solar collector - flat plate anaylsis

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NATIONAL INSTITUTE OF TECHNOLOGY DEPARTMENT OF MECHANICAL ENGINEERING MAJOR PROJECT REPORT - PART 1 (7 TH SEMESTER END EVALUATION) DESIGN OF LOW COST SOLAR DRYER FOR FISH DRYING UNDER THE GUIDANCE OF: Dr. T.P. ASHOK BABU SUBMITTED BY: ASHWATHI SHENOI (08M106) HARSHA GOPARAJU (08M117) SHARATH N YALE (08M141) SURESH NEHRA (08M151) VIJAY KUMAR (08M160) SUBMITTED ON: 17 th November 2011

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Page 1: Solar Collector - Flat Plate Anaylsis

NATIONAL INSTITUTE OF TECHNOLOGY

DEPARTMENT OF MECHANICAL ENGINEERING

MAJOR PROJECT REPORT - PART 1

(7TH SEMESTER END EVALUATION)

DESIGN OF LOW COST SOLAR DRYER FOR

FISH DRYING

UNDER THE GUIDANCE OF:

Dr. T.P. ASHOK BABU

SUBMITTED BY:

ASHWATHI SHENOI (08M106) HARSHA GOPARAJU (08M117) SHARATH N YALE (08M141) SURESH NEHRA (08M151) VIJAY KUMAR (08M160)

SUBMITTED ON:

17th November 2011

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Contents

ACKNOWLEDGEMENT ................................................................................................................ 1

OBJECTIVES ................................................................................................................................ 2

CHAPTER 01: INTRODUCTION .................................................................................................... 3

CHAPTER 02: LITERATURE SURVEY ............................................................................................ 5

PATENTS REVIEWED ............................................................................................................... 5

DATA SURVEY AND COLLECTION ............................................................................................ 8

Solar Irradiance data........................................................................................................... 8

Food related data ............................................................................................................... 9

PERFORMANCE PARAMETERS COMMONLY CONSIDERED FOR EVALUATION .................... 10

CONCLUSIONS: ..................................................................................................................... 11

CHAPTER 03: SOLAR DRYER - WORKING PRINCIPLES .............................................................. 12

CHAPTER 04: SOLAR FLAT PLATE COLLECTOR ......................................................................... 15

Introduction ...................................................................................................................... 15

Absorber Plate .................................................................................................................. 15

Cover Sheets ..................................................................................................................... 17

Collector Performance ...................................................................................................... 18

HEAT ENERGY BALANCE ....................................................................................................... 19

CHAPTER 05: SOLAR DRYER DESIGN ........................................................................................ 22

Design Procedure.............................................................................................................. 22

Equations used for design of dryer................................................................................... 22

MATLAB PROGRAM ................................................................................................................. 25

APPENDIX A: MATLAB Program Results – Optimum Result .................................................... 26

APPENDIX B: Calculation of Collector area for above conditions ............................................ 27

APPENDIX C: Materials List ...................................................................................................... 28

APPENDIX D: PROJECT TIME-LINE ............................................................................................ 29

REFERENCES: ............................................................................................................................ 30

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ACKNOWLEDGEMENT

We would like express our sincere gratitude to Dr. T.P. Ashok Babu whose

valuable guidance greatly assisted us in our task.

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OBJECTIVES

The aim of the project is to design, build and test the working of a low cost solar dryer

(mixed type) for use in fish drying along the coasts of Mangalore.

The objectives are as follows:

To study the various physical and thermal parameters that influences the performance of

the solar dryer.

Write a program that will help us perform number of iterations to see how changing various

parameters affect the dryer efficiency and collector area required.

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CHAPTER 01: INTRODUCTION

Open air sun drying is still the most common method used to preserve agricultural crop and

sea food. The marine fish production potential of Karnataka is estimated at around 4.25 lakh

M.T. per annum. In Karnataka, mackerals, sardines, anchovics and other elupeids form the

dominent pelagic fishing while catfishes, Sciaenids, Perches, sharks and rays etc. constitute

the deep sea fishing. Soles, and prawns form the major demersal fishery.

