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Solar Thermal Power May Make Sun-Powered Grid a Reality
It’s solar’s new dawn. For five decades solar technologies have delivered more promises than power. Now, new Breakthrough Award–winning innovations are exiting the lab and plugging into the grid—turning sunlight into serious energy.
Published in the November 2008 issue.
Each Stirling Energy SunCatcher dish can produce 60,000 kilowatt-hours
of electricity a year—enough to power a dozen U.S. homes. (Photograph
by Jamey Stillings)
Planted in the New Mexico desert
near Albuquerque, the six solar dish engines of the Solar Thermal Test
Facility at Sandia National Laboratories look a bit like giant, highly
reflective satellite dishes. Each one is a mosaic of 82 mirrors that
fit together to form a 38-ft-wide parabola. The mirrors’ precise
curvature focuses light onto a 7-in. area. At its most intense spot,
the heat is equivalent to a blistering 13,000 suns, producing a flux 13
times greater than the space shuttle experiences during re-entry.
“That’ll melt almost anything known to man,” says Sandia engineer Chuck
Andraka. “It’s incredibly hot.”
The heat is used to run a Stirling engine, an elegant 192-year-old
technology that creates mechanical energy from an external heat source,
as opposed to the internal fuel combustion that powers most automobile
engines. Hydrogen gas in a Stirling engine’s four 95 cc cylinders
expands and contracts as it is heated and cooled, driving pistons to
turn a small electric generator. The configuration of the dish and
engine represent the fruit of more than a decade of steady
improvements, developed in collaboration with Arizona-based Stirling
On a crisp morning this past January, Andraka and his colleagues fired
up Dish No. 3. The temperature was around freezing, and the sky was 8
percent brighter than average—the contrast between the cold air and the
hot sun helps the engine run more efficiently. When power began to flow
from the 25-kilowatt system, it did so with the highest conversion
efficiency ever recorded in a commercial solar device: 31.25 percent of
the energy shining onto the giant dish flowed into the grid.
To Bruce Osborn, president and CEO of Stirling Energy, this merely
confirmed something that he already knew: The system, which his company
calls the SunCatcher, was ready to exit the laboratory. “The rocket
science is already done,” he says. The challenge remaining is to turn
the prototypes into a low-cost, mass-producible design—“just a question
of good, old-fashioned engineering,” according to Osborn. To that end,
Stirling Energy signed the two largest solar energy contracts in
history with two Southern California utilities, promising to build up
to 70,000 SunCatchers and provide power for a million homes.
Construction starts next year.
Big promises from solar power companies are nothing new. “It is
stern work to thrust your hand into the sun and pull out a spark of
immortal flame to warm the hearts of men,” an AT&T publicity film
crowed after the invention of the silicon photovoltaic (PV) cell in
1954. “Yet in this modern age, men have at last harnessed the sun.”
Well, sort of. The Bell Solar Battery, as it was called, had some
successes—powering the first communications satellite, in 1962, for
instance—but hopes of cheap, plentiful energy have remained elusive.
PV cells and concentrating solar thermal (CST), the two basic methods
for harnessing the sun’s power, have made great strides since those
early days. But inflation in the cost of raw materials, such as
silicon, combined with decades of cheap fossil fuels has kept overall
solar energy consumption in the U.S. at 0.08 percent. And a series of
new technologies that looked promising in the lab have proved
impractical on the open market, leaving many observers to conclude that
the age of solar energy will always remain just around the corner.
Meanwhile, though, almost under the radar, a few solar technologies
have reached maturity. A type of silicon-free solar panel, half as
expensive as silicon cells, has rapidly turned Arizona-based First
Solar into the biggest solar-panel maker in the country. And along with
Stirling Energy’s SunCatcher, new CST designs promise to provide a
steady flow of solar electricity—even at night.
(Illustration by Dogo)
says Reese Tisdale, a senior analyst at Emerging Energy Research, based
in Cambridge, Mass. “It’s large-scale and it’s [usually] steam-powered,
so it’s not so different from the gas- and coal-fired plants they’re
familiar with.” The idea is not new—in fact, nine CST plants with a
combined capacity of 354 megawatts have been operating in the Mojave
Desert since their construction between 1984 and 1991, powering the
homes of 500,000 Californians and proving the design’s reliability. (An
average coal plant produces about 670 Mw.) The plants use a “parabolic
trough” design, with more than 900,000 mirrors, shaped like a
skateboarder’s half-pipe in vast arrays over 1500 acres of desert. The
mirrors adjust to track the sun across the sky, reflecting and
concentrating its rays onto liquid-filled pipes. The hot liquid, in
this case oil, then boils water, which produces steam to spin a
Progress on CST plants ground to a halt after natural gas prices
plummeted in the 1990s. It wasn’t until last year that the next major
plant in the United States opened: a 64-Mw parabolic trough system in
Boulder City, Nev., called Nevada Solar One, built by the Spanish
company Acciona. Now there are 13 other plants, totaling 5100 Mw, in
advanced planning stages in Florida, Arizona and California; most
will use parabolic troughs. Stirling Energy pursued a different kind of
system, one that offers more flexibility and better efficiency.
