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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 respectively, and the inlet and outlet temperatures of heat source are 20 °C and 15 °C respectively [34]. For facilitating the test parameter control, the inlet temperature and flow rate of heat source was main- tained at 20 °C and 178 g s 1 respectively for both R22 and R744/R290 blend during the tests. Meanwhile, the evaporating temperature for R22 was maintained at 10 °C by adjusting the degrees of opening of ex- pansion valve. A suitable test condition should be adopted for a fair comparison of the performances using the pure refrigerant and the zeotropic blends. In consideration of large temperature glide as shown in Fig. 1(b) and optimum condition, the temperature at the evaporator inlet for R744/R290 blends was set to 0 °C with the maximum mean value of evaporating temperature around 10 °C as comparable with that using R22. The mean evaporating temperature of the blend is defined as the arithmetic mean value of the inlet temperature and dew point of the blend in the evaporator [31]. When the variations of the measured temperature, pressure, mass flow rate and compressor power consumption were within ± 0.2 K, ± 5 kPa, ± 0.2 g s 1 and ± 0.02 kW, respectively, the system was considered reaching the steady state and the experimental data were collected every 20 s for at least 30 min. 4. Results and discussion In this study, the heat pump system performance comparisons of R744/R290 blends at six compositions with R22 were conducted in a test rig as described in Fig. 2. The optimal refrigerant charge with the maximum COP of the system was obtained under nominal working condition and the optimal charge of working fluid was ascertained by experiment with various charging quantities in step of 100 g. Table 2 shows the comparison of the experimental performances of the heat pump water heaters with R22 and R744/R290 under optimal charge of 12%/88% by mass, Mopt for short, under the nominal working condi- tion. And the experimental performances of the heat pump water hea- ters with R290 or R744 for domestic hot water in the literature are listed in Table 3. However, in consideration of the deviation of the inlet temperature of heat sink as shown in Table 3, the experimental results of the R744 transcritical heat pump water heaters were just listed for reference instead of being compared with those of Mopt heat pump system in this study, due to the inlet temperature of heat sink has great impact on the heating COP of R744 transcritical heat pump system [37]. As shown in Table 2, the optimal refrigerant charge of the Mopt heat pump system is 1.0 kg, which is 47.37% lower than that of R22 because of smaller density of the HC than that of the halocarbon [38]. The decrease of the refrigerant charge benefits in the reduction of the greenhouse effect. The allowable charge of flammable refrigerant for heat pump sys- tems can be found in many related standards. Taking EN378 as an ex- ample, the allowable charge are based in particular on the location of the heat pump system, the room size, and the lower flammable limit of the flammable refrigerant [39]. If the system is installed in the outdoor, the allowable refrigerant charge will be significantly increased. This is the reason that the German company Dimplex has manufactured R290 heat pumps with the charge changing from 1 to 2.5 kg [40]. And the refrigerant charge can be significantly reduce by designing compact system, minimizing internal volume or adopting the falling film phase change heat transfer technology [41–43]. Hence, in view of the system safety, the heat pump system with R744/R290 (12%/88%) should be Table 2 Comparison of test results for R22 and Mopt at nominal working condition of heat pump. Table 3 Experimental results of the relevant references. Parameters COP R290 4.51 R744 3.06 R744 4.21 Qh (kW) 6.556 2.319 – Pdis (MPa) tdis (°C) 1.844 71.5 8.510 86.83 8.750 – r wcom (kW) Ref 2.690 1.455 [35] 1.955 0.758 [36]a – – [37]b Parameters R22 Mopt Diff. (%) COP Qh (kW) 4.262 4.696 4.731 5.518 11.00 17.50 Pdis (MPa) 1.9420 2.1588 11.16 tdis (°C) 95.39 69.93 – r 3.220 2.683 16.68 wcom (kW) 1.102 1.166 5.81 mr (kg) 1.90 1.00 47.37 a The heat sink at the inlet of the gas cooler was local tap water. b The inlet temperature of heat sink was 12 °C. placed outside or designed for minimum refrigerant charge. 4.1. In uence of refrigerant concentration The results of the comparison test on the heat pump system with R744/R290 mixtures and the reference fluid under the nominal working condition are presented as the function of the concentration of R744 in Fig. 3. The durability (lifetime) and stability of the heat pump system with the blends can be examined indirectly by some operational parameters, such as the compressor discharge pressure (discharge pressure for short), suction pressure, the compressor discharge temperature (dis- charge temperature for short), pressure ratio and compressor power consumption. Fig. 3(a) illustrates that the discharge pressure of the mixtures in- creases drastically first and then mildly along with the increase of concentration of R744, mainly as a result of the reduction of normal boiling point of the R744/R290 blend as described in Fig. 1(b). The blend achieves respectively higher discharge pressure than the R22 and R290 does at the mass fraction of R744 above 0.08 and the full in- vestigated range and the discharge pressure of Mopt is 0.2168 MPa and 0.3148 MPa higher than that of R22 and R290 respectively. The change in the pressure ratio is contrary to that of the discharge pressure. This is largely due to the rapid rise in the suction pressure of the blends with the increase of Xf. It can be found that the Mopt presents respectively a slightly higher suction pressure but a lower pressure ratio in compar- ison with those of R22. One of the important operational parameters of the compressor is discharge temperature, which has an obvious effect on the stability of the blends and lubricant as well as the lifetime of the compressor. The discharge temperature is a strong correlation with the pressure ratio, refrigerant concentration, specific heat ratio, suction temperature and isentropic efficiency of compressor, etc. It can be seen from Fig. 3(b) that the increase of mass fraction of R744 in the blends results in the reduction of discharge temperature, which is mainly because the pressure ratios drops with the increase of Xf as shown in Fig. 3(a). The blend presents a much lower discharge temperature in comparison to that of R22 in the full investigated range but a slightly lower discharge temperature than that of R290 in the range of Xf = 0.12–0.16. And the discharge temperature of Mopt is 25.46 K lower than that of R22 but slightly lower than that of R290. This is beneficial to prolong the compressor life and improve the stability of the blends and lubricant. The outlet temperature of heat source drops firstly and then rises gently with the increase of Xf, but the amplitude of variation is very small. It can be discovered that the compressor power increases almost linearly with the rise of Xf. The reason is because the increment of R744 results in the rises of both mass flow rate and the discharge pressure of the 5PDF Image | heat pump water heater using R744
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