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F. Ju et al. , Q W H U Q D W L R Q D O - R X U Q D O R I 7 K H U P D O 6 replacement in various heat pump, air conditioning and refrigeration systems. The aim is simply to mitigate the flammability of R290, but the benefits are multiple that the operating pressure of R744 system is re- duced and the system efficiency is improved by taking advantage of the temperature glide. Fan et al. [19] conducted a simulation study on the performance of a heat pump water heater system using R744/R290 as the substitute for R22 and indicated that the mixture with mass concentration 20%/80% of R744/R290 has the best performance, which presents respectively 12.62% and 34.24% increments in heating COP (coefficient of perfor- mance) and volumetric heating capacity with respect to those of R22. Dai et al. [20] theoretically investigated the feasibility of binary blends of R744 with ten low-GWP refrigerants used as substitutes in heat pump water heater system and discovered that the mixture R744/R290 pre- sents the promising results at two optimum concentrations for higher heating COP. Hakkaki-Fard et al. [21] performed a numerical simula- tion study on the feasibility of R744 blends used as a substitute for R410A in a cold climate air-source heat pump and concluded that the heating COP of heat pump with R744/R290 (5%/95%, by mass) is higher but the heating capacity is low in comparison with that of R410A; while the results are opposite with R744/R32 (20%/80%, by mass). Zhang et al. [22] theoretically and experimentally studied the performance of a transcritical system with R744/R290 mixture and the results demonstrated that both volumetric heating capacity and the heating COP of the R744/R290 (95%/5%, by mass) are comparable to those of pure R744 with a decrease of heat rejection pressure, but the heat rejection pressure is still very high. Kim et al. [23] performed an experimental study on the air conditioning system using blend of R744/ R290 and demonstrated that the addition of R290 to R744 can promote the system energy efficiency and reduce the discharge pressure, with better temperature matching in the heat exchangers. Kim and Kim [24] conducted theoretical and experimental investigations on the perfor- mance of an autocascade refrigeration system with R744/R290 blend. Niu and Zhang [25] and Di Nicola et al. [26] experimentally and the- oretically investigated the performance of R744/R290 for cascade re- frigeration systems respectively. The above studies indicate that R744/R290 has the potential to be a long term substitute in heat pump, air-conditioning and various re- frigeration systems, but for heat pump water heater systems using R744/R290 the investigation was only carried out by our research team with theoretical research [19], and the feasibility of R744/R290 as the substitute work medium for heat pump water heater systems needs to be testified by experimental research. Hence, in this paper, the feasi- bility verification of R744/R290 blend used as a substitute for R22 in an instant heat pump water heater system with large temperature lift of heat sink for domestic hot water supply was experimentally in- vestigated in a water to water heat pump testing rig. The emphasis was on the enhancement in heating COP and heating capacity of heat pump using zeotropic blend R744/R290 by replacing the conventional cycle to the Lorenz cycle [27,28]. 2. Refrigerant properties The ODP and GWP of R22 are respectively 0.05 and 1810, while R744/R290 blend presents the excellent environmental performances with null ODP and negligible (< 20) GWP. Fig. 1 shows the physical characteristics of R22 and R744/R290 blends at six compositions. Some parameters, such as normal boiling point, critical temperature, critical pressure and temperature glide, were computed by using REFPROP 9.1 [29]. The lower critical temperature of the mixture leads to the higher heating capacity due to the lower suction specific volume under the same evaporation temperature but reduced heating COP for the ex- cessive superheat at compressor outlet, reduced condensation section and increased flash gas loss at throttle valve outlet. Thus, a modest critical temperature is necessary with consideration of the heating COP and heating capacity. It is noted from Fig. 1(a) that the increase of R744 in the mixture leads to the lower critical temperature for its lower critical temperature of 31.1 °C. R744/R290 blends at six compositions perform the slightly lower critical temperatures than that of R22, but they still meet the demand in view of the operational condition of the heat pump in this study. It also can be found that the higher critical pressure of R744 results in the rise of critical pressure of the mixtures with the increase of R744. It can be speculated that the discharge pressure of the heat pump system using R744/R290 may increase with the rise of R744 under the same operating conditions, mainly due to the reduced critical temperature and raised critical pressure. Hence, the content of R744 in the blend should be controlled to ensure the ac- ceptable discharge pressure can be obtained. The normal boiling point of a zeotropic mixture is defined as the arithmetic mean value of the bubble point and dew point under normal atmospheric pressure. It is clear from Fig. 1(b) that the normal boiling point of the R744/R290 blend, as the criterion that judge the applicable temperature zone, decreases with the rise of mass fraction of R744 which has a low normal boiling point of 78 °C. If the system has a leakage, the firstly leaked R744 with lower normal boiling point might reduce the danger. Generally, the refrigerant with lower normal boiling point has the higher discharge pressure under the same operating temperature. It can be predict that the R744/R290 blend will presents the higher discharge pressure in comparison with that of either R290 or R22 because of the lower normal boiling point as shown in Fig. 1(b). In addition, the temperature glide of the R744/R290 blend increases with the rise of mass fraction of R744. The optimum temperature glide can compromise the requirement of temperature variations in both heat exchangers of a heat pump system with improved temperature matching between hot and cold heat transfer fluids. Thus, the use of the mixtures can enhance the COP or exergy efficiency of heat pump system by reducing the mean temperature differences between heat transfer fluids [30]. For the binary blend with high temperature glide, the two compo- nents of the blend have quite different normal boiling points. Hence, the system is charged firstly with the lower vapor pressure (higher normal boiling point) refrigerant and then the higher one. If the system has a leakage, the component with lower normal boiling point leaks firstly, this results in the variation in mixture concentration. Thus, for system maintenance, the recharge of the mixture may be a challenge. 3. Experiments 3.1. Experimental setup A fully instrumented water-water heat pump system was developed for a precise performance assessment and comparison of pure R22 and R744/R290 blends. As shown in Fig. 2, the testing apparatus is com- posed of three closed loops: refrigerant loop, heat sink loop and heat source loop. The refrigerant loop contains a rolling rotor compressor, a con- denser, an expansion valve, an evaporator, a dry filter and two sight glasses. A compressor with a constant speed designed for R22 was used for the tests. In consideration of the safe operation of the compressor, the maximum allowable discharge pressure and discharge temperature of the heat pump are 2.996 MPa and 110 °C respectively and the al- lowable suction pressure range is 0.296–1.044 MPa, which were pro- vided by the compressor manufacturer. The condenser and evaporator were made of respectively eighteen and eight pieces of 1.2-m pre- manufactured concentric copper tubes connected together. Compared with the R22 heat pump system, the system was retrofitted with the condenser and evaporator of about 14% and 20% longer tube length, respectively. For the condenser, the outer diameter and the thickness of the outer and the inner tubes are 16.0 mm × 1.5 mm and 9.52 mm × 0.8 mm respectively. For the evaporator, the outer diameter and the thickness of the outer and the inner tubes are 2PDF Image | heat pump water heater using R744
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