Concentrated Solar Power (CSP) plants are probably the most technologically advanced and efficient form of generating solar energy on a large scale. Currently there are only a handful in the world (although that number is expected to grow) and one of them (inventively called PS10) majestically stands in Andalucia, Spain. It is 100 metres tall and surrounded by rows of giant mirrors (heliostats), each roughly the size of half a tennis court. The heliostats reflect the sun’s energy onto the central tower where it is then converted into enough electricity to power 6,000 homes. CSP’s could potentially generate enough clean, renewable energy to power the entire US, based on the assumption that two commodities are available in abundance; land and sunlight.
Current designs of CSP plants position the heliostat mirrors in an expanding formation, placing the mirror behind in the gap between the two mirrors in front, rather like seats in a cinema. Whilst this pattern reflects a high proportion of sunlight, it is highly inefficient resulting in higher-than-necessary shadowing throughout the day. Researchers at MIT, along with RWTH Aachen University in Germany, have recently calculated a design that reduces the amount of land required to build the plant by 20%, whilst increasing the amount of sunlight the mirrors can collect, and therefore the power generation potential. A win-win situation!
The team consisted of Alexander Mitsos, the Rockwell International Assistant Professor of Mechanical Engineering, and Corey Noone of MIT along with Manuel Torrilhon of RWTH Aachen. They used numerical optimisation to calculate the most theoretically efficient layout of the heliostats and found that they could bring the mirrors closer together and thereby reduce the land area of the site, without affecting the sunlight reflecting efficiency. The resulting layout resembled a spiral, very similar to certain patterns found in nature. Inspired by this discovery the team looked for more examples to follow in nature; they chose the sunflower.
The petals of a sunflower are arranged in a special spiral pattern commonly found in nature and known as a Fermat Spiral, a design that has fascinated mathematicians for centuries. Each petal is turned at a “magical” angle of 137 degrees with respects to its neighbour. This has allowed the “footprint” to be further reduced up to 20% of the original PS10; but even better, the spiral pattern reduces the actual number of heliostats needed and the shading they cast on one another, increasing the total efficiency of sunlight reflection. The researchers published their results in the journal Solar Energy, and have recently filed for patent protection.
Frank Burkholder, an engineer with the National Renewable Energy Laboratory, says that “the heliostat field presently contributes to about a third of the direct cost of most (CSP) plants … because heliostats are costly, their spacing relative to each other and the tower … is important. If care isn’t taken in their placement, they can shade and block each other and reduce the amount energy delivered significantly.” This new design could generate the same annual energy whilst taking up “far less” land area. As Mitos says, “concentrated solar thermal energy needs huge areas. 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.”
By. James Burgess of Oilprice.com