The absorption chiller

As a general term, the word 'sorption' describes the processes that result in the enrichment of a substance within a given phase (absorption), or to a boundary surface/interface between two phases (adsorption). The absorption chiller, a compact, ready-for-connection unit, is located, for example, in a separate container in the immediate vicinity of the CHP unit. The salt lithium bromide is usually used as absorption medium for refrigeration systems. The coolant is water.

Rather than a mechanical compressor like the ones used in compression refrigeration systems, absorption chillers operate on the basis of a so-called thermal compressor. Thus, the absorption chiller unit has no moving parts and is therefore not subject to any significant wear and tear. The coolant is compressed through absorption and decompressed through desorption. The system exploits the temperature dependency of the physical solubility of two substances. The unrestricted solubility of each of the substances in the other and in any mixed ration of the two substances is therefore an indispensable prerequisite. Other system components as well as the basic thermodynamic principles are largely identical in both absorption chillers and compression refrigeration systems.

The functionality of the thermal compressor requires a substance pairing consisting of coolant and sorption or solution media. The coolant evaporated within the evaporator as a result of the heat absorbed from the refrigeration medium is absorbed in the absorber via the concentrated sorption medium. The solution heat that this releases is dissipated with the aid of cooling water, as otherwise the coolant's absorption capacity would fall dramatically. A solution medium pump then pumps the solution, now enriched with the coolant, into the generator (also known as the extractor) in which heat from an external source (for example, from a CHP system) is added. As a result of the coolant's lower boiling point compared with that of the sorption medium, the coolant is boiled off through the added heat. The resulting refrigerant vapour is channelled to the condenser and follows the familiar route through the cooling circuit. What remains is the solution, enriched with sorption medium and depleted of coolant, which can then be resprayed over the refrigerant vapour within the absorber. Usually there is also a thermal exchange between the refrigerant depleted and refrigerant enriched solution, which results in an energy saving and therefore contributes to an improved efficiency level.

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