Alternative interlayer for photovoltaic modules

PVB benefits from the transfer of expertise from the glass industry

Interest in photovoltaic systems for buildings has been growing rapidly in the leading industrialized nations for several years now. The continuing rise in the cost of fossil fuels is accelerating the search for alternatives and the intensified further development of renewable energy sources. Solar power plants are constantly surpassing each other in setting new performance records.

Building-integrated photovoltaic systems (BIPV systems) have been developed against this backdrop. To exploit the potential of these PV systems even more efficiently, architects and engineers are constantly on the lookout for new ways of cost-effectively installing PV systems in glass façades and roofs.
This trend is being supported by political decisions in the various OECD nations to allow and make it easier for property owners to feed electricity from these systems into the national grid. Compared to the grid and electricity prices of established suppliers, solar power today is still two to four times as expensive. However, experts expect solar power to become competitive within the coming decade.

The main obstacles to the use of solar power from buildings and façades are still of a technical nature. This can be shade, the pitch or orientation of the building, or the lack of standards relating to building connections.

A possible solution to this problem is PV systems integrated in façades and glass roofs that meet the needs of “space efficiency” and are attractive to look at. Today, for safety reasons, overhead glazing, such as façade glazing, glass roofs, glass staircases or glass balustrades, in many countries may only be made of laminated safety glass. A further argument in favour of laminated safety glass is that PV modules are in themselves laminated glass systems (solar cells encapsulated in glass). An ideal complement is the resin polyvinyl butyral (PVB), which has been employed for decades now as the interlayer between the glass plies of laminated safety glass.

Encapsulation of solar cells
Manufacturers today warranty PV modules for 20 years and more – an outcome of the efforts in the recent decades to improve the longevity of modules and solar cells. An important factor in this is the encapsulation material that protects the solar cell proper from damage and ensures a long service life.
The currently favoured material is ethylene vinyl acetate (EVA) because of its flow behaviour during the lamination process and its amenability to single-stage lamination. The alternative materials include acrylic- or polyurethane-based casting resins.

An alternative to today’s standard PV modules is dual-ply modules in which the cells are encapsulated in TROSIFOL SOLAR, a PVB film for PV systems in laminated safety glass. Launched by Kuraray Europe GmbH in 2004, it was at the time the world’s only PVB film for PV systems in laminated safety glass. TROSIFOL, the PVB film from Kuraray, has in fact developed remarkably well on the market over the last ten years. In addition, PVB has outstanding optical properties, modifiable adhesion to glass, high impact resistance and exceptional UV and temperature resistance. Applications of laminated safety glass with PVB stretch back to the Thirties.

The differences between the rubber-elastic elastomer EVA and soft thermoplastic PVB become most obvious in the laminate. An EVA-based glass laminate, for instance, requires an extra glass ply in order to achieve the same standard of safety as a laminate with PVB. In addition, PVB shows stronger resistance to external influences and shows superior behaviour after breakage. And it is precisely these properties that make PVB ideal for applications in PV modules for façades and roofs.
Furthermore, PVB is inexpensive and permits thinner – and hence lighter – laminated safety glass.

TROSIFOL SOLAR was designed specifically for the requirements of solar cells in PVB laminated safety glass. Special importance was attached to effectively encapsulating and lastingly protecting the fragile and pressure-sensitive solar cell. The key to this is the modified viscosity and surface roughness of the film for superior performance in the vacuum lamination process. For larger module formats, it is now possible to supplement the standard vacuum bag process (deairing) with conventional autoclaving.
Other advantages of TROSIFOL SOLAR PVB film over EVA films include:

• PVB can be stored for up to 4 years until use; EVA only 6 months.
• Greater resistance to impression and lack of flow at the edges – hence no contamination of the modules during lamination
• Reproducible lamination thanks to the lack of crosslinking of PVB (there is no need for PVB to crosslink)
• The use of other, more cost-effective lamination processes
• Better long-term behaviour in terms of UV and temperature resistance (familiar for decades from PVB’s use in laminated safety glass)
• It can be combined with noise-abating or coloured films in order to satisfy aesthetic or functional needs.

Manufacturers of photovoltaic modules and laminated safety glass in Europe are already cooperating closely and marketing PV systems with PVB films. The resultant transfer of knowledge and the use of familiar one- and two-stage lamination processes now also permit the production of larger PV systems than before. The more efficient exploitation of space compared to the glass sizes used to date reduces the price of PV modules and may encourage the more widespread use of renewable energy from sunlight. An outstanding example is the Austrian company Ertex Solar which exhibited at INTERSOLAR 2005 what was then the world’s biggest PV module measuring 5,100 x 2,450 mm – with TROSIFOL SOLAR PVB film.

Rising rates of efficiency
It is not only the sizes that are increasing, but also the rates of efficiency. Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg have now converted sunlight into electricity with an efficiency of 37.6 per cent, setting a new European record at the beginning of July 2008. This result was achieved with the aid of so-called “concentrator solar cells” made of III-V semiconductors – solar cells that so far have been mainly used in space. Ongoing development work by the Fraunhofer researchers will now also permit the cost-effective utilization of such solar cells on earth.

Thin-film technology
Photovoltaic technology integrated in building façades and roofs will become an increasingly important development, and not only in Germany. It will be interesting to note the impact of the latest technological breakthrough – that of the development and commercial-scale introduction of thin-film technology. By definition, thin films of materials (roughly 2-3 micrometres) are applied with a variety of methods to a substrate and subsequently processed. These films on glass or metal act as solar cells as an alternative to cells made of amorphous or crystalline silicon. Amorphous silicon, which achieves an efficiency of 6 to 8 per cent, currently claims the biggest slice of the market. Crystalline silicon, such as microcrystalline silicon, combined with amorphous silicon achieves a higher efficiency of up to 14 per cent. Although mono- and polycrystalline modules are even more efficient at between 12 and 16 per cent, thin-film PV modules are much cheaper, and this is expected to encourage their widespread use – in the private sector as well – in the coming decade. For architects this opens up entirely new applications with previously undreamt-of scope for the design of façades and of glazed surfaces in general.