Thermal energy storage refers to technologies that store energy in a thermal reservoir for later use.

The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) than that of the ambient environment.

A full thermal reservoir holds 1,010 MW h of heat, enough to run the turbine for about 7.5 hours at full-load, in case it rains or after sunset.

Lakes, oceans and rivers often serve as thermal reservoirs in geophysical processes, such as the weather.

In atmospheric science, large air masses in the atmosphere often function as thermal reservoirs.

But two thermal reservoirs are needed, because transfer of energy as heat is irreversible.

In this simplest kind of convection, the carrier body exchanges heat successively with the respective thermal reservoirs.

The efficiency solely depends on the temperature difference between the hot and cold thermal reservoirs.

Phonons in the metal are typically well-coupled to substrate phonons and act as a thermal reservoir.

Nevertheless, Equation 3 is extremely useful for determining the maximum efficiency that could ever be expected for a given set of thermal reservoirs.