03.15.10 Hayward Authors Technical Solar Article

Chicago, IL — March 15, 2010 — VentureLab partner Laurence Hayward has recently authored the article, "Improving the Commercial Viability of Concentrator Lens Technology," for publication in various solar journals. The text of the article follows.

Improving the Commercial Viability of Concentrator Lens TechnologyRecent Advances in Manufacturing Processes Can HelpBy Laurence K. Hayward, VentureLab, March 2010
According to the 2009 Global PV Industry Report, demand for solar energy has grown approximately 30% annually for the past 15 years. Still, solar energy contributes less than 1% to the world’s energy supply. The ongoing challenge for commercializing solar technology is cost. Although increased supply has reduced silicon prices, total costs including materials, component assembly, installation, and ongoing maintenance remain high. The upfront nature of the capital expense is a challenge. The footprint requirement can be a limitation. And except in select areas, conversion efficiency aren’t at the levels yet desired. Without subsidies, adoption will be protracted.
Not to be deterred, solar companies are tackling the aforementioned challenges. One critical element in doing so is to find better methods of manufacture. The industry requires processes that can not only produce solar components to exacting demands, but also processes that scale up efficiently as production needs rise – all while reducing costs.
Companies using Concentrator technology, both the Concentrator Photovoltaic (CPV) and Concentrator Solar Power (CSP) companies, continue to achieve new records of conversion efficiency. Several companies or institutions in CPV now claim efficiencies greater than 40%. However, it is one thing to achieve a high conversion efficiency, it’s yet another to create economic efficiency. Among other things, the latter requires practical manufacturing technology.
In this article, we will discuss manufacturing technology in the context of the CPV solar segment and the use of concentrating optics, but some of these same technologies can be applied to other segments including the use of Fresnel mirrors in CSP.
In CPV, the emerging standard is the III-V cell, which is comprised of elements from the III and V columns of the periodic table, such as gallium indium arsenide, gallium indium phosphide and germanium. Using multiple layers of different elements increases the range of acceptable wavelengths of absorption for the cells and allows them to capture more energy from sunlight. A challenge with multi-junction cells, however, is that they are considerably more expensive than conventional silicon cells, further exacerbating the issue of cost.
However, there is potential to improve the efficiency-to-cost ratio by using multi-junction cells in combination with concentrating optics to focus sunlight onto a relatively small area of solar cells. In simple terms, optical devices of area x are used to harness sunlight and focus them onto a smaller area y of semiconductor material with a concentration ratio is x / y. Properly designed, CPV may require only hundredths of a fraction of semiconductor materials compared to traditional PV cells, paving the way for lower cost solar modules that consist mostly of relatively inexpensive mirrors or lenses (See Figure 1 as an example of a Fresnel lens concentrator). Costs decrease as concentration ratios increase. Current technology is able to achieve concentrations of more than 1,000 times.
There are three main types of solar concentrators used. They include:
1. Reflective concentrators – using curved mirrors to focus sunlight on a parabolic or dish reflector. These mirrors are typically made from coated glass or metal foils.2. Refractive concentrators – using Fresnel lenses to improve the performance of solar systems by concentrating the maximum level of sunlight on the PV cells. The structure of the lens varies by application and can be plane, cylindrical or dome shaped. They are typically made from glass or polymer.3. Hybrid concentrators – Fresnel lenses are often incorporated with supporting elements using refraction or total internal reflection. The compound parabolic concentrator is one example. It has a wide acceptance angle that can capture a relatively large amount of sunlight without the need for complex structures and tracking systems, although they still might be used in conjunction.
Tracking systems are an important component to concentrator technology to ensure the focus of the sun meets the acceptance angle of the optics. At low concentrations (less than 500 times), a one-axis tracking system may be sufficient. At higher concentrations, a two-axis tracking system is more typical to ensure maximum exposure to sunlight. When supporting systems such as the parabolic concentrator mentioned above are used, the requirements for tracking system accuracy and complexity can be less demanding.
For both reflective and refractive concentrators, one of the major challenges is how to structure the optics into the solar panel. Injection or compression molding lenses of any material can result in long-term problems due to the polymers used and the residual stress inherent in these processes, which can lead to optical distortions over time. These processes also may have trouble meeting the tolerances required for precision optics and holding such tolerances over long periods of time under severe environmental stress.
Another challenge is developing efficient production technologies to bring down costs. Injection molding is challenged by two factors: relatively high ongoing tooling costs and a batch process that results in many individual parts or pieces that require handling and assembly.
The use of thin films is emerging as an economical method of production. They use less material, have a thinner active area, and are well-suited for large-scale automated production and packaging. Manufactures of solar systems often purchase “mirror film” from suppliers and attach it to parabolic or dish systems.
For Fresnel lenses, one method is to mold optically clear silicone rubber, which has thermal stability at high temperatures, and laminate it to a glass substrate. However, weight and cost can be an issue in some applications and there are limited long-term studies demonstrating the weathering of silicone adhered to glass. Another option is to use a selection of polymers with a demonstrated history of holding up under environmental stresses of the natural elements. The traffic control industry has more than 30 years of experience demonstrating the weathering capability of polymers used to reflect light on traffic signs. These products use Polymethyl methacrylate (PMMA), a transparent thermoplastic also known as “acrylic”. Several of the longest running CPV installations have also used PMMA. However, it is important to note that there are many formulations of PMMA and selecting the correct form can be critical to long term performance.
This is an area where Robert Pricone, founder of 10x Technology, has a great deal of experience. According to Mr. Pricone, “The solar industry stands to benefit from the ground paved by other industries in the use of polymers that can hold up to the demands of high stress environmental conditions. Polymer selection, in combination with the method of forming the optics, is a key element in designing lens that efficiently transmit light for many years.”
Robert is the founder of an emerging company based in Libertyville, IL that uses a continuous roll-to-roll manufacturing process to produce micro-structured polymer substrates. The process enables the production solar concentrator lenses in thin films, which can then be laminated to thicker substrates – the objective of which is to permit high volume manufacturing and lower the per square foot cost of concentrator lenses. 10x is not working alone; it recently formed a partnership with Evonik, a global supplier of specialty chemicals that supplies structural plastics for use in solar panels. The companies are marketing concentrator lenses using 10x micro-replication technology and Evonik Plexiglass. 10x is also collaborating with LPI, a world leader in the field of no nimaging optics, to develop the optical components.
Concentrator technology is an important element in the advancement of solar. While companies are taking different approaches to achieve new breakthroughs, a common need is a manufacturing platform that increases efficiency, reliability and cost-competitiveness. Help appears to be on the way.
To learn more about manufacturing technologies for solar concentrators, contact Laurence Hayward at VentureLab Inc. (lkh@thventurelab.com, 847.739.0100x2)
About VentureLabVentureLab provides business and financial advisory services to early-stage companies. VentureLab works with business owners and entrepreneurs to identify their core needs, evaluate the company’s strengths and opportunities, build plans to facilitate the greatest return on investment, and execute those plans to raise capital, form strategic alliances, and grow the company. Headquartered in Northbrook, IL, VentureLab serves clients in the Midwest and throughout the United States. www.theventurelab.com
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