Development and modelling of highly-efficient PVT collectors with low-emissivity coatings

Photovoltaic-thermal (PVT) collectors are hybrid collectors which make, in principle, optimal use of the solar resource by co-generating electricity and heat in a single module. Spectrally selective but transparent low-emissivity (low-e) coatings are a suitable measure to reduce thermal losses. In this paper the development of PVT collectors with low-e coatings by means of experiment and simulation is presented. A highly transparent low-e coating based on silver, specifically optimized for the application in PVT collectors, is developed. By combining layers of low-refractive SiO2 and high-refractive TiO2 a low emissivity of ɛ373K = 0.13 is achieved while maintaining high solar transmittance of τAM1.5 = 0.79. This coating is applied in a glazed, flat plate, liquid-type PVT collector. Thermal and electrical performance were tested and compared to a PVT collector with the same design yet without low-e coating. A numerical model of the PVT collector is presented showing good agreement of simulation and experiment. With this model the impact of low-e coatings on thermal and electrical performance is analysed and discussed. The newly developed low-e coating reduces the collector heat losses by 82% while the electrical efficiency drops by only 3%.