Integrated analysis of refrigerant charge and seasonal performance for a residential expander-enhanced vapor compression system

Space cooling and heating systems in buildings use nearly 33% of the world’s energy. To make heat pump (HP) systems more energy-efficient, especially during part-load operation, it is important to use variable-speed compressors. However, part-load conditions come with their own set of problems, primarily because the ideal amount of refrigerant varies based on both indoor and outdoor temperatures. Another difficulty is accurately controlling the expansion device when both operating conditions and refrigerant charge levels fluctuate. In this study, we examined an R-410A based 5-ton (17.6 kW) HP system that includes a variable-speed rolling piston compressor and an expander/separator replacing a traditional expansion valve. This expander/separator also recovers some energy during the expansion process. A validated steady-state HP model in cooling mode was used to simulate performance, with refrigerant charge gradually increased from 4.2 kg to 5.6 kg in steps of 0.01 kg across six different operating scenarios. The analysis revealed that the system achieved its highest coefficient of performance (COP) when the charge was around 5 kg, though this optimal point shifted slightly (±0.03) based on operating parameters. While the cooling output rose steadily with added refrigerant, extra charge led to flooding in the expander/separator, which in turn lowered the energy recovery output. Furthermore, using the optimized refrigerant charge of 5 kg, the cycle was simulated to match the building loads for typical residential buildings in a mixed-humid climate (West Lafayette, IN, USA), and in an extreme hot-dry climate (Kuwait City, Kuwait) during summer months. Comparisons were made between an R-410A based 5-ton (17.6 kW) baseline-TXV cycle and the expander cycle. Similar overall hourly COPs were reported from these simulations. However, the cooling capacity from both locations was higher for the expander system than the TXV-system by 0.3 to 2 kW. As the HP was undersized for an extremely hot-dry climate zone, the expander was unable to generate power at peak temperature periods in Kuwait, given the expander over-flooded with refrigerant as opposed to the mixed-humid case. These results highlight the need for an effective system to manage refrigerant charge, as maintaining the right charge level under different environmental conditions is essential, especially with upcoming cooling technologies expected to use low-Global Warming Potential (GWP) refrigerants. In addition, there is a need to optimize the expander designs depending on the climate zones to achieve maximum COP improvements and power generation.