different connectors used in the assembly of the circuit.

Figure 6.11: Usage of Header Rows (Left), Deans Connectors (Center) and Bullet Connectors (Right) in Assembly of Control System Circuit

A wire colour convention was also defined for the different LDRs and ground in order to avoid

confusion when assembling the circuit. This convention is defined in table 6.1.


43

Table 6.1: Wire Colour Convention

Colour Convention

Grey Top LDR

Black Bottom LDR

White Left LDR

Red Right LDR

Brown Ground

Having already set up the hardware, the control system program was written using the Arduino IDE.

The Arduino could then be connected to the rest of the circuit and the control system could be

calibrated. Calibration of the control system was important as it allowed the team to account for the

errors and different characteristics of the control system components. This could be done fairly easily

by using the Arduino IDE to modify tolerances and incorporate zero errors in the program. Figure

6.12 is a screenshot of the Arduino IDE.

Figure 6.12: Screenshot of the Arduino IDE

A full copy of the control system program calibrated for use at a solar intensity of 850W/m2 can be

found in the Appendix E.

6.2.6. Painting

Painting was carried out using brushes. Brushes were preferred to spray paint or baked paint because

the costs associated with the two latter processes would have been excessive considering the budget

limit. All metal components were degreased using paint thinner and painted under ventilated

conditions. The wooden plates, being quite large, did not fit into the painting room of the IDEA’s

workshop. For that reason, they were painted outdoors under sunny conditions.

The hot end of the Stirling engine, reflecting a large amount of the focused light, was painted black

with a spray of High Temperature Resistant paint (stove paint, matt black). This can be seen in

fig.6.13. The hot end was sand blasted prior to being painted in order to obtain a good adhesion to


44

the steel. The manufacturer (ThermaCure9) specified that the paint could sustain temperatures up to

650°C. However, testing revealed that this paint could not resist the temperature experienced by the

hot end during operation.

Figure 6.13: Hot end of the Stirling engine after painting

6.3. General Assembly

Following all design amendments during manufacture and assembly phases, the final design is

presented in fig.6.14.

Figure 6.14: Assembly of Final Design

9 Thermacure. http://www.thermacure.co.uk/ [Accessed 04/06/2011]


45


46

7. Testing

In order to measure and quantify the level of success of the project, a range of tests were conducted.

The motivations for the tests undertaken are outlined in the following section as well as the

respective results and comments.

7.1. Performance Tests

7.1.1. No Load Speed with Lens Partially Covered

In order to determine the performance of the lens, it was decided to record the minimum solar

energy input required to sustain continuous operation of the Stirling engine. Rather than testing the

solar collector at different levels of solar radiation, different energy input levels were achieved by

covering parts of the lens. Testing was conducted on Friday 3 June from 12:10pm to 1:40pm. The lens

was partially covered in sections of area fraction 1/8 and the no load speed was recorded together

with the solar radiation at each instant. The speed was measured using a laser tachometer (Concorde

CT6) and the solar radiation data was taken from the Westminster station of London Grid for Learning

Weather Monitoring System.

It was found that the Stirling engine would run when 5/8 or less of the lens was covered. Figure 7.1

presents the relationship between the no load speed and the effective solar radiation intensity

( ).

Figure 7.1: Speed of Stirling Engine at Different Values of Energy Input.

7.1.2. Control System Update Frequency

In order to determine how frequently the control system would need to update the tracker position

to sustain the Stirling engine it was decided that the engine characteristics needed to be known in

greater detail. The importance of this test comes from the fact that the Stirling engine needs initial

0

100

200

300

400

500

600

700

800

900

1000

0 200 400 600 800 1000

Ro

tati

on

al s

pee

d (

rev/

min

)

Effective solar radiation intensity (W/m²)


47

energy input to the flywheel to run. If the engine cools down and stops human input is required to

restart it.

A performance test in operating conditions was conducted at 2:55pm on Saturday 4 June. Solar

radiation intensity was 761 W/m2 and the engine was running with no load at 700 rev/min. At this

speed the lens was covered entirely with cardboard to prevent any sunlight from hitting the Stirling

engine. This situation simulated a dense cloud or alternatively a large error in the tracker

performance. The engine was observed to slow down and eventually stop after 1 minute 45 seconds.

It was therefore decided that the delay between cycles needed to be shorter than this period in order

to minimise the risk of the engine grinding to a halt without possibility of recovery, and that the initial

proposed delay time of 5 minutes was too long.

7.1.3. Transient Speed Test

A test was conducted to determine the stability of engine speed over time. A laser tachometer was

used to measure the rotational speed. The focal point was manually positioned on the Stirling engine

hot end to ensure that the results were independent of the solar tracker performance.

Figure 7.2: Speed of Stirling Engine at No Load

As demonstrated in fig.7.2, the engine speed dropped when clouds covered the sun and the engine

accelerated when exposed to direct sunlight. It was also noted that acceleration at around 750

rev/min was slower than the acceleration at lower speeds. This is likely to be explained by the

dynamics of the system. At approximately 750 rev/min the steel frame started vibrating with large

amplitude, most likely excited by the running speed of the engine.

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25 30 35

Stir

ling

engi

ne

spe

ed

(re

v/m

in)

Time (minutes)


48

7.1.4. Continuous Tracking

A tracking test was performed to determine whether the control system would successfully track the

sun and focus sunlight onto the hot end of the Stirling engine and thus provide sufficient energy input

to sustain continuous operation without human input.

The test was performed at 12:30pm on Saturday 4 June 2011 and the solar radiation intensity was

900 W/m2. The program code was calibrated to position the focal point at the Stirling engine hot end.

Having achievement adequate focal point position, the engine was allowed 90 seconds to heat up

and the flywheel was subsequently set into motion. The engine was left running and the control

system updated the tracker position with 5 seconds intervals. The control successfully tracked the

sun and the lens provided sufficient energy for the engine to run without stopping for 38 minutes 55

seconds. At this point a large cloud covered the sun for more than 2 minutes. With no energy input

for the period of cloud coverage there was not enough angular momentum in the flywheel to sustain

continuous operation of the engine.

