PHOTON International The Photovoltaic Magazine

The return of laminated glass modules with PVB

During the early years of the PV industry, many manufacturers built their solar modules like safety glass. But the polyvinyl butyral (PVB) foil used for this clouded upon contact with water vapor, which explains the subsequent switch to ethylene vinyl acetate (EVA). Now, several module manufacturers are working on a comeback for PVB.

	© WulfmeierSolar GmbH
Renaissance: Wulfmeier Solar uses PVB in its modules rather than the more conventional EVA foil. Hence the modules can be used in overhead glazing in Germany without undergoing any special tests – for instance, as pictured here in this entrance hall in Bielefeld.

Solar modules are exposed to weather like almost no other high-tech product, and still are expected to have extremely long life-spans. Manufacturers generally guarantee that their products will produce at lease 80 percent of their nominal power after 20 to 25 years of operation, with total life-spans being somewhere beyond 30 years.

Such a long life expectancy is achieved with durable packing: the upper layer needs to be translucent, scratch-resistant, and weatherproof, which is why it normally consists of glass. Directly under that layer are the solar cells wrapped in an encapsulation material. The back-side of the module is usually a compound foil made of polyvinylfluoride (PVF), which is also known under the brand name Tedlar from Dupont, and polyethylene terephtalate (PET). However, often the second module layer is also made of glass. The encapsulation material is not only used to embed the solar cells, but it also ensures that the front- and back-side stick together. That's why the encapsulation material cannot contain any aggressive components that could attack cell parts. It has to electrically isolate the cells from one another, protect them from water vapor and oxygen, and on top of all of this, maintain this protection for decades.

The problem with cloudiness

In the early period of PV module production, Siemens Solar or AEG, for instance, used foils made of thermoplastic polyvinyl butyral (PVB) as encapsulation material. PVB had already been used for laminated glass in the construction and automobile industries. It possesses good adhesion qualities and the desired durability. Unfortunately, the material has one unattractive characteristic: if PVB absorbs moisture, the foils cloud up. Clouding means reduced translucence, and that means a significant drop in module power. »This effect was even noticeable on the laminated glass windows of old German train cars,« reports Helge Schmidhuber from Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany: »Over time, upon contact with moisture, the PVB foils became a cloudy white.«

To prevent this from happening to solar modules, manufacturers switched foils, generally settling with ethylene vinyl acetate (EVA). It functioned so well in combination with a PET/PVF-based back-foil, that the industry felt confident enough to issue power guarantees for more than 20 years.

Nevertheless, a section of the PV industry was unsatisfied with EVA. The production of so-called glass-glass modules proved problematic: for the best possible cohesion, the different layers are »baked« in huge laminators to inextricably fuse together the units. But if the front- and back-sides consist of glass, which is a poor conductor of heat, then it can take quite a while to reach the necessary 130° to 150 °C. And inhomogeneous temperature distributions, which are getting more frequent if one attempts to accelerate the heating-up process, can often lead to a premature melting of the foil at the module's edges.

If the softened synthetic material leaks out during the lamination process, it not only disfigures the module, but in the worst-case scenario, the contaminated parts of the laminator become unusable and have to be replaced. Manufacturers that specialize in glass-glass modules, like Saint-Gobain Glass Solar GmbH, avoid this problem by using casting resin made of polyurethane (PU) for encapsulation. But in comparison, this process is much more complex and difficult to control.

There's another disadvantage in using EVA in glass-glass modules: if architects want to use these translucent electricity generators in roofs, one has to pay special attention to the country's building regulations. In Germany, for example, building regulations vary in each federal state, though in the case of »overhead glazing structures« there is agreement. To ensure that no splinters rain down in the event that the glass breaks, the regulations specifically require the use of laminated glass with PVB foil. »The wording of the building regulation is the result of heavy lobby work by the PVB and glass industries,« comments Bernd Koll from HT Troplast GmbH, a manufacturer of PVB foils based in Troisdorf near Cologne. The regulation's wording indirectly prohibits the use of casting resin. This not only applies to normal glass roofs but also for those made of solar modules. That's why Saint-Gobain Glass has to conduct an expensive individual inspection of its special modules when they are installed in overhead structures.

The return to PVB?

Structure of a solar module:

© Deutsche BP AG, Geschäftsbereich BP Solar

If one wants to avoid these inspections, thereby making it less expensive to integrate solar energy generators into glass roofs in Germany, then one has to use glass-PVB laminates. As a matter of fact, some manufacturers and researchers are reconsidering the use of PVB, since the earlier characteristics of the material »cannot be compared with today's PVB foils, since they have been further developed and much improved,« explains Koll.

Fraunhofer researcher Schmidhuber has also tested the suitability of PVB as an encapsulation material and published his results as part of his 2003 dissertation, titled Encapsulation of Crystalline Silicon Solar Cells. In his work, he confirms the material's penchant for holding water: following a combination of exposure to heat and moisture, Schmidhuber could prove the presence of water vapor, and adhesion on glass deteriorated. But in a 70-day test under UV-light and heat, the EVA foil performed worse: it tended to detach earlier from the front glass. As the modules in the tests also had a PET/Tedlar compound back-foil that allowed the penetration of small amounts of water vapor, Schmidhuber summarized his results with a resounding »Yes to PVB« – at least for modules with diffusion-blocking back-sides: if the back-side doesn't allow water to come into contact with the PVB, there can't be any trouble.

Moreover, says Schmidhuber, in the meantime one can observe the high quality of the synthetic material by looking at the PVB laminated glass used in today's automobiles. Car windows no longer cloud up – even if their corners aren't specially sealed. »The tiny bit of water that gets in over the edges after a rain,« says Schmidhuber, »dries up again.« And after all, the process is reversible: when the water dries up, the edges of the foil once again show their full adhesive strength. This has also been confirmed by Bernd Koll, who points to Bielefeld-based Wulfmeier Solar GmbH, whose glass/PVB/glass modules have passed IEC 61215 testing and therefore the accompanying damp-heat test. Hence PVB is slowly gaining ground again: RWE Schott Solar uses the foil for a portion of its amorphous thin-film modules, as does module assembler Glaswerke Arnold GmbH & Co. KG (which processes RWE Schott's raw modules).

The rediscovered material has another advantage up its sleeve. Because of the current enormous demand for modules, EVA manufacturers are gradually finding it difficult to keep up with deliveries, whereas the PVB industry has plenty of its material available for laminated glass. »The current market is driving customers right into our arms,« says Koll happily. The common claim that the synthetic material's water content leads to corrosion of solar cell contacts is no longer a valid argument; it's just a preconceived notion about PVB, he adds, since after the laminating process the foil hardly contains more water than EVA, and in quantities that cannot cause any damage.

Whereas Troplast is trying to establish its PVB foil »Trosifol Solar« on the market, other foil competitors to EVA are moving into position: the thermoplastic PU Desmopan from Bayer Materialscience AG is quick and easy to process (see PI 5/2004, p. 26) and the silicone-based product under development by US company Dow Corning is highly flexible and UV-resistant. »Yet,« says Schmidhuber, »module producers are extremely conservative when it comes to innovations.« In the end, they take a considerable risk by offering such long guarantees. They tend to stick to what's proven reliable in the past.