|Sunlight incident on a solar panel generates heat as well as electricity.|
A PV module exposed to sunlight generates heat as well as electricity. For a typical commercial PV module operating at its maximum power point, only 15 to 20% of the incident sunlight is converted into electricity, with much of the remainder being converted into heat. The factors which affect the heating of the module are:
- the reflection from the top surface of the module;
- the electrical operating point of the module;
- absorption of sunlight by the PV module in regions which are not covered by solar cells;
- absorption of low energy (infrared) light in the module or solar cells; and
- the packing density of the solar cells.
Front Surface Reflection
Light reflected from the front surface of the module does not contribute to the electrical power generated. Such light is considered an electrical loss mechanism which needs to be minimized. Neither does reflected light contribute to heating of the PV module. The maximum temperature rise of the module is therefore calculated as the incident power multiplied by the reflection. For typical PV modules with a glass top surface, the reflected light contains about 4% of the incident energy.
Operating Point and Efficiency of the Module
The operating point and efficiency of the solar cell determine the fraction of the light absorbed by the solar cell that is converted into electricity. If the solar cell is operating at short-circuit current or at open-circuit voltage, then it is generating no electricity and hence all the power absorbed by the solar cell is converted into heat.
Absorption of Light by the PV Module
The amount of light absorbed by the parts of the module other than the solar cells will also contribute to the heating of the module. How much light is absorbed and how much is reflected is determined by the color and material of the rear backing layer of the module.
Absorption of Infra-red Light
Light which has an energy below that of the band gap of the solar cells cannot contribute to electrical power, but if it is absorbed by the solar cells or by the module, this light will contribute to heating. The aluminium at the rear of the solar cell tends to absorb this infrared light. In solar cells which do not have full aluminium coverage at the rear of the solar cell, the infrared may pass through the solar cell and exit from the module.
Packing Factor of the Solar Cells
Solar cells are specifically designed to be efficient absorbers of solar radiation. The cells will generate significant amounts of heat, usually higher than the module encapsulation and rear backing layer. Therefore, a higher packing factor of solar cells increases the generated heat per unit area.