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What You Should Know and Keep In Mind About Solar Panel Degradation

What is solar panel degradation? What influence does it have on the operation of solar PV systems? Which factors affect solar panel degradation? This article gives you the answers to these questions.

Solar panel degradation is defined as a process, which occurs over time and leads to a reduction of performance of solar PV panels. The degradation of solar PV panels should be considered when calculating the yield of a solar panel system.

Causes of solar panel degradation

A decrease in the performance of solar PV panels can result from different causes.

These include, among other things, the effect of Shading on solar PV panels, the roof pitch angle, the ambient temperature and the solar panel degradation, which means the purely technical impairment of solar PV modules in their ability to convert solar radiation into photovoltaic solar power.

Types of solar panel degradation

In principle, the solar panel degradation can be distinguished into the light-induced, aging-related and potential induced degradation. Whereas the light-induced and the potential induced degradation have a one-time effect, the aging-related degradation is a continuous process.

The light-induced degradation (LID) occurs for the first time after the solar panel installation, namely, when the solar PV panels come into contact for the first time with sunlight, and it concerns only amorphous modules (Thin-film solar panels).

However, the potential induced degradation can occur in all solar panel types. In contrast, the aging-related degradation is based on natural aging phenomena of the solar PV panels.

Light-induced degradation (LID) of solar PV panels

Not all solar panel types are affected equally by light-induced degradation. It is especially high in solar PV modules, which are made from amorphous silicon, thin-film solar panels. These respond to the first solar radiation with a reduction of their photoconductivity, whereby their performance is affected.

However, the so-called initial degradation appears only in the first 1.000 operating hours of the thin-film solar panels. After a period of about 1.000 working hours, the efficiency of the solar PV panels stabilizes at a lower level and remains approximately constant.

The rapid output decrease by some percentage points is taken into account from the solar module manufacturers by the calculation of the nominal output of solar PV modules.

Causes of light-induced initial degradation

The initial degradation of solar PV panels leads back to the Staebler-Wronski effect. This effect describes the changes in amorphous silicon cells, which are caused by sunlight.

This effect is not observed in CIS-cells. Here, the usual degradation runs like that in the crystalline solar panels.

Appearance of light-induced initial degradation

The effect appears increasingly in the photovoltaic solar cells of p-type silicon, which has been doped with Boron during the ingot production. This concerns the vast majority of modules because they are made from this type of silicon.

Through the economical use of Boron, the effect can be reduced. Another possibility to influence this effect is to let it diffuse oxygen as little as possible into the liquid silicon.

On the contrary, the light-induced initial degradation is negligible in crystalline solar panels. In the early days after putting them into operation, the initial degradation of crystalline solar panels was only 1-2% (depending on the material used).

This decrease is considered by the declaration of the solar panel efficiency. This means the given nominal output in the solar panel datasheet relates to the operation after the initial degradation.

Potential induced degradation (PID) of solar PV panels

Also, there is solar panel degradation, which is caused by the type of PV installation. In solar panel systems with higher system voltage and transformerless inverters, a loss of performance is detected, which can cause, according to the investigation of the Fraunhofer-Center for Silicon Photovoltaics (CSP), losses of up to 30%.

This effect is called potential induced degradation (PID). The reason for this effect lies in the so-called leakage currents. So, the potential induced degradation often occurs on unearthed PV solar Systems by the negative voltage in relation to the earth potential, and by high system voltages, high temperatures and high humidity can be favored.

The potential induced degradation can be prevented by using a solar power inverter, which has the possibility of earthing of a positive or negative pole.

In addition, solar module manufacturers also test their modules for PID resistance. Independent PID tests are provided by among other things, the Underwriters Laboratories (UL), the Photovoltaic-Institute of Berlin (PI).

Aging-related degradation of solar PV panels

The aging-related degradation arises from wear signs in the solar PV panels. Here, it is the effect of external factors on the solar PV panels, which affect the performance.

According to Fraunhofer ISE, monocrystalline and polycrystalline solar panels have similar degradation values of about 0.1 percent a year. Others are based on loss of performance of about 0.5 percent a year. The output losses of light-induced degradation are clearly higher.

Causes of aging-related solar panel degradation

Crystalline hardening in semiconductive material is held responsible for aging-related degradation. They are produced by chemical reactions. Then, intercrystalline recombination processes hinder the electron flow. However, until now these phenomena cannot be removed via an economically viable way.

In addition to crystalline hardening, there are other factors, which are held responsible for affecting the performance of solar modules. These include the contamination of photovoltaic solar cells, shadowing effects and aging signs of other elements of solar modules.

Solar panel cleaning helps against pollution. This is recommended, especially for solar PV systems which have a flat set-up angle or are located close to dust sources or leaf shedding.

Degradation and performance warranties of the solar module manufacturers

Because the aging effects of solar PV systems are difficult to influence, solar module manufacturers often give a performance warranty on their solar PV panels of 20 to 25 years, many of them even 30 years. Normally, solar PV systems are efficient even up to 35 or 40 years.