Energy Storage Technologies Questions and Answers

Answer: C

Correct answer: 3.0 Wh

The energy in a capacitor is

Three watt hours from a device the size of a large drink can shows the limitation: a lithium ion cell of the same volume holds perhaps thirty times as much. What the supercapacitor offers instead is the ability to deliver it in a second and to do so a million times.

Answer: D

Mass times specific heat times temperature difference is 10 000 times 4.18 times 40, or 1.672 \( \times \) 10\( ^6 \) kilojoules, which is 464 kilowatt hours. A forty degree swing is generous for a chilled water store.

Answer: A

Correct answer: 27.8 kWh

Latent heat storage holds \( mL \):

The attraction of a phase change material is that all of this is absorbed and released at essentially one temperature, so the store delivers at a constant useful temperature instead of drifting down as a sensible store does.

Answer: B

Correct answer: 5 units

Because only 80 per cent of what is bought comes back, more must be bought than is delivered:

The store therefore only pays if the selling price exceeds five units, so the price spread must be at least 25 per cent before arbitrage breaks even, before any allowance for the wear the cycling costs. Multiplying by the efficiency instead of dividing gives 3.2, which would have the losses making the energy cheaper.

Answer: C

Correct answer: 1-Q, 2-R, 3-P

Each technology occupies a different place on the duration axis. A flywheel discharges in seconds to minutes and cannot hold energy longer, since windage and bearing losses drain it. A lithium ion battery is economic over hours, which suits daily cycling and peak shaving. Hydrogen is the only one of the three that can hold energy for weeks or months without loss, which is why its poor efficiency is tolerated for seasonal duty.

Answer: D

The form of storage determines the response time, the standing loss and the cost, so it is the first thing to establish about any medium. Mechanical, electrical, electrochemical and thermal stores behave quite differently.

Answer: A

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: the flow battery’s tanks and stack size the energy and the power independently, and ice storage works on the latent heat of fusion of water. Statement 2 is the reverse of the truth - hydrogen returns only about 30 to 40 per cent of what it takes in against a lithium ion battery’s 90, and it is chosen for duration rather than efficiency.

Answer: B

Correct answer: a sodium sulphur battery

A sodium sulphur cell is an electrochemical store, holding energy in a chemical reaction between molten sodium and sulphur at about 300 degrees. The other three are mechanical: a flywheel holds kinetic energy, a pumped hydro scheme gravitational potential energy, and a compressed air cavern the energy of a compressed gas.

Answer: C

Correct answer: a flywheel

Every feature of the duty points to a flywheel: the power is high and the duration very short, so little energy is needed; the cycle count is enormous, and a flywheel suffers no degradation from cycling where a battery would be worn out within months; and the response required is immediate. Hydrogen and thermochemical stores are far too slow, and a pumped hydro scheme cannot start and stop hundreds of times a day.

Answer: D

Correct answer: a lithium ion battery

The duty is 20 MWh delivered over four hours, once daily, for some five thousand cycles - a classic daily energy shifting requirement. Lithium ion is economic at that duration, achieves round trip efficiencies around 90 per cent, and will reach five thousand cycles if the depth of discharge is limited. Supercapacitors, flywheels and superconducting magnetic stores are all power devices whose energy capacity would be absurdly expensive over four hours.