FEEG6007 FUEL CELLS & PHOTOVOLTAIC SYSTEMS 1 SEMESTER 1 EXAMINATION 2016/17
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FEEG6007
SEMESTER 1 EXAMINATION 2016/17
FUEL CELLS & PHOTOVOLTAIC SYSTEMS 1
SECTION A
Q1. Write illustrated notes on TWO of the following:
(a) Write the general equation for the cell voltage of a typical battery that illustrate the influence of the standard electrode potential of each reaction and the IR drop. Explain the polarization curve that results from this equation. Include the appropriateness of using reference electrodes to study the half-cell reactions and indicate how the potential they measure, relates to the overall cell voltage. What are the typical reference electrodes that can be used to measure the potential of the half-cell reactions and how do they relate to the standard hydrogen reference electrode?
(b) In a typical hydrogen/air fuel cell, describe the cell and its components as well as the operational parameters. State clearly any advantages and disadvantages and the importance of hydrogen.
(c) what is the meaning of the Nernst-Planck equation? Describe the significance of each term in the equation in relation to the transport of ions through an ion exchange membrane. Use diagrams to illustrate.
Your answer should include sketches, reactions and mathematical expressions, where appropriate. [2 × 12.5 Marks]
Q2. Answer ALL parts of this question.
(a) Describe the key features of a redox flow battery. Include in your description examples of fully balanced equations for the reactions taking place at the electrodes. [5 Marks]
(b) What are the major requirements of the electrodes and ion exchange membrane in a redox flow cell? [5 Marks]
(c) Calculate the equilibrium cell voltage and the minimum Gibbs free energy necessary to start charging the vanadium – vanadium redox flow battery, per mole of vanadium. Use the following equations and potentials:
Negative electrode: V(III) + e- 不 V(II); Ee = -0.26 V vs. SHE
Positive electrode: V(IV) - e- 不 V(V); Ee = +1.00 V vs. SHE [6 marks]
(d) Assume a stack with 100 electrode bipolar cells of 80 cm2 geometrical area in each electrode, and a current density of 20 mA cm-2 , calculate the kilograms of V(II) produced in 24 hours. State clearly any assumptions made in the calculation. [5 marks]
(e) Discuss why the initial cell voltage during charge is typically 1.5 V and increases with time. What can be done to decrease this applied cell potential so the energy spent during charge is lower? [4 marks]
Faraday constant = 96 485 C mol-1 ;
Molar mass of vanadium = 50.94 g mol-1 .
Q3. Answer ALL parts of this question.
Methanol is consumedat a rate of 32 g h-1 in a methanol-air fuel cell.
(a) At an open-circuit cell voltage of 0.756 V, what is the available maximum theoretical energy (in kWh kg-1) from the cell? [5 Marks]
(b) What is the main cathode reaction, anode reaction and cell reaction? What are the possible secondary reactions or processes that lower the current efficiency? [6 Marks]
(c) In 35 hours,what is the maximum energy available (in kJ) from
the cell? What assumptions you need to make to calculate the current and what is the value? [9 marks]
(e) State the major limitations of methanol fuel cells and what can be done to improve the energy efficiency. [5 Marks]
Faraday constant = 96 485 C mol-1 ;
Molar mass of methanol = 32.04 g mol-1;
1 kJ 1 = 2.76 × 10-4 kW h
SECTION B
Q4. Answer ALL parts of this question.
(a) A beam of light at a wavelength of 600 nm, with a spectral width of 5 nm and spectral irradiance of 1200 W m-2 μm-1 , illuminates an area of 10 cm2 .
(i) What voltage could this beam generate if converted to electricity? [3 Marks]
(ii) What current could this beam produce? [6 Marks]
(b) Figure 1 (on the following page) shows the graph of the maximum current density that can be generated by a single- junction solar cell as a function of the semiconductor bandgap, under illumination with the standard solar spectrum. Using this graph, determine the maximum conversion efficiency of a crystalline silicon solar cell. [6 Marks]
(c) Using the graph in Fig. 1, determine the maximum efficiency of a double-junction solar cell manufactured from semiconductors with bandgaps of 1 eV and 2 eV. [6 Marks]
(d) Describe briefly how the following changes would affect the parameters of the current-voltage characteristic of a solar cell:
(i) Reduction in quantum efficiency
(ii) Increase in series resistance [4 Marks]
Bandgap of crystalline silicon = 1.12 eV
Wherever appropriate, use a fill factor equal to 0.85.
Figure 1
Q5. Answer ALL parts of this question.
(a) Describe the principal steps in the manufacture of a crystalline silicon solar cell. [17 marks]
(b) A 15% efficient solar cell is manufactured from a 350 μm thick silicon wafer. Assuming 200 μm kerfloss in wafering, what is the silicon usage (in grams per Wp) in the cell manufacture? [ 5 marks]
(c) Compare and contrast a typical crystalline silicon solar cell with a typical cadmium telluride solar cell with respect to:
(i) Efficiency
(ii) Amount of material used in manufacture [ 3 marks]
Silicon density = 2.33 g/cm3
Q6. Answer ALL parts of this question
(a) Concisely discuss the principal features of the following solar energy systems and applications:
- Solar-powered lighthouse
- Concentrator systems
- Building integrated systems in Southampton [ 12 Marks]
(b) A photovoltaic module consisting of 36 crystalline silicon solar
cells has the following parameters: Voc = 22 V; Isc = 3.5 A; Pmax = 64 W. The operation of this module can be described by an ideal current-voltage characteristic.
(i) Determine the dark saturation current of this module under standard test conditions at the standard test temperature. [5 Marks]
(ii) How many such modules would you need to power a daily load of 300 Wh in a location with daily solar radiation of 5 kWh m-2 , for a system that operates with a maximum power-point tracker? [3 Marks]
(iii) The module is installed as part of an array where external wiring introduces a series resistance Rs = 0.7 ohm. If the current at the maximum power point is equal to 90% of the short-circuit current, what power will the installed module produce under standard test conditions? [5 Marks]
Boltzmann constant kB = 8.61 ×10-5 eV K-1
Conversion to absolute temperature scale: [K] = [°C] + 273.15
Electron charge 1.602 × 10-19 C
2024-01-18