Experimental investigation on the performance of propane (R290) air-source heat pumps working in conditions of frost formation

The global push towards net zero carbon emissions increases each year. The Heating Ventilation and Air Conditioning (HVAC) systems are also undergoing a significant transformation because of this push by switching from its traditional heating systems, such as furnaces and boilers, which are typically based on fossil fuels, to heat pumps, which are a more sustainable solution. Among these, the demand for air source heat pump (ASHP) has been increasing because of their relative flexibility in installation compared to other types of heat pumps. In ASHP, heat is extracted from the ambient air and delivered to the building. When the air temperature goes below 0°C, the surface of the outer unit extracting heat from outside can freeze. The formation of frost on the surface of the outdoor heat exchanger affects the overall performance of the system. To address this, defrosting is required to remove the frost from its surface, yet they come at a cost, as each defrost cycle requires energy and temporarily changes from heating to cooling energy supply. Thus, it decreases its energy efficiency and an increase in operational costs. The existing defrosting technologies are energy expensive and causes inconvenience. So, understanding the factors that influence frost formation and optimizing defrost cycles are critical to improving ASHP performance, particularly in colder climates, where frost buildup occurs more frequently. So in the thesis, an experimental analysis has been conducted to investigate the influence of various operating conditions on frost formation at the Cooling \& Heating Test Lab of the Swegon Spa Company. The experimental study examined the effects of operational conditions such as ambient air temperature, Relative Humidity (RH), air velocity to optimize the existing defrosting technology used by Swegon for their heat pump systems by optimization of the time to trigger the defrost and the time interval between the defrosting. By varying these parameters, we aim to analyze their effects on the Coefficient of Performance (COP), heat transfer rate, and the frequency of defrost cycles required under different environmental conditions. The experiments results revealed that low ambient temperature increases the rate of frost accumulation, high relative humidity accelerates frost formation on the heat exchanger surface area, higher air velocity through the heat exchanger decelerates the frost formation speed. Adjusting the defrost cycle interval has a significant impact on energy consumption, system performance, and operational costs. The results of these experiments provide valuable insight into the optimization of the defrosting technology used for ASHP systems under frosting conditions. The data suggest that by fine-tuning parameters such as airflow rate and defrosting intervals, performance of the ASHP can be improved, leading to energy savings. This work highlights potential strategies for minimizing energy losses in ASHP systems, contributing to the development of more resilient, energy-efficient HVAC systems capable of supporting the transition to sustainable heating solutions in alignment with net zero carbon emissions goals.