Minimising the Risk of Water Hammer and Other Problems at the Beginning of Stagnation of Solar Thermal Plants – a Theoretical Approach

Solar plants are increasingly used not only for hot tap water heating but also for the assistance of space heating. These plants produce much more energy in summer than needed, which often results in stagnation. Because of stagnation-temperatures of today’s selective collectors up to 200°C, the collector fluid evaporates. In several plants a high noise level and a vibration of the plant during this evaporation phase is reported. This is due to the occurrence of water hammer in the system, when liquid collector fluid passes areas where the fluid was already evaporated and superheated. The remaining vapor bubbles deflate rapidly and the liquid phases collide with high velocity, which results in a rapid pressure increase. This paper describes the theory of condensate-induced water hammer and conditions of solar plants, under which this can happen. A simulation model for the evaporation phase of the collector is presented to give a deeper understanding about the influences of solar radiation, the size of the tubing, and the size of the expansion device on the process of the evaporation. Three hydraulic layouts of the collector array are discussed for their potential to produce water hammer.