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| Absorption cycleThe affinity between the absorption cycle and the vapour compression cycle can be seen in the Figure below. An absorption heat pump contains an evaporator, condenser and expansion valve which operate in exactly the same way as those in the vapour compression system: Heat is taken in at the evaporator causing the working fluid (refrigerant) to evaporate at low pressure, and heat is released by condensation at high pressure. This type of heat pump does not necessarily need fossil fuel energy. It can use any thermal source such as waste heat and solar energy instead.
In the absorption cycle, however, there is a secondary circuit around which a liquid absorbent or solvent flows. The evaporated refrigerant vapour is absorbed into this at low pressure, liberating the heat of condensation and solution. The solvent, now diluted by refrigerant, is raised to the high pressure by a liquid pump. High pressure refrigerant vapour is then produced by the addition of heat to the mixture in the generator. The resulting heat output is, as in the case of the vapour compression cycle, greater than that added from primary energy sources. In the absorption cycle, however, there is a secondary circuit around which a liquid absorbent or solvent flows. The evaporated refrigerant vapour is absorbed into this at low pressure, liberating the heat of condensation and solution. The solvent, now diluted by refrigerant, is raised to the high pressure by a liquid pump. High pressure refrigerant vapour is then produced by the addition of heat to the mixture in the generator. The resulting heat output is, as in the case of the vapour compression cycle, greater than that added from primary energy sources. The absorption cycle heat pump may at first sight appear much more complicated than the vapour compression system. However, it is essentially a collection of heat exchangers, and this offers (a) considerable scope for cost reduction. Cost and size reductions are the goal of several national and international absorption cyle programmes. A second feature of the absorption cycle is the frequently environmentally friendly nature of the refrigerant/solvet fluid pair. Most commonly these are ammonia/water or water/lithium bromide. Another sorption system is the solid-gas adsorption heat pump. Although currently less well developed than systems using liquid absorbents, this offers opportunities not normally associated with more conventional heat pump systems. These include integral heat (and cold) storage and the ability to produce heat at temperature levels of up to 200-300°C. This latter feature makes the solid-gas system highly appropriate for industrial process applications. Source: Brochure "Heat Pumps - an option for an energy efficient and clean society" of the Commission of the European Communities. | |
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