Researchers at Sandia National Laboratories have successfully demonstrated a prototype machine that uses the sun’s energy to convert water and carbon dioxide into the molecular building blocks that make up transportation fuels. The “Sunshine to Petrol” system could ultimately prove a practical way to recycle CO₂ from power and industrial plants into gasoline, diesel, and jet fuel, assuming the process can become at least twice as efficient as natural photosynthesis.
Until recently, the system had only been validated in a laboratory in small batches. A hand-built demonstration machine was successfully tested this fall. “This is a first-of-its-kind prototype we’re evaluating,” says Sandia researcher Rich Diver, inventor of the device.
“In the short term we see this as an alternative to sequestration,” says James Miller, a chemical engineer with Sandia’s advanced materials laboratory. Instead of just pumping CO2 underground for permanent storage, Miller says, the sun’s abundant energy can be used to achieve “reverse combustion” that essentially turns carbon dioxide back into a fuel. “It’s a productive utilization of CO2 that you might capture from a coal plant, a brewery, and similar concentrated sources.”
The cylindrical metal machine, called the Counter-Rotating-Ring Receiver Reactor Recuperator (CR5), relies on concentrated solar heat to trigger a thermo-chemical reaction in an iron-rich composite material. The material is designed to give up an oxygen molecule when exposed to extreme heat, and then retrieve an oxygen molecule once it cools down.
The machine is designed with a chamber on each side. One side is hot, the other cool. Running through the center is a set of 14 Frisbee-like rings rotating at one revolution per minute. The outer edge of each ring is made up of an iron oxide composite supported by a zirconium matrix. Scientists use a solar concentrator to heat the inside of one chamber to 1,500 º C, causing the iron oxide on one side of the ring to give up oxygen molecules. As the affected side of the ring rotates to the opposite chamber, it begins to cool down and carbon dioxide is pumped in. This cooling allows the iron oxide to steal back oxygen molecules from the CO₂, leaving behind carbon monoxide. The process is continually repeated, turning an incoming supply of CO2 into an outgoing stream of carbon monoxide.
Miller says the same process can be used to produce hydrogen, the only difference being that water, instead of carbon dioxide, is pumped into the second chamber. The two separately retrieved gases–hydrogen and carbon monoxide–are then mixed together to make syngas, which can be used to make a “drop-in replacement” for traditional fuels, says Miller.