Physical Property Calculation and Refrigeration Cycle Analysis of Mixed Refrigerant R32/R290

Abstract

The adoption of eco-friendly refrigerants in air conditioning systems is crucial for advancing low-carbon architecture. The current refrigerant R410A, with its high global warming potential, underscores the need for sustainable alternatives that balance cooling efficiency and environmental impact. This study investigates a binary mixture of R32 and R290 as a potential replacement for R410A. Using the Peng–Robinson equation of state, the thermodynamic properties of the mixed refrigerant were calculated post-temperature glide, analyzing variations across different mixing ratios. A specific ratio of 0.3:0.7 (R32:R290) was identified as optimal, offering a balance between safety and performance, closely matching R410A’s properties. Simulations of the refrigeration cycle were conducted to assess the effects of condensation and evaporation temperatures, as well as subcooling and superheating, on system performance. Key findings reveal that the 0.3:0.7 mixture not only meets safety standards for central air conditioning but also demonstrates efficiency comparable to R410A. These results provide a robust theoretical foundation for the development of low-carbon air conditioning technologies, highlighting the potential of R32/R290 mixtures in reducing environmental impact while maintaining performance.

1. Introduction

Refrigeration is widely regarded as one of the most significant technological achievements of the 20th century, alongside space exploration, the internet, and computers [1,2]. The refrigeration and air conditioning (R&AC) sector alone accounts for approximately 20% of global energy consumption [3,4]. In China, building energy consumption represents about 45% of the nation’s total energy usage [5], with heating and air conditioning systems contributing 50% to 70% of this building-related energy demand [6]. Refrigerants, which are substances or mixtures [7] used in heat cycles, play a critical role in these systems by undergoing phase transitions between gaseous and liquid states to facilitate heat transfer.

Over the past two decades, significant efforts have been made to explore a wide range of refrigerants, including ether, CO₂, and SO₂ (first-generation refrigerants); NH₃ and water (second-generation refrigerants); HCFCs and HFCs (third-generation refrigerants); and HFOs (fourth-generation refrigerants), among others [8]. However, these refrigerants face critical challenges, as emissions from refrigeration systems contribute to environmental issues such as global warming and ozone layer depletion. In response to these concerns, technological and economic advancements have driven the development of air conditioning refrigerants with zero ozone depletion potential (ODP) and low global warming potential (GWP) [9].

Prior to the implementation of the Kyoto Protocol in 1997, research on environmentally friendly refrigerant alternatives primarily focused on protecting the ozone layer [10], which led to the development of HFCs. However, after the protocol’s adoption, the focus shifted towards identifying refrigerants that not only had zero ODP but also minimized contributions to global warming. Despite these efforts, the options for non-toxic, non-flammable refrigerants with zero ozone depletion remain limited. Candidates such as R23 and R134a [11], for instance, have been unable to match the thermal efficiency of older-generation chlorofluorocarbons (CFCs) and HCFCs.

No single refrigerant can simultaneously meet the dual criteria of operational efficiency and environmental sustainability. While some natural refrigerants like CO₂ have extremely low GWP values, achieving both operational suitability and environmental sustainability remains a significant challenge due to trade-offs in thermodynamic properties, safety, and efficiency. To address this challenge, researchers and policymakers worldwide have adopted a strategy of blending multiple refrigerants, combining their optimal properties to achieve both zero ODP and low GWP. While mixed refrigerants show promise as replacements for traditional options—offering thermal efficiency and cooling performance that meet zero ODP requirements—their use in central air conditioning systems still poses significant environmental concerns. For instance, R32, a widely favored refrigerant due to its operational suitability, has a GWP of 675, far exceeding the Montreal Protocol’s recommended threshold of below 150 [12].

