Energy Storage Technologies Questions and Answers

Answer: C

It must absorb heat by melting during the day at a temperature the occupants tolerate and freeze again in the cool of the night. A material melting outside that band never changes phase and contributes only its sensible heat.

Answer: D

Correct answer: at night, when the chiller runs at low tariff

Ice storage moves the cooling load in time: the chiller runs overnight, when electricity is cheapest and the ambient temperature is lowest so the machine works most efficiently, and freezes a store of water. During the afternoon the ice melts to meet the cooling demand, and the chiller runs little or not at all. The saving comes from the tariff difference and from a smaller chiller, since it no longer has to meet the instantaneous peak.

Answer: A

The 334 kilojoules per kilogram of fusion dwarfs the sensible heat available over a chilled water swing of a few degrees, so the tank is several times smaller. The penalty is a lower evaporating temperature and a poorer chiller efficiency.

Answer: B

A reversible reaction is driven one way by heat and runs back when the products are recombined, and the separated products can be kept indefinitely at ambient temperature. That makes it the only medium credible for seasonal storage.

Answer: C

Correct answer: almost no loss while standing

Seasonal storage requires energy to survive months, which rules out every medium with a standing loss - sensible heat leaks away, batteries self discharge. In a thermochemical store the energy is held in the separated products of a reversible reaction, and provided they are kept apart there is essentially no loss at all, so summer heat can be recovered in winter. Its efficiency and cost are worse than the alternatives; the absence of standing loss is what makes it uniquely suited to the duty.

Answer: D

Correct answer: a fuel cell or a turbine

Electrolysis splits water into hydrogen and oxygen on charge; the hydrogen is stored, and on discharge it is recombined with oxygen, either electrochemically in a fuel cell or by combustion in a turbine or engine. Each conversion carries a loss, which is why the round trip efficiency is only about 30 to 40 per cent - well below any battery.

Answer: A

Correct answer: can be stored in bulk for a long period

Hydrogen’s poor efficiency is accepted because it does the one thing batteries cannot: it stores very large quantities for very long periods, in salt caverns or as a liquid or a chemical carrier, with no self discharge. For seasonal balancing or for a multi-week wind lull, the alternative is not a less efficient store but no store at all. Its volumetric energy density is in fact poor, which is why it must be compressed or liquefied.

Answer: B

Correct answer: hydrogen or methane injected into the gas network

Power to gas uses surplus electricity to electrolyse water, and either injects the hydrogen directly into the gas network within the permitted limit, or combines it with carbon dioxide to make synthetic methane which the network can take without restriction. The attraction is the gas network’s own enormous storage capacity, far beyond anything the electricity system possesses, already built and paid for.

Answer: C

Correct answer: during the hours of highest demand

Peak shaving discharges the store when demand is greatest, so the peak drawn from the network or from generation is reduced. For a consumer it cuts the maximum demand charge, which on an industrial tariff may be a large part of the bill; for a network it defers the reinforcement that the peak would otherwise require. The store is recharged during the off-peak hours.

Answer: D

Correct answer: selling it when it is dear

Arbitrage exploits the difference between prices at different times: buy at the overnight minimum, sell at the evening peak. The spread must exceed the round trip losses and the wear on the store to be worth doing, which is why a store of 80 per cent efficiency needs prices to differ by more than a quarter before it breaks even - the calculation set out later in this topic.