Module temperature model

A module temperature model estimates how hot a PV module runs from local conditions: irradiance on the module, ambient air temperature, wind speed and sometimes wind direction. Designers need it to predict annual energy production, using hourly irradiance, temperature and wind data from weather databases or models. The Sandia empirical model predicts back-surface module temperature with two fitted coefficients. Coefficient a sets the upper temperature limit at low wind and high irradiance, and coefficient b sets how fast temperature falls as wind increases. The coefficients depend on module construction, mounting style (open rack, close roof mount or insulated back) and where wind is measured, which by standard practice is at a height of 10 m. Cell temperature is found by adding a temperature difference, scaled by irradiance, to the back-surface temperature. This difference is typically 2 to 3 °C for open-rack flat-plate modules and zero for modules with an insulated back. The simple Sandia model is accurate to about ±5°C, which changes predicted power by less than 3%. Other common models are the Ross model, which uses ambient temperature and irradiance, and the Faiman model, which adds cooling by wind. Module temperature matters for service life because it speeds up most degradation processes. After installation, measured module temperature, ambient temperature and wind speed can be used to refit the model for the site. The fitted model then supports temperature normalization, expected-yield calculations and the temperature-corrected performance ratio.