Solar Thermal for Local Energy
By Peter Asmus
A boom in sales of solar photovoltaics (PV) and small wind turbine
systems highlights the role these renewable, distributed generation
technologies can play in today’s volatile power markets.
At a time when electricity rates for local governments could rise
as much as 50 percent, these on-site renewable energy technologies
offer greater reliability, price stability, and environmental
benefits than the electricity purchased from the grid.
The demand for these technologies, however, has risen so suddenly
that local governments may have a hard time purchasing these systems
in a timely fashion to help meet this summer’s peak demands
for electricity.
Last year, sales of solar PV systems jumped 44%. The numbers
for 2001 will, no doubt, echo these huge leaps in sales. Small
wind turbine vendors also report brisk sales. Bergey Windpower
sold 40 units in January alone. That compares to total sales of
six in 2000. According to Sandy Miller, a California Energy Commission
economist, applications for buy-down rebates for solar PV and
small wind turbines in January and February already topped the
application totals for all of 2000.
These rebates, which were increased by the California Energy
Commission on May 16th from $2.50 per watt to $4.50 per watt for
renewable energy systems larger than 10 kW, can cover half of
the installation costs of emerging renewable systems that also
include fuel cells (that do not rely upon fossil fuels) and solar
thermal electric technologies. For more information, call (800)
555-7794, or visit http://www.energy.ca.gov/renewables/.
The buy-down rebate amounts were increased for all four of these
clean, cutting edge power generation technologies because these
additions to the state’s severely stressed power grid could
help reduce the threat of daily blackouts this August, when California
has traditionally witnessed the highest demand for electricity.
While the state rebates have helped fuel the increasing popularity
of solar PV and small wind turbine technologies, not one California
consumer has used the buy-down rebates to reduce the costs of
a new solar thermal electric system.
Interestingly enough, some experts predict that solar thermal
electric systems could be an even better technology to address
California’s power supply shortage than solar PV. A variety
of solar thermal electric technologies are not only more efficient
in their conversion of sunlight into electricity, but they offer
additional applications that are particularly well suited to local
governments.
History of the Technology
The intense energy of the sun has long been used to heat liquids.
As far back as the late 1800s, relying upon the sun to heat water
was common practice in the southwestern United States. Photos
can be found showing pioneer families proudly showing off new
homes equipped with solar water heaters. At one point, almost
a quarter of the residents of Los Angeles relied upon the sun
to heat their water with rooftop solar thermal systems.
Over the past 20 years, solar electricity generation technologies
have grown by leaps and bounds, registering annual growth rates
between 25% and 41%. Costs have also fallen by 80%. Global solar
electric generation technologies contribute roughly 2,000 MW of
electricity today.
 While
solar PV technologies are better known to the general public,
California actually gets far more of its electricity from solar
thermal electric power plants. Nine distinct solar thermal power
plants located in the Mojave Desert total 360 MW, by far the largest
central solar power station in the world. For comparison, solar
PV systems installed in California generate less than 50 MW.
Solar thermal power plants were installed in the 1980s in response
to a series of changes in federal and state laws that encouraged
a greater reliance upon alternative fuels. These facilities rely
upon curved mirrored troughs that concentrate sunlight. The sun
heats a liquid that creates steam to turn a traditional turbine.
A more efficient technology is called the ”dish stirling,”
which is powered by an entirely new kind of engine. Instead of
the internal combustion engine, which relies upon an explosion
inside the engine walls to turn pistons, the dish stirling engine
relies upon the sun to heat tubes filled with hydrogen that turn
the crankshaft.
Solar PV panels register efficiencies ranging from 9 to 15 percent.
The solar thermal trough rankine cycle facilities currently operating
in the Mojave are approximately 22 percent. Stirling solar dishes
have been measured at efficiencies as high as 30 percent.
Solar Trough Systems
Though Duke Power of Raleigh, North Carolina-based Duke Power
has received a great deal of negative press regarding its profits
derived from selling electricity from fossil fuel power plants
into California’s power market, its Duke Solar subsidiary
is currently marketing an improved version of the rankine cycle
parabolic trough technology for utility-scale central station
applications. The World Bank and the Global Environmental Fund
have chosen this solar thermal trough technology as its preferred
green power technology and is funding Duke Solar projects in Mexico,
Egypt, India and Morocco.
