Photovoltaic (PV) modules typically operate at approximately 30 °C above ambient temperature on clear sunny days, irrespective of their location. Since the average annual daytime temperature is typically higher than 20 °C in most locations, where PV modules are installed, operating temperatures can exceed 50 °C on clear sunny days. This translates to a 12% reduction in nameplate power for crystalline silicon modules. In addition, thermally induced degradation mechanisms have a higher probability of occurrence when operating temperatures increase, thereby reducing the module lifetime. The operating temperatures are impacted by the selection of packaging materials, e.g., backsheets and encapsulants. This paper demonstrates a significant reduction in the operating temperature of single-cell modules with innovative thermally conductive backsheet (TCB) materials vis-à-vis a baseline Tedlar/polyester/Tedlar (TPT) backsheet. Field results demonstrate that the nominal operating cell temperature of the TCB coupons is approximately 1 °C lower than those of conventional TPT coupons. The daily average module operating temperature of TCB coupons was as much as 3 °C cooler compared with the TPT coupons in summer months. Reducing the module temperature by 3 °C results in a 1.5% relative efficiency increase. Finally, an empirical thermal model to predict the cell temperature for each backsheet type and a physical thermal model using ANSYS were developed and presented in this paper.

Encapsulant discoloration is a well-known field degradation mode of crystalline-silicon photovoltaic modules, particularly in the hot climate zones. The discoloration rate is influenced by several weathering factors, such as UV light, module temperature, and humidity, as well as the permeability of oxygen into the module. In this article, a rate dependence model employing the modified Arrhenius equations to predict the degradation rate for encapsulant discoloration in different climates is presented. Two modeling approaches are introduced, which utilize the field and accelerated UV testing degradation data in conjunction with the field meteorological data to determine the acceleration factor for encapsulant browning. A novel method of accelerated UV stress testing at three simultaneous module temperatures in a single environmental chamber test run is implemented to estimate the activation energy for browning. The test was performed on three field-retrieved modules to capture the wear-out failure mechanism. The degradation in short-circuit current Isc rather than maximum power is used as a decisive parameter for the discoloration analysis. Furthermore, the developed model has been used to predict the Isc degradation rate for the Arizona field characterized by a hot and dry climate and is validated against the field-measured value. It has also been applied to other climate types, e.g., the cold and dry climate of New York.

The operating temperatures of photovoltaic (PV) modules can be impacted by the selection of specific packaging materials, e.g., backsheets and encapsulants. This research focuses on the evaluation of operating temperature reduction of one-cell modules by comparing conventional Tedlar/polyester/Tedlar (TPT) backsheet with novel thermally conductive backsheets (TCBs) materials. A large number of one-cell modules with two TCB types (TCB_A and TCB_B) and baseline TPT type were fabricated and installed in three different climatic conditions of the hot-dry desert in Arizona (high and low wind speed locations) and North Carolina (temperate with low wind speed location). In this study, these two TCBs were compared with conventional TPT backsheet in terms of performance, lifetime and levelized cost of energy (LCOE). The field results were analyzed for thermal performance of TCBs compared to TPT at three sites for two and half years. This study concludes that the thermal and electrical performances of the PV modules can be improved by using TCB_A in hot and dry climate sites and TCB_B at temperate climate sites. Therefore, the lifetime of TCB-based modules is expected to be higher than TPT-based modules. Using backsheet-specific power degradation levels and assuming the same cost for both types of backsheets, the LCOE of modules using TCBs is estimated to be lower than that of TPT.

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.

One of the major defects that can cause significant power loss is the degradation of interconnect metallization system (IMS) comprised of cell metallization and solder bonds of cell and string interconnects. Weak cell interconnect solder bonds between copper ribbon and busbar of cells result in series resistance increase which affects the fill factor causing a power drop. In this paper, the results obtained from series resistance and peel test experiments performed on the cells extracted from modules exposed in three different climates (Arizona - Hot and Dry, Mexico - Warm and Humid, and California - Temperate) for more than 18 years are presented. Finally, climate specific thermal modelling was performed for those sites over 20 years to calculate the accumulated thermal fatigue and to evaluate its correlation, if any, with series resistance.