However, being a highly perishable food product, fish can be stored for long periods of time

only by proper refrigeration or drying. Since most of the fishermen living at the coastal

belt are below the poverty line therefore refrigeration is distinct dream to them. The

only alternative available is drying in the open air, in an uncontrolled and unhygienic

environment. To reduce the processing losses during the drying and to retain the quality

of dried product, it is necessary to dry the fish in the close chamber with preventing

product from dust, insect, larva, birds and animal.

The coastal region of Karnataka is blessed with ample sunlight all the year round. There is

tremendous potential for utilization of this solar energy. It is therefore expected that the

design of a simple and economical solar dryer could contribute greatly in solving the local

fishermen's problem.

Solar dryers may be classified according to the mode of air flow as natural convection and

forced convection dryers. Natural convection dryers do not require a fan to blow the air

through the dryer. Solar drying may also be classified into direct, indirect and mixed-modes.

In direct solar dryers the air heater contains the materials and solar energy passes through a

transparent cover and is absorbed by the materials. Essentially, the heat required for drying

is provided by radiation to the upper layers and subsequent conduction into the material

bed. In indirect dryers, solar energy is collected in a separate solar collector (air heater) and

the heated air then passes through the material bed, while in the mixed-mode type of dryer,

the heated air from a separate solar collector is passed through a material bed and at the

same time, the drying cabinet or chamber absorbs solar energy directly through the

transparent walls or roof.

Hence we will be focusing our efforts on designing and fabricating a simple direct natural

convention dryer for Mangalore. The use of solar technology has often been suggested for

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the dried fruit industry both to reduce energy costs and economically speed up drying,

which would be beneficial to final quality, dried grapes, okra, tomato and onion using solar

energy. They concluded that drying time reduced significantly resulting in a higher product

quality in terms of color and reconstitution properties. They also believe that as compared

to oil or gas heated dryers, solar drying facilities are economical for small holders, especially

under favorable meteorological conditions.

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CHAPTER 02: LITERATURE SURVEY

After extensive survey of literature and collection of papers we have seen that it would be

most economical to first develop a prototype of a simple cabinet chamber type of solar

dryer, as the more complex designs would increase the cost quite considerably. Once we

have accomplished the successful working of a basic design prototype, we would like to look

at modifications to the design, which can improve performance and reduce costs further.

PATENTS REVIEWED

The following paragraphs describe in brief the various patents that were surveyed,

pertaining to drying technology, solar dryers and food preservation.

PATENT TITLE:

Drying Method and Apparatus for Drying Prunes, Fish, Brewers Grain, Shelled Corn and the

like.

PATENT NUMBER:

US4326341

DESCRIPTION:

Developed a method and apparatus for drying moisture-containing material by use of a

vacuum tank. The tank is partially filled with cold liquid, such as water, and the material to

be dried is supported in the tank above the liquid surface. Surface condensing means are

located inside the tank. Air is first evacuated from the chamber by first filling the tank

completely with liquid. Then by means of a pumping mechanism, the liquid and the

condensate are removed from the chamber, due to drop in pressure the moisture from the

material is also evaporated and condensed on the provided means.

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PATENT TITLE:

Convection Powered Solar Food Dryer

PATENT NUMBER:

US4501074

DESCRIPTION:

Developed a solar powered food dryer that may be used indoors. Convection column is used

to produce a draft of air. By means of this draft, air is drawn from a solar collector through a

food drying chamber in such a way that the hot air from the collector is drawn over the food

in the drying chamber and hot air ducting of the air stream provides a means of remotely

locating the drying chamber to a sanitary and convenient place while leaving both the air

heater and the draft generating convection column in a place exposed to solar radiation.

PATENT TITLE:

Process of Dehydrating Biological Products

PATENT NUMBER:

US6068874

DESCRIPTION:

Makes use of a closed system for dehydration using a heat exchanger and dehydration

chamber connected by appropriate conduits. Provision of blowers as controls for the system

parameters of the process. Advantages of the design include, retention of aroma and

natural flavor of the original food, and ease of transportation unlike refrigerated foods etc.

Main disadvantage is the high cost of equipment and uneconomical for small/individual

scale of drying.

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PATENT TITLE:

Vegetable Product Drying

PATENT NUMBER:

US6922908

DESCRIPTION:

Utilizes a thermal collector that is constructed and arranged to convert solar energy to heat

energy, a heat transfer system and a housing that defines a drying chamber. The thermal

collector is positioned toward a light source and is thermally connected to the heat transfer

system, which in turn is in communication with the drying chamber. Convection forces are

used to move air inside the chamber and photovoltaic energy is also used to enhance the

effect.

