March 23, 2010 - Harnessing the sun’s energy to produce electricity is once again garnering interest from eager consumers looking for a cheaper, environmentally friendly power source. During the last energy crisis of the 1970s, solar power gained attention as an alternative to more costly fossil fuels, but the high cost of producing solar energy at that time made it impractical for most consumers, especially for large installations. However, according to the Solar Energy Industries Association (SEIA), because of current concerns about rising costs, energy security, and supplies, coupled with new state and federal incentives, the solar energy industry has seen tremendous growth since 2005. The US now ranks fourth in the world for cumulative installed solar capacity after Germany, Spain and Japan. Solar energy is the cleanest, most abundant, renewable energy source available and making solar systems more cost-effective will expand consumer availability.

One of the methods for harnessing the sun’s rays is through photovoltaics (PV) -- converting light into electricity. Photovoltaic devices consist of an array of cells containing various materials (such as an active silicon layer) that turn solar radiation into direct current electricity. To produce enough power for very large applications, such as a home or a commercial business, cells are electrically connected to form photovoltaic modules or solar panels.

Structural Adhesive Advantages

Although solar energy manufacturing is expanding in the US, along with the number of installed solar systems, manufacturers and distributors are continuing to seek out new methods of cutting manufacturing and installation costs. One solution that has the potential to streamline costs, decrease maintenance, and improve durability and product life expectancy of the solar energy systems is the transition from mechanical fasteners to structural adhesives for solar panel manufacturing and installation.

As an alternative to mechanical fasteners, structural adhesives offer the benefits of reduced stress points, leaks and corrosion, resistance to extreme environmental conditions, and enhanced sealant properties. Structural adhesive bonding is an established joining method, proven in a variety of end-use markets, including cars, trucks, specialty vehicles, and boats. By switching to structural adhesives, the PV panel industry can realize significant cost savings in both manufacturing and installation.

Structural adhesives have demonstrated excellent environmental resistance in real-world installations. For example, an acrylic adhesive was used to bond corner clips to reinforce the metal frame of a panel, channels for sealants, and module support doublers used on a large heliostat. The adhesive had excellent durability with no failures after more than 20 years of operation. Similarly, adhesives have been successfully used for many years to construct aluminum satellite dishes, commercial signage, and military transport vehicles, where thermal and mechanical stresses create fatigue conditions.

Accelerated fatigue testing has also shown how resistant adhesives can be to the mechanical and thermal stresses that are imposed on assemblies. Figure 1 shows the number of cycles that an adhesively bonded specimen withstood as various loads were applied to the joint. The bonded specimen surpassed 10,000,000 cycles of repeated loadings of 900 lb/in1.

In addition to thermal and mechanical stresses, assemblies have to deal with harsh environmental conditions. Proper selection of the adhesive for bonding the substrates is the key. Data in Figure 2 shows that a bonded aluminum structure can have excellent resistance to corrosive environments. In this case, an accelerated corrosion test from the automotive industry, VDA 621-415, was used. The adhesive bond was unaffected after 8 cycles of the VDA test.

Manufacturing with Adhesives

There are many areas of solar panel construction where structural adhesives would be a viable alternative to mechanical fasteners including the assembly of PV panels and in the supporting framework structure. For example, in photovoltaic cell production, the active silicon layer is often sandwiched between two glass panels. A metallic or composite frame encloses the panels; the frame connects to a rack or framework structure that supports the panels. The racks may be affixed to singleor dual-active tracking systems that allow the panels to follow the sun.

Case in point: the manufacture of PV panel systems. An alternative to mechanical fasteners such as U-bolts and screws, structural adhesives can be applied between the glass and the metallic or composite frame, and also used to attach the frame to the rack. Structural adhesives offer a process-friendly solution that can save time and money, while increasing product quality. Depending on the assembly process or the speed of the assembly process, a manufacturer can apply the adhesives from a manual cartridge or a very sophisticated, high-speed robotic system.

Installing with Adhesives

But it’s not only photovoltaic panel manufacturers that can reap benefits from switching to structural adhesives; the installers of these systems will also find advantages in using adhesives in place of mechanical fasteners. For example, consider the number of mechanical fasteners used on the average photovoltaic array. In many cases, there are hundreds of bolts connecting the array to the racking and tracking systems. These bolts require continual maintenance in order to ensure the operating integrity of the solar panel system. Further, since installers offer panel warranties to their customers, usually from 5-20 years, for the installation, they require the consumer to purchase an agreement plan. Under the plan, the installer returns to the installation every 30-90 days to tighten the mechanical fasteners -- and that could be hundreds of fasteners. Although the consumer can choose to maintain the fasteners on his/her own, if the consumer does not comply with the maintenance requirements, the warranty is voided. Reducing the complexity and duration of the maintenance checks would not only reduce costs, but the installer could pass along a portion of the savings to the consumer. A typical commercial installation might have 100-400 solar panels or more, at $1200-$1300 per panel, with the final cost reaching upwards of a half million dollars. Any cost savings to the consumer would make solar panel systems much more attractive as an energy option.

The mechanical fasteners, used to secure the photovoltaic arrays to the supporting racks and tracking systems, loosen over time due to wind-induced and/ or structural vibration. Structural adhesives not only eliminate the problem of loosening fasteners, but also eliminate the need to visit an installation site for maintenance follow-ups.

Another concern for solar panel installers is the weight of the system. Structural adhesives eliminate the weight of mechanical fasteners and improve stress distribution, while providing a clean, streamlined appearance. Further, another potential benefit to PV installers and manufacturers is a reduction in inventory as they can replace their huge inventory of bolts, washers and screws with one structural adhesive kit.

An Easy Choice

Although structural adhesives are a proven, established substitute for mechanical fastening, some manufacturers and installers might still have a few doubts about their viability in solar panel systems. Solar panel systems must be able to withstand extreme heat, high winds and severe weather conditions for the life of the installation. However, structural adhesives have proven to maintain high bond strength performance in harsh environments, and are therefore ideal for use in assemblies that will be exposed to extreme environmental conditions. Further, skilled labor is not necessary, and in less than one half-hour, a person can be taught how to mix the adhesives and use the bonding equipment. By switching to structural adhesives, installers will reap the benefits of less installation time and decreased maintenance, while improving product durability. In addition to labor savings, the cost savings can be significant when using structural adhesives for solar panel assembly and installation. When you consider the number of mechanical fasteners required to secure an array to a rack or tracking system and then compare that to a single structural adhesive, which requires few installation hours and no follow-up maintenance, the choice becomes easy.

Saving Solar Energy

According to the SEIA, “the greatest challenge the US solar market faces is scaling up production and distribution of solar energy technology to drive the price down to be on par with traditional fossil fuel sources.” As a less expensive, environmentally-friendly energy source, solar power is slated to become even more prominent in the US as other energy supplies become more costly and less obtainable.

Conclusion

For photovoltaic panel manufacturers, using structural adhesives in place of mechanical fasteners will help them to build a better product, faster, more simply and with less cost. Solar panel installers will find using structural adhesives can eliminate the problem of loosening fasteners while providing the benefits of little or no maintenance and less labor requirements. For the solar energy industry to be really successful, it is imperative that it continues to lower costs, whether at the module level or the installation level. Structural adhesives offer the potential for cost-savings, along with the reassurance of product durability and environmental stability.