Performance assessment of air-to-water heat pumps in alpine regions under present and future climate: Impacts of start-up and defrosting cycles

Air-to-water heat pump (AWHP) is a key technology for decarbonising building heating, yet their deployment in Alpine regions remains limited due to performance degradation at low source temperatures. These regions are climate-change hot-spots characterised by rapid warming and complex topography, raising the question of whether future climate change will alleviate or intensify the operational constraints affecting AWHPs. This study quantifies the combined impact of start-up and defrosting losses on AWHP performance under present and future climate conditions. An integrated TRNSYS-based model was developed, incorporating experimentally derived degradation correlations for start-up and defrosting cycles. A single-family house equipped with an inverter-driven AWHP was simulated for five Alpine municipalities in Northern Italy, across elevations from 83 to 652 m above sea level. In the base year, start-up operation accumulates up to 40 to 70 h per month during transitional and winter periods, while defrosting reaches up to 30 to 40 cycles per month at higher elevations. These transient effects reduce the seasonal coefficient of performance (SCOP) by 4 to 6% due to start-up alone and up to 11% when combined with defrosting, leading to primary energy increases of 7 to 12%. By 2050, climate warming reduces defrosting frequency by up to 33 to 100% in transitional months, while start-up hours remain significant due to increased part-load operation. As a result, SCOP increases by about 2% in 2030 and 6% in 2050, with primary energy consumption decreasing by up to 10%. Explicit modelling of operational dynamics is essential for realistic AWHP assessment.