Perhaps we shouldn't be surprised that the fast-growing, brilliantly bright sunflower offers guidance on how best to gather solar energy. Researchers from MIT and RWTH Aachen University in Germany recognized that possibility, and they ran with it. They're reporting now that by mimicking the spiral pattern of the sunflower in laying out the heliostats in power tower concentrating solar power (CSP) plants, they can reduce both the amount of land and the number of heliostats needed.
Power tower CSP plants use big mirrors -- heliostats -- to direct sunlight at the top of a tower several hundred feet high. The energy is received by the tower and used to heat water, generating steam and then power. There are plants like this already operating in Europe and several are either under construction or planned for construction in the United States, including some that use molten salts to store the heat that's been gathered so it can be used to produce power long after the sun goes down.
The researchers used the first power tower plant ever built, Abengoa's PS10 in Andalucia, Spain, as the model for their theoretical work as they sought to find ways to optimize the heliostat layout to improve efficiency. Some initial efforts to squeeze the mirrors together yielded encouraging results; the amount of land the mirrors took up was trimmed by 10 percent without degrading the efficiency of the mirrors. And the researchers noted something interesting about the resulting pattern: It had some spiral elements similar to layouts in nature. That's when the sunflower-shaped light bulb went off.
In pressing on with their work, the researchers decided to copy an essential element to the arrangement of sunflower florets, a pattern called Fermat's spiral. In this arrangement, according to MIT, "each sunflower floret is turned at a 'golden angle' -- about 137 degrees -- with respect to its neighboring floret." The researchers tried that with the heliostats, with each mirror angled about 137 degrees relative to its neighbor. And what did they find? "The numerically optimized layout takes up 20 percent less space than the PS10 layout," MIT said. "What's more, the spiral pattern reduced shading and blocking and increased total efficiency compared with PS10's radially staggered configuration."
It's not difficult to imagine how reducing the number of heliostats needed can be a boon for CSP technology. PS10 is a small plant, using 600 mirrors -- but the Ivanpah plant now under construction in California, for example, will take up around 3,500 acres and use 173,500 heliostats.
"The heliostat field presently contributes to about a third of the direct cost of most [CSP] plants," Frank Burkholder, an engineer with the National Renewable Energy Laboratory, told MIT. "Because heliostats are costly, their spacing relative to each other and the tower ... is important. If care isn't taken in their placement, they can shade and block each other and reduce the amount energy delivered significantly."
It might cost a little something for developers to use the sunflower pattern, however; while nature gave the researchers the idea, and they've shared it with the world in the journal Solar Energy, they've also applied for patent protection for the layout.
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