Solar Cell Towers:
Onward and Upward,
not Outward


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Solar Cell Towers: Onward and Upward not Outward - Renewable Energy

Researchers around the world have focused on improving the performance of photovoltaic cells and bringing down their cost.

But very little attention has been paid to the best ways of arranging those cells, which are typically placed on a rooftop or other surface, or sometimes attached to motorized structures that keep the cells pointed toward the sun as it crosses the sky.

Now, a team of researchers at the Massachusetts Institute of Technology, Cambridge, MA, has come up with a different approach: building cubes or towers that extend the solar cells upward in three-dimensional configurations. The results from the structures the team has tested show power output ranging from double to more than 20 times that of fixed flat panels with the same base area, says Jeffrey Grossman, associate professor of power engineering at MIT, who worked on the project.

Solar Cell Towers: Onward and Upward not Outward - Renewable Energy

Jeffrey Grossman, associate professor of power engineering at MIT.

Those findings were based on both computer modeling and outdoor testing of modules, he added.

The biggest boosts in power were seen in the situations where improvements are most needed: in locations far from the equator, in winter months, and on cloudier days, Grossman says. "I think this concept could become an important part of the future of photovoltaics," he said.

Greater Surface Area

The basic physical reason for the improvement in power output—and for the more uniform output over time—is that the structures' vertical surfaces can collect much more sunlight during mornings, evenings, and winters, when the sun is closer to the horizon, adds Marco Bernardi, a graduate student in MIT's Department of Materials Science and Engineering, who also worked on the project.

Solar Cell Towers: Onward and Upward not Outward - Renewable Energy

Two small-scale versions of three-dimensional photovoltaic arrays were among those tested by Jeffrey Grossman and his team on an MIT rooftop to measure their daily electrical output. Image courtesy: MIT: Allegra Boverman

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

Greater Cost but Greater Output

While the cost of a given amount of energy generated by such 3-D modules exceeds that of ordinary flat panels when broken out by capital costs, 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, Grossman said.

These improvements make power output more predictable and uniform, which could make integration with the power grid easier than with conventional systems, he adds.

Solar cells have become less expensive than accompanying support structures, wiring, and installation, Grossman says. 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, he added. Who knows? The solar tower may one day be as familiar as today's cell phone tower.

Read the latest issue of Mechanical Engineering.

I think this concept could become an important part of the future of photovoltaics.

Jeffrey Grossman, associate professor of power engineering, MIT

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July 2012

by Jean Thilmany, Associate Editor, Mechanical Engineering