Electricity generated by renewables, such as wind energy and solar power, is based on variable resources. Electricity storage can optimise the energy flows between supply and demand and therefore enable a higher contribution of renewable energy in our electricity mix.
Renewables may also not be fully available at the moment when demand is higher or they may supply an excess when the demand is lower, thus creating an imbalance. Electricity storage can overcome the mismatch between output and demand (the so-called time-shifting) and it can smooth out fluctuations in supply without calling on other back-up capacities. It can also save a supplier from penalties when forecast supply cannot be met (the so-called forecast hedging). The principal electricity storage technologies include hydropower with storage,compressed air energy storage, flow batteries, hydrogen-based energy systems,secondary batteries, flywheels, super capacitors, and superconducting magnetic energy storage.
A wide range of technologies is available to store electricity, including those based on mechanical, chemical and physical principles. Ultimately, the main services that the storage has to provide will dictate the best-adapted technology. For example, there are energy-related applications, where the electricity storage system is designed to discharge for several hours, with a nominal storage capacity of 10 to 500 MW and a time response of 1 to 5 minutes. On the other hand, there are power-related applications, such as maintaining grid frequency, suppressing fluctuations and stabilising voltages, that discharge for between a few seconds to less than an hour and require a response time of a few milliseconds.
The most mature storage technology is hydropower – with either reservoir storage or hydro-pumped storage. The basic principle is to store energy as the potential energy between two reservoirs at different elevations. The wide deployment of hydropower in Europe offers a significant technology base for regulating variable electricity production. The average plant size in the EU-27 is about 270 MW, but can reach 1800 MW, as in France (Is