Thermal energy storage systems involve changing the temperature or phase of a medium to store energy, allowing for the generation of energy at a different time from its use. This allows for optimizing the varying cost of energy based on the time of use rates, demand charges, and real-time pricing utility incen.
There are two basic energy storage strategies: latent heat systems and sensible heat systems. Latent heat systems use stratification within the tank to create thermal layering of stored water, with colder water being denser and settling toward the bottom of the tank while warmer water naturally rises to the top. This is done using diffusers within the tank on the inlet and outlet piping.
Chilled water storage tanks require a large footprint to store the large volume of water required for these systems, approximately 15 cubic feet per ton hour for a 15° F 8.3 de C temperature difference. The greater the delta T of the water, the smaller the tank can be. Partial storage systems utilize chilled water to supplement main chiller equipment when they reach their full capacity and additional cooling is needed.
Latent heat transfer strategies are more complex, with several strategies for producing ice. One such strategy is to circulate a glycol solution through coils submerged within the tank, which then accumulates on the outside of the coil. A heat exchanger separates the primary and secondary loops, and a three-way valve and control sequence control the flow of water to and from the tank.
Ice storage systems take less room for storage than chilled water systems due to their greater capacity to store energy per unit area. Storage volumes range from 2 to 4 cubic feet per ton hour for an ice system compared to 15 cubic feet per ton hour for a chilled water system.
Utilities often structure their rates for the usage of electrical power to coincide with their need to reduce loads during peak usage periods. Producing ice or chilled water at night can also increase the efficiency of the overall system as ambient temperatures are cooler at night, especially with the use of aircooled chillers.
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