1. Introduction
R134a is an HFC (hydrofluorocarbon) refrigerant that is widely used in automotive air conditioning and water chillers with centrifugal compressors. However, its value of global warming potential (GWP) is 1300, which contributes to global warming [
1,
2]. With increasing global environmental requirements, the usage amount of R134a has been requested to reduce [
3,
4]. One kind of replacement is HFO (Hydrofluoroolefin) refrigerants. This kind of refrigerant has attracted extensive attention and is applied in some applications due to its 0 ozone depletion potential (ODP) and low GWP. Additionally, the representatives of HFOs include R1234yf, R1234ze(E), and R1233zd(E) [
5,
6].
R1234yf is mainly used to replace R134a in automobile air conditioners, while R1234ze(E) or R1233zd(E) is commonly used to replace R134a in centrifugal chillers [
7,
8]. At present, some companies are developing centrifugal chillers using R1234ze(E) and R1233zd(E) as refrigerants, such as Danfoss, Klima-Therm Company, Carrier, Mitsubishi Heavy Industries, Trane, and Gree [
9,
10].
Many scholars studied the replacement of R134a refrigerant. Aral et al. [
11] developed an automotive air conditioning (AAC) and automotive heat pump (AHP) system employing refrigerants R134a and R1234yf. They tested them in cooling and heating operation modes under broad ranges of compressor speed and air inlet temperature conditions. Additionally, the results revealed that R1234yf performs better in heating relative to cooling mode. It can be used as a replacement for R134a in not only AAC but also AHP systems at the expense of slightly lower energy effectiveness. Qi [
12] performed thermodynamic analysis for the R1234yf mobile air-condition (MAC) system associated with the R134a system under three typical vehicle operating conditions. The performance improvement potentials by superheating, subcooling, and compressor performance were mainly focused on and discussed. It was concluded that adding an internal heat exchanger and improving compressor efficiencies would be good options for future R1234yf MAC system enhancement. Mota-Babiloni et al. [
13] presented an energy performance evaluation of two low-GWP refrigerants, R1234yf and R1234ze(E), as drop-in replacements for R134a in a vapor compression system using a reciprocating compressor. Additionally, the volumetric efficiency, cooling capacity, and coefficient of performance (COP) were analyzed. Results showed that the COP values were about 7% lower for R1234yf and 6% lower for R1234ze than those obtained using R134a. Additionally, the use of an internal heat exchanger reduced the COP differences for both replacements. Chen et al. [
14] investigated the performance of R1234yf and R134a in an oil-free vapor compression refrigeration (VCR) system. The results showed that R1234yf is similar to R134a in terms of operating pressure and temperature, but R1234yf has an 11% and 16% deterioration in cooling capacity and COP, respectively.
Many scholars compared the boiling and condensation heat transfer coefficients of R1234yf, R1234ze(E), and R1233zd(E) with those of R134a. Most scholars showed that the boiling and condensation heat transfer coefficients of R1234yf, R1234ze(E), and R1233zd(E) are lower than those of R134a. For example, Yang et al. [
15,
16] provided an experimental analysis of flow boiling and condensation heat transfer of R1234yf and R134a in a small circular tube. The test results showed that the boiling and condensation heat transfer coefficients of R1234yf are lower than those of R134a. Nagata et al. [
17] comparatively assessed the free convective condensation and pool boiling heat transfer coefficients (HTCs) of 1234ze(E) on a smooth horizontal tube made of copper with an outer diameter of 19.12 mm. Results showed that the condensation HTC and pool boiling HTC of R1234ze(E) are slightly lower than that of R134a. Ubara et al. [
18] experimentally evaluated the falling film evaporation and pool boiling heat transfer coefficients of R1233zd(E) on a single horizontal tube. The experimental results for R1233zd(E) were compared with those for R134a. The results showed that the pool boiling and falling film evaporation heat transfer coefficients were lower for R1233zd(E) than for R134a. Therefore, in the water chiller, it is necessary to consider increasing the heat transfer area of the evaporator and condenser when R1234yf, R1234ze(E) and R1233zd(E) are used to replace R134 as the refrigerant.
In spite of a number of studies mentioned above, research on the drop-in refrigerant replacement of R134a centrifugal chiller is rare at present. HFOs are considered new generation refrigerants with the most potential to replace R134a, with broad application prospects. At present, there are still R134a centrifugal chillers in use in the market. With the gradual prohibition of R134a, it is of great significance to study the refrigerant direct replacement technology for the existing R134a centrifugal chillers. Moreover, due to the difference in working principles from the positive displacement (screw, scroll, reciprocating, etc.) refrigeration compressor, the centrifugal refrigeration compressor is more sensitive to the physical properties of the refrigerant.
Among HFO refrigerants, R1234yf, R1234ze(E), and R1233zd(E) are the suitable refrigerants that are researched and applied in centrifugal water chillers. However, R1233zd(E) cannot be used as a drop-in replacement refrigerant for R134a through the analysis in the next section. Therefore, this paper took the centrifugal refrigeration compressor as the research object and used the CFD numerical method to simulate and analyze the compressor using R134a, R1234yf, and R1234ze(E), respectively. Parameters such as the pressure ratio, power, and isentropic efficiency of the centrifugal compressor were calculated. The R134a centrifugal chiller was selected, the refrigerant was directly replaced by R1234yf and R1234ze(E), and the refrigeration performance (including cooling capacity and COP) was compared and analyzed.
2. Comparative Analysis of Refrigerant Physical Properties
In consideration of the thermophysical properties of refrigerants, R1234yf, R1234ze(E), and R1233zd(E) can be considered the drop-in replacement refrigerants of R134a [
19,
20].
