
Compression of air creates heat; the air is warmer after compression. Expansion removes heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, then the efficiency of the storage improves considerably. There are several ways in which a CAES system can deal with heat. Air storage can be , diabatic, , or near-isothermal. [pdf]

The different kinds of thermal energy storage can be divided into three separate categories: sensible heat, latent heat, and thermo-chemical heat storage. Each of these has different advantages and disadvantages that determine their applications. storage (SHS) is the most straightforward method. It simply means the temperature of some medium is either increased or decreased. This type of storage is the most commerciall. [pdf]
Researchers from Solar Energy Institute at UPM are developing a new energy storage system in which the entry energy, either from solar energy or surplus electricity from a renewable power generation, is stored in the form of heat in molten silicon at very high temperature, around 1400 °C.
“In theory, this is the linchpin to enabling renewable energy to power the entire grid.” MIT engineers have designed a system that would store renewable energy in the form of molten, white-hot silicon, and could potentially deliver that energy to the grid on demand.
Molten salts can be employed as a thermal energy storage method to retain thermal energy. Presently, this is a commercially used technology to store the heat collected by concentrated solar power (e.g., from a solar tower or solar trough).
The new MIT storage concept taps renewable energy to produce heat, which is then stored as white-hot molten silicon. The U.S. researchers have dubbed the technology Thermal Energy Grid Storage – Multi-Junction Photovoltaics. The technology uses two large 10-meter wide graphite tanks, which are heavily insulated and filled with liquid silicon.
A novel system has been created that allows the storage energy in molten silicon which is the most abundant element in Earth's crust.
The sensible heat of molten salt is also used for storing solar energy at a high temperature, termed molten-salt technology or molten salt energy storage (MSES). Molten salts can be employed as a thermal energy storage method to retain thermal energy.

Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance; full-cycle lifetimes quoted for flywheels range from in excess of 10 , up to 10 , cycles of use), high (100–130 W·h/kg, or 360–500 kJ/kg), and large maximum power output. The (ratio of energy out per energy in) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 to 1. [pdf]
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