For example, a rear surface with a lower packing density and reduced thermal resistance may make a temperature difference of 5☌ or more. Module design, including module materials and packing density, have a major impact on the NOCT. Module temperature will be lower than this when wind velocity is high, but higher under still conditions. An approximate expression for calculating the cell temperature is given by 2: The best module operated at a NOCT of 33☌, the worst at 58☌ and the typical module at 48☌ respectively. Temperature increases, above ambient levels, with increasing solar irradiance for different module types 1. The best case includes aluminium fins at the rear of the module for cooling which reduces the thermal resistance and increases the surface area for convection. The NOCT for best case, worst case and average PV modules are shown below. The equations for solar radiation 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 Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open circuited cells in a module under the conditions as listed below: In order to determine the power output of the solar cell, it is important to determine the expected operating temperature of the PV module. However, when operating in the field, they typically operate at higher temperatures and at somewhat lower insolation conditions. A PV module will be typically rated at 25 ☌ under 1 kW/m 2.
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