Fuel from water, CO2 and sunlight

New solar reactor proves feasibility of solar chemical processes

PhD student Philipp Furler and Professor Aldo Steinfeld are pursuing an experiment with their solar thermochemical reactor for the production of fuel from water and CO2 at the ETH Zurich high-flux solar simulator. © Peter Rüegg / ETH Zurich
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An American-Swiss research team has developed a solar reactor that uses fuel and concentrated sunlight to produce fuel from water and carbon dioxide. As reported in "Science", the efficiencies are two orders of magnitude higher than in previous photocatalytic processes. An industrial application could be possible from 2020, according to the researchers.

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Solar energy is clean and available indefinitely; but it is not permanently available and distributed unevenly over the earth's surface. That's why scientists around the world are asking themselves: How can you store solar energy to transport it from the sunniest spots on earth to industrialized centers, where most of the energy is needed? This question motivates researchers to search for recipes on how sunlight can be converted into chemical energy sources, in the form of liquid fuels that can be stored for long periods of time and transported over long distances.

A research team from the California Institute of Technology (Caltech), the Paul Scherrer Institute (PSI) and the ETH Zurich have now succeeded in developing such a recipe and the associated solar reactor. With a radically new process, water (H2O) and carbon dioxide (CO2) are converted into a mixture of hydrogen (H2) and carbon monoxide (CO). This combination is called syngas and is a precursor of gasoline, kerosene and other liquid fuels.

Cerium oxide as a catalyst

The underlying idea is to split up water and CO2 in a two-step process using solar energy. In a first step, the scientists steer concentrated sunlight through a shutter sealed with a quartz glass into the solar reactor. In the cavity there is a cylinder of cerium oxide, which is reduced at a temperature of 1, 500 ° C. The material releases oxygen atoms from the structure. display

In the second step, the reduced cerium oxide is allowed to react with water vapor and CO2 at about 900 ° C .; In doing so, the water and CO2 molecules are broken up. The thereby released oxygen atoms are integrated into the material structure, so that the cerium oxide is present again in the initial form and the cycle can be restarted. What remains is pure syngas from H2 and CO.

Power of 1, 500 suns

The scientists tested their reactor prototype on the high-flux solar simulator of the PSI. They used a radiation intensity that corresponds to the power of 1, 500 suns. The conversion efficiency of solar energy into fuel was 0.8 percent. This value results from the calorific value of the produced syngas, divided by the input of radiant energy.

These efficiencies are two orders of magnitude higher than those achieved by conventional photocatalytic methods for CO2 cleavage, Al explains Aldo Steinfeld, Professor f Renewable energy sources at ETH Zurich. "The results that we publish in Science demonstrate the feasibility of solar-powered thermochemical processes for the production of carbon dioxide and water."

First industrial use 2020

Steinfeld and his group are currently working on optimizing the solar reactor in such a way that it can also be used on a large scale - in the megawatt range - in solar tower systems. Such systems are already in commercial use for power generation. Steinfeld believes that great efforts are needed before his solar reactor technology is put to practical use. However, in 2020 we should be ready for the first industrial solar fuel system to go into operation and make a key contribution to sustainable energy generation in the future . (Science, 2010; doi: 10.1126 / science.1197834)

(ETH Zurich, 07.01.2011 - NPO)