PHOTON International The Photovoltaic Magazine

Very NICE – a module concept

At the 19^th European PV Conference, Parisian-headquartered company Apollon Solar presented its NICE concept for manufacturing solar modules employing a simple production method based on insulation glass windows. The modules are manufactured without EVA and Tedlar sheets, and the electrical contacts are created without soldering. Apollon predicts that these innovations will result in significant cost reductions when compared with conventional lamination processes.

	© Apollon Solar
NICE prototype: The soldered cell string contacts are later replaced by flat junction boxes that will be attached to the module.

Windows with insulation or heat protection glass are nothing special. Usually they consist of two spaced panes of glass. The gap between the two panes is filled with either air or inert gas. The spacer, generally an aluminum or stainless steel band, is combined with a seal that prevents water vapor from entering and the gas from escaping. Otherwise, the insulation would deteriorate. Insulation glass has been around since the 1930s, and has been used in home construction since the 1950s.

The concept of manufacturing solar modules using this process seems quite obvious. If windows can withstand the burden of weather day in, day out, and still maintain their gas filling for decades, this type of construction would seem well-suited to the encapsulation of solar cells. In fact, Adolf Goetzberger, former head of the Freiburg-based Fraunhofer Institute for Solar Energy Systems, worked on this concept in the beginning of the 1990s. Around the same time, Jürgen Schmid's working group at the Institute of Solar Energy Supply Technology (ISET) in Kassel, Germany began considering R&D in the same direction. »If you could leave out the EVA sheet, that would solve one module aging problem,« says Schmid today.

But research conducted back then never left the conceptual stage or resulted in prototypes. Then came Henning Nagel from the Institute for Solar Energy Research (ISFH) in Hameln, Germany, who tackled the subject anew. He encapsulated a silicon solar cell in an »insulation glass window« whose front pane was coated on both sides with a two-fold anti-reflection layer. The layer system mitigates one disadvantage of the module concept: on its journey from the front pane to the solar cell, light crosses the gas-filled space. The refraction indices of glass and the gas are different, which leads to an increase in light reflection. About 4 percent of light is lost as a result of Fresnel reflection at every interface between glass and air. Nagel's layer system enables the refraction index to change incrementally as light passes through each layer, whereby reflection is reduced and thus more light reaches the solar cell. In fact, Nagel discovered that the cell efficiency of a silicon solar cell hardly changes after incorporation into a module – it fell from 21.1 to 21.0 percent. Henning Nagel presented his research results at the 2nd PV World Conference in Vienna in 1998.

A new attempt

The industrial application of this technique is now the aim of French company Apollon Solar SAS, based in Paris. The firm introduced its concept called NICE – New Industrial Solar Cell Encapsulation – for the first time at the 19^th European PV Conference in their hometown Paris in June. The poster presentation promptly received an award from the conference organizers for »outstanding innovation in the area of module technology« (see PI 7/2004, p. 34).

Especially interesting is the fact that the NICE process not only forgoes EVA and Tedlar sheets, but also the soldering of solar cells. This could mean considerable cost reductions: Apollon liberally estimates that direct module production costs (without cells and investment costs for the production line) would be 23 euro cents (29¢) per W. By comparison, manufacturing costs for conventional solar modules are currently around €2.40 ($2.99) per W. Whereas around €1.70 ($2.12) of that is spent on the solar cells, a further 40 euro cents (50¢) goes for other materials like glass and sheets. The labor costs, including general costs, equal around 30 euro cents (37¢). Hence, manufacturing costs for Apollon modules could amount to €2.10 ($2.61) per W.

The production of a NICE module begins with deposition of adhesive on the rear sheet, for instance a glass pane. This glue fixes the consecutively placed electrical connectors and solar cells in the appropriate position. The contacts for transport of electricity are created by laying 150 to 200 µm thick tin solder coated ribbons between the solar cells. Apollon has developed an external electrical connector that has the same thickness as the module sandwich, and is positioned and sealed during assembly. It contains the protective diodes and a plug-in connector for the module interconnection and is connected to the module strings' metal ribbon, which is sealed by the so-called Thermo Plastic Spacer (TPS).

