|The CR5 thermochemical engine is the basis of the Sunshine to Petrol project. Click to enlarge.|
Researchers at Sandia National Laboratories are extending work on the development of a device for the solar thermochemical production of hydrogen from the splitting of water to recycling CO2 into liquid hydrocarbon fuels.
The prototype device—the Counter Rotating Ring Receiver Reactor Recuperator (CR5)—will be applied to breaking the carbon-oxygen bond in carbon dioxide to produce carbon monoxide and oxygen. Combining the CO stream with the hydrogen resulting from the splitting of water by a CR5 device, an integrated “Sunshine to Petrol” (S2P) system could then synthesize a liquid combustible hydrocarbon fuel.
As originally developed for hydrogen production, the CR5 is a stack of rings made of a reactive ferrite material, consisting of iron oxide mixed with a metal oxide such as cobalt, magnesium, or nickel oxide. Every other ring rotates in opposite directions. Concentrated solar heat is reflected through a small hole onto one side of the stack of rings. The side of the rings in the sunlit area is hot, while the other side is relatively cold. As the rotating rings pass each other in between these regions, the hot rings heat up the cooler rings, and the colder rings cool down the hot rings. This arrangement results in a conservation of heat entering the system, limiting the energy input required from the sunlight.
Hydrogen production with these materials involves two chemical reactions: a high temperature (1,550°C) thermal reduction to produce oxygen followed by a lower temperature (1,100°C) water oxidation to produce hydrogen.
One of the keys to the device is the material used in the rings. For hydrogen production, the team found that suspending the ferrite material in zirconia, a refractory oxide that withstands high temperatures, delivered a high yield of hydrogen “quickly and repeatedly,” even after forming the mixture into complex solid shapes. Without using the zirconia, the ferrite material doesn’t hold together well; it essentially forms a slag and stops reacting.
The ferrite/zirconia structures are laid line-by-line using robocasting, a method developed and perfected by other team members that relies on robotics for computer-controlled deposition of materials through a syringe. The materials flow like toothpaste and are deposited in thin sequential layers onto a base to build up complex shapes.
Over the past year, the Sandia researchers have shown proof of concept of S2P and are completing a prototype device that will use concentrated solar energy to split carbon dioxide or water.
Rich Diver is the inventor of the CR5 device. Co-researchers on the project are Jim E. Miller and Nathan Siegel. Project champion is Ellen B. Stechel, manager of Sandia’s Fuels and Energy Transitions Department. Stechel says that researchers have known for a long time that theoretically it might be possible to recycle carbon dioxide, but many thought it could not be made practical, either technically or economically.
Funding for Sunshine to Petrol come from Sandia’s internal Laboratory Directed Research and Development (LDRD) program. The research has also attracted interest and some funding from DoD/DARPA (Defense Advanced Research Projects Agency).
Miller says that while the first step would be to capture the carbon dioxide from sources where it is concentrated, ultimate goal would be to snatch it out of the air. A S2P system that includes atmospheric carbon dioxide capture could produce carbon-neutral liquid fuels.
The research team has already proven that the chemistry works repeatedly through multiple cycles without losing performance and on a short enough cycle time for a practical device. The prototype should be completed by early next year. Initial tests will break down water into hydrogen and oxygen. That will be followed by tests that similarly break down carbon dioxide to carbon monoxide and oxygen.
A commercial S2P device is probably at least 15 to 20 years away, according to Stechel.
Development of Solar-Powered Thermochemical Production of Hydrogen from Water (DOE Hydrogen Program FY 2007 Annual Progress Report)