Distributed Cooling, Thermal Storage
A building or process having a large number of separated cooling loads could have a refrigeration system for each of these loads but it is usually more convenient to concentrate the cooling into one plant. The cooling effect of a central refrigerating system can be distributed by a heat-transferring liquid or secondary refrigerant. Where the working temperatures are always above 0°C, such as in air-conditioning, water is commonly used. At temperatures below this, nonfreezing liquids are used.
Domestic applications of thermal storage are very familiar. Ice cubes are used to cool drinks. Plastic containers holding a phase change material are frozen by placing them in the domestic deep-freeze cabinet. Once frozen, they can be used for the short-term storage of cold foods and drinks.
In commercial use, thermal storage is used to smooth cooling loads or take advantage of off-peak electricity tariffs. It can be used to make available large cooling capacities with limited plant size and typical applications include:
1. To make ice over a period of several hours and then use ice water for the cooling of a batch of warm milk on a dairy farm. This is also used at main creameries, to reduce peak electricity loads. The available water is very close to freezing point, which is the ideal temperature for milk cooling.
2. To run the refrigeration or heat pump system at night, or other times when electricity is cheaper. It is also in use in areas where the electricity supply is unreliable. Where the cold water is to be used at a higher
Temperature, such as in air-conditioning, the circuit requires three-way blending valves.
3. As hold-over cooling plates in transport (see Chapter 17).
Posted in Refrigeration and Air Conditioning