代写FEEG6008 Advanced Photovoltaic Fuel Cells & Batteries SEMESTER 2 EXAMINATION 2022/2023调试Haskel
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Title: Advanced Photovoltaic Fuel Cells & Batteries
8 hours (online open book) including up/download time. Recommended duration 120 minutes (On-line)
Attempt SIX QUESTIONS
Use a single file but make sure that the answers are well identifiable with the question number.
Marks in brackets are for guidance only
Numerical questions:
Marks will not be awarded unless full working solution is shown.
A Set of Physical Constants is provided in each problem if required. However, if needed please use any reference book that you need.
SECTION A
Q1. Answer ALL parts of this question.
(a) A 100 cm2 standard crystalline silicon solar cell has the following parameters:
Electron diffusion constant (D) in the base = 40 cm2 sec-1 . Minority carrier lifetime (τ ) in the base = 5 μsec.
Reflection coefficient (R) from the top surface = 3%. Emitter thickness (xn) = 1 μm.
Depletion layer thickness (xdr) = 0.3 μm.
Equilibrium electron density in the base = 105 cm-3 .
The wavelength dependent absorption coefficient (aλ ) of the silicon solar cell can be found from blackboard.
(i) Assume that there is no recombination in the emitter (its collection probability is 1.0) and the base thickness is infinite.
Under standard testing conditions, determine:
− the simplified equation for the wave-length dependent internal
quantum efficiency (IQEcell(λ)) of the cell;
− the photogenerated current produced by the cell;
− the dark saturation current;
− the open circuit voltage. [15 Marks]
(ii) If the recombination rate in the emitter is approaching infinite, determine the photogenerated current for the cell. [5 Marks]
(b) A schematic diagram of HIT silicon solar cell is shown in the Figure below.
Discuss the features which contribute to its high efficiency, and the key considerations which affect their design. Wherever appropriate, indicate which spectral region you would expect to show an improvement in the quantum efficiency for each feature. [5 Marks]
Q2. Answer ALL parts of this question.
(a) A standard crystalline silicon solar cell of area 100 cm2 with contacts to front and rear has a top n-type diffused emitter of thickness xn = 1.0 µm and a depletion layer of thickness xd.r =0.5 µm. The base, which can be assumed to be of infinite thickness, is doped with boron at concentration of 5 × 1015 cm-3 . The diffusion constant of electrons in the base is D = 30 cm2 sec-1 minority-carrier lifetime in the base is 8 µsec. The cell is illuminated by long-wavelength light which excites charge-carriers uniformly, with a constant (depth-independent) generation function g = 2 × 1018 sec-1 cm-3 .
(i) What is the rate of generation of the photoelectron-hole pairs (in sec-1) in the depletion region? [3 marks]
(ii) What is the probability of an electron generated in the base, 120 µm from the junction, to be collected at the junction? [3 marks]
(iii) Neglecting the contribution from the emitter, what is the photogenerated current from the cell? [6 marks]
(iv) Using the intrinsic carrier concentration in silicon equal to 1010
cm-3 , determine the dark saturation current from the base. [3 marks]
(b) The materials listed in the following table are available for fabrication of binary organic solar cells.
|
Materials |
HOMO/VBM (eV) |
LUMO/CBM (eV) |
Materials |
Work function(eV) |
|
P3HT |
- 6.6 |
- 3.2 |
ITO/glass |
- 4.8 |
|
PCBM |
- 5.2 |
- 4.2 |
Ag |
- 4.3 |
|
TiO2 |
- 7.5 |
- 4.1 |
|
|
|
MoO3 |
- 5.3 |
- 2.3 |
|
|
(i) Drawing a schematic energy diagram for a bilayer organic solar
cell, briefly discuss its working principle and interpret the roles of buffer layers in term of their band-structures. [6 Marks]
(b) Propose three effective methods to further improve the bilayer organic solar cell’s efficiency and stability. You may introduce new chemicals except for using new organic semiconductor light absorbers. [4 Marks]
Q3. Answer ALL parts of this question.
(a) Describe working principle of Dye Sensitised Solar Cell (DSSC). Draw a schematic energy diagram of a standard DSSC, indicating energy levels of all participating compounds, and the directions of energy and charge transfer. Provide the list of the reaction processes and their names. Show the structure and typical dimensions of standard DSSC. [3 marks]
(b) Discuss the main requirements for the dye in DSSC when searching for new one. Rank their priority. [4 marks]
(c) A newly developed DSSC semiconductor is based on ZnO nanostructures with triangular prism shape. The prism is characterised by average triangle side of a = 10 nm and height of h = 20 nm. Calculate the specific surface area of the nanostructured material (in m2 g-1) and the surface area of 1 cm2 electrode composed by aggregated nanoprisms forming 10 µm thick layer with porosity 60%. The density of solid ZnO is 5.6 g cm-3 . [5 marks]
(d) Describe working principle of amorphous silicon (a-Si:H) solar cell. Draw its layered structure and architecture, indicating the typical thickness and composition of each layer and its functions. Sketch the energy diagram of the a-Si:H solar cell and indicate the direction of charge carriers transfer. Discuss specific features, advantages and disadvantages of a-Si:H solar cell. [5 marks]
(e) Consider a 4-junction solar cell having following architecture
Semiconductor A/ Semiconductor B / Semiconductor C / Semiconductor D with band gaps 2.00 eV, 1.45 eV, 1.01 eV and 0.77 eV, respectively.
