Sorbents are materials that have an ability to attract and hold other gases or liquids. Desiccants are sorbents that have a particular affinity for water. The process of attracting and holding moisture is described as either absorption or adsorption, depending on whether the desiccant undergoes a chemical change as it takes on moisture. Absorption changes the desiccant the way, for example, table salt changes from a solid to a liquid as it absorbs moisture. Adsorption, on the other hand, does not change the desiccant except by the addition of the weight of water vapor, similar in some ways to a sponge soaking up water (ASHRAE, 2005).
Compared to an ordinary cooling cycle, the basic idea of an absorption system is to avoid compression work. This is done by using a suitable working pair: a refrigerant and a solution that can absorb the refrigerant.
Absorption systems are similar to vapor-compression air-conditioning systems but differ in the pressurization stage. In general, an evaporating refrigerant is absorbed by an absorbent on the low-pressure side. Combinations include lithium bromide-water (LiBr-H2O), where water vapor is the refrigerant, and ammonia-water (NH3-H2O) systems, where ammonia is the refrigerant (Keith et al., 1996).
Adsorption cooling is the other group of sorption air conditioners that utilizes an agent (the adsorbent) to adsorb the moisture from the air (or dry any other gas or liquid), then uses the evaporative cooling effect to produce cooling. Solar energy can be used to regenerate the drying agent. Solid adsorbents include silica gels, zeolites, synthetic zeolites, activated alumina, carbons, and synthetic polymers (ASHRAE, 2005). Liquid adsorbents can be triethylene glycol solutions of lithium chloride and lithium bromide solutions.
More details of these systems are given in separate sections further on.
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