1. Introduction
With the accelerated substitution of renewable energy for coal and the rapidly developing electrification of terminal energy systems, the global heat pump market is rapidly expanding [
1]. A heat pump is an efficient and energy-saving device that fully utilizes low-grade thermal energy and has enormous potential in the field of energy conversion. It was rated as one of the top ten breakthrough technologies by the Massachusetts Institute of Technology Review in 2024. Compared to ordinary heat pumps, high-temperature heat pumps that use waste heat as their main driving heat source have a wider range of applications due to their higher outlet water temperature. They can not only be used for household heating but also in industrial scenarios such as drying, distillation, and semiconductor manufacturing, playing a positive role in industrial carbon reduction [
2,
3].
At present, high-temperature heat pumps are mainly divided into two categories from the perspective of refrigerants and circulation methods. The first category is transcritical cycle heat pumps with R744 (carbon dioxide) as the main working fluid, and the other is subcritical cycle heat pumps with HFC refrigerants (hydrofluorocarbon, R134a, R152a, R245fa, etc.) as the main working fluids [
4]. In transcritical cycle heat pumps, R744 achieves good thermal performance and is environmentally friendly, and the large temperature drop on the exothermic side makes it easier to heat the target. However, the high pressure at the exothermic end, leads to high costs and unstable operation [
5]. In subcritical cycle heat pumps, the main problem is that when the heating temperature is high, the system performance is lower, and the GWP of the working fluid is generally higher. In response to these issues, many scholars have conducted research on these two types of heat pump at different levels.
Wang et al. [
6] evaluated the application of R744/R41 in transcritical cycle heat pump systems. The results indicated that the mixture is a good alternative to pure R744 and can effectively reduce the optimal high pressure of the system and improve the system COP. Compared to pure R744, the optimal pressure of R744/R41 (50/50) can be reduced by 28.62%, and COP can be increased by 20.52%. Sun et al. [
7] studied the heating performance of R744/R32 in water–water heat pumps using experimental methods and explored the influence of different factors on system performance. The results showed that when the mass fraction of R744 was 0.6, the heating performance coefficient of the system increased by 23.3% and the discharge pressure of the system significantly decreased. Yu et al. [
8] conducted experimental research on the application of R744/R41 in automotive heat pump systems. The results indicated that the charge amount of the working fluid was an important factor affecting system performance, and the use of the mixture not only effectively reduced discharge pressure but also increased the heating COP of the system by up to 14.5%. Zhang et al. [
9] studied a transcritical heat pump using an R744/R290 mixture as refrigerant through experimental and simulation methods and analyzed the influence of different factors on the optimal discharge pressure of the system. The results indicated that when the mass fraction of R744 was greater than 0.78, the system had the optimal discharge pressure and obtained the maximum COP. In addition, the mass fraction of the mixture, the outlet temperature of the gas cooler, and the evaporation temperature have a significant impact on the discharge pressure. Yu et al. [
10] experimentally studied the effect of R744 and R290 on the performance of a transcritical heat pump under different ratios. The results showed that when the mass fraction of R744 was 0.6, the COP of the system reached its highest level—29.4% higher than that of a pure R744 system—and the optimal pressure was reduced by 40%. Dai et al. [
11] proposed a heat pump dryer system using a R744/low-GWP working fluid mixture and compared it with traditional boiler drying systems. The results showed that under the influence of the R744/low-GWP working fluid mixture, the performance coefficient of the heat pump dryer was significantly improved, and the discharge pressure was reduced. When the ratio of R744/R32 was 0.1/0.9 mass ratio, the COP of system reached the highest level of 4.29, which is 7.42% higher than that under pure R744. Tian et al. [
12] designed a transcritical heat pump with an expander and studied the effects of different factors on system performance using experimental methods. The results showed that under the same working conditions, compared with the R744 transcritical system without an expander, the COP of the R744 transcritical system with an expander increased by about 6–10%. Sun et al. [
