Renewable remarks: Acrylic optics in concentrating photovoltaics, focusing on reliability
Published: December 19th, 2011
Concentrated photovoltaics (CPV) use materials like glass and plastic to concentrate solar radiation onto a small, high efficiency photovoltaic cell to be converted into electricity. As this growing market becomes more standardized and commercialized, the industry is still split about whether plastics are suitable for reliable use in these outdoor environments.
In 2010, NREL released a report about the durability and lifetime of PMMA lenses indicating that some materials had higher degrees of yellowing and hazing leading to reduced transmission of light over time. A presentation at the 2011 Solarexpo by Frauenhofer indicated that CPV optics are still regarded as potential system lifetime limiters.
Some leading CPV manufacturers, such as Amonix, use plastic-based fresnel lenses, while others such as Solenergy, remain hesitant and continue to include all-glass, silicone on glass, or other glass-based materials for optics in their systems.
Since the NREL report was released, much testing and focus has been placed upon understanding and overcoming lifetime and reliability issues of using acrylic for high performance optics outdoors.
As more companies are designing CPV systems for better reliability and lifetime from the outset, customized testing now goes beyond the 20-year outdoor performance data that plastics manufacturers provide from usage in other markets. PMMA used in CPV systems now undergoes tests that among other specialty variables involve concentrated light intensity, sandblasting, and/or accelerated aging.
Evonik Industries, who manufacturers acrylic and silicone-on-glass materials for the CPV industry, has made many improvements to their UV resistant and temperature stabilized acrylics, offering a variety of options and protection systems such as uv absorbers, stabilizers, and combinations that help reduce the amount of degradation due to heat, humidity, and time.
Their customers use this material with a hot vacuum embossing process to produce CPV lens optics that are integrated into different types of manufactured CPV systems. Evonik recently released news that they are now supplying large sized lenses, and their improved PMMA fresnel lens material has been installed in "over 10 MW of electricity from concentrating photovoltaics in 2011", or about half of the total estimated volume installed in all of 2010.
Case Western Reserve University reported at the EERE's 2011 Solar Reliability Workshop that during natural weathering of Evonik's Acrylite in Low Concentration PV systems, transmission was minimally affected after a decade of lifetime testing, and humidity did not affect the amount of light transmission when tested in Arizona and Florida. This supports Evonik's findings of no yellowing over a decade in Arizona, and "virtually no yellowing" in Florida's more humid environment.
Concentrated photovoltaics represent an industry that is still expanding. Though not every CPV need is currently best served through the use of plastics, customized solutions are increasingly being found that make it a material worth considering.
About the Author: Debbie Sniderman writes, owns, and consults with VI Ventures (www.vivllc.com), an R&D and manufacturing consulting company for renewable energy products and technologies. She can be contacted at firstname.lastname@example.org.