Energy Storage Technologies Questions and Answers

Answer: A

Correct answer: matched instant by instant

Electricity is not stored in the network itself, so generation must equal demand plus losses at every moment; any imbalance appears at once as a change in system frequency. Traditionally the matching was done by dispatching generators up and down, and storage is the alternative that lets energy be moved in time rather than requiring a plant to follow the load.

Answer: B

Correct answer: variable and not dispatchable

Wind and solar output follows the weather rather than the operator’s instruction, so it cannot be called up when needed nor held back when not. The system must therefore absorb surpluses and cover shortfalls by other means, and as the share of such generation rises the size and duration of those excursions grow. Storage is how the energy is moved from when it is produced to when it is wanted.

Answer: C

Correct answer: net demand after solar generation is subtracted

Net demand is what the remaining generators must supply. On a system with much solar it falls steeply through the middle of the day and rises sharply in the evening as the sun sets and domestic demand peaks, giving a curve whose belly and steep neck resemble a duck. The problems it poses are the depth of the midday minimum and the steepness of the evening ramp, both of which storage addresses directly.

Answer: D

Correct answer: the potential energy of raised water

Surplus electricity drives pumps that lift water from a lower reservoir to an upper one, and the energy is held as gravitational potential energy until the water is released back through turbines. The stored quantity is

so it depends on the volume and the head, and nothing else. It remains by far the largest form of grid storage in service worldwide.

Answer: A

Correct answer: two reservoirs separated by a height difference

The scheme needs somewhere to put the water at each end and a head between them, and since the energy stored is proportional to that head, the greater the difference the smaller the reservoirs need be. Suitable sites are mountainous, and largely already developed in the countries that have them, which is the principal constraint on building more. No river flow is required, since the same water circulates.

Answer: B

Correct answer: 75 to 80 per cent

Losses arise in the pumps and turbines, in the pipework and in the electrical machines, and a modern reversible pump-turbine scheme returns about three quarters to four fifths of what it took in. That is high for a bulk store of this size, and combined with a life measured in many decades it is what has kept pumped hydro dominant despite its site constraints.

Answer: C

A solution-mined salt cavern gives the enormous volume needed at a cost no manufactured vessel could approach, which is why the technology is as site-limited as pumped hydro. Above-ground vessels suit only small schemes.

Answer: D

Correct answer: heated the air and that heat was rejected

Compressing air heats it, and in a diabatic plant that heat is thrown away in an aftercooler so the air can be stored cold in the cavern. On discharge the air is therefore cold, and expanding it directly would chill the turbine to destruction, so fuel is burnt to reheat it. That fuel is why the plant is not a pure store. Adiabatic designs retain the compression heat in a thermal store and return it on discharge, removing the need for fuel.

Answer: A

Keeping the compression heat in a thermal store and returning it to the air before expansion removes the need for any fuel and lifts the round trip efficiency substantially. The thermal store is the difficult and costly component.

Answer: B

Correct answer: the kinetic energy of a rotating mass

A flywheel holds energy in the rotation of its rotor:

Modern units use a composite rotor in a vacuum on magnetic bearings, spinning at tens of thousands of revolutions per minute, with a motor-generator to put energy in and take it out. The stored energy is limited by the tensile strength of the rim, which sets the maximum tip speed.