Thermal energy storage is one of the many new technologies that allow for increased energy efficiency at large facilities with a large demand for energy. Chillers cool water during off-peak hours, which is then stored in insulated tanks. These tanks are usually large, vertical, pressurized vessels, but atmospheric UL-142 storage tanks are also used. During peak hours, the stored, cooled water is used for space conditioning using pumps, control valves, and fans. This process reduces both demands on the grid during peak hours and energy costs. In addition to using stored, chilled water for supplemental space conditioning during times of high demand, thermal energy storage systems can also be used as a backup space conditioning system should the normal system fail.

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http://www.highlandtank.com/thermal-energy-storage-tanks