PVT collector technologies in solar thermal systems: A systematic assessment of electrical and thermal yields with the novel characteristic temperature approach

Photovoltaic-thermal PVT collectors generate solar electricity and heat in a single component. The annual electrical and thermal yields depend in particular on the efficiency of the PVT collector technology, climate, and the type of solar thermal system. This paper provides a systematic comparison of the performance of unglazed, glazed, and PVT collectors with low-emissivity coatings. The yields of these PVT technologies are assessed in four types of solar thermal systems including domestic hot water systems and combi systems in single and multi-family homes at four European locations. For this purpose, an empirical PVT performance model is presented, validated and implemented into TRNSYS. Analyzing the simulation results, the assessed PVT collector technologies reach either high electrical or high thermal yields. This compromise originates from the technological trade-off between a good optical efficiency versus a high level of insulation. Thermally optimized glazed PVT collectors with low-emissivity coatings achieve the highest overall yields and the best energetic utilization of the collector area. These collectors achieve up to three times the electrical output compared to a side-by-side installation of flat plate collectors and PV modules with equal thermal output. The variations between the investigated systems can be attributed to the governing operating temperatures of the PVT collector which are found to be of central relevance. A novel approach puts these temperatures into the focus and introduces the characteristic temperature Tchar as new indicator with a strong correlation to both electrical and thermal yields. This approach can be used to assess electrical and thermal yields of PVT systems and select a suitable collector technology for a given application.