Solar Simulator: Reactor could make fuel from solar energy and CO2

(Image: Aldo Steinfield/Swiss Federal Institute of Technology)

Imagine filling up your car on an environmentally-friendly fuel produced from sunlight, water and carbon dioxide from the air.

That goal may now be a step closer following successful tests of a new solar-powered reactor.

Researchers around the world have recently been experimenting with different catalysts capable of producing hydrocarbon fuels from water and carbon dioxide when heated by concentrated sunlight.

Such a fuel would not only reduce greenhouse gas emissions, but could be used with little change to our existing cars and infrastructure.

Now William Chueh at the California Institute of Technology in Pasadena and colleagues have developed a reactor that uses cerium oxide as a catalyst. Cerium oxide is an abundant material suitable for commercial-scale fuel production, but it hadn't been demonstrated in a reactor under realistic conditions.

In Chueh's reactor, concentrated solar energy enters the chamber through a window. Once inside the chamber, the sunlight is reflected several times to capture as much of the solar energy as possible. It is used to heat a 35 millimetre-diameter cylinder of cerium oxide to around 1500 degrees Celsius. This causes the cerium to release an oxygen atom.

The temperature in the chamber is then reduced to around 900 degrees Celsius, and carbon dioxide is pumped into the chamber through an inlet. The cerium grabs an oxygen atom from the carbon dioxide to replace the one that it has lost, producing carbon monoxide and cerium oxide.

The carbon monoxide is then removed from the chamber, which is heated back up to 1500 degrees Celsius and the whole cycle is repeated.

The same process is also used to generate hydrogen from steam. The team has successfully run the process to produce the two gases in over 500 cycles.

The efficiency of the device is low, at around 0.4 per cent, but much of this is due to heat loss through the reactor walls and aperture, which can be dealt with through improvements to the device's insulation and design, the team say. Efficiencies of up to 19 per cent should then be possible, they add.

Carbon monoxide and hydrogen can be converted into a synthetic liquid using a technique such as the Fischer-Tropsch process, in which they are heated in the presence of an iron-based catalyst to produce hydrocarbon fuels.

Ultimately, such plants could use carbon dioxide from the air to produce fuel. Team member Aldo Steinfeld at the Swiss Federal Institute of Technology, Zurich, has already demonstrated that a similar process can be used to remove CO2 from the atmosphere.

He and his team used concentrated sunlight to heat calcium oxide to 400 degrees Celsius, causing it to react with CO2 in the air to form calcium carbonate. When heated again, this time to 800 degrees Celsius, the calcium carbonate releases a pure stream of CO2 that can then be used in the solar fuel reactor.

Journal reference: Science, DOI: 10.1126/science.1197834