Renewable energy systems comprise a large number of electrical connections and hence are vulnerable to energy losses at the contact points. In order to minimise these losses, long-lasting and secure cable connections with low contact resistances are required. Meanwhile, connectors are needed to facilitate and ease the installation process. Further, in the case of solar photovoltaic (PV) systems, junction boxes are required to aggregate multiple strings from modules, which in turn can reduce the number of cables fed into inverters. While the cost of these components is not very significant, they can have a significant impact on the overall yield of a project. Notably, the cost of replacing or reinstalling these components is more significant than the first time installation costs.
Renewable Watch takes a look at the evolving market trends and innovations in the connectors, cables and junction box space…
Cables and connectors
In a solar PV installation, DC cabling is provided between solar PV panels and inverters (including junction boxes), while AC cabling is between the inverter and the substation. Besides, earthing arrangements need to be made. For this, low-voltage power cables are used from the panel to the combiner box, from the combiner box to the inverter and from the inverter to the transformer. Medium-voltage power cables are used for transferring electricity from the transformer to the substation, while high voltage and extra high voltage ones from the substation to the grid.
Modules in a solar PV set-up are pre-attached to connectors to facilitate plant assembly and prevent wrong connections. These connectors are used to connect the modules in a string and all strings to the inverter or junction box. Connectors provide secure and touch-proof connections between components.
Solar cables have to withstand high temperatures, ultraviolet radiation, rain, humidity, dirt, etc., apart from being subjected to various kinds of stresses, pressures, tensions and volatilities. High operating temperatures can lead to wiring losses in the system. The conducting wire of a cable is therefore typically insulated for protection, with many solar PV installers using neoprene cables. However, these cables are susceptible to cracks and damages in harsh environments. Hence, insulation made from electron-beam cross-linked polymers that do not melt or flow even at high temperatures is gaining traction. Cross-linking provides resistance to long-term heat and stress cracks.
Both copper and aluminium conductors are used for solar projects. However, the aluminium ones have a price advantage and are hence used more often despite the fact that aluminium is less tensile and leads to higher power losses. Moreover, since aluminium is not a poor conductor of electricity, the wires are much thicker as compared to copper. Like cables, PV connectors are exposed to harsh weather conditions that they must withstand, and not disconnect under excessive loads. They therefore need to have a firm locking mechanism.
Wind power projects have higher cabling requirements as compared to solar projects. A typical wind energy set-up may deploy up to 50 types of cables varying in voltage, insulation, strength, etc., depending on the function they will be required to perform. For instance, cables are required to transmit power from a turbine generator to the ground-based equipment. Medium voltage cables are used for the collection and transmission of power from multiple turbines to the substation while overhead and underground transmission conductors are used to transfer power from the substation to the grid. In the case of offshore projects, cables are required for connecting the turbines as well as for exporting electricity to the nearest onshore grid. Meanwhile, electricity is transported to the shore through cables along the ocean floor.
Over the past couple of years, there has been an increasing focus on sophisticated data collection, monitoring and analysis to improve the reliability and performance of wind projects and facilitate grid integration. In this context, fibre optic cables are gaining traction as they are useful for data transfer and communication among turbine components. Like other cables used in wind installations, these need to be resistant to harsh external conditions and meet the engineering requirements. In an offshore scenario, the cable signal should also be able to withstand magnetic fields that change with electric currents.
Junction boxes
Junction boxes are enclosures installed at the back of solar modules to provide an electrical interface with inverters through connectors and cables. The proper functioning of junction boxes is essential as even a minor fault in their components (bypass diodes) or failure in their operations can affect the project’s power generation capacity significantly.
The junction boxes are attached to the back of the modules through adhesives or sealants. These adhesive solutions should ensure reliable bonding in different climatic conditions and must be resistant to ultraviolet radiation. They must also have a low moisture vapour transmission rate and be non-conductive so as to avoid short circuits. In the past, solutions such as silicone adhesive have been used due to their strong bonding with glass, metals and plastic. The adhesive also exhibits long-term durability and the ability to withstand harsh weather conditions. On the downside, the curing time of silicone adhesive can be very high (several hours), which significantly delays the panel production process. However, innovation in silicones has resulted in the introduction of fast-fixture adhesives like the Henkel Corporation’s Loctite silicone, which ensures fixture in two to three minutes. Other solutions used for mounting junction boxes are adhesive tapes such as polyethylene and polyurethane foam tapes and acrylic foam tapes, which, unlike liquid adhesives, do not require investments in dispensing technology or time for curing.
Innovations in solar PV module manufacturing and the increasing pressure on manufacturers to cut costs and improve reliability have led to innovations in the electrical junction boxes space as well. For instance, the REC Group, as part of its REC TwinPeak Series, has introduced split junction boxes, which are significantly different from traditional junction boxes. The latter typically comprise an enclosure that houses three bypass diodes, which enable the connection of the solar PV modules to the rest of the PV system through cables. Meanwhile, in a split junction box, three bypass diodes are installed in three smaller boxes, each attached with a single cable. According to the company, the split junction box configuration uses one less cross connector than a standard junction box, which reduces the internal panel resistance and helps save space. This space can be used to provide a larger gap between cells, increasing the internal reflection of light, thereby resulting in higher absorption of sunlight. Internal tests have shown a reduction of 15-20 ºC in heat build-up behind the three new boxes as compared to the single box on a standard panel. This reduction in temperature can help increase the reliability of PV modules and produce higher output.
Meanwhile, several solar equipment manufacturers, including GreenPeak Technologies, have introduced smart junction boxes that enable wireless communication between the PV modules and a central unit, which can track, monitor and control the performance of individual modules. These boxes allow the monitoring of voltage, current and temperature parameters, and determine whether service or maintenance is required. In the case of a fire or electrical emergency, the PV modules are shut down instantly, thereby preventing further damage. GreenPeak has also developed solar panel monitoring applications based on the ZigBee communication standard, which can be used instead of Wi-Fi technology as it consumes less power. Similarly, Molex offers a smart junction box called SolarSpec. Its base is attached to the back of the module and it features a removable cover assembly. Like other smart junction boxes, SolarSpec provides features such as system safety, arc detection, security monitoring and performance tracking. The company claims that the unique modular concept of SolarSpec provides maximum flexibility and easy upgrade of PV panel functionality as the printed circuit board always resides in the removable junction box cover. This ensures easy access to components, which is helpful during repair and maintenance.
