Optimization of systems with the combination of ground-source heat pump and solar collectors in dwellings

The use of ground-source heat pumps for heating and domestic hot water in dwellings is increasing rapidly in Sweden. The heat pump extracts heat from the ground by a borehole, normally between 80-180 m deep. 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. 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 is when the solar heat may be used directly for domestic hot water during summertime and used for recharging 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 may increase with solar recharging, because of the increased operating time of the circulation pumps. Another advantage with solar heat in combination with heat pumps is when the boreholes or neighbouring installations 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 recharge from solar heat.