Creating electricity, letting light through

A photovoltaic membrane of 4,296 square meters stretches over 33 robust steel supports like a thin cloth. Through it, you can see clouds drifting by. At irregular, seemingly random intervals, a transparent glass element allows a clear view of the sky. When the students and teachers of the Herwig Blankertz School move into their new space in the former Pommern barracks in Wolfhagen, Germany, next school year, they will pass through this converted tank hangar, whose atmosphere is created by light coming through transparent PV modules.
Architecture firm HHS of Kassel kept the tank hangar’s steel structure, replaced the asbestos sheet roofing with solar modules, and created classrooms underneath. The structured solar modules let ten percent of the sunlight trickle into the building. The giant roofing skin gives off a striking impression. From below, it looks almost delicate. “Thin film modules create a very fine structure. We wanted this delicateness for our project,” explains project manager Markus Meinlschmidt of HHS. In addition, the firm wanted the photovoltaic surface to have some disruptions so it doesn’t seem perfect and uniform. To this end, the designers replaced an occasional solar module with a clear glass element, creating an effect of “broken tiles on a barn roof” – random patches of light and clear views.
Italy and France in particular are breathing life into the market for transparent modules with higher compensation for building-integrated photovoltaics. According to a study by market research firm EuPD Research, they constitute a big share – 59 percent – of the relatively small French market. In Germany, too, the total volume of photovoltaic expansion is expected to grow from three percent today to seven percent in 2012. Indeed, the market is becoming more interesting not only for architects, who can design “lighter” constructions, often with thin film modules as in the school in Wolfhagen, but also for manufacturers, who are developing products for new structural possibilities.
Glass-glass modules, in which crystalline cells are placed at varying distances from each other, are well known and widely used. Light comes into a building through the variably spaces gaps. The architects at HHS also continue to use this technology. It is still attractive, since the hand-sized cells create a distinctive pattern of shadows that architects can use in interior spaces. One example of this technique is the façade of inverter manufacturer SMA’s headquarters. There, double glazed elements with monocrystalline cells in various formats compose the exterior. The result is a glass façade with starkly contrasting areas of light and darkness.
Another well-established way to “play” with transparency is to perforate crystalline cells, as Sunways does. The company perforates their monocrystalline cells with 64 holes, each 5 x 5 millimeters square. This pattern creates a transparency of ten percent and allows an even amount of light to come through across the entire module surface. According to the company, efficiency is still 13.7 percent.
Renowned architect Frank O. Gehry used these cells for a building made entirely of glass, designed for Novartis in Basel. The glass structure is entirely covered in solar cells. This thick cover of cells, which allows only ten percent of the sunlight into the building, prevents overheating while still letting excellent quality light shine in evenly across all façade surfaces. For this project, the California architect ordered solar cells with round perforations at a diameter of only two millimeters.
It’s not all about function. While transparent modules provide both protection from the sun and produce electricity, they also have the advantage over conventional sun protection – a particular quality of light; sunlight filtered through structured solar modules retains its natural color. If the layering density is well matched to the location, using colored glass in a façade or atrium roof may not be necessary.

Stripes, points and rhombuses

Of course, structures do not always have to be small to let in an even amount of light. “What we do with photovoltaics is exactly the opposite of what people expect from a window,” says Christof Erban of Schüco. If you look out through a photovoltaic façade from the inside, you notice dark areas framed by lighter areas. So why not use this effect and play with the forms? Erban, working on new patterns for façade modules based on crystalline cells, is placing thin, dark strips of cells on larger clear glass surfaces to form a weaving pattern of square frames.
Apparently, the process is easier with thin film modules. In a second step after depositing the photovoltaically active semiconductor layers, a laser can make the extensively coated glass panes transparent by removing the black or reddish photovoltaic treatment in some places on the raw module. Stripes, rhombuses, points and freehand designs can thus be created on the active surface. Output losses are roughly equal to the amount of surface removed. A module with ten percent transparency therefore produces one tenth less than a comparable opaque module.
“For the project with the converted tank hangar in Wolfhagen, we had the choice between modules with crystalline cells and thin film,” explains HHS architect Meinlschmidt. Together with their clients, the architects decided on thin film amorphous silicon modules, which cover 90 percent of the surface area. When people look out of the building, they get a rough view of the surroundings. As the current roof is not optimally facing sunward, thin film technology promises higher yields as a percentage of nominal output, since thin film modules are good at turning even diffuse sunlight into electricity.
The 100 x 60 centimeter glass elements lie on a secondary structure made of timber rafters. While retaining ledges secure the long sides, only bands of silicon, in the direction rainwater flows, cover the glass joints. The modules’ wiring runs down aluminum profiles parallel to the drip molding to the inverters, which are secured to the steel stanchions. The 7,160 thin film modules made of amorphous silicon manufactured by Schott Solar of Alzenau, Germany, bring the system to a total of 220 kilowatts peak output.

New solutions with thin film

Thin film cells also easily allow a gradient in the transparency from dark to light, as can be seen on a school façade composed of CIS modules from Würth Solar. These 132 modules, with a basis of copper, indium, and selenium, were designed especially for use in double glazing. Twenty black opaque modules were placed under 108 transparent modules, whose line structure becomes finer as it goes upwards, increasing the amount of light let through. The 92 square meter façade reaches a peak output of seven kilowatts, which comes out to a power density of 76 watts per square meter. In comparison, the opaque CIS modules manage 111 watts per square meter. An array’s yield depends on the degree of transparency. Würth Solar offers up to 50 percent, but that is a fairly rare figure. More common are transparent areas of ten or thirty percent of a module’s surface.
Timo Bauer, product manager at Würth Solar, thinks semi-transparency will continue to be a hot topic. He completed several projects last year in cooperation with architects. The process always looks about the same. “We charge one-time fees for creating the layout,” Bauer explains. “Then we create an individual design through discussions with the client.” The most popular transparent module from the company is composed of alternating clear and opaque areas with a width of three centimeters each. Bauer himself is currently working on designing a more delicate striped structure. “I’m thinking of two to three millimeter thin strips that would create a homogenous image without a zebra effect,” he says.
So far, custom design of transparent areas has only been possible with CIS modules from Würth Solar, although laser technology can make any thin film technique transparent. Schott Solar, for example, manufactures its thin film module ASI Thru with ten percent transparency in only one size – 60 x 100 centimeters – and doesn’t vary the grid pattern for the transparency. When window fitters use modules in large double glazed panes, they place them next to each other, leaving the sides visible. From its offices in Wiesbaden, Germany, Japan’s Kaneka markets modules that are also just under one square meter.
Signet Solar’s European Headquarters in Mochau, Germany, has other plans. Signet produces modules of about six square meters, with edge lengths of up to 2.2 x 2.6 meters. In collaboration with laser specialist Jenoptik of Jena and an industrial partner from the window manufacturing industry, Signet is preparing to enter the market in 2010 with transparent modules in variable patterns. The company is also testing the market beforehand to make sure its products will meet customer demand. “With Jenoptik’s laser technology, individual settings for module stripping can be easily implemented,” explains Signet sales manager Matthias Gerhardt. “But no matter what the flexibility, in the end, what counts is value for the buck,” Gerhardt adds. In other words, not only should the module look good, but also produce electricity. To this end, Signet Solar is aiming for a maximum transparency of 20 percent. Architects and building owners will therefore have an even broader palette of photovoltaic options to choose from in the future.