Power Engineering

Sun In, Hydrogen Out

A Duke University engineer thinks he's found a better way to make hydrogen from sunlight for fuel cells. Some people might find the rooftop approach over their heads.

Roofs across the world hold photovoltaic solar panels to convert sunlight into electricity. Now a Duke University engineer believes a hybrid system can wring even more useful energy out of the sun's rays.

The idea is this. Instead of systems based on standard solar panels, Duke University assistant engineering professor Nico Hotz proposes a version in which sunlight heats a combination of water and methanol in a maze of glass tubes on a rooftop. After a couple of catalytic reactions, the system produces hydrogen more efficiently it turns out than current technology and without significant impurities. The resulting hydrogen can be stored and used on demand in fuel cells.

For his analysis, Hotz compared his hybrid system to three different technologies in terms of their "exergetic" performance. Exergy is a way of describing how much of a given quantity of energy can theoretically be converted to useful work.

"The hybrid system achieved exergetic efficiencies of 28.5 percent in the summer and 18.5 percent in the winter," said Hotz. He compared that to 5 to 15 percent for conventional systems in the summer and 2.5 to 5 percent in the winter.

Hotz's comparisons took place during July and February to measure each system's performance during summer and winter months.

The hybrid system begins with collecting sunlight, just like other systems. But although the hybrid device might look like a traditional solar collector from a distance, it actually is a series of copper tubes coated with a thin layer of aluminum and aluminum oxide and partly filled with catalytic nanoparticles. A combination of water and methanol flows through the tubes, which are sealed in a vacuum.

This set-up allows up to 95 percent of the sunlight to be absorbed with little lost in the form of heat. Hotz called this a crucial factor because it permits temperatures of well over 200 C within the tubes. By comparison, a standard solar collector can heat water to between 60 C and 70 C.

Once the evaporated liquid hits those temperatures, tiny amounts of a catalyst are added. This, in turn, produces hydrogen. The combination of high temperature and catalyst turns out to be an efficient hydrogen producer. The resulting hydrogen can be directed to a fuel cell to provide electricity. Or it can be compressed and stored in a tank to provide power later.

The three systems examined in the analysis were a standard photovoltaic cell which converts sunlight directly into electricity to split water electrolytically into hydrogen and oxygen; a photocatalytic system producing hydrogen similar to Hotz's system (but said to be simpler and not yet mature); and a system in which photovoltaic cells turn sunlight into electricity which is then stored in different types of batteries (lithium ion proved the most efficient).

A cost analysis found the hybrid solar-methanol system was the least-expensive solution. It rang up total installation costs of $7,900 if designed to meet summer requirements. Hotz said that price tag is still much higher than a conventional fossil fuel-fired generator.

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