A photovoltaic module is usually rated at 1 kW/m2 at 25°C. However, when operating in the field, they usually operate at higher temperatures and in slightly lower sunlight conditions. In order to determine the power output of the solar cell, the expected operating temperature of the solar cell must be determined. The nominal operating cell temperature (NOCT) is defined as the temperature reached by an open-circuit solar cell in portable solar panels under the following conditions.

Battery surface irradiance =800 W/m2

Air temperature =20°C

Wind speed =1 m/s

Mount = back open

The equations for the temperature difference between solar radiation and the solar panel and the air show that at a given wind speed, the conduction and convection losses are linearly related to the incident solar illumination, provided that the thermal resistance and heat transfer coefficient do not become strong with temperature changes. Best case, worst case and average PV module NOCT. In the best case, aluminum fins at the rear of the assembly are used for cooling, reducing thermal resistance and increasing the surface area for convection.

The best solar charging panels run at 33°C NOCT, the worst solar PV modules run at 58°C, and typical solar panels run at 48°C. An approximate expression for calculating the cell temperature is:

Cell temperature = air temperature +(NOCT-20)/80*S, where S= sunshine amount, unit is mW/cm2. When the wind speed is high, the solar PV module temperature will be lower than this value, but higher at rest.

Influence of solar module design on NOCT

Solar module design, including solar panel material and packaging density, can have a significant impact on NOCT. For example, a rear surface with lower packing density and reduced thermal resistance may result in a temperature difference of 5 ° C or more. However, the NOCT of most solar panels is very similar around 40-45 ° C. The uncertainty in measuring the NOCT is usually higher than the variation in the NOCT between modules.

Influence of installation conditions

Both thermal and convective heat transfer are significantly affected by the installation conditions of photovoltaic modules. A rear surface that cannot exchange heat with the environment (i.e. a covered rear surface, such as mounted directly on a roof, with no air clearance), will have a virtually infinite rear thermal resistance. Again, under these conditions, convection is limited to that coming from the front of the solar panels. As a result, integrated roof installations result in higher operating temperatures, often increasing the temperature of components by 10 ° C or more.