Solar Colletors Combined with Ground-Source Heat Pumps in Dwellings - Analyses of System Performance

Abstract: The use of ground-source heat pumps for heating buildings and domestic hot water in dwellings is increasing rapidly in Sweden. The heat pump extracts heat from the ground by a U-pipe in a vertical borehole. In order to reduce the electricity demand in the system, the combination with solar collectors is introduced. This system may be designed in many ways and the advantages differ a lot. Solar heat can also be used for recharging of boreholes when neighbouring boreholes are thermally influencing each other. In order to analyze different systems with combinations of solar collectors and ground source heat pumps, computer simulations have been carried out with the simulation program TRNSYS. Large differences were found between the systems. The optimal design in a new system is to use solar heat is directly for domestic hot water during summertime and (depending on the depth of the borehole) also for recharging of the borehole during wintertime. The advantage is related to the rate of heat extraction from the borehole as well as the whole design of the system. The demand of electricity for the circulation pumps increase with solar recharging, because of the increased operating time. For new, highly efficient circulation pumps this disadvantage decreases and longer operating times for recharging can be accepted. In existing ground-source heat pump systems the advantage with solar heat in the system depends on the system design. In systems with undersized boreholes the advantage with recharging the borehole with solar heat is large. The optimal system is when solar heat is used directly for domestic hot water during summertime and for recharging of the borehole during wintertime, but for extremely short boreholes, recharging all solar heat is the optimal system. Solar heat in combination with ground-source heat pumps gives an advantage when the neighbouring boreholes are drilled so close that they are thermally influencing each other. This may lead to decreasing temperatures in the ground, which gives decreased performance of the heat pump and increased use of electricity. The net annual heat extraction from the ground is reduced by recharging with solar heat.