Siliconne Rubber: Relying on glass as a component material for CPV system

After years of research, concentrating photovoltaics (CPV) technology is gearing up to achieve what has been promised over the years. For a technology that only exploits normal direct solar irradiation, the challenge has been to move it out of the research phase and prove its worth. Benefits of concentrating are related to cost, and also manufacturability and reliability. By Hriday Malik.

CPV, which features an optical system that focuses a large area of sunlight onto each cell, is all about minimising the ratio of expensive components such as the solar cells. Instead, the focus is on optimising the use of cheaper components, such as the glass, used in the collection system. Companies like the US-based SolFocus highlight that concentrating systems are mechanical assemblies. They can make use of inexpensive, field-proven materials and manufacturing techniques. An efficient way of overcoming the supply constraints of specialised materials such as PV cells is to use materials such as glass. Unique challenges From glass manufacturers’ perspective, when they break-down the cost of traditional c-Si and thin-film modules (whether silicon-based, CIS/CIGS or CdTe), glass is a relatively inexpensive component of the system compared to the semi-conductors and other components. However, the economics are a bit different when it comes to CPV. This is because as both the cost and conversion efficiencies of the modules increase, so does the importance of the performance of all the other components. Because the cells used in CPV applications are generally both significantly more expensive and significantly more efficient than standard PV technologies, the economics are all about trying to get as much of the usable spectrum of light as possible to the cell or module and to protect that very expensive cell from any environmental or other damage. The slight difference in CPV versus traditional PV applications is that the performance of the glass is far more important than just the cost, says Scott Thomsen, vice president and chief technology officer of Guardian group. “That’s not to say that cost is not still a key driver of the decision to use glass. However, it is somewhat less of a consideration overall given the premium placed on performance, durability, system longevity and the protection of the cells themselves,” explains Thomsen. According to another manufacturer, Perrysburg, Ohio-based Glasstech, when it comes to reflector shapes, glass has proven to be a viable, effective substrate in focusing sunlight. The tighter the tolerances achieved, the more energy driven into the prescribed target area for conversion. Though the glass used in glass reflector technology is a low-iron glass, supply continues to grow for this product and it is available in the marketplace. The capability of Glasstech systems, according to its director for solar energy systems, Mike Ondrus, is to process glass in high volumes at fast throughput rates and produce formed and strengthened finished product to become advantageous to the customer in driving down production costs and minimising installation and in-field losses which all contribute to driving down cost per watt. Usage of glass CPV technology companies have been looking at developing CPV products to generate electrical power at a cost that will reach grid parity in countries with good levels of sunlight by as early as 2010. The UK-based Silicon CPV plc is one of them. Germany’s Concentrix Solar GmbH, too, is making rapid progress. Glass is still by far the best transparent cover and therefore used by two of the three most competitive CPV companies, says Dr. Andreas Gombert, chief technology officer of Freiburg, Germany-based Concentrix. For its part, Concentrix also uses glass due to its low thermal expansion and cost. According to Silicon CPV’s CEO Prof. Humayun Mughal, the use of glass in CPV systems varies considerably from one manufacturer to another. “For example at Silicon CPV, we use half the amount of glass compared to equivalent flat plat PV whereas Concentrix Solar uses twice as much as equivalent flat plat PV. In my view glass will be a relatively small but an important component in future CPV systems,” said Prof. Mughal. He added that there will be a need for glass with high transmitance and low reflection properties specially formulated to deal with a very different optical arrangement, compared to flat plat PV, that exists in CPV systems. There will be a need for low cost glass with anti-reflection coating. According to Guardian’s Thomsen, generally the advancements fall into the categories of the lenses and reflectors, along with their performance and configuration within the system. The ultimate goals, of course, are to concentrate the maximum amount of light in the tightest or most accurate concentrating path to achieve the highest concentrating ratios using the fewest components and least space possible -- all at the lowest cost possible. “The contribution glass makes regarding these goals is related to technologies that enhance total solar reflectivity and focus precision (e.g. very high transmission glass, superior reflective coatings, custom and precision bending of mirrors to optimize focus, etc.),” said Thomsen. For instance, manufacturing methods and designs which enable the use of very thin glass reflectors can dramatically improve reflectivity and efficiency. “In fact, Guardian is pioneering several thin, high-efficiency monolithic and laminated reflector designs for CPV which provide not only best-in-class reflectivity but which are also customised to meet the specific needs of each application or technology,” Thomsen said. Mass production There are two main types of concentrating optical systems: refractive types that use Fresnel lenses, and reflective systems that use one or more mirrors. Mirror-based systems rely on reflected light and Fresnel and other lens-based systems use refracted light, in both cases the transmission of the usable light through the system and to the receiver is one of the most critical issues. High-performance solar glass is considered to be a perfect system component because of the higher solar transmission in both systems and higher reflectance in the mirror-based systems. According to Glasstech’s Ondrus, reflective technology and low-iron glass of varying thicknesses are being used for CPV systems. Glass can be bent and formed into the shape with the potential of being thermally strengthened and used as part of a structural support, depending on the application. Glasstech systems are well suited to bend glass reflector shapes to precise tolerances and at high volume to reduce costs. Concentrix’s Dr. Gombert explains that Concentrix’s technology comprises fresnel lenses replicated in silicone rubber under a glass plate (instead of the widespread hot embossed acrylics), bent glass is also a substrate for mirrors, providing a better durability of the coating. One of the critical factors is related to mass production of highly reliable CPV devices in order to drive the costs down. Prof. Mughal says CPV technology is still at low volume stage and the supply chain has not really been tested. Only when the technology moves into GigaWatt level, the strains in the supply chain will become visible. “This technology has the potential to be most cost effective at the utility level deployment. The glass will largely be used for either system protection from the environment or as a substrate,” said Prof. Mughal. It is certainly true that there are advantages to trading off more widely available materials like glass and mirrors for relatively expensive PV cell materials. However, when it comes to comparison between, say, a traditional (non-concentrating) c-Si PV system vs. a CPV system, it could get more complicated depending on the CPV technology. For example, while many of the CPV systems may require smaller quantities of PV cell material, some of those materials themselves can be very expensive and hard to come by, particularly in the form of finished goods ready for assembly into modules. “So,in general it is a benefit to trade-off more widely available components for those that might be in short supply but I would put a caveat on stating that as a rule,” says Thomsen. At the same time, Thomsen says “mass production” is music to “our ears”. Just as in many other industries, the real efficiencies in manufacturing glass products come from large scale production campaigns. In addition, product quality and consistency also becomes easier to maintain at a lower cost as volume goes up. Mutually beneficial relationship Prof. Mughal says the glass manufacturers should work closely with CPV system designers to produce the type and quality of glass needed to give highest optical performance at the lowest possible cost in order to achieve grid parity using CPV technology. Guardian’s Thomsen says the industry is ready to provide the glass necessary to scale up with higher volumes of production of these PV and CPV products. At the same time, he acknowledges that in the short-term, the greatest challenge lies in dealing with all the different and competing CPV technology platforms and customer requirements. “It seems that each company or CPV technology platform requires a slightly different - in some cases, extremely unique- type and configuration of glass. Until they reach high-volume production, these competing platforms result in very high R&D and prototyping costs along with higher production costs that result from short production campaigns,” said Thomsen. In the longer-term, there remain opportunities to incrementally and continually improve the transmission of the glass itself, as well as the reflectivity and accuracy of mirrors. “The real technical challenges will probably come in making progress in other areas. These fall into many categories. For example, anti-reflective coatings are becoming more widely available today and Guardian is launching such a coating on our EcoGuard Pattern glass. One challenge for the industry, however, is to develop next-generation anti-reflective coatings that provide the gains in transmission while also being able to stand-up to long-term and often very hostile environmental conditions,” concluded Thomsen.