Comparative Study for Time-specific Ross Coefficient and Overall Ross Coefficient for Estimation of Photovoltaic Module Temperature

Ross coefficient is a popular method used to estimate a photovoltaic (PV) module temperature. Generally, it is related to the PV module temperature, ambient temperature and solar irradiance. However, the overall Ross coefficient, which is usually obtained through a large quantity of data for a long period of measurement, can create a significant error in the temperature estimation. This paper introduces a method to obtain a number of specific Ross coefficients at different time of a day, and it is denoted as time-specific Ross coefficient. Two PV modules mounted on two types of mounting structures located at two different sites were used to generate the required data for the characteristic analysis. The values of the coefficients were also compared to that of overall Ross coefficient. It is found that time-specific Ross coefficient indeed varies significantly over time, deviated far from the overall Ross coefficient. For metal-deck roof case, the time-specific Ross coefficient varies from 0.004 °C/(W/m
²
) to 0.032 °C/(W/m
²
) while the overall 50-day Ross coefficient is 0.0286 °C/(W/m
²
). The patterns of variation are explained here. It is also found that the y-intercept points of the regression lines of the datasets show clear variation across time, as high as 12 °C in the noontime. This y-intercept point should be included with the time-specific coefficients for the module temperature-irradiance model. Finally, an evaluation of both types of Ross coefficients with the measured PV module temperature proves that time-specific Ross coefficients can give a higher accuracy in predicting the module temperature.