Electrical energy can be stored in the temperature difference between something hot and the environment at ambient temperature. The higher the thermal capacity of the hot material, and the higher its temperature, the better both in terms of the cost of the installation, and the percentage of stored energy that can be recovered and converted back to electricity when required. If the hot material has a phase change (say solid to liquid) at a convenient temperature and a large latent heat involved in the phase change, then the system has the advantage of working over a constant temperature range, which makes it both simpler and more efficient. 1414 Degrees is a company in Australia developing such systems using silicon, which has a phase change (solid to liquid) at 1414°C with a very high latent heat of melting.
Energy can also be stored by liquifying air when there’s surplus energy, then storing the liquid air until the energy is needed, when the liquid air is warmed back to ambient temperature, producing high pressure air which drives turbines. This is also a thermal energy storage system, this time using the environment as the “hot” mass, and the liquid air as the cold mass. In this case the cold mass has a phase change (gas to liquid) again achieving a constant temperature range to work over. Highview Power is a company in London developing such systems.
Colocating these two systems could substantially improve the efficiency of both. For the explanation of this, see A Bit of Thermodynamics.
There’s a group at Newcastle University exploring another option, already using both a cold and a hot store rather than the environment as a heat source/sink, but working between less extreme temperatures. They have a 600kWh, 150kW system up and running: Grid-scale pumped heat energy storage system.
Former nuclear or fossil fuel power station sites would be ideal locations for such systems. They are already industrial sites, so no new ground would be being broken, they already have a workforce with relevant skills, and they already have high capacity connections to the national electricity grid. The energy storage system could utilize the whole capacity of the connection to accept energy for storing when supplies from wind and sun exceed demand, or to deliver energy when they fall short, or both.
A relatively minor point is that these facilities could also be solar farms, and probably wind farms too. For example, the area of the Hinkley Point site is 135 hectares, which could host 330 MW worth of solar panels if they covered the whole site (and there’s no conflict with the energy storage system in doing that), producing about 35 MW (mean). It could also have six 2.5 MW wind turbines, capable of generating 15 MW (peak) or about 4 MW (mean).
This is only 1/80 of the generating capacity of the proposed reactors for the same site, but the main point of the project would be the thermal energy storage system, which could have the same capacity as the proposed reactors, at considerably lower cost.
One by-product of the cryogenic storage is solid carbon dioxide, frozen out of the air before the air liquifies. This has to be removed to prevent it fouling the machinery. The quantity may be sufficient to be worth using as feedstock for synthetic fuel production, see Chemical Energy Storage, but that would also be only a minor contributor to the value of the project.
See also Storing Electrical Energy for other methods that may be more appropriate at certain sites.