Optimised thermosyphon solar hot water heater — simulation, design and experimental analysis

Thermosyphon systems represent 70–80 % of the solar-thermal capacity installed world-wide. Outside of China — the world’s largest thermosyphon system market — thermosyphon systems using flat-plate collectors are the dominant configuration. In Southern Europe conventionally-designed thermosyphon systems with a collector area of about 2.0 m² and a storage tank of 180 l are able to deliver about 70 % of the annually needed hot water of a four person household. However, research into the performance of thermosyphon systems has stagnated in recent years, hence the primary objective of this research was to evaluate in detail the physical factors affecting the performance of flat plate thermosyphon systems hence pointing the way towards improved performance. The investigation consisted of the implementation of a detailed dynamic system simulation, which included the development of new component models, the validation of the new component models via laboratory testing, the use of the system model in a sensitivity analysis of the significance of individual component performance and finally the testing of an improved prototype thermosyphon system. Eighteen different geometrical and physical parameters were investigated in the sensitivity analysis to find the most significant design factors. The sensitivity analysis incorporated three different European locations, but the dependency of the optimal thermosyphon system configuration on the location was found to be rather weak. The research concluded that a performance improvement from 70 % to 85 % of the hot water demand of a four-person household could be achieved with an improved system configuration. The improved system prototype addressed cost reduction by the material selection for the storage tank and its built-in components as well as the selection of aluminium for the solar absorber and the use of prefabricated parts for the system support. Reverse thermosyphoning was prevented by adjusting the tank and collector inlet/outlet co-locations, while the availability of hot water was enhanced by improved hydraulic design of the storage tank. Suggestions for further work included the development of more detailed component models and long-term testing of improved system configurations.