Today’s Solutions: July 01, 2022

Martin Lindsay, a London-based businessman, parked his Jaguar in the wrong spot. He left it for several hours beneath a concave skyscraper at 20 Fenchurch Street, and came back to find that the sun, reflected off the building, had melted his car. We have much to gain from Lindsay’s misfortune. A system by scientists at the University of Colorado, described in an article published in the journal Science, suggests that energy from the sun, focused on a reactor instead of a luxury automobile, could power the reduction of water into its components—oxygen and, more important, hydrogen, useable as fuel. Who wouldn’t want energy from water and sunlight?
Hydrogen is one of those alternative fuels that holds great 
promise, especially for the auto industry, but remains out of reach for practical use due to some technological barriers engineers have yet to crack. For starters, widespread use would require a new 
distribution system.
The University of Colorado team, led by professors Alan Weimer and Charles Musgrave, deals with a different hurdle: Water stays water for a reason, and splitting it is energy intensive. “In our process, we’re taking concentrated sunlight and directly driving a chemical reaction,” says Weimer. By simplifying the process, the researchers can skip several steps and make the reaction more efficient—and thus, more useful.
The crux of the technology is a catalyst—in this case, a metal oxide called hercynite, a mix of aluminum, cobalt, iron and oxygen. When steam is pushed through a 1350 C reactor filled with hercynite, the oxygen atoms get stuck to the catalyst, and the hydrogen floats free. When you remove the hydrogen and cut off the steam, the oxygen escapes from the hercynite and the cycle is ready to repeat.
“In the hercynite, the material actually undergoes a phase change, and we can drive that phase change with a change in the chemical potential of oxygen,” says Musgrave. “Basically, introducing steam into the 
system increases the chemical potential of oxygen, and that drives oxygen into the material and causes the chemical reaction.”
That’s not the type of phase change we typically envision—from solid to liquid to gas and vice versa—but rather different solid phases of the material, depending on how the atoms in it are arranged. Because the catalyst drops and picks up oxygen atoms based on its phase rather than its temperature, Weimer and Musgrave were able to run the whole process at a constant temperature, saving considerable energy.
It’s a step that makes water splitting a lot more realistic. And water splitting, as an efficient means of isolating pure hydrogen, makes a hydrogen-based fuel economy a lot more likely. The advantages are many: Hydrogen burns clean—no carbon here, and the only output is water; hydrogen does not break down in transport and it keeps indefinitely in storage; hydrogen could replace gasoline in automobiles (goodbye, smog).
Many challenges remain. As a gas, hydrogen can’t just be pumped into existing internal combustion 
engines; it needs specialized containers. By volume, 
it’s not as energy dense as gasoline. But with a cheap source of the gas, these problems will be addressed. Right now, the U.S. Department of Energy is funding research to develop hydrogen distribution systems, 
vehicles with hydrogen fuel cells and hydrogen production. (The University of Colorado study was funded in part by the Department of Energy and the National 
Science Foundation.)
Not only have Weimer and Musgrave streamlined the water-splitting reaction, they’ve envisioned an 
elegant apparatus to run the reactor: Toss it on top of a 450-foot pole in the desert, focus sunlight on it via acres of computer-controlled mirrors (optimized to move with the sun) and run the whole system on solar power. “They have all these mirrors arranged, and oriented to reflect light onto a solar absorber, which takes that light and basically heats up the reactor with that light, like a magnifying glass heats up an ant,” says Musgrave. Or like a building melts a Jaguar.
These “solar-power towers” aren’t exactly new. Power companies have been using similar towers to generate energy in other ways, including driving ordinary turbines, in heavy-sun areas from South Africa to Spain to the Southwest U.S. California’s 3,500-acre Ivanpah Solar Electric Generating System is one such example: Scheduled for completion this year, it uses 300,000 mirrors, their movement optimized based on the location of the sun, to produce 377 megawatts, enough to power 140,000 homes. Though it contains a turbine rather than a solar thermal reactor, the tower would look virtually the same.
Photo: National Renewable Energy Laboratory/Wikimedia

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