Assignment Detail

Answer ALL questions.
Each question carries marks distributed as shown. [ ] Note that a number of the numerical questions are set based on the use of the last two digits of your student number. If the last two digits are 00 then use the value of 10. State what the two digits are at the beginning of each calculation.
Part 1. This part relates to the electricity grid.
a) Describe with the aid of a diagram the structure of the national electricity grid, showing the generators and loads at different levels. Give examples of the kind of generator and load that could exist at each level including their typical average power generation and use size. [5]
b) Explain the reason for the way in which a high voltage transmission wire is constructed. Sketch the cross-section of a typical cable bundle. What is the consequence of too much current passing through such a wire, and how can a single cable failure lead to whole regions of the power grid going down? [5]
c) Sketch the typical 24 hour load profile for the UK. Explain the reason for the shape of the profile. With social isolation and home working imposed by the covid-19 pandemic, draw how you would expect the profile to change and state what factors could affect it. What factors lead to variations in national load profiles for different regions of the world and seasons? [5]
d) What factors would you expect to affect the resilience of the national grid? How could  you improve the technologies? Using the example of at least five power generation technologies, describe how their dispatchability varies and put them in order of dispatchability. Explain your answer briefly. [5]
e) Taking the last two digits of your student number to represent the power being carried by a transmission cable in kW (i.e. student number 10269459 would represent 59 MW), calculate the distance in km that would result in 4% power loss due to Ohmic (Joule) heating in the wire if the transmission voltage is 100 kV . Assume that the cable is made of a material with a resistivity of 2.8 ×10-8 m and a cross-sectional area of 100 mm2. [5]

f) Using a Sankey diagram, represent the components and flows of a national energy system with the following attributes: – Energy into system: natural gas (40%), coal (10%), renewable electricity
(25%), oil (25%). – Of the natural gas that comes into the system, 50% goes direct to heating
the domestic sector and 50% into power stations for electricity generation. – All of the coal goes to electricity generation. For both the coal and the natural gas that goes to electricity generation, the power station efficiency is 40%.
– Of the power that comes out of the power stations, 50% goes to the domestic sector and 50% goes to the industrial sector.
– The renewable electricity has a transmission efficiency of 96%. With 75% of the delivered electricity going to the transport sector for electric vehicles and 25% going to the domestic sector.
– All of the oil is processed in refineries to petroleum with an efficiency of 80%. All of the petroleum goes into the transport sector. State the overall efficiency of the system and show potential locations on the
diagram where storage can be introduces and the nature of what that storage could be. [10]

Part 2. This part relates to fuel cell systems.
a) Considering the piping and instrumentation diagram (P&ID) of the polymer electrolyte fuel cell (PEFC) system shown in Figure Q2. i) What temperature would you expect at the input of the high
temperature radiator?

ii) What is the purpose of the membrane humidifier and where does it obtain its water from?
iii) Describe what happens to the gas stream exiting the anode. What are the possible routes for this exit gas and what is the purpose of the purge valve?
iv) Explain why the air compressor is integrated with a cathode exit gas expander.
v) What is the purpose of the air pre-cooler, and why is it required? [5]
b) Taking the last two digits of your student number to represent the power of the stack in kW (i.e. student number 10269459 would represent 59 kW), if the open circuit voltage is 1 V, the area specific resistance is 0.3 cm2, the current density is 1.2 A cm-2 and the area of each cell is 100 cm2, calculate the
following:

i) operating voltage.
ii) Power of each cell.
iii) Number of cells in the stack and overall stack voltage.
iv) Hydrogen consumption (in g hour-1) if operated at a hydrogen stoichiometry of 1.5.
v) Assuming all inefficiencies result in heat generation, how much heat would need to be dissipated from the stack if it were operating at 45% electrical efficiency? [10]
c) Take the last two digits of your student number to represent the current density of a fuel cell (i.e. student number 10269459 would represent 0.59 A cm-2). Assume that the water generated remains in the liquid phase and no water is ejected from the stack during operation, calculate the time required for
1 mg cm-2 liquid water to accumulate in a fuel cell. [5]
d) Explain the processes that determine water dynamics in a polymer electrolyte fuel cell. Use a diagram to help explain the different processes, where water is generated and the direction and mechanism by which it is transported. [5]
e) Assuming that on average 1.5 molecules of water are transported with every proton in the polymer electrolyte due to electro-osmotic drag. Compare the total water delivered to the cathode by electro-osmotic drag to that generated by reaction at a given current density, assuming the membrane is in contact with vapour-phase water only. [5]

f) The mass transport of reactant gas to the electrode catalyst layer is a key factor limiting fuel cell performance at high current density. Describe the function and structure of the gas diffusion layer (GDL) and discuss the design factors that influence mass transport of reactant in the GDL.
What are the practical limitations that dictate the design of the GDL? [5]
Figure Q2. Piping and instrumentation diagram of a  polymer electrolyte fuel cells system
Part 3. This part relates to energy economics and policy. a) What is the purpose of a national Energy Policy? What instruments are available for governments to set policy and influence the market?
Describe briefly how each method operates. [5]
b) What is a Learning Curve and how can they be used to estimate future cost of energy? Take the example of solar photovoltaics (PV) and with the aid of a sketch, describe how its learning curve has evolved. What factors have influenced the cost reduction of solar PV? [5]
c) With the aid of a diagram, for an energy technology other than batteries, describe an energy technology Hype Cycle. What factors determine where a technology is on the cycle and how it
progresses through the cycle?  [5]

Part 4. This question relates to electric and hybrid vehicle.
a) Provide an explanation for each of the following: i) Why is it not possible in practice to abstract 100% of the available kinetic energy in the form of regenerative braking?
ii) What is the purpose of a differential gear in a road vehicle?
iii) What are the benefits of a series/parallel hybrid over a parallel hybrid?
iv) When considering vehicle dynamics and generating a power cycle from a drive cycle it is common to consider the aerodynamics, rolling resistance and acceleration factors. In practice, what other factors will influence the power demand of a vehicle?
v) What requires the highest battery power density, a pure electric (battery) vehicle or hybrid (battery) electric vehicle, and why? [5]
b) Taking the last two digits of your student number to be the velocity of a vehicle in km h-1 (i.e. student number 10269459 would represent 59 km h-1), calculate the power required at the power source if the vehicle has the following properties: weight = 1500 kg, drag coefficient = 0.3, rolling resistance coefficient = 0.01, frontal area = 2 m2 and power train efficiency = 90%. [5]

c) With the aid of a diagram, show the forces on a vehicle going up a hill. Using simple trigonometry, show how the force opposing climbing the hill can be found. If the same vehicle were to climb a hill with a 1 in 9 gradient, by how much would the power required to drive the vehicle at the same speed have to increase? [5]
END