Thermal performance of a trapezoidal-shaped solar collector/energy store

A simple, low-cost solar water heater has been developed for operation in Mediterranean Europe or regions of similar latitude (40-45° north). It takes the form of a trapezoidal-shaped water store in direct contact with an inclined flat-plate solar collector assembly. This cross-section induces thermal stratification in the water store, and provides sufficient energy storage to meet typical daily hot-water demand. Its thermal performance is critically dependent on the waterside convective heat-transfer coefficient on the backward-reclining collector plate; previously evaluated by Cruz et al. (Cruz JMS, Hammond GP, Reis AJPS. Buoyancy-driven convective heat exchange in a trapezoidal-shaped solar collector/thermal store. In: Proc. of the 5th ASME/JSME Joint Thermal Engineering Conf., San Diego, CA (ASME, New York), 1999, 9 pp). In the current design, the absorber plate inclination to the horizontal was fixed at 45° (close to the local latitude) in order to yield maximum solar gain over a typical year. The energy saving provided by the solar collector/thermal store demonstrator largely depends on the amount of thermal stratification within the trapezoidal storage cavity. This was evaluated via both computation and measurements of the temperature field. A thermal network analysis model was then used to assess the energy-saving potential of the composite system. It indicated that a 30-70% reduction in daily load could be obtained in contrast to direct, electrical or gas, heating: the smaller saving occurred at times of greatest use or hot-water take-off.