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FEEG6007

SEMESTER 1 EXAMINATION 2018/19

FUEL CELLS AND PHOTOVOLTAIC SYSTEMS

SECTION A

Q1. Write illustrated notes on TWO of the following:

(a)  Draw the graph of a typical cell voltage vs. current density dependence (polarization curve) for a fuel cell and explain the three main characteristic regions. Use equations that define the cell voltage and labeled diagrams to illustrate your answer. What are the major losses of energy? [12.5 Marks]

(b)    Explain the concept of cyclic voltammetry and the criteria for a reversible system.  Show an example that includes a detailed labelled current vs. potential curve. Explain how the concentration of the electroactive species on the surface of the electrode change as the potential moves away from the equilibrium value. [12.5 Marks]

(c)  Explain the principle of redox flow cells. Are they suitable to store large amounts of electrical power? Why? What is the driving force? Illustrate your answer with some examples. [12.5 Marks]

(d)  What is the purpose of an ion exchange membrane? Give examples where these membranes are used. Explain which is the main transport mode of ions inside the ionic membranes and why. Mathematical expressions and diagrams should be used. [12.5 Marks]

(e)  Explain the difference between galvanic (self-driven) and electrolytic (driven) cells. Give examples of each system and explain the direction of the reaction in terms of Gibbs free energy. [12.5 Marks]

ii)      Using the Nernst equation, what is the effect on the cell potential when metallic Zn increases? [4 Marks]

iii)     What is the standard cell potential of the cell, E0cell? [4 Marks]

iv)    What is the real cell voltage of the cell at 25°C when the concentration of [Zn2+] is 0.5 M? The partial pressure of hydrogen, pH2 is 10 atm and the pH is 6. [5 Marks]

Faraday constant, F = 96 485 C mol-1

Gas rate constant, R = 80314 J mol-1 K-1

273.15°C = 0 K; 25°C 298.15 K

F/RT = 38.92 V-1

RT/F = 0.02569 V

2.303 RT/F= 0.05916 V

Q3.Answer ALL parts of this question.

A pilot-scale borohydride – air fuel cell consumes 100 g of fuel per hour.

(a) State the most likely anode-, cathode- and cell reactions and potentials. [6 Marks]

(b) If the open-circuit potential of a single cell during operation is 1 V, calculate the theoretical maximum energy in kJ mol-1 and kW h kg-1 available from the cell. [5 Marks]

(c)  Calculate the maximum energy available in kJ from the stack in 15 hours. [4 marks]

(d) Calculate the cell current, stating all assumptions. [5 Marks]

(e) What is the importance of the cell design for a successful operation of a borohydride fuel cell? State the  strategies to achieve  high energy from a stack. [5 Marks]

Faraday constant, F = 96485 C mol-1 ;

Molecular weight of so-diu m borohydride 37.83 g mol-1 ;

Conversion: 2.78 x 10-4 kWh = 1 kJ

E0BH4  = -1.24 V vs. SHE, E0O2   (in alkaline solutions) = 0.4 V vs. SHE.

(b)    A  photovoltaic module, consisting of 36 crystalline silicon solar cells, has an open - circuit voltage of 21 V and a short - circuit current of 3 A. The  series resistance of one  cell  has  been determined to be 40 mΩ. If the fill factor ffo  of a similar ideal solar cell (without series resistance) is 0.892, what is the power of this module at the maximum power point? [5 Marks]

(c)    What is the dark saturation current of one cell in the module in Part (b) under standard test conditions? [4 Marks]

(d)    How many of the modules in Part (b) and how large battery (in Wh) would you need to power a daily load of 300 Wh in a location with daily solar radiation of 4.5 kWh m-2 , for a system that operates without a maximum power-point tracker, having 6 days of autonomy ? [4 Marks]