Résumé

A general exergy based-design method for optimization of heat pump/refrigeration systems is proposed. It is based on a concept of overall temperature level of a flow-energy, to propose a general expression of overall exergy efficiency and losses of any heat pump/refrigeration cycle. Explicit and general relations of exergy efficiency and coefficient of performances are given to evaluate de performance of such cycles regarding the selection of working fluids, the characteristic of equipment (pinches on evaporators and condensers, performance characteristics of compressors and expansion valve) and the design methods for optimization. Rigorously we introduce the overall and complete exergy efficiency for the most general cases where two energy services are provided, like producing simultaneously refrigeration and heating services or when the cycle is located in a temperature domain far from the atmospheric temperature. This complete exergy efficiency is determined by considering losses in the various components of the cycle and permits to analyse the various cases of heat pump systems including frigopump and thermopump with or without cogeneration systems. Such a method will facilitate the use of exergy theory in a way to highlight the existing link and relationship between energy and exergy losses of heat pump systems. Results of using such a method will be shown for simple and advanced cycles. Results show that the coefficient of performance of a heat pump/refrigeration installation does not necessarily depend on the reference atmospheric temperature but only on the intrinsic parameters relating to the choice of cycle, the operating conditions and the components of the machine. These parameters are obviously chosen according to the temperature levels of the available sources.

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