General Thermal Energy Storage
Thermal energy storage can be a valuable attribute to district energy systems. It offers the opportunity to meet peak demands without installing new production equipment. By charging cold storage (water or ice) during the cheaper off-peak nighttime the district energy provider can shift production away from the higher cost daytime. Thermal storage also allows equipment to operate during cooler parts of the day and closer to full load which both contribute to less power consumption.
The two common types of media are water and ice for cold storage and water for hot storage. For cold storage water storage is typically less expensive than ice, but since storage is sensible heat only, and limited by the system temperature differential, large storage tanks are needed. In contrast, the heat of fusion of ice means more energy can be stored in a smaller volume. Also supply temperatures can be much lower with ice than with water storage which can benefit the overall system differential temperature (delta T between return and supply). In hot storage, the storage temperature must be below boiling 212F (100C) and for practical reasons is kept below 185F (85C).
Where the geology is suitable, cold or hot water can be stored on a seasonal basis in underground aquifers and used for heating or cooling. Underground thermal energy storage (aquifer or borehole) are open systems and should not be confused with closed “geo-exchange” systems like ground source heat pumps. Geo-exchange systems use the ground as a radiator to dissipate heat, but underground storage uses groundwater like a battery and stores energy in one period (winter) for use in another (summer).