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HERIOT-WATT UNIVERSITY

MSc in Energy

MSc in Renewable Energy Engineering

MSc in Renewable Energy & Distributed Generation

MSc in Materials for Sustainable & Renewable Energy

MSc in Photonics

BSc Physics with Environmental Sciences

MEng Mechanical Engineering & Energy Engineering

School of Engineering and Physical Sciences

B51GE1: Renewable Energy Technologies

Tuesday 14 December 2010

09.30 - 11.30

Instructions to Candidates:

Answer ALLquestions from Section A, each weighted at 4 marks

Answer ONEquestion from Section B, weighted at 40 marks

Answer ONEquestion from Section C, weighted at 40 marks

List of Data Tables:For all questions, use the supplied data sheet. Where no information is given in the data

sheet, make sensible assumptions and explain or justify those assumptions in your answer.

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B51GE1 Renewable Energy Technologies 2 out of 8

SECTION A

ANSWER ALL QUESTIONS [20 marks]

A1. Evaluate whether a prototype wind turbine with a rotor diameter of 5 m is likely to

produce an output of 7.5 kW at a wind speed of 10 m/s. For this purpose use typical

values for the mechanical and electrical efficiency of the machine as well as for the air

density. [4 marks]

A2. A beam of light with intensity of 3 mW and a wavelength of 743 nm is striking a

Sunpower solar cell. The solar cell has an efficiency of 22%. Estimate the number

of photons incident on the cell. Ignore any loss mechanism during the process.

[4 marks]

A3: A dry Granite rock is situated at a depth of 7km with a temperature gradient of40Ckm-1. The minimum useful temperature is 140K above the surface temperature

and the density and specific heat of the rock is 2700kgm-3and 820 Jkg-1K-1

respectively.

(a) Calculate the useful heat content per square kilometre of dry rock.

[2 marks]

(b) Also calculate the useful heat extraction rate initially and after 10 years of operation?

[2 marks]

A4. A flat plate collector of area 2m2has been installed in Edinburgh. The collector

manufacturer has provided the following equation (overall efficiency) in steady-statecondition:

0.8 8T

fiT

a

G

Calculate the useful heat which will be collected by the solar collector during four

hours of a day assuming that the following parameters are constant: incident solar

irradiance on collector surface 650 W/m2, collector water inlet temperature 20C and

ambient temperature 15C. [4 marks]

A5. The Wave Dragon is a focussing and over-topping wave energy converter with a wavecapturing width of 300 m, reducing it to a width of 100 m at the reservoir. It is placed

in a sea of energy period 6 s and a significant wave height of 3.5 m.

(a) Assuming no losses, estimate the wave height at the reservoir [2 marks]

(b) Calculate its efficiency, if it produces an output of 7 MW from this sea. [2 marks]

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SECTION B

ANSWER ONE QUESTION[40 marks]

B1A remote cottage located at Exeter, England (5043`N, 332`W) has the loads listed in the

table given below.

Appliance type No Type Power (W) Daytime run (hour) Night time run (Hour)

Lamps 5 DC 11W each 0 5

Television 1 AC 75W 2 4

Computer 1 AC 160W 4 3

Radio 1 DC 25W 3 1

Water pump 1 AC 60W (6A

startcurrent)

1 1

Oven 1 AC 1200W 2 1

The yearly average solar radiation availability on PV plane in Exeter is 1200kWhm-2year-1.

The energy for this cottage is powered by a string of batteries each having 12V and 125Ah

supply when charged fully. The following properties are applied:

The battery efficiency is 73% with a depth of discharge of 70%

Each PV panel (dimension of 1m 1m) produces 80Watts when incident solar

radiation intensity is 1000Wm-2.Each PV system has the same efficiency of 18% and is temperature independent.

Each PV panel has open circuit voltage (Voc) of 16V and operating voltage at load is

80% of the Voc.

Short circuit current of each PV panel is 5.5A

Daily average sunshine hours are 5 hours

The system operating voltage is 24V

(a) Estimate the average daily load and the peak power to be satisfied by the PV system.

