Working-fluid selection for a frost-free air-source heat pump: A case study integrating thermophysical, corrosion, and economic trade-offs

This case study evaluates working-fluid selection for frost-free air-source heat pumps (FFASHPs), in which the circulating fluid affects heat and mass transfer, pumping demand, material compatibility, and life-cycle cost. Most studies compare single fluids or isolated metrics, so a practical comparison under the same antifreeze requirement is still needed. This study evaluates calcium chloride (CaCl 2 ), ethylene glycol (EG), and potassium formate (HCOOK) as representative inorganic salt, organic compound, and organic salt candidates under a unified antifreeze requirement. A coupled FFASHP model is benchmarked against winter experimental data, key solution-property correlations are checked against source data, electrochemical Tafel polarization tests are used to quantify corrosion, and ten-year life-cycle cost with one-factor sensitivity analysis is assessed. Results show that the HCOOK-based system achieves a unit COP of 3.40 and a system COP of 2.55 during the heating season, within 0.37% and 1.03% of the best conventional options, respectively. Its corrosion rate on A3 carbon steel is 0.00125 mm year −1 , equal to 23.5% and 54.6% of the rates for CaCl 2 and EG. For T2 copper, HCOOK is less corrosive than CaCl 2 but 21.1% higher than EG. Its ten-year life-cycle cost is only 2.0% and 0.9% higher than those of CaCl 2 and EG. A multi-criteria evaluation further ranks HCOOK first under both equal and entropy weighting and across all 120 tested weight combinations. HCOOK therefore offers a balanced engineering alternative when performance, durability, and economics are considered together.