Temperatures of building applied photovoltaic (BAPV) modules: Air gap effects

Building applied photovoltaics (BAPV) is a major application sector for photovoltaics (PV). Due to the negative temperature coefficient of power output, the performance of a PV module decreases as the temperature of the module increases. In hot climatic conditions like Arizona, the BAPV module temperature can reach as high as 90-95°C during peak summer. Considering a typical 0.5%/°C power drop for crystalline silicon modules, about 30% performance drop would be expected during peak summer because of the difference between rated temperature (25°C) and operating temperature (∼90°C) of the modules. In order to predict the performance of PV modules, it becomes necessary to predict the module temperature. The module temperature is dictated by air gap between module and roof surface, irradiance, ambient temperature, wind speed, and wind direction. Based on the temperature and weather data collected over a year in Arizona, a mathematical thermal model has been developed and presented in this paper to predict module temperature for five different air gaps (0, 1, 2, 3 and 4 inches) as well as modules with a thermally insulated (R30) back. The thermally insulated back is expected to serve as the worst case temperature a BAPV module could ever experience. This paper also provides key technical details on: the specially built simulated rooftop structure; mounting configuration of PV modules on the rooftop structure; LabVIEW program developed for data acquisition; and a data processing program for an easy data analysis.