13] proposed a novel R744 transcritical cycle system with a mechanical superheat-assisted cycle and compared it with two different R744 transcritical systems. The results showed that compared with the traditional R744 system and the R744 system (with expander), the COP of the new system was increased by 15.72% and 12.19%, respectively, and the optimal pressure was reduced by 2.3% and 1.3%, respectively. Ye et al. [
14] established a theoretical model to study the effects of gas coolers and internal heat exchangers on the optimal discharge pressure and performance of an R744 transcritical system. The results showed that under the influence of the subcooling of the internal heat exchanger, the COP of the system was increased by 26.3%, and the optimal discharge pressure was also reduced. Qin et al. [
15] evaluated an R744 transcritical system from two perspectives, namely the actual operational thermal efficiency of the internal heat exchanger and the actual growth rate of COP. The results showed that under the given operating conditions, the addition of an internal heat exchanger significantly reduced the exergy efficiency of the system, and the influence of the internal heat exchanger on the exergy efficiency decreased with the increase in discharge pressure and ambient temperature. Zendehboudi et al. [
16] proposed a theoretical model to analyze the impact of different factors on the performance of R744 heat pump systems. The results showed that increasing the outlet water temperature from 60 °C to 80 °C resulted in a nearly 14% decrease in optimal COP and a 12.2% increase in optimal discharge pressure. When the evaporation temperature was increased from −10 °C to 10 °C, the optimal COP increased by about 63%, while the optimal discharge pressure remained almost unchanged. Dai et al. [
17] proposed a transcritical R744 heat pump system with a mechanical subcooling-assisted cycle and established a mathematical model to study its annual energy and economic performance. The results showed that under the given operating conditions, the COP of the system can be increased by 24.4%, and the discharge pressure can be reduced by 2.093 MPa. Compared with traditional R744 heat pump systems, the proposed new system has lower power costs. Zendehboudi et al. [
18] proposed a CO
2 heat pump with an integrated tri-partite gas cooler and conducted a comprehensive performance analysis on it. The results showed that when the ambient temperature and inlet water temperature are both 10 °C, combining SH with DHW can reduce the discharge pressure of the system by 7.9% and increase the COP by 7.5%, and compared with the DHW system, for every 5 °C increase in ambient temperature, the total exergy destruction cost rate of the DHW + SH system decreases by an average of 7.7%.
In the field of subcritical cycle heat pumps, Sun et al. [
19] proposed a new type of mixed refrigerant to replace R134a for high-temperature heat pumps and explored the optimal ratio of mixed refrigerant and the influence of different factors on system performance through experimental and simulation methods. The results showed that the GWP of the new refrigerant is lower, and the maximum COP of the system can be increased by 20.52%, with a charge amount of only 80.95% of R134a. Carlos et al. [
20] conducted a feasibility study on replacing HFC-245fa with HCFO (hydrochlorofluoroolefins)-1224yd (z), HCFO-1233zd (E), and HFO (hydrofluoroolefins)-1336mzz (z) in high-temperature heat pump systems. The results showed that compared with R245fa, the system COP of HCFO-1233zd (E), HFO-1336mzz (z), and HCFO-1224yd (z) increased by approximately 27%, 21%, and 17%, respectively. Meanwhile, when using HFO-1336mzz (z), the compressor and installation size of the system are larger, while when using HCFO-1233zd (E), the emission reduction of the system is higher. Adrian et al. [
21] conducted experimental research, for the first time, on the application of HFO-1234ze (E) and its non-combustible mixture, R515B, instead of HFC-134a in medium- and high-temperature heat pumps. The results indicated that the system COP of HFO-1234ze (E) and R515B is equivalent or slightly higher (up to 5%), and considering other factors, R515B is a suitable alternative to HFO-1234ze (E). Carlos et al. [
22] evaluated the potential of integrating high-temperature heat pumps into district heating networks (DHNs). The results indicated that when DHN acts as a heat exchanger, the COP of the integrated system is 3.2 to 5.4, while when DHN is a heat source, the COP of the integrated system is 2.8 to 5.7. Meanwhile, HCFO-1233zd (E) and HCFO-1224yd (Z) are considered the most promising low-GWP refrigerants to replace HFC-245fa. Carlos et al. [