PATENT TITLE:

Wood Drying Solar Greenhouse

PATENT NUMBER:

US7748137

DESCRIPTION:

Constructed a solar greenhouse, with one wall or panel that is transparent to solar

radiation, where as the other walls are made highly resistant to convective and conductive

heat transfer. A simple control system is used in order to maintain the interior temperature

and humidity of the greenhouse within pre-determined ranges in order to enhance and

improve the drying process. This also provides opportunity for controlled drying of the

material, thanks to the humidity regulator.

In addition to surveying existing patents in this technology, we also surveyed similar projects

and research work that has been undertaken in the field of solar dryer design.

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DATA SURVEY AND COLLECTION

To perform the calculations above, the following datai were collected:

Solar Irradiance data

Month Solar Irradiance on a horizontal

surface (kw/m^2/day)

(At optimum 77⁰ tilt of collector) (kwh/m^2/day)

Collector adjusted

throughout the year

(kw/m^2/day)

Average Temperature ⁰C

Relative Humidity %

January 5.65 6.87 0 26 62

February 6.33 7.1 7.11 27 66

March 6.81 6.92 6.92 28 68

April 6.84 6.27 6.84 29 71

May 5.85 5.7 5.9 29 71

June 4.37 4.3 4.42 27 87

July 4.25 4.15 4.28 26 89

August 4.87 4.62 4.88 26 88

September 5.44 5.29 5.29 26 85

October 5.19 5.57 5.52 27 79

November 5.27 6.09 0 27 73

December 5.44 6.76 6.97 27 65

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Global Solar Irradiance = 1813 kwh/m^2/year

Diffuse solar Irradiance = 860 kwh/m^2/year

Average ambient temperature = 27.1 ⁰C

Average relative humidity = 75%

Average Wind speed = 2 m/s

Food related data

The design of the solar dryer is directly dependent on the nature of the food substances to

be dried and the extent to which the drying process should be carried out. For this reason,

data regarding the water content of the fishes suitable for drying found in the region have

been tabulated.

Fish drying:

The water content of freshly caught fish differs from one species to the other. The table

below shows the water content for different fish species that are commonly dried in the

Mangalore region.

Fish species

Water content before drying (%)w/w

Mori- Rahu Hybrid 69.265 Labeo rohita 72.810 Cyprinus carpio 65.605 Thalla Rahu Hybrid 69.250 Cirrhinus Mrigala 69.500 H Molitrix 72.680 Catla Catla 68.840 S.Sihama 77.800

Water content after drying is nearly same for all species and is classified into three types.

1. Ordinary cure has water content 44-48%.(most common)

2. Semi-dry product has water content 40-44%.

3. Dry Product has water content 38-40%.

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PERFORMANCE PARAMETERS COMMONLY CONSIDERED FOR EVALUATION

The table below shows the most common parameters of the dryer that were considered for

performance evaluation in the different papers that were surveyedii:

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CONCLUSIONS:

Some of the modifications to existing designs that could enhance the performance of a

simple dryer are listed below:

Use of locally grown material for construction of drying chamber so that it provides natural insulation against heat losses by convection and conduction and at the same time reduces costs of material.

Use of mosquito nets as trays for better performance in humid regions, by exposing greater surface area of the food to the dry air.

To make the design more ergonomic, the dryer can be built onto a mobile platform for ease of transport.

Implement a method to ensure that losses from the edges of the door to the drying chamber are reduced to a bare minimum.

Provision for rotation of trays by 180 degrees daily to ensure uniform drying.

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CHAPTER 03: SOLAR DRYER - WORKING PRINCIPLES

BASIC CONCEPT:

We propose to design and construct a mixed type solar dryer. A simple sketch is shown

above to give a conceptual understanding of the process.

Due to the temperature and pressure difference a natural convection current is set up as indicated by the arrows in red, so that air enters the flat plate solar collector at the bottom vent.

As it passes through the collector, the air gets heated up to 60o- 100o C depending on the collector specification.

This heated air then exits the top vent which is connected to the drying chamber, which in itself is built to function as a mini-greenhouse.

