Energy Storage Technologies Questions and Answers

Answer: A

Correct answer: both the assertion and the reason are true and the reason explains the assertion

Both are true and the reason is the trade being made. Hydrogen returns only 30 to 40 per cent of what it takes in, but the store itself - a salt cavern, or a depleted gas field - costs very little per kilowatt hour held and loses nothing while standing. For seasonal balancing the cost of capacity dominates and the cost of losses does not, so a cheap inefficient store beats an efficient expensive one. The reverse holds for daily cycling, which is why batteries win there.

Answer: B

More energy per cycle is indeed delivered, but cycle life falls faster than the depth rises, so the total energy the battery delivers over its whole life may be less. The optimum is a moderate depth, not the deepest permitted.

Answer: C

Correct answer: in a brief high powered burst

Braking a vehicle returns a large amount of energy in a few seconds, which is a very high power for a short time. A supercapacitor absorbs that burst without difficulty and can do so at every stop for the life of the vehicle; a battery asked to accept the same current repeatedly would be heated and degraded, and its charge acceptance is limited in any case. The usual arrangement pairs the two, the capacitor taking the peaks and the battery holding the energy.

Answer: D

Correct answer: reducing the number of energy conversions in the path

Every conversion carries a loss, and the round trip efficiency is the product of them all - which is precisely why hydrogen, with electrolysis, compression and a fuel cell in the chain, does so much worse than a battery with an inverter at each end. The other three all reduce efficiency: deeper discharge and higher current increase the internal losses, and standing time allows self discharge to take its share.