The key to building an efficient concentrated solar power plant that uses less land may just lie within the face of a sunflower. Researchers at Massachusets Institute of Technology in collaboration with RWTH Aachen University in Germany found they could reduce the footprint of a CSP install by nearly 16 percent percent and increase the amount of sunlight collected by arranging the mirrors -- otherwise known as heliostats -- in a pattern similar to the spirals on the face of a sunflower. The researchers, who have recently filed for patent protection for their design, published the results in the journal Solar Energy.
Concentrated solar power (CSP) uses mirrors or lenses to harness and concentrate the sun’s heat onto a small area to produce steam, which is used to drive turbines and generate electricity. There are a number of different solar thermal technologies including power towers, which use mirrors to concentrate sunlight on to a small area atop a tower that contains fluid such as oil. BrightSource Energy is one company that has developed solar thermal power systems in the United States.
Available land and shadows are the primary challenges with CSP. Typically, CSP plants have mirrors arranged around a central tower in concentric circles. The mirrors are staggered so every other row is aligned. But this pattern creates shading, which in turn, reduce efficiency.
Solar thermal developers have tinkered with their own designs to combat the problem. For example, BrightSource plans to use higher towers, which should allow it to pack more of its sun-tracking mirrors into a smaller area, for its Rio Mesa project in California.
The golden angle
The MIT researchers may have found a better way after developing a computational model to evaluate the efficiency of mirror layouts. In some of their initial tinkering, the researchers were able to theoretically reduce land use by 10 percent by bringing the layout closer together. The pattern contained some spiral elements similar to those found in nature. And nature, specifically the sunflower, is where the researchers turned.
Take a look at the face of a sunflower and you'll notice the florets are arranged in a spiral pattern, known as the Fermat or parabolic spiral. (Play the animation from Flickr user Rob Ireton for a look at a growing sunflower and how it maintains the same pattern). The criss-crossing spiral can be represented by a fraction describing the angle. The angle of each floret in a sunflower is relative to the next at a degree of 137.51, otherwise known as the golden angle. The result is a pattern of interconnecting spirals, where the number of left and right spirals are successive Fibonacci numbers. The Fibonacci sequence can be found in pine cones, branching plants, seed heads and even sea shell shapes.
Researchers copied this pattern and angled each mirror at about 137 degrees relative to its neighbor, according to MIT. The result was a compact, efficient pattern. The upshot? Less land and boosted efficiency means potentially lower costs for CSP projects, which can run into the hundreds of millions of dollars.