Performance analysis of transcritical CO2 and common medium low-temperature air energy heat pump

To comply with the development of the “dual-carbon” goal, a new refrigerant for heat pump products is necessary. Firstly, mathematical models of enhanced vapor injection air energy heat pump and transcritical CO2 heat pump systems were established, and they were calculated using thermodynamic methods. The results show that when CO2 is used as the refrigerant, the discharge temperature is 30% and 22.7%, which is higher than that of R410A and R134a, and 4.9% lower than that of R32. At the compressor outlet, the volume flow rate of CO2 is 59.0%, 60.8%, and 64.2% less than R410A, R32, and R134a, respectively. In terms of power consumption, the difference between CO2 and R410 A is within 6%. The power consumption of the CO2 system is 0% ∼ 6.4% higher than that of the R32 system and 29.2% ∼ 46.5% lower than that of the R134a system. The COP of the CO2 system is within 6% of that of the R410A and R32 systems, which is 41% ∼ 87% higher than that of the R134a system. When the ratio of CO2 / R32 is 0 ∼ 0.2 and 0.9, the COP of the mixed refrigerant is better than that of the single refrigerant CO2.


Introduction
The proposal of the "double carbon plan" regards the air energy heat pump as the best heat source for clean heating and speeds up the exploration of cleaner quality [1].According to statistics in [2], by the end of 2022, the total heating area in the north has reached 23.8 billion square meters, and clean energy heating accounts for 75%.For severe cold regions, the traditional air energy heat pump has the problems of high exhaust temperature, insufficient heating capacity, and low performance [3] [4].The heating performance of the enhanced vapor injection air source heat pump system is better, so it has attracted more attention [5~7].At present, the refrigerants used in heat pump products are mainly R410A, R32, and R134a.The global warming potential (GWP) values of the three are high.The Kigali Amendment to the Montreal Protocol stipulates that the use of high GWP refrigerant should be reduced in heat pump products [8].Therefore, the exploration of new refrigerants has become the focus of research.
Based on this, the mathematical models of enhanced vapor injection air source heat pump and transcritical CO2 heat pump system are established, and the system performance of CO2 in enhanced vapor injection air source heat pump is studied.The system performance is compared with the system performance of commonly used refrigerants in heat pump products, which provides a theoretical basis for the substitutability of CO2 and its mixed refrigerant to conventional refrigerant.

System cycle principle
The low-temperature air source heat pump (cold water) unit is used as the heating unit.Its main feature is that it has a more enhanced vapor injection technique than ordinary air source heat pump, and is more applicable in low-temperature environments.
The schematic diagram of the system is shown in Figure 1 (a).The air source heat pump cycle system uses air as the low-level heat energy.The refrigerant absorbs the heat in the air in the evaporator (8-1) and becomes the superheated refrigerant to be inhaled by the compressor.After the first-stage compression (1-2), it is fully mixed with the refrigerant of the auxiliary circulation circuit (2-3) and enters the second-stage compression (3)(4).After the compression, it enters the condenser (or gas cooler) and heats the cooling water (4-5).The cooled refrigerant enters the flasher (5-6) through EEV1 throttling and depressurization to the intermediate state, and the refrigerant is separated by gas and liquid in the flasher.The saturated gaseous refrigerant enters the compressor through the auxiliary circulation circuit to supplement the refrigerant in the compressor (6-9) to complete the auxiliary circulation process.The saturated liquid refrigerant is reduced to a low-temperature and low-pressure refrigerant (7-8) through EEV2 throttling and enters the evaporator to absorb heat to complete the cycle.The cycle pressure-enthalpy diagram of the conventional refrigerant in the enhanced vapor injection air source heat pump system is shown in the solid line in Figure 1

System mathematical model
In the compressor, the enhanced vapor injection technology divides the space in the compression chamber into a first-stage compression chamber and a second-stage compression chamber.The exhaust temperature of each stage in the compression process is shown in Formula (1): where out T is the first stage compression exhaust temperature, ℃; in T is the compression suction temperature, ℃; out P is the first stage compression exhaust pressure, kPa; in P is the suction pressure of the compressor, kPa; n is a polytropic index.
The volume flow rate of the refrigerant at the high-pressure side can be seen in Formula (2): where v q is high-pressure side volume flow, m 3 /s; Q is system heating capacity, kW; h is the enthalpy value of the refrigerant, J/kg; θ is the density of refrigerant, kg/m 3 ; the subscript 1~9 (1'~9') represents each state point.
The main power consumption equipment of the system is the compressor.The power consumption of the system is calculated by the state point physical properties of the inlet and outlet of the compressor.The mathematical model of the power consumption is shown in Formula (3): where W is system power consumption, kW; m q is the mass flow rate of the refrigerant, m 3 /s.The value of evaluating the economy of the system is the coefficient of performance.Through the calculation of the total heating capacity and power consumption of the system, the calculation of the heating coefficient of performance is shown in Formula (4): where COP is the coefficient of performance.
For the calculation of the physical properties of the mixed refrigerant, the He temperature formula [9] combined with the Soave-Redlich-Kwong (SRK) state equation [10] is used to calculate the critical temperature and critical pressure of the mixed refrigerant.
Firstly, the critical temperature of the mixed refrigerant is calculated as shown in Formula (5): where c T is the critical temperature of the refrigerant, ℃; x is the molar fraction of refrigerant; σ is a binary interaction coefficient; i , j , m is the type of refrigerant.
The critical pressure is calculated by the critical temperature obtained as shown in Formula (6): where P is critical pressure, kPa; R is a gas constant, J/(mol•K); T is the refrigerant temperature, ℃; v is volume, m 3 ; b is the volume parameter of the state equation; a is the gravitational parameter of the state equation; D is a constant of state equation.
5 .0 (9) where ij k is a binary interaction constant.In the formula, i a , j a , and i b are determined by the properties of the components of the mixed refrigerant as shown in Formulas (10) ~ (12): In the calculation process, the coefficients involved are:

