Flower power

A flower that already is named for the sun has provided scientists with some clues about how to capture solar rays more efficiently.

The sunflower, native to the Americas, is known for its huge, fiery blossoms—which actually comprise 1,000 to 2,000 separate petals, or florets, joined together by a receptacle base. Generally, each floret is oriented toward the next by the golden angle, 137.5 degrees— producing a pattern of interconnecting spirals,

Now, U.S. researchers at the MIT in Cambridge, in collaboration with scientists at RWTH Aachen University in Aachen, North Rhine-Westphalia, Germany, have found that—by arranging the mirrors or heliostats in a solar installation to duplicate the spirals on the face of a sunflower— they can realize several advantages.

Most importantly, duplicating the spirals reduces the "footprint" of the heliostats by 20 percent, facilitating a more compact layout and minimizing heliostat shading and blocking by neighboring mirrors. Therefore, the spiral arrangement of the heliostats reduces the amount of land required to build a concentrated solar power (CSP) plant, while increasing the amount of sunlight its mirrors collect.

The first "spiral trial" was done just outside Seville, in the desert region of Andalucía, Spain, at a sprawling CSP known as PS10. A 100-meter tower is the focus of attention at the center of the site, surrounded by rows of giant mirrors rippling outward. More than 600 of these mirrors, each the size of half a tennis court, track the sun throughout the day, concentrating its rays on the tower, where the sun’s heat is converted to electricity — enough to power 6,000 homes.

Until recently, at PS10 and other CSP plants worldwide, mirrors have been arranged around the central tower in concentric circles. The spacing between mirrors is similar to the seats in a movie theatre—staggered so that every other row is aligned. However, this pattern results in higher-than-necessary shadowing and blocking throughout the day, reducing the reflection of light from mirrors to the tower. ??The MIT team looked to optimize the pattern to increase a plant’s overall efficiency. The institute’s Rockwell International Assistant Professor of Mechanical Engineering, Alexander Mitsos, along with student Corey Noone, collaborated with Manuel Torrilhon of RWTH Aachen, where Mitsos was a researcher prior to joining the MIT faculty. ??

Mitsos’ lab developed a computational model to evaluate the efficiency of concentric-circle heliostat layouts. The model divided each mirror into discrete sections and calculated the amount of light each section reflected at any given moment. The researchers then tested the model on the existing commercial-scale PS10 plant. The group found that the CSP plant experienced a significant amount of shading and blocking each day, despite the staggered layout of its mirrors.

To increase the plant’s theoretical efficiency, Noone and Mitsos tinkered with the pattern of heliostats, using numerical optimization to first bring the fanned-out layout closer together. This narrower layout, the model calculated, reduced the amount of land the mirrors took up by 10 percent without affecting the mirrors’ efficiency in reflecting light. The resulting pattern had some spiral elements similar to layouts in nature.??

So the MIT team, working with Torrilhon, looked to nature for inspiration — specifically, to the sunflower. The florets of a sunflower are arranged in a spiralling pattern, known as a Fermat spiral, that appears in many natural objects and has long fascinated mathematicians: The ancient Greeks even applied the patterns to buildings and other architectural structures.

After the researchers arranged the heliostats in sunflower-like spirals, they achieved success—less space, more power. Mitsos says arranging a CSP plant in such a spiral pattern could reduce the amount of land and the number of heliostats required to generate an equivalent amount of energy, which could result in significant cost savings.

"Concentrated solar thermal energy needs huge areas," Mitsos says. "If we’re talking about going to 100 percent or even 10 percent renewables, we will need huge areas, so we better use them efficiently." ??The researchers published their results in the journal, Solar Energy, and have recently filed for patent protection.