Researchers in Italy analyzed air-to-water heat pump performance in Alpine regions, focusing on how start-up and defrost cycles affect efficiency under current and future climate conditions. Their simulations show that while warmer temperatures slightly reduce defrosting losses and modestly improve efficiency, start-up cycling losses remain significant and continue to limit overall performance gains.
Scientists from Italy’s University of Trento have investigated the impact of start-up and defrosting cycles on a PV-linked air-to-water heat pump (AWHP) located in Alpine regions, which are climate-change hotspots characterized by rapid warming and complex topography.
“The novelty of this research lies in the explicit integration of experimentally derived and validated start-up and defrosting degradation correlations into a dynamic TRNSYS framework, where AWHP performance can be assessed under present and future climate conditions in any location globally,” corresponding author Fabian Eze told pv magazine.
Eze further explained that Alpine climates are characterized by complex topography, which causes AWHPs to experience unique operational dynamics, such as frequent on–off cycling and defrost cycles.
“These transient effects often lead to performance degradation, but in conventional simulations they are often overlooked. Consequently, AWHP performance is consistently overestimated, resulting in unrealistic dynamic performance assessments,” he said.
In their simulation study, the AWHP was coupled with a 4 kW PV system, an electricity grid connection, a 3 kWh battery energy storage system (BESS), a 277 L combined heating/cooling thermal storage tank, and a 156 L domestic hot water tank. The AWHP used R32 as a refrigerant and was inverter-driven. It had rated cooling and heating capacities of 8.73 kW and 9 kW, respectively, corresponding to electrical input powers of 2.11 kW and 1.89 kW for cooling and heating operation.
The research team simulated AWHP operation under projected conditions for 2030 and 2050.
Image: University of Trento, Energy, CC BY 4.0
The systems were simulated for installation in a single-family residential dwelling with a total heated floor area of 140 m², divided into four thermal zones. The houses were assumed to be located in five municipalities in the Italian province of Trentino: Arco, Trento, Storo, Cavedine, and Cles. These municipalities span different elevations of 83 m, 185 m, 384 m, 549 m, and 652 m, respectively.
The base climate year was 2016, with future weather data generated using the Test Reference Years model. Between 2016 and 2030, the projected increase in average air temperature ranges from 0.48 C in Arco to 0.6 C in Cles. By 2050, warming becomes more pronounced, with increases of 1.24 C in Arco and 1.50 C in Cles.
“One of the most surprising findings is that start-up and defrosting effects are not minor corrections,” said Eze. “Start-up alone reduces the seasonal coefficient of performance (SCOP) by about 4% to 6%, while the combined effects of start-up and defrosting reduce SCOP by 9% to 11% and increase annual primary energy consumption by up to 12%.”
The researchers explained that, although future climate change reduces defrosting frequency, start-up losses remain significant because part-load cycling continues. “For example, by 2050, SCOP is expected to increase by about 6%, leading to a reduction in annual primary energy consumption of up to 10%. These outcomes will help inform future technical design of AWHP systems and policy development for building decarbonization in these regions,” Eze said.
All in all, under future climate scenarios, rising temperatures are projected to reduce defrosting frequency, which slightly improves seasonal efficiency. However, this benefit is only partial, as start-up losses driven by part-load cycling persist and continue to affect overall system performance. As a result, while climate change may modestly enhance heat pump efficiency in colder regions such as the Alps, the improvement is limited and does not offset the importance of accurate dynamic modeling.
The findings emphasize the need for more detailed simulation approaches that capture real operating conditions, as well as for careful system design and policy planning to support reliable and efficient electrified heating solutions in a decarbonizing energy system.
The research work was presented in “Performance assessment of air-to-water heat pumps in alpine regions under present and future climate: Impacts of start-up and defrosting cycles,” published in Energy. “Our future work will focus on validating the framework with site-specific monitored data and expanding the analysis to other building types, such as multi-family and commercial buildings,” concluded Eze.
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