In response to these challenges, researchers and experts have turned their attention to the development of natural refrigerants [13,14], such as ammonia (NH₃), carbon dioxide (CO₂), hydrocarbons (HCs), water, and air, as well as low-GWP synthetic refrigerants, including HFCs (e.g., R152a, R32, and R161), HCs, hydrofluoroolefins (HFOs, such as R290, R600, and RE170), and unsaturated compounds like olefins. While these alternatives meet the criteria of zero ODP and low GWP, they introduce new challenges related to toxicity and flammability. To mitigate flammability risks, flame-retardant additives are often employed; however, these additives can paradoxically exhibit high GWP, creating a complex trade-off in refrigerant selection and development [15]. Recent studies have explored the use of Al₂O₃ nanolubricants in refrigeration systems, demonstrating their potential to enhance thermal efficiency, reduce energy consumption, and improve cooling performance in air conditioners and household refrigerators [16,17,18,19]. Today, environmental impact is considered equally important as thermal efficiency in refrigerant selection. Pfeiffer et al. used R454B as a replacement for R410A as the refrigerant in a vapor-injected rotary compressor [4]. New refrigerants must align with global commitments to combat climate change and protect the ozone layer [20]. Consequently, the latest criteria for refrigerant selection prioritize options with minimal ODP and very low GWP, reflecting the urgent need for sustainable cooling solutions.

The refrigerant R32 is widely utilized due to its exceptional thermodynamic properties, high latent heat capacity, and chemical stability at elevated temperatures [21]. However, its performance at low temperatures can lead to increased compressor ratios and elevated exhaust temperatures, which adversely affect system efficiency. While R32 boasts a zero ODP, its GWP of 676 [22] poses a challenge in meeting stringent national ‘dual carbon’ targets. In contrast, R290 exhibits an ODP of 0 and an exceptionally low GWP of 3 [23,24], positioning it as a highly sustainable option amid the escalating global warming crisis. R290 is distinguished by its superior thermal conductivity, high latent heat of evaporation, and low molecular weight, which enhance fluid dynamics, reduce transport pressure, and lower compressor load. These attributes not only improve compressor durability but also enable a 30% reduction in refrigerant charges compared to conventional refrigerants. Additionally, R290 is compatible with standard lubricants and mechanical materials without disrupting the natural hydrocarbon balance.

Despite its environmental and thermodynamic advantages, R290’s flammability, safety concerns, and charging volume limitations restrict its application primarily to residential air conditioning systems. To address these challenges, blending R290 with R32 in precise ratios offers a promising solution. This study proposes a hybrid R32/R290 refrigerant mixture designed to overcome the limitations of each individual refrigerant. By investigating the thermophysical properties of R32/R290 blends across various ratios, the study aims to validate their potential as a replacement for R410a. Using a vapor compression refrigeration simulation model, the cooling performance of different R32/R290 mixtures is evaluated, with a focus on optimizing environmental sustainability and system efficiency.

2. Materials and Methods

2.1. Mathematical Model

The refrigerants R290 and R32 exhibit a significant difference in boiling points. For non-azeotropic refrigerant mixtures, this difference causes the more readily condensable components to liquefy first during cooling at constant pressure [25,26]. This shift in composition leads to a phenomenon known as temperature glide, which must be carefully accounted for in refrigeration system designs to ensure optimal efficiency.

A temperature glide arises when two refrigerants with distinct boiling points are mixed in varying proportions. Traditional methods, such as the Refprop approach for retrieving refrigerant data, have become inadequate in such cases, necessitating the development of a new model to evaluate thermophysical properties. This study builds on the gas–liquid phase equilibrium experiments conducted by Gong Maoqiu and colleagues [27], applying the Peng–Robinson equation of state to analyze how temperature glide, normal boiling points, and saturation pressures vary with different R32/R290 mixing ratios.

As outlined by Poling et al. [28], the family of cubic equations of state (cEoS) can be expressed in a common structure, with modifications to the attraction term tabulated alongside adjustable parameters. Johannes Gernert et al. [29] extended this approach by deriving the analytic derivatives of these equations of state in a form compatible with the multi-parameter Helmholtz-energy-explicit EoS. Within this framework, the cEoS is expressed as follows:

$$p={\displaystyle \frac{RT}{v-b}}-{\displaystyle \frac{a\left(T\right)}{\left(v+{\Delta }_{1}b\right)\left(v+{\Delta }_{2}b\right)}}$$

(1)

where ${\Delta }_1$ and ${\Delta }_2$ are different for each EoS, being ${\Delta }_1=1+\sqrt{2}$ and ${\Delta }_2=1-\sqrt{2}$ for the PR EoS. The PR EoS is a modification of the SRK EoS that allows for better predictions of molar volumes in the liquid region and a better representation of the vapor–liquid equilibrium for many mixtures. These features have made the EoS one of the most used cEoS today. Although other cEoS have been developed, none have demonstrated a clear general advantage in thermodynamic property predictions [30].

