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Applications of the thermal energy storage (TES) principle (storage of sensible heat or latent heat, or heat storage in reversible chemical reactions) in power systems are evaluated. Load leveling behind the meter, load following at conventional thermal power plants, solar thermal power generation, and waste heat utilization are the principal TES applications considered. Specific TES examples discussed include: storage heaters for electric-resistance space heating, air conditioning TES in the form of chilled water or eutectic salt baths, hot water TES, and trans-seasonal storage in heated water in confined aquifers.

The term thermal energy storage can apply to any storage function for which the principal inputs and outputs are thermal energy, whether as “hotness” or as “coolness”. Generally, hotness storage technologies have received more research and development support because they have more uses and cover a wider range of temperatures, but for building applications the storage of coolness (commonly called cool storage) has acquired at least equal prominence in regions where comfort cooling is a major use of energy.

Thermal energy storage (TES) is the term usually applied to any storage function for which the principal inputs and outputs are thermal energy. In building applications it may be “hotness” storage for space heating or domestic water heating, “coolness” storage for air conditioning, or storage at room temperature for stabilization within the comfort zone. It can substantially improve the economics of conventional space conditioning, and in solar applications it is virtually indispensible.