Once all the components are positioned, the encapsulation process begins. A ribbon of glue and sealing material between 5 and 10 mm wide is deposited on the perimeter of the second glass pane, which serves as the module's front sheet. The front and back sheets are then placed on top of each other and sandwiched together. Finally, the module interior is flooded with inert gas, mainly to prevent solar cells and connectors from oxidation.

The insulation glass industry as role model

The insulating and adhesive material TPS is a derivative from the family of isobutylenes. During deposition the TPS is heated to 120 °C and can be applied to the glass sheet with a nozzle. This production step, developed by the insulation glass industry, can be achieved in a fully automated assembly line and thus holds the potential for further cost reduction. By comparison, a conventional glass/Tedlar module has to remain in the laminator for 10 to 15 minutes. That not only means that the device undergoes a longer lamination process, but it also usually requires manual work (such as removal of debordering EVA), this costs time and labor – and therefore money.

© Apollon Solar

Nicer? While conventional solar modules use EVA to encapsulate cells, with the NICE concept of French company Apollon the cells are placed in a gas-filled space between two glass panes.

One way the NICE process differs from the production of insulation glass is that the hollow space between the front and back glass sheets exists in a vacuum of around 200 mbar. This pressure difference presses the sandwich (front glass sheet / contact / cells / contact / glue / back glass sheet) together and ensures uninterruptible electrical connections. Apollon says that the connectors are profiled in a way to allow for a spring effect in order to lower the pressure impact on the cells and to compensate for local thickness variations of cells. For this reason a copper alloy is used for the connector ribbon which according to Apollon shows both a higher mechanical rigidity and more elastic behavior than standard copper. At the time of the conference, a specialized vacuum-capable press was still under development by German company Lenhardt Maschinenbau.

Nevertheless, using TPS does not come without its risks, since the thermoplastic material can warp as a result of pressure or heat from solar radiation. This type of TPS warping is well known among window builders. Torsten Kaneneks, employee at adhesive and insulation material company Henkel Teroson GmbH in Heidelberg, explains that this effect generally occurs when the Thermoplast's maximum temperature of around 80 °C is exceeded. For comparison: if you take the operating temperature of the black absorber from a thermal sun collector, it can easily exceed 150 °C. Hence one could imagine that the TPS, which is also black, could exceed the 80 °C point. Whether this will pose a problem, Apollon Solar wouldn't say.

To improve the mechanical qualities, mentioned Apollon's Roland Einhaus in his Paris paper, one could incorporate an additional polysulfide insulation, as is done in insulation glass window production. This type of insulation also decreases the infiltration of water vapor, though according to Kaneneks it can only be used with temperatures up to 70 °C, since it cannot withstand UV radiation above that point. That's why window builders keep the polysulfide behind the frame. Prior to the market introduction of NICE modules, the company should take into account the experiences of window builders in order to prevent theoretical weaknesses from leading to difficulties during system operation.

The idea of developing the NICE concept for industrial production has a certain allure: modules without EVA sheet don't run the risk of power losses as a result of yellowed sheets, which over the course of years increasingly block out light. And this module type can also be repaired: simply warm it, open it, switch defective cells or contacts, and then re-seal it. After all, cells and electrical connections are no longer embedded in the sheet. And if they could not be repaired, at least the recycling of old modules would be easier than with current laminates.

Apollon Solar is working to prepare its NICE concept for industrial production. The company says it sent »a number of modules« for damp heat and thermo-cycle tests according to IEC 6121 and the products passed »without losses.« Apollon Solar has recently built a consortium with partners from the European Commission's Joint Research Center in Ispra, Italy, the Fraunhofer Institute for Solar Energy Systems, Lenhardt Maschinenbau, and RWE Schott Solar to demonstrate the industrial applicability of the concept. The construction of a pilot system with a 25 MW production capacity should be the final proof for this new module technology. However, a construction schedule has yet to be set.