Energy Storage Technologies Questions and Answers

Answer: A

Correct answer: withdrawn from it

Depth of discharge is the fraction taken out; state of charge is the fraction remaining, and the two sum to one. A cell at 30 per cent depth of discharge is at 70 per cent state of charge. The distinction matters because cycle life is quoted against depth of discharge, and a battery cycled shallowly lasts far longer than one cycled deeply.

Answer: B

Correct answer: extends its cycle life at the cost of usable capacity

Cycle life rises steeply as the depth of discharge is reduced - a lithium ion cell cycled to 80 per cent may last a few thousand cycles, and one cycled to 20 per cent tens of thousands. The price is that a larger battery must be bought to deliver the same usable energy. The economic optimum balances the two, and it is why a storage system is almost always specified with a usable capacity well below its nameplate.

Answer: C

Correct answer: its rated capacity in ampere hours

The C-rate normalises current to capacity, so 1C is the current that would discharge the battery in one hour, 0.5C in two hours, and 2C in half an hour. Expressing the current this way makes limits comparable between cells of any size, which is why manufacturers specify maximum charge and discharge rates in C rather than in amperes.

Answer: D

Correct answer: while the battery stands with no load connected

Self discharge is the loss of charge to internal side reactions when nothing is drawing current. Lithium ion loses a few per cent a month, lead acid rather more, and both rates rise sharply with temperature. It matters for standby duties and for anything left unused for long periods, and it is one reason no electrochemical store is suitable for seasonal storage.

Answer: A

Correct answer: fast it can be delivered

Energy density is capacity per unit mass or volume - how much is stored; power density is the rate at which it can be moved in or out. The two are largely independent and often opposed: a supercapacitor has a very high power density and a low energy density, while a fuel or a battery has the reverse. Matching the duty to the right corner of that trade is the whole of storage selection.

Answer: B

Correct answer: as charge separated at an electrode surface

A supercapacitor stores charge physically, in the electrical double layer that forms at the interface between a very high area electrode and the electrolyte. Because no chemical reaction is involved, charge and discharge are almost instantaneous and the device withstands hundreds of thousands of cycles - but the energy stored, \( \tfrac{1}{2}CV^2 \), is small compared with a battery of the same size.

Answer: C

It can absorb or release its content almost instantly, which suits regenerative braking and pulse loads, but holds only enough for seconds of duty. Batteries and capacitors are often combined so each does what it does best.

Answer: D

Current circulates in the coil with no resistive loss for as long as the temperature is maintained, and the energy is released almost instantaneously. The cryogenic plant makes it expensive and its standby load significant.

Answer: A

Correct answer: raising the temperature of a material

Sensible heat storage holds energy in the temperature rise of a medium - water, rock, molten salt, concrete - and the quantity is \( mc\Delta T \). It is simple and cheap, but the store is only useful over the temperature range available and it loses heat continuously to its surroundings. Latent storage, which uses a phase change, holds more in a smaller volume and at a nearly constant temperature.

Answer: B

The latent heat of fusion is large compared with the sensible heat over any practical temperature swing, and it is absorbed without the store getting warmer. Discharging at a fixed temperature suits many process duties.