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PE, ASHRAE Presidential Fellow, LEED AP BD+C, BEAP, MCIBSE
Chilled Water TES Hydraulics
ASHRAE Journal, February 2015
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Thermal energy storage (TES) is an effective means of shifting cooling electrical load from peak to off-peak electrical rates. Chilled water is the most common form of TES, using concrete or steel tanks to store chilled water at 39°F (4°C), which is the temperature at which water density is highest, encouraging stratification within the tank. Under normal conditions, a chilled water TES 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 returned to the top of the tank. Due to the increased density of colder water, a stable stratification of layers of water can be obtained.
Placing an open chilled water thermal energy storage tank in a chilled water system has several ramifications on the hydraulic performance of the system. This month, I intend to point out engineering issues that must be addressed under various scenarios. These are all real examples from actual facilities that I have designed, peer reviews I performed of designs by others, or retrofits of designs by others.
TES tanks are seldom ASME-rated pressure vessels due to the high cost, so they must be vented to atmosphere. This establishes the reference pressure for the system; essentially the TES tank is serving as a vented expansion tank. Heat exchangers could be used to physically isolate the TES tank from the distribution system, but this is typically not preferred since chilled water ΔT will suffer with heat exchangers and additional pumps would be required. This column will focus on how to control for pressure in chilled water systems with atmospheric TES tanks without heat exchangers.