Influence of sizing strategy and control rules on the energy saving potential of heat pump hybrid systems in a residential building

Hybrid heat pump systems are a suitable solution to mitigate the well-known disadvantages of air-source heat pumps, such as energy losses linked to defrost cycles and a significant reduction of their heating capacity during the most severe part of the season. In hybrid systems the heat pump is sized to satisfy only a fraction of the building peak load and is coupled to a second heater (back-up device), which helps the heat pump during the coldest part of the winter. In this paper, a series of dynamic simulations has been performed to calculate the seasonal performance of hybrid systems based on an air-to-water heat pump and to assess the optimal configuration of the system. Results point out that the energy performance of these systems strongly depends on the heat pump sizing, the back-up device typology and the control algorithm used for the activation of the heat generators. It is demonstrated that the adoption of hybrid systems in which the heat pump is coupled to a gas boiler allows to obtain relevant primary energy savings. The overall seasonal efficiency can be increased up to 6% and 22%, if compared to monovalent systems respectively based on a heat pump or a gas boiler, only if the heaters are activated following an alternative operating mode, with a cut-off temperature selected between the design and the bivalent temperature. On the contrary, if the back-up device of the hybrid system is an electric resistance, the heaters have to work in parallel during the whole heating season and the only achievable advantage is that the heat pump can be slightly under-sized with respect to the nominal building load.