Successful tracking for 40 minutes corresponds to a rotation of 10° in the horizontal plane. It can

therefore be concluded that the performance of the control system is good enough to allow for

continuous tracking and therefore ability to sustain the Stirling engine. This is a key result and will be

discussed further in sec.9.

7.1.5. Tracking System Accuracy

The accuracy of tracking was tested to verify that the control system would adequately move the

frame to face the sun at all times.

Fig 7.3: Tracking Accuracy Test

The control program was run 22 times at 15 second intervals to determine the stability of the system.

Testing started at 5:15 pm after calibration of the control code for a radiation intensity of 530 W/m2.

The error was measured by visual inspection since handling of equipment close to the focal point


49

would be too dangerous due to the very high temperatures achieved. Having defined the centre of

the Stirling engine hot end as the datum, it was observed that all measurements were within

of the desired position. This level of accuracy ensured that the focal point was never located outside

the engine nor on the cold end. Figure 7.4 shows the results from the test.

Figure 7.4: Position of focal point relative to ideal position

7.2. Design Tests

In the Product Design Specification several constraints were enforced on the design to ensure that

the final product would be user friendly and practical. Portability, assembly and dismantling tests

were undertaken in order to confirm that the design requirements had been met.

7.2.1. Time of Assembly

In order to quantify ease of assembly it was required to measure the time taken to assemble the

structure from the individual components. Two people equipped with a hammer, an Allen key set, an

adjustable spanner and a screw driver took 25 minutes to assemble the product. Disassembly could

be done by two people with the same tools in 20 minutes.

7.2.2. Compact Design

It was desired to verify that the assembly could be easily stored whenever not in use. The product

was dismantled and the components ordered to occupy a minimum of space. Figure 7.5 shows the

space required to store the product. The base plates are 1220 mm in diameter and the maximum pile

height is 225 mm.

0

1

2

3

4

5

0 5 10 15 20 25

Erro

r (c

m)

Tracking Try

Pan

Tilt


50

Figure 7.5: Position of Focal Point Relative to Ideal Position

7.2.3. Portability

The PDS required the final design to be light enough to be carried and transported by two people.

This was tried in order to confirm that the target had been met.

Figure 7.6: Portability Test


51

8. Expenses

The team achieved the goal of maintaining the project budget of £600.

Table 8.1: Table of Expenses and Parts

Item Supplier Price (Ea) (£) QTY Total incl. Shipping

Control System

LDR Farnell 327700 1.59 4 6.36

NPN Transistor ZTX688B RS Stock No. 669-7537 0.53 5 2.65

Arduino Uno MCU Board Farnell 16.26 1 16.26

Prototyping board Farnell RE520-HP – PCB 2.31 2 4.62

Diode 1000V RS Stock No. 628-9546 0.029 10 0.29

USB Printer Cable Ebay 0.99 1 0.99

Solder Farnell 1.17 1 1.17

Relay 12V DC Farnell 1.90 1 1.90

Header Row Right Angle Cool Components 1.54 1 2.14

ATMega 328 MCU Cool Components 6.00 1 6.60

Sabertooth Motor Driver 2X10A Robotbits 57.95 1 57.95

18 AWG zip wire Robot Market Place 1.99 3m 1.99

6AA Batteries Ebay 4.20 1 4.70

Desoldering Braid Farnell 1.08 1 1.08

Deans Connectors Ebay 3.47 2 6.94

Bullet Connectors Farnell 1.78 1 1.78

Bullet Connectors Jacks Hardware 4.00 2 8.00

Rocker Switch Farnell C6053ALNAE 0.84 1 0.84

DC Power plug 2.1mm Farnell 1812750 0.292 1 0.292

10A Figure 8 Zip Wire 3m Maplin 5.07 1 5.07

3.5mm Heatshrink 1m Maplin 2.09 1 2.09

Control System Total 133.712

Transmission, Motors & Battery

Geared DC Motor 12V Robot Market Place AME 214 16.56 2 67.00

3/8” Pitch Timing Pulley - 13 HPC Gears 17.18 2 41.86

3/8” Pitch Timing Belt – 647.70 mm HPC Gears 5.03 2 10.06

Epoxy Farnell 3.96 1 3.96

Set screws, keys ME Stores / RS 5.00 1 5.00

Battery 12V 9Ah - VARTA 509 014 Ebay 17.45 1 24.44

Bicycle Tube Cycle Surgery 5.00 1 5.00

Transmission, Motors & Battery Total 157.32

Frames

Fresnel Lens 1m x 0.75m Green Power Science 117.72 1 170

Exterior Ply 12 mm 8 x 4 ft Jennor Timber 27.99 1 27.99

4 pcs 31mm plate fix wheel castors Ebay 3.25 1 4.85

Steel 12m 3/4" x 3/4" x 1.5 mm + 6m 7/8" x 7/8" x 1.5 mm

Doré Metal Service 50.00 3 57.00

M10 x 1m studding ZP Farnell 1.10 1 1.10

M10 Flange Clamps x 3 Ebay 1.31 1 3.81

Paint Matt Black Farnell 4.03 1 4.03

Double Sided Foam Tape Jacks Hardware 3.29 1 3.29

Electrical Black Tape Jacks Hardware 2.59 1 2.59

Fasteners ME Stores 12.78 - 12.78

Gloves/Brushes/Cable Ties ME Stores 10.15 - 10.15

Frames Total 297.59

TOTAL 588.62


52

9. Discussion

The following section of the report is a discussion on how the team achieved its goals and the key

findings from the Solar Powered Stirling Engine project.

9.1. Assessment of Fulfilment of PDS Criteria

The Product Design Specification had been outlined in the Project Plan Report. Table 9.1 identifies

the key attributes of the final product recognised at the initiation of the project and the outcomes as

a method of assessing the degree to which certain objectives were achieved. Where appropriate, the

team aimed to cite quantitative data from test results.

Table 9.1: Comparison between PDS and Final Project Outcomes

Aspect Objective Criteria Outcome

Performance

Sustaining Stirling engine

Useful power output from solar collector must sustain Stirling engine when sunlight intensity exceeds 600 W/m²

Manufactured solar collector capable of sustaining Stirling engine operation under sunlight intensity of 300W/m2, exceeding expectations.

