In order to create a renewable carbon economy, you can use solar power to split carbon dioxide. Combine the resulting carbon monoxide with hydrogen and you have the beginnings of a solar fuel that could one day replace oil.
Since 2008, a European consortium led by Athanasios Konstandopoulos at the Centre for Research and Technology Hellas, Thessaloníki, Greece, has been operating a 100-kilowatt pilot plant that generates hydrogen from a combination of sunlight and steam. The plant is sited at a concentrating solar power tower – the Plataforma Solar de Almería, in Almería, Spain – which houses a series of mirrors to concentrate the sun's rays onto solar panels beneath.
The same technology can also be used to split CO2 – the resulting CO can be combined with the hydrogen to form hydrocarbon fuel, they say.
The pilot plant contains a ceramic reactor riddled with a honeycomb network of channels coated in a mixed iron and cerium oxide. Concentrated solar energy heats the reactor to around 1200 °C, releasing oxygen gas, which is pumped away. The reactor is then cooled to around 800 °C before steam is fed through the honeycomb – the depleted material steals back oxygen and in the process converts the steam into hydrogen gas.
The team has run the pilot plant in several week-long bursts since its launch as part of the European Commission-funded Hydrosol II project. They claim that it is possible to convert up to 30 per cent of the steam into hydrogen.
Now, Konstandopoulos and colleagues have successfully used the same reactor technology and process to split carbon dioxide into carbon monoxide in the lab. Two reactors running simultaneously could generate hydrogen and carbon monoxide, which could be combined into synthetic fuel using one of two established chemical processes, says Konstandopoulos.
In the Sabatier process the two gases are heated at high pressure in the presence of a nickel catalyst to produce methane or methanol, while in the Fischer-Tropsch process an iron-based catalyst is used to generate liquid hydrocarbon fuels.
The process would help to make better use of the CO2 captured from power plants, which otherwise might simply be buried underground. Konstandopoulos says it could also solve the problem of storing and transporting hydrogen once it is produced – a problem that could prevent the development of a hydrogen economy.
"Hydrocarbons are the best energy carriers that we have available – nature has already proven that," he says. "We just have to find a way not to use them as our primary energy source."
Generating hydrocarbons this way would also mean few changes are needed to cars and existing fuel infrastructure, he says.
Other teams are investigating different reactor designs for producing solar fuel, including rotating rings of cerium oxide. A team led by Aldo Steinfeld at the Swiss Federal Institute of Technology, Zurich, has built a 10-kilowatt plant in which steam and carbon dioxide are reacted with zinc oxide to produce synthetic gas in one step. They plan to test a 100-kilowatt version next year.
Konstandopoulos and colleagues are now working to scale up their technology and build a 1 megawatt hydrogen-producing plant, in a project known as Hydrosol 3D.