Energy Storage on a Power System

Abstract

The storage of energy on power systems is nearly as old as power systems themselves. Pumped water storage has always been widely used on power systems and is still the only method that is economic today, although many others have been tried, including fuels cells, compressed air and batteries.

Devices for power correction and balancing (e.g. capacitor banks and batteries) may physically involve the storage of energy (just as a mobile phone does) but it is misleading to describe them as methods of power system storage, (just as it would be misleading to call a School bus a fuel transportation system, even though it does transport fuel).

Economic storage on a power system must rely on arbitrage, buying electrical power when it is cheap and selling when it is dear. In practice, this means a store must buy power at night and sell it during the day.

There is no solar power at night (by definition), so solar power cannot be stored economically on a well-run power system. Also renewables (and nuclear) are installed commercially to save fuel but fuel costs the same at night as it does during the day, so there is no arbitrage on fuel-savers to justify storage. It is not economic to provide for (or associate storage with) wind power.

When a power system has different sorts of plant generating—coal, gas, nuclear, wind, etc.—any power being put into storage is from the plant that would need to be switched off (because less power was needed) if storage ceased (e.g. because the store became full or failed). On a well-run power system, that always has to be the plant with the highest fuel/running cost. The wind blows free and has zero fuel/running cost, so wind is never (normally) stored unless there is no other plant on line, that is, wind power is the last to be stored.

There are two significant exceptions to this. A high-performance plant has high temperature welds that can be damaged by thermal recycling, so such a plant does not want to be a turned off and may be prepared to pay to be allowed to continue running. Then it effectively has a negative running cost which can beat the zero running cost for wind. Nuclear plant can suffer in the same way because, although the temperatures are not particularly high, radiation can significantly increase thermal fatigue at quite modest temperatures.

Another exception can be combined heat and power: in the middle of a cold night there may be a demand for heat but no light and therefore no power. A CHP system has to produce both at the same time and may therefore be prepared to pay to remain online with its power generation.

Electric vehicles provide storage but cannot easily be recharged rapidly without the provision of huge power levels. With normal amounts of power available, a vehicle has to be recharged all night. For many commercial applications, such as milk floats in urban areas, this is no problem but for more general use it will raise difficulties.