Reliability Stable and robust tracking system that effectively follows the path of the sun without user input

Control system and transmission system designed allow successful tracking of the sun to an accuracy of ±1 cm.

Temperature Operate between ambient temperatures 0<T<40°C

Not assessed in entire range. Solar collector tested working at temperature of 18 – 28°C.

Humidity Operate at 25-100% relative humidity

Not assessed in entire range. Solar collector worked in 45-75% humidity.

Cost Minimise Must not exceed £600 Total project expenditure £588.62.

Life Maximise Must exceed 20000h No suitable test available.

Size and weight Sturdy construction

Must withstand windy conditions Solar collector capable of operation under wind speeds of at least 10m/s

Easily rotatable solar collector

Must not stall tracking mechanism

Transmission system with plain bearings allow for smooth, and automatic rotation of solar collector without human input.

Portability Must be possible to carry using no more than two people

Solar collector designed to be flat-packable for easy transportation and storage. Can easily be transported by two people.

Manufacture Ease of manufacture

Minimise dependence on CNC manufacture

CNC not required for manufacture.

Availability of components

Maximise use of standard engineering components

Fresnel lens and motors imported from the USA. Sabertooth 2X10A motor driver no longer available for commercial purchase. All other components can be readily purchased in the UK.


53

Assembly Ease of assembly Minimise part count 273 Parts

Design Time

Minimise Finish by 21 January 2011

Design of product completed before preliminary design review on 28 January 2011. Few changes made to final design during manufacturing phase.

Patents

Ensure legality Avoid infringement

Solar collector system designed found not to infringe on any existing patents based on patent search conducted.

Aesthetics Professional Must be well-engineered

Product deemed to be aesthetically pleasing and professional.

Safety

High

House and waterproof electrical circuitry

Electrical circuitry protected from water by silicon and a protective housing. System still functioning after splash test.

Solar radiation protection for user Aluminium Heat Shield

House Stirling engine Aluminium Heat Shield

Recycling

Compliant Use recyclable materials

Bulk of frame manufactured from mild steel which is easily recycled. Wood used for based plates is bio-degradable. Rechargeable batteries used.

Maintenance

Easy

Parts must be easily accessible for replacement

Product can be easily disassembled for maintenance in 20mins

Material must be easy to clean Product can be easily disassembled for cleaning.

As can be seen from the table, most of the requirements initially identified by the team have been

met. Some of the criteria such as product life and humidity range are difficult to test and therefore to

supply numerical data for.

9.2. Management

Engineering Skills Developed

This project required the utilisation of a wide range of engineering skills. At the initial stages, it was

evident that some members of the team were especially talented in certain areas of engineering,

allowing for engineering tasks to be split accordingly. As the project progressed, team members grew

in engineering knowledge and gained practical experience, allowing the supplementation of theory

learnt in the classroom.

Firstly, the team put design skills into practice in order to arrive at a final design that was robust,

performed to specification, and was aesthetically pleasing. Ideation techniques were used in order to

better draw out and incorporate ideas from all group members. In accordance with the total design

process, market analysis was conducted and the product was truly designed with the end user in


54

mind, exposing the team to challenges that may be faced by an engineer working to design a product

for a customer.

In assessing the Stirling engine, it was necessary to have a command of heat transfer and

thermodynamics. This project introduced the team to a new and fairly complex Stirling cycle, which

no members of the team had previously encountered. Tests were designed to familiarise the team

with the externally combusted cycle and the engine, and determine the required conditions under

which the Stirling engine would run. Finally, the group was capable of suggesting changes which

could be made to the engine in order to allow more efficient performance.

The manufacturing section of the project allowed the team to utilise the skills developed in the ME1

manufacturing course. In fact, it called not only for the manufacture of very small, high tolerance

components, but also large structural members, which require different types of skills. Upon final

assembly of the product, analytical skills were developed in the identification, and rectification of

problems which the group had not previously anticipated during the design phase.

Design of the frame of the product exposed the team to the wide range of materials available for

purchase and use in engineering applications. It taught the team to choose materials based on

certain parameters, and the conditions under which the materials would be required to perform. The

team also learnt to conduct stress analysis on the design and to implement precautions to prevent

failure under predicted modes.

Finally, the team developed mechatronics skills in the design and construction of a control system

and belt transmission system. Team members learnt a new computer language in order to program

an Arduino Microcontroller Unit and faced several problems in the assembly of the control system

circuit. However, the team finally overcame these problems, picking up electronics skills along the

way. The team also managed to design a transmission system under budget constraint, implementing

mechanisms to prevent slippage in an economical fashion.

Publicity

The team’s enthusiasm about both the project, and the pertinence of solar energy under current

environmental conditions, is reflected in the publicity efforts that were undertaken. In the earlier

stages, the team exceeded expectations, and developed and still maintain a team website aimed at

creating an awareness of the project and increasing general public interest in concentrated solar

power systems. The website www.icsolar.co.uk contains information about the solar collector, the

techniques used throughout the project, some components used in its design, and some pictures and


55

videos taken during the course of the project. The team responded diligently to enquiries made by

members of the public through the website’s contact form. The team also took efforts to ensure that

the website was easily discoverable through search engines like Google by performing search engine

optimisation on the website.

The team’s publicity efforts attracted the attention of an entrepreneur by the name of Mr. Serge

Crasnianski who expressed an interest in the team’s project. Mr. Crasnianski was working on Stirling

engines at the time that this report was written and contacted the group to obtain some information

about the project and the Stirling engine used. The group assisted Mr. Crasnianski who responded by

inviting a representative from the group to the Intersolar Europe exhibition in Munich. Unfortunately,

due to constraints imposed by the project submission deadlines, the team was unable to take Mr.

Crasnianski up on his offer.

Project Management

The group tried to ensure smooth progress of the project by adhering to the project Gantt chart

which was written in the initial stages of the project. Along the way, it was soon found that other

commitments ensured that some of the targets implied by the project Gantt chart were unrealistic. It

was during these times that the group saw greatest deviation from the project Gantt chart. It was

made clear, however, that strict deadlines imposed by the college would have to be adhered to, and

all reports were submitted on time.

