The water temperature of the heat pump water heater developed in China can generally reach the requirement of 50e, but the high pressure of the compressor is too high, and the COP of the unit is relatively low, especially in winter. At present, the technical level of China's heat pump water heaters is relatively low. Under the impetus of the market, research on heat pump water heaters has become one of the hot research. Among them, the working fluid is an important research topic of heat pump water heater research.
In the preferred working conditions of air source heat pump water heaters, the following principles must be met: 1) in line with China's environmental protection policy, that is, ODP and GWP values ​​are small; 2) suitable pressure within the working temperature range, the compression ratio should not be too large; ) Good chemical stability and thermal stability; 4) Has the largest possible unit volume cooling capacity; 5) Small density, low viscosity, high thermal conductivity; 6) Non-toxic, non-flammable, non-explosive, safe to use; 7) The raw material source is sufficient, the manufacturing process is simple and the price is cheap; 8) The higher COP is obtained under the premise of ensuring the outlet water temperature (5560e); 9) The compressor of R22 can be used.
From the physical property analysis, R410A has a lower critical temperature, which limits the water temperature of the water heater; R407C also has a relatively low critical temperature, even if it can reach the required outlet temperature, it will result in lower COP; R417A Compared with R22, it has larger molar mass, lower critical temperature and lower critical pressure; R134a has larger molar mass than R22, lower critical pressure, and farther away from R22 critical pressure; TJR01 Molar mass, critical pressure, ODP and R22 are almost the same, and the critical temperature is higher, which is conducive to the production of hot water at higher temperatures. The working fluids R407C, R417A, R410A, and TJR01 have temperature slip, especially the TJR01 has a large temperature slip, which is favorable for heat exchange.
Safety of medium and high temperature heat pump working medium The working medium of medium and high temperature air source heat pump water heater can not endanger human health and property safety. Regarding the safety requirements of working fluids, ANSI PASHRAE 342 1997 specifies the safety classification of refrigerants <9> as shown. Among them, the literature <10> also describes the safety indicators: acute toxicity indicators include life- or health-impaired limits (IDLH), 50% test animal lethal concentration (LG50), minimum impact observation concentration (LOEL), no The observed observed concentration (NOEL) and 50% of the tested animals were affected by the effective concentration (EC50); the chronic toxicity indicators included the safety threshold 2 time-weighted average (TLV2TWA) and the allowable exposure limit (PEL). According to the safety analysis indicators, the selected medium and high temperature heat pump water heaters were analyzed.
From the analysis, it can be seen that from the safety data, the TLV2TWA index of TJR01 working fluid is lower than other working fluids, and the other two indicators are slightly higher than other working fluids; from the safety classification, the difference is not big; from the toxicity data, TJR01 is obviously superior Other working fluids. Overall, TJR01 has a high level of security.
Solubility of medium and high temperature heat pump working fluid In order to reduce the friction between the moving parts of the compressor and the leakage between the gaps, a certain amount of lubricating oil is added to the compressor. The working fluid and the lubricating oil are mixed in the system, and whether they are mutually soluble has a certain influence on the heat pump unit and the circulation system.
If the refrigerant is incompatible with the lubricating oil, the lubricating oil may deposit in the heat exchanger and form an oil film on the heat exchange surface to reduce the heat transfer effect. When directly filling other working fluids in the compressor for R22 air conditioning, it is necessary to consider whether the lubricating oil is compatible. Because R134a and R22 are incompatible with the lubricating oil, it cannot be directly filled with R22. The mixed working fluids R417A and R407C also contain R134a, so it is not suitable for direct filling. The currently used hydrocarbons R290, isobutane R600a, etc. can be compatible with the mineral lubricating oil commonly used in heat pump units.
From the perspective of oil solubility, R410a and TJR01 have obvious advantages.
The cycle performance analysis of medium and high temperature heat pump working medium is selected in the medium of high temperature air source heat pump water heater. Under the premise of ensuring the green environmental protection and hot water effluent temperature requirements, it is also necessary to use energy-saving working fluids as much as possible to make the heat pump water heater The COP is large to highlight its advantages of efficient economy. In this paper, the cycle parameters of each group of working fluids are calculated by CSD equation <11>.
CSD equation p=RTv@1+y+y2-y3(1-y)3-av(v+b)(1)y=b4v(2) where: p, v are the pressure specific volume of the refrigerant; , b is the correlation function of temperature T; a(T)=a0exp(a1T+a2T2); b(T)=b0+b1T+b2T2, where: a0, a1, a2, b0, b1, b2 are each work The qualitative physical parameters are optimized by the known saturated vapor pressure, liquid phase and vapor relative volume of the material.
When applied to mixed working fluids, the mixing rule is: b=bixi(3)a=aijxixj(4) where: aij=(1-kij)(aiaj)0.5,kij=kji,kij=kji=0 in theoretical calculation When setting the following parameters: compressor efficiency Gcs = 0.75, evaporation temperature (equivalent evaporation temperature) te = 7e, superheating degree HHsh = 5e, subcooling degree $ Hsc = 5e. for temperature slip The mixed working fluid, the evaporation temperature and the condensing temperature take the equivalent value, that is, the average value of the temperature slip in the respective processes of evaporation and condensation. Calculation results, 6.
It can be seen that when the condensation temperature is 5061e, the COP of R134a is slightly larger than that of R22; when the condensation temperature is greater than 61e, the COP of R22 is slightly larger than that of R134a. The working fluids which are slightly lower than the COP of R22 are TJR01, R407C, R417A, and R410A, and the COP of R410A is much lower than that of R22.
From the comparison of exhaust pressure: R410A and R407C exhaust pressure are larger than R22, especially R410A; R417 exhaust pressure is closer to R22, but slightly lower than R22; and RTJR01 and R134a exhaust pressure are relatively close, and both It is much lower than R22.
From the temperature slip in the process, R22, R410A and R134a have no temperature slip, the heat transfer effect of the heat exchanger is poor, and TJR01, R407C and R417A have temperature slip, in which the temperature slip of TJR01 is large, The heat exchanger has the best heat transfer effect.
According to the above cycle performance analysis, the pressure ratio of COP and compressor of TJR01 is closer to R22, and the compressor exhaust temperature and exhaust pressure of TJR01 are lower than R22. The advantage is that TJR01 has a larger temperature. Slip, the best heat transfer effect, this advantage is unmatched by several other working fluids.
Conclusion 1) The medium-high temperature air source heat pump water heater is optimized according to the general working principle and the special and practical requirements of the medium-high temperature air source heat pump water heater. 2) TJR01 replaces R22, which can ensure the required outlet temperature, minimize the compressor discharge pressure and exhaust temperature, improve the COP of the unit, and ensure that the R22 compressor operates under normal conditions.
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