More Power from Rooftop Solar  

Technology Review has an article on an interesting approach for boosting generation on rooftop solar PV installations - More Power from Rooftop Solar.

A startup called TenKsolar, based in Minneapolis, says it can increase the amount of solar power generated on rooftops by 25 to 50 percent, and also reduce the overall cost of solar power by changing the way solar cells are wired together and adding inexpensive reflectors to gather more light.

TenKsolar says its systems can produce power for as little as eight cents a kilowatt-hour in sunny locations. That's significantly more expensive than electricity from typical coal or natural-gas power plants, but it is less than the average price of electricity in the United States.

Solar cells have become more efficient in recent years, but much of the improvement has gone to waste because of the way solar cells are put together in solar panels, the way the panels are wired together, and the way the electricity is converted into AC power for use in homes or on the grid. Typically, the power output from a string of solar cells is limited by the lowest-performing cell. So if a shadow falls on just one cell in a panel, the power output of the whole system drops dramatically. And failure at any point in the string can shut down the whole system.

TenKsolar has opted for a more complex wiring system—inspired by a reliable type of computer memory known as RAID (for "redundant array of independent disks"), in which hard disks are connected in ways that maintain performance even if some fail. TenKsolar's design allows current to take many different paths through a solar-panel array, thus avoiding bottlenecks at low-performing cells and making it possible to extract far more of the electricity that the cells produce.

The wiring also makes it practical to attach reflectors to solar panels to gather more light. When solar panels are installed on flat roofs, they're typically mounted on racks that angle them toward the sun, and spaced apart to keep them from shading each other over the course of the day. Reflectors increase the amount of light that hits a solar array, but they reflect the sunlight unevenly. So in a conventional solar array, the output is limited by the cell receiving the least amount of reflected light. The new system can capture all the energy from the extra, reflected light. "The small added cost we put in on the electronics is paid back, plus a bunch, from the fact that we basically take in all of this reflected light," says Dallas Meyer, founder and president of TenKsolar. "We've architected a system that's completely redundant from the cell down to the inverter," he says. "If anything fails in the system, it basically has very low impact on the power production of the array."

The reflectors use a film made by 3M that reflects only selected wavelengths of light, reducing visible glare. The material also reflects less infrared light, which can overheat a solar panel and reduce its performance.