The team faced difficulty in following the Gantt chart during the manufacturing phase of the project.

Despite having planned in advance, the team was unable to correctly predict the impact of the

workshop timetable on the manufacturing timeline. Furthermore, the product comprises several

different components which had to be manufactured in several different workshops which added to

the complexity of the manufacture process. The team accounted for these issues by scheduling the

manufacture process so that the parts which required a certain type of machine that was in a

workshop which closed early (such as the mill), or a certain type of manufacturing process which

relied on external help (like welding), were completed earlier. This left the team with more time for

independent work in workshops which had later closing hours. In the end, the team exceeded the

scheduled manufacture completion date by approximately one week.

The team ensured smooth project progress by keeping meticulous accounts of all meeting minutes.

All files were also stored on the cloud and backed up regularly to minimise the risk of loss of work. By

the end of the project, it was safe to say that each team member cultivated an awareness of each of


56

the other team members working habits and styles, enabling the team to function much more

efficiently than it did at the beginning of the project.

Health and Safety

The team took health and safety very seriously throughout the progress of the project. Therefore the

project was completed without any injuries either to members of the team or the public. College

rules and health and safety guidelines were abided by and COSHH forms were filled in where

necessary. Being aware of the dangers of the project caused by the electrical transmission systems

and the high temperatures at the focal point of the lens, several precautions were taken to ensure

that the test was conducted in a safe and isolated environment, with relevant members of staff

notified, with warning signs in place to notify the general public, and with contingency plans in place

in case of safety breaches.

Budget

The team had initially anticipated some difficulty with adhering to the budget of £600. This was

overcome by importing expensive components such as the lens and the motor from the USA where

cheaper suppliers could be found, and allocating extra time for delivery. Careful budgeting allowed

the team to keep to the budget, spending a total of £588.62.

Some examples of measures taken to cut down expenditure included the manufacture of wooden

pulleys instead of purchasing large timing pulleys which would have been very costly and using LDRs

instead of solar cells. The team also made sure to purchase exactly the correct amount of material, in

suitable diameters to ensure little wastage, and used scrap metal available in the student and

research workshops where possible.

9.3. Results and Improvements

The results from sec.7.1.2. suggest that the Stirling engine will continue to run for approximately 1

min and 45 seconds from the moment the focal point is removed from the hot end. It was decided

that the tracking system should update more frequently so that an erroneous measurement would

not result in stalling the Stirling engine. The team suggests that depending on the light intensity and

the anticipated cloud coverage, delay should be adjusted to minimise power consumption without

risking stall. Delay could therefore be short on cloudy days but should not exceed 90 seconds even

under sunny conditions.

As reported in sec.7.1.3. large vibrations were observed at engine speeds around 750 rev/min. This

frequency corresponds to 12.5 Hz and is likely to be a natural frequency of the steel frame. When the

Stirling engine was running at speeds close to 750 rev/min it was clear that further acceleration was

slow and required high radiation intensity. In order to reduce the energy lost to vibration, damping


57

should be added to the system. Alternatively, dynamic analysis could be performed and design

changes implemented to reduce the natural frequency so that the Stirling engine could be easily

accelerated through resonance and achieve speeds exceeding 800 and 900 rev/min.

9.4. Mass Production

The current design involves several features that make it unsuitable for mass production. However,

by making suitable design amendments and introducing different manufacture processes, large scale

production could be much more attractive.

To begin with, the part count of 273 is high and could be reduced. For instance, various ad hoc

additions such as spacers, extra stiffening etc. could be reduced. Secondly, the design was developed

keeping in mind that all components were to be manufactured by team members, with the exception

of welding steel square tube that was required to be done by a technician. If the product were to be

mass produced, more automatic manufacturing methods would be available, in particular CNC

turning and milling. As a consequence, other design options would become available and priority

would turn to minimisation of human labour.

If large scale manufacturing was available, the product could be suitably redesigned to avoid the

need for welding and thus minimising the manual labour time. If welding could be avoided, the entire

frame could be made of aluminium alloy. This would not only make the frame lighter, but also make

it more resistant to corrosion, making painting unnecessary. The lighter frame would allow for the

use of smaller motors and even a weaker motor driver, reducing cost further.

One way of achieving a structure without welds would be to use thermoplastic joints onto which the

aluminium bars could be slid. With large scale production facilities, these joints could be injection

moulded and customised to accommodate the structure bars at the required angle. A suitable

thermoplastic for this application would be polypropylene.

Fig 9.1: Square Tube Connectors10

10 ESEDirect. http://www.esedirect.co.uk/p-808-square-tube-connectors-joints.aspx [Accessed 31/05/2011]


58

When pricing the mass produced version one would have to account for several additive price

effects. To begin with the bulk of the material used was standard metal square tubes and sheets

which can both be acquired very cheaply in large quantities. Secondly, the control system

components (the motor driver, motors, Arduino board etc.) could very possibly be negotiated to a

lower price when bought in large quantities. However, the real bottleneck would be the lens. The

Fresnel lens used in the product was a second hand lens previously used in another application. As a

consequence it was possible to procure the lens at a greatly reduced price. Since second hand lenses

become available when deemed unfit for other applications, the supply of lenses will be limited in

numbers. If lenses were to be manufactured and used directly, a much higher price would be paid

(most likely in excess of $1000). However, considering that the lens is made of acrylic (a relative

cheap material), it is very likely that a big discount could be negotiated if large quantities are ordered.

In addition, many components could be replaced. For instance, the large pulleys could possibly be

made from thermoplastic (e.g. polyoxymethylene) which would be cheaper in large scale production.

The pulleys could also be moulded to match the timing belt which would increase traction and thus

decrease the tension and torque requirements. As a consequence, less powerful motors would be

required. To save further on cost, the bronze bushes could be also replaced by polyoxymethylene

equivalents. When considering replacing metals with polymers in pulley and bearing applications,

detailed calculations must be undertaken to ensure sufficient strength.

