systems: A classical use of this refrigeration engineering procedure is the refrigerator. In this coldness-machine the cooling agent initially absorbs heat energy from the inside of the refrigerator on a low energy-level (low pressure, low temperature) and thereby evaporates. By means of the additional compression of the cooling agent, its energy density rises to a higher energy level (high pressure, high temperature). This energy can then be dispensed to the surrounding area via a condenser. During the process, the cooling agent condenses and is then relaxed via an expansion valve until it reaches its initial energy-level once again (low pressure, low temperature). Many procedures in refrigeration engineering are based on this principle. An evening cooling in this procedure, however, can only be guaranteed through a constant supply of electrical or mechanical energy in the compressor.
Absorption refrigeration engineering: A different procedure is used in absorption refrigeration engineering. Instead of mechanical energy, thermal energy is used. In the cooling circuit of the cooling-machine, a pair of materials is used that is completely soluble over a wide temperature range, such as water in certain saline solutions. In a first step, the gaseous cooling agent (water) is condensed. Hereby heat can be dispensed into the surrounding area. Then the liquid water is vaporised in a low pressure area. The needed vaporisation energy is taken from the cooling agent and thus coldness develops. In a further low pressure area, the steam is cooled and – while dispensing heat – returns to its fluid state and is solved with the salt. The watery saline solution is separated via external heat supply, whereby pure water as a cooling agent and a concentrated saline solution as a solvent are regained. The circular process is completed and can begin anew. In this method of refrigeration engineering, the coldness is thereby created under a steady supply of thermal energy.