Thermal Storage - Chilled Water
A chilled water thermal storage system uses the sensible heat in a body of water to store BTUs. Simply put, water passing through a chilled water coil warms as it absorbs BTUs from and subsequently cools the surrounding air. Given its specific heat of 1 Btu/lb F, about 10 cu ft of water are required to absorb 12,000 BTUs and provide 1 ton-hour of cooling if the coil successfully raises the water temperature by 20°F.
By contrast, the same ton-hour of cooling can be provided with just 1.5 cu ft of ice, since each pound of ice absorbs 144 BTUs as it melts. Therefore, a thermal storage system that uses chilled water rather than ice will require 6 to 7 times more installed storage volume.
This graph plots the cost of thermal storage components as a function of the ton-hours of cooling stored. The sizable cost penalty imposed by the significantly larger storage tank volume required for chilled water is readily apparent in a cost-line comparison with ice storage. Keep in mind, however, that the cost of the water storage tank is a function of its surface area, while the capacity of the tank is a function of its volume. Therefore, as a system requires very large chilled water storage tanks, the per-ton-hour cost of the storage tank actually decreases. Consequently, it appears that chilled water may be competitive with ice in applications that require more than 10,000 ton-hours of thermal storage.
A chilled water storage system is really just a simple variation of a decoupled chiller system. Since the same fluid water is used to both store and transfer heat, very few accessories must be added to the system. This gives chilled water storage its principle advantage: It's easy to put in place.
As shown in this schematic, a decoupled system separates the production and distribution of chilled water. The balance of flow between the constant volume production of chilled water and its variable volume distribution is handled with a bypass pipe commonly called a "decoupler." The decoupler bypasses surplus chilled water when production exceeds distribution and borrows return water when distribution exceeds supply. In effect, the decoupler pipe itself can serve as a chilled water storage tank if its volume is large enough.
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