A significant disadvantage of the current design is the low power output achieved from the Stirling

engine. At present, the system is unlikely to deliver positive net power since the motors will consume

more power than delivered by the Stirling engine. An essential part of the route to commercialisation

would be to acquire professionally built Stirling engines with high thermal efficiency.

9.5. Sustainability Considerations

Using concentrated solar power to produce mechanical work gave the project a strong emphasis on

environmental sustainability. During the progress of the project, adverse effects on the environment

were minimised by paying special attention when choosing components and materials for the

product, thus maintaining the sustainable profile. All batteries procured were rechargeable to avoid

unnecessary waste and long term additional spending. In addition, materials were ordered from

suppliers that could deliver in suitable quantities so that waste could be minimised. Where possible,

scrap materials were sourced in the Imperial College Mechanical Engineering Department, avoiding

purchase of new materials, which has been beneficial for the environment and the project budget.

The energy consumption of the control system and DC motors has been minimised by running the

system intermittently instead of continuously.


59

10. Conclusion

Fuelled by the current energy trends, technologies utilising solar power has been gaining increasing

pertinence. Team DMT04 set out to design a solar collector capable of sustaining an existing Stirling

engine throughout the day without human input.

Despite the relatively low solar intensity in the United Kingdom, the team succeeded in producing a

system that could successfully track the sun and sustain the Stirling engine through the usage of a

large Fresnel lens. The system included a frame, which was designed to position the hot end of the

Stirling engine at the focal point of the lens. A digital control system was designed to control both the

pan and tilt mechanisms of the frame, allowing the solar collector to track the sun.

Most of the requirements laid out in the initial product design specifications were met. Test results

showed a positive correlation between solar intensity and engine speed. It was found that a

minimum solar intensity of 300 W/m2 was required to sustain the Stirling engine. Furthermore, the

control system designed incurred a maximum focal point error of ±10 mm, and proved accurate

enough to ensure continuous operation of the Stirling engine in the absence of cloud interference.

Lastly, the team successfully adhered to the project budget of £600, spending only £588.62.

It is suggested that the possible future developments described by team DMT04 be implemented in

order to encourage the advancement of this technology.


60

11. Future Developments

The Solar Powered Stirling Engine project was undertaken as a continuation of a previous team’s

work creating a Stirling engine. DMT04 has therefore considered how their project could be

developed further in future DMT projects and the implications of such developments.

It was clear from the start that the Stirling engine had a low power output, so future development

could entail the procurement or construction of a more efficient Stirling engine. Efficiency could be

further improved by increasing the temperature difference between the hot and cold ends. This can

be achieved either through a reduction in temperature of the cold end or an increase in temperature

of the hot end. The cold end of the engine could be cooled by forced convection, enhanced finning or

installation of a water cooling system. Methods for attaining a higher temperature of the hot end

through the minimisation of radiation and convective heat losses should also be considered by future

groups.

For future development, other Stirling engine types could be investigated. In addition to the alpha

and beta models introduced in sec.2 one might consider the Free-piston Stirling engine (FPSE)11. In

contrast to the gamma configuration Stirling engine used in the project, the FPSE has a piston which

drives a linear alternator as opposed to a flywheel. With fewer moving parts, the FPSE does not

require lubrication and high-pressure seals, and friction and wear are substantially reduced.

With regards to the control system, it is recommended that solar cells, which could be used instead

of light dependent resistors, be experimented with. These are active transducers which use the

photovoltaic effect to produce a voltage which varies linearly according to light intensity. These solar

cells could possibly even be used to power the control system, eliminating the need for batteries and

charging, making it truly self sustaining.

The Stirling engine is not self starting, and requires a kick to impart an initial momentum to the

engine. It is feasible to connect a DC machine to the flywheel of the Stirling engine. It can then, be

used in conjunction with the Arduino MCU to provide the initial kick to the Stirling engine, and as a

generator to create usable electrical output when the Stirling engine is running continuously.

Furthermore, excess power output could be stored for later consumption by for example recharging

batteries.

11 Lane,N. W. & Beale, W. T. (1996) A Free-Piston Stirling Engine-Alternator for Solar Electric Power. Sunpower, USA.


61

12. References

[1] International Energy Agency (IEA). http://www.iea.org/index_info.asp?id=1959 [Accessed

02/06/2011]

[2] Department of Energy and Climate Change. www.news.bbc.co.uk/1/hi/sci/tech/8150919.stm

[Accessed 20/05/2011]

[3] Space Future. http://www.spacefuture.com/power/introduction.shtml [Accessed 22/05/2011]

[4] Wikipedia. http://en.wikipedia.org/wiki/Stirling_engine [Accessed 07/05/2011]

[5] Guardian. http://www.guardian.co.uk/environment/2009/may/26/solarpower-

renewableenergy [Accessed 18/11/2010]

[6] Wikipedia. http://en.wikipedia.org/wiki/Concentrated_solar_power [Accessed 18/11/2010]

[7] Solar Central. http://solarcentral.org/ [Accessed 19/11/2010]

[8] Green Power Science. http://greenpowerscience.com/ [Accessed 12/12/2010]

[9] Thermacure. http://www.thermacure.co.uk/ [Accessed 04/06/2011]

[10] ESEDirect. http://www.esedirect.co.uk/p-808-square-tube-connectors-joints.aspx [Accessed

31/05/2011]

[11] Laser Physics. http://www.laserphysics.co.uk/laser_beam_dump.html [Accessed 26/05/2011]

[12] Lane,N. W. & Beale, W. T. (1996) A Free-Piston Stirling Engine-Alternator for Solar Electric

Power. Sunpower, USA.

http://www.news.bbc.co.uk/1/hi/sci/tech/8150919.stm

http://www.spacefuture.com/power/introduction.shtml

http://www.guardian.co.uk/environment/2009/may/26/solarpower-renewableenergy%20%5bAccessed%2018/11/2010

http://www.guardian.co.uk/environment/2009/may/26/solarpower-renewableenergy%20%5bAccessed%2018/11/2010

http://en.wikipedia.org/wiki/Concentrated_solar_power%20%5bAccessed%2018/11/2010

http://solarcentral.org/


62

13. Acknowledgements

Team DMT04 would like to acknowledge the following people for their contribution to the successful completion of the project:

