Greetings to all my friends out there looking out for storing solar energy
This was one of the articles that I came across
Hope you all will like it..
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| The molten salts stored in the two tanks
pictured here amidst the rows of troughs at the Andasol 1 power plant in
Spain will allow solar energy to produce electricity even at night. |
Near Granada, Spain, more than 28,000 metric tons of salt is now coursing through pipes at the Andasol 1 power plant. That salt will be used to solve a pressing if obvious problem for solar power: What do you do when the sun is not shining and at night?
The answer: store sunlight as heat energy for such a rainy day.
Part of a so-called parabolic trough solar-thermal power plant,
the salts will soon help the facility light up the night—literally.
Because most salts only melt at high temperatures (table salt, for
example, melts at around 1472 degrees Fahrenheit, or 800 degrees
Celsius) and do not turn to vapor until they get considerably
hotter—they can be used to store a lot of the sun's energy as heat.
Simply use the sunlight to heat up the salts and put those molten salts
in proximity to water
via a heat exchanger. Hot steam can then be made to turn turbines
without losing too much of the original absorbed solar energy.
The salts—a mixture of sodium and potassium nitrate, otherwise used as
fertilizers—allow enough of the sun's heat to be stored that the power
plant can pump out electricity for nearly eight hours after the sun
starts to set. "It's enough for 7.5 hours to produce energy with full
capacity of 50 megawatts," says Sven Moormann, a spokesman for Solar Millennium, AG,
the German solar company that developed the Andasol plant. "The hours
of production are nearly double [those of a solar-thermal] power plant
without storage and we have the possibility to plan our electricity
production."
Using mirrors to concentrate the sun's energy is an old trick—the ancient Chinese and Greeks both used it to start fires—and modern power plants employing it might provide a significant source of renewable energy without any greenhouse gas emissions.
That is a step forward in its own right, but such power plants are
limited to generating energy only when there is sunshine. So engineers
have tried a number of different technologies to store the sun's energy
so that such power plants can be more broadly employed. They have tried batteries but too much of the energy that goes in is not returned, and they tend to be too expensive, according to an analysis from the National Renewable Energy Laboratory
(NREL) in Golden, Colo. Compressing air or pumping water uphill are
more promising, but the opportunities to do that are limited by the
number of caverns and the availability of water and reservoirs.
Melting salts at temperatures above 435 degrees Fahrenheit (224 degrees
Celsius), however, can deliver back as much as 93 percent of the energy,
plus the salts are ubiquitous because of their application as fertilizers.
"There's a term called round-trip efficiency. Basically, it's a measure
of how much electricity is produced if the thermal energy that's
generated