Mix a pile of manure with some zinc oxide, angle a few giant mirrors towards the mixture, turn on the sun and steam the result. It may not sound appetising, but Michael Epstein and his colleagues at the Weizmann Institute of Science, in Israel, think that this recipe represents a novel way of collecting solar energy to generate what many hope will be the fuel of the future − hydrogen.
Readers who remember their chemistry lessons may recall mixing zinc with hydrochloric acid in a test tube and standing by, lighted splint in hand, ready to ignite the hydrogen that is given off. (Metallic zinc reacts with the chlorine in the acid, leaving hydrogen behind.) Zinc reacts similarly with water − or, rather, steam − in this case stripping the oxygen from H2O and once again, leaving the hydrogen. Industrialising that process, though, relies on finding a cheap way of turning the zinc oxide that results back into metallic zinc, so that the material can be recycled. And this, courtesy of the Weizmann Institute's Solar Tower laboratory, is what Dr Epstein has done.
The tower's 64 seven-metre-wide mirrors track the sun and focus its rays into a beam with a power of up to 300 kilowatts (see picture). In Dr Epstein's experiment, which he outlined at the recent International Solar Energy Society conference in Orlando, Florida, the beam was used to heat a mixture of zinc oxide and charcoal. The charcoal (which is pure carbon) reacted with the oxygen in the zinc oxide, releasing the zinc. This instantly vaporised and was then extracted and condensed into powder.
At the moment, the cheapest way of making hydrogen is a process called reformation, which also uses steam, but reacts it with natural gas, a fossil fuel. Dr Epstein thinks that if his process were scaled up, it would cost about the same as reformation. It would also have the advantage over reformation that no fossil fuel need be involved (the charcoal can be made from agricultural waste, such as manure), and so there is no net contribution of climate-changing carbon dioxide to the atmosphere. And the other way of using solar power to make hydrogen − generating electricity using solar cells and then using that electricity to split water into its component gases − is vastly less efficient than Dr Epsteins's method.
In the meantime, the powdered zinc produced can be employed in a different form of energy technology − zinc-air batteries. These are used to power certain sorts of electronic device. So, even if your car never runs on second-hand solar energy, one day your laptop might.
