A technical note on solar thermal applications of semi-transparent liquid films

• new solar collector based on liquid films is proposed • collector was tested experimentally at 25°C and 0.13-0.74 sun • the thermal efficiency of water films is 22% due to evaporation • the thermal efficiency of oil films is 55-85 With over 500 GWth of installed total capacity, solar thermal generation provides approximately one-third of the world’s solar power. However, this important branch is based on the last-century thermal technology, flat plate solar collectors, which demonstrate low efficiency in moderate-to-cold climates. This contribution proposes a concept of a film solar collector (FSC) based on thin semi-transparent liquid layers cooling the collector’s absorbing surface. A prototype collector was tested at 0.13-0.74 sun and air temperatures of ∼25°C using water, an edible oil, and a red-colored oil to enhance the film’s volumetric absorption. Due to evaporation, the water-based FSC has a low thermal efficiency of 20%. The colored oil increased efficiency to 55%, and the semi- transparent oil film collected about 85% of solar heat.