Glass is a problem. It breaks easily, particularly when formed into wide, thin panes. In order to make solar panels, you have to move around a lot of glass panes. In particular, the process of laminating glass and substrates during the solar module manufacturing process invites trouble in many forms.

Many manufacturers of solar modules are turning to glass-glass systems, where the photovoltaic layer is sandwiched between two glass panes. In March, for example, SolarWorld introduced its Sunmodule Protect glass-glass solar panels. Advocates say the glass-glass combination produces cells that are less susceptible to shear stress, thereby reducing cell breakage. Glass-glass modules can be manufactured both as thin-film modules and with crystalline silicon cells.

However, more glass in the module means more glass in the factory. In order to get around this problem, manufacturers, process engineers and material developers are working on various techniques to minimize opportunities for gravity and stress to spoil the work.

Handle with care

According to Sven Kramer, director of photovoltaic sales at Robert Burkle GmbH, his company employs a two-step process in its lamination systems - in part to minimize manual glass handling and reduce stress on glass that is handled through automation. The first step is vacuum lamination for sealing the solar collection membrane between glass layers, resulting in a module assembly. The second step involves a flat hot press for performing the final lamination using upper and lower heating platens that make direct, flush contact with the module.

Using this process, the module stack - a glass sandwich - is transported into the machine by means of a conveyor. A second conveyor removes the completed module from the laminator. The conveyor lines are automated, with manual handling of glass required before and after but not during the lamination process. When the module stack is moved during the process, it is done by means of pins.

“By using pins to lift the module, we avoid pinching the module edges,” Kramer says. “This technique avoids generating stresses that can lead to breakage during the lamination process.”

In addition, the lifting mechanism eliminates the need for a supporting frame for the module sandwich. This frameless approach eliminates another point of manual intervention for assembly and cleaning. Thus, another opportunity for glass breakage is avoided, Kramer says.

Komax Solar’s XLam laminator also features a pin lifting system. In February 2011, Komax signed an agreement with Yingkou Jinchen that added laminators to its product range.

Spire offers its automated Spi-Laminator series of photovoltaic laminators that can be integrated into an automated module production line or operated in manual mode. The laminators can be equipped with automated loading and unloading of glass to reduce handling.

Robotics integrator QComp Technologies Inc. produces conveyors for glass-handling applications. According to the company, the straight conveyor is designed for precise and non-marking transportation of flat glass. The pop-up transfer conveyor is intended for 90-degree transfer from one straight section to another, and the cantilevered-finger conveyor allows for bottom-side glass placement with an integrated robot tool.

The company also produces a line of automatic glass and module load/unload and palletizing systems. The glass load/unload system uses a six-axis ABB robot to unload glass or substrates from A-frame or L-frame racks and place them onto the production line. The system utilizes a vision technology and tool developed by QComp.

Raise your glass

In order to improve the performance of solar modules, many manufacturers are turning to specialized glass for use in lamination. When light hits glass surfaces, a significant part is reflected and - depending on the glass thickness - also absorbed. According to industry estimates, up to 8% of sunlight is lost due to reflection and absorption, which represents a direct reduction in PV system performance. To counter this, an anti-reflective coating layer is applied to the glass.

According to Burkle, a coating of anti-reflective materials can improve PV performance by up to 3% over untreated glass. The company’s e.a.sy-Coater systems apply anti-reflective layers in a continuous process by means of rubber-covered application rollers that transfer the liquid coating material onto the glass panel surface. The rubber in the rollers is grooved to help achieve a homogeneous coating layer. After the coating process, the coated glass can optionally be pre-dried in a convection oven at temperatures of up to 150° C.

Pittsburgh-based solar glass maker PPG Industries produces its Solarphire AR line of anti-reflective coated glass that is heat-strengthened in thicknesses of less than 3 mm. According to PPG, when incorporated into solar modules, heat-strengthened thin glass permits more sunlight to reach the active layer, which enhances the conversion of sunlight into energy and increases power output. The glass’ surface-compression strength exceeds that of fully tempered glass (greater than 10,000 pounds per square inch), making it suitable in solar applications that require resistance to wind loads, hail impact and other environmental hazards. These same properties improve the glass’ survivability during manufacturing processes, such as lamination.

In March 2011, PPG received a $3.1 million grant from the U.S. Department of Energy (DOE) to develop the materials, coating designs and manufacturing processes necessary to commercialize a new type of glass for the cadmium telluride PV module manufacturing industry. The grant is part of a $20 million investment by the DOE in the Solar Energy Technologies Program. The PPG award will be delivered through the Photovoltaic Supply Chain and Cross-Cutting Technologies program, which aims to accelerate the development of unique PV products or processes with the aid of related technologies from non-solar companies.

As glass becomes thinner and anti-reflective coatings improve, the solar sector is likely to see an increase in the acceptance of glass-glass applications. Moreover, related PV technologies like building-integrated photovoltaics and other thin-film uses should also increase demand over time. S