Using Table 1 below, estimate the maximum theoretical efficiency of this solar cell assuming a current matching regime and a fill factor equal to 0.8.
Calculate the thicknesses of the semiconductor A and Semiconductor B layers in order to absorb 95 % of the incident light for each subcell spectral range, using the values of absorption coefficients near the band gap as α 1 = 5 µm-1 and α2 = 2 µm-1 for semiconductor A and B respectively. [4 marks]
Table 1. The maximum current density that can be generated by a single-junction solar cell made from a semiconductor with bandgap Eg under AM1.5 sun radiation.
(f) Calculate how much the thickness of the second layer (Semiconductor B) can be reduced without affecting the total current of the cell. [4 marks]
SECTION B
Q4. Answer ALL parts of this question.
(a) Consider the negative electrode of lithium battery, which operates on following semi-reaction:
x Li+ + x e- + TiO2 = LixTiO2 (E0 = 1.5 V vs. Li+/Li)
Calculate the specific charge/discharge capacity of the negative electrode (in mA h g-1) for x = 3. (Atomic masses of Li, Ti and O are 6.94, 47.87 and 15.99 g mol-1 respectively, Faraday constant, F = 96485 C mol-1). [4 marks]
(b) Consider the positive electrode of lithium battery, which operates on following semi-reaction:
Li1-xCoO2 + x Li+ + xe- 不 LiCoO2 (E0 = 4.2 V vs. Li+/Li)
Calculate the specific charge/discharge capacity of the negative electrode (in mA h g-1) for x = 0.5. (The atomic mass of Co is 58.93 g mol-1). [4 marks]
(c) Schematically draw the complete lithium cell assembled from
the negative and the positive electrodes from (a) and (b).
(i) Show all components of the cell and write the overall electrochemical reaction.
(ii) Calculate the maximal operational voltage of the cell.
(iii) Determine the charge/discharge capacity of the assembled cell.
(iv) Taking into account that the mass of active the electrode components comprises only 40% of the total mass of the
cell,calculate the mass of the 2500 mA h cell.
(v) What does the other 60% of the mass consisting of? (vi) What is the specific energy of this cell in Wh kg-1? [6 marks]
(d) Compare electrochemical batteries and supercapacitors in terms of electricity storage. What are the advantages and disadvantages of each example. Explain the difference between EDLC and pseudo-capacitors. Illustrate the difference using charge discharge curves. [5 marks]
(e) Explain the most common mechanisms of lithium-cobalt oxide battery degradation. At which operational conditions are they most likely to occur? For each mechanism indicate measures which can inhibit the degradation [6 marks]
Q5. Answer ALL parts of this question.
(a) Calculate the limiting current for the reduction of Fe3+ and Sn4+ ions contained in a 1 moldm-3 HCl solution at concentrations of 2.0 × 10-3 mol dm-3 and 1.0 × 10-3 mol dm-3 , respectively. The reduction was at a flat platinum rotating disc electrode of 0.30 cm2 geometrical area. The mass transport coefficient of Fe2+ and Sn4+ at the same rotation rate is the same, i.e., 10-2 cm s-1 . [5 marks]
(b) Make a sketch of how the current vs. potential curve should look like when the potential is scanned from +1.3 V to -0.40 V vs. SHE. The diagram should be labelled and quantitatively correct, assume that there is no change of concentration of Fe2+ and Sn4+ during the potential scan. [10 marks]
(c) In a metal plating company, you need to measure the reduction kinetics of a single electron reaction where the reactant is a soluble species. The diffusion coefficient is not known. Can you use a rotating disk electrode be to make the measurement effectively? If so, how would you proceed? If not, why not? Please include the equation or equations that you should use and the implications of the mass transfer and the current distribution. [10 marks]
Q6. Answer All parts of this question
a) Assume an electrolyser that consumes 100 kW power at a current density of 1500 A m-2 to generate hydrogen gas operating at 80 0C at 98.5% faradic efficiency. The equilibrium voltage at the operating temperature is 1.18 V but during the operation, away from the equilibrium, the potential difference between anode and cathode is 1.85 V. If the surface area of the electrodes is 1 m2 calculate the following:
i) The operating current and voltage of the stack
ii) The rate of hydrogen production
iii) The energy efficiency of the electrolyser [Mark 20]
b) The catalyst load of a gas diffusion fuel cell electrode w is typically in the order of 20 mg cm-2 . Assume that the hydrogen adsorption Qad on the catalyst, determined by cyclic voltammetry in 0.5 mol dm-3 H2SO4 , was 0.1C. The geometric area of the electrode, Ag was 1 cm2 and that the area of the catalyst available, typically platinum determined by BET, used
to manufacture the fuel cell electrode was 3 m2 Pt g-1 . What is the percentage of platinum utilization? [5 Marks]