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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 blend. And the variation of the compressor power is very limited with the blend. Fig. 3(c) shows the variation of the COP of the heat pump system with R744/R290 versus R744 mass fraction Xf. The COP of the R744/ R290 blend increases sharply at first then gently before reaching the peak and then decrease slowly with the increase of Xf. The blend pre- sents a higher COP than that of R22 and R290 both in the range of Xf = 0.08–0.16. The main reason is that the use of blend can sig- nificantly decrease the irreversible loss in the condenser in comparison with that of R22 for their temperature glides. The better match of heat transfer between refrigerant and the secondary fluids can increase system energy and exergy efficiencies. And for R744/R290 blend, the maximum COP of 4.731 is obtained at the optimal mass fractions of 12%/88%, which is improved by 11.00% and 4.90% compared with that of R22 and R290 respectively. The curve of COP obtained from the experiment shows the similar variation tendency as that indicated in our previous theoretical study [19] and the Mopt presents a 1.00% higher COP than that of theoretical result. However, the optimal mass fraction (12%/88%) deviates from the theoretical result (20%/80%) which might be mainly caused by variation of real compressor effi- ciency, control strategy, lubrication or some improper assumptions for the heat pump model. Therefore, the heat pump model in Ref. [19] can be further modified with these experimental results of the heat pump system using R744/R290 to predict more precise results under different working conditions. The other major performance parameter of the heat pump system is heating capacity. In order to avoid making significant design changes on the original system, the alternative mixtures should achieve a similar or higher heating capacity than that of R22, which contributes to the improvement of the seasonal performance of heat pump system, espe- cially for winter condition. Fig. 3(c) also shows that the heating capa- city of the blend increases quickly firstly and then slowly with the rise of Xf. This is largely because the critical temperature of the blend drops with the increase of Xf as shown in Fig. 1(a) and the critical temperature reduction results in lower suction specific volume (higher vapor den- sity) under the same evaporating temperature, which nearly leads to higher heating capacity, such as the analysis in Section 2. It also reveals that the heating capacity of the R744/R290 blend in the range of Xf = 0.10–0.16 is higher than that of R22 and Mopt performs an addi- tion of 17.50%. Hence, the use of Mopt can improve the reliability and comfort of the original R22 heat pump system with minor modifications such as the increase of the areas of the heat exchangers as indicated by Hakkaki-Fard et al. [21]. The temperature profiles of the heat transfer fluids, refrigerants (Mopt and R22) and the secondary fluids in the condenser and eva- porator are respectively shown in Figs. 4 and 5. The temperatures are presented at inlet and outlet and at the bent tubes and connecting tubes for both refrigerant and secondary fluids in the heat exchangers re- spectively. As seen in Fig. 4, the heat transfer curves of both Mopt and R22 include three different sloped heat rejection processes of superheat vapor cooling section, condensation section and subcooled liquid cooling section. Fig. 4(a) shows that the temperatures of heat sinks rise slowly in the range of Lc = 18–21.6 m due to the low heat transfer coefficient of the transfer fluids and small temperature difference be- tween two transfer fluids. For R22, the almost constant temperature during the condensation process and the quite high discharge tem- perature result in the heat transfer sections with a large temperature difference between refrigerant and heat sink in the range of Lc = 0–2.4 m and Lc = 12–16.8 m, respectively. This leads to the tem- perature mismatch of two heat transfer fluids. It can also be noted that the temperature profile of Mopt has a virtually parallel matching with temperature profile of heat sink in the range of Lc = 7.2–15.6 m be- cause of its large temperature glide of about 21 K under the test con- dition, which is more clearly shown in Fig. 4(b) with relative heat as abscissa. It is not difficult to infer that the Mopt obtains a better temperature match with the heat sink than R22 does, due to the heat transfer curve of Mopt includes three different sloped heat rejection processes with larger temperature glide and lower discharge tempera- ture. Thus, the irreversible loss of Mopt in the condenser is lower that of R22, which makes a contribution to the augment of the system energy efficiency. The augment of the system energy efficiency relies on both pro- cesses of evaporation and condensation. The temperature difference of heat source is usually much smaller than that of heat sink, while the zeotropic blend has similar temperature glides in phase change pro- cesses. Fig. 5(a) shows that the temperature rise of Mopt in evaporation is much greater than the temperature drop of heat source, even though it undergoes partial evaporation, since at the inlet of evaporator the state is in two-phase flow zone from the expansion valve. Fig. 5(b) shows that the heat transfer pinch points appear at the exit of the evaporator for both Mopt and R22. However, the heat transfer pinch point may also appear at the inlet for R22 and at the middle of eva- poration for Mopt, because of the convex temperature profile of non- linear temperature evaporation of the blend. Since Mopt has a greater temperature difference between outlet and inlet of the evaporator than R22 does, with a greater logarithmic mean heat transfer temperature difference between temperature profiles of heat source than that of R22, the irreversible loss in the evaporator with Mopt might greater than that with R22. Moreover, the evaporator area should be enlarged to avoid the wet compression of the compressor. The optimum concentration of blend in application is usually a compromise with appropriate temperature glide to fit both the tem- perature rise of heat sink in condenser and the temperature drop of heat source in evaporator by improving the temperature match between heat transfer fluids. 4.2. In uence of heat sink outlet temperature All the above discussed is based on the setting condition of fixed heat sink outlet temperature of 55 °C. However, the hot water with different temperatures will be supplied for different users or different purposes. If the heat sink outlet temperature maintains a low value for a long time, the legionella would multiply rapidly in the heat sink water tank and cause health hazards. Therefore, the hot water outlet tem- perature is advised to be more than 60 °C in view of killing the legio- nella for the health. However, the rise of heat sink outlet temperature results in the decrease of the heating COP because of the increment of the condensing temperature. Thus, it is necessary to investigate the impact of the heat sink outlet temperature on the cycle performance of the heat pump. The comparative performance tests were conducted for the heat pump system with Mopt and R22 at the heat sink outlet tem- perature ranging from 45 °C to 65 °C with step 5 K and the comparison results are demonstrated in Fig. 6. Fig. 6(a) demonstrates that the increments of the discharge pres- sures are nearly linear for both Mopt and R22 with the rise of heat sink outlet temperature, which are largely caused by raising the condensa- tion temperatures. It also can be observed that the discharge pressure of Mopt is always slightly higher than that of R22 at the investigated temperature range and the pressure differences between Mopt and R22 are lower than 0.265 MPa with minor fluctuation. Fig. 6(a) also shows that Mopt has a smaller pressure ratio than R22 does and the pressure ratios of both Mopt and R22 increase with the rise of heat sink outlet temperature because the discharge pressure increases with the rise of heat sink outlet temperature while the suction pressures keep almost unchanged. Fig. 6(b) shows that the discharge temperatures of both Mopt and R22 increase with the increase of the heat sink outlet temperature, which is mainly because of the rise of the pressure ratios. The discharge temperature of Mopt is 24.35–28.03K lower than that of R22 in the studied range of heat sink outlet temperature. When the heat sink outlet temperature was at 65 °C, the discharge temperature of R22 reached 7PDF Image | heat pump water heater using R744
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