The hot air absorbs the moisture from the food and flows out, thus setting up a natural convection current.

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For improving the wind draught by natural convection, a chimney is built above the drying

cabinet, so that the air that exits the collector enters the bottom of the chimney and is

forced upwards due to the pressure difference between the top and bottom of the chimney.

The schematic diagram of this set up and a simple block diagram representing general air

flow:

The figure below shows how the air flows when a chimney has been constructed atop the

drying chamber:

PREPARE FISH:

REMOVE GILLS

AND WASH

SOAK IN SALT

WATER SOLUTION

REMOVE AND COAT

WITH COARSE SALT

PLACE IN A THIN

LAYER ON THE

TRAYS IN DRYER

REMOVE AFTER

PRESCRIBED

DRYING TIME

Air In

Collector

Air Out

Chimney Trays for

food

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The main factors that influence the drying rate are Temperature, Relative Humidity and Air

Flow rateiii.

High temperature, High air flow rate and Low humidity are desired for increasing the drying

rate. However since increased air flow rate tends to decrease temperature, are objective is

to optimize the collector efficiency and area in order to achieve as high drying rate as

possible.

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CHAPTER 04: SOLAR FLAT PLATE COLLECTOR

Introduction - A flat plate collector is one of three main types of solar collectors, which are

key components of active solar heating systems. The other main types are evacuated tube

collectors and batch solar heaters. Flat plate collector consist of dark flat plate absorber of

solar energy, a transparent cover that allows solar energy to pass through but reduces heat

losses, a heat transport fluid to remove heat from absorber, and a heat insulating backing.

The absorber consists of a thin absorber sheet often backed by a grid or coil of fluid tubing

placed in an insulated casing with a glass or polycarbonate cover.

The construction of a flat-plate collector is shown in Figure. The basic parts noted are a full-

aperture absorber, transparent or translucent cover sheets, and an insulated box. The

absorber is usually a sheet of high-thermal-conductivity metal with tubes or ducts either

integral or attached. Its surface is painted or coated to maximize radiant energy absorption

and in some cases to minimize radiant emission. The cover sheets, called glazing, let

sunlight pass through to the absorber but insulate the space above the absorber to prohibit

cool air from flowing into this space. The insulated box provides structure and sealing and

reduces heat loss from the back or sides of the collector.

Absorber Plate

The main element of a flat-plate collector is the absorber plate. It covers the full aperture area of the collector and must perform three functions: absorb the maximum possible amount of solar irradiance, conduct this heat into the working fluid at a minimum temperature difference, and lose a minimum amount of heat back to the surroundings.

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Absorption Solar irradiance passing through the glazing is absorbed directly on the absorber plate without intermediate reflection as in concentrating collectors. Surface coatings that have a high absorptance for short-wavelength (visible) light, are used on the absorber. Usually these coatings appear dull or "flat," indicating that they will absorb radiation coming from all directions equally well. Either paint or plating is used, and the resulting black surface will typically absorb over 95 percent of the incident solar radiation.

Fin Heat Removal The second function of the absorber plate is to transfer the absorbed energy into a heat-transfer fluid at a minimum temperature difference. This is normally done by conducting the absorbed heat to tubes or ducts that contain the heat-transfer fluid. The heat-transfer fluid may either be a liquid (water or water with antifreeze) or gas (air). The important design criterion here is to provide sufficient heat transfer capability that the difference between the temperature of the absorber surface and the working fluid is not excessive; otherwise, the heat loss from the absorber would be excessive. High heat-transfer rates are usually accomplished at the expense of pumping power and absorber plate material.

The following are important points in designing a good ‘tube and sheet’ absorber:

1. The fin (absorber sheet) must he made of a material with high thermal conductivity. 2. The fin should be thick to minimize the temperature difference required to transfer

heat to its base (tube). 3. Tubes should not be spaced too far apart; otherwise, a higher temperature

difference between the tip of the fin (midway between the tubes) and the base will result.

4. Tubes should be thin-walled and of a high-thermal -conductivity material. 5. The tube should be brazed or welded to the absorber sheet to minimize thermal

contact resistance. 6. The tube and absorber sheet should be of similar material to prevent galvanic

corrosion between them.

