Simulations of combined solar thermal and heat pump systems for domestic hot water and space heating
The system combination of solar thermal collectors and heat pumps is a very attractive option for increasing the renewable energy usage at worldwide level for heating and domestic hot water preparation. In this work parallel and series combined solar and heat pump systems are analyzed within the IEA SHC Task44/ HPP Annex38 reference conditions for different buildings and a typical Central European climate. Three combined systems have been studied in detail: solar and air source heat pump, solar and ground source heat pump, and exclusively solar source heat pump in combination with an ice storage. Numerical calculations have been performed using two simulation platforms: TRNSYS-17 and Polysun-6®. Comparisons between the two simulation environments have been also provided. Moreover, a reference case without solar has been used to determine the potential efficiency benefits of using solar collectors compared to a system with a heat pump alone in the specified climate and for covering the specified heat load. Simulations presented in this work show that differences in system performance up to 4% can be expected between TRNSYS-17 and Polysun-6® for air source based systems, and higher discrepancies, up to 14% are obtained for ground source based systems. Comparisons between combined solar thermal and heat pump systems with their respective "heat pump only" reference solutions show that the absolute electricity savings of air source are usually higher compared to ground source based systems. Systems using large ice storages are able to reach seasonal performance factors in the range of 5, which is of the order of performance of combined solar and ground source heat pump systems.