Sunflower power

At first – or even at second glance – the Fermat spiral doesn’t seem to have much to do with solar energy. It’s what mathematicians see when they study the mesh-type arrangement of the florets on a daisy or sunflower. For the truly initiated, the exceptional mesh of spirals occurs in Fibonacci numbers.

But here is where science begins to learn from nature. On a sunflower each floret is turned at an angle of about 137 degrees to its neighbour in what’s known as the “golden angle”. And this angle could turn out to be pure gold for the future of solar energy.

Concentrated solar power plants – vast parks of mirrors, or heliostats, known as CSPs – reflects the sun to a single high tower that then converts the energy into electricity.

Usually, one heliostat produces around enough energy to power ten homes and, according to researchers, CSPs could keep the entire US in power all year round without resorting to harmful/non-renewable forms of power generation like coal and gas.

The trouble is there’s not enough space, as the Solar Energy Journal points out. Each mirror takes up the equivalent of half a tennis court. To put just 600 mirrors together as in the PS10 site in the desert of Andalucia just outside Seville, one of several CSPs around the world, it requires a lot of free space.

However, the technology is so promising that MIT and Germany’s RWTH Aachen University have recently collaborated to try to reduce its energy footprint. Using a process known as numerical optimisation, they organised the mirrors in a way they hoped would prove more far more efficient then normal practices.

Subsequent tests showed they were on the right track. But the researchers found that the rearrangement bore similarities to the pattern of spirals on a sunflower. The team led by MIT engineer, Prof Alexander Mitsos, decided to take things a step further and mimic nature using the “golden angle”. In short, a mesh of spirals.

Measurements showed the sunflower-based arrangement not only gathered more energy for the same number of mirrors but reduced “blocking and shading” as the sun moved during the day. Thus the space required for CSPs fell by 20 percent, improving the long-term prospects for these plants. “Concentrated solar thermal energy needs huge areas,” explains Mitsos, “so we’d better use them efficiently.”

This unexpected application of the Fermat spiral doesn’t surprise proponents of biomimicry, the replication of nature in science. As India’s Dr A Jagadeesh Nellore, an authority in wind power, commented on the research: “The diversity of nature can help in many aspects of engineering and solve problems of mankind”. As an unknown poet once wrote: “Like a single sunbeam on a warm summer day, there is an exuberance and brilliance in a sunflower.” And of course, heat.