Dr. Ricardo Martinez-Botas

Mr. Harminder Flora

Mr. Gordon Addy

Mr Paul Woodward

Mr Dave Murphy

Mr. Asanka Munasinghe

Dr. Fred Marquis

Dr. Ferdinando Rodriguez y Baena

Mr. David Sweeney

Mr. Mark Holloway

Mr. Dave Robb

DMT12_2010


63

Appendix A: Calculations – Stress Analysis of Frame Bars

Thinner square tube

Bar dimensions:

Cross sectional area:

Weight of bars:

Second moment of Area:

Thicker square tube

Bar dimensions:

Cross sectional area:

Weight of bars:


Diagonal Bars

Length of bars:

Weight of steel frame:

Lens and Stirling Engine:

Total weight:

Length of load carrying bar:

Force per load carrying bar:

Maximum bending moment:


Maximum stress:

It is proposed that a 12 mm hole is drilled through the bar. Stress concentration factor for the hole is estimated at and a new second moment of area is calculated.


Maximum stress:

A-bars (midpoint bars)

Bar length of front frame half:

Bar length of rear frame half:

Weight of Lens:

Weight front half of frame:

Weight of Stirling Engine:


64

Weight rear half of frame:

Force per load carrying bar:

Maximum bending moment:


Maximum stress:


65

Appendix B: Calculations – Battery Life

Assumptions made throughout battery life calculations:

8 hours of sunlight of intensity above 600W/m2 on a given average day.

Discharge current of 32mA for the 6AA batteries

6 x AA Batteries

Battery life of AA Battery =

Total Battery life of 6AA Batteries = 7

Discharge time of 6AA batteries per day =

Battery life in days =


66

Appendix C: Calculations – Control

Fig C.1: Circuit Diagram for Determination of R1 Resistor

Relay coil resistance,

By Ohm’s law, Vr=IR

Current that will flow through relay,

Transistor forward current gain,

At 5V, 2.0mA,

Base current,

From E12 series, use a 15kΩ resistor.


67

Appendix D: Calculations – Transmission

The figures below show the components and variable considered when performing initial torque

calculations.

Figure D1: Torque Analysis of the Upper Transmission System

Figure D2: Torque Analysis of the Lower Transmission System


68

Figure D3 shows the specifications of the motors that will be used.

Figure D3: Motor AME 214 Specifications

The different models of timing pulleys available from HPC gears were used to obtain the graph below.

The graph shows the maximum tensile force in the belt and the price of the transmission system as a

function of the gear ratio (between 2 and 4) and of the pulley diameter (between 30.32 mm and

54.57 mm).

Figure D4: Maximum Tensile Force in the Belt and Price of one Transmission System versus the Gear Ratio for Various Small Pulley Diameters

0

5

10

15

20

25

30

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

2 3 4

Pri

ce (

Po

un

ds

Ste

rlin

g £

)

Forc

e (

N)

Gear Ratio

Force

Price


69

The graph was constructed from the table below. Here, the table only shows data for a gear ratio of

5.

Table D1: Geometry and Force Analysis of the Belt for a Gear Ratio of 5

Gear Ratio 5

Diameter 1 (mm) 30.32 33.35 36.38 39.41 42.45 45.48 48.51 51.54 54.57

Diameter 2 (mm) 90.96 100.05 109.14 118.23 127.35 136.44 145.53 154.62 163.71

Distance btw Axis (mm) 260.00 260.00 260.00 260.00 260.00 260.00 260.00 260.00 260.00

Delta 1 (deg) 6.70 7.37 8.04 8.72 9.40 10.07 10.75 11.43 12.12

Delta 2 (deg) 83.30 82.63 81.96 81.28 80.60 79.93 79.25 78.57 77.88

Alpha (deg) 166.61 165.26 163.91 162.56 161.21 159.85 158.49 157.13 155.77

Friction Coefficient 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30

Torque (Nm) 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00

Tension 1 (N) 679.99 621.37 572.56 531.32 495.90 465.36 438.68 415.19 394.35

Tension 2 (N) 284.22 261.55 242.71 226.82 213.21 201.51 191.31 182.36 174.45

Verif 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00

Length 581.00 587.13 593.28 599.43 605.60 611.77 617.94 624.12 630.30

Price belt 6.37 6.37 6.37 6.37 6.37 6.37 6.37 6.37 6.37

Price pulley 19.05 19.24 19.59 19.86 21.69 22.96 23.40 24.26 24.92

Price total 25.42 25.61 25.96 26.23 28.06 29.33 29.77 30.63 31.29


70

Appendix E: Control Program

//Define I/O pin positions int OutT=11; //Output Tilt - Motor Driver S1 int OutP=10; //Output Pan - Motor Driver S2; int OutRe=13; //Output to Relay Circuit

//Define initial values int BaseU=0;//Set initial LDR readings to 0 int BaseD=0; int BaseL=0; int BaseR=0; int DifT; //Define integer for difference between top and bottom int DifP ; //Define integer for difference between left and right int tol=1; //tolerance value (can take any value between 1 and 1024) int n=1; //loop counter for main loop int n1=1; //loop counter for pan int n2=1; //loop counter for tilt int Tiltz=-144; //-ve to make spot go up int Panz=35; //+ve to make spot go left int Ta=157; //PWM for tilt int Tb=97; int Pa=214; //PWM for pan int Pb=40; void setup() pinMode(A0,INPUT);//Define analogue pins as input pinMode(A1,INPUT); pinMode(A2,INPUT); pinMode(A3,INPUT); pinMode(OutT,OUTPUT); //Tilt output pinMode(OutP,OUTPUT); //Pan output pinMode(OutRe,OUTPUT); //Relay output void loop() n=1; //Reset loop counter while (n<=2) //repeat 2 times n++; //adds 1 to loop counter n1=1; //Reset loop counters n2=1; analogWrite(OutT, 127); //PWM takes values between 1 and 256 analogWrite(OutP, 127); digitalWrite(OutRe, HIGH); //Relay on //tilt mechanism BaseU=analogRead(A0); //Reads in UP analogue voltage (this is a number between 0 and 1024) BaseD=analogRead(A1); //Reads in DOWN analogue voltage DifT=BaseU-BaseD;


