Energy storage devices are “charged” when they absorb energy, either directly from renewable generation devices or indirectly from the electricity grid. They “discharge” when they deliver the stored energy back into the grid.
Charge and discharge normally require power conversion devices, to transform electrical energy (AC or DC) into a different form of electrical, thermal, mechanical or chemical form of energy.
Energy storage can be used among to store surplus energy from intermittent renewable sources, such as solar PV and wind power, until it is required.
There exist many different forms of energy storage: chemical (batteries, natural gas, hydrogen, electrolysis), mechanical (pumped storage plants, compressed air energy storage, pumped heat electrical storage, compressed storage of liquid air) and thermal (molten salt, bulk goods).
Dependent on the needs of the specific application or use case, various solutions are possible depending on whether a larger number of small, local storage facilities or a smaller number of large, central facilities are to be used.
The two main parameters to differentiate energy storage solutions are:
• Power: can reach from a few kW (e.g. in end user applications like residential PV) to MW (large scale generation plants) up to hundreds of MW or even GW for centralised bulk energy storage devices.
• Time: storage may perform charge or discharge functions over a few seconds or minutes (e.g. for grid services like frequency stabilisation), minutes to a few hours (smoothing or time shift of renewable generation), up to days and weeks (balancing long term fluctuations in generation and consumption). Multiplying power by time delivers the capacity or energy content of the storage.