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It is commonly believed that a PV power station can operate normally and continuously generate electricity for more than 25 years after the station is complete. However, in real life cases, sometimes power generation capacity and even the lifetime of PV power plants are observed to be decreased due to some problems in after-sales stage. Among which, hotspot effect is a commonly occurred and thorny problem in the operation and maintenance of PV power plants that troubles many operation and maintenance personnel and investors. Therefore, this article is written to introduce the causes of hotspot effect and what we can do to mitigate its harm.
The harm of Hotspot
The impact and harm of hot spots on modules can be severe. When hot spots occur, it will first reduce the ability of modules in receiving lights and therefore affects the power generation of the entire power station, and reduce revenue for investors. When the temperature rises beyond a certain limit, partial burning will occur on PV modules which lead to dark spots, solder joint melting, materials aging, diode breakdown and other permanent damage, and may even lead to more serious safety hazards, such as partial failure of modules and even fires in PV station. According to statistics, the severe hot spot effect will reduce the life length of PV modules by more than 30%.
The cause of Hotspot
When the cells of the module are partially shaded by such as dust, fallen leaves, shadows and etc., the shaded cells cannot receive solar light, which decrease the power generation capacity of cells. The current of the shaded cells will be lower than that of the unaffected cell that is normally generating electricity around it, and will be forced to enter a reverse bias state and dissipate power. As a result, the shaded cells will change from a power generation state to an energy-consuming state, which will then lead to a high temperature in the shaded area on the module. This is what we called the hot spot effect. Therefore, you can tell why hotspot is commonly seen in project sites. Sometimes, only a falling leaf or bird droppings on modules can become the “culprit” of module damage.
Actions to mitigate Hotspot risk
Even though hotspot effect is common and the harm is severe, there are many ways to mitigate hot spot risk and reduce the corresponding effect.
First of all, in terms of module design, LONGi, including some top manufacturers in the industry, has taken effective measures to reduce the risk of hot spots in module design stage. For example, bypass diodes are connected in parallel next to cell string. LONGi currently uses three bypass diodes for each module. Under normal circumstances, the bypass diode is in the reverse bias voltage and does not affect the normal operation of the module. When the module is partially shaded, the other cells will promote the shaded area in reverse bias to become resistance, and the diode is turned on to remove the shaded cell strings from the whole module circuit. Therefore, reduce the impact of the hot spot on the power generation of the module. Ideally, if a bypass diode can be connected in parallel on each single cell, the hot spot problem can be eliminated.
Secondly, the quality of cell also directly affects the probability of hot spot occurrence. Quality issues of cell, such as cracks, weak soldering, and grid breakage, are one of the factors that cause the hot spot risk on the module. Therefore, LONGi has strictly controlled the cell quality in module production. For example, two strict quality inspections, both AI and manual, are implemented before encapsulation and before packaging to prevent the quality issues mentioned above. Moreover, LONGi also control the distribution of leak current of cells within a module to ensure the leak current is evenly distributed which avoids high concentration of leak current and thereby effectively reduces the risk of hot spots.
Lastly, regular and appropriate operation and maintenance in PV station is also significant to prevent hot spots risk. Regular cleaning of module to ensure no dust deposition on module surface. Timely remove weeds and other obstacles near modules to avoid shading on module surface can greatly reduce the possibility of hot spots caused by external factors.In conclusion, hotspot risk can be greatly mitigated through multiple measurements. With optimal product design, strict quality control and appropriate maintenance, hot spot effect can be effectively controlled. |
