Once the cells are sorted by power output, another researcher, Adam Stokes, strings them together with a tool that solders flat strips of metal called busbars to electrical contacts on their front and back. The lab can test different ways to connect the cells, varying factors such as the number and type of busbars and then measuring the resulting performance to determine whether any extra costs are worthwhile.
Researchers sandwich a short string of solar cells between glass and a protective film, a process designed to keep the cells dry. This panel will be small enough to fit in one of the specialized chambers the lab uses to test new materials being considered for adoption by the solar industry.
A large laminating machine operated by Dan Doble, group leader for the PV Modules Group at Fraunhofer, seals solar cells inside a protective package. To earn back their cost, solar panels must perform well for decades, often under extreme conditions. If even a small amount of water vapor enters the panel, it can corrode contacts and degrade its performance.
This chamber can subject solar panels to a wide range of temperatures and humidity levels. It includes a device invented at the Fraunhofer lab that presses on the surface of a panel by inflating a rubber bladder, simulating pressure from a load of snow. Solar power may be associated with warm, sunny climates, but some of the biggest markets are in snowy places such as Germany.
Researchers Dan Doble and Carola Völker lower a solar panel into a tank of water to test how well the circuitry within it is sealed. A current of at least 500 volts are applied to the circuits, and an electrical lead in the water detects any current leakage. The test can help determine whether the panels are likely to survive exposure to extreme temperatures and mechanical pressure. The researchers also study micrographs to detect damage.
In most solar panels, a hole is cut in a protective envelope surrounding the solar cells to allow a connection to an outside circuit. To speed manufacturing and avoid allowing water to leak in, the lab is developing a device (right) that can be installed before the cells are encapsulated. The yellow tabs can be inserted between a sheet of encapsulant and the cells and sealed in place during a standard lamination step. The cables sticking out of the device are connected to similar cables in neighboring solar panels on a roof before the panels are connected to an inverter and the power grid. In the current design, this is done by hand, but in a future design, the devices will snap together, allowing the panels to be installed quickly and cheaply.