Question

Chose the correct answer(s) from the multiple choice lists below 1. Exergy of a system is a) b) always increasing e) guided by the first and second laws of thermodynamics for reversible processes only Un d) limited by the second law of thermodynamics for all processes. processes, it is destroyed partly or completely). e) equal to the Gibs free energy of the system. 211two electrodes, one made of u (E.-3.06 V) and another made of Cu ΟΕ.+034V placed in an alkaline electrolyte and connected to a voitmeter e a) The potential at the Li electrode is higher than the potential at the copper b) Cu is the positive electrode. c) Phase potential is equal across the whole cell. d) Li is the cathode. e) none of the above 3. The power of a flywheel system is determined by a) the size of the electrical machine b) the size of the flywheel. c) the energy density of the flywheel. d) none of the above. 4. Suppose that water is pumped up at night with an efficiency of 95% and released with an efficiency of 85% in the morning. If energy loss on the electric grid is of 8%, what is the energy efficiency of sending the electric energy to the storage facility at night and have it back in the morning? a) 85% b)68% c) 74% d)80% e) 92% 5. The work done by a compressor in raising the pressure of 100kg of water from 1 to 200 atmospheres is: a) 200 MJ b) 20 MJ c) 2 MJ d) 200 kJ e) 20 kJ 6. An industrial manufacturer needs to heat up 1000 tonnes of material by 250 K in a time period of 1000 seconds. The material has a specific heat of 1 kJkg K-1, What is the mass flow-rate of gas (heat comes from burning natural gas) that must be supplied to the heating system? The heat content of natural gas is 50 MJ kg-1. a) 200gs- b) 2kgs-1 c) 2m2s-1 9s-1 d) 5 kgs- e) 5kghr-1 0406451: Energy Storage Problem 2 car of mass 1500 kg brakes from 50 kmh-1 in 3.5 seconds when meeting a red light. Based n the intermediate values given in Table 1 below, determine 1. the mass needed for a lithium-ion battery and a supercapacitor; which of the two is a better option? 2. the distance each of the energy storage technology can take you at 50 kmh-1 (requires 10 W); comment. Table 1: Specific energy and power of lithium-ion batteries and supercapacitors for transportation Specific energy (kWh kg-) Specific power (kWkg Lithium-ion batteries 0.15-0.25 Supercapacitor 2-4 0.1-0.5 0.002-0.005 Problem 3 Wind turbines need supercapacitors for system control and power buffer if wind fluctuations The requirement for the superca 300 V. The current i pacitors is that they must handle a voltage change of 220 V to s 13 A. The charging and discharging cycles can last for up to 8 seconds. the supercapacitors tolerates 2.7 V and has a maximum capacitance of 50 F and an Each of internal resistance of 50 m2. 1. What is the capacitance needed for the system? 2. The supercapacitors are co upled in series; what is the equivalent capacitance of this arrangement? 3. How many series like the ones in 2. should be in par allel to provide the needed capacitance? 4. Considering that the maximum current is applied at the highest applied voltage, what is the voltage that remains stored in such a circuit? 5. What is the energy efficiency of the circuit? Problem 4 Consider a stationary, closed compressed air energy storage vessel. The work potential of the air at the compressed s recovered using reversible transformation until equilibrium with the atmosphere. Considering the atmosphere (pa Ta) as reference, the exergy Er of the vessel at a compressed state (Ps. V) is given by: tate (pr, T) is its exergy Ey which equals the work which could be Et = mc.rr-T) + pavy-va) _ mnTa(Cpln T -RIn Pal 1. Simplity equation 1, assuming that air behaves like an ideal gas, and is initially at ambient temperature 2. Write the exergy efficiency y of the system as it is being discharged from state (pi. V to (pr. Vf), as function of pa, pi and the pressure ratio PR- pflpi (0

Answer 1

1) exergy is limited by second law of thermodynamics for all the processes. D option