Model verification
The reliability of the mathematical model is verified by the experimental data of the coefficient of performance of the heat pump system.The average error in the calculation range of the CO2 model is 2.3%, and the average error of R410A is 5% as shown in Figure 2.

Performance of single refrigerant system
System performance of single refrigerant.During the calculation process, the system operation is in a stable state, ignoring the heat loss of system equipment, pipelines, etc.; the temperature of the heat source range is -30℃ ~ 5℃; the heating capacity of the system is 10 kW.
The exhaust temperature results of the system using different refrigerants are shown in Figure 3 (a).The heat pump system was calculated with R410A, R32, R134a, and CO2 in the set temperature of the heat source range.The results show that with the increase in temperature of the heat source, the exhaust temperature decreases.This is mainly because the higher the temperature of the heat source, the greater the suction pressure of the compressor.When the condensing pressure is constant, the pressure ratio is reduced, the efficiency of the compressor is increased, the degree of deviation of the IOP Publishing doi:10.1088/1742-6596/2771/1/0120015 compression process from the isentropic process is reduced, and the exhaust temperature of the compressor is lower.
The change in the volume flow of the refrigerant at the inlet and outlet of the compressor is shown in the solid line of Figure 3 (b).The heating capacity is constant.Within the set temperature of the heat source range, the volume flow rate of the suction port is sorted as follows: R134a > R410A > R32 > CO2.The order of exhaust port volume flow from large to small is R134a > R32 > R410A > CO2.
The power consumption and COP of different refrigerants are shown in the solid and dotted lines of Figure 3 (c), respectively.With the increase in temperature of the heat source, the power consumption gradually decreases.In the phase change process in the circulation system, when the latent heat of vaporization of the refrigerant is smaller, the heat recovery efficiency of the heat is better.Therefore, with the increase in temperature of the heat source, the evaporation temperature increases, and the suction pressure of the corresponding compressor increases.Under the condition of fixed exhaust pressure, the pressure ratio decreases, the power consumption required to complete the compression process decreases, and the corresponding COP shows an increasing trend.

Performance of mixed refrigerant system
In the conventional refrigerant, R32 refrigerant with a low GWP value is selected to mix with CO2 to form a binary mixed refrigerant.R32 itself has low flammability, so CO2 can play a flame-retardant role, and the safety after mixing is better.Therefore, CO2 and R32 were selected for mixing, and their thermophysical properties and heat pump system performance were analyzed.The critical temperature, critical pressure, and boiling point of CO2 / R32 vary with the mass ratio of CO2 in the mixture as shown in Figure 4.In the performance calculation of the mixed refrigerant system, it is assumed that the heat source temperature is -18℃, and 10 kW of heat is produced.In the transcritical cycle, the high-pressure side pressure is 7.5 MPa.The COP of the system with different mixing ratios under the same working condition is calculated, and the results are shown in Figure 5.

Conclusions
(1) Under the same heating capacity, with the increase of temperature of the heat source, compared with other conventional refrigerants, the exhaust temperature of CO2 refrigerant decreases greatly, and the volume flow rate is 59.8%, 60.6%, and 65.5% less than that of R410A, R32, and R134a, indicating that the volume of equipment required to use CO2 refrigerant is smaller.
(2) In the temperature of the heat source range of -30℃ ~ 5℃, power consumption: CO2 and R410A, R32 difference within 6%, R134a power consumption is the largest, the other three are 40% 80% smaller than R134a; in terms of COP: CO2 is less than 6% different from R410A and R32, R134a has the worst performance, and the other three are 4% ~ 80% higher than R134a.
(3) When the mass ratio of CO2 in the mixed refrigerant CO2 / R32 is 0 ~ 0.2 and 0.9, the COP is higher than that of the single refrigerant CO2, and the safety of the mixed system is better.When the mass ratio of CO2 is less than 0.5, the operating pressure of the system is lower.

Figure 1 .
Figure 1.Principle diagram of enhanced vapor injection air source heat pump cycle.

Figure 2 .
Figure 2. Comparison of experimental and simulated COP.

Figure 3 .
Figure 3.The performance of the system varies with the temperature of the heat source.

Table 1 .
The parameters of thermal properties, environmental protection, and safety of each refrigerant are shown in Table1, and the pressure involved is absolute.Physical properties parameters of refrigerant.