Synthetic Fuel From a Solar Collector

Solar energy powers greenhouse-gas-free synthetic fuel production in Sandia experiment

PHOTO: Randy Montoya

7 January 2008—At first blush, you might lump claims about a machine that supposedly turns sunshine, air, and water into fuel in the same category as e-mails insisting that someone in Nigeria will pay you handsomely to help free up a large sum of money. But researchers at the U.S. Department of Energy’s Sandia National Laboratories, in Albuquerque, say they have created a device that can break water into hydrogen and oxygen using sunlight, or in a another reaction convert carbon dioxide, to carbon monoxide that combines with hydrogen to make hydrocarbons such as methanol, ethanol, and even gasoline or diesel fuel. The technology holds the promise of using the same resources as biomass-to-fuel schemes but with potentially greater efficiency, according to the researchers.

The machine, called the Counter Rotating Ring Receiver Reactor Recuperator, or CR5, is essentially a stack of rings or disks, each outfitted with a dozen fins around its circumference that are constructed of a reactive metal oxide (rust to you and me) in a matrix of material that can withstand high temperatures. As a ring rotates, the reactive material passes into a chamber irradiated by a solar collector where temperatures exceed 1500 °C. This is hot enough to trigger a reaction that liberates oxygen from the rust, changing the metal’s chemical structure at the same time. The ring rotates the metal oxide (at roughly 1 revolution per minute) 180 degrees to a reaction chamber where, at relatively cool temperatures (around 1000 °C), the scorched rust is exposed to superheated steam. The metal oxide and water react in a way that effectively strips the oxygen from the water and restores the rust to its original form, yielding free hydrogen in the process. Then the cycle begins again.

In another CR5 stack, carbon dioxide is split into carbon monoxide and oxygen in the solar chamber. The carbon monoxide could then be used to make synthetic hydrocarbon fuels by combining it with hydrogen from the first CR5 stack, using any of a number of commercial processes.

”The first step would be to capture the carbon dioxide from sources where it is concentrated, such as power plants and industrial smokestacks,” says James E. Miller, a chemical engineer who is one of the lead researchers on Sandia’s ”Sunshine to Petrol” project. ”But the ultimate goal would be to snatch it out of the air” to yield carbon-neutral liquid fuel, he says.

The device gets the ”Counter Rotating” part of its name from the fact that each ring in a stack rotates in the direction opposite that of the rings adjacent to it. A single dc motor and a set of gears, which are isolated from the scorching heat in the machine, turn the rings. The design conserves energy by allowing the part of one ring that is exiting the sizzling solar chamber to transfer heat to the parts of its neighboring rings that, having just left the cooler steam chamber, need to be reheated in order to release the oxygen they just acquired. These transfers, say the Sandia scientists, boost the machine’s maximum theoretical efficiency at turning the sun’s heat into chemicals from 36 percent to 76 percent.

The researchers say the aim is to build a reactor about the size of an oil drum. Such a generator could transform 22 kilograms of carbon dioxide and 18 kg of water into roughly 9.5 liters of liquid fuel each day, according to Sandia researchers. They envision that commercial refineries would have arrays of synthetic fuel generators.

But the CR5 is probably 15 to 20 years from commercialization, says Ellen B. Stechel, manager of Sandia’s fuels and energy transitions department, under which this research is being conducted. ”There is no precise calculation underlying that number,” says Miller. ”Just a sense of where solar thermal is in terms of acceptance, and the improvements we have to make in areas such as surface-to-volume ratio of the reaction material that affects reaction rates, and oxygen exchange, not to mention the status of systems for capturing carbon dioxide.”

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