Although not extensively talked about, junction boxes are a crucial component of solar panels. These devices are pre-installed on the back of solar modules with silicon adhesives and are designed to protect the electrical systems on the panels from the environment. Moreover, junction boxes prevent wires and conductors on solar panels from corroding and provide temperature resistance, water resistance, ozone resistance, etc. All in all, these devices enhance the efficiency of solar panels by optimising and stabilising the power generated by the system, making the power safe for further transportation and use.
As panel technologies advance at a rapid pace, the role of the junction box is increasingly coming into focus. It is thus important to review the progress in junction box technology in recent years.
Design and material considerations
A junction box typically contains diodes that help in connecting panels together. Essentially, it wires four connectors together and is the output interface of the solar panel. These diodes also keep power flowing in one direction, and prevent it from being fed back to the panels. Similar to a battery, a junction box has two ends, positive and negative, and these can be connected to panels either serially or in parallel.
The four types of junction boxes are silicon junction boxes, non-silicon junction boxes, curtain junction boxes and explosion-proof junction boxes. Building-integrated solar PV junction boxes are commonly used for solar power systems that are integrated into building structures. Regardless of the type, junction boxes are produced in two ways. The first is the soldering and potting method, wherein foils protruding from the solar panel are soldered to the diodes in the junction box. The junction box is then “potted” with an adhesive to allow thermal transfer of heat, and to keep the solder joint in place and prevent it from falling. Once enough time has passed for sufficient curing of the potting material, the panel can be used for installations. The second method is the clamping mechanism, which simply attaches the foil to the wires, without any fumes or clean-up required. While clamping boxes are more expensive, the labour required for the soldering and potting method is often higher.
A material that is commonly used for manufacturing junction boxes is polyphenylene oxide, a high temperature thermoplastic. However, there are some concerns with its durability, especially with more powerful modules. This is because reverse currents could lead to the diode reaching temperatures as high as 100 °C, which is about the same relative temperature index of polyphenylene oxide.
Need for upgrades
The solar market boom has been driven by a fall in module and cell prices as well as massive improvement in the efficiency of PV technology. Solar projects across the world have shifted from polycrystalline solar panels to monocrystalline ones, and the developments made with passivated emitter and rear cell technology have seen the industry chasing the 25 per cent efficiency mark. With these advancements, a lot rides on the humble junction box to ensure the stability of the network. While the role of the junction box has not changed, its diodes now have to do much more work in dealing with more powerful solar panels. This has even led to an increased rate of failure due to burnt bypass diodes, low power and high energy stress on the system. The melting of plastic components, broken latches and the separation of external connectors are other negative factors.
A solar testing organisation, PV Evolution Labs (PVEL) recently released its PV module reliability scorecard 2021. While it acknowledged the increasing efficiency of solar modules and reduction in power costs, it also found that the reliability of modules has fallen. Most notably, it found that one in three manufacturers experienced safety failures due to junction box defects; the majority of these failures were observed before testing. PVEL carried out a similar study last year, which found only one in five failures of this kind. Furthermore, the new study found that 26 per cent of the bills of materials (BOMs) reported at least one failure, 6 per cent higher than the previous year.
All junction boxes must pass through quality inspection checks from testing companies such as Technischer Uberwachungs Verein and Underwriters Laboratories. In India, domestic junction box manufacturers have to adhere to guidelines set by the Ministry of New and Renewable Energy. However, as Tara Doyle, chief commercial officer, PVEL, noted, “Between supply chain instability and the ever-present push for lower prices, one cannot assume that every module sold under a given model type uses tested BOM components. Buyers must specify their desired BOM in supply contracts to achieve this.”
Innovations in technology
Cool bypass switches have emerged as a solution to issues of excess heat generation, replacing traditional diodes in junction boxes to reduce the heat generated from high-power modules. The device can be switched on and off. The switch opens the circuit when panels are trying to pull energy, allowing heat to dissipate. The drawback, however, is that cool bypass switches are much more expensive than traditional diodes. Another innovation in recent years is the development of split junction boxes, which are designed for bifacial modules.
Many players have also made efforts to redesign junction boxes altogether. TE Connectivity is one such company. It has looked into building a better device that can cope with newer solar panels. The company has brought out junction boxes with printed circuit boards that can be integrated into solar panels with monitoring, optimisation and rapid shutdown abilities. The Shanghai-based developer has also tried to create a compact and cost-effective design whereby the connecting wires are directly encapsulated in a polymer resin, which saves assembly costs and ensures durable encapsulation of the diode. TE Connectivity developed its Zytel technology with the DuPont material to increase the junction box’s resistance to heat. The material is a 25 per cent glass-fibre reinforced, flame-retardant polyamide 66 resin with a significantly higher electrical relative temperature index at 130 °C and a UL-94 flammability rating of 5VA.
In the Indian subcontinent, DhaSh has emerged as one of the leading innovators in the field of solar junction boxes, despite being established only in 2017. In addition to offering a comprehensive portfolio of solar junction boxes, the company has developed 15A/20A/24A-rated junction boxes with thermal conductivity that can fulfil the requirements of high-power modules as well as 12V (72 cell) solar panels. It has also developed ultra-thin split junction boxes for bifacial solar modules. These devices occupy less space and cast less shade on the rear solar cells, improving output. DhaSh currently has a junction box manufacturing capacity of 2 GW, and aims to take this capacity to 5 GW in 2021.
China is, unsurprisingly, the global leader in junction box production, meeting about 45 per cent of the global demand for the device, which is estimated to be about 112 million. India accounts for about 5 million of this demand but still has some way to go to be cost-competitive with global manufacturing hubs such as China, Thailand and Malaysia. That said, domestic manufacturers have the advantage of local presence and high quality product offerings, which are often appreciated among consumers in the home country.
The factors that led to the manufacturing boom in China included low-cost finance, with interest rates of 1-4 per cent and a $30 billion credit line, tax incentives, free land, and subsidised and uninterrupted electricity. These incentives promoted the production of solar modules, which, in turn, led to a manufacturing boom of junction boxes and glasses. The case of China suggests that a similar backward integration will happen in India once cell and module manufacturing capacity is set up.
India is making a strong push in setting up a robust domestic manufacturing base for the solar industry with the Make in India initiative. The launch of the production-linked incentive scheme, which targets 10 GW of integrated solar PV manufacturing capacity by 2023-24, is a ray of hope for the country. Notably, it is expected to spur demand for balance of system components such as junction boxes.
The worldwide market for solar junction boxes is expected to grow rapidly. Although this is a promising sign, it is merely a representation of the fast growth of the solar segment. The increased rate of failure of solar junction boxes is a call for manufacturers to innovate and develop more reliable models of these small yet crucial devices. Further, there must be stringent enforcement of testing standards for devices, ensuring that only tested junction boxes are sold in the market.
While domestic manufacturing in India holds promise, there are still crucial segments where solar manufacturers are dependent on the import of critical raw materials. Going forward, it is important to identify the segments in which manufacturing can be carried out domestically. That said, existing manufacturers in India must make technological innovations so that the country’s junction boxes are on par with the rest of the globe.
By Rithvik Kumar