Environmental concerns have driven refrigerant changes since the 1980s. The Montreal Protocol (1987) affected chlorine-containing refrigerants, and the Kigali Amendment (2016) addressed global warming concerns by restricting the use of high–global warming potential (GWP) refrigerants.
The options to replace R-410A have been reduced by these measures, and replacements are mostly non-azeotropic blends with large temperature glides. Among the alternatives, R-32, R-454B, and R‑454A are near-term options with GWP <750. Long-term options would likely have GWP <150, which requires the use of high-glide blends such as R-454C and R-455A (Table 1). These refrigerants also have lower volumetric capacity and pressure. Therefore, they will require significant changes to heat exchanger (HX) designs. Therefore, this study focuses on optimizing the whole system by employing HXs with smaller diameters (5 mm) and optimized circuitry.
The US Department of Energy’s Oak Ridge National Laboratory’s Heat Pump Design Model  was used to simulate the performance of heat pumps. This model has been validated using experimental data . REFPROP 10.0  was used to calculate refrigerant properties. To ensure proper simulation of 5 mm tube HXs, air-side correlations that were developed for small-diameter tubes  were implemented. For the 9 mm R-410A baseline system, a model from Wang et al.  was used.
Low–global warming potential refrigerants can significantly reduce the direct emissions of CO2 equivalents originating from HVAC systems. However, high-efficiency systems are needed to reduce indirect CO2 emissions. In this study, an R‑410A residential 5-Ton heat pump was optimized using R‑32, R‑454A, R‑454B, R‑454C, and R‑455A. Among these options, R‑455A and R‑454C have the lowest global warming potential but have a lower volumetric capacity and high glide. Optimization results using 5 mm tube heat exchangers showed 12.4% to 19.1% efficiency improvements and 13% to 33% reduction in overall lifetime CO2 emissions.
The optimal 5 mm tube heat exchangers obtained from this research can fit into the original R-410A system frame, which helps to minimize changes in manufacturing and installation, thus reducing impacts on manufacturers and end-users. The proposed approach establishes a production and installation path to produce cost-effective low-GWP reversible heat pumps.
This study has clearly shown the usefulness of Artificial Intelligence, i.e., optimization, in designing the next generation A/C systems. Still, significant challenges remain as other components like the compressor will also need to be properly designed for the new low-GWP refrigerants.
Samuel Yana Motta, Distinguished R&D Scientist, Oak Ridge National Laboratory