abstract
The district heating sector in Germany needs to be supplied increasingly from renewable sources. Large-scale solar thermal systems can make a significant contribution to this process. The paper analyzes 38 concepts of feasibility studies as well as 30 realized systems by investigating relevant designing parameters. Network supply temperatures of around 75 °C and 80 °C are very common for the feed-in of solar thermal energy. The achievable solar fraction is strongly dependent on the storage capacity. A doubling of solar fraction typically requires a tenfold increase in storage volume. The installed storage capacity is often smaller than 100 l/m2. Large seasonal storages, as usual in Denmark, are very seldom in Germany so far. Thus, stagnation events become more relevant in large-scale systems, as we report from the realized projects. Systems with solar fractions of around 20%, which are typical for district heating networks especially in countryside regions, achieve up to 50 stag-nation days per year, a representative stagnation day is analyzed for one monitored system. Finally, the paper addresses possible causes of stagnation and discusses a prevention strategy by using heat pipe collectors with inherent temperature limitation.