[10 marks]

(b)

Calculate the number of batteries required for the cottage. [4 marks]

(c)

Calculate the number of PV panels required for this cottage. [6 marks]

(d) Draw a suitable I-V curve of one PV panel and indicate the short circuit current, open

circuit voltage, current at maximum power point, voltage at maximum power point.

Calculate the fill factor of this PV panel when the solar radiation intensity is

1000Wm-2. Also draw the I-V curve for two panels (only outline drawing in the same

scale is required) connected in series. [12 marks]

(e)

Calculate how many batteries will be connected in series and how many batteries will

be connected in parallel. [3 marks]

(f) Estimate number of PV panels will be connected in series and parallel respectively

[3 marks]

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B2A wind-based stand-alone system is used for the satisfaction of a remote consumer who is

unable to obtain grid connection. The energy produced by the wind turbine (rated power

20kW) is used so as to satisfy a constant 10-hour electrical load of 12kW (from 5.00 to

15.00) as well as a constant 3-hour electrical load of 15kW (00.00-3.00) (see also Table 1),while the rest of the energy production, after being rectified, is stored in a battery bank of

48Volts. The electrical power output of the existing wind turbine is given by the non-

dimensional power curve (Figure 1), based on the mean wind speed measurements provided

for the day of study, given in Table 1.

In this context, you are asked to:

(a) Present the main components of a typical wind-based stand-alone system and describe

the operational principles of the configuration. [10 marks]

(b) Present the daily electricity production and electricity consumption profile of the

system examined on an hourly basis for the entire day, i.e. 24 hours and estimate the

capacity factor of the wind turbine. [10 marks]

(c) Estimate the energy stored (in Ah) during the day in the battery bank (48Volts) if the

overall efficiency of the storage branch is equal to 80%. [10 marks]

(d) Check whether the satisfaction of the same electrical loads is possible for the day after

as well, if a calm-spell period of 24 hours occurs. For this purpose you should check if

the energy stored in the battery bank is adequate, taking into account that the

discharge efficiency of the batteries is equal to 90%. [10 marks]

Figure 1: The non-dimensional power curve of the wind turbine

Table 1: Mean Hourly Wind Speed and Load Consumption for the Day of Study

Hour of the Day 0 to 3 3 to 5 5 to 10 10 to 15 15 to 19 19 to 24

Mean Wind Speed (m/s) 10 7 8 9 10 8

Consumption Load (kW) 15 0 12 12 0 0

Non Dimensional Power Curve of a 20-kW WT

0

0,2

0,4

0,6

0,8

1

1,2

0 2 4 6 8 10 12 14 16 18 20 22

Wind Speed (m/sec)

Non-DimensionalPowerOutput

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B3One of the main industries of a small island is sugar production. It exports annually

400 000 tonnes of sugar (sucrose: C12H22O11 ) but has to import 300 000 tonnes of

petroleum based fuels, of which 2/3 are used for transportation and 1/3 for power generation

in diesel generators

Calorific values of

- petroleum fuels: hf=48 GJ per tonne.

-

Bioethanol (C2H5OH): he= 30 GJ per tonne

- Biogas: hg=x * 50 GJ/tonne, wherexis the methane content of biogas by mass.

(NB: tonne is the SI tonne = 1000 kg)

(a)

Derive a chemical reaction equation for the fermentation of sucrose with water to

ethanol and carbon dioxide, and calculate how much (tonnes) bioethanol could be

obtained from fermenting one tonne of sugar.[8 marks]

(b)

Derive a chemical reaction equation for converting sucrose and water into biogas and

determine how much energy (GJ/tonne) could be obtained from one tonne of sugar in

the form of biogas.

[8 marks]

(c) Estimate by how much the fuel imports can be replaced by local bioethanol and

biogas production. Use the following assumptions:

- 1 tonne of bioethanol can replace 0.8 tonnes of petroleum fuels

- Biogas can be used in gas turbines to replace diesel generators

i. Calculate energy equivalence between bioethanol and petroleum fuels (i.e.

compare the replacement value given in mass terms with a replacement value

given in GJ) and explain differences.

[4 marks]

ii. Derive an equation for tonne of fossil fuel replaced as a function tonne of sugar

used and ratio of conversion to bioethanol over biogas. [10 marks]

(d)

Propose a solution to minimise the fuel imports.

