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What affect the max output power of PV modules?
- Jul 31, 2018 -

Photovoltaic components are the core of the photovoltaic power generation system, which is used to convert the solar energy into electric energy and store it in the battery or drive the load. For photovoltaic modules, output power is very important. What are the factors affecting the maximum output power of PV modules?

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1. Temperature characteristics of photovoltaic modules

Photovoltaic modules typically have three temperature coefficients: open circuit voltage, short circuit current, and peak power. As the temperature increases, the output power of the photovoltaic module decreases. The peak temperature coefficient of the mainstream crystalline silicon photovoltaic modules in the market is about -0.38~0.44%/°C, that is, for every degree of temperature increase, the power generation of photovoltaic modules is reduced by about 0.38%. The temperature coefficient of thin film solar cells will be much better. For example, the temperature coefficient of copper indium gallium selenide (CIGS) is only -0.1~0.3%, and the temperature coefficient of cadmium telluride (CdTe) is about -0.25%, which is superior to crystalline silicon cells.

2. Aging attenuation

Slow power attenuation occurs in long-term applications of photovoltaic modules. The maximum attenuation for the first year is about 3%, and the annual decay rate for the next 24 years is about 0.7%. From this calculation, the actual power of the PV module after 25 years can still reach about 80% of the initial power.

There are two main reasons for aging decay:

1) The attenuation caused by the aging of the battery itself is mainly affected by the type of battery and the production process of the battery.

2) The attenuation caused by the aging of the packaging material is mainly affected by the production process of the component, the packaging material and the environment of the place of use. Ultraviolet radiation is an important cause of deterioration in the performance of the main material. Long-term exposure to ultraviolet light causes aging and yellowing of EVA and backsheet (TPE structure), resulting in a decrease in the transmittance of the module, resulting in a decrease in power. In addition, cracking, hot spots, sand abrasion, etc. are common factors that accelerate component power attenuation.

This requires component manufacturers to strictly control the selection of EVA and backplane to reduce component power attenuation caused by aging of auxiliary materials.

3. Initial photoinduced attenuation of components

The initial photoinduced attenuation of the photovoltaic module, that is, the photovoltaic module output power decreased significantly during the first few days of initial use, but then stabilized. Different types of batteries have different degrees of photo-attenuation:

In P-type (boron-doped) crystalline silicon (single crystal/polycrystalline) silicon wafers, light or current injection causes the formation of a boron-oxygen complex in the silicon wafer, which reduces the lifetime of the minority carriers, thereby causing partial photo-generated carriers to recombine and reduce Battery efficiency, causing photoinduced attenuation.

In the half year period of the initial use of amorphous silicon solar cells, the photoelectric conversion efficiency will be greatly reduced, and finally stabilized at about 70% to 85% of the initial conversion efficiency.

For HIT and CIGS solar cells, there is almost no photoinduced attenuation.

4. Dust and rain cover

Large-scale photovoltaic power plants are generally built in the Gobi area. The sand is large and there is little precipitation. At the same time, the frequency of cleaning will not be too high. After long-term use, the efficiency loss will be about 8%.

5. Component series mismatch

The series mismatch of PV modules can be explained by the barrel effect. The amount of water in the barrel is limited by the shortest plank, and the output current of the PV module is limited by the lowest current in the series assembly. In fact, there will be a certain power deviation between the components, so the number of mismatched components will cause a certain amount of power loss.

The above five points are the main factors that affect the maximum output power of the photovoltaic cell components, and will cause long-term power loss. Therefore, the later operation and maintenance of the photovoltaic power station is very important, which can effectively reduce the loss caused by the failure.