Energy Storage Technologies Questions and Answers

Answer: C

Correct answer: high power for a short time

A flywheel can absorb or deliver very large power almost instantly and can be cycled millions of times without degradation, which makes it excellent for frequency regulation, for ride-through of momentary interruptions, and for repetitive duties. What it cannot do is hold energy: bearing and windage losses drain it in hours, so it is a power device rather than an energy one.

Answer: D

Correct answer: four

Stored energy varies with the square of the angular velocity:

so doubling the speed quadruples the energy. This is why flywheel design pushes speed rather than mass - and why the limit is the strength of the rim material, since the stress also rises with the square of the speed. Composite rotors are used because their strength-to-density ratio permits a higher tip speed than steel.

Answer: A

Correct answer: low cost per unit of energy

Lead acid remains the cheapest practical rechargeable chemistry per kilowatt hour, and its materials are abundant and almost entirely recycled. Everything else about it is inferior to lithium ion: it is heavy, its cycle life is short if it is deeply discharged, and it deteriorates if left standing discharged. Cost is what keeps it in service for standby and starting duties.

Answer: B

Correct answer: left standing in a discharged state

Discharging converts the active material to lead sulphate in a fine, readily reversible form. Left in that state, the crystals coarsen into hard sulphate that will not dissolve on recharge, so capacity is permanently lost. This is why a lead acid battery must be recharged promptly and kept on float, and why a car battery left flat over a winter is often ruined.

Answer: C

It stores several times the energy of lead acid for the same mass and survives thousands of cycles, which is what made portable electronics and electric vehicles possible. It is intolerant of abuse and needs a management system.

Answer: D

Correct answer: keep every cell within its safe voltage and temperature

Lithium ion cells tolerate no abuse: overcharging drives lithium plating and can lead to thermal runaway, over-discharging damages the electrodes irreversibly, and both are made worse by heat. In a series string the weakest cell reaches its limit first, so the management system monitors each cell individually, balances them, limits current and temperature, and disconnects the pack if any parameter goes out of range. It is a safety system, not merely a performance one.

Answer: A

Above a threshold the internal reactions release heat faster than the cell can shed it, and the temperature climbs without limit until the cell vents or burns. It carries its own oxidant, which is why smothering the fire is ineffective.

Answer: B

Correct answer: electrolyte held in external tanks

In a flow battery the active species are dissolved in liquid electrolytes stored in separate tanks and pumped through a stack where the reactions occur. The consequence is that energy and power are decoupled: the tanks determine how much is stored, the stack how fast it can be delivered, and each can be sized independently. That is the flow battery’s distinctive advantage over a conventional cell, where the same electrodes do both jobs.

Answer: C

Doubling the tank volume doubles the hours of discharge and costs nothing in stack, while doubling the stack doubles the power at the same duration. In every sealed cell the two are fixed together by the cell's construction.

Answer: D

Both the sodium and the sulphur are liquid at the operating temperature and the beta alumina electrolyte conducts sodium ions only when hot. The heating parasitic load is a real cost of keeping such a battery on standby.