Thermal Energy Storage


Thermal energy storage (TES) involves both heating and cooling energy. A heating TES add heat to a storage medium, a cooling TES extracts heat from a storage medium, which is stored in a reservoir.
Cool TES systems are used e.g. for air-conditioning: At night inexpensive off-peak power is used to create a reservoir of cool material, and during the day the reservoir is tapped to provide cooling capacity.


Storage medium Capacity(40K) Note
Ice 334,000 J/kg 144 Btu/pound + for low temperatures
+ low space requirement
Chilled water 40,000 J/kg 18 Btu/pound o for conventional temperatures
- high space requirement
Phase-change 160,000 J/kg 60 Btu/pound o for conventional temperatures
+ low space requirement

Btu=British thermal unit

Phase-change material systems are more expensive than chilled water sytems, but they are easy to maintain and need less space.


The storage medium is cooled in the cooler and stored in the insulated reservoir for later use. When cooling is needed, the evaporator produces it by tapping the reservoir.
Two reservoirs are not always required, in some cases the same reservoir can be used. If ice is produced, an evaporator is not always necessary; for air-conditioning, the air is cooled enough by flowing through the reservoir.


Christoph Böhm TI4
Gregor Lotti NT4
Patrick Lotti NT4