A novel solar thermal and ground source heat pump system that harnesses the excess heat of the collectors during summer by an Organic Rankine Cycle (ORC) is simulated. For the ORC the heat pump process is reversed. In this case the scroll compressor of the heat pump runs as a scroll expander and the working fluid is condensed in the ground heat exchanger. Compared to a conventional solar thermal system the only additional investments for the combined system are a pump, valves and upgraded controls. The goal of the study is to simulate and optimize such a system. A brief overview of the applied models and the evolutionary algorithm for the optimization is given. A system with 12 m(2) of flat plate collectors installed in a single family house is simulated for the locations Ankara, Denver and Bochum. The ORC benefits add up to 20-140 kWh/a, which reduces the net electricity demand of the system by 1-9%. Overall 180-520 (sic) are saved over a period of 20 years, which can be enough to cover the additional investments. (c) 2015 Elsevier Ltd. All rights reserved.