Solar cells for future-oriented façades

Voltarlux, the photovoltaic glass based on thin-film modules and produced by Arnold Glas, was integrated into the façade of the Kulturhaus Milbertshofen cultural centre.
Pictures: Arnold Glas

Thin-film solar cells can be easily integrated into glass façades. They produce power even without direct exposure to sunlight and may also be used to create shade.

At a thickness of only 3 to 5 µm, thin-film solar cells are 50 to 100 times thinner than a human hair, yet they are very efficient. They are capable of producing up to 60 kilowatt hours of electrical energy per square metre and year. In the area of solar architecture, the razor-thin solar cells offer a very broad potential of use, since they may be inconspicuously integrated into facades or glass roofs and offer, in addition to environmentally friendly power production, also effective protection from the sun.

Growing market share

According to forecasts by the Munich-based company iSuppli Deutschland GmbH, the market share of thin-film solar modules will increase from ten to 18 percent by 2011. The Association of Machinery and Plant Manufacturers even predicts that in 2010, power created by thin-film modules will cost the same as power from conventional power plants. Thus the future outlook for this technology is bright. The glass refiner Arnold Glass from Baden-Württemberg reports considerable success with Voltarlux, a glass variety that produces power and creates shade at the same time. Voltarlux is currently being used in the building of the Munich cultural centre Kulturhaus Milbertshofen. The thin-film solar cells integrated into the glass façade produce an average of 3,000 kilowatt hours of power per year.

Lighter in weight than traditional modules

Voltarlux was developed in a cooperation between Arnold Glas and the thin-film-module specialists at SCHOTTsolar. A thin layer of amorphous silicon, the most common material for thin-film cells, is deposited onto a glass pane like a film-coating. Since the window glass serves as the carrier material of the thin-film modules, no further stabilizer is required. Therefore, the thin-film modules, weighing 20 to 30 mg/m², are significantly lighter than wafer-based modules, which are cut as wafers from a silicon bar and may weigh up to 1 kg/m².

Power production without direct exposure to the sun

At a rate of about 5 to 7 percent, the efficiency level of amorphous silicon-based thin-film solar cells is currently still lower than that of thick-layer modules of mono-crystalline silicon (12 – 14%) or multi-crystalline silicon (10 - 12%), however, efficiency levels of 10 percent have already been achieved in current laboratory tests with further optimised coatings. Experts assume that efficiency levels as high as 12 percent are possible. Thus, the thin-film modules would achieve almost the same level as produced by thick-layer solar modules. Another advantage is that thin-film cells produce power even without direct exposure to the sun and are hardly temperature sensitive. While the common, wafer-based modules produce optimal power generation only with best-possible sun exposure and an angle of inclination of 35 degrees, the thin-film modules are fully efficient even at angles of less than 10 degrees. They are therefore optimally suited for the conditions especially in Germany and the other Northern European countries. Costs are also a factor in favour of thin-film technology. “In 2008, the production costs for thin-film modules are between € 0.65 and 1.30 per watt of output,” says Dr. Hennig Wicht, Senior Director of iSuppli Germany. “But as early as 2009, the costs will be about 10 percent lower.“ The reason, he says, can be found mainly in improved efficiency of the modules.

The razor thin solar cells applied to the glass can only be seen at close inspection.

Suitable for shading

The solar glass created by Glas Arnold and SCHOTTsolar features another useful characteristic. The solar modules absorb a portion of the solar radiation and allow only about ten percent of the light to pass through the pane to the inside. Although this sounds like very little, in practice it creates a daylight atmosphere with a pleasant shading effect. The view from the inside to the outside is nearly free of obstacles.

For galleries and museums this integrated shading effect of the solar glass is an invaluable advantage, since old paintings and art prints must be protected from intensive exposure to light. According to Arnold Glas Company, the integration of thin-film solar modules makes sense even for private homes, which generally offer less area than large-scale public buildings. Roofs, for example, provide a large area for the integration of solar solutions. Used as a replacement for roofing tiles, the semi-transparent thin-film solar modules do not only offer a completely new aesthetic dimension but also afford unique views of the sky above.

Solar technology at glasstec 2008

The motto of glasstec 2008 is “Glass and Energy". It offers far reaching insights into the advances and possibilities of thin-film technology. In addition to Arnold Glas (Hall 11, stand B26), other companies and institutions will present their latest developments at the foremost international trade fair of the glass industry. Very exciting in this connection is also the Architectural Congress, which will be held on 24 October with the motto “Transparency! Glass and Façade Technology“ simultaneously with glasstec at the CCD Ost (East) at Messe Düsseldorf.