The PR EoS for a pure fluid, expressed explicitly in terms of pressure, has the following form:

$$p={\displaystyle \frac{RT}{v-b}}-{\displaystyle \frac{a_{c}a\left(T_r,w\right)}{v\left(v+b\right)+b\left(v-b\right)}}$$

(2)

where the parameters are expressed as follows:

$$a_c=0.45724\left({\displaystyle \frac{R^2{T}_c^2}{p_c}}\right)$$

(3)

$$b=0.07780\left({\displaystyle \frac{R{T}_c}{P_c}}\right)$$

(4)

$$a\left(T_r,w\right)={\left[1+m\left(w\right)\left(1-\sqrt{T_r}\right)\right]}^2$$

(5)

where the term m is a function of the acentric factor and is given as follows:

$$m\left(w\right)=0.37464+1.5422w-0.26992w^2$$

(6)

The parameters a_c and b as defined here are fluid dependent. In the case of fluid mixtures, a mixing rule is necessary. See the work by reference [31] for a list of common mixing rules. The classical mixing rule is that of van der Waals, which can be augmented by one or two adjustable parameters. These parameters need to be fitted to the experimental data for each fluid pair. In the scope of this work, the van der Waals mixing rule without adjustable parameters is used for the Peng–Robinson model:

$$a_c={\displaystyle {\sum }_i{\displaystyle {\sum }_j{x}_i{x}_j{a}_{ij}}}{a}_{ij}=\sqrt{a_{c,i}{a}_{c,j}}$$

(7)

$$b={\displaystyle {\sum }_i{\displaystyle {\sum }_j{x}_i{x}_j{b}_{ij}}}{b}_{ij}={\displaystyle \frac{b_i+b_j}{2}}$$

(8)

Developing C language programs to determine the thermodynamic properties of innovative refrigerants is advantageous due to their adaptability, swift performance, and high-quality code output. The programming process is both efficient and versatile, offering a variety of problem-solving strategies. The methodology for such a C program is outlined in Figure 1. The specific steps are as follows:

(1) Input the critical temperatures T_ci and T_cj, critical pressures P_ci and P_c, liquid-phase mole fractions x_i and x_j, binary interaction coefficient k_ij, and step size 0.0001 for the single components of the mixed refrigerant.

(2) Calculate the cohesive energy term and co-volume term for pure substances using the following formulas:

(3) Calculate the critical temperature and critical pressure for each single-component refrigerant separately.

(4) Calculate the intermolecular attraction constant and intermolecular repulsion constant for each pure refrigerant.

(5) Given the mixing ratio, use the mixing rule to calculate the intermolecular attraction constant and repulsion constant for the mixture.

(6) Calculate the latent heat of vaporization, saturation temperature, saturation pressure, and specific heat of the mixture.

(7) Calculate the gas-phase mole fraction of the mixed components and check whether the convergence condition is satisfied. If satisfied, proceed to step (8); if not, return to step (3) and repeat the cycle until the convergence condition is met.

(8) Output the thermodynamic parameters of the mixed refrigerant.

2.2. Model Validation

To validate the accuracy of the Peng–Robinson equation of the state model, this study conducts a verification using the refrigerant R410a as a benchmark. The experimental data for R410a were obtained exclusively from the REFPROP database (version 10.0), maintained by the National Institute of Standards and Technology (NIST). Recognized as one of the most authoritative sources in refrigerant property research, the REFPROP database provides data that have undergone rigorous experimental validation and uncertainty analysis. The computational results, as illustrated in Figure 2, demonstrate the model’s performance and alignment with the reference data.

Figure 2 illustrates the comparison between the saturated vapor pressures calculated using this model and the reference data for R410a at a pressure of 0.1 MPa. The results reveal a maximum discrepancy of 4.8% in the calculated saturated vapor pressure, which is within the acceptable threshold of 5%. The close agreement between the calculated values from the phase equilibrium model and the actual measurements validates the model’s accuracy and demonstrates its suitability for evaluating the thermodynamic properties of novel refrigerant blends.

