Wednesday August 9, 2017 0 comments
GOLDEN -- Innovation is forever marching forward, but sometimes it's necessary to take a small step backward in the interest of greater accuracy.
Revisiting previous solar-to-hydrogen research enabled scientists at the Energy Department's National Renewable Energy Laboratory (NREL) to improve the efficiency of a novel process that directly converts sunlight into hydrogen -- and recapture a world record first set at NREL in 1998.
Earlier this year, the NREL team (James Young, Myles Steiner, Henning Döscher, Ryan France, John Turner, and Todd Deutsch) published the results of its record-setting research -- which achieved 16.2% solar-to-hydrogen conversion efficiency -- in the journal Nature Energy.
It was a fitting way to mark NREL's 40th anniversary: Turner, a research fellow who set the initial record, has been at the laboratory since its early days as the Solar Energy Research Institute, where it all started with a desire to harness the sun.
Turner set the 1998 record using photoelectrochemical (PEC) water-splitting, a technology that was in its early stages at the time and is now poised to enable cheap and sustainable hydrogen production in the future.
A PEC system operates by directly injecting electrons, created when sunlight hits photovoltaic (PV) cells immersed in acidic electrolyte, to power a water electrolysis reaction.
Turner's early success with PEC relied on tandem solar cell technology, developed at NREL in the early 1990s. The tandem cell uses two semiconductors to capture more of the light that hits it, generating the higher voltages required for the reaction. Turner modified the cell to operate in a water-splitting environment.
"Instead of making electricity and running it through a wire into an electrolyzer, you basically integrate the electrolyzer directly onto the surface of the semiconductor and you can split water that way," Turner said.
"You can do that with high efficiency."
The 12.4% solar-to-hydrogen efficiency record Turner set using this method was unmatched for decades.
"While the efficiency was important, the most significant thing to me was the use of the tandem device configuration to get to higher water-splitting efficiencies than had previously been obtained," Turner said.
"I would characterize it as a quick-and-dirty measurement to show the viability of the concept. There were no standardized solar-to-hydrogen measurement procedures available at the time, and 18 years later, they still did not formally exist."
According to the Energy Department, the United States produces nearly 10 million metric tons of hydrogen each year. Hydrogen is used for various applications including ammonia production, which in turn is used for fertilizer, and can also be used to power fuel cell electric vehicles.
A metric ton of hydrogen is enough to send each of 200 fuel cell vehicles on a 350-mile trip, but to make that much in a day would require a field of PEC devices. Exactly how many depends on the efficiency of the devices. If the efficiency is 15%, the devices must fill a space the size of five football fields. Raising the efficiency to 25% brings the size requirement down to three football fields.
Finding a cost-effective process to produce hydrogen is also of particular importance. The Energy Department would like to see the cost of hydrogen fall to less than $4 per gasoline gallon equivalent (1 kg of hydrogen is the energy equivalent of a gallon of gasoline) by 2020.
This would make hydrogen-fueled vehicles competitive on a cost-per-mile basis with gasoline-fueled hybrid electric vehicles, NREL said.