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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Energy Conversion and Management, Volume 44, Issue 2, January 2003, Pages 267–282
This paper presents the performance analysis of an air-to-water vapor compression heat pump system using pure refrigerants and zeotropic refrigerant mixtures. The heat pump system is composed of a compressor, condenser, air cooled evaporator, expansion valve, a receiver tank, a superheater/subcooler, refrigerant mixture unit and some auxiliary and measurement devices. The study focuses on the second law efficiency characteristics of the heat pump system. Comparisons are made between the pure refrigerants and refrigerant mixtures on the basis of the COP and second law efficiency. Also, the effect of the evaporator source inlet temperature on the COP and second law efficiency is presented. It was found that the mixture ratio affects the COP and second law efficiency significantly, and the COP and second law efficiency for the pure refrigerants could be improved by using an appropriate mixture of the refrigerants.
Since the action of chlorine atoms liberated from chlorofluorocarbons (CFCs) used in refrigeration technology as catalysts in ozone depleting reactions and their contribution to the greenhouse effect have been discovered, many actions have been performed by different countries and international organizations to reduce the production and consumption of CFCs. According to these regulations, fully halogenated halocarbons, known as CFCs, will ultimately be phased out in all developed countries, while developing countries will benefit from a more relaxed phase out schedule  and . There are two types of measures to reduce CFCs emissions: short term measures and long term measures. Short term measures include a more accurate design of the plants, better maintenance operations and by recycling the fluids whenever possible. Long term measures involve: (i) substitution of the actual refrigerants with non-polluting ones that could meet the requirements of absence of toxicity, flammability and all exigences from the thermodynamic and thermophysical points of view and (ii) use of alternative refrigeration systems as compared to the vapor compression ones, such as air refrigerating machines, steam jet refrigerating machines, absorption refrigerating machines using different circuit types, reverse Stirling refrigerating machines etc. The new refrigerants can be grouped into three main categories: natural refrigerants, HFCs and mixtures of (environmentally friendly) refrigerants ,  and . Considering the negative role of CFCs on ozone depletion and the greenhouse effect as well as the positive impact of heat pumps for reduction of CO2-emissions, research and development work has not only to be concentrated on environmentally benign fluids with no or negligible ozone depletion potential (ODP) and global warming potential but also on improved energy efficiency, heat pump performance and lower primary energy consumption compared with conventional heating systems. The interest in the use of zeotropic mixtures of refrigerants in compression heat pumps and other vapor compression cycles has grown in recent years. In heat pump systems, zeotropic mixtures have several advantages over pure refrigerants , ,  and : (1) Improved system performance: A zeotropic mixture is one whose components do not form a zeotrop, i.e. it never performs as a pure fluid. Isobaric phase change operations do not occur at constant temperature. Rather, there is a range of temperatures over which the mixture exists in a two phase state. Therefore, the temperature of a zeotropic mixture during evaporation or condensation at constant pressures will vary. By approximating this temperature variation in the working fluid to the temperature profile of the source or sink fluid, the average temperature difference is reduced. The irreversibilities of the components are also reduced due to the reduction of this temperature difference, resulting in improved system performance. (2) Possibility of continuous capacity control: The capacity of a particular system could potentially be varied by changing the circulating composition of the working fluid, thus ensuring the matching of the heating capacity to the consumer’s heat consumption curve. Controlling the capacity of a heat pump system would reduce on/off cycling when operating at off design conditions, thereby diminishing transient system losses and easing equipment sizing problems for applications with widely varying loads. (3) Increased capacity at low ambient temperature. (4) Use of zeotropic mixtures may help in the research for environmentally safe refrigerants. A large number of zeotropic binary mixtures may be formed with refrigerants that are suitable for compression type heat pumps . In this study, R12, R22 and R114 were used as pure refrigerants. Therefore, the characteristics, including ODP, greenhouse warming effect and phase out schedule, are given in the following paragraph and summarized in Table 1. Table 1. Properties of R12, R114 and R22 refrigerants  and  Fluid Formula Boiling (°C) ODP GEP Main uses Flammability Safety group R12 CCl2F2 −29.8 1 2.5 R/BA/AP No 1 R114 C2Cl2F4 +3.5 1 1.2–3.7 R/BA No 1 R22 CHClF2 −40.8 0.05 0.34 R No 1 ODP: ozone depletion power; GEP: greenhouse effect power; R: refrigerants (in refrigeration systems or heat pumps); BA: blowing agents in expanded foams; AP: aerosol propellents. Table options R12 is a fully halogenated refrigerant (CFC) working within evaporation temperatures from +10 to −20 °C and condensation temperatures up to +90 °C. R12, with their two chlorine atoms, have a greater environmental impact than refrigerant 22, which has only one chlorine atom. According to the Montreal protocol adopted in 1987, R12 is one of the most dangerous refrigerants on the basis of ozone depleting potential and greenhouse effects. R22 is a partly halogenated refrigerant (HCFC) with a lifetime of approximately 20 years and ODP of about 5% as compared to that of R11 and R12. R22 has a phase out schedule of 2020 world wide and 2015 in the European Union. Thus, a replacement for R22 is not apparently very urgent, and the Copenhagen and Vienna amendments to the Montreal Protocol established HCFCs as controlled substances, although some nations approved a more accelerated phase out schedule , ,  and . The use of refrigerant mixtures in heat pumps affects the parameters: (i) COP, (ii) heating capacity, (iii) variation of the heating capacity with load, (iv) pressure levels in the condenser and evaporator, (v) adjustment of series-manufactured equipment to various heating capacity needs and (vi) acceptance of higher temperature gradients in the heat sink and heat source . The variation of these parameters in heat pumps with various refrigerant mixtures has been studied by several investigators. There is relatively little information in the open literature on second law evaluation of heat pumps with zeotropic refrigerant mixtures. Although R11 and R114 are among the refrigerants to be controlled due to their potential for ozone damage, the purpose of this study was to evaluate heat pumps on the basis of second law efficiency, and therefore, this investigation focused on the second law efficiency characteristics of a vapor compression heat pump using pure refrigerants and zeotropic refrigerant mixtures. Experiments were conducted using an air-to-water heat pump system. R12, R22 and R114 and various mass mixtures of R12/R22 and R12/R114 were selected as working fluids. The effect of evaporator source inlet temperature on the COP and second law efficiency is also presented. The results of the COP and second law efficiency for pure refrigerants and zeotropic mixtures are compared.
نتیجه گیری انگلیسی
In the present study, the COP equation for zeotropic refrigerant mixtures is derived, and the second law efficiency obtained by using this equation is employed for evaluating the performance characteristics of pure refrigerants R12, R22 and R114 and various mixtures R12/R114 and R12/R22 in an air-to-water heat pump system. A comparative study of the pure refrigerants and the mixtures is made on the basis of the COP and second law efficiency. Also, the effect of the evaporator source inlet temperature on the COP and second law efficiency is studied. The results show that: (a) The COP and ηII increase with increasing evaporator source inlet temperature for the pure refrigerants and refrigerant mixtures. (b) The mixture ratio affects significantly the COP and second law efficiency of heat pumps. (c) The COP and ηII of pure refrigerants can be improved by using appropriate mixtures of the refrigerants. (d) Second law efficiency can be used to identify the highest efficiency mixtures of refrigerants. Considering the phase out schedule of CFCs and HCFCs, it is very important to evaluate refrigerant mixtures which do not deplete ozone and which do not have greenhouse effects. Therefore, zeotropic refrigerant mixtures which have smaller ODP, minimum greenhouse effect, higher COP and higher second law efficiency can be a substitute for CFC and HCFC refrigerants.