When air is the heat-transfer fluid, often the back side of the absorber plate usually forms one surface of a duct and heat is transferred through the absorber sheet to the air over the entire back surface of the absorber. A thin, rather than thick, absorber sheet of high-thermal-conductivity material will enhance this heat-transfer process. The internal air passage must be designed to provide a sufficiently high airflow velocity past the back of the absorber to give adequate heat transfer without producing a high pressure drop across the collector. Low heat-transfer rates cause the absorber plate to become significantly hotter than the heat-transfer fluid, which increases heat loss.

Emittance Because the temperature of the absorber surface is above ambient temperature, the surface re-radiates some of the heat it has absorbed back to the surroundings. This loss mechanism is a function of the emittance of the surface for low-temperature, long-wavelength (infrared) radiation. The dilemma is that many coatings that enhance the absorption of sunlight (short-wavelength radiation) also enhance the long wavelength radiation loss from the surface. This is true for most dull black paints.

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A class of coatings, mostly produced by metallic plating processes, will produce an absorber surface that is a good absorber of short-wavelength solar irradiance but a poor emitter of long-wavelength radiant energy. Flat-plate absorbers that have selective surfaces typically lose less heat when operating at high temperature. However, the absorptance of selective coatings is seldom as high as for non-selective coatings, and a trade-off must be made based on whether the increased high-temperature performance overshadows the reduced low-temperature performance and expense of the selective coating.

Cover Sheets

The absorber is usually covered with one or more transparent or translucent cover sheets to reduce convective heat loss. In the absence of a cover sheet, heat is lost from the absorber as a result of not only forced convection caused by local wind, but also natural convective air currents created because the absorber is hotter than ambient air. The cover sheet forms a trapped air space above the absorber, thereby reducing these losses. However, convective loss is not completely eliminated because a convective current is set up between the absorber and the cover sheet, transferring heat from the absorber to the cover sheet. External convection then cools the cover sheet, producing a net heat loss from the absorber. In addition, heat loss is reduced because of the thermal resistance of the added air space.

The number of cover sheets on commercial flat-plate collectors varies from none to three or more. Collectors with no cover sheet have high efficiencies when operated at temperatures very near ambient temperature. This is because incoming energy is not lost by absorption or reflection by the cover sheet. When no cover sheet is used, however, a considerable amount of the incident energy is lost during operation at temperatures much above ambient or at low solar irradiance levels.

In regions of average mid-latitude temperatures and solar radiation, collectors with no glazing are generally used for applications to 32ºC (90ºF), single-glazed collectors are used for applications to 70ºC (158º F), and double-glazing is used in applications above 70ºC (158ºF). Collector efficiency increases with increasing solar irradiance level but decreases with increasing operating temperature. In regions of low average solar irradiance or extremely low temperatures, therefore, double-glazed collectors are used in applications where single-glazed collectors should be used normally and single-glazed collectors for unglazed applications. Also, selective absorber surfaces become more worthwhile.

Materials Because of its superior resistance to the environment, glass is used as the outer cover sheet on most commercial collectors. Usually the glass is tempered, with a low iron content and 3.2-6.4 mm (0.12-0.25 in.) thick. The surface may be either smooth, making the glass transparent, or with a surface pattern, making it translucent. Both types have a transmittance of around 90 per cent.

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Plastic cover sheets are sometimes used for the second cover sheet when two sheets are required. Installation of the plastic sheet beneath the glass protects the plastic from the environment. Glass also does not transmit UV radiation and thus protects the plastic, which is usually sensitive to this portion of the solar spectrum. Rigid sheets of acrylic-or fiberglass-reinforced polymers are in use, as are stretched films of polyvinyl fluoride. Some of these plastic cover sheets have a transmittance approaching that of low iron glass. A major drawback of this scheme is the potential for overheating the plastic sheet at collector stagnation (no-flow) temperatures. Solar radiation is absorbed by absorbed by absorber plate and transferred to the fluid that circulates through the collector in tubes.

Collector Performance

Orientation The orientation of a flat-plate collector is a concern in system design. The

designer must decide on both the collector azimuth and tilt angles or to install the collectors

horizontally.

Azimuth The most obvious azimuth for a fixed surface in the northern hemisphere is south facing. This will give equal amounts of energy before and after noon and usually the maximum daily total energy collected. There are a number of reasons why the system designer may not select this azimuth. It may be simply that the building or land orientation makes it desirable to rotate the azimuth axis to fit the installation conditions. Other performance related factors can affect the collector field orientation.

