No reflection please

Solar power can give conventional energy a run for its money. That is not something that is too far-fetched to postulate. After all, we do have the sun shining on us everyday (unless you are experiencing the winter solstice). Still, the story is such that not all of this glorious sunlight can be absorbed by our hardworking solar cells. Photovoltaic modules have to deal with reduced conversion efficiency even before the light has a chance to reach the solar cell. The protective glass cover of the module plays a role in the reflection of some of the incident sunlight. The story would be different if the cells were sitting naked on a rooftop. However, that would mean exposing them to environmental stress, and at the mercy of deposits from the sky – think of dust, hailstones, birds, stray golf balls – that can irreversibly damage the cells. Hence, a protective layer such as glass is necessary.

Glass, ARC and solar

Wayne Boor, Manager, Solar Technology Transfer, PPG Industries, states that according to industry forecasters solar coating will account for ten to 20 percent of flat glass sales, if not more, by 2015. U.S. based PPG has been working with the solar industry forerunners to increase the efficiencies of solar specific, anti-reflective coated glass. Boor stresses that glass substrates have to be more transparent to solar energy. Glass manufacturers may or may not be able to dance to the tunes of the solar sector completely. At the recent Solarpeq conference in Düsseldorf, Germany, there was a hint of the opinion that the glass manufacturers may not be ready to change their attitudes towards the solar sector. As Scott Thomsen, Group Vice President of Guardian Industries was quoted saying, the PV market is still not important enough for people in the glass industry (see pv magazine, 11/2010).
Theodoor Scheerder, Scheuten Solar’s Director of Sales and Marketing, sees it otherwise. “For me, I see no real obstacle (for glass and solar industries to collaborate and accommodate one anothers’ needs). The adoption rate is growing very fast. The main change will be to implement high speed coating deposition technologies for one-sided coatings,” he says. On the other hand, there are some solar manufacturers who made the move to keep their solar specific AR coated glass ‘in house’ by manufacturing it themselves. Scheuten Solar is one. Scheuten uses f-glass on its modules. F-glass, in Saxony-Anhalt, is the collaboration and sharing of knowledge between Scheuten Solar and glass maker Interpane. This has enabled the crews to understand each others’ needs relatively well in order to come up with the apparently successful design for the optimal AR coated glass. Scheerder points out, “For Scheuten itself, fitting solar and glass comes naturally as we have the technology in-house. By the joint venture and the operation that we have in the Solar Valley in Germany, our company is situated right in the middle where solar manufacturers sit and demand for glass. That was also in the vision for the development of the f-glass facility there. It has been built with the insight to provide the right glass for the solar business.”
The coming together of solar and glass manufacturers to produce AR coated solar glass with the optimum fit for modules has become an increasing trend. Other glass-solar ‘marriages’ include AGC Glass and Tenesol, to manufacture customized laminated-glass modules, as well as Australian dye-sensitized solar cell materials supplier Dyesol and glass manufacturers Pilkington North America, to form DyeTec Solar. On the production technology side, solar industry equipment supplier Beneq joined forces with flat glass producer Glaston to develop glass coating technologies, of which ARC equipment are a part. Beneq manufactures mostly aerosol coating and atomic layer deposition equipment.
These partnerships mean that the solar glass industry is strengthened and the solutions for ARCs are being improved on. When the two sectors collaborate and allow for better information exchange, and even more if they are housed under the same roof, then costs of solar glass coatings as well as reflections themselves, can be brought down. Still, not every manufacturer has the luxury of ‘in-house’ manufacture of solar glass with the optimal ARC. These module manufacturers look to leading industry glass and coating suppliers to protect their cells as well as to push that transmission boundary.

