Effect of Fresnel lens as cover in a passive solar water heater with some air gap between cover and absorber plate

A passive solar heater can be used to heat air in a room or water for some household needs. Passive solar heater can reduce the energy consumption by converting solar radiation energy to be thermal energy. Basically, a simple passive solar heater consists of absorber plate, thermal insulation, working fluid, cover, and structure to hold everything. This paper will
discuss about the effect of Fresnel lens used as the cover and the air gap between cover and absorber plate on a passive solar water heater. The results of the experiment are that the commercial Fresnel lens could not gave the highest water temperature nor efficiency for passive solar water heater. The highest efficiency with glass cover and five-cm air gap was 22.8%. The smaller the air gap, the higher the water temperature in a passive solar water heater.
The highest water temperature with glass cover and five-cm air gap was 63.8C. The result of this experiment gives new challenge for the next research: we do the experiment in a solar water heater where water is circulated in a pipe and design until manufacture the new Fresnel lens.