10 August 2011
Duke University engineer Nico Hotz has proposed and analyzed a hybrid system in which sunlight heats a mixture of water and methanol in a maze of tubes on a rooftop. After two catalytic reactions, the system produces hydrogen that can be stored and used on demand in fuel cells.
A schematic of the hybrid system. Credit: Nico Hotz.
The hybrid device contains 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 vacuum-sealed tubes.
"This set-up allows up to 95% of the sunlight to be absorbed with very little being lost as heat to the surroundings," Hotz said. "This is crucial because it permits us to achieve temperatures of well over 200 ºC within the tubes. By comparison, a standard solar collector can only heat water between 60 and 7 ºC."
Once the evaporated liquid achieves a high enough temperature, tiny amounts of a catalyst are added. The combination of high temperature and catalysts produces hydrogen very efficiently, Hotz said. The hydrogen can be immediately directed to a fuel cell to provide electricity to a building during the day, or compressed and stored in a tank to provide power later.
Hotz compared the theoretical exergetic performance of the hybrid system to three technologies that can produce hydrogen: a standard photovoltaic cell converting sunlight directly into electricity to then split water into hydrogen and oxygen; a photocatalytic system producing hydrogen in a manner similar to Hotz's system; and a system in which photovoltaic cells turn sunlight into electricity that is stored in different types of batteries (with lithium ion being the most efficient).
"The hybrid system achieved exergetic efficiencies of 28.5 percent in the summer and 18.5 percent in the winter, compared to 5 to 15 percent for the conventional systems in the summer, and 2.5 to 5 percent in the winter," said Hotz. His system also came out on top for the cost analysis, costing only $7,900 for installing a system that fulfills requirements for the summer. He is now constructing one of the systems to see if the theoretical efficiencies hold true experimentally.
"The installation costs per year including the fuel costs, and the price per amount of electricity produced, however showed that the (hybrid) solar scenarios can compete with the fossil fuel-based system to some degree," he said. The paper describing the results of Hotz's analysis "Exergetic Analysis of Hybrid Energy Conversion and Storage Scenarios for Use of Solar Power in Stationary Applications" was named the top paper during the ASME Energy Sustainability Fuel Cell 2011 conference in Washington, D.C.
Written by Nancy Lamontagne, Contributing Editor - US