16 January 2012
Concentrated solar power (CSP) plants require ample supply of land and sunlight. Now researchers at the Massachusetts Institute of Technology (MIT), in collaboration with RWTH Aachen University in Germany, optimized the use of both by mimicking the spiral pattern of the face of a sunflower in laying out the big concentric mirrors—or heliostats—that reflect sunlight to the top of a power tower.
Alexander Mitsos, Rockwell International Assistant Professor of Mechanical Engineering, and his team tested their theoretical work on an existing commercial-scale CSP plant: PS10 outside Seville in Andalucía, Spain. In their existing outward rippling configuration, PS10’s more than 600 mirrors experience a significant amount of shading and blocking throughout the day.
The florets of a sunflower are arranged in what’s known as a Fermat spiral, where each floret is turned at a “golden angle” of about 137 degrees with respect to its neighboring floret. Mitsos’ team plugged PS10’s specs into their computational model to see what theoretically would happen if the heliostats were arranged in the same spiral pattern seen in the center of a sunflower. The group was able to shrink the plant’s numerically optimized footprint by 20%. “Our analysis and reported savings are based on a computer model,” Mitsos says. “The model has been validated with SolTrace and results in the literature.” (SolTrace, incidentally, is a software tool developed at the National Renewable Energy Laboratory (NREL) to model CSP systems and analyze their optical performance.) “In particular, we reproduce the PS10 efficiency with very good accuracy,” he explains.
What’s more, the sunflower pattern significantly reduced shading and blocking and increased the solar plant’s total efficiency. By just how much? “The answer depends on whether you want to simultaneously improve both coverage and efficiency or only efficiency. In the former case we get 0.4 percentage points. In the latter closer to 1 percentage point,” says the MIT professor.
At this point, it remains undetermined whether the sunflower principle applies to different plant layouts other than circular CSP plants. “Our idea is directly applicable to central-receiver plants, that is, the ones using heliostats,” Mitsos clarifies, adding that they were exploring generalizations, but had nothing to report for the moment. In the meantime, Mitsos’ group has applied for a patent. What else has yet to happen before the world will see its first sunflower-inspired CSP plant design? “We need to find a partner,” Mitsos says. “Most likely a company that is willing to take our idea, validate it with their models, and test it out in the field.” As to whether they have received any interest from the industry, as expected, the MIT researcher responds, “We are talking with companies, but I cannot share any predictions or details at this point.”
The PS10 plant in Spain was merely a hypothetical reference point for research and won’t be reconfigured based on Mitsos’ findings. However, “In principle, moving the heliostats is possible, but would incur extra costs—unlike when this is done at the design phase,” he says.