71

while(n1<200 && (abs(DifT)>tol) ) //While loop with 2 conditions. Controls tilt n1++; //Increase loop counter by 1 BaseU=analogRead(A0); BaseD=analogRead(A1); DifT=BaseU-BaseD-Tiltz; if (DifT > 0) // Sun on top < Sun on Bottom analogWrite(OutT, Ta); delay (50); else analogWrite(OutT, Tb); delay (50); analogWrite(OutT, 127); //Reset pin 11 to 0rpm //pan mechanism BaseL=analogRead(A2); //Reads in LEFT analogue voltage BaseR=analogRead(A3); //Reads in RIGHT analogue voltage DifP=BaseL-BaseR; while(n2<200 && (abs(DifP)>tol) ) //While loop with 2 conditions. n2++; BaseL=analogRead(A2); BaseR=analogRead(A3); DifP=BaseL-BaseR-Panz; if (DifP > 0)//Sun on left < Sun on right? analogWrite(OutP, Pa); delay(50); else analogWrite(OutP, Pb); delay(50); analogWrite(OutP, 127); //Reset pin 10 to 0rpm digitalWrite(OutRe, LOW); //Relay off. No power to driver. digitalWrite(OutP, LOW); //No logic output from pin10 digitalWrite(OutT, LOW);//No logic output from pin11 delay(100000); //delay till next loop in milliseconds


72

Appendix F: Control Flowchart

Figure F1: Pan Sub-Process Flowchart


73

Individual Critiques

Andrew Tan

Participating in the DMT project has been a valuable experience for me, and I thoroughly enjoyed

being a member of Team ICsolar. Despite already having been acquainted with the other members of

my 4-man team, it took a while before we each came to an understanding of each other’s working

styles. The project we chose was sufficiently challenging to keep us occupied, and solar energy is a

topic of particular interest to me, having come from Singapore – a country which has spent large

amounts of money on solar energy research. Throughout the project, the group not only formed

bonds in school whilst working on the project, but whilst engaging in social activities outside of

school.

The team did not adhere strictly to the minimum requirements of the project, but saw it as an

opportunity to practise skills which could prove useful during our future careers. We sought to create

publicity for the project by creating a team website, wrote to magazines to try and get our project

published, and looked for sponsors. The actual project was conducted as professionally as possible,

as if working for a company, and all meetings were taken very seriously. I also cherished the fact that

our group was given a fair amount of independence from the supervisors to try out our own ideas.

This project also gave us the opportunity to develop the important skill of being able interact with

people outside of the college.

At the beginning stages of the project, it was decided that I should undertake the role of project

manager. This was one of the easiest projects that I have had to manage as each member of my

group was well aware of their individual roles, acted extremely responsibly, and needed little

encouragement to achieve the goals of the project. However, as always, it is difficult to manage a

team of peers and it took a while before I felt that everybody was truly comfortable with their place

within the group. This is a useful skill which this project has helped me to improve. As project

manager, it was my duty to organise meetings, manage the budget, try to mitigate risks, and ensure

that the project proceeded in a smooth and timely fashion. Despite the fact that every group member

was capable of making decisions on their own, I found that it was still important to have a project

manager that could tie them all together.

After some time, group members became confident enough to voice their opinions. I participated in

heated discussions with other group members concerning key aspects of the project, and found that

these discussions were good for addressing conflicting views and essential to project progress. Good

project management did not require that all group members agreed completely on every aspect of

the project, but that some group members were mature enough to allow ideas which they may

initially not have supported to come to fruition. In order to be effective, it was important to always


74

pick the correct person for the job, and not assign roles to people that they were not confident in,

even if this required some people within the group to assume a slightly higher workload than others

during certain stages in the project. Ultimately, however, I found that every person contributed

practically equally to the success of the project.

If I were to perform the project again, I would like to ensure better time management as this is

something which I feel I could have orchestrated better. During some critical stages of the project,

time was lost when all group members were called together for meetings which, despite having a

clear purpose, did not require that all group members were present, and in which some group

members did not have a role to play. This was ideally to allow every group member to feel that they

were participating in the project. However, this practice actually wasted time which idle group

members could have better spent elsewhere. This could be avoided by splitting up tasks properly,

allowing each group member to complete a certain amount of work independently, and organising

meetings only for consolidation.

Overall, I would say that this project has not only been useful for the development of my technical

engineering skills. It has been quite enjoyable, and has been an excellent platform for the

development of competencies that will eventually be used in my future career.

Maira Bana

The success of a project can be attributed to many factors, be it good design, skilled manufacture or

completeness in approach; but centre to all of these is a strong team, a team that works as a whole

and as individuals, with members who are in sync but continue to challenge one another. DMT04

consisted of four enthusiastic and diligent students, making for a strong group dynamic and a very

enjoyable project.

The project was chosen after the team formed, and it was clear that all members were excited to

work on a solar-related project. The idea of continuing the work of last year’s DMT Stirling Engine

project was appealing due to the quality of the product.

From the start, the team realised that organisation would be key to achieving our goals. We identified

four positions of responsibility and assigned them to members according to individuals’ strengths and

interests. This meant that all aspects of the project could be managed effectively by different

members, while being overseen by the Project Manager. We were keen to maintain organisation

through the recording of Meeting Minutes, a task that was shared equally between all team

members.

The team aimed to never compromise on quality, and this was, as anticipated, difficult at times.

However, on reflection, it seems that the team found the most elegant and feasible solutions to


75

problems encountered. Early on in the project, the team was concerned about exceeding the budget,

but careful reconsideration and sourcing of items meant staying within the cost limit.

The execution of the project could have been improved with respect to timeliness. Whilst we had

planned our timings, and succeeded in meeting deadlines, there was a phase during the project when

we were pressed for time.

