Published on January 12th, 2012 | by Ravinder Casley Gera1
How the Humble Sunflower Holds the Key to More-Efficient Solar Power
Regular readers are well aware of concentrating solar power (CSP), the growing technology which uses the sun’s energy to heat fluid (not unlike a steam engine) rather than using photovoltaic cells to convert it directly to electricity. If you’re not up to speed, we recently published a quick primer on CSP and why it could be vital to tomorrow’s energy mix.
But every CSP installation requires hundreds of mirrors to catch the sun’s rays and direct them towards the central tower, where the fluid is stored. The problem is, those mirrors take up a lot of space. That’s why, so far, the majority of CSP installations have been in fairly out-of-town desert locations.
But, now, a team of researchers at MIT have come up with a way of packing the mirrors — known as heliostats — in more tightly, while still efficiently directing the sun’s rays to the central tower. And the solution takes its inspiration from a very appropriate source in nature — the sunflower.
At most CSP sites, the heliostats — typically several hundred of them, each the size of half a tennis court — are arranged in concentric circles. As MIT explains, “The spacing between mirrors is similar to the seats in a movie theater, 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.” Good sunlight wasted, in other words.
Working with scientists at RWTH Aachen University in Germany, the MIT team — led by assistant professor Alexander Mitsos — developed a computer program to calculate the efficiency of various layouts of heliostats, and tested it using the details of a real-life CSP plant, the PS10 plant outside Seville, Spain. And they quickly discovered that the most efficient pattern closely resembled the buds in the center of a sunflower. We’ll let the boffins explain:
The florets of a sunflower are arranged in a spiraling 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. Mathematicians have found that each sunflower floret is turned at a “golden angle” — about 137 degrees — with respect to its neighboring floret.
The researchers devised a spiral field with its heliostats rearranged to resemble a sunflower, with each mirror angled about 137 degrees relative to its neighbor. The numerically optimized layout takes up 20 percent less space than the PS10 layout. What’s more, the spiral pattern reduced shading and blocking and increased total efficiency compared with PS10’s radially staggered configuration.
Remember, its’ not the petals we’re talking about, it’s the florets. A quick Google Image Search will give you the idea.
The MIT folks aren’t clear yet on exactly how much energy efficiency will be improved using this method, as it’s yet to be tried out on a real-life CSP installation. (Would you be willing to move 600 giant mirrors around for an experiment?) But they do predict space savings of around 20% compared to a radial layout, with at least some increase in power generation, which sounds pretty cool to us. So, hopefully, we’ll see commercial CSP plants adopting this layout in the near future. After all, if we’re really going to carpet the entire Sahara with CSP installations, we might as well pack ‘em in as efficiently as possible.