Development of a Thermal Model for Photovoltaic Modules and Analysis of NOCT Guidelines

Abstract

The temperature of a photovoltaic module is typically required as an input to models that predict the module’s performance. Some common models use the nominal operating cell temperature (NOCT), as by the manufacturer. This paper develops a thermal model and uses it to analyze NOCT testing standards. Specifically, the standard correction factor charts found in the ASTM E1036 and IEC 61215 standards are evaluated. Results show that the correction charts were likely created assuming laminar flow correlations, while validation efforts and the fact that wind is often characterized by turbulence even at low wind speeds suggest that turbulent flow models may be more appropriate. In addition, the results presented in this paper show that the standard NOCT charts do not account for the backside insulation of photovoltaic (PV) arrays. These results suggest that the standard correction charts are inaccurate for any mounting types that differ from the open rack configuration. The paper concludes with recommendations to improve the usefulness of the NOCT.