Bruce Osborn started his research career at Ford Motor Co., and the
key advantage of his solar dish is one his former employers would
understand. “Henry Ford used to say you can have your car in any color
as long as it’s black,” Osborn says, “and that’s our approach, too.”
The planned 900-Mw Stirling Solar Two plant near San Diego will
eventually have as many as 36,000 identical dishes, and the 82 mirror
panels that make up each dish come in only two shapes. That design
choice causes a slight decrease in power output, in exchange for the
advantages of low-cost mass production.
The long mirrors in parabolic trough plants are designed to focus
incoming sunlight onto a narrow, liquid-filled tube that runs parallel
to the array. At the Nevada Solar One plant, 180,000 mirrors help heat
a mineral-oil transfer fluid to 735 F.
Since each 25-kw SunCatcher has its own Stirling engine producing
electricity, there’s no single point of failure. “If something goes
wrong with one dish, it doesn’t matter,” Osborn says. In contrast, the
thousands of mirrors in a parabolic trough plant all feed a central
turbine, so when the turbine is down for maintenance, power production
stops. The SunCatcher design also shortens the wait for power during
construction: Electricity will flow once the first 40 are built—a
“solar group” that can churn out 1 Mw.
The breakthrough efficiency of the dish results from focusing the
sun’s rays on a single spot instead of on a long pipe, which allows
temperatures to reach 1450 F, compared to 750 F for parabolic troughs.
In addition, the Stirling engine has a relatively flat efficiency
curve: It produces close to maximum output even when the sun is
obscured or low in the sky. So while the record 1-hour efficiency
achieved earlier this year was 31.25 percent, the SunCatcher’s
full-year, sunrise-to-sunset efficiency is still a respectable 24 to
25 percent, roughly double that of parabolic trough systems.
Another twist on CST designs confronts the challenge that dogs
every solar power scheme: “When the sun sets, that’s it for the day,”
as Tisdale puts it. “But in Arizona in midsummer, it’s hot as hades, so
people have their a/c cranked until 9 or 10 in the evening.” A hot
liquid can be stored more efficiently than electricity; the analogy
used by one industry executive is that a $5 thermos can hold as much
energy in the form of heat as a $150 laptop battery can store
electrochemically. Two 50-Mw plants that should begin operations by the
end of this year in Spain will operate on this principle, using what
amounts to a giant thermos filled with molten salt.
In the U.S., a thermal storage facility is scheduled for completion
in Gila Bend, Ariz., in 2011. The 280-Mw Solana plant, being built by
Spanish company Abengoa Solar, will use a parabolic trough design, but
will incorporate a thermal storage tank that can keep the plant running
for 6 hours with no sun. “We could design a plant that runs 24 hours a
day,” says Fred Morse, an adviser for Abengoa who was formerly the
Department of Energy’s solar czar, “but that would make no economic
sense.” Instead, the plant is designed to cover Arizona’s peak
energy-use periods, when power is most expensive.
A Matter of Scale
(Illustration by Dogo)
Energy plants provides an answer to skeptics who doubt whether a few
rooftop panels here and there can ever play a meaningful role in the
world’s energy portfolio. But size also creates its own set of
problems. For one thing, the power has to be transmitted to where it’s
needed, and the empty deserts best suited for sprawling CST plants tend
to be in the middle of nowhere. The site of Stirling Energy’s future
plant for the San Diego market currently has enough transmission
capacity for 300 Mw, or 12,000 dishes. The remaining 24,000 dishes will
be built only if San Diego Gas & Electric is able to complete a
proposed 150-mile transmission line between the plant and the city.
Water use is another issue. CST plants with steam turbines can
require hundreds of millions of gallons of water to cool their
condensers—a challenge in regions where water is already at a premium.
In this respect, Stirling Energy’s hydrogen-based system has a
significant advantage, since it only uses water to rinse the mirrors
every few weeks. Osborn estimates that the San Diego plant, when
producing power for 500,000 households, would use the same amount of
water as 33 average homes.
Utility-scale solar power also requires enormous capital, which keeps
it out of reach of people in the developing world, where such solutions
are desperately needed. That’s a challenge RawSolar, an MIT spinoff, is
trying to meet with a dish that is just 12 ft. wide, and simple and
cheap enough to make for stand-alone operation. The nonprofit Solar
Turbine Group, another MIT spinoff, built an even more bare-bones
mini-CST system in Lesotho last summer, using spare car parts for the
The most natural fit for small-scale solar, though, is the good old
photovoltaic cell. It takes in sunlight and spits out electricity with
no moving parts, requires no water and can be situated wherever
electricity is needed, to avoid transmission losses. PV panels can
generate useful amounts of electricity even in the weaker sunlight of
northern states where big CST plants aren’t practical. Also, they’re
ideal for homeowners, since they are simple to install and maintain—in
fact, integrated building materials like PV roof tiles will make new
homes even easier to connect.