# Thermal Time Constant of PV Roof Tiles Working under Different Conditions  
[PERSON]  
1 Faculty of Electrical Engineering, Institute of Electrical Engineering and Electronics, Poznan University of Technology, st. Piotrowo 3a, 60-965 Poznan, Poland; [EMAIL_ADDRESS]  
1 Faculty of Electrical Engineering, Institute of Electrical Engineering and Electronics, Poznan University of Technology, st. Piotrowo 3a, 60-965 Poznan, Poland; ryszard. The dimensions of PV roof tiles are selected in such a way that they can be fitted in between traditional roof covering items (i.e., ceramic tiles or bitumen covering). The standard height of PV roof tiles is ca. 40 cm, while their width ranges from several centimetres to several metres. They replace several ceramic tiles or a row or top layer of asphalt roofing felt. The smaller the item, the easier it is to fill the whole roof with PV elements, which increases the cost of a PV system compared to larger roof tiles [4; 5]. PV cells in photovoltaic roof tiles are exposed to more severe working conditions compared to traditional PV panels installed on a supporting structure several centimetres above the roof surface. The lack of natural air movement (wind) on the back of the PV module, which reduces the temperature of PV cells, contributes to a lower power-generating efficiency of solar roof tiles. The heat released by the bottom side of the roof tiles is accumulated in the air gap between the bottom surface of the roof and the attic insulating layer in the case of rigid PV roof tiles with construction similar to the construction of traditional PV panels or PV ceramic roof tiles, or it may be transferred further into the roof structure as in the case of elastic PV roof tiles fixed (glued) to the roof construction (e.g., on the top layer of asphalt roofing felt and boards). The degree and rate of heat exchange from the PV roof tile to the roof, environment, or attic depends on the type of PV roof tile, its installation, the particular insulation materials of the roof and attic use. The bottom side of PV roof tiles can only be cooled and vented by air in the spaces between roof slats. The authors in [6; 7] indicate a lack of confirmatory data concerning the efficiency of PV roof tiles, which prevents the popularisation of this technology. The authors in [8; 9; 10] present different studies on the productivity and energy loss quantities in building-integrated photovoltaic (BIPV) systems composed of different photovoltaic items (roof tiles, glass panes, shutters, wall-mounted items etc.). A number of studies have been carried out on methods of PV cell ventilation (when the cells are integrated with a roof or wall), heat removal and their use and impact on the energy efficiency of the system. In [11; 12] it was revealed that adding a ventilating duct of a relevant size behind PV items increases their energy-generation efficiency. [PERSON] [13] demonstrated that the optimum height of the gap between the panel and the roof is about 0.125 m, regardless of the inclination angle. The authors used CFD modelling to present the results of studies for different PV modules' installation parameters and scenarios in [14; 15; 16]. In order to analyse the temperature changes of PV cells, [PERSON] et al. [7] examined panels installed on a roof with an air gap (standard installation on a supporting structure), built-in in a cold roof (with no thermal insulation), built-in in a hot roof (with mineral wool insulation) and installed on a roof with American-style shingles. Additionally, for comparison, a panel was fixed to a structure which enabled its free cooling by the wind, and a weather station was installed to collect data on insolation, ambient temperature, wind force and precipitation in the UK. The author demonstrated that PV cells installed on American-style wood shingles reached the highest temperatures because heat exchange in their case was most difficult, and the heat released at the back of the panel heated the PV cells further [PERSON] et al.