On the influence of hydronic distribution loop on energy performance and indoor thermal comfort for air-to-water heat pump systems in residential buildings

Air-to-water heat pumps (AWHPs) are an efficient technology to provide thermal energy for space heating and domestic hot water production in residential buildings, which cover about 30% of the Italian overall energy consumption. Despite of a series of benefits, such as low investment costs and large availability of the external source (ambient air), airsource heat pumps are affected by several drawbacks, the most important of which is the frosting phenomena. Nowadays, the most widespread defrosting technique is reverse-cycle defrost (RCD): the indoor heat exchanger of the AWHP operates as an evaporator, extracting thermal energy from the heated space to melt the ice layer. For this reason, during the defrosting process no heating is provided and the indoor air temperature may significantly drop; therefore, the thermal comfort of building occupants is negatively affected. Very few studies demonstrating that the correct design of the hydronic distribution loop can mitigate the adverse effect of defrosting cycles on indoor thermal comfort can be found in literature; hence, the aim of this paper is to optimize position and size of a thermal storage tank inserted within the hydronic loop in order to minimize the impact of defrosting cycles on the heating system performance. This work analyses the performance of an inverter-driven AWHP coupled to a residential building located in Bologna (Northern Italy) by means of a simulation model developed with TRNSYS; furthermore, the heat pump dynamic model takes into account the reverse cycle operating mode during defrosting cycles and the energy losses linked to on-off cycling. Results show that defrosting cycles have a negative, significant impact on the seasonal performance factor of the system, which is reduced up to 10% with respect to an ideal case where frosting phenomena is not considered. Furthermore, defrosting transients cause a relevant decrease of the indoor air temperature (about 1°C), linked to the worsening of indoor thermal comfort conditions, in systems characterized by low thermal inertia (water volume lower than 5 l/kW); on the contrary, the influence of defrosting cycles on indoor conditions can be considered as negligible for a water content in the distribution loop larger than 10 l/kW, especially if the thermal storage is placed on the supply loop (i.e. between the heat pump and the terminal units).