Development of a passive solar water heater designed for downward heat transfer

Abstract

Due to regulatory height restrictions, lowering the installation height of passive solar water heating systems by lowering the water tank is an attractive proposition. Conventional passive systems use the thermosyphon principle to transfer heat upwards to the water tank. A review of available literature identified possible mechanisms to passively transfer heat downwards. A design for a solar water heater is proposed based on the periodic two-phase thermosyphon concept. The working principle of the proposed system is outlined, using the natural cycle of solar radiation as the periodic input driving its operation. A comprehensive theoretical framework is laid out, which is then used to develop a numerical model. The numerical model is tested for varying elevation differences between the collector and water tank, with and without auxiliary heating. Furthermore, the system’s periodic stability is tested by introducing perturbations and analysing the system’s response. A full-scale setup is constructed to experimentally test the system in real conditions using commercially available flat-plate collectors and a water tank. The results from the numerical model show that the system can function periodically— however, it is susceptible to stagnating when it experiences changes to ideal operating conditions. This effect is exacerbated with increasing elevation differences, with elevations over 1.5 m requiring auxiliary heating to sustain periodicity. Restarting a system from a stagnated state required the use of additional heating or pumping, negating the benefits of the proposed system. The experimental results showed good qualitative agreement with the numerical model, supporting its use as a predictive tool for the system. Periodicity could not be attained experimentally due to leaks compromising system performance.