23] determined the optimal choice for high-temperature heat pumps in different scenarios by comparing eight advanced cycle configurations and nine low-GWP refrigerants from the perspectives of energy, economy, and environment. The results indicated that a two-stage cascade is more suitable for high-temperature rise scenarios, single-stage cycles with economizers and parallel compression are more suitable for low-temperature rise scenarios, and the system COP and heat balance of HCFO-1233zd (E) and HCFO-1224yd (Z) are better. Meanwhile, compared to natural gas boilers, advanced high-temperature heat pump configurations can effectively reduce carbon emissions. Yan et al. [
24] proposed a novel performance prediction model that only requires knowledge of the critical parameters of the working fluid to quickly predict the performance coefficient of the working fluid used in the system. The results showed that the results of the model are consistent with those of the Peng–Robinson equation of state (PR-EOS) and REFPROP 10.0. Kondou et al. [
25] conducted an exploratory evaluation of heat pump heat recovery systems using environmentally friendly refrigerants and compared the system performance of refrigerants R717, R365mfc, R1234ze (E), and R1234ze. The results indicated that the cascade cycle using R1234ze (Z) and R365mfc has a relatively high COP and provides practical benefits. Wu et al. [
26] studied the system performance of six refrigerants used in high-temperature heat pumps through experimental and simulation methods, including R718, HCs (hydrocarbon, R600, R601), HFOs (R1234ze (Z), R1336mzz (Z)), and HFC (R245fa) as refrigerants. The results indicated that R718 achieves the best system performance and Carnot efficiency (COP/COPCarnot). Meanwhile, compared to R1336mz (Z), R600, and R245fa, R718 has its unique advantages in high-temperature heat pump applications. Byrne et al. [
27] introduced a heat pump that can achieve simultaneous heating and cooling in hotels, luxury residences, and small office buildings and found that hydrofluorocarbons achieve good performance. Longo et al. [
28] studied the heat transfer and pressure drop characteristics of refrigerant HFO-1234ze (Z) during condensation through experimental methods and compared it with HFC-236fa, HFC-134a, HC-600a, and HFO-12334ze (E). The results indicated that at the same mass flow rate, the heat transfer coefficients of HFO-1234ze (Z) are higher than those of all refrigerants currently used in heat pumps, and its frictional pressure drop is similar to that of HC-600a. Mikielewicz et al. [
29] studied the system performance of different low-GWP refrigerants for high-temperature heat pumps. The results indicated that although pentane and R365mfc have the best system COP, the flammability of pentane and the high price of R365mfc are important factors hindering their future implementation. Sulaiman et al. [
30] presented a series of theoretical simulation results using low-GWP refrigerants for high-temperature heat pumps and developed a steady-state thermodynamic model for a single-stage high-temperature heat pump with an internal heat exchanger (IHX). The results indicated that the proposed model is highly reliable and that HCFO-1233zd (E) and HFO-1336mzz (Z) are the most likely substitutes for HFC-245fa and HFC-365mfc, respectively.
In summary, although a considerable amount of research has been conducted to improve the overall performance of high-temperature heat pumps from the perspectives of refrigerant and system structure, all of them focus on individual types of high-temperature heat pumps, and there are few reports comparing the two types of high-temperature heat pumps. Therefore, to fill the current research gap, this paper proposes the research concept of comparing and evaluating the performance and economy of different types of high-temperature heat pumps. This not only promotes energy conservation, carbon reduction, and the development of heat pump technology but also provides guidance for users to choose which heat pump is most suitable.
This paper first builds two test benches and a simulation model for two types of high-temperature heat pump. After verifying the accuracy of the model, the optimal new refrigerants suitable for transcritical heat pumps and subcritical heat pumps are determined. Finally, the performance and economy of the two types of high-temperature heat pump are compared through improvements at the refrigerant level, and the impact of key factors on system performance is analyzed through experimental methods.