Overheating prevention in large-scale solar thermal systems based on heat pipe collectors

Solar thermal systems can efficiently provide renewable heat but are often affected by overheating and stagnation events, particularly during periods of low heat demand. This study investigates the use of temperature-limiting heat pipe collectors with a maximum collector temperature of 135 °C (HP135) as a solution to prevent overheating in large-scale installations. We developed a TRNSYS model of a solar thermal system supplying a district heating network with a representative solar fraction of 18 % to compare both the stagnation behavior and the annual solar yield of the HP135 configuration to a conventional heat pipe collector system without temperature limitation (unlimited). The results show that the HP135 configuration reduces the solar gain in the range from 0.9 % up to 2.6 %, in dependency of the applied heat load profile. At the same time, 18 to 28 stagnation days are avoided. Furthermore, collector field oversizing can be realized without stagnation-related operating restrictions. When doubling the collector area, solar gain increases by 62 % for both collector configurations, while the temperature-limited configuration avoids more than 40 stagnation days. Overall, temperature-limiting collectors improve the operational reliability of large-scale solar thermal systems without the need for additional components or control strategies.