Coatings

An optimal ARC should increase light over the entire solar spectrum. Because the wavelengths of sunlight vary over a broad range and are not confined to one. To optimize the generous energy given by the sun, none of these wavelengths should technically be wasted. Normally, the ARC thickness is tuned for the glass used and controlled to one quarter of the targeted wavelength to optimize anti-reflection capabilities. Since sunlight has a broad wavelength, multi-layer coatings are often employed. The angle at which the light is incident on the surface affects the light reflected from the surface. The position of the sun changes throughout the day, consequently, the incident angle. Thus, an ARC would have to ensure the reduction of reflection and increase in transmission through the day.
pv magazine spoke to Olivier Mal, Marketing Director of AGC Solar. On the practical side of things, Mal suggests that one has to examine the initial performance as well when looking out for the AR ability of the module. This is the increase at watt-peak level. He also suggests looking at on-field performance, the durability performance, hydrophillicity and aesthetics. He adds that from a manufacturer’s point of view, the ARC or the coating technology has to be able to process on the lines as well.
According to the Berlin-Brandenburg Association for Solar Energy (DGS), the standard thickness of a solar glass that protects the crystalline silicon cells, is related to the size of the module and usually varies between three to six millimeters. In the case of unusually large or customized modules, it can be as thick as ten millimeters. To cut back on the reflective effect of the glass, manufacturers developed various ARCs for glass for PV modules. The DGS offers a few examples of common ARCs. Solar glass, for example, is immersed in a bath of a porous layer of silicon dioxide or Sol-Gel.
ARCs can also be composed of sputtered-on silica and silicon nitride. Mal says, “It is possible to reduce the reflection at the air-glass interface by adapting the refractive index of the top layer of the glass. Treating the surface of the glass in order to generate a smoother transition between the refractive index of air and glass usually does this. The thickness of this transition layer is typically of an order of magnitude of 100 to 200 nanometers (0.1 to 0.2 micrometers).” Mal adds that the Solar Plus Anti-Reflective Coating (SPARC) that AGC Solar has created increases light transmission up to 2.4 percent, when measured at normal incidence. As it is usually in practice, SPARC’s light transmission is then further increased at lower incidence. “As a result, the energy output of the module in real condition is increased by up to five percent,” Mal evaluates. SPARC is also hydrophillic. This means that water forms a uniform film on the glass surface that removes soiling and deters the formation of drying marks as well. Plus, knowing how important looks can be for certain customers, SPARC apparently can also preserve the aesthetics of the glass surface as well.
According to Scheerder, ARCs were once dishonored as they were deemed as not being long- lasting. A solution came in the form of f-glass solarfloat HT. “The composition is a kind of silicon dioxide; it can be called ‘a quartz-like layer.’ This material is so hard that it is robust and seals off the total layer. Thanks to the nanopore structure to the glass, it has a gradual interface and solves the air to glass light change, giving it its light collection property. You typically lose four percent and this is now largely reduced.” Scheuten Solar uses the high transmission glass on its new P6-60 and P6-66 photovoltaic modules. The glass that is manufactured by f-glass, has a quartz-hard ARC that provides up to three percent higher energy yield. The silicon dioxide layer is grown on float glass, with nanosized structures. Tests done by the company show that the solarfloat HT AR coated surface, after three weeks of damp-heat tests, resulted in protecting the float glass better from glass corrosion than the non-coated surface. The state of the glass, after cleaning, showed the results even clearer.
DSM Functional Coatings tackled the ARC optimization puzzle their own way. Hugo Schoot, DSM’s Business Development Manager explains it to pv magazine. Traditional single-layer ARCs consist of solid silica nanoparticles that are glued together with a binder so that the spaces between act as nanopores. Too much binder can reduce nanoporosity and thereby AR performance. Too little can lead to poor mechanical strength and low durability. The open structure of the surface makes the silica layer of the coating vulnerable to hydrolysis that can result in the deterioration of optical and mechanical properties. DSM’s KhepriCoat turns the structure of the coating inside out and the nanopores are then formed inside hollow particles. DSM uses core-shell particles that have a polymer core and a silica shell. During coating, a 100 to 150 nanometer layer of material is deposited on the glass. Thereafter, the spaces between the deposited core shell particles are filled with a silica binder. During thermal hardening, the polymer cores of the particles are removed, leaving a silica layer with a high proportion of binder and internal nanoporosity. The KhepriCoat has been one of the contributing factors for the 17 percent efficiency of Norwegian manufacturer REC’s multi-crystalline modules.

Production technology

On the production side, module manufacturers will also have to seek fitting automation. Mal elaborates that there are several existing processes on the market for ARC deposition. Chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma enhanced and atomic layer deposition techniques are part of the smorgasbord. The ways are plenty and the module manufacturer has to figure out the optimal methodology in terms of efficiency and cost for his ARC choice. After all, this step of production is a contribution to the end-cost of the module.
Sputtering (see pv magazine, 10/2010), has according to previous research high deposition rates of up to 150 nanometers per minute, very uniform coating and is a well-developed technology. This gives it some clear advantages. And, according to a paper presented at the 14th Workshop on Crystalline Silicon Solar Cells and Modules in Colorado, in the U.S., it is estimated that non-uniformity of depositions can lead to a photocurrent density loss of about one milliampere per square centimeter. Sputtering, good as it is, does not come cheap, apparently. Other technologies and manufacturers who offer their deposition solutions for ARCs are challenging this method in terms cost.
One deposition art, pressure nozzles, has advantageous cost factors. But nozzles often raises eyebrows when used in the same sentence as efficient coating. Nozzles can cause significant overspray, clogging, have poor deposition control and inconsistent uniformity. U.S. based Ultrasonic System’s Robert LePage says that any type of nozzle, air atomizing or ultrasonic, delivers a spray pattern that has a conical shape, maybe elliptical at best. He adds that the pattern is going to be heavier in the middle and lighter at the edges. Imagine a drop of liquid falling onto a surface and this gets easier to visualize. “That is why spray patterns with nozzles are non-uniform and more difficult to control, compared to nozzle-less spraying,” adds LePage. Spray coating is one cost-effective alternative. LePage elaborates on his company’s system, the PV 480.
Glass panels up to 48 inches wide (1.2192 meters) can be fed in and the proprietary Ultrasonic spray head technology coats uniform, thin films. Speed can also be a factor for manufacturers. So, how fast does the PV 480 roll? LePage explains, “Up to six feet (1.8288 meters) per minute. We have no problem with processing a 600 by 1,200 millimeter panel every 30 seconds. USI’s ultrasonic spray head technology is nozzle-less. We have an ultrasonic transducer that vibrates at a fixed frequency. The coating is metered to the side of the vibrating transducer and the ultrasonic energy does the job of atomizing the coating-liquid into a very fine mist. An independent air director shapes and controls the mist into a uniform, flat, rectangular spray pattern. The spray pattern can be controlled somewhere between a 100 and 200 millimeters wide.” Cost-wise, compared to sputtering or plasma enhanced deposition of ARC, nozzle-less spraying of ARCs can be seen as an alternative.
Sono-Tek is, however, trying to dispel the nozzle-curse. Sono-Tek claims that with their HyperSonic system, the ultrasonic nozzles are able to have minimal overspray, non-clogging performance and uniform coatings. According to Sono-Tek, the drop size control is done by varying the nozzle frequency and tightening drop distribution to provide results that are uniform and precise. The issue of large applications can apparently be addressed by the incorporation of a robust, high-speed reciprocator (up to two meters per second) to the nozzles. Nevertheless, as mentioned before, the sector has plenty of options to offer.
ARCs hold an important key in module prices and efficiencies. Most solar manufacturers have found the right partnerships or have started their own ARC research and development in house. However, the trend seems to be that more solar manufacturers are preferring to keep things under their own roofs.