By Jan H. Schut
Sunlight is made into electricity either directly by entering solar photovoltaic (PV) modules or indirectly by bouncing off solar thermal concentrators to power generators. The New Technology Forum on “Polymers in Solar and Flexible Films” at SPE’s ANTEC conference May 5 in Boston is the first plastics venue to focus on both solar technologies (http://www.4spe.org/sites/default/files/antec11-ntf-gunes.pdf). It will also introduce the growing area of solar thermal concentration to a plastics audience for the first time. Two out of six sessions present plastics in solar concentration. A seventh session (which won’t be given because the presenter changed jobs) would also have covered a new polymer mirror film for solar concentration.
Both solar technologies have been around for over 25 years, during which time PV module technology developed steadily, while thermal concentrator installations stalled. One giant concentrator power system (Solar Energy Generating Systems in the Mojave Desert in Calif.) with nine units was built over six years from 1984 to 1990 and for the next 25 years was the only plant in the world of any size. That’s now changing rapidly. Over 20 new concentrator plants started up in the past two years bringing world capacity to 1265.65 MW and 30 more are under construction for another 2214.4 MW (en.wikipedia.org/wiki/List_of_solar_thermal_power_stations).
THEY DON’T PUT THEM WHERE THE SUN DON’T SHINE
Solar thermal concentrators use large flat or curved mirrors to reflect sunlight onto thermal collectors, which heat water or other liquid to make steam to power turbine generators. Curved mirror dishes can also concentrate sunlight onto thermal absorbers to heat gases, which drive engines and generators. Solar thermal concentrators use all light wavelengths and are about 30% efficient in light-to electricity conversion, but they can store heat or pressure, so they can continue to generate electricity at night. Solar mirrors are programmed to track the sun and reflect the maximum amount of sunlight onto thermal collectors, typically pipes filled with fluid. If pipes plug up or flow stops, mirrors automatically defocus and stop tracking the sun, so collectors don’t overheat. Concentrators are typically installed in blue sky desert conditions, mostly in the Southwestern U.S. and Spain.
Because solar concentrators are most effective when built in the desert, any plastic used has to deal with extreme UV irradiation and sand. Timothy Hebrink, a lead research specialist at 3M Co., St. Paul, Minn. (www.mmm.com) will introduce two new polymer mirror films for concentrators for the first time in presenting “Durable Solar Films for Reducing the Levelized Cost of Energy.” One is a patent-pending mirror film (WO 2010/078105 A1 and US # 2009283133) with hundreds of biaxially oriented birefringent nano layers. The mirror effect in nano layered films is made by bouncing light waves off the interfaces between nano layers of alternating materials with different refractive indices. The new film offers higher reflectivity and better UV stability than previous 3M mirror films, which were made of PEN and PMMA. The new solar mirror film is made of PET high refractive index layers, which are easier to UV stabilize than PEN, and incorporate fluoropolymers for greater UV stability. Commercial less than a year with one large customer, it was introduced previously at solar power shows, but this is its first presentation to a plastics audience.
3M is also re-introducing a solar thermal mirror film, which 3M first developed 20 years ago, abandoned 15 years ago when solar concentrators were developing so slowly, and then began work on again five years ago. It’s a highly reflective silver-coated PMMA film, originally called ECP 305 (energy control product) and now called SMF 1100 (solar mirror film). The new version, commercialized in the past year, has the advantage of UV stability proven in the field for over 15 years, whereas the new nano-layered mirror film is still undergoing accelerated weather testing and has only a few years of real outdoor exposure.
You won’t get to hear Gary Jorgensen describe the latest “Development and Properties of Abrasion-Resistant Hard-Coated Polymer Film Mirrors” for the first time to a plastics audience. But you can read about it here. Jorgensen is co-inventor of a patented (U.S. Pat. # 6989924 and 7612937) multi-polymer mirror film, called ReflecTech, developed in a partnership between the National Renewable Energy Laboratory in Golden, Colo. (www.nrel.gov), and SkyFuel Inc. in Arvada, Colo. (www.skyfuel.com). Jorgensen, formerly a principal scientist at the NREL, recently became principal material scientist at SkyFuel and won’t be able to attend the conference. ReflecTech’s latest development is use of a commercially available UV-cured hard coat from Red Spot Paint & Varnish Co., Evansville, Ind., (www.redspot.com), which allows the polymer mirrors to be contact cleaned. Coated ReflecTech film will be commercial later this year. Uncoated ReflecTech film, which has been commercial for three years, is cleaned with pressured water. Both films are applied to 50-mil thick aluminum sheets 5 ft wide and up to 45 ft long to make solar troughs. Advantages of plastic mirrors over glass are resistance to breakage, lower cost, lighter weight, and one-piece installation in thermal concentrators vs. aligning multiple glass mirrors. SkyFuel makes and sells the troughs, while its ReflecTech subsidiary (www.reflectechsolar.com) sells the film.
There are also new opportunities for plastic films in glass solar mirrors. Philip J. Lingle, technical fellow at Guardian Industries Corp., Carleton, Mich. (www.guardian.com), which makes glass solar mirrors, will present an overview of “Concentrating Solar Power: The Other Solar Option.” In particular, Lingle says glass mirrors could benefit from high temperature protective plastic films, which don’t exist today, to reduce breakability and replace protective coatings. Guardian, which also makes automotive windshields and safety glass, makes solar mirrors much the same way–applying reflective silver to glass, then adding a single layer of PVB (polyvinyl butyral) adhesive film, followed by another sheet of glass. This sandwich is first bent, then autoclaved at high temperature (120 C for up to 4 hours) to remove residual moisture and finally coated for moisture and chemical protection. Laminating two sheets of glass with PVB to make parabolic mirrors up to 8 x 12 ft is very expensive. Replacing laminated glass mirrors with a single glass mirror protected against breakage by plastic films could significantly reduce cost and weight. Using plastic film for corrosion resistance would also be easier to apply than hard coating.
