Credit: Chris Ballance via Flickr

The solar modules currently on the market mainly consist of glass panels, EVA polymer glue, photovoltaic cells, back sheets, and aluminum frames. The resulting heat resistance is high, so why is it burning? The problem may come from the transmission circuit and the inverter under the module, which often bursts into flames due to abnormally high temperature.

Before discussing the cause of the fire, a closer look at how solar modules work is imperative. The solar module converts sunlight into direct current, which is converted into alternating current by the inverter. Only then the electricity can be connected to the feeder lines or simply for self-consumption. To achieve such a complicated process, there are bound to be lots of lines and equipment involved for transmission and distribution purposes. If it is not regularly checked or ignored, short circuits or even fire are bound to happen.

Problems may also occur during the process of photovoltaic energy conversions, such as hot spots, cracks, and yellowing, which reduce the power generation and damages the solar cells and modules. The so-called hot spot is a localized area of elevated temperature in the solar module. There is a multitude of factors that could cause hot spots: the quality of PV cells, improper installations, and maintenances, shading situations or even bird excrement, etc. The damaged or blocked areas of a PV cell would heat up.

The manufacturers usually connect a bypass diode in the junction box to provide an additional current path, which reduces the effects of hot spots and protects the module from incinerating immediately. However, bypass diodes also need to be replaced. The current will also heat up when passing through the bypass diode, increasing the temperature of the junction box.

Regarding the solar inverters, which serve as DC/AC inverters, there are also several protection mechanisms for them, such as DC isolators: anti-islanding protection. The main functions of this protection mechanism are electrical separation, prevention of fire, or electric shock. Anti-Islanding protection is also a switch for solar energy. Moreover, all of the distributed solar power generators with grid-connections are required to install anti-islanding protection. The grid-connected inverters must immediately disconnect from the grid in the case of a power outage. This protection mechanism prevents the maintenance personnel or other people from suffering an accidental electric shock.

There should be no danger in most cases, provided that the installation is properly executed. Léonie Green, head of the new market and promotion of Britain’s Solar Trade Association (STA) in the UK, described that the risk of solar modules catching fire is one in 80,000, which is lower than other electrical equipment.

However, all the previous efforts will be in vain, if no monitoring and maintenance have been regularly carried out. Department of British Energy and Industrial Strategy (DBEIS) has conducted investigations regarding 80 fire accidents resulted from solar power. According to the findings, the DC isolator is the most at risk of fire. Most likely because the DC isolators are also electromechanical equipment. 26 of the 80 accidents are caused by poor designs and installations, while the DC connector has the second-highest rate of bursting into flames.

Britain’s Building Research Establishment (BRE) has also published a similar report. It is intended to establish correlations among 50 solar power-related accidents. The report shows that 36% were due to problematic installations of solar power systems, while 12% were due to product failures. Only 5% were results of faulty designs. The remaining 47% of accidents cannot be attributed to solar modules.

Japan has also published a review in January. There have been 127 fire accidents that are related to solar modules in Japan over the last 10 years. 70% of them were caused by a lack of regular inspections.

Walmart, the US retail giant, filed a lawsuit against Tesla in the New York Supreme Court recently, claiming that Tesla's solar modules have been negligently installed and maintained, causing at least seven of its stores to ignite and demanding compensation from Tesla. Walmart has also sought damages from Tesla regarding the stores and commodities which are affected and destroyed. Tesla's solar modules also have to be removed from the roofs of 240 Walmart stores.

All of the said examples have shown the importance of inspection and maintenance. Nowadays many manufacturers have also adopted the technology of infrared thermal imaging aerial photography to detect abnormal high temperature on the module, in addition to the manual inspections performed by staff members. Monitoring equipment is also deployed to observe whether there is an abnormally high temperature. So the modules can be shut down as soon as required.

However, if a fire does occur, it cannot be stressed enough that the solar modules can still generate electricity, even when they are seriously damaged, inflamed or flooded. The exposure to sunlight or even the light from the fire alone can keep the solar modules up and running. Therefore it is absolutely imperative to inform the power company and solar energy services provider to cut off the power immediately when it has been established that solar modules have caught on fire. While the professionals are on-site, be sure to ask them to determine whether the electricity is still being distributed between the inverters. Pouring water on an electrical fire is prohibited at all times.

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