Another factor causing the collectors not to be oriented toward the south is the presence of a blockage (mountain or building) that shades morning or afternoon sunlight. In this case the optimum orientation may call for rotation away from the blockage. Likewise, either persistent morning or afternoon cloud cover may cause the designer to orient the field azimuth for optimum energy collection.

Tilt The most logical tilt angle for the fixed flat-plate collector is to tilt the surface from horizontal by an angle equal to the latitude angle. At this tilt, if the collector is facing south, the sun will be normal to the collector at noon twice a year (at the equinoxes). Also, the noontime sun will only vary above and below this position by a maximum angle of 23.5 degrees.

As with collector azimuth orientation, shadowing objects may be considered and the collector tilted less because of blockage of solar irradiance when the altitude of the sun is low. Another factor that may affect the tilt orientation is the climate. If the particular region has considerable cloud cover during the winter, the collectors would be tilted to maximize summer energy collection.

One final factor that could influence the approximate tilting of the collector is the systems operating threshold. If the system needs a high value of solar irradiance to begin operation, tilting the collectors closer to vertical may provide more energy to the system during start-up. Note that a wide variation in tilt angle makes little difference in the irradiation received. This implies that the collector tilt optimization is not critical and that even horizontal

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surfaces may be an appropriate design choice if the cost of installation is considerably less for this orientation.

HEAT ENERGY BALANCE

The model calculates the useful heat gain from the iterative solution of basic heat transfer equations of top loss and equates the same with the convective heat transfer from the absorber plate to the air using proper heat transfer correlations for the smooth duct air heater. The back loss from the collector is calculated from the iterative solution of the heat balance equation for the back surface, the edge loss is estimatediv.

Fig: Heat Balance on solar air heater

The heat balance on the air heater gives the distribution of incident solar radiation I into useful heat gain Q and the heat loss QL. The useful heat gain or heat collection rate can be expressed as

Q = AI (τα) – QL = AI (τα) – UL (Tp – Ta) (1) Where A = area of absorber plate τα = transmittance-absorptance product of the glass cover absorber plate Combination QL = Heat loss from the collector is sum of losses from top Qt, back Qb, and edge Qe of the collector as shown above.

UL = QL (2) (Tp – Ta) UL = Overall heat loss coefficient

TP = Mean absorber plate temperature

Ta = Ambient temperature

The collected heat is transferred to the air flowing through the air heater duct. Thus,

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Q = mcp (To − Ti) = GAcp (To − Ti) (3) Where m = mass flow rate of the air G = m/A mass flow rate per unit area of absorber plate area So effective heat gain is

Q = hA (Tp – Tm) (4) Tm = mean temperature of the air in heater duct h = heat transfer coefficient Heat transfer from absorber plate at a mean temperature Tp to the inner surface of the glass at temperature Tgi takes place by radiation and convection. Hence,

Qtpg = A [σ (Tp4

– Tgi

4) (1/εp+1/εg− 1) −1 + hpg (Tp-Tgi)] (5)

Where εp and εg are the emissivity of the absorber plate and the glass cover, respectively, and hpg is the convection heat transfer coefficient. The conduction heat transfer through the glass cover of thickness δg is given by

Qtg =kgA (Tgi – Tgo)

δg (6) Where kg is the thermal conductivity of the glass and Tgo is temperature of the outer surface of the glass. From the outer surface of the glass cover, the heat is rejected by radiation to the sky at temperature Ts and by convection to the ambient. Hence,

Qtgo = A *σ εg (Tgo4 – Ts

4) + hw (Tgo- Ta)] (7)

hw = wind heat transfer coefficient 5-10 W/m-k Ts = Sky temperature same as ambient In equilibrium we have Qtpg = Qtg = Qtgo = Qt Back and Edge Losses - The back loss from the collector is calculated from the following equation

Qb = A (Tb − Ta) (8) (δ/ki + 1/hw)

Where δ is the insulation thickness, ki is the thermal conductivity of the insulating material and Tb is the temperature of the bottom surface of the collector duct, which has been estimated from the iterative solution of the heat balance equation detailed below. Heat transfer by radiation from the heated absorber plate to the duct bottom surface Qpb is calculated from

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Qpb= σ (Tp

4-Tb4) (1/ εpi+1/ εb-1)-1 (9)

The heat is lost from bottom of the plate to surrounding that is Qba which comes equal to the Qpb. Edge loss

Qe = .5Ae (Tp-Ta) (10)

Knowing the useful heat gain the outlet air temperature is given by

To = Ti + Q/ (mcp) (11)

Thermal Efficiency η = Q/IA (12) Where, I = incident solar radiation.

