The Golden Angle: How it Could Mean Green Energy for Everyone

Out of the dust of a cracker dry field near Seville in southern Spain, rises an ivory tower of hope for the end of fossil fuels and the beginning of enough clean, renewable energy to power the entire countries. It is Abengoa’s Planta Solar 10 (PS10), Europe’s first commercial concentrating solar power (CSP) tower and now MIT researchers, in collaboration with RWTH Aachen University in Germany, have discovered that Mother Nature has been holding the key to making it more efficient — using less land, fewer heliostats, and generating more energy.

The 600+ mirrors, or heliostats, each the size of half a tennis court, gathered around the feet of PS10 have up until now, been staggered, theater seating style, with every other row lining up. Their job: to reflect the fierce Mediterranean sun up to a central pillar where it heats water, generating steam, and from that, power. But the layout, researchers found, led to shading and blocking as the sun tracked across the sky.

Attempts to increase the theoretical efficiency of the plant found that by squeezing the concentric circles of mirrors in closer, 10% of the energy field could be shaved off without interfering with the light being reflected –  no small feat in and of itself.  But when researchers spotted a familiar pattern in the resulting layout, things really began to heat up.

The MIT team of Alexander Mitos, the Rockwell Assistant Professor of Mechanical Engineering and Corey Noone SM ’11, working with Manuel Torrilhon of RWTH Aachen, observed that the mirrors were fanned out in what appeared to be a Fermat spiral, a pattern similar to those found in everything from nautilus shells and ram’s horns to hurricanes and, most interesting to the researchers, sunflowers.

The “golden angle” that creates this pattern – two sets of spirals, moving in opposite directions – is a 137 degree turn between each of the florets. By mimicking this placement and aligning the heliostats at that same angle, researchers were able to save 20 percent of the space needed in the PS10 layout.  “What’s more,” adds MIT, is that “the spiral pattern reduced shading and blocking and increased total efficiency compared with PS10’s radially staggered configuration.”

Looks like Mother Nature was on to something.

While these findings haven’t yet been tested in the real world, the results, which are being published in the journal Solar Energy, are promising. According to Frank Burkholder, an engineer with the National Renewable Energy Laboratory (NREL), “The heliostat field presently contributes to about a third of the direct cost of most plants.” For CSP projects like the Ivanpah plant now under construction in California, which will take up around 3,500 acres and use 173,500 heliostats, this could be a boon.

Mitos says “Concentrated solar thermal energy needs huge area. 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.”

Laura can be reached at laura.eliza.mitchell(at)gmail.com