Scientists have built a reactor that could one day lead to the efficient production of fuel from sunlight. With solar energy having long been the potential savior for our dwindling energy resources, the fact that it cannot be transported from place to place has thwarted its widespread use. Now, however, a new process that demonstrates how solar-driven thermochemical approaches can be technologically viable may bring that ideal a little closer.
A research team led by Professor Sossina Haile, a professor at Steele Laboratories, California Institute of Technology (Caltech), has developed a prototype reactor with a quartz window and a special cavity able to absorb concentrated sunlight through a type of magnifying glass to focus the sun's rays.
The study, a joint effort between Caltech and ETH Zurich and published in Science [Chueh et al. Science (2010) doi: 10.1126/science.1197834], uses ceria as a key ingredient for concentrating solar energy and using it to efficiently convert carbon dioxide and water into fuels. Ceria, an oxide of the most common of the rare earth metals cerium, is used in ceramics, stone and glass polishing, and in the walls of self-cleaning ovens. As ceria can ‘exhale’ oxygen from its crystalline framework at very high temperatures and then ‘inhale’ oxygen back in at lower temperatures, it is ideal to drive the solar-powered reactions.
Dr Haile points out that “What is special about the material is that it doesn't release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves. When we cool it back down, the material's thermodynamically preferred state is to pull oxygen back into the structure.”
The inhaled oxygen is stripped off of carbon dioxide and/or water gas molecules pumped into the reactor, which produces carbon monoxide and/or hydrogen gas, with the latter able to be used for fuelling hydrogen fuel cells. The carbon monoxide, when combined with the hydrogen gas, can also be used to create synthetic gas, the precursor to liquid hydrocarbon fuels. Once the ceria is oxygenated to full capacity, it can then be re-heated, beginning the cycle again.
In order to achieve this effect, the reactor’s temperature must reach almost 3000 oF. The researchers were able to use photons for this at the Paul Scherrer Institute in Switzerland, where the reactor was installed on a large solar simulator capable of delivering the heat required.
The research could lead to its adoption for large-scale energy plants, bringing the possibility of solar-derived power being available at any time of day or night. Another potential application is using the carbon dioxide emissions from coal-powered electric plants for conversion into transportation fuels. However, the reactor design and materials will need to undergo further improvement to allow this approach to be commercially viable.
Laurie Donaldson