Solar energy: flower power

In matters of clever design, nature has often got there first

SOLAR-POWER stations take up a lot of room. They need either vast arrays of photovoltaic panels, which convert sunlight directly into electricity, or of mirrors, which direct it towards a boiler, in order to raise steam and drive a generator. The space these arrays occupy could often be used for other purposes.

Two researchers from the Massachusetts Institute of Technology have now devised a better and more compact way of laying out arrays of mirrors. Slightly to their chagrin, however, and somehow appropriately, they found when they had done the calculations that sunflowers had got there first.

Alexander Mitsos and Corey Noone started with the observation that existing concentrated solar-power plants, as those which drive boilers are known, usually have their mirrors arranged in a way that resembles the seating in a cinema. The mirrors are placed in concentric semicircles facing a tower, on top of which the boiler and the turbine sit. That arrangement, however, sometimes results in the mirrors shading each other as the sun’s position in the sky changes, even though the mirrors are usually attached to robotic arms that track the sun as it moves.

According to their report in Solar Energy, Dr Mitsos and Mr Noone found that they could do better. They divided each of the mirrors in a real power plant, PS10, in southern Spain into about 100 pieces. (Or, rather, they divided a computer representation of each mirror.) They then plugged each of those pieces into a computer model that calculated all of the energy losses by noting points where mirrors were not optimally oriented to the sun and places where they hindered one another by blocking incoming or reflected rays. It then rejigged them into a better arrangement.

Fermat’s conjecture

Previous efforts have been directed mainly at stopping the mirrors shading each other, which tends to mean spreading them out. Dr Mitsos and Mr Noone also wanted to save space. In trying to do so they stumbled on an unusual arrangement that had the desired effect. When they showed this layout to a third researcher, Manuel Torrilhon of Aachen University in Germany, he recognised the spiral patterns within it, and this prompted the trio to test a design specifically modelled on nature.

That design was a pattern known as a Fermat spiral, in which each element is set at a constant angle of 137° to the previous one. It is most familiar as the arrangement of the florets that make up a sunflower head. When the three researchers programmed their model to arrange PS10’s mirrors in front of the tower in a segment from such a spiral, they both improved the efficiency of the collection process and saved space. The improvement in efficiency was, admittedly, quite small (about half a percent), but the space saving was significant—almost 16%.

If solar power is to make up much of the world’s electricity output in future, as supporters of alternative energy hope it will, a lot of land will be needed for the power stations. Reducing that requirement by a sixth, as this discovery promises, would be a big gain. It would also show that if you look hard enough, there really is nothing new under the sun.

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