NEW FILMS FOR ‘THIN FILM’ SOLAR MODULES
Several sessions in the forum will present new plastic films for solar PV modules. PV modules are made of interconnecting cells that convert light into electricity by passing it through positive and negative conductive layers, which absorb photons and give off electrons. PV modules are generally smaller and less efficient than solar mirrors. Typically only 6% to 23% of light hitting a PV cell is converted into electricity, though specialized modules can go as high as 40%, and they don’t use long wavelength IR. Most of the available solar energy is lost as heat. Electricity output from PV modules also drops when modules get too hot, for example during the middle of the day. Installations, however, are flexible, ranging in size from large “solar fields” with hundreds of modules and rooftop installations with dozens of them down to single modules for roadside lights and small swatches to recharge handheld devices.
Thin film and flexible solar modules are the technology to watch because they’re made in a continuous process, so they’re lighter weight and less expensive than traditional solar modules made with crystalline silicon (c-Si) in a batch process. Two companies will introduce new high temperature polymeric films for “thin film” solar, so called not because it uses plastic films, but because thin films of semi-conducting material are sputtered onto a substrate, usually metal. Thin film panels use any of three semiconductor materials–a-Si (amorphous silicon), CdTe (cadmium telluride), or CIGS (copper-indium-gallium selenide)–followed by plastic encapsulation sheets, a glass or plastic front sheet and a glass or plastic back sheet. Substrates for thin film PV have to be dimensionally stable at high temperatures in roll-to-roll processes, so they are typically ceramic or metal foils. Any polymers used in thin film solar panels have to withstand very high manufacturing temperatures for 30-40 minutes because of the semiconductor materials.
One contender for a polymer substrate is a “High temperature polyimide film for roll-to-roll copper-indium-gallium-selenide (CIGS) deposition,” presented by Salah Boussaad, senior research physicist at DuPont Co., Wilmington, Del. DuPont’s patent-applied-for (WO 2009/142938) reinforced polyimide for photovoltaics, called PV 9200, is intended to withstand production temperatures of 450 to 500 C. The latest test data shows it can withstand up to 460 C–enough for CIGS modules. DuPont’s patent application describes reinforcement with a nano filler with an aspect ratio of more than 3:1. DuPont’s reinforced polyimide film would be the first to challenge metal and ceramic substrates for CIGS modules. The big advantage of a plastic substrate would be that it’s not electrically conductive, so the PV cells don’t have to be electrically isolated, saving cost.
The other new high temperature plastic for “thin film” modules targets replacing glass front sheets. Dong Zhang, vice president of research and product development at Akron Polymer Systems Inc., Akron, Ohio (www.akronpolysys.com), a developer of new polymers spunoff in 2002 from the University of Akron, will introduce its first “New High Performance Polymer Films for Flexible Displays and Photovoltaics.” The patent-pending polymer, called TN 6, can be solution cast into high temperature film for front sheets for light weight cadmium telluride solar modules. TN 6 has a Tg of >400 C, transparency of >80% and coefficient of thermal expansion of <40 ppm/degree C). Most solar modules are built from the back sheet forward, but cadmium telluride modules are built from the front back, then treated at very high temperature (>350 C). TN 6 film can withstand 390 C for up to an hour during module manufacturing. It’s expected to be commercial in two years. One market would be lighter weight solar modules for shopping centers with flat roofs, which also have to carry a weight of snow.
Xiong Gong, assistant professor of polymer engineering at the University of Akron, will present the physics behind a particularly famous all-organic “thin film” solar module and explain how it can be as efficient as inorganic ones in “Printable Polymer Photovoltaic Cells for ‘Plastic’ Electronics.” The all-organic semiconductor layers are based on the work of Nobel laureate Alan Heeger at the University of Santa Barbara, Calif., under whom Gong did post doctoral work. Heeger’s technology has been commercialized under license by Konarka Technologies Inc. in Lowell, Mass. (www.konarka.com). Konarka prints the ink-like semiconductor material onto a plastic substrate (DuPont’s mylar), using a solution of polymers and conductive nano-carbon clusters or “buckyballs.” Konarka’s thin film solar materials, called Power Plastic, target flexible applications like tent covers, clothing and computer bags. The latter were commercialized a year and a half ago.
3M’s Hebrink will also introduce new micro replication film for solar modules with an “anti-reflection” micro surface structure for the first time. 3M’s micro replication films, which have been made for decades, use high precision micro structures on a plastic substrate. A micro replication pattern, for example, increases reflectivity on films for traffic signs, while a micro Fresnel lens pattern focuses light entering high concentration photovoltaic modules. (These use triple junction gallium arsenide PV cells with 40% efficiency, typically for PV utilities in the desert.) Prismic Fresnel lenses were originally invented in glass to increase light transmission from lighthouses. 3M’s new micro-replication film for photovoltaics will use a special surface geometry with patent-pending material combinations to minimize surface reflections, letting more light into modules. It increases power output 5-10%, but won’t be commercially available until its durability has been more thoroughly tested.
An even bigger increase in solar module output, however, comes from combining PV modules with light wave selective nano-layered mirror film. Mirror films can be designed to reflect only the wavelengths of light that are used by a PV cell. 3M’s “Cool Mirror” film for PV modules, for example, selectively transmits IR heat away from the module and reflects useful wavelengths into the module. This increases power generation 50%-100%, Hebrink says, and modules don’t overheat. That’s brilliant!