Simulation and performance analysis of combined parallel solar thermal and ground or air source heat pump systems

The combination of solar thermal (ST) systems and heat pumps (HP) is a promising hybrid heating system concept for efficient energy supply of buildings. In this work, a simulation study on the performance of combined ST and HP systems and a comparison with systems without ST collectors for different types of buildings and climates is presented. With regard to the dominating market-available systems, the evaluation is limited on systems with parallel integration of ST collectors. For a better transparency of the results, the simulation models and the main model parameterization are explained in detail. The results of the simulations show that the performance of HP systems can significantly be increased by adding a ST system. The relative increase of the performance decreases with increasing space heating demand of the building and is lower for climates with low ambient temperatures. In contrast, in most cases the absolute electricity savings increase with increasing space heating demand and are higher for systems with air source HPs. The seasonal performance factor (SPF) of parallel solar thermal and ground source heat pump (SGSHP-P) systems is between 0.5-1.1 (Strasbourg) and 1.0-2.0 (Helsinki) higher than the SPF of parallel solar thermal and air source heat pump (SASHP-P) systems with the same ST collector area. Hence, the difference between the performance of SASHP-P and SGSHP-P systems increases with increasing heat load and decreasing ambient temperatures of the location. However, in case of moderate climates like Strasbourg, SASHP-P systems can achieve the same or even higher values of SPF than ground source heat pump (GSHP) systems without ST collectors. Especially for buildings with low energy demand for space heating, the SPF is higher (up to 78.5% with a collector area of 25 m2) and the systems can compete with each other. In a nutshell, in particular if the investment costs of a borehole heat exchanger and a ST system are similar or if there is no possibility to use a GSHP system with borehole heat exchanger, using a SASHP-P system may be a useful opportunity for a heating system concept with high energy performance and low electricity demand.