Cracking water with sunlight

28 March 2008

A power plant that makes hydrogen by splitting water with concentrated sunlight

launches in Almeria, Spain, on 31 March. It's a glimpse into a possible carbon-free

future that uses solar-driven chemical reactions to produce the gas.

The reactor, Hydrosol II, is the largest pilot-scale project of its kind, though hundreds

of thermochemical water splitting schemes have been sketched out on paper and

tested in laboratories. The system will take in half a litre of water every minute and

should produce around 3 kilograms of hydrogen an hour - equivalent to a thermal

output of 100kW, explains project coordinator Athanasios Konstandopolous, who

works for the Chemical Process Engineering Research Institute based in Thessaloniki,

Greece.

That's small fry compared to the tonnes of hydrogen produced every day by reforming

natural gas, but the concept does avoid using up fossil fuels and emitting carbon

dioxide - a must if hydrogen is to be a truly environmentally-friendly source of energy.

The pilot plant is the scaled-up version of a concept which has been tested in the solar

furnace of the German Aerospace Centre (DLR), Cologne, for four years, and which

shared the European Commission's 2006 Descartes prize for scientific research. Industrial

R&D partners Johnson Matthey Fuel Cells and Stobbe Tech Ceramics (Denmark)

have joined the German, Greek and Spanish research teams making up the

Hydrosol consortium. So far the whole programme has required only 7 million of

funding, half of which came from the EU. If the larger system works and is economically

feasible, the researchers hope to secure funding for a 1MW mass production

plant, Konstandopoulos says.

Drive it off

At the core of the reactor are two honeycomb-like ceramic chambers coated with

oxygen-deficient ferrite structures containing zinc and nickel. At high enough temperatures

(800-1200°C) these materials strip water of its oxygen, leaving hydrogen

gas to bubble away (Zn0.xNi(1-0.x)Fe2O4 + yH2O Zn0.xNi(1-0.x)Fe2O4+y + yH2). The

oxidised materials must then be recycled, driving off their collected oxygen as gas, in

a separate reaction step at 1000-1200°C.

As Christian Sattler of Cologne's DLR explains, the high temperatures required are

achieved by focusing sunlight onto the chambers, using a field of silvered mirrors

that track the sun's movement. The hydrogen-producing (water-splitting) and oxygenproducing

(recycling) steps take place in two parallel chambers, so that there is no

need to separate hydrogen and oxygen gases. When each chamber's metal oxides

have completed their reaction, their functions are swapped over - so that hydrogen is

produced almost continuously, rather than in batches. Crucially, this modular approach

means the system can easily be scaled up even further.

Sattler says that hundreds of similar thermochemical routes to hydrogen have been

mooted, and tested, over the last decade. Among other popular options are zinc/zinc

oxide cycles run at much higher temperatures. But much of the funding - particularly

from the US Department of Energy's (DOE) hydrogen programme - has been focused

not on metal oxide reactions, but on more complicated lower temperature cycles involving

sulfur and iodine chemistry, because these might be powered by advanced

nuclear reactors (which generate temperatures of only 800-1000°C, not as high as solar

concentrators). 'I don't think there will be one best way, but the Hydrosol II project

is the closest to a mass production scale,' he says.

'It's the biggest,' confirms Alan Weimer, who works on the US DOE's Solar Thermochemical

Hydrogen Team (STCH) at the University of Colorado.

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