This study evaluates how the assumed temperature of water supplied from the mains affects the energy and economic performance of a residential solar thermal system for domestic hot water preparation. The system consists of flat plate solar collectors connected to a storage tank through a lower internal heat exchanger and a natural gas boiler supplying heat through an upper internal heat exchanger. A numerical model was developed and intrinsically validated using manufacturer data and then simulated in transient conditions in TRNSYS environment. Two approaches were compared: a time-varying mains water temperature profile and a constant value equal to the mean ambient temperature. Simulations were performed for thirty four European capitals using typical meteorological data and national household gas prices. Results show that constant assumptions can overestimate annual savings and distort solar contribution indicators across climates. The study reveals that the mean difference in operation cost between the time-varying and constant mains water temperature cases among the considered locations was +36.6% for the proposed system.
Rafał Figaj (Sun,) studied this question.