New Solar Technology Concentrates on Cost,
Efficiency
NREL - January 13, 2009
It looks like a giant funhouse mirror. But the big new dish atop South Table
Mountain could be a renewable energy breakthrough that helps make
concentrated solar power more affordable and appealing to utilities and
their customers.
For the next several months, NREL engineers will be testing the performance
of SkyTrough, an innovative parabolic trough that is coated with a gleaming
reflective skin instead of mirrored glass.
NREL offers leading-edge testing and performance analysis for advanced solar
technologies and other renewable energy designs.
The SkyTrough was developed by SkyFuel, an Albuquerque–based manufacturer
with a research facility near NREL in Arvada, CO.
The unit's lightweight glass-free are mirrors made of sheet metal beneath
ReflecTech® mirror film.
This highly-reflective, silver-metalized film is lighter and less expensive
than the breakable glass mirrors that are traditionally used. The film is a
joint invention of NREL and ReflecTech™ and exclusively licensed from NREL.
The glossy laminate is comprised of multiple layers of polymer films with an
inner layer of pure silver to provide for a highly reflective surface that
also protects the silver layer from oxidation
In commercial use, a SkyTrough could measure as large as 375 feet long and
20 feet high. One SkyTrough would supply enough electricity for 125 homes.
The test model is smaller, but uses the same technologies.
"It's unlike any parabolic trough design used so far," said NREL senior
engineer Keith Gawlik. "Our new facility is designed to test the optical
efficiency of the unit, which they can't do on their own at SkyFuel."
How Parabolic-Trough Systems Work
Parabolic-trough systems concentrate the sun's energy through long U-shaped
mirrors. The mirrors are tilted toward the sun, focusing sunlight on a
vacuum pipe that runs down the center of the trough.
The tube contains heat-transfer oil that absorbs the focused sunlight and
reaches temperatures of 400 degrees Celsius. The hot oil then is used to
boil water in a conventional steam generator to produce electricity.
As the sun moves across the horizon, the troughs follow its trajectory by
rotating along their axes with the help of tracking motors. This keeps the
collectors oriented towards the sun to maximize the system's performance
throughout the day.
SkyTrough Testing at NREL
The SkyTrough itself is mounted on NREL's Large Payload Solar Tracker. It
supports solar components that require 2-axis tracking. The tracker is
capable of carrying a maximum vertical load of 9,000 pounds with a tracking
accuracy of 1 milliradian.
The NREL tests will center on validating the SkyTrough's optical
performance. A key step in concentrating solar power (CSP) is making sure
the light collected in the parabolic trough is accurately converted and
focused on the receiver tube so it can heat the transfer oil efficiently.
"Lots of things come into play when focusing light," Gawlik said. "We have
to consider the reflectivity of the surface, the accuracy of the surface and
then aiming all of the light into the narrow focal line of the receiver
tube."
Typically, a parabolic trough operates at nearly 80 percent optical
efficiency, and SkyFuel expects its design to function at least as well,
while being less expensive to manufacture, transport and maintain.
The NREL test will span portions of at least three seasons to explore the
unit's performance under a variety of weather conditions and sun angles.
"There is a cascade of opportunities to lose some light at every step in the
process," Gawlik said. "That's why we field test the whole unit and get
solid data over a number of months. It removes the uncertainty in the final
efficiency result."
Future Concentrating Solar Power Plants
The NREL tests will not include actual electricity generation because that
step in the process uses conventional steam turbine technology. But it is
that hybrid combination of the renewable and the conventional that makes CSP
appealing to utilities as a source of cleaner bulk power during peak and
intermediate load periods.
Parabolic trough power plants also require relatively large tracts of nearly
level open land with strong solar characteristics, as well transmission
lines and other infrastructure. These factors make Southwestern states the
leading candidates for additional CSP installations; in California, the
Solar Energy Generating Systems plants have been operating for two decades.
By 2015, the Western Governors' Association estimates that 4 gigawatts of
new concentrating solar power plants could be built in the United States.
— Joseph B. Verrengia
|