The Photovoltaic (PV) industry has developed very rapidly over the last decade. Today, more and more PV systems have been installed for both residential and commercial use due to the decreasing price and massive financial support. As a result of technological innovation, the efficiency and reliability of PV modules have been greatly improved.
PV technology has become competitive with fossil due to the decreasing price and non-polluting aspects. Massive financial support and technological innovation are two big drivers that keep the PV market growing.
However, it is known that some PV modules degrade rapidly and their actual outputs are much lower than normal ones. In addition to power losses, some of modules reveal a number of safety issues caused by cell damage and packaging material degradation. It cannot be ignored that module degradation has become a potential issue that could constrain the PV industry’s development. There are four basic categories of module degradation. They are cell failure, module failure, packaging material degradation and power output decrease. In addition, each category of degradation includes several different cases.
All of the above problems that affect module performance are referred to as module degradation. Normally, the lifecycle of a PV module ranges from 20 to 30 years, which means the rate of module degradation should be less than 1% per year. Nevertheless, it is reported that some modules initially exceed the guaranteed level. Rapid module degradation can lead to short module lifecycle and high replacement cost. Therefore, it is necessary to provide effective troubleshooting techniques and proper module performance measurements for reducing module degradation.
t was found that hot spot is the most common cell failure issue mainly resulting from cracked and shaded cells which can cause the overheated spot. It has been proved that solar cells that are in a long-term overheated status could experience power decrease and damage the whole structure permanently. In addition to hot spot, it has been demonstrated that micro-cracks in cell could destroy the whole module structure and drastically shorten module lifecycle. Micro-cracks in cell often occur in the manufacturing process or transportation. Also, it has been seen that thermal stress and hail damage could result in micro-cracks. This study also introduces two major types of module failure caused by soiling and shading. Both types of module failure can lead to considerable power losses.
In general, module troubleshooting is divided into four steps. The first step is visual inspection in order to detect bubbles, delamination, encapsulant discoloration, glass breakage and obvious cell cracks. The next step is thermal analysis using an infrared camera to detect hot spots or an abnormal area of a module which has a much higher temperature. A bypass diode is an effective way to eliminate hot spots but it can result in extra power losses. In addition, the LIT technique combined with IR imaging is used to detect shunt defects. Further analysis consists of EL and PL imaging techniques which can detect invisible defects such as micro-cracks. Although the RUV technique has high accuracy in micro-crack detection, it is relatively expensive and complex.
The final step is outdoor PV module performance measurements. Mapping measured results to STC is one of the most important steps in the outdoor PV module performance measurement because the results measured in different weather conditions are difficult to compare and need to be normalized to the same condition for comparison. In terms of the normalized results, it was found that the four measured modules have different rates of power degradation in a 12-year period. The SX-75 module is the only one whose power output is still higher than its IGM of 70 W while the other three modules have degraded below to their minimum tolerance. It was also discovered that the performance of the SX-75 module is quite good with the lowest degradation rate of 0.66 %/year while the situation for the PW750/70 module is the worst and has resulted in considerable power losses of over 20 W, equal to a degradation rate of 2.67 %/year, after 12 years of exposure.
However, it is inevitable that experimental errors induced by changed meteorological conditions and the inaccuracy of equipments can affect the accuracy of the measured results. So it is recommend that a specific simulation be used in future research.
Source: Zihang Ding, PV Module Troubleshooting and Measurement, MSc in Renewable Energy of Murdoch University, 2012
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