Solar power

Solar power
Glowing after dark

Aug 7th 2008
From The Economist print edition
Looking to leaves for a way to store solar power after sunset

PLANTS absorb sunlight, produce energy, consume carbon dioxide and exhale oxygen. It is a perfect model for power generation, but copying Mother Nature is difficult. Although research into solar power has come a long way, sunset still poses a problem. Storing solar-made electricity in batteries can be expensive and inefficient. So to be really successful, the solar industry needs another way to keep power for use at night. Now researchers have found a chemical cocktail that might do the trick.

By adding cobalt and phosphates to water and passing a mild current through the solution with a glass electrode, Matthew Kanan and Daniel Nocera of the Massachusetts Institute of Technology (MIT) were able to break water apart and force oxygen to bubble to the surface. Protons left behind by the oxygen migrated to a second electrode made of platinum and formed into hydrogen.
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Although the process appears simple, it is difficult in practice. Given enough electricity, there are already ways to break water apart. The problem is that the amount of energy required may significantly exceed that which the hydrogen can subsequently be used to generate. Scientists have tried adding chemicals to reduce the amount of electricity needed, but most of the ingredients have been rare and expensive. By using easily obtainable cobalt and phosphates, the MIT work could make it a lot more viable to obtain hydrogen directly from solar cells.

The researchers suggest splitting water into hydrogen and oxygen during the day whenever electricity from solar cells is not needed for anything else. At night the hydrogen could be burned or run through a fuel cell to create power.

The process, reported in Science, is similar to how plants work. When they do not need energy immediately, plants transform it into sugar which is stored. When energy is required, the sugar is used—regardless of whether the sun is up or down. Although their experimental work is yet to be commercialised, the MIT researchers suggest that within ten years solar cells using cobalt-based reactions to store energy could help to power some buildings.