Hybrid Solar System Makes Rooftop Hydrogen
August 9th, 2011
By Richard Merritt
DURHAM, N.C. – While roofs across the world sport photovoltaic solar
panels to convert sunlight into electricity, a Duke University
engineer believes a novel hybrid system can wring even more useful
energy out of the sun's rays.
Instead of systems based on standard solar panels, Duke engineer
Nico Hotz proposes a hybrid option in which sunlight heats a
combination of water and methanol in a maze of glass tubes on a
rooftop. After two catalytic reactions, the system produces hydrogen
much more efficiently than current technology without significant
impurities. The resulting hydrogen can be stored and used on demand
in fuel cells.
For his analysis, Hotz compared the 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, compared to 5 to 15
percent for the conventional systems in the summer, and 2.5 to 5
percent in the winter," said Hotz, assistant professor of mechanical
engineering and materials science at Duke’s Pratt School of
Engineering.
The paper describing the results of Hotz's analysis was named the
top paper during the ASME Energy Sustainability Fuel Cell 2011
conference in Washington, D.C. Hotz recently joined the Duke faculty
after completing post-graduate work at the University of
California-Berkeley, where he analyzed a model of the new system. He
is currently constructing one of the systems at Duke to test whether
or not the theoretical efficiencies are born out experimentally.
Hotz's comparisons took place during the months of July and February
in order to measure each system’s performance during summer and
winter months.
Like other solar-based systems, the hybrid system begins with the
collection of sunlight. Then things get different. While the hybrid
device might look like a traditional solar collector from the
distance, it is actually 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 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 degrees Celsius within the tubes. By comparison, a
standard solar collector can only heat water between 60 and 70
degrees Celsius."
Once the evaporated liquid achieves these higher temperatures, tiny
amounts of a catalyst are added, which produces hydrogen. This
combination of high temperature and added catalysts produces
hydrogen very efficiently, Hotz said. The resulting hydrogen can
then 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.
The three systems examined in the analysis were the standard
photovoltaic cell which converts sunlight directly into electricity
to then split water electrolytically into hydrogen and oxygen; a
photocatalytic system producing hydrogen similar to Hotz's system,
but simpler and not mature yet; and a system in which photovoltaic
cells turn sunlight into electricity which is then stored in
different types of batteries (with lithium ion being the most
efficient).
"We performed a cost analysis and found that the hybrid
solar-methanol is the least expensive solution, considering the
total installation costs of $7,900 if designed to fulfill the
requirements in summer, although this is still much more expensive
than a conventional fossil fuel-fed generator," Hotz said.
Costs and efficiencies of systems can vary widely depending on
location – since the roof-mounted collectors that could provide all
the building's needs in summer might not be enough for winter. A
rooftop system large enough to supply all of a winter’s electrical
needs would produce more energy than needed in summer, so the owner
could decide to shut down portions of the rooftop structure or, if
possible, sell excess energy back to the grid.
"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," Hotz said. 'In summer, the first and third
scenarios, as well as the hybrid system, are cheaper than a propane-
or diesel-combusting generator."
This could be an important consideration, especially if a structure
is to be located in a remote area where traditional forms of energy
would be too difficult or expensive to obtain.
Hotz’s research was supported by the Swiss National Science Fund.
Joining him in the study were UC-Berkeley's Heng Pan and Costas
Grigoropoulos, as well as Seung H. Ko of the Korea Advanced
Institute of Science and Technology, Daejon.
Source: www.pratt.duke.edu