Thermal Storage - Chilled Water - Pros and Cons

General


Chilled water thermal storage systems offer a number of attractive benefits. Because of the decreasing unit cost of the tanks, chilled water storage can be economically attractive in larger systems. These systems also allow the chiller to operate at peak efficiency during the storage cycle. And, since the storage medium chilled is the same fluid that is cooled in the chiller and warmed in the cooling coil, few accessories are required. Note as well, that storing a large volume of water on site can be a valuable asset for fire/life safety systems. In fact, some system designs use sprinkler system water in their design.

Of course, the disadvantages of chilled water storage - most of which relate to the tank - must also be recognized. The storage tank's design, weight, location and space requirements can pose some unusual problems...along with tank leakage. In addition, storage tank costs can vary significantly because the tank is constructed on site. And, don't forget water treatment cost. The water stored is used in the chilled water system as well.

Perhaps the most significant problem with chilled water as a storage medium is inherent to the chilled water system itself. To be effective, chilled water storage systems must raise the return water temperature to relatively high values. If the chilled water distribution system cannot achieve this, the Btu storage capacity of the tank is severely impaired. Continual monitoring and disciplined maintenance of the chilled water valves and controllers are required to assure that chilled water always returns to the tank at the warmest possible temperature.

The chilled water thermal storage system's installed cost curve shows the significance of storage tank expense. While somewhat prohibitive for most applications under l0,000 ton-hours, the decreasing unit cost of chilled water storage systems can be very attractive for large central plants and industrial installations.