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CHAPTER 05: SOLAR DRYER DESIGN

The following points were considered in the design of the direct natural convection solar

dryer system

· The amount of moisture to be removed from a given quantity of food product.

· The daily sunshine hours for the selection of the total drying time.

· The quantity of air needed for drying.

· Daily solar radiation to determine energy received by the dryer per day.

· Wind speed for the calculation of air vent dimensions.

Design Procedure

The size of the dryer was determined as a function of the drying area needed per kilogram load. The drying temperature was established as a function of the maximum limit of temperature the food product might support. From the climatic data of Table 1 the mean average day temperature is 27.1ºC and relative humidity is 75 %. From the psychometric chart the humidity ratio is 0.017 kgwv/kgda. The optimal drying temperature of fish products was found to be 46ºC and final moisture content of fish for storage is 16 % wet basis.

Equations used for design of dryerv

The amount of moisture to be removed from the product, mw [kg] was calculated using the

following equation:

mw = mp(Mi - Mf)/(100 –Mf)

(1)

Where mp[kg] is the initial mass of product to be dried;

Mi [%] and Mf [%] wet basis are the initial moisture content and the final moisture

content, respectively.

Final relative humidity or equilibrium relative humidity, ERH [%], was calculated

using sorption isotherms equation

aw = 1 - exp[-exp(0.914+0.5639lnM)] (2a)

M = Mf/(100 - Mf) (2b)

ERH = 100aw

(3)

Where aw [-] is the water activity; M [kgw/kgs] dry basis.

The quantity of heat required to evaporate the water would be

Q = mwhfg

(4)

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Where Q [kJ] is the amount of energy required for the drying process

hfg [kJ/kg wv] = the latent heat of evaporation. The amount needed is a function of

temperature and moisture content of the fish. The latent heat of vaporization was

calculated using equation

hfg = 4186(597 – 0.56tpr)

(5)

Where tpr [oC] is the product temperature.

Moreover, the total heat energy, E [kJ] required to evaporate water was calculated as

follows

E = ṁa(hf - hi) τd / 3600 (6)

Where ṁ [kg/s] = the mass flow rate of air;

hf [kJ/kgda] and hi [kJ/kgda] are the final and initial enthalpy of drying and ambient air,

respectively;

τd [s] is the drying time.

The enthalpy, h [kJ/kgda] of moist air at temperature td [ºC] can be approximated as

h = 1.007td + ω*251.2131+1.5524td]

(7)

Average drying rate, dr [kg/s], was determined from the mass of moisture to be removed by

solar heat and drying time by the following equation

dr = mr/τd

(8)

The mass of air needed for drying was calculated using equation

ṁa = dr/(wf-wi) (9)

Where wf and wi are final and initial humidity ratios.

From the total useful heat energy required to evaporate moisture and the net radiation received by the tilted collector, the solar drying system collector area, Ac [m

2], can be calculated from the following equation Ac = Q/Ihτdη (10) Where I [kJ/m2/s] is the total global radiation on the horizontal surface during the drying period η *%+ is the collector efficiency and range from 30 to 50 %.

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(11)

The air vent area, Av [m2] can be calculated by

(12)

Ws speed of wind (m/s)

The length of air vent, Lv [m], will be equal to the length of the dryer. The width of the air

vent, Bv [m], can be given by

Bv = Av/Lv (13)

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MATLAB PROGRAM

After preliminary manual calculations, an iterative MATLAB program was written in order to determine the

following. The algorithm used is shown above.

