Chilled Water Storage
Chilled water is the most common and simplest form of TES, using concrete or steel tanks to store chilled water at 40 F to 42 F (4.4 C to 5.6 C) that is generated with conventional chillers. Under normal conditions a chilled-water storage tank is always filled with water. During discharge, cold water is pumped from the bottom of the tank, while an equal amount of warm return water is supplied to the top of the tank. Due to the different densities for water at different temperatures, a stable stratification of layers of water can be obtained. Where space is cost-effectively available for chilled water storage, the economies of scale for this technology can provide significant economic advantages over ice storage.
Ice thermal storage uses the latent heat of fusion of water. The storage volume depends on the final proportion of the ice to water in a fully charged tank. The ice storage requires ¼ or less volume than chilled water storage for the same capacity. The ice making chillers, on the other hand, operate with lower efficiencies than conventional chillers (COP’s in the range of 2.5 to 4.1 versus 5.0 to 5.9 for chilled water). Thus, the economic benefits with ice storage systems rely, to a great extent, on the lower off-peak electrical rates.