The MIT team initially used a computer algorithm to explore a wide variety of possible configurations, and developed analytic software that can test any given configuration under a whole range of latitudes, seasons, and weather. Then, to confirm their model’s predictions, they built and tested three different arrangements of solar cells on the roof of an MIT laboratory building for several weeks.

While the cost of a given amount of energy generated by such 3-D modules exceeds that of conventional flat panels, the expense is partially balanced by a much higher energy output for a given footprint, as well as much more uniform power output over the course of a day, over the seasons of the year, and in the face of blockage from clouds or shadows. These improvements make power output more predictable and uniform, which could make integration with the power grid easier than with conventional systems, the authors said.

The basic physical reason for the improvement in power output — and for the more uniform output over time — is that the 3-D structures’ vertical surfaces can collect much more sunlight during mornings, evenings, and winters, when the sun is closer to the horizon, said co-author Marco Bernardi, a graduate student in MIT’s Department of Materials Science and Engineering.

The time is ripe for such an innovation, Grossman added, because solar cells have become less expensive than accompanying support structures, wiring, and installation. As the cost of the cells themselves continues to decline more quickly than these other costs, the advantages of 3-D systems will grow accordingly.

“Even 10 years ago, this idea wouldn’t have been economically justified because the modules cost so much,” Grossman said. But now, “the cost for silicon cells is a fraction of the total cost, a trend that will continue downward in the near future.” Currently, up to 65 percent of the cost of photovoltaic energy is associated with installation, permission for use of land, and other components besides the cells themselves.

Although computer modeling by Grossman and his colleagues showed that the biggest advantage would come from complex shapes — such as a cube where each face is dimpled inward — these would be difficult to manufacture, said co-author Nicola Ferralis, a research scientist. The algorithms can also be used to optimize and simplify shapes with little loss of energy. It turns out the difference in power output between such optimized shapes and a simpler cube is only about 10 to 15 percent — a difference that is dwarfed by the greatly improved performance of 3-D shapes in general, he said. The team analyzed both simpler cubic and more complex accordion-like shapes in their rooftop experimental tests.

Publication date: 05/07/2012