01
One approach to improving the efficiency of HAVC systems is the use of refrigerants such as CFCs and HCFCs. However, leaks of these refrigerants cause ozone layer depletion, leading to their quick ban under the Montreal Protocol. This prompted a shift to practical alternatives like HFCs and R134a. Yet, HFCs were found to have high global warming potential (GWP) when released into the atmosphere, driving another transition to substitutes like R1234yf. Recently, the European Chemicals Agency (ECHA) announced an impending ban on PFAS, which will lead to the gradual phase-out of R1234yf refrigerant currently used in vehicles. As a result, natural refrigerants such as R290 (propane) and R744 (carbon dioxide) have emerged.
Various studies on R290 and its blends have shown superior performance in both air conditioning and heat pump operations. Due to its lower boiling point, R290-based heat pump systems can operate at lower ambient temperatures compared to R1234yf.
A major issue with R290 refrigerant is its flammability.
This necessitates the design of a new indirect refrigerant and coolant system, which can transfer heating and cooling to the cabin and powertrain components without refrigerant entering the cabin.
Additionally, due to the flammability of A3, the charging mass of R290 refrigerant in the system is limited to 150g (at the time of this study) to avoid refrigerant gas concentration.
02
Experimental Research
The figure below shows the mechanism of the newly developed R290 heat pump system, which is divided into a refrigerant system and a coolant system.
The experimental setup of the R290 indirect refrigerant system is shown in the figure below. The system consists of a refrigeration unit, which acts as an independent refrigeration system. Then, the coolant is used as the working fluid to transfer heat loads to the cabin HVAC and powertrain components, and extract heat from the environment for the operation of the heat pump.
The refrigerant system consists of four main components and pressure-temperature (PT) sensors. Data from the PT sensors helps determine the state of the refrigerant at different positions in the refrigerant circuit.
The cabin HVAC is heated using coolant, which absorbs heat from the condenser. The refrigerant expanded by the water condenser undergoes expansion through an electronic expansion valve, cooling the refrigerant and reducing pressure.
Cold coolant in the radiator is used to recover heat from the environment. After recovering heat from the environment, the hot coolant passes through a battery cooler, where it exchanges heat with the cold refrigerant.
The battery cooler compresses refrigerant vapor, and the compressor delivers the hot refrigerant vapor to the water condenser, completing the cycle. The refrigerant tank is equipped with a 33cc variable-speed scroll compressor.
The independent refrigerant unit is charged with 90g of R290 refrigerant. The charge determination test is conducted to identify the amount of refrigerant required to achieve the heat pump’s target.
To compare R290 with current refrigerants, the same indirect system was tested with R1234yf under the same test matrix.
The system is equipped with pressure, temperature, and flow sensors to measure the state of refrigerant and coolant at different positions throughout the circuit. The table below shows the accuracy of the measurement results.
The figure below shows the schematic diagram of thermocouple arrangement.
03
Results
The data shows that the indirect heat pump system has achieved all test matrix vehicle targets. The maximum condenser heating of the refrigerant unit varies linearly with ambient temperature, from 9.6kW at 0°C to 6.5kW at -20°C. The losses between the condenser and HVAC are measured at 0.4-0.5kW, which quantifies the indirect nature of the system.
For different ambient test cases, the COP of the heat pump system is around 1.5. Under maximum performance conditions, the maximum COP reached at 0°C is 1.97.
Heating capacity of the R290 heat pump system’s condenser and system COP: At 0°C, the maximum condenser heating capacity is 9.8kW. At this point, the maximum value calculated by the system code is 1.97. Under maximum performance load conditions, the condenser’s heating capacity varies linearly with ambient temperature.
Air inlet and exhaust temperature distribution between radiators of the R290 system at an ambient temperature of -15°C.
Comparison of HVAC Heating Capabilities Between R290 and R1234yf
The comparison shows that under similar boundary conditions, the heating capacity of R290 is improved compared to R1234yf. Since the boiling point of R1234yf is lower than -15°C, heating capacity data for it cannot be obtained.
Comparison of Condenser Heating Capabilities Between R290 and R1234yf.The indirect nature of the system results in heat losses of 400-500W between the condenser and HVAC.
Comparison of Compressor Power Between R290 and R1234yf
The comparison shows that the compressor in the R290 unit has higher power than that in R1234yf. For the same evaporator and condenser temperatures, the R290 system operates at higher pressures, which in turn increases the enthalpy and thus the power consumption of the compressor.
In the case of R290, the significant improvement in heating capacity did not directly translate into similar improvements in system COPs.
This can be explained by the figure below, which shows the power consumed by the compressor in the cases of R290 and R1234yf. In the case of R290, the compressor power is higher than that of R1234yf. This is likely because the compressor used in the refrigerant system was designed and adjusted for R1234yf refrigerant, and it was not optimized for the R290 system.
The R290 heat pump system can recover ambient heat at temperatures below -15°C, which is different from the R1234yf system due to its lower boiling point. This will help reduce the vehicle’s dependence on alternative heat sources (such as resistive coolant or air heaters) under low-temperature conditions.
04
Conclusion
The investigation shows that both refrigerants met the heating performance targets. The experiment quantified the indirect system losses as 0.4-0.5kW.
The magnitude of losses indicates the improvement range in system COP by enhancing heat transfer between the refrigerant and coolant heat exchangers. System COP can also be enhanced by optimizing the compressor and oil charge. The system COP with R290 refrigerant is approximately 10% higher than that of R1234yf, making it an excellent substitute for current systems. The fact that R290 can recover ambient heat at temperatures below -15°C due to its low boiling point further supports this claim. Since heat pumps are more efficient than resistive heaters, using a heat pump with R290 at lower temperatures will reduce battery energy consumption and significantly improve the driving range of electric vehicles.
The next step of the research is to test the indirect structure and control logic of the refrigerant and all refrigerant and powertrain components on the test bench.
R290 Modular Heat Pump Architecture
