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Review

Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field

by
Hongzeng Ji
*,
Jinchen Pei
,
Jingyang Cai
,
Chen Ding
,
Fen Guo
and
Yichun Wang
School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Energies 2023, 16(10), 4011; https://doi.org/10.3390/en16104011
Submission received: 4 April 2023 / Revised: 23 April 2023 / Accepted: 28 April 2023 / Published: 10 May 2023
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Abstract

:
Refrigerant substitution is an urgent need in the context of reducing carbon emissions and slowing global warming. CO2 is now being proposed as a promising solution based on its excellent properties and system performance, especially in low-temperature environments. This paper presents an overview of recent advances in system configuration and operation characteristics to improve the performance of transcritical CO2 heat pump and refrigeration systems. The paper first introduces the basic research background, system cycle, and thermodynamic characteristics. Secondly, CO2 cycle improvements with single modifications and modification combinations are reviewed. Then, some important operation characteristics and control methods are discussed. Additionally, the paper provides a detailed description of the development of transcritical CO2 heat pump and refrigeration systems in the vehicle field. At the end of this review, conclusions and opportunities for future work in this field are presented.

1. Introduction

Though the substitution of hydrofluorocarbons (HFCs) for hydrochlorofluorocarbons (HCFCs) has been beneficial for ozone layer protection, HFCs contribute considerably to global warming due to their high global warming potential (GWP) [1]. The Kigali Amendment to the Montreal Protocol and the Paris Agreement further restrict the use of HFCs (such as HFC-32 or HFC-134a) since 2019 to achieve carbon neutrality and slow the rate of global warming [2]. Thus, refrigerants with substantially lower GWP, such as HFC-1234yf, HFC-1234ze, or CO2, have recently received increased attention. Among these low-GWP refrigerants, CO2 shows such potential that some people regard it as an ultimate solution. CO2 is safe and clean because of its properties of being non-toxic and non-flammable, with 0 ozone depletion potential (ODP) and 1 GWP [3]. CO2 also exhibits competitive cooling performance and outstanding heating performance compared with other refrigerants [4]. As a common byproduct of many industrial processes, CO2 is cheap and easy to obtain. These unique properties make CO2 a promising refrigerant in refrigeration and heat pump systems.
As a natural working fluid, carbon dioxide has been used as a refrigerant since the 19th century. However, it was replaced with synthetic refrigerants such as chlorofluorocarbons (CFCs) and HCFCs due to its poor subcritical cycle performance. The CO2 refrigerant did not draw any further attention from researchers until a new transcritical CO2 cycle was proposed by Pettersen and Lorentzen in the 1990s [5]. They validated it in a prototype and pointed out that the CO2 transcritical system has several advantages in terms of cost, performance, and dimensions. Since then, research into the use of the CO2 transcritical cycle in various applications has continued.
There are several review papers that focus on CO2 refrigerant or heat pump systems, including cycle design and improvements, heat transfer characteristics, components, or applications. Ma et al. submitted a comprehensive review including the novel CO2 transcritical cycle, component improvements, and CO2 heat transfer characteristics in the supercritical or two-phase region [6]. They suggested that CO2 should be a more common refrigerant for the further restriction of harmful emissions. Their article mainly focused on cycle modifications and components. Ehsan et al. reviewed the supercritical CO2 heat transfer and pressure drop characteristics in various applications [7]. The tube diameter, heat flux, and operating mass velocity were considered to be important factors that influence heat transfer effectiveness. Yu et al. presented a review that focused on the cycle modifications made in recent years and demonstrated the merits or drawbacks of different improvements [8], but they did not consider the combination of different modifications. Dilshad et al. also introduced the changing process of refrigerants and different CO2 cycle modification measures in their review. However, only applications for commercial use worldwide were highlighted in their review [9]. Barta et al. summarized that CO2 vapor compression cycles are highly effective for stationary applications in nearly all climates. In transportation applications, the transcritical CO2 cycle is competitive but without general recognition [10]. Soylemez et al. provided an overview of the application of the CO2 refrigeration system on fishing vessels. Three major operating conditions—low seawater temperatures, high freezing capacity, and additional freezing capacity in high seawater temperatures—are considered suitable for CO2 refrigeration system application [11]. Gullo et al. concluded that the multi-ejector solution is the main driving force for the spread of CO2 supermarket refrigeration systems, but stores in areas of high ambient temperatures lack confidence in CO2 refrigeration systems [12]. Bruno et al. presented a comprehensive review that included cycle modifications, exergy analysis, modeling tools, and operation considerations [13]. Although that article was more comprehensive than other reviews, its focus was not on modification combinations or operation optimization. Some other reviews related to CO2 refrigeration and heat pump systems include refs. [14,15].
The abovementioned review papers mainly focused on cycle modifications and performance analysis but contain little content about CO2 cycle operation characteristics, control strategies, and configuration modification combinations. Thus, this paper aims to provide a comprehensive, state-of-the-art review of these topics for the CO2 cycle. This review will discuss refrigerant charges, operation process characteristics, and multi-modification configurations. In addition, we present considerable information about the development of CO2 heat pump systems in the vehicle field, considering the enhanced attention. Guidance for research on CO2 cycle operation optimization and configuration modification combinations are provided.
This paper starts with an introduction to the basic thermodynamic characteristics and description of the transcritical CO2 cycle in Section 2. This is followed by a comprehensive presentation in Section 3 on system configuration improvements, especially modification combinations. Then, the operation characteristics and control methods of the transcritical CO2 cycle including modification configurations are discussed in Section 4. In Section 5, the applications of CO2 refrigerant or heat pump systems in the vehicle field are discussed. Finally, we present our opinions on the outlook of CO2 refrigeration and heat pump fields.

2. Thermodynamic Characteristics and Cycle Introduction

A basic transcritical CO2 cycle employs the same devices as a conventional cycle, namely, a compressor, condenser, expansion valve, and evaporator. Thus, the thermodynamic diagrams are similar. The p–h diagrams of the transcritical CO2 cycle and conventional subcritical cycle are shown in Figure 1.
Processes 1–2 and 1′–2′ are isentropic compression processes, processes 3–4 and 3′–4′ are adiabatic expansion processes, and processes 4–1 and 4′–1′ are evaporation processes under constant pressure. The main difference between the two cycles is the heat rejection process. In the conventional subcritical cycle, the heat rejection process (process 2–3) occurs below the critical point (31.2 °C, 7.38 MPa) in the subcritical region. This process is usually referred to as the condensation process since it involves a phase transition in the two-phase region. In contrast, in the transcritical CO2 cycle, the heat rejection process (process 2′–3′), which is called the gas cooling process, takes place above the critical point in the supercritical region [16]. During this process, the distinction between liquid and gas phases vanishes, and the temperature of the refrigerant continues to decrease due to the absence of the two-phase region. This temperature glide feature of CO2 is advantageous as it fits the temperature profile of the coolant side, contributing to the transcritical CO2 cycle’s higher efficiency [17]. Another noteworthy advantage of the CO2 cycle is its high compression efficiency resulting from its lower compression ratio [18]. Additionally, the large-volume refrigeration capacity of CO2 makes for more compact heat exchanger [19].
In contrast to the subcritical region, the pressure and temperature of the refrigerant in the supercritical region are two independent variables [20]. Thus, the optimum high pressure and refrigerant temperature at the outlet of the gas cooler are the two most important cycle characteristics in the transcritical CO2 cycle for improving system performance.
With a drastic change in pressure along the isothermal curve as it approaches the critical point, an optimum coefficient of performance (COP) exists at a constant refrigerant outlet temperature of the gas cooler, a phenomenon reported in several papers. Sarkar and Agrawals optimized a parallel-compression transcritical CO2 cycle and developed a formula related to the temperature at the evaporator and gas cooler exit [21]. Chen and Gus reported that the compressor discharge pressure has a significant influence on the system performance of a CO2 refrigerant system, whereas the evaporating temperature has little effect on the optimum high pressure [22]. Zhang et al. validated the existence of an optimum high pressure in a CO2 heat pump system with two throttle valves [23]. Optimizing high pressure is an important and common parameter for operational optimization. Aprea and Maiorinos claimed that optimizing high pressure is an easy way to improve CO2 air conditioning system performance [24].
Furthermore, a literature review of the optimum high pressure reveals that the temperature of the gas cooler exit is an important influence factor on the optimum high pressure and system performance. Figure 2 shows variations in the optimum high pressure and COP with changing gas cooler exit temperature at a 15 °C evaporating temperature.
The system COP increases with the decrease in the gas cooler exit temperature. Conversely, the optimum high pressure increases as the gas cooler exit temperature increases. This phenomenon is particularly well-documented in high ambient temperature conditions. Both Laipradit et al. [25] and Sawalha [26] have previously reported on this phenomenon in their works. Liang et al. presented that the gas cooler exit temperature accounts for 96.38% of the variation in the optimum high pressure for the transcritical CO2 cycle using response surface methodology [27]. Consequently, the ability of the transcritical CO2 cycle to provide effective cooling is a persistent and widespread challenge.

3. Transcritical CO2 Cycle Improvements

The unique characteristics of the transcritical CO2 cycle necessitate modifications to CO2 heat pump or refrigeration systems in order to enhance system efficiency. These modifications can be broadly classified into three categories.
The first category involves decreasing the gas cooler outlet temperature. As explained in the thermodynamic analysis of the transcritical CO2 cycle in Section 2, the gas cooler outlet temperature has a significant impact on the system COP. Decreasing this temperature can increase system COP significantly, particularly in high ambient temperatures. To achieve this, common modifications include the use of an internal heat exchanger (IHX) [28,29], a cascade cycle [30], thermoelectric subcooling [31], and magnetic subcooling [32].
The second category involves improving the throttling process. In the transcritical cycle, the pressure difference on either side of the throttling process is considerable, resulting in higher exergy loss compared to other common refrigerant cycles. To reduce the exergy loss, measures such as the use of an ejector [33] or an expander [34] are considered effective.
The third category involves improving the compression process. When the evaporative temperature decreases, the density of CO2 vapor also decreases, resulting in a small mass flow rate in a transcritical CO2 cycle system operating in a low-temperature environment. Flash gas bypass [35] and parallel compression [36] are commonly used to address this issue. Alternatively, a two-stage compression process [37] can provide a more efficient compression process for lower pressure ratios and can serve as another improvement. These above improvements have been proved to be useful and are commonly used. Bellos and Tzivanidiss compared the performance of transcritical CO2 refrigeration systems with internal heat exchangers, mechanical subcooling, parallel compressors, and two-stage compression. Their simulation results showed that all of the modifications make system performance better than the base system and that the mechanical subcooling performs the best [38]. Liu et al. replaced the auxiliary compressor and high pressure control valve with an ejector. This replacement reduces the maximum power consumption by 22.8% in the transcritical refrigeration cycle [39]. A detailed description of these improvements can be found in the comprehensive reviews of Yu et al. [8] or Bruno et al. [13]. It should be noted that some new modifications based on these existing systems have been proposed and proved to be effective, such as the integrated mechanical subcooling system [40] and multi-ejector expansion system [41]. This section highlights the effectiveness of the combined use of these individual modification technologies.
Although every single improvement has been proved to be effective in promoting performance, the specific process influenced by each modification varies. Furthermore, the effects of combining different modifications are still unclear. Consequently, the system performance influenced by combinations of two or more modifications has been studied by many researchers. For example, as an easy and useful measurement, IHXs are taken as a familiar improvement in all sorts of transcritical CO2 cycles; thus, the simultaneous use of an IHX and other modifications is one of the most common combinations. The combination of an IHX and an ejector contributes more to the system COP than using either of them alone [42]. However, not all combinations of IHXs and other modifications result in improved system performance. Adding an IHX can actually decrease the COP of a basic cycle with an expander, as shown in several studies [43].
Single and combination modifications are categorized and visualized in Figure 3. Every circle represents a category of modification and their intersection signifies a combination of modifications.
Table 1 summarizes some of the combination modifications applied to the transcritical CO2 cycle and their corresponding major findings, particularly in energy efficiency. The modifications were implemented in a cycle consisting of a compressor, a gas cooler, an expansion valve, and an evaporator. The “+” means a combination of two modifications.
It should be noted that ejectors (EJEs) and IHXs are the two commonly used modifications to improve the transcritical CO2 cycle performance because of their simplicity and low cost. The vortex tube (VT) has not been extensively studied with other modifications due to the lack of research. Combinations of modifications in the same category are relatively uncommon compared to those consisting of modifications from different categories. These modifications are mainly employed in refrigeration scenarios where cooling performance under high-temperature environments is poor. Therefore, thorough evaluation is necessary before implementing these modification systems in other settings.

4. Operation Characteristics Analysis and Control Methods

4.1. Optimal-Pressure-Seeking Methods

Due to the great significance of optimum high pressure in system performance, the search for optimum high pressure has been a heated research area in the transcritical CO2 cycle. Inokuty utilized a graphical method to transform the search for the optimum pressure into a curve comparison in the p-h diagram of CO2 [65]. The assumption underlying the research is that the pressure and temperature at the end of refrigeration and the temperature at the beginning of throttling expansion are known values. Kauf considered the graphical method as time-consuming; thus, he proposed a correlation to determine the optimum pressure based on the ambient temperature or the gas cooler exit refrigerant temperature [66]. Liao et al. found that the value of optimum high pressure is influenced by several key factors, including compressor performance, evaporation temperature, and gas cooler outlet temperature [67]. They established a model involving these parameters and validated it with a standard deviation of less than 1%. Several similar correlations have also been developed separately by different researchers [68,69,70]. Cabello et al. compared the correlations proposed by Liao, Chen, Kauf, and Sarkar and the results showed that Sarkar’s correlation provides the most precise results [71]. Yin et al. selected four characteristic variables—the evaporative temperature, the gas cooler exit temperature, the ambient temperature, and the water temperature of the outlet—as important parameters using the group method of data handling (GMDH). The discharge pressure is obtained through particle swarm optimization-back propagation (PSO-BP) [72]. This method was later validated by Song et al. experimentally [73]. Model predictive control (MPC) was also applied in optimum high pressure search [74]. These offline methods, which rely on prior data, are precise enough in the corresponding systems and operation conditions, but only in particular conditions. The predefined parameters in such empirical correlations are influenced strongly by the system and operating conditions, and therefore, when applied to other systems or conditions, these models may fail to satisfy the requirement. Figure 4 shows a comparison between the optimal value and some correlation results. The maximum COP deviation is −5.4%. Cecchinato et al. proposed that the correlations obtained from a specific system must be assessed before they are used in another system or condition [75]. Shao et al. considered the influence of high pressure on COP under different working conditions and suggested that a constrained optimal high pressure equation can result in a minor COP decrease despite significant pressure reduction [76]. Yang et al. optimized the optimum pressure correlation through minimizing the difference between the actual COP and the maximum COP [77].
In recent years, many new online methods have been applied to the optimum high pressure search under a wide operation range. In contrast to offline methods, online methods generally do not rely on models or correlations. Online methods obtain the optimal high pressure through real-time optimization techniques, such as based on the system transient response or by comparing the results prior to and following optimization. A correlation-free control method based on the steepest descent method was validated by Zhang and Zhang [78]. According to the relationship between the COP and compressor energy consumption, Penarrocha et al. transformed the target of maximizing the COP to minimizing energy consumption with the ‘perturb and observe’ algorithm [79]. Cecchinato et al. obtained a system model using an artificial neural network (ANN) and determined the optimal pressure in real time via PSO [80]. Extremum seeking control (ESC) has been utilized for optimal pressure seeking in several applications [81,82,83,84]. The searching process of ESC is shown in Figure 5. The settling time in the start phase and sudden change phase are about 5000 s and 1000 s, respectively. Kim et al. suggested that ESC could be time-consuming because it depends on the system’s transient response [85]. These online methods present excellent performance but require a lengthy searching process. Thus, their practical use remains unrealistic.
To combine the technical merits of offline methods and online methods, hybrid methods have become the focus of attention. Okasha et al. employed a non-dominated sorting genetic algorithm-II (NSGA-II) technique to find the best high pressure and then optimized it with an online optimizer [86]. Ji et al. compared a hybrid method with traditional offline and online methods, and the new method presented high efficiency and fast speed under various operating conditions [87]. The hybrid method has potential to be a feasible solution for optimum high pressure search with fast speed and high precision. The variation of optimum-high-pressure-seeking methods is shown in Table 2.
Little attention has been given to the transition between subcritical and transcritical cycles in the literature. Jin et al. activated the optimal gas cooler pressure control only when the high pressure exceeded the minimum operating pressure in winter and fell below the maximum operating pressure in summer for efficiency and safety reasons [88]. Ge and Tassou proposed a floating discharge pressure control strategy in which the setting value is a function of ambient temperature [89]. Shao and Zhang found that the optimal high pressure appears in the subcritical region adjacent to the critical point [90]. Considering pressure control devices, the TXV (thermostatic expansion valve) is not suitable in the transcritical CO2 cycle for controlling varied objects. Presently, the EXV (electric expansion valve) has emerged as the main pressure-adjustable component due to its precision and large regulating range [91]. The adjustable needle [92] and vortex control [93] are receiving great interest for their high work recovery efficiency.

4.2. Refrigerant Charge Characteristics

Irrespective of system types and operating conditions, the charge amount of the refrigerant is a crucial performance-determining factor [94,95]. In the case of CO2, system performance is more sensitive to refrigerant charge amount [96]. Figure 6 shows the COP ratio variation caused by charge deviation in R22, R410A, R407C, and CO2 systems.
Aprea et al. presented that the COP and cooling capacity reach a maximum value at the optimum charge. The exergy destruction in the compressor contributes the most to total exergy loss [97]. Wang et al. comprehensively analyzed the influence of refrigerant charge on energy, exergy, economy, and the environment [98]. Hazarika et al. quantified the influence of refrigerant charge amount on system performance and showed the effects of gas cooler face velocity [99]. Zhang et al. and Wang et al. investigated the refrigerant charge characteristic in transcritical CO2 heat pumps with EXVs and capillary tubes, respectively [100,101]. Wang et al. studied the effect of refrigerant charge in an electric bus CO2 air conditioning system and supposed that overcharge has a more substantial impact than insufficient charge on system exergy efficiency [102]. Yin et al. discussed the refrigerant charge amount under different working conditions and proposed that the optimum charge is not a definite value, but a range of charge amounts [103]. Kim mentioned that varying the refrigerant charge or inside volume of the high side are proper methods to control the high side pressure [20]. A system with a low-pressure buffer or medium-pressure buffer was introduced to control high side charge. Changing the high side volume is also an effective way to regulate the high side pressure. He et al. analyzed the refrigerant charge distribution in a transcritical CO2 heat pump water heater [104]. The simulation results showed that the optimum COP reaches at an optimal high pressure while the high pressure is influenced by the refrigerant charge amount. A system charge management method based on heat exchanger resizing or additional gas cooler volume has been proposed to improve system performance without controlling the high pressure. Refrigerant migration and distribution characteristics are not only important to this method, but also the main reason for cycling loss [105]. Wang et al. studied the CO2 transient migration behavior under the period of start-up and shutdown experimentally with the help of a visualized accumulator [106]. The results show that the system reveals rapid refrigerant migration and transient response because CO2 exists in a superheated vapor state after the shutdown of the system for CO2 mobile air conditioning (MAC). Thus, better transient performance and a higher energy-saving effect are achieved. A simulation study of refrigerant distribution and migration characteristics was executed by Wang et al. [107]. The ambient temperature and air volume flow rate on both the gas cooler side and evaporator side affect the CO2 distribution, evidently, and should be treated properly during the system design and operation. Refrigerant leakage, responsible for over 20% of the lost refrigerant charge escaping into the atmosphere annually, results in significant system performance degradation [108]. To the best of the authors’ knowledge, the refrigerant charge fault detection of the CO2 system is rarely reported in papers. Uren et al. applied a method based on the energy graph on CO2 transcritical heat pump fault diagnosis [109]. Yin recommended that the discharge temperature, suction temperature, and the temperature at the inlet of EXV are the best parameters for appropriate charge judgment [103].

4.3. Operation Control in Modified Cycles

Optimal pressure and refrigerant charge amount are two common characteristics in the operation process of all transcritical CO2 cycles. In the modified transcritical CO2 cycle, some extra parameters are needed to control for better performance.
The system using the ejector has limitations on high pressure control and expansion work recovery due to the constant ejector geometry. Liu et al. investigated the performance enhancement with the substitution of a controllable ejector in a transcritical CO2 cycle [110,111]. The ejector has two adjustable parameters, suction nozzle throat area and motive nozzle throat area. They found that the ejection ratio and COP are dependent on the motive nozzle throat diameter. The motive nozzle efficiency and the suction nozzle efficiency are affected by their respective diameter to mixing section diameter ratios. To improve the adaptability of the ejector, a multi-ejector system with several parallel working ejectors was proposed. Every ejector can be activated independently, offering several possible combinations under different operation conditions [112].
For parallel compression or multi-stage compression systems, the interstage pressure provides an additional degree of freedom to optimize system performance. Cecchinato et al. demonstrated that the inter-stage cooling during compression is more beneficial than staged throttling in transcritical CO2 cycles. Moreover, they noted that the optimal intermediate pressure is sensitive to cooling agent temperature [113]. Singh et al. showed that the optimum interstage pressure is not the geometric mean interstage pressure in a multi-stage refrigeration system, no matter with flash gas intercooler (FGI), flash gas bypass (FGB), or intercooler (IC). Additionally, they found that the percentage deviation is influenced by gas cooler outlet temperature [47].
Nebot et al. described an integrated mechanical subcooling CO2 refrigeration cycle that replaces the dedicated mechanical cycle with an internal cycle. They pointed out that not only the discharge pressure but also the subcooling degree are needed for optimization. The subcooling degree is influenced by both the evaporation temperature and the environment temperature. Nonetheless, high pressure has a greater impact on the COP than the subcooling degree [114]. A similar system was presented by Song et al. and the system performance was analyzed via simulation. The medium temperature, defined as the EXV inlet temperature, affects the main and sub-cycles’ performance in opposite ways [115]. Figure 7 shows the COP variation of different cycles under different medium temperatures.
They further presented that the COP of the main cycle does not follow a particular pattern with the intermediate temperature under different operating conditions. Conversely, the sub-cycle COP increases with the increase of intermediate temperature. Less than 5% variation of the whole system COP was observed under different operating conditions [116].

5. Application of CO2 Cycle in the Vehicle Field

The use of transcritical CO2 cycles in automotive AC systems dates back to thirty years ago [5]. From then on, several simulations or experiments were executed by different researchers [117,118,119]. Mathur compared the thermodynamic performance of automotive air conditioning (AC) systems using R134a and CO2 as working fluids [120]. The study revealed that the COP of the CO2 system is approximately 12% lower than that of the R134a system. Liu et al. found that the lubricant type and refrigerant mass charge affect the CO2 system performance significantly [121]. Niu et al. validated a wet-compression absorption CO2 refrigeration system with a sustainable cooling capacity for vehicle AC. The high pressure reduces to less than 35 bar [122]. These studies demonstrate that CO2 is an acceptable but not very excellent refrigerant for vehicle air conditioning. Hence R134a plays the primary role in vehicle refrigerants.
However, with the development of electric vehicles, the heat shortage challenge stands out in extremely cold environments. The R134a heat pump or positive temperature coefficient (PTC) consumes enormous amounts of energy; thus, the driving range of electric vehicles decreases rapidly [123,124]. CO2 draws researchers’ attention again for its excellent heating performance.
Tamura et al. tested the heating performance of a CO2 heat pump system in a highly efficient automobile with an engine. In comparison with the R134a/PTC system, the relative COP of the CO2 system was 1.31 [125]. A comparative study between R134a and CO2 heat pump systems with equal size components was presented by Dong et al. [126]. The CO2 system was modified with an IHX; thus, the cooling performance improved. Their results showed that the CO2 system provides a competitive cooling capacity. Under 40 °C operating conditions, the CO2 system reached 3.2 kW with a COP of 1.6, while the R134a system reached 3.2 kW with a COP of 1.41, as depicted in Figure 8. Additionally, the CO2 system achieved a COP of 2.16 with 7502 W heating capacity under −20 °C.
Wang et al. considered the impact of battery heating/cooling demand on the performance of the CO2 heat pump system further [127]. Ko et al. analyzed the start-up and the pull-down processes of a CO2 AC system for an electric vehicle [128]. The pull-down time and energy consumption increase with the decrease of target temperature or the increase of initial temperature. Increasing the initial RH leads to more energy consumption and almost unchanged pull-down time. Apart from the research about system performance [106,129], several studies focused on the design and optimization of components. For example, flow unsteadiness and tangential leakage were proposed for compressor performance improvement [130,131]. Additionally, the heat transfer performance of a micro-channel gas cooler was analyzed by simulation and experiments [132,133]. A configuration called series gas cooler was introduced to improve the heat exchange capacity on the high-pressure side in heating mode [134]. Mahvi et al. delayed the frosting process with a superhydrophobic heat exchanger, but the results showed that the defrosting process is unavoidable [135]. Due to the performance degradation caused by frosting, some necessary defrosting methods and control strategies are applied to vehicle CO2 heat pump systems. Wang et al. proposed a frost-free control strategy and demonstrated the application scope. However, the study does not consider the influence of defrosting on performance [136]. Steiner and Rieberers simulated the reverse cycle and hot gas defrosting process and found that there exists an optimal valve-opening and ideal point of time in the two methods to conduct the defrost operation for electric vehicles, respectively [137,138].
To improve system performance or reduce energy consumption, several modified transcritical CO2 cycles were applied for electric vehicles. The IHX has become a routine alternative for its significant improvement in the cooling mode under high-temperature environments. Fang used two needle valves to adjust IHX effectiveness. The experiment results showed that the IHX enhances the cooling COP all the time, but the cooling capacity reveals in a parabola form as the IHX effectiveness increases. It should be noted that the discharge temperature grows rapidly with the increment of IHX effectiveness [139]. The ejector was also considered an appropriate and easy-to-use option for performance improvement [140]. However, the non-adjustable feature of the ejector is not suitable for vehicles, which operate in a wide range of temperature, no matter heating or cooling. Thus Zou et al. designed a CO2 heat pump system with dual ejectors. The two ejectors were optimized based on the weight of operating conditions and used in cooling mode and heating mode separately [141]. The system demonstrated better adaptability to different environmental conditions and presented large improvements under both heating and cooling modes. In another study, intermediate cooling enhanced the heating capacity and COP significantly in the temperature range of −20 °C to 0 °C and contributed a significant increase in cooling capacity and maximum COP at 45 °C, compared to the basic CO2 system. A lower discharge temperature was reached when intermediate cooling was applied and this characteristic extended the application scope of the CO2 heat pump system effectively [142,143]. The application of a vapor-injection compressor could make the heating capacity and COP of the transcritical CO2 cycle greater than the basic cycle, but the cooling performance was not reported [144]. Some mixture refrigerants, such as CO2–propane and CO2–R41, were also utilized to improve the performance of automotive heat pump systems, and the experimental results showed positive changes in performance and parameters [145,146]. Yu et al. combined the CO2–propane mixture refrigerant with an auto-cascade heat pump system, and the modification improved the system performance in heating mode compared with the basic pure CO2 single-stage system [147]. Due to the increased cooling demand taken by the fast charge and discharge of the battery, more modifications need to be employed in the vehicle transcritical CO2 cycle to improve the cooling performance. To the best of our knowledge, no one has studied the direct refrigerant cooling of CO2 for batteries in electric vehicles.
Besides pure electric vehicles, CO2 heat pump systems were also applied in fuel cell electric vehicles, buses, freight transport vehicles, and railways. The cooling performance and heating performance in fuel cell vehicles were evaluated [148,149,150]. Kim et al. considered the influence of heat exchanger arrangements on system performance, as shown in Figure 9. The new radiator-front arrangement increased the heating performance with a decrement in the cooling performance because of preheated air [151].
Wang et al. conducted a refrigerant charge experimental study in an electric bus and pointed out that the lack of refrigerant charge enhances the irreversible losses in both exchangers significantly [102]. Song et al. compared the CO2 system with the R407C system from the standpoint of energy, economy, and environment and the results revealed that the CO2 system has an environmental advantage in most areas of the world. The CO2 system and R407C system have their respective advantages in their respective suitable regions considering energy consumption and economy [152]. Shi et al. designed a combined CO2 system for refrigerated trucks that can achieve refrigeration and power generation, leading to a 7.4% and 4.9% fuel economy increase under refrigeration and freezing conditions, respectively [153]. Fabris et al. found that ejector efficiency increases with the increase of ambient temperature in a CO2 system with an ejector [154].

6. Conclusions and Suggestions for Future Work

After a long time of stillness, CO2 comes into focus again for its clean, low-carbon, and economical characteristics. This review summarizes recent advances in modification combination, operating characteristics, control methods, and development in the vehicle field relevant to transcritical CO2 heat pump and refrigeration cycles. CO2 heat pump or refrigeration cycle performance can be improved through optimizing the system configuration and operating characteristics. This study is expected to facilitate the development and further investigation of this field. With a deeper understanding of the transcritical cycle and better improvement of performance, CO2 would become a proper choice in many fields. The main conclusions and suggestions for the future are as follows:
  • Previous studies about combination modifications in CO2 systems have mainly focused on simulating for commercial refrigeration; more validation experiments and applications in other scenes are needed.
  • Different combinations yield improvements in several aspects of system performance; the selection of combinations depends on the comprehensive assessment of operating conditions, major improvement objectives, cost, climate, and control complexity.
  • Online methods of optimal pressure seeking require a long time with good performance, while offline methods acquire inaccurate results with a fast speed. A hybrid method combining the online and offline methods may be a feasible solution that provides both good performance and fast speed.
  • Although the effect of charge amount on transcritical CO2 system performance has been investigated extensively, more research on refrigerant migration and distribution characteristics is needed. Additionally, refrigerant leakage detection methods for transcritical CO2 systems are imperative due to the exceptionally high pressure.
  • The transcritical CO2 cycle can provide enough heat with a high COP in low-temperature environments. However, the cooling performance in high-temperature environments is doubtful, especially with the increasing demand for cooling in electric vehicles. More evaluation and employment of vehicle operating conditions are necessary to find beneficial modification combinations.

Author Contributions

Conceptualization: H.J. and F.G.; methodology: Y.W.; writing—original draft preparation: H.J.; writing—review and editing: C.D. and J.C.; visualization: J.P.; supervision: J.P.; project administration: F.G.; funding acquisition: Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data sharing not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

HFCsHydrofluorocarbonsFGIFlash gas intercooler
FGBFlash gas bypass
HCFCsHydrochlorofluorocarbonsICIntercooler
GWPGlobal warming potentialRHRelative humidity
ODPOzone depletion potentialEJEEjector
CFCsChlorofluorocarbonEXPExpander
COPCoefficient of performanceTSTwo stage compression
IHXInternal heat exchangerTS(M/F)ITwo stage compression with (multi/external) intercooler
GMDHGroup Method of Data HandlePCParallel compression
PSO-BPParticle swarm optimization-back propagationBSBooster system
ANNArtificial neural networkCCCascade cycle
ESCExtremum seeking controlTESThermoelectric subcooling
MPCModel predictive controlDMSDedicated mechanical subcooling
NSGA-IINon-dominated sorting genetic algorithms-IIECEvaporative cooling
TXVThermostatic expansion valvePCRParallel compression with recooler
EXVElectric expansion valveSAPCCSolar absorption partial cascade cycle
MACMobile air conditioningVTVortex tube

References

  1. The Kigali Amendment. The Amendment to the Montreal Protocol Agreed by the Twenty-Eighth Meeting of the Parties (Kigali, 10–15 October 2016) | Ozone Secretariat. 2016. Available online: https://ozone.unep.org/treaties/montreal-protocol/amendments/kigali-amendment-2016-amendment-montreal-protocol-agreed (accessed on 3 September 2021).
  2. Fang, X.; Ravishankara, A.R.; Velders, G.J.M.; Molina, M.J.; Su, S.; Zhang, J.; Hu, J.; Prinn, R.G. Changes in Emissions of Ozone-Depleting Substances from China Due to Implementation of the Montreal Protocol. Environ. Sci. Technol. 2018, 52, 11359–11366. [Google Scholar] [CrossRef] [PubMed]
  3. Brodribb, P.; McCann, M. Cold Hard Facts 3. Prepared by Expert Group & Thinkwell Australia Pty Ltd. Canberra for the Australian Government, Department of the Environment and Energy, (DoEE) Energy Innovation and Ozone Protection Branch, International Climate Change and Energy Innovation Division. 2018. Available online: http://www.environment.gov.au/protection/ozone/publications/cold-hard-facts-3 (accessed on 27 April 2023).
  4. Nekså, P.; Rekstad, H.; Zakeri, G.; Schiefloe, P.A. CO2-heat pump water heater: Characteristics, system design and experimental results. Int. J. Refrig. 1998, 21, 172–179. [Google Scholar]
  5. Pettersen, J.; Lorentzen, G. A New, Efficient and Environmentally Benign System for Automobile Air Conditioning; SAE Technical Paper 931129; SAE International: Warrendale, PA, USA, 1993. [Google Scholar] [CrossRef]
  6. Ma, Y.; Liu, Z.; Tian, H. A review of transcritical carbon dioxide heat pump and refrigeration cycles. Energy 2013, 55, 156–172. [Google Scholar] [CrossRef]
  7. Ehsan, M.M.; Guan, Z.; Klimenko, A.Y. A comprehensive review on heat transfer and pressure drop characteristics and correlations with supercritical CO2 under heating and cooling applications. Renew. Sustain. Energy Rev. 2018, 92, 658–675. [Google Scholar] [CrossRef]
  8. Yu, B.; Yang, J.; Wang, D.; Shi, J.; Chen, J. An updated review of recent advances on modified technologies in transcritical CO2 refrigeration cycle. Energy 2019, 189, 116147. [Google Scholar] [CrossRef]
  9. Dilshad, S.; Kalair, A.R.; Khan, N. Review of carbon dioxide (CO2) based heating and cooling technologies: Past, present, and future outlook. Int. J. Energy Res. 2019, 44, 1408–1463. [Google Scholar] [CrossRef]
  10. Barta, R.B.; Groll, E.A.; Ziviani, D. Review of stationary and transport CO2 refrigeration and air conditioning technologies. Appl. Therm. Eng. 2021, 185, 116422. [Google Scholar] [CrossRef]
  11. Söylemez, E.; Widell, K.N.; Gabrielii, C.H.; Ladam, Y.; Lund, T.; Hafner, A. Overview of the development and status of carbon dioxide (R-744) refrigeration systems onboard fishing vessels. Int. J. Refrig. 2022, 140, 198–212. [Google Scholar] [CrossRef]
  12. Gullo, P.; Hafner, A.; Banasiak, K. Transcritical R744 refrigeration systems for supermarket applications: Current status and future perspectives. Int. J. Refrig. 2018, 93, 269–310. [Google Scholar] [CrossRef]
  13. Bruno, F.; Belusko, M.; Halawa, E. CO2 Refrigeration and Heat Pump Systems—A Comprehensive Review. Energies 2019, 12, 2959. [Google Scholar] [CrossRef]
  14. Kasaeian, A.; Hosseini, S.M.; Sheikhpour, M.; Mahian, O.; Yan, W.-M.; Wongwises, S. Applications of eco-friendly refrigerants and nanorefrigerants: A review. Renew. Sustain. Energy Rev. 2018, 96, 91–99. [Google Scholar] [CrossRef]
  15. Qi, Z. Advances on air conditioning and heat pump system in electric vehicles—A review. Renew. Sustain. Energy Rev. 2014, 38, 754–764. [Google Scholar] [CrossRef]
  16. Groll, E.A.; Kim, J.-H. Review article: Review of recent advances toward transcritical CO2 cycle technology. HVAC&R Res. 2007, 13, 499–520. [Google Scholar]
  17. Cecchinato, L.; Corradi, M.; Fornasieri, E.; Zamboni, L. Carbon dioxide as refrigerant for tap water heat pumps: A comparison with the traditional solution. Int. J. Refrig. 2005, 28, 1250–1258. [Google Scholar] [CrossRef]
  18. Oritz, T.M.; Li, D.; Groll, E.A. Evaluation of the Performance Potential of CO2 as a Refrigerant in Air-to-Air Air Conditioners and Heat Pumps: System Modeling and Analysis; Final Report; United States Department of Commerce: Washington, DC, USA, 2003.
  19. Oh, H.-K.; Son, C.-H. New correlation to predict the heat transfer coefficient in-tube cooling of supercritical CO2 in horizontal macro-tubes. Exp. Therm. Fluid Sci. 2010, 34, 1230–1241. [Google Scholar] [CrossRef]
  20. Kim, M.H.; Pettersen, J.; Bullard, C.W. Fundamental process and system design issues in CO2 vapor compression systems. Prog. Energy Combust. Sci. 2004, 30, 119–174. [Google Scholar] [CrossRef]
  21. Sarkar, J.; Agrawal, N. Performance optimization of transcritical CO2 cycle with parallel compression economization. Int. J. Therm. Sci. 2010, 49, 838–843. [Google Scholar] [CrossRef]
  22. Chen, Y.; Gu, J. The optimum high pressure for CO2 transcritical refrigeration systems with internal heat exchangers. Int. J. Refrig. 2005, 28, 1238–1249. [Google Scholar] [CrossRef]
  23. Zhang, X.; Fan, X.; Wang, F.; Shen, H. Theoretical and experimental studies on optimum heat rejection pressure for a CO2 heat pump system. Appl. Therm. Eng. 2010, 30, 2537–2544. [Google Scholar] [CrossRef]
  24. Aprea, C.; Maiorino, A. Heat rejection pressure optimization for a carbon dioxide split system: An experimental study. Appl. Energy 2009, 86, 2373–2380. [Google Scholar] [CrossRef]
  25. Laipradit, P.; Tiansuwan, J.; Kiatsiriroat, T.; Aye, L. Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant. Int. J. Energy Res. 2008, 32, 356–366. [Google Scholar] [CrossRef]
  26. Sawalha, S. Carbon Dioxide in Supermarket Refrigeration. Doctoral Thesis, Royal Institute of Technology (KTH), Stockholm, Sweden, 2008. [Google Scholar]
  27. Liang, X.-Y.; He, Y.-J.; Cheng, J.-H.; Shao, L.-L.; Zhang, C.-L. Difference analysis on optimal high pressure of transcritical CO2 cycle in different applications. Int. J. Refrig. 2019, 106, 384–391. [Google Scholar] [CrossRef]
  28. Boewe, D.E.; Bullard, C.W.; Yin, J.M.; Hrnjak, P.S. Contribution of Internal Heat Exchanger to Transcritical R-744 Cycle Performance. HVAC&R Res. 2001, 7, 155–168. [Google Scholar]
  29. Qin, X.; Wang, D.; Jin, Z.; Wang, J.; Zhang, G.; Li, H. A comprehensive investigation on the effect of internal heat exchanger based on a novel evaluation method in the transcritical CO2 heat pump system. Renew. Energy 2021, 178, 574–586. [Google Scholar] [CrossRef]
  30. Yao, L.; Li, M.; Hu, Y.; Wang, Q.; Liu, X. Comparative study of upgraded CO2 transcritical air source heat pump systems with different heat sinks. Appl. Therm. Eng. 2021, 184, 116289. [Google Scholar] [CrossRef]
  31. Casi, A.; Aranguren, P.; Araiz, M.; Sanchez, D.; Cabello, R.; Astrain, D. Experimental evaluation of a transcritical CO2 refrigeration facility working with an internal heat exchanger and a thermoelectric subcooler: Performance assessment and comparative. Int. J. Refrig. 2022, 141, 66–75. [Google Scholar] [CrossRef]
  32. Nebot-Andrés, L.; Del Duca, M.G.; Aprea, C.; Žerovnik, A.; Tušek, J.; Llopis, R.; Maiorino, A. Improving efficiency of transcritical CO2 cycles through a magnetic refrigeration subcooling system. Energy Convers. Manag. 2022, 265, 115766. [Google Scholar] [CrossRef]
  33. Haida, M.; Banasiak, K.; Smolka, J.; Hafner, A.; Eikevik, T.M. Experimental analysis of the R744 vapour compression rack equipped with the multi-ejector expansion work recovery module. Int. J. Refrig. 2016, 64, 93–107. [Google Scholar] [CrossRef]
  34. Singh, S.; Dasgupta, M.S. Evaluation of research on CO2 trans-critical work recovery expander using multi attribute decision making methods. Renew. Sustain. Energy Rev. 2016, 59, 119–129. [Google Scholar] [CrossRef]
  35. Elbel, S.; Hrnjak, P. Flash gas bypass for improving the performance of transcritical R744 systems that use microchannel evaporators. Int. J. Refrig. 2004, 27, 724–735. [Google Scholar] [CrossRef]
  36. Gullo, P.; Elmegaard, B.; Cortella, G. Advanced exergy analysis of a R744 booster refrigeration system with parallel compression. Energy 2016, 107, 562–571. [Google Scholar] [CrossRef]
  37. Hwang, Y.H.; Celik, A.; Radermacher, R. Performance of CO2 Cycles with a Two-Stage Compressor. In Proceedings of the International Refrigeration and Air Conditioning Conference, West Lafayette, IN, USA, 12–15 July 2004. [Google Scholar]
  38. Bellos, E.; Tzivanidis, C. A comparative study of CO2 refrigeration systems. Energy Convers. Manag. X 2019, 1, 100002. [Google Scholar] [CrossRef]
  39. Liu, G.; Wang, Z.; Zhao, H.; Abdulwahid, A.A. R744 ejector simulation based on homogeneous equilibrium model and its application in trans-critical refrigeration system. Appl. Therm. Eng. 2022, 201, 117791. [Google Scholar] [CrossRef]
  40. Nebot-Andrés, L.; Calleja-Anta, D.; Sánchez, D.; Cabello, R.; Llopis, R. Experimental assessment of dedicated and integrated mechanical subcooling systems vs parallel compression in transcritical CO2 refrigeration plants. Energy Convers. Manag. 2022, 252, 115051. [Google Scholar] [CrossRef]
  41. Banasiak, K.; Hafner, A.; Kriezi, E.E.; Madsen, K.B.; Birkelund, M.; Fredslund, K.; Olsson, R. Development and performance mapping of a multi-ejector expansion work recovery pack for R744 vapour compression units. Int. J. Refrig. 2015, 57, 265–276. [Google Scholar] [CrossRef]
  42. Nakagawa, M.; Marasigan, A.R.; Matsukawa, T. Experimental analysis on the effect of internal heat exchanger in transcritical CO2 refrigeration cycle with two-phase ejector. Int. J. Refrig. 2011, 34, 1577–1586. [Google Scholar] [CrossRef]
  43. Shariatzadeh, O.J.; Abolhassani, S.S.; Rahmani, M.; Nejad, M.Z. Comparison of transcritical CO2 refrigeration cycle with expander and throttling valve including/excluding internal heat exchanger: Exergy and energy points of view. Appl. Therm. Eng. 2016, 93, 779–787. [Google Scholar] [CrossRef]
  44. Zhang, Z.; Tian, L.; Chen, Y.; Tong, L. Effect of an Internal Heat Exchanger on Performance of the Transcritical Carbon Dioxide Refrigeration Cycle with an Expander. Entropy 2014, 16, 5919–5934. [Google Scholar] [CrossRef]
  45. Pan, S.; Nie, S.; Li, J.; He, S. Thermodynamic performance study on a novel trans-critical CO2 heat pump cycle integrated with expander and internal heat exchanger. Proc. Inst. Mech. Eng. Part E J. Process. Mech. Eng. 2022, 236, 2639–2650. [Google Scholar] [CrossRef]
  46. Patil, O.S.; Shet, S.A.; Jadhao, M.; Agrawal, N. Energetic and exergetic studies of modified CO2 transcritical refrigeration cycles. Int. J. Low-Carbon Technol. 2021, 16, 171–180. [Google Scholar] [CrossRef]
  47. Singh, S.; Purohit, N.; Dasgupta, M.S. Comparative study of cycle modification strategies for trans-critical CO2 refrigeration cycle for warm climatic conditions. Case Stud. Therm. Eng. 2016, 7, 78–91. [Google Scholar] [CrossRef]
  48. Liu, J.; Liu, Y.; Yu, J. Performance analysis of a modified dual-ejector and dual-evaporator transcritical CO2 refrigeration cycle for supermarket application. Int. J. Refrig. 2021, 131, 109–118. [Google Scholar] [CrossRef]
  49. Liu, X.; Fu, R.; Wang, Z.; Lin, L.; Sun, Z.; Li, X. Thermodynamic analysis of transcritical CO2 refrigeration cycle integrated with thermoelectric subcooler and ejector. Energy Convers. Manag. 2019, 188, 354–365. [Google Scholar] [CrossRef]
  50. Manjili, F.E.; Yavari, M.A. Performance of a new two-stage multi-intercooling transcritical CO2 ejector refrigeration cycle. Appl. Therm. Eng. 2012, 40, 202–209. [Google Scholar] [CrossRef]
  51. De Carvalho, B.Y.K.; Melo, C.; Pereira, R.H. An experimental study on the use of variable capacity two-stage compressors in transcritical carbon dioxide light commercial refrigerating systems. Int. J. Refrig. 2019, 106, 604–615. [Google Scholar] [CrossRef]
  52. Purohit, N.; Gullo, P.; Dasgupta, M.S. Comparative Assessment of Low-GWP Based Refrigerating Plants Operating in Hot Climates. Energy Procedia 2017, 109, 138–145. [Google Scholar] [CrossRef]
  53. Gullo, P.; Elmegaard, B.; Cortella, G. Energy and environmental performance assessment of R744 booster supermarket refrigeration systems operating in warm climates. Int. J. Refrig. 2016, 64, 61–79. [Google Scholar] [CrossRef]
  54. Goodarzi, M.; Gheibi, A.; Motamedian, M. Comparative analysis of an improved two-stage multi-inter-cooling ejector-expansion trans-critical CO2 refrigeration cycle. Appl. Therm. Eng. 2015, 81, 58–65. [Google Scholar] [CrossRef]
  55. Sun, Z.; Wang, C.; Liang, Y.; Sun, H.; Liu, S.; Dai, B. Theoretical study on a novel CO2 Two-stage compression refrigeration system with parallel compression and solar absorption partial cascade refrigeration system. Energy Convers. Manag. 2020, 204, 112278. [Google Scholar] [CrossRef]
  56. Zhang, Z.; Wang, H.; Tian, L.; Huang, C. Thermodynamic analysis of double-compression flash intercooling transcritical CO2 refrigeration cycle. J. Supercrit. Fluids 2016, 109, 100–108. [Google Scholar] [CrossRef]
  57. Zeng, M.Q.; Zhang, X.L.; Mo, F.Y.; Zhang, X.R. Thermodynamic analysis of the effect of internal heat exchanger on the dual-ejector transcritical CO2 cycle for low-temperature refrigeration. Int. J. Energy Res. 2022, 46, 12702–12721. [Google Scholar] [CrossRef]
  58. Gullo, P. Impact and quantification of various individual thermodynamic improvements for transcritical R744 supermarket refrigeration systems based on advanced exergy analysis. Energy Convers. Manag. 2021, 229, 113684. [Google Scholar] [CrossRef]
  59. Singh, S.; Hafner, A.; Maiya, M.; Banasiak, K.; Neksa, P. Multiejector CO2 cooling system with evaporative gascooler for a supermarket application in tropical regions. Appl. Therm. Eng. 2021, 190, 116766. [Google Scholar] [CrossRef]
  60. Kwan, T.H.; Shen, Y.; Wu, Z.; Yao, Q. Performance analysis of the thermoelectric device as the internal heat exchanger of the trans-critical carbon dioxide cycle. Energy Convers. Manag. 2020, 208, 112585. [Google Scholar] [CrossRef]
  61. Tsamos, K.M.; Amaris, C.; Mylona, Z.; Tassou, S. Analysis of Typical Booster Configuration, Parallel-Compressor Booster Configuration and R717/R744 Cascade Refrigeration System for Food Retail Applications. Part 2: Energy Performance in Various Climate Conditions. Energy Procedia 2019, 161, 268–274. [Google Scholar] [CrossRef]
  62. Fu, R.; Wang, J.; Zheng, M.; Yu, K.; Liu, X.; Li, X. Thermodynamic Analysis of Transcritical CO2 Ejector Expansion Refrigeration Cycle with Dedicated Mechanical Subcooling. Entropy 2019, 21, 874. [Google Scholar] [CrossRef]
  63. Manjili, F.E.; Cheraghi, M. Performance of a new two-stage transcritical CO2 refrigeration cycle with two ejectors. Appl. Therm. Eng. 2019, 156, 402–409. [Google Scholar] [CrossRef]
  64. Xing, M.; Yu, J.; Liu, X. Thermodynamic analysis on a two-stage transcritical CO2 heat pump cycle with double ejectors. Energy Convers. Manag. 2014, 88, 677–683. [Google Scholar] [CrossRef]
  65. Inokuty, H. Approximate graphical method of finding compression pressure of CO2 refrigerating machine for maximum coefficient of performance. Trans. Jpn. Soc. Mech. Eng. 1923, 26, 1–12. [Google Scholar]
  66. Kauf, F. Determination of the optimum high pressure for transcritical CO2-refrigeration cycles. Int. J. Therm. Sci. 1999, 38, 325–330. [Google Scholar] [CrossRef]
  67. Liao, S.; Zhao, T.; Jakobsen, A. A correlation of optimal heat rejection pressures in transcritical carbon dioxide cycles. Appl. Therm. Eng. 2000, 20, 831–841. [Google Scholar] [CrossRef]
  68. Sawalha, S. Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions. Int. J. Refrig. 2008, 31, 516–524. [Google Scholar] [CrossRef]
  69. Sarkar, J.; Bhattacharyya, S.; Gopal, M.R. Optimization of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications. Int. J. Refrig. 2004, 27, 830–838. [Google Scholar] [CrossRef]
  70. Qi, P.-C.; He, Y.-L.; Wang, X.-L.; Meng, X.-Z. Experimental investigation of the optimal heat rejection pressure for a transcritical CO2 heat pump water heater. Appl. Therm. Eng. 2013, 56, 120–125. [Google Scholar] [CrossRef]
  71. Cabello, R.; Sánchez, D.; Llopis, R.; Torrella, E. Experimental evaluation of the energy efficiency of a CO2 refrigerating plant working in transcritical conditions. Appl. Therm. Eng. 2008, 28, 1596–1604. [Google Scholar] [CrossRef]
  72. Yin, X.; Cao, F.; Wang, J.; Li, M.; Wang, X. Investigations on optimal discharge pressure in CO2 heat pumps using the GMDH and PSO-BP type neural network—Part A: Theoretical modeling. Int. J. Refrig. 2019, 106, 549–557. [Google Scholar] [CrossRef]
  73. Song, Y.; Yang, D.; Li, M.; Cao, F. Investigations on optimal discharge pressure in CO2 heat pumps using the GMDH and PSO-BP type neural network—Part B: Experimental study. Int. J. Refrig. 2019, 106, 248–257. [Google Scholar] [CrossRef]
  74. Wang, W.; Zhao, Z.; Zhou, Q.; Qiao, Y.; Cao, F. Model predictive control for the operation of a transcritical CO2 air source heat pump water heater. Appl. Energy 2021, 300, 117339. [Google Scholar] [CrossRef]
  75. Cecchinato, L.; Corradi, M.; Minetto, S. A critical approach to the determination of optimal heat rejection pressure in transcritical systems. Appl. Therm. Eng. 2010, 30, 1812–1823. [Google Scholar] [CrossRef]
  76. Shao, L.-L.; Zhang, Z.-Y.; Zhang, C.-L. Constrained optimal high pressure equation of CO2 transcritical cycle. Appl. Therm. Eng. 2018, 128, 173–178. [Google Scholar] [CrossRef]
  77. Yang, L.; Li, H.; Cai, S.-W.; Shao, L.-L.; Zhang, C.-L. Minimizing COP loss from optimal high pressure correlation for transcritical CO2 cycle. Appl. Therm. Eng. 2015, 89, 656–662. [Google Scholar] [CrossRef]
  78. Zhang, W.-J.; Zhang, C.-L. A correlation-free on-line optimal control method of heat rejection pressures in CO2 transcritical systems. Int. J. Refrig. 2011, 34, 844–850. [Google Scholar] [CrossRef]
  79. Penarrocha, I.; Llopis, R.; Tarrega, L.; Sanchez, D.; Cabello, R. A new approach to optimize the energy efficiency of CO2 transcritical refrigeration plants. Appl. Therm. Eng. 2014, 67, 137–146. [Google Scholar] [CrossRef]
  80. Cecchinato, L.; Corradi, M.; Cosi, G.; Minetto, S.; Rampazzo, M. A real-time algorithm for the determination of R744 systems optimal high pressure. Int. J. Refrig. 2012, 35, 817–826. [Google Scholar] [CrossRef]
  81. Hu, B.; Li, Y.; Cao, F.; Xing, Z. Extremum seeking control of COP optimization for air-source transcritical CO2 heat pump water heater system. Appl. Energy 2015, 147, 361–372. [Google Scholar] [CrossRef]
  82. Cui, C.; Zong, S.; Song, Y.; Yin, X.; Cao, F. Experimental investigation of the extreme seeking control on a transcritical CO2 heat pump water heater. Int. J. Refrig. 2022, 133, 111–122. [Google Scholar] [CrossRef]
  83. Rampazzo, M.; Cervato, A.; Corazzol, C.; Mattiello, L.; Beghi, A.; Cecchinato, L.; Virzi, A. Energy-efficient operation of transcritical and subcritical CO2 inverse cycles via Extremum Seeking Control. J. Process. Control. 2019, 81, 87–97. [Google Scholar] [CrossRef]
  84. Hu, B.; Li, Y.; Wang, R.; Cao, F.; Xing, Z. Real-time minimization of power consumption for air-source transcritical CO2 heat pump water heater system. Int. J. Refrig. 2018, 85, 395–408. [Google Scholar] [CrossRef]
  85. Kim, M.S.; Kang, D.H.; Kim, M.S.; Kim, M. Investigation on the optimal control of gas cooler pressure for a CO2 refrigeration system with an internal heat exchanger. Int. J. Refrig. 2017, 77, 48–59. [Google Scholar] [CrossRef]
  86. Okasha, A.; Müller, N.; Deb, K. Bi-objective optimization of transcritical CO2 heat pump systems. Energy 2022, 247, 123469. [Google Scholar] [CrossRef]
  87. Ji, H.; Pei, J.; Niu, J.; Ding, C.; Guo, F.; Wang, Y. Hybrid offline and online strategy for optimal high pressure seeking of the transcritical CO2 heat pump system in an electric vehicle. Appl. Therm. Eng. 2023, 228, 120514. [Google Scholar] [CrossRef]
  88. Jin, Z.; Eikevik, T.M.; Neksa, P.; Hafner, A. A steady and quasi-steady state analysis on the CO2 hybrid ground-coupled heat pumping system. Int. J. Refrig. 2017, 76, 29–41. [Google Scholar] [CrossRef]
  89. Ge, Y.; Tassou, S. Control optimisation of CO2 cycles for medium temperature retail food refrigeration systems. Int. J. Refrig. 2009, 32, 1376–1388. [Google Scholar] [CrossRef]
  90. Shao, L.-L.; Zhang, C.-L. Thermodynamic transition from subcritical to transcritical CO2 cycle. Int. J. Refrig. 2016, 64, 123–129. [Google Scholar] [CrossRef]
  91. Ekren, O.; Sahin, S.; Isler, Y. Comparison of different controllers for variable speed compressor and electronic expansion valve. Int. J. Refrig. 2010, 33, 1161–1168. [Google Scholar] [CrossRef]
  92. Lawrence, N.; Elbel, S. Experimental investigation on control methods and strategies for off-design operation of the transcritical R744 two-phase ejector cycle. Int. J. Refrig. 2019, 106, 570–582. [Google Scholar] [CrossRef]
  93. Zhu, J.; Elbel, S. Experimental investigation into the influence of vortex control on transcritical R744 ejector and cycle performance. Appl. Therm. Eng. 2020, 164, 114418. [Google Scholar] [CrossRef]
  94. Choi, H.; Cho, H.; Choi, J.M. Refrigerant amount detection algorithm for a ground source heat pump unit. Renew. Energy 2012, 42, 111–117. [Google Scholar] [CrossRef]
  95. Kim, W.; Braun, J.E. Evaluation of the impacts of refrigerant charge on air conditioner and heat pump performance. Int. J. Refrig. 2012, 35, 1805–1814. [Google Scholar] [CrossRef]
  96. Cho, H.; Ryu, C.; Kim, Y.; Kim, H.Y. Effects of refrigerant charge amount on the performance of a transcritical CO2 heat pump. Int. J. Refrig. 2005, 28, 1266–1273. [Google Scholar] [CrossRef]
  97. Aprea, C.; Greco, A.; Maiorino, A. An experimental study on charge optimization of a trans-critical CO2 cycle. Int. J. Environ. Sci. Technol. 2015, 12, 1097–1106. [Google Scholar]
  98. Wang, Y.; Ye, Z.; Song, Y.; Yin, X.; Cao, F. Energy, exergy, economic and environmental analysis of refrigerant charge in air source transcritical carbon dioxide heat pump water heater. Energy Convers. Manag. 2020, 223, 113209. [Google Scholar] [CrossRef]
  99. Hazarika, M.M.; Ramgopal, M.; Bhattacharyya, S. Studies on a transcritical R744 based summer air-conditioning unit: Impact of refrigerant charge on system performance. Int. J. Refrig. 2018, 89, 22–39. [Google Scholar] [CrossRef]
  100. Zhang, Z.; Dong, X.; Ren, Z.; Lai, T.; Hou, Y. Influence of Refrigerant Charge Amount and EEV Opening on the Performance of a Transcritical CO2 Heat Pump Water Heater. Energies 2017, 10, 1521. [Google Scholar] [CrossRef]
  101. Wang, D.; Lu, Y.; Tao, L. Optimal combination of capillary tube geometry and refrigerant charge on a small CO2 water-source heat pump water heater. Int. J. Refrig. 2018, 88, 626–636. [Google Scholar] [CrossRef]
  102. Wang, H.; Li, S.; Song, Y.; Yin, X.; Cao, F.; Gullo, P. Experimental Thermodynamic Investigation on the Refrigerant Charge in a Transcritical CO2 Electric Bus Air Conditioning System. Appl. Sci. 2021, 11, 5614. [Google Scholar] [CrossRef]
  103. Yin, X.; Wang, A.; Fang, J.; Cao, F.; Wang, X. Coupled effect of operation conditions and refrigerant charge on the performance of a transcritical CO2 automotive air conditioning system. Int. J. Refrig. 2021, 123, 72–80. [Google Scholar] [CrossRef]
  104. He, Y.-J.; Liang, X.-Y.; Cheng, J.-H.; Shao, L.-L.; Zhang, C.-L. Approaching optimum COP by refrigerant charge management in transcritical CO2 heat pump water heater. Int. J. Refrig. 2020, 118, 161–172. [Google Scholar] [CrossRef]
  105. Li, B.; Peuker, S.; Hrnjak, P.; Alleyne, A.G. Refrigerant mass migration modeling and simulation for air conditioning systems. Appl. Therm. Eng. 2011, 31, 1770–1779. [Google Scholar] [CrossRef]
  106. Wang, D.; Zhang, Z.; Yu, B.; Wang, X.; Shi, J.; Chen, J. Experimental research on charge determination and accumulator behavior in trans-critical CO2 mobile air-conditioning system. Energy 2019, 183, 106–115. [Google Scholar]
  107. Wang, A.; Yin, X.; Fang, J.; Cao, F. Refrigerant distributions and dynamic migration characteristics of the transcritical CO2 air conditioning system. Int. J. Refrig. 2021, 130, 233–241. [Google Scholar] [CrossRef]
  108. Tassou, S.A.; Grace, I.N. Fault diagnosis and refrigerant leak detection in vapor compression refrigeration systems. Int. J. Refrig. 2005, 28, 680–688. [Google Scholar] [CrossRef]
  109. Uren, K.R.; Schoor, G.; Eldik, M.; Bruin, J.J.A. An Energy Graph-Based Approach to Fault Diagnosis of a Transcritical CO2 Heat Pump. Energies 2020, 13, 1783. [Google Scholar] [CrossRef]
  110. Liu, F.; Li, Y.; Groll, E.A. Performance enhancement of CO2 air conditioner with a controllable ejector. Int. J. Refrig. 2012, 35, 1604–1616. [Google Scholar]
  111. Liu, F.; Groll, E.A.; Li, D. Investigation on performance of variable geometry ejectors for CO2 refrigeration cycles. Energy 2012, 45, 829–839. [Google Scholar] [CrossRef]
  112. Boccardi, G.; Botticella, F.; Lillo, G.; Mastrullo, R.; Mauro, A.; Trinchieri, R. Experimental investigation on the performance of a transcritical CO2 heat pump with multi-ejector expansion system. Int. J. Refrig. 2017, 82, 389–400. [Google Scholar] [CrossRef]
  113. Cecchinato, L.; Chiarello, M.; Corradi, M.; Fornasieri, E.; Minetto, S.; Stringari, P.; Zilio, C. Thermodynamic analysis of different two-stage transcritical carbon dioxide cycles. Int. J. Refrig. 2009, 32, 1058–1067. [Google Scholar] [CrossRef]
  114. Nebot-Andrés, L.; Calleja-Anta, D.; Sánchez, D.; Cabello, R.; Llopis, R. Thermodynamic Analysis of a CO2 Refrigeration Cycle with Integrated Mechanical Subcooling. Energies 2020, 13, 4. [Google Scholar] [CrossRef]
  115. Song, Y.; Cui, C.; Li, M.; Cao, F. Investigation on the effects of the optimal medium-temperature on the system performance in a transcritical CO2 system with a dedicated transcritical CO2 subcooler. Appl. Therm. Eng. 2020, 168, 114846. [Google Scholar] [CrossRef]
  116. Song, Y.; Wang, H.; Cao, F. Investigation of the Impact Factors on the Optimal Intermediate Temperature in a Dual Transcritical CO2 System with a Dedicated Transcritical CO2 Subcooler. Energies 2020, 13, 309. [Google Scholar] [CrossRef]
  117. Kim, S.C.; Won, J.P.; Kim, M.S. Effects of operating parameters on the performance of a CO2 air conditioning system for vehicles. Appl. Therm. Eng. 2009, 29, 2408–2416. [Google Scholar] [CrossRef]
  118. Alkan, A.; Hosoz, M. Comparative performance of an automotive air conditioning system using fixed and variable capacity compressors. Int. J. Refrig. 2010, 33, 487–495. [Google Scholar] [CrossRef]
  119. Lee, H.-S.; Lee, M.-Y. Cooling Performance Characteristics on Mobile Air-Conditioning System for Hybrid Electric Vehicles. Adv. Mech. Eng. 2013, 5, 282313. [Google Scholar] [CrossRef]
  120. Mathur, G.D. Carbon dioxide as an alternative refrigerant for automotive air conditioning systems. In Proceedings of the 35th Intersociety Energy Conversion Engineering Conference & Exhibit (IECEC), Las Vegas, NV, USA, 24–28 July 2000; Volume 1, pp. 371–379. [Google Scholar]
  121. Liu, H.; Chen, J.; Chen, Z. Experimental investigation of a CO2 automotive air conditioner. Int. J. Refrig. 2005, 28, 1293–1301. [Google Scholar] [CrossRef]
  122. Niu, Y.; Chen, J.; Chen, Z.; Chen, H. Construction and testing of a wet-compression absorption carbon dioxide refrigeration system for vehicle air conditioner. Appl. Therm. Eng. 2007, 27, 31–36. [Google Scholar]
  123. Kondo, T.; Katayama, A.; Suetake, H.; Morishita, M. Development of automotive air-conditioning systems byheat pump technology. Mitsubishi Heavy Ind. Tech. Rev. 2011, 48, 27–32. [Google Scholar]
  124. Hosoz, M.; Direk, M. Performance evaluation of an integrated automotive air conditioning and heat pump system. Energy Convers. Manag. 2006, 47, 545–559. [Google Scholar] [CrossRef]
  125. Tamura, T.; Yakumaru, Y.; Nishiwaki, F. Experimental study on automotive cooling and heating air conditioning system using CO2 as a refrigerant. Int. J. Refrig. 2005, 28, 1302–1307. [Google Scholar] [CrossRef]
  126. Dong, J.; Wang, Y.; Jia, S.; Zhang, X.; Huang, L. Experimental study of R744 heat pump system for electric vehicle application. Appl. Therm. Eng. 2021, 183, 116191. [Google Scholar]
  127. Wang, Y.; Dong, J.; Jia, S.; Huang, L. Experimental comparison of R744 and R134a heat pump systems for electric vehicle application. Int. J. Refrig. 2020, 121, 10–22. [Google Scholar] [CrossRef]
  128. Ko, J.; Thu, K.; Miyazaki, T. Transient analysis of an electric vehicle air-conditioning system using CO2 for start-up and cabin pull-down operations. Appl. Therm. Eng. 2021, 190, 116825. [Google Scholar] [CrossRef]
  129. Wang, D.; Yu, B.; Li, W.; Shi, J.; Chen, J. Heating performance evaluation of a CO2 heat pump system for an electrical vehicle at cold ambient temperatures. Appl. Therm. Eng. 2018, 142, 656–664. [Google Scholar] [CrossRef]
  130. Zheng, S.; Wei, M.; Hu, C.; Song, P.; Tian, R. Flow characteristics of tangential leakage in a scroll compressor for automobile heat pump with CO2. Sci. China Technol. Sci. 2021, 64, 971–983. [Google Scholar] [CrossRef]
  131. Zheng, S.; Wei, M.; Song, P.; Hu, C.; Tian, R. Thermodynamics and flow unsteadiness analysis of trans-critical CO2 in a scroll compressor for mobile heat pump air-conditioning system. Appl. Therm. Eng. 2020, 175, 115368. [Google Scholar] [CrossRef]
  132. Li, J.; Jia, J.; Huang, L.; Wang, S. Experimental and numerical study of an integrated fin and micro-channel gas cooler for a CO2 automotive air-conditioning. Appl. Therm. Eng. 2017, 116, 636–647. [Google Scholar] [CrossRef]
  133. Wang, D.; Wang, Y.; Yu, B.; Shi, J.; Chen, J. Numerical study on heat transfer performance of micro-channel gas coolers for automobile CO2 heat pump systems. Int. J. Refrig. 2019, 106, 639–649. [Google Scholar] [CrossRef]
  134. Wang, Y.; Wang, D.; Yu, B.; Shi, J.; Chen, J. Experimental and numerical investigation of a CO2 heat pump system for electrical vehicle with series gas cooler configuration. Int. J. Refrig. 2019, 100, 156–166. [Google Scholar] [CrossRef]
  135. Mahvi, A.J.; Boyina, K.; Musser, A.; Elbel, S.; Miljkovic, N. Superhydrophobic heat exchangers delay frost formation and enhance efficency of electric vehicle heat pumps. Int. J. Heat Mass Transf. 2021, 172, 121162. [Google Scholar] [CrossRef]
  136. Wang, A.; Yin, X.; Fang, J.; Jia, F.; Song, Y.; Cao, F. A novel frost-free control strategy and its energy evaluation of the CO2 heat pump system. Appl. Therm. Eng. 2022, 201, 117745A. [Google Scholar] [CrossRef]
  137. Steiner, A.; Rieberer, R. Parametric analysis of the defrosting process of a reversible heat pump system for electric vehicles. Appl. Therm. Eng. 2013, 61, 393–400. [Google Scholar] [CrossRef]
  138. Steiner, A.; Rieberer, R. Simulation based identification of the ideal defrost start time for a heat pump system for electric vehicles. Int. J. Refrig. 2015, 57, 87–93. [Google Scholar] [CrossRef]
  139. Fang, J.; Yin, X.; Wang, A.; Sun, X.; Liu, Y.; Cao, F. Cooling performance enhancement for the automobile transcritical CO2 air conditioning system with various internal heat exchanger effectiveness. Appl. Therm. Eng. 2021, 196, 117274. [Google Scholar] [CrossRef]
  140. Li, H.; Zhang, Z.; Song, X.; Chen, J. Experimental Study of a Trans-Critical CO2 Mobile Air Conditioning System with an Ejector. J. Shanghai Jiaotong Univ. 2021, 55, 179–187. [Google Scholar]
  141. Zou, H.; Yang, T.; Tang, M.; Tian, C.; Butrymowicz, D. Ejector optimization and performance analysis of electric vehicle CO2 heat pump with dual ejectors. Energy 2022, 239, 122452. [Google Scholar] [CrossRef]
  142. Chen, Y.; Zou, H.; Dong, J.; Wu, J.; Xu, H.; Tian, C. Experimental investigation on the heating performance of a CO2 heat pump system with intermediate cooling for electric vehicles. Appl. Therm. Eng. 2021, 182, 116039. [Google Scholar] [CrossRef]
  143. Chen, Y.; Zou, H.; Dong, J.; Xu, H.; Tian, C.; Butrymowicz, D. Experimental investigation on refrigeration performance of a CO2 system with intermediate cooling for automobiles. Appl. Therm. Eng. 2020, 174, 115267. [Google Scholar] [CrossRef]
  144. Peng, X.; Wang, D.; Wang, G.; Yang, Y.; Xiang, S. Numerical investigation on the heating performance of a transcritical CO2 vapor-injection heat pump system. Appl. Therm. Eng. 2020, 166, 114656. [Google Scholar] [CrossRef]
  145. Yu, B.; Wang, D.; Liu, C.; Jiang, F.; Shi, J.; Chen, J. Performance improvements evaluation of an automobile air conditioning system using CO2-propane mixture as a refrigerant. Int. J. Refrig. 2018, 88, 172–181. [Google Scholar] [CrossRef]
  146. Yu, B.; Yang, J.; Wang, D.; Shi, J.; Guo, Z.; Chen, J. Experimental energetic analysis of CO2/R41 blends in automobile air-conditioning and heat pump systems. Appl. Energy 2019, 239, 1142–1153. [Google Scholar] [CrossRef]
  147. Yu, B.; Yang, J.; Wang, D.; Shi, J.; Chen, J. Modeling and theoretical analysis of a CO2-propane autocascade heat pump for electrical vehicle heating. Int. J. Refrig. 2018, 95, 146–155. [Google Scholar] [CrossRef]
  148. Lee, H.-S.; Won, J.-P.; Cho, C.-W.; Kim, Y.-C.; Lee, M.-Y. Heating performance characteristics of stack coolant source heat pump using R744 for fuel cell electric vehicles. J. Mech. Sci. Technol. 2012, 26, 2065–2071. [Google Scholar] [CrossRef]
  149. Kim, S.C.; Park, J.C.; Kim, M.S. Performance characteristics of a supplementary stack-cooling system for fuel-cell vehicles using a carbon dioxide air-conditioning unit. Int. J. Automot. Technol. 2010, 11, 893–900. [Google Scholar] [CrossRef]
  150. Kim, S.C.; Won, J.P.; Park, Y.S.; Lim, T.W.; Kim, M.S. Performance evaluation of a stack cooling system using CO2 air conditioner in fuel cell vehicles. Int. J. Refrig. 2009, 32, 70–77. [Google Scholar] [CrossRef]
  151. Kim, S.C.; Kim, M.S.; Hwang, I.C.; Lim, T.W. Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements. Int. J. Refrig. 2007, 30, 1195–1206. [Google Scholar] [CrossRef]
  152. Song, Y.; Wang, H.; Ma, Y.; Yin, X.; Cao, F. Energetic, economic, environmental investigation of carbon dioxide as the refrigeration alternative in new energy bus/railway vehicles’ air conditioning systems. Appl. Energy 2022, 305, 117830. [Google Scholar] [CrossRef]
  153. Shi, L.; Tian, H.; Shu, G. Multi-mode analysis of a CO2-based combined refrigeration and power cycle for engine waste heat recovery. Appl. Energy 2020, 264, 114670. [Google Scholar] [CrossRef]
  154. Fabris, F.; Artuso, P.; Marinetti, S.; Minetto, S.; Rossetti, A. Dynamic modelling of a CO2 transport refrigeration unit with multiple configurations. Appl. Therm. Eng. 2021, 189, 116749. [Google Scholar] [CrossRef]
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Figure 1. The p–h diagram of the transcritical CO2 cycle(1′2′3′4′) and conventional subcritical cycle(1234). 1, 1′—inlet of compressor; 2, 2′—outlet of compressor; 3, 3′—inlet of expansion valve; 4, 4′—outlet of expansion valve.
Figure 1. The p–h diagram of the transcritical CO2 cycle(1′2′3′4′) and conventional subcritical cycle(1234). 1, 1′—inlet of compressor; 2, 2′—outlet of compressor; 3, 3′—inlet of expansion valve; 4, 4′—outlet of expansion valve.
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Figure 2. Variation of optimum high pressure and COP with the change of gas cooler exit temperature [23].
Figure 2. Variation of optimum high pressure and COP with the change of gas cooler exit temperature [23].
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Figure 3. Three categories of mainstream improvement measures and their combinations for transcritical CO2 cycle.
Figure 3. Three categories of mainstream improvement measures and their combinations for transcritical CO2 cycle.
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Figure 4. COP as a function of the cycle high pressure and water inlet temperature for the baseline coaxial heat exchanger [75].
Figure 4. COP as a function of the cycle high pressure and water inlet temperature for the baseline coaxial heat exchanger [75].
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Figure 5. ESC simulation results under variable ambient conditions [84].
Figure 5. ESC simulation results under variable ambient conditions [84].
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Figure 6. COP ratios of R22, R410A, R407C, and CO2 systems with deviation from optimal charge [96].
Figure 6. COP ratios of R22, R410A, R407C, and CO2 systems with deviation from optimal charge [96].
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Figure 7. The COP variation of different cycles with the increase of medium temperature [115].
Figure 7. The COP variation of different cycles with the increase of medium temperature [115].
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Figure 8. Cooling performance comparison of R134a and R744 system [126]. (a) the compressor speed is equal to 5600 RPM, (b) the cooling capacity is equal to 3.2 kw.
Figure 8. Cooling performance comparison of R134a and R744 system [126]. (a) the compressor speed is equal to 5600 RPM, (b) the cooling capacity is equal to 3.2 kw.
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Figure 9. The CO2 heat pump system configuration considering the arrangement of heat exchanger [151].
Figure 9. The CO2 heat pump system configuration considering the arrangement of heat exchanger [151].
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Table
Table 1. Review of combination modifications applied to the transcritical CO2 cycle.
Table 1. Review of combination modifications applied to the transcritical CO2 cycle.
Combination ModificationsReferencesMajor FindingsType of Study
and Applications
TS + FGB[21]
  • PC has a similar performance to PCR, better than TS + FGB.
  • PC is more beneficial for lower temperature applications compared with PCR.
Simulation
Refrigeration
IHX + EJE[42]
  • The additional IHX is beneficial to both the basic cycle and ejector system’s COP.
  • The maximum COP is about 27% higher than the basic cycle.
Experimental
Refrigeration
EXP + IHX[43,44,45,46]
  • The use of an IHX increases the COP of the throttling valve cycle while decreasing the COP of the expander cycle.
  • The new cycle presents 9% less COP improvement than the cycle with an expander only.
  • The modification increases the COP by a maximum of 31.49% in the warm period and the heat capacity by a maximum of 12% in the cold period.
Simulation
Refrigeration/Heat pump [45]
TS + FGB
EXP + IHX		[47]
  • Compared with TS + FGB, the TSFI system performs better regardless of the evaporator temperature.
  • The application of the expander produces a larger COP due to work recovery; the expander with an IHX has a lower COP compared with the expander system but has a higher COP compared with the IHX system.
Simulation
Refrigeration
Dual EJE + IHX[48]
  • The compressor pressure ratio is reduced by up to 19.1% and the COP is 15.9–27.1% higher than the IHX system.
Simulation
Refrigeration
EJE + TES[49]
  • The new modification has the highest COP (+39.34%) and lowest optimum discharge pressure (−8.01%) among the studied cycles.
Simulation
Refrigeration
TSMI + EJE[50]
  • Compared with the EJE system and EJE + IHX system, the new system increases the maximum COP by 15.3% and 19.6%.
Simulation
Refrigeration
TSI + IHX + FGB[51]
  • The new cycle improves the COP slightly more than the cycle with an IHX only, but it can decrease the discharge temperature and achieve a higher cooling capacity.
Experimental
Refrigeration
BS + PC + DMS [52,53]
  • The CC behaves better than other configurations at high ambient temperatures at the cost of worse performance below the ambient temperature of 14 °C.
  • The system with DMS presents better performance than the system with PC at high temperatures.
  • High subcooling can also make the performance of DMS in warm climates better.
Simulation
Refrigeration
TSMI + EJE + IHX[54]
  • The new cycle behaves better under low-pressure working conditions.
Simulation
Refrigeration
TS + PC + SAPCC [55]
  • The new system has the greatest COP and the energy-saving share is improved with the increase of solar radiation.
  • The improvements are influenced by location and climate.
Simulation
Refrigeration
TSFI + EXP[56]
  • The TSFI system has a larger COP than the TSI system.
  • The complete substitution of the expander for the throttling valve leads to a smaller improvement in the TSFI system compared to the improvement in the TSI system.
Simulation
Refrigeration
Dual EXP + IHX + TS[57]
  • Compared with the TSI system with or without an IHX, the COP of these new modifications is significantly improved. However, after multi-parameter optimization, the addition of an IHX is harmful to the new system’s COP in low-temperature refrigeration, regardless of the IHX’s position
Simulation
Refrigeration
PC +EJE + BS[58]
  • PC and ejector are profitable to decrease exergy destruction and can be used for a better exergy efficiency simultaneously. Enhancing the parallel compressor efficiency can make system performance better. An overfed evaporator has little effect on exergy destruction decrease.
Simulation
Refrigeration
PC + BS + multi EJE + IHX + EC[59]
  • With an IHX and EC, the COP is improved by up to 40% and the power input ratio is reduced by up to 26%.
Experimental
Refrigeration
TES + IHX[60]
  • The COP improvement is less when the thermoelectric works as a generator compared with the cycle modified with an IHX only.
  • The system COP is reduced when the thermoelectric works as a cooler because the thermoelectric consumes additional power.
Simulation
Refrigeration
FGB + CC + BS[61]
  • Considering annual energy consumption, the PC + BS system is suitable for moderate conditions while the FGB + PC + BS system is suitable for warm conditions.
  • The PC + BS system is suitable for convenience stores due to lower energy consumption under both moderate and warm climates.
Simulation
Refrigeration
EJE + DMS[62]
  • The system with an ejector and DMS presents better performance than the control group (TES + EJE), especially in warm and hot regions.
  • The performance is influenced by the mass ratio of the mixed refrigerant in DMS.
simulation
TS + two EJE[63]
  • The COP is improved by up to 80% compared with the basic system when the new system works under high ambient temperatures.
  • The COP ratio of the new system to the conventional cycle rises above 2 in specific situations
Simulation
Refrigeration
TS + IHX + two EJE[64]
  • Without an IHX, the COP is 10.4% on average and the volumetric heating capacity is 3.6–8.2% higher than the TS system under different evaporation temperatures.
  • With an IHX, the COP is 10.5–30.6% and the volumetric heating capacity is 6.6–25.9% higher than the TS system.
Simulation
Refrigeration
Table
Table 2. Optimal pressure seeking methods analysis.
Table 2. Optimal pressure seeking methods analysis.
Offline MethodsOnline MethodsHybrid Methods
MethodsGraphical method
Correlations
ANN		Steepest descend method
Perturb and observe
PSO
ESC
MPC		Offline target value and online optimize
SpeedFastSlowFast
AccuracyLowHighHigh
DevelopmentEnergies 16 04011 i001
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Ji, H.; Pei, J.; Cai, J.; Ding, C.; Guo, F.; Wang, Y. Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field. Energies 2023, 16, 4011. https://doi.org/10.3390/en16104011

AMA Style

Ji H, Pei J, Cai J, Ding C, Guo F, Wang Y. Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field. Energies. 2023; 16(10):4011. https://doi.org/10.3390/en16104011

Chicago/Turabian Style

Ji, Hongzeng, Jinchen Pei, Jingyang Cai, Chen Ding, Fen Guo, and Yichun Wang. 2023. "Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field" Energies 16, no. 10: 4011. https://doi.org/10.3390/en16104011

APA Style

Ji, H., Pei, J., Cai, J., Ding, C., Guo, F., & Wang, Y. (2023). Review of Recent Advances in Transcritical CO2 Heat Pump and Refrigeration Cycles and Their Development in the Vehicle Field. Energies, 16(10), 4011. https://doi.org/10.3390/en16104011

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