Stagnation in Large-Scale Solar Thermal Systems? Experiences From Recent Installations as Motivation for Heat Pipe Collectors in a Novel System Design

Large-scale solar thermal systems supplying heating networks can make a significant contribution to the decarbonization of the heating sector and are becoming increasingly relevant in practice. The first part of the paper analyses 30 realized systems in Germany focusing on collector area, heat storage capacity and stagnation events. More than 60 % of these systems have heat storage capacities smaller than 100 l/m² collector area. Nevertheless, a third of the systems reaches an annual solar fraction of over 20 %. Our investigations for such dimensioned systems report a number of 20 to 43 stagnation days per year, depending on the storage capacity. The relevance of stagnation load motivates our research activities to avoid its negative consequences in large-scale systems. The second part of the paper describes the development of evacuated tube collectors with inherent overheating prevention as well as the outdoor tests we carried out with various prototypes at our sun tracker. The collector concepts are based on heat pipes with temperature limitation to 125-135 °C in the solar circuit and are therefore able to avoid evaporation of the heat transfer medium. Due to lower stagnation loads, we investigate the possibility of using polymeric pipes instead of steel pipes for the solar circuit and discuss an integrated system concept. The innovative collector and system design are implemented in a demonstration plant that we will monitor during the construction phase and in operation.