When employed in utility-scale solar farm configurations, these
power plants can generate electricity at a cost somewhere between
7 to 14 cents per kilowatt/hour during summer peaks, when wholesale
costs can reach as high as $2 per kilowatt hour. Buy-down rebates
can reduce these costs significantly.
Duke Solar is currently offering solar thermal electricity generators
as small as 6 kilowatts and as large as 1 MW. Because of better
concentrators, these organic rankine cycle micro-power plants
can increase fluid temperatures to as high as 750°F, thereby
boosting efficiency. Still, solar PV technologies are generally
favored in solar applications under 100 kW in size because solar
PV technology is so passive, with virtually no moving parts.
Once a solar electric generation system is as large as 100 kW,
however, the additional installation and maintenance requirements
with some solar thermal begins to be offset by economies of scale.
Generally speaking, the larger the solar thermal power plant,
the more cost effective it will be.
“If a system is 1 MW or larger, then solar thermal is a
much better deal than solar PV,” said John Schaefer, Duke
Solar’s California marketing representative. “These
units can be economic at smaller scales if the waste heat can
be used in a cogeneration mode. With the price of natural gas
in California today, this could be appealing to those local governments
relying upon natural gas for heating.”
Schaefer added that the new collectors no longer have to track
the sun like the first generation technology employed in Mojave,
and therefore can work nicely on rooftops. Ground mounted systems
still utilize tracking mechanisms to boost electricity production
levels.
“Our challenge has been to find a single customer to commit
to 50 MW or more,” Schaefer said. He added that he was currently
working on aggregating demand for solar thermal electric systems.
“Municipalities would be ideal candidates.” For more
information, see http://www.dukesolar.com.
Dish Stirling Technologies
The other primary solar thermal electricity generation option
is a variation on dish stirling technology. Instead of the troughs,
this technology relies upon a cluster of dishes arranged in a
circle. These dishes reflect sunlight into a single small collector
that is mounted a short distance from the center of the dish cluster.
These systems, like solar thermal troughs, can run on gaseous
fuels absent adequate solar fuel. They are more efficient than
troughs because they have a more sophisticated solar tracking
system that has two instead of just one long axis.
Another advantage of the dish stirling technology is that it
is modular. Typically dish stirling systems come in the size of
25 kW, which is larger than necessary for most homes, but is often
a good size for many local government applications. Proposals
for 1 MW clusters to be sited in the deserts of southern California
and Arizona have been forwarded to the Los Angeles Department
of Water and Power and other entities. Since water requirements
are minimal and the dish stirling technology is not as maintenance
intensive as the rankine cycle, considerable commercial potential
exists.
Stirling Energy Systems of Phoenix, Arizona has teamed up with
Boeing to commercialize the “Solar Dish Stirling” (SDS)
technology. Two 25 kW systems have been installed at Boeing’s
Huntington Beach facility and have registered availability numbers
of above 95%. In other words, the generators were operating 95%
of the time when solar resource was adequate to generate electricity.
The company’s SDS technology has held the world’s
efficiency record for solar conversion since 1984, but has yet
to be installed in a commercial application anywhere. For more
information on SDS technology, visit http://www.stirlingenergy.com.
Science Application International Corporation of Phoenix, Arizona
offers a SUNDISH technology. Instead of relying upon almost 90
rectangular mirrors clustered in a circle (as is the case with
SDS) that cover 91 square feet, the SUNDISH has larger round dishes
to reflect sun back to a single collector point and covers 108
square feet.
Two 25 kW SUNDISH systems have been installed in Phoenix at
Arizona Public Service. The company’s first sale was to the
Salt River Project’s landfill gas site on the Pima-Maricopa
Indian Reservation site in Arizona. SUNDISH systems can rely upon
methane as a fuel to supplement solar generation. They can also
rely upon other back-up fuels.
For more details, e-mail Barry Butler at barry.l.butler@saic.com.
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