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Thermal energy (chilled water or hot water) is produced during periods of off-peak electrical demand (or usage), collected in a thermal energy storage tank, then withdrawn and distributed to the facility during on peak periods. Warm and chilled water enters and exits the tank through diffusers located at the top and bottom of the tank. These diffusers are designed to eliminate turbulence and allow the water in the tank to stratify, with the colder (and denser) water at the bottom and the warmer water at the top. A narrow and sharply defined thermocline, or transistion layer of water, will form between the warm and cold water regions.

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The illustration shows how a TES tank would operate at the beginning of the daily discharging phase (refer to On Peak Cooling Mode). During the discharge mode, the chillers and associated condensing equipment are de-energized, and chilled water from the TES tank is circulated to the facility for cooling.

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When the discharge operation is complete, the tank will contain mostly warm water, and is ready for the start of the “charging” mode (Refer to Off-Peak Cooling Mode illustration). In the charging phase, warm water is withdrawn through the top diffuser, sent to the chiller plant, and then returns cold into the tank through the bottom diffuser after being cooled by a chiller system. When that process is ended, one thermal energy storage cycle is complete, and the tank again is ready to be discharged during the On Peak Cooling Mode.