[10 marks]

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SECTION C

ANSWER ONE QUESTION[40 marks]

C1.

There is an increased scientific interest demonstrated during the recent years for the energy

evaluation of RES-based configurations on a life-cycle basis, including also wind energy

technology. Although a lot of research work has been carried out in the field of grid-

connected wind turbines, wind stand-alone systems used for the satisfaction of remote

consumers have not yet been evaluated in depth.

(a) Present the various stages that should according to your opinion be considered for the

estimation of the life-cycle energy requirements of a wind farm as well as any other

environmental impacts accruing from its installation and operation.

[5 marks]

(b) Estimate the life-cycle energy requirements (20-year operation) of a representative

wind stand-alone system (neglecting the energy content of the balance of system

components) using the information provided in the following table and compare this

with the respective results for a typical diesel engine set used to satisfy the same

remote consumer on an annual basis (both the diesel and the fuel embodied energy

should be considered).

[10 marks]

(c) Accordingly estimate the energy payback period of the wind stand-alone system (i.e.

the ratio of primary energy requirements on a life-cycle basis to the annual energy

production of the system). Note that for this to be achieved, the electrical energy

production should be converted into primary (for this purpose one should use the

electricity conversion efficiency of the alternative conventional energy solution that is

currently corresponding to the diesel engine set). [10 marks]

(d) Re-estimate the energy payback period of the system adopting the useful energy

production of the wind stand-alone the system, i.e. the amount of energy that is

actually consumed by the remote consumer. [5 marks]

(e) Comment on the results obtained and discuss on the sustainable character of big scalegrid connected plants and small scale stand-alone systems based on RES. [10 marks]

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Wind Stand-alone Diesel Engine Remote Consumer Embodied Energy Coefficients

WT rated power:8.5 kW

Rated power:4 kW

Peak load demand:3.5 kW

WT:3000 kWh/kW

Battery capacity:15000 Ah

Efficiency: 25%Energy consumption:

4.7 MWh /yearBattery:4.5 kWh/Ah

WT service period:20 years

Service period:5 years

Diesel:600 kWh/kW (without the fuel)

Battery service period:5 years

WT capacity factor:35%

C2.(a) Discuss the role which solar energy might play towards the renewable energy targets

in the European Union. Identify high solar radiation areas and low solar radiation

areas in the EU and identify key technologies to be applied, such as Photovoltaic

power generation, solar thermal power generation, building integrated Photovoltaics

and domestic hot water systems.

[25 marks]

(b) The UK is known to be cloudy with maritime climatic conditions. Identify keyrenewable sources to be applied to meet the UKs renewable energy target by 2020.

Discuss the possibilities of solar thermal technologys role in these climatic

conditions. Calculate the area of solar energy devices required to contribute 20% ofUKs total energy demand (assume the UKs annual energy demand is 400 TWh and

the efficiency of typical solar energy devices is 20% and the average solar radiation is

1000 kWm-2year-1). [15 marks]

C3. A headline in The Guardian(UK) from 18 October 2010 is:Severn barrage

ditched as new nuclear plants get green l ight. The article continues with: The

UK government today dropped plans to build a 10-mile barrage across the

Severn estuary to generate green energy from tides.

(a) Describe the principle of tidal (range) power generation and common components in

a tidal power station.

[5 marks]

(b)

Taking an available head of 10 m, an optimum flow rate of 40 000 m3/s and a

nondimensional specific speed of suitable turbines (rotating at 93.75 rpm) of 4.0 for

the Severn barrage, estimate

i.

The power generation potential from the site, [2 marks]

ii.

the power output from one turbine and the number of turbines required to match the

generation potential. [3 marks]

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iii. Assuming a capacity factor of 30%, the electricity generated in a year.

[2 marks]

iv. Compare this to the installed capacity and expected annual electricity production

from a current typical nuclear power plant with installed capacity of 1.4 GW and aload factor or 90%. [3 marks]

(c) Discuss advantages and disadvantages associated with building and operating a tidal

barrage.

[15 marks]

(d)

Describe and discuss alternative approaches to generating power from tides other

than large barrages.

[10 marks]

---------- End of Paper ----------

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