It was clear from the start that there would not always be agreement within the team, but these

situations were managed well. Contestable decisions were not made without discussion, and this

made for comfortable relationships between the team members.

My role within the team was Editor-in-Chief. This involved leading the team in the writing of reports

and poster design. It was important to ensure equal designation of tasks, and to then edit reports as

a whole to ensure they read well. Due to health and safety reasons, I played no part in the

manufacture of the product other than assisting in the building of the control system. The other

three members of the team therefore had a lot of work to do. Without permission to enter the

workshop, I found that I became slightly out-of-touch with changes in design and the progress of the

team. In the future, I would therefore aim to have better communication with the team during the

manufacturing phase.

The enthusiasm of the team was evident throughout the project. The ICSolar website is the perfect

example of how we aimed to publicise our work and spark an interest in solar technologies in others.

Looking back, I can say that it has been a pleasure working on such an interesting and exciting

project, with such talented individuals and such a strong team.

Charles Peurois

The Design Make and Test project has been very beneficial to me. The project enabled me to improve

my organisational, project management and time management skills. In my opinion, the key reason

for the success of the project was the fact that goals, roles and responsibilities were clearly outlined

from the start of the project. The early organisation of the project was indeed very advantageous

since all the important decisions were drawn from a solid foundation. Another very important

characteristic of team ICSolar was that the work assigned to each team member corresponded to the

field in which they were most motivated, experienced and likely to perform well. The different

working styles and backgrounds of each member allowed for a wide range of opinions. Group

discussions were indeed very interesting and enriching. All members felt comfortable explaining what

their thoughts were and were keen on contributing to discussions.

I found the engineering discipline of the project really interesting, especially because renewable

energies and solar power are becoming increasingly important in today’s society. The work load was

satisfying and I truly enjoyed participating in this project.


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My role in the team was manufacturing manager. As such, a big part of my responsibilities was to

ensure that the designs agreed upon during meetings were sensible and practical from a

manufacturing point of view. I also had to supervise the manufacturing stage of the project. I found

that role very exciting because the integration of design and manufacture has interested me for a

long time. However, manufacturing is not the only area in which I contributed and I think that is why

the DMT project is a very enriching experience. I also had the opportunity to assist other members in

their tasks, such as CAD modelling, and to work on creating the website for our project. Furthermore,

it enhanced my knowledge in engineering, especially in the fields of solar energies and mechatronics.

This task diversity made my experience much more rewarding and enjoyable.

In my next group project I would like to insist more on keeping up with the timeline of the project.

Even though this DMT was planned reasonably and the required goals were achieved, I felt at some

times in the project that the team was behind schedule or that valuable time was lost. This was

particularly obvious before taking an important step forward, like starting manufacturing. Although

the timeline was clearly defined by the Gantt chart at the beginning of the project, the use of such

charts could have been optimised and more regular reference to this timeline would have ensured a

smoother operation of the project. Another aspect to which I would pay more attention in the future

is to perform preliminary test of important parts delivered by suppliers. Since there is always a

degree of uncertainty in the quality or reliability of ordered components, it is important to verify that

core components comply with the specifications or expected values so that no surprises are found

after manufacturing.

In summary, this really was practical experience of what a professional project might be like. I

sincerely believe that it has prepared me well for some challenges that I might come across at the

start of my professional career or postgraduate education.

Marcus Ulmefors

The management of the project evolved naturally without the need of employing an artificial

structure or enforcing strict roles. Having said that, every area of the project was the responsibility of

one of the team members. No task fell between two chairs or was forgotten, which can happen if

responsibility is unclear. However, no team member was ever left completely alone with a large

portion of the project. There was always a second person with deep knowledge of that section that

could be a discussion partner and develop ideas. No major problems were encountered in the

internal management of the group, partly because the team members knew each other beforehand

and knew what would be expected from them. One person being ultimately responsible but having

secondary support has worked very well and is something I will use in the future. With team

members taking different electives throughout the year it sometimes proved difficult to plan


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meetings with the group and both supervisors present. However, the quality of the work never

suffered since all team members were always eager to contribute to the development of the project.

This is perhaps best exemplified in the creation of the team website which was not part of the

objectives but was both fun and incidentally resulted in promising industrial contacts.

Personally, my largest area of responsibility was initially the mechanical design. This involved material

selection, solid modelling, dimensioning and creation of drawings. All team members were of course

heavily involved in both the conceptual design and detail design phases, but it was my responsibility

to provide information on whether materials could be procured quickly and within budget. Secondly,

the combination of having spent 10 weeks on a UROP placement last summer in addition to my role

as a student representative proved useful because of the knowledge acquired about the Department

during this time. Having direct connections with academics, administrative staff and technicians

saved time since we would know quicker who to address when a question arose. This was particularly

useful towards the end of the project when planning time, location and procedures of testing.

At the end of the project we found ourselves in a rush because we were dependent on sunny days to

conduct our testing. The London climate obviously would not provide a long period of uninterrupted

sunshine and we were probably lucky to get some good sun hours on suitable dates at the very end

of the project. Perhaps we should have tried to have a bigger margin of error to account for a long

streak of clouds and rain. An additional lesson learnt came from the focal length surprise. Although

having conducted a test that confirmed the manufacturer's description, we found ourselves having to

reconsider the frame design which wasn't very flexible since it was already welded. A good solution

was found but next time I will definitely be more careful and test more meticulously the data

provided by suppliers.

This project has run very smoothly in terms of management (although there have been many

engineering challenges), very likely because all team members knew each other beforehand. All team

members have been very comfortable voicing their opinions which has benefitted the progress of the

project. Looking back at the project from start to finish, I am very impressed with what has been

achieved. The project's very open brief gave a lot of design freedom at the early stages. Having seen

the diverging concepts come together to form a complex system of mechanics, transmission,

mechatronics and programming that actually works and meets set targets has been greatly

rewarding. In coming projects the most important thing to keep in mind will be to be ready for more

difficult group dynamics and the fact that the luxury of choosing my colleagues cannot be taken for

granted.

dmt04 - ulmefors€¦ · of solar power and stirling engine technologies. the dmt04 team was ......

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