A PV module will be typically rated at 25 °C under 1 kW/m2. However, when operating in the field, they typically operate at higher temperatures and at somewhat lower insolation conditions. In order to determine the power output of the solar cell, it is important to determine the expected operating temperature of the PV module. The Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open circuited cells in a module under the conditions as listed below:
Irradiance on cell surface = 800 W/m2
Air Temperature = 20°C
Wind Velocity = 1 m/s
Mounting = open back side.
The equations for solar radiation and and temperature difference between the module and air show that both conduction and convective losses are linear with incident solar insolation for a given wind speed, provided that the thermal resistance and heat transfer coefficient do not vary strongly with temperature. The NOCT for best case, worst case and average PV modules are shown below. The best case include aluminium fins at the rear of the module for cooling which reduces the thermal resistance and increases the surface area for convection.
The best module operated at a NOCT of 33°C, the worst at 58°C and the typical module at 48°C respectively. An approximate expression for calculating the cell temperature is given by (Ross):
S = insolation in mW/cm2. Module temperature will be lower than this when wind velocity is high, but higher under still conditions.
Module design, including module materials and packing density, have a major impact on the NOCT. For example, a rear surface with a lower packing density and reduced thermal resistance may make a temperature difference of 5°C or more.
Both conductive and convective heat transfer are significantly affected by the mounting conditions of the PV module. A rear surface which cannot exchange heat with the ambient (i.e., a covered rear surface such as that directly mounted on a roof with no air gap), will effectively have an infinite rear thermal resistance. Similarly, convection in these conditions is limited to the convection from the front of the module. Roof integrated mounting thus causes higher operating temperature, often increasing the temperature of the modules by 10°C.