Absorption Refrigeration

Desiccant Cooling

Another type of air-conditioning system uses a salt solution to cool spaces (Fig. 25-3). In the absorption cycle, water vapor is attracted to a concentrated salt solution, which absorbs water from the evaporator vessel. The water cools rapidly as it is evaporated into the evaporator vessel. The water is now diluting the salt solution in the evaporator. The diluted salt solution is drawn off from the vessel continually, sprayed into a piece of equipment called a generator that boils excess water off, and returned to repeat the absorption cycle. The steam that boils off condenses at a condenser with cool water or air, and returns to the evaporator vessel. The cooled water left in the evaporator can be tapped through a heat exchanger as a source of chilled water.

Absorption cycles are about half as efficient as compression cycles. Energy for the system can come from the sun or from high-temperature waste heat from steam or hot water. Even though an absorption cycle is less efficient, it may use less energy, since it can use lower-grade heat to run a generator, as opposed to the electricity the compression cycle uses for its compressor.

Cooling with desiccants does not use any refrigerants with CFCs. Desiccants are porous materials, such as silica gel, activated alumina, and synthetic polymers with a high affinity for water vapor, that lower humidity without overcooling the air. In active desiccant systems, desiccants are heated with natural gas or solar energy to drive out the moisture that they have removed from the air. Passive systems use the heat from the building's exhaust air to release and vent moisture removed from incoming air.

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