Design of a Heat Pump System Using an R744/R290 Refrigerant Mixture
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친환경 냉매 특성 연구 및 히트펌프 설계
Study of the Characteristics of Eco-friendly Refrigerants and Heat Pump Design
◇Research Background
As global policies for climate change mitigation and environmental protection strengthen, the refrigeration industry has increasingly focused on developing eco-friendly alternative refrigerants. Since the adoption of the Kigali Amendment in 2016, the use of HFC refrigerants has been gradually restricted, bringing natural refrigerants to the forefront as promising alternatives. Natural refrigerants offer the advantages of low Global Warming Potential (GWP) and minimal environmental impact compared to synthetic refrigerants.
Among natural refrigerants, R744 (carbon dioxide, CO₂) has attracted significant attention as an environmentally friendly alternative due to its excellent heating performance and a very low GWP of 1. Studies report that when the outdoor temperature is –5°C and –10°C, R744 provides 32.69% and 45.93% higher heating performance, respectively, compared to R134a. However, R744 has disadvantages such as a low critical temperature and high operating pressure, which reduce compatibility with conventional heat pump systems.
Meanwhile, R290 (propane) is recognized for its superior heating performance and high energy efficiency (COP) in low-temperature environments. Research indicates that R290 offers excellent performance at temperatures above –10°C and has high potential for use in electric vehicle heat pump systems. However, R290 is highly flammable, requiring minimized charge amounts to ensure safety.
To overcome these limitations and maximize the strengths of each refrigerant, studies have explored the use of R744/R290 refrigerant mixtures. Mixing the two refrigerants is expected to reduce R744’s high operating pressure while mitigating the flammability issues of R290. However, research on how mixture ratios and charge amounts affect system performance remains limited. Therefore, this study aims to evaluate the performance characteristics of R744/R290 mixtures at various mixing ratios and charge quantities to identify optimal conditions for vehicle heat pump systems.
◇ Research Methods and System Description
This study evaluates the heating and cooling performance of a vehicle heat pump system using R744/R290 mixed refrigerants. Various mixture ratios and charge amounts were applied, and the resulting performance differences were analyzed through simulations.
2.1. Overview of the Heat Pump System
The heat pump system used in this study is based on a Direct Expansion (DX) cycle. The system consists of major components including an outdoor heat exchanger (condenser), indoor heat exchanger (evaporator), compressor, and expansion valve. Thermodynamic calculations were carried out for each component of the system
2.2. Refrigerant Mixing Ratios and Operating Conditions
The mixing ratios of R744 and R290 were varied from 1:9 to 9:1 to compare performance differences. Heating-mode evaluations were conducted at outdoor temperatures of –15°C, –10°C, –5°C, and 0°C, while cooling-mode performance was measured at 30°C, 35°C, and 40°C. To ensure consistency, the refrigerant mass flow rate was fixed at 0.2 m³/s throughout the experiments.
2.3. Performance Evaluation Methods
To evaluate the performance of the heat pump system, the following analytical methods were applied:
Compressor performance: Predicted using the ARI 10-coefficient compressor map method under varying mixture ratios and operating conditions.
Heat exchanger performance: Assessed using the ε-NTU method to evaluate heat transfer performance in both evaporator and condenser.
Refrigerant charge amount: Estimated using Zivi’s void fraction model, with additional correction factors applied to account for mixture properties.
◇ Research Results
This study compared the heating and cooling performance of the heat pump system under various R744/R290 mixture ratios and charge amounts. The results showed significant performance variations depending on the refrigerant mixture ratio. As the proportion of R744 increased, heating performance improved, especially in low-temperature conditions below –10°C. Conversely, higher proportions of R290 enhanced cooling performance, achieving high energy efficiency at elevated temperatures between 35°C and 40°C.
Overall, the optimal refrigerant mixture for vehicle heat pump systems was found at a specific ratio where R744 and R290 are effectively balanced. This study confirms that selecting optimal mixture ratios according to temperature conditions is crucial for maximizing system energy efficiency.
This study applied R744/R290 mixed refrigerants to a vehicle heat pump system and analyzed system performance under various temperature conditions. The results demonstrate that refrigerant mixture ratio significantly influences heat pump performance and that an optimal ratio exists for achieving maximum efficiency. Additionally, refrigerant charge amount plays a key role in performance and must be carefully considered during system design.
Future work should include experimental validation under real vehicle conditions to obtain more precise performance data and to evaluate safety. The findings of this study are expected to contribute to improving energy efficiency in vehicle heat pump systems and promoting the use of environmentally friendly refrigerants.
