Effect of the choice of the balance point on the mean seasonal COP of electric air-to-water heat pumps without inverter

A MATLAB numerical code to determine the effects of the choice of the balance point and of the thermal storage (puffer) volume on the mean seasonal COP of an on-off electric air-to-water heat pump is presented. The heat pump is used to heat a building and is provided with a puffer and a back-up system (electrical resistances). The code employs as main input parameters: the hourly values of the external air temperature, the heat loss coefficient of the building, the volume and the imposed maximum and minimum temperature of the storage water, the technical data of the heat pump as functions of the source temperatures. The code evaluates the hourly values of the energy lost by the building, of the average temperature of the water in the puffer, of the energy supplied to the building by the heat pump and by the back-up system. By varying the selected values of the balance point and of the storage volume, it is possible to calculate their optimal values in order to maximize the seasonal COP. The results show that a correct choice of the balance point yields a significant increase of the mean seasonal COP of the system, while the effect of the puffer volume is less considerable.