IMPROVEMENT OF THE PERFORMANCE OF PVT COLLECTORS

In a photovoltaic-thermal (PVT) collector one tries to collect heat from solar cells not used in the photovoltaic conversion process, as good as possible e.g. for heating of tap water. The thermal efficiency of the current PVT collector designs is on the one hand limited because of the limited absorption factor (around 85% compared to 95% for a black absorber) and on the other hand by the relatively high emissivity (85% compared to 12% for a spectrally selective coated absorber) of encapsulated solar cells. This paper deals with the investigation of the possibilities to improve the annual electrical and thermal yield of systems with PVT collectors by the application of anti-reflective (AR) coatings and lowemissivity (low-e) coatings. The investigation was carried out using an advanced optical model for the absorption factor of solar cells together with a system simulation program. Both a system for Solar Domestic Hot Water (SDHW) heating and for combined SDHW and Room Heating (RH) were considered. It turns out that application of AR coatings in the case of covered PVT collectors is quite favorable for both the annual thermal efficiency and the annual electrical efficiency. The gain in the annual thermal efficiency can be up to 5% relative, whereas the gain in annual electrical efficiency can be up to 10% relative. The application of a low-e coating is favorable for the annual thermal efficiency but reduces the annual electrical efficiency. Though this reduction can be limited by application of an AR coating on top of the low-e coating, the remaining reduction (up to 10%) is too large to justify application of a low-e coating in many cases.