Evaluating the efficiency, economics, and environmental impact of hybrid heat pumps assisted by biomass boilers for Spanish climate zones

Hybrid air-water heat pump systems combine the efficiency of the former with the boiler backup system when external conditions are unsuitable. For the rural domestic sector, the use of biomass boilers as backup also allows the sustainable use of local natural resources. The appropriate choice of size for both units is a complex decision that depends not only on demand and climate but also on other parameters, such as the storage tank and the control strategy. Specifically with biomass boilers, this choice has not been sufficiently addressed in the scientific literature. For this reason, a dynamic model of this hybrid scheme has been created, the analysis of which has been carried out with energy, environmental and economic indicators. This model has been applied to a single-family home in three areas in Spain with a higher demand for heating than cooling and with different efficiency standards, comparing different sizes of inverter heat pump with commercial features to cover the required demand together with the boiler. With all the reservations given by the variability of the case studies analyzed, it can be stated that the approximate value of a heat pump with a nominal capacity of 60–70 % of the peak demand of the house is the most cost-effective option as it obtains the best combined seasonal performance, reduced environmental impact, lower operating costs and in its life cycle. With this design, relevant participation of the boiler is assured, contributing to low unitary emissions and allowing the heat pump to operate at its best efficiency, also reducing its number of starts and stops. Parameters such as greater storage or a lower pump stop temperature also improve its performance, within limits close to the reference situation for Spain, also considering its prices for biomass and electricity consumed.