3. Results

To improve the accuracy and efficiency of refrigeration cycle performance simulations, this study replaces the labor-intensive and error-prone manual parameter entry with a MATLAB 2014a-based thermodynamic cycle simulation tool. This tool not only enhances precision but also significantly streamlines the simulation process. Building on the thermophysical properties of refrigerants R32 and R290, as detailed in

Table 1. Main parameters of refrigerants R32 and R290.

Parameters	R32	R290
Chemical formula	CH2F2	C3H8
Vapor pressure/MPa	1.518	0.953
Standard boiling point/°C	−51.7	−42.1
Critical temperature/°C	78.3	96.7
Critical pressure/MPa	5.81	4.25
Latent heat of vaporization/kJ/(kg·°C)	390.5	430.2
Safety level	A2	A3

, a MATLAB-driven phase equilibrium model for R32/R290 mixtures is developed. This model analyzes how variations in the mixture ratio influence key physical parameters, enabling the identification of an optimal R32/R290 blend ratio.

The selection of blend ratios was guided by a comprehensive evaluation of thermodynamic properties, safety considerations, system compatibility, and performance optimization [32]. For example, studies indicate that a blend ratio of 0.4:0.6 (R32:R290) achieves optimal thermodynamic performance and cooling capacity [33], while a higher R32 proportion (>0.6) is recommended to address safety concerns [34]. Conversely, a 0.5:0.5 ratio is often preferred for balancing system compatibility and performance optimization. To ensure a thorough analysis, this study evaluates a wide range of R32 proportions, from 0.1 to 1.0, providing a robust foundation for identifying the most effective blend.

3.1. Dew Point and Bubble Point

Dew point and bubble point temperatures are critical parameters that define the state of refrigerants and have significant implications for the performance of air conditioning and refrigeration systems. The dew point temperature is closely associated with a system’s dehumidification capacity and directly impacts indoor air comfort. Conversely, the bubble point temperature plays a key role in achieving high refrigeration cycle efficiency and optimizing the energy efficiency ratio (EER). Effective management and regulation of these temperatures are essential for the efficient design and operation of air conditioning systems. For pure refrigerants such as R32 and R290, the dew point and bubble point temperatures coincide at various pressures. However, in blended refrigerants, a temperature glide causes a separation between these temperatures, as illustrated in Figure 3.

Figure 3 illustrates the relationship between the dew point and bubble point temperatures for varying R32 concentrations under standard atmospheric pressure (curve a), as well as at pressures of 1.0 MPa (curve b), 2.0 MPa (curve c), and 3.0 MPa. The analysis reveals a consistent decrease in both dew point and bubble point temperatures as the R32 fraction increases, regardless of the pressure. This trend is attributed to the inherently lower saturation temperature of R32 compared to R290 at equivalent pressures. As the proportion of R32 rises, its influence on the mixture’s thermodynamic properties becomes more pronounced. Additionally, the temperature glide—defined as the difference between the dew point and bubble point temperatures—initially increases before decreasing at higher R32 concentrations. This behavior is further detailed in Figure 4.

Figure 4 demonstrates the relationship between the temperature glide and the proportion of R32 under varying pressures. The results show that as the R32 concentration increases, the temperature glide initially rises, reaching its peak at a 50% R32 composition, before gradually decreasing. Furthermore, the magnitude of the temperature glide increases with higher pressures, peaking at 2.0 MPa. Beyond this pressure, the temperature glide begins to decline. These findings suggest that operating air conditioning systems with mixed refrigerants at lower pressure levels can help mitigate the effects of temperature glide, thereby optimizing system performance.

3.2. Boiling Point

When designing air conditioning systems, the standard boiling point is a critical parameter to consider, as a lower boiling point generally correlates with improved cooling performance. As shown in Figure 5, the standard boiling point of the R32/R290 mixture decreases progressively as the proportion of R32 increases, reaching its minimum value when the mixture consists entirely of R32. This trend arises because R32 inherently has a lower standard boiling point than R290 at equivalent pressures. Consequently, increasing the R32 fraction amplifies its influence on the mixture’s boiling behavior, enhancing overall cooling efficiency.