Table 1 compare the main physical property parameters of R134a with the refrigerants R1234yf, R1234ze(E), and R1233zd(E) [
21,
22,
23]. It can be seen that R1234yf, R1234ze(E), and R1233zd(E) have extremely low GWP compared with R134a.
The saturation lines of four refrigerants on the pressure-specific enthalpy diagram are shown as thin lines in
Figure 1a. It can be seen that the saturation lines of R1234ze(E) and R1234yf are similar to that of R134a, and the saturation line of R1234ze(E) has the smallest deviation from that of R134a. The saturation line of R1233zd(E) is quite different from that of R134a. Under the same pressure, it has a relatively higher specific enthalpy value than R134a.
The working conditions are set as evaporation temperature 5.5 °C, condensation temperature 37 °C, superheating temperature 3 °C, and subcooling temperature 4 °C. The theoretical cycles of the four refrigerants are represented on the pressure-enthalpy diagram (shown as the thick lines in
Figure 1a). Pressure is the operating pressure of the system and will affect the working process of the compressor. Specific enthalpy directly affects system performance. It can be seen that the pressure of R1234yf is the closest to that of R134a, but the specific enthalpy value of R1234yf in the gas phase region is smaller than that of R134a; the specific enthalpy value of R1234ze(E) is the closest to that of R134a, but its pressure is lower than that of R134a; the difference between R1233zd(E) and R134a is large, the pressure of R1233zd(E) is lower than that of R134a.
Figure 1b show the saturation pressure of four refrigerants at different temperatures. The curve of R1234yf and R134a is the closest, followed by R1234ze(E). At the same temperature, the saturation pressure of R1233zd(E) is lower than that of R134a. Additionally, with the increase in temperature, the difference between them increases gradually.
Due to the low pressure of R1233zd(E), the volume flow rate under the same cooling capacity is quite different from that of other refrigerants, leading to a significant reduction in the rotational speed of the compressor impeller and a large difference in impeller design size [
24]. Because the physical properties of R1233zd(E) and R134a are quite different, it is difficult to directly replace R1233zd(E) in the original centrifugal chillers using R134a. Therefore, this paper does not consider the drop-in replacement of R1233zd(E) in the R134a centrifugal chiller. The working fluids of R1234yf and R1234ze(E) have similar physical properties to R134a, and they can potentially be used as drop-in refrigerants.
For centrifugal chillers, if we want to replace the refrigerant without changing the compressor, the cooling capacity per unit volume of the replaced refrigerant should be similar to that of the original refrigerant. Under the mentioned working conditions above, the cooling capacity per unit volume is calculated using the following Equation (1):
where
q0 (kJ·kg
−1) is the unit cooling capacity;
ρ1 (kg·m
−3) is the density of the compressor inlet.
The cooling capacity per unit volume of the four refrigerant units is shown in
Table 2. It can be seen that the cooling capacity per unit volume of R1234yf and R1234ze(E) is not much different from that of the R134a unit, the cooling capacity per unit volume of R1234yf is 0.95 times that of R134a, and the cooling capacity per unit volume of R1234ze(E) is 0.75 times that of R134a. At the same time, the cooling capacity per unit volume of R1233zd(E) is quite different from that of R134a, which is 0.22 times that of R134a. Therefore, it may be feasible to replace R134a with R1234yf or R1234ze(E) directly.
6. Conclusions
In this paper, the CFD numerical method is used to simulate the internal flow of the centrifugal refrigeration compressor, which uses R134a, R1234yf, and R1234ze(E) as refrigerants, respectively. For the R134a centrifugal chiller, the refrigerant is drop-in replaced by R1234yf and R1234ze(E), and the refrigeration performance is compared and analyzed by thermodynamic analysis method under the set working conditions. The main conclusions are as follows: the mass flow rate range of the R1234ze(E) compressor is moved to the left and is smaller than that of the R134a compressor; that of the R1234yf compressor is moved to the right and is larger than that of the R134a compressor. The pressure ratio curve of R1234ze(E) and R1234yf is higher than that of R134a.
At the same mass flow rate, the power of the R1234ze(E) compressor is 3.5% lower than that of the R134a compressor. Additionally, the power of the R1234yf compressor is 13.7% larger than that of the R134a compressor.
At the same rotational speed, in the R134a centrifugal chiller, although using R1234yf to directly replace R134a can obtain higher cooling capacity, the COP is reduced by about 12.5%; using R1234ze(E) to replace R134a can reduce the cooling capacity under certain conditions, the COP is reduced by about 7.0%.
When the evaporation temperature, condensation temperature, and cooling capacity of the R134a unit and the R1234ze(E) unit are the same, the COP of the R1234ze(E) unit is reduced by about 5.14% on average than that of the R134a unit. Additionally, the COP of the R1234yf unit is reduced by about 8.93% on average compared to that of the R134a unit. Therefore, in the R134a centrifugal chiller, compared with R1234yf, directly replacing R134a with R1234ze(E) can obtain better refrigeration performance.
The model can run stably in the experiment of the R134a centrifugal water chiller. The experimental results are relatively close to the calculated results in this paper, which verifies that the results in this paper are reliable.
In this paper, only the drop-in replacement of centrifugal compressors in the R134a centrifugal chiller is studied, and the replacement of heat exchangers and other parts in the system is not considered. Hence, research on the whole system of the drop-in replacement of R134a should be carried out in the future. In addition, relevant experimental research should be carried out in the future.