Final temperature of air exiting flat plate collector

Heat energy needed to remove the desired amount of moisture from fish

Collector area required for the given load

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APPENDIX A: MATLAB Program Results – Optimum Result Area = 2*4.3 = 8.6 m2

Temperature Out = 324 K

Mean Air Temperature = 312 K

Efficiency = 36%

Heat Absorbed = 2477 W

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APPENDIX B: Calculation of Collector area for above conditions

Optimal drying temperature for fish = 46oC

Mean Average day temperature = 27.1oC

Relative Humidity = 75%

Average water content of fish before drying = 70% (w/w)

Average water content of fish after drying = 16% (w/w)

From literature average drying time td = 20 hrs in two days

For preliminary calculations assume collector efficiency = 36%

The amount of moisture to be removed from the product, mw [kg] was calculated using

the following equation:

Mw = mp (Mi - Mf)/(100 –Mf)

= 15kg

Initial relative humidity = 75%

To calculate Final relative humidity:

Water evaporated (dry basis) M = Mf/(100 - Mf) = 16/84 = 0.19

Water activity aw = 1 - exp[ -exp(0.914+0.5639lnM)] = 0.624

Equilibrium Relative Humidity ERH = 100aw = 62.4%

Latent heat of vaporization at given temperature of product = hfg = 4186(597 – 0.56tpr)

= 2.39MJ

Hence quantity of heat needed to evaporate the water =

Q = mwhfg = 15 * 2.39 = 35.6 MJ

Collector area is given as Ac = Q/Ihτdη

= 7.6 m2

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APPENDIX C: Materials List The proposed materials to be used to construct the basic structure of the dryer are:

Wood

Cement

Metal

Mortar

Proposed material options that are under consideration for absorber plate are (however for

prototype the easiest and cheapest in the market will be selected):

Aluminium

Galvanized Iron or Steel

Corrugated Metal Sheets

Proposed material for use as insulation:

Glass wool

Plywood

Thermo Cole

Proposed glazing material:

Glass

Proposed material for construction of chimney:

PVC pipes

Cement/Mud

Proposed material for use as trays in the dryer to hold the food:

Mosquito nets

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APPENDIX D: PROJECT TIME-LINE

August 2011– Literature Review

September 2011 1st to15th – Data Gathering

September 15th to 30th – Preliminary design

October to November – Program and analysis

January 2012 – Acquiring of materials

February 2012 – Construction

March 2012 – Analysis and Further development

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REFERENCES:

1. Patents mentioned in the literature survey

2. Performance Study of Solar Air Heater Having Absorber Plate with Half-Perforated

Baffles by B. K.Maheshwari, Rajendra Karwa, and S. K. Gharai

3. Design and Fabrication of a Direct Natural Convection Solar Dryer for Tapioca by

Diemuodeke E. OGHENERUONA Momoh O.L. YUSUF

4. Design and Construction of A Solar Dryer for Mango Slices by EL- Amin Omda

Mohamed Akoy , Mohamed Ayoub Ismail , El-Fadil Adam Ahmed, W. Luecke

5. Design and Fabrication of a Convective Fish Dryer. By C.A. Komolafe, I.O.

Ogunleye, A.O.D. Adejumo

6. Performance Evaluation Of A Solar Tunnel Dryer For Chilli Drying by GAUHAR

A. MASTEKBAYEVA, M. AUGUSTUS LEON and S. KUMAR

7. Low cost solar dryer for fish by S. H. Sengar, Y. P. Khandetod and A. G. Mohod

8. Design and construction of a solar drying system, a cylindrical section and analysis

of the performance of the thermal drying system by Ahmed Abed Gatea

9. Dehydration of food crops using a Solar Dryer with Convective Heat Flow by

Akwasi Ayensu

10. Solar Drying by V. Belessiotis, E. Delyannis

11. Karnataka Marine Conservation Department, Govt. of Karnataka.

12. International Scholarly Research Network, Renewable Energy –Resources.

i Mangalore Climate Data ii A comprehensive procedure for performance evaluation of solar food dryers.( M. Augustus Leon, S. Kumar,

S.C. Bhattacharya) iii Design Construction and Use of Indirect, Through Pass Solar Food Dryer. (Dennis Scanlin)

iv Performance Study of Solar Air Heater Having Absorber Plate with Half-Perforated Baffles(B. K.Maheshwari,

Rajendra Karwa, and S. K. Gharai)

v Design and Fabrication of a Direct Natural Convection Solar Dryer for Tapioca (Diemuodeke E.

OGHENERUONA Momoh O.L. YUSUF)