In air conditioning systems, the saturation pressure of the refrigerant is a critical parameter for both system design and operational efficiency. The system relies on the refrigerant evaporating at low pressure to absorb heat and condensing at high pressure to release heat, thereby enabling the heat exchange cycle. Figure 6 illustrates the relationship between the bubble point and dew point pressures of an R32/R290 refrigerant mixture at different temperatures, plotted against the R32 concentration. The data reveal that at a fixed temperature, increasing the proportion of R32 leads to a corresponding rise in both dew point and bubble point pressures. This trend is attributed to the fact that R32 exhibits a higher saturation pressure than R290 at the same temperature, and as its concentration increases, its influence on the mixture’s thermodynamic properties becomes more pronounced. As a result, to ensure safety, the system’s pressure thresholds must be designed to accommodate higher pressures compared to systems using pure R290.

Figure 6 demonstrates that both bubble point and dew point pressures initially increase and then decrease as the proportion of R32 rises, reaching a peak at a 60% R32 composition. The saturation pressure of the refrigerant blend is calculated as the arithmetic mean of the dew point and bubble point pressures, with its variation illustrated in Figure 7. Analysis of the graph reveals that the saturation pressure decreases as temperatures drop. At a constant temperature, the saturation pressure of the blend increases with a higher R32 concentration. Notably, this increase becomes more pronounced at elevated temperatures, suggesting that the rate of saturation pressure rise is significantly higher under higher temperature conditions.

3.3. Latent Heat of Vaporization

The latent heat of vaporization, defined as the energy absorbed by a refrigerant during its phase transition from liquid to gas, is a critical factor in air conditioning systems. In the evaporator, the refrigerant absorbs heat from the indoor air to facilitate vaporization, and a higher latent heat of vaporization enables the refrigerant to absorb more heat per unit mass, thereby enhancing cooling efficiency. As illustrated in Figure 8, the latent heat of vaporization for the R32/R290 mixture varies with the proportion of R32. The data show that the latent heat increases with higher R32 concentrations, although the rate of increase gradually diminishes. Furthermore, at a fixed mixture ratio, higher pressures result in lower latent heat of vaporization values. To maximize refrigeration performance, it is essential to maintain low pressure in the evaporator, ensuring effective heat absorption and optimal cooling efficiency.

3.4. Thermal Conductivity

Thermal conductivity is a key parameter influencing the heat exchange efficiency of refrigerants in heat exchangers. Figure 9 presents the thermal conductivity trends for the liquid and vapor phases of mixed refrigerants within the temperature range of −35 to 0 °C at standard atmospheric pressure. The results show that the thermal conductivity of the liquid phase decreases as the temperature rises, indicating a reduced capacity for heat conduction at higher temperatures. In contrast, the thermal conductivity of the vapor phase increases with temperature, suggesting enhanced heat transfer performance under warmer conditions. Notably, the thermal conductivity of the vapor phase is approximately an order of magnitude lower than that of the liquid phase. For pure refrigerants such as R32 and R290, the thermal conductivity of both phases decreases uniformly with increasing temperature. In mixed refrigerants, however, the liquid phase thermal conductivity initially decreases gradually, but then experiences a sharp decline within the temperature range between the bubble and dew points. Conversely, the vapor phase thermal conductivity exhibits an almost linear increase with temperature. A unique observation is that while pure refrigerants show no intersection in thermal conductivity between their liquid and vapor phases across the temperature range, mixed refrigerants demonstrate a specific temperature at which the thermal conductivities of the two phases converge.

4. Refrigeration Cycle Performance Analysis

Figure 10 presents a simulation model based on a vapor-compression refrigeration system, which consists of four primary components: the compressor, evaporator, condenser, and expansion valve. In the evaporator, the low-temperature, low-pressure liquid refrigerant absorbs heat and vaporizes, producing low-pressure vapor that is drawn into the compressor. The compressor then pressurizes the refrigerant into high-temperature, high-pressure vapor, which flows into the condenser. Here, the refrigerant releases heat and condenses into a high-pressure liquid. After passing through the expansion valve, where its pressure is reduced, the refrigerant becomes a low-temperature, low-pressure mixture of vapor and liquid. The liquid portion of this mixture is vaporized again in the evaporator, completing the cycle and enabling continuous refrigeration.

To optimize the vapor-compression refrigeration cycle, a comprehensive analysis of key parameters during evaporation, compression, condensation, and expansion is essential. Strategic adjustments to these interdependent parameters can significantly enhance system efficiency. Critical factors such as condensing and evaporating temperatures, as well as degrees of subcooling and superheating, play a pivotal role in cycle management and are closely tied to the properties of the refrigerant used. When analyzing cycles involving mixed refrigerants, it is necessary not only to compare cooling capacity and efficiency but also to evaluate how these key parameters influence the overall performance of the refrigeration system.

4.1. The Verification of the Refrigeration Cycle

The correctness of the established refrigeration cycle was verified using the experimental results of R410A from reference [35]. The thermodynamic parameter calculation model had already been validated with R410A in Part 2. The calculated R410A parameters were introduced into the refrigeration cycle and validated under the experimental conditions from the reference. The condensing temperature was maintained at 38 °C, while the evaporating temperature varied from 5 °C to 13 °C. The results, shown in Figure 11, indicated a maximum relative error of 2.27% at an evaporating temperature of 5 °C, which was within the acceptable limit of 5%. This confirmed the accuracy of the refrigeration cycle model.

4.2. Simulation Results of the Refrigeration Cycle

The unit cooling capacity is a key performance metric for refrigeration systems, representing the amount of heat absorbed per unit of refrigerant flow under specific conditions. When the components of a refrigeration cycle are fixed, the choice of refrigerant influences both thermodynamic and environmental properties, thereby affecting the unit cooling capacity. Figure 12 illustrates the trend of unit cooling capacity as the proportion of R290 in the refrigerant mixture increases. The data show a slight but consistent rise in unit cooling capacity with higher R290 concentrations, although the overall increase is relatively small. This indicates that while the R290 ratio has a measurable impact on unit cooling capacity, its effect is limited.

Another crucial parameter for evaluating refrigeration system performance is the coefficient of performance (COP), which measures the efficiency of cooling equipment or heat pumps by calculating the ratio of cooling output to power input under specific operating conditions. Figure 13 demonstrates the relationship between COP and the proportion of R290. The results reveal a clear trend: as the R290 share increases, the COP consistently decreases, with the decline becoming more pronounced at higher R290 ratios.

When comparing unit cooling capacity and COP, it is evident that the unit cooling capacity remains relatively stable across different R32/R290 mix ratios. In contrast, the COP exhibits a significant decrease, with the difference between extremes approaching a factor of 1.5. This substantial variation underscores the importance of COP as a critical metric for assessing the efficiency and effectiveness of refrigeration systems.

Refrigerants R32 and R290, classified as mildly flammable gases, pose a potential explosion risk in the event of a leak. Refrigerants operating under higher working pressures are more susceptible to leakage due to increased stress on system components and sealing mechanisms. This phenomenon is particularly critical at the compressor outlet, where the pressure is significantly elevated compared to other parts of the system [36,37,38]. As a result, discharge temperature serves as a key performance indicator for evaluating refrigerant blends. Figure 14 illustrates the variation in the compressor exhaust temperature as the proportion of R290 in the mixture increases. The data reveal that increasing the R290 concentration initially lowers the exhaust temperature, which then rises again, with a narrow temperature range of only 5.714 °C between the highest and lowest values. The optimal discharge temperature occurs at an R32–R290 mix ratio of 0.3:0.7, with the lowest recorded temperature of 102.125 °C.

5. Discussion

The ratio of R32 to R290 in a blended refrigerant significantly impacts not only the performance of refrigeration cycle systems but also the interaction of key parameters across different system components. This study examines several representative mixing ratios to analyze the effects of variations in condensing temperature, evaporating temperature, subcooling, and superheating on the overall performance of refrigeration systems.

5.1. Condensing Temperature

The condensing temperature is defined as the temperature at which the refrigerant undergoes a phase transition from gas to liquid within the refrigeration system, releasing heat into the surrounding environment. Effective understanding and control of the condensing temperature are crucial for optimizing refrigeration system efficiency, as it directly influences energy consumption and overall cooling performance. Figure 15 illustrates the relationship between COP values and condensing temperatures at different blend ratios, highlighting the impact of this parameter on system performance.

Figure 15 illustrates the relationship between the COP for refrigeration and the condensing temperature for pure R32, pure R290, and their standard mixtures. The results demonstrate that at a fixed condensing temperature, the COP reaches its maximum when pure R32 is used as the refrigerant. The addition of R290, even in small amounts, results in a gradual decline in COP. Furthermore, the COP shows a consistent linear decrease with increasing condensing temperature, regardless of the R32/R290 blend ratio. Higher condensing temperatures are found to significantly reduce the efficiency of the refrigeration cycle. Therefore, to achieve an optimal COP, careful consideration of the condensing temperature is essential during the design and operation of refrigeration systems.

5.2. Evaporation Temperature

The evaporation temperature is a critical parameter in refrigeration systems, representing the temperature at which the refrigerant absorbs heat and undergoes a phase transition from liquid to vapor in the evaporator. This temperature has a direct influence on the system’s overall efficiency and performance. In refrigeration system design, identifying the optimal evaporation temperature is crucial for maximizing operational effectiveness. This study investigates the cooling performance of various refrigerant mixtures across a range of evaporation temperatures, with the results illustrated in Figure 16.

Figure 16 illustrates the relationship between the COP for refrigeration and the evaporation temperature for pure R32, pure R290, and their standard mixtures. The results demonstrate that at a fixed condensing temperature, the COP reaches its maximum when pure R32 is used as the refrigerant. The addition of R290, even in small amounts, results in a gradual decline in COP. Furthermore, the COP shows a consistent linear increase with rising evaporation temperature, regardless of the R32/R290 blend ratio. Higher evaporation temperatures are directly correlated with improved refrigeration efficiency. Therefore, to achieve an optimal COP, it is essential to maximize the evaporation temperature within practical design constraints.

The analysis reveals that variations in evaporation temperature have a relatively small impact on the refrigeration system’s COP. For example, in a 1:1 mixture, the difference in COP between the lowest and highest evaporation temperatures is only 8.0%. Additionally, the evaporation temperature significantly affects not only the cooling efficiency but also the compressor’s exhaust temperature, as illustrated in Figure 17. The compressor’s discharge temperature exhibits irregular fluctuations with varying R32/R290 blend ratios but consistently trends downward. The lowest discharge temperature is achieved at an evaporation temperature of 5 °C, representing the optimal condition. Given that both R32 and R290 have an A2 safety classification, a lower compressor discharge temperature is advantageous for the safe operation of the refrigeration cycle. Therefore, to ensure system safety, it is essential to maintain the evaporation temperature at approximately 5 °C.

6. Conclusions

This study first addresses the temperature glide phenomenon in R32/R290 non-azeotropic mixtures, which arises from differences in boiling points, by developing a thermophysical property calculation model based on the Peng–Robinson equation of state. This model overcomes the limitations of traditional REFPROP methods under temperature glide conditions. Second, a C++ calculation program was developed based on this model, enabling efficient and accurate computation of key thermophysical parameters, such as enthalpy, entropy, specific heat, and latent heat of vaporization. This program provides a reliable tool for analyzing the performance of non-azeotropic mixtures. Finally, the program was used to systematically identify the optimal mixing ratios for R32/R290 mixtures, significantly reducing experimental and computational costs while ensuring performance, efficiency, and safety. This offers a scientific foundation for selecting mixing ratios in practical applications. The specific conclusions are as follows:

(1) A significant temperature glide is observed in the R32/R290 binary mixture. As the proportion of R32 increases, the temperature glide initially rises and then declines, reaching its maximum when the mixture ratio is 1:1. Additionally, the temperature glide is influenced by operating pressure, with lower pressures resulting in reduced temperature glide values. Therefore, it is essential to ensure that air conditioning systems operate at lower pressures to minimize temperature glide effects.

(2) A central air conditioning simulation system was developed using C++ to evaluate the unit cooling capacity and COP values for different R32/R290 mixture ratios. The results show that as the proportion of R32 increases, the unit cooling capacity gradually rises, while the COP value decreases. From a safety perspective, considering the impact of exhaust temperature, the optimal R32/R290 ratio is 0.3:0.7.

(3) The key factors influencing the refrigeration process in air conditioning systems are evaporating temperature, condensing temperature, superheat, and subcooling degree. The findings reveal that changes in condensing temperature have the most significant impact on refrigeration performance.

7. Advantages and Challenges of R32/R290 Mixed Refrigerant

A comparison of the key thermodynamic properties and refrigeration capacity between the R32/R290 mixed refrigerant (Nantong Huaxin center air conditioner, Nantong, China) (with a mixing ratio of 0.3:0.7) and R410A is presented in

Table 2. A comparison of the main thermodynamic properties and refrigeration capacity between the mixed refrigerant R32/R290 (with a mixing ratio of 0.3:0.7) and R410A.

Refrigerant	R410a	R32/R290 (0.3/0.7)
Standard boiling point (°C)	−41.2	−39.4
Critical pressure (MPa)	4.98	4.72
Critical temperature (°C)	96.13	91.18
Latent heat of vaporization (kJ/kg)	380.59	418.57
Refrigeration capacity (kJ/kg)	177.48	275.08
Exhaust temperature (°C)	95.839	102.128

. As shown in the table, the R32/R290 mixture demonstrates high similarity to R410A in terms of standard boiling point, critical temperature, and critical pressure, making it an excellent alternative to R410A from a thermodynamic perspective. Moreover, the latent heat of vaporization of the R32/R290 mixture is significantly higher than that of R410A, indicating its superior refrigeration capacity. This conclusion is further supported by the comparison of refrigeration capacity per unit volume. However, the R32/R290 mixture has a higher discharge temperature than R410A and is classified as flammable, necessitating careful attention to safety during its application.

7.1. Advantages and Social Contributions of R32/R290 Mixed Refrigerant

By comparing the thermodynamic properties and refrigeration performance of the R32/R290 mixed refrigerant with R410a, the following advantages of the mixed refrigerant can be identified:

(1) Enhanced energy efficiency

The R32/R290 mixed refrigerant exhibits superior thermodynamic properties compared to the widely used R410a, particularly in terms of unit refrigeration capacity. This improvement in performance can lead to more energy-efficient air conditioning systems, reducing overall energy consumption in buildings and lowering operational costs for both consumers and businesses.

(2) Reduced environmental impact

The environmental benefits of adopting R32/R290 are significant. Both R32 and R290 have substantially lower GWP compared to R410a, which is known for its high GWP. Transitioning to this mixed refrigerant can help mitigate the climate impact of refrigeration systems. Furthermore, R290 is a natural refrigerant with zero ODP, further enhancing its environmental sustainability.

(3) Economic and social benefits

By improving the efficiency and environmental performance of central air conditioning systems, this research supports broader societal goals, such as reducing greenhouse gas emissions and promoting sustainable development. It also aligns with global regulatory trends that are increasingly phasing out high-GWP refrigerants, helping industries comply with environmental regulations and avoid potential penalties.

7.2. Challenges of R32/R290 Mixed Refrigerant

From the above comparison, the application of mixed refrigerants presents challenges in the following three areas:

(1) Higher discharge temperature

The discharge temperature of the mixed refrigerant is significantly higher than that of R410A, potentially leading to increased operating temperatures for the compressor and other system components. This could result in elevated thermal loads and greater wear risks. To address these challenges, system design optimizations, such as intermediate cooling or the use of advanced compressor materials, are necessary.

(2) Flammability concerns

The R32/R290 mixed refrigerant is flammable, which introduces safety risks in practical applications. In particular, leaks could pose fire or explosion hazards. Therefore, strict compliance with safety regulations is critical, including enhanced leak detection systems, the use of explosion-proof equipment, and ensuring adequate ventilation.

(3) System compatibility and stability

While the mixed refrigerant shows high similarity to R410A in terms of thermodynamic properties, its compatibility and long-term stability in real-world systems require further investigation. For instance, the mixed refrigerant may interact with lubricants, sealing materials, or piping components in existing systems, potentially impacting performance and longevity. As a result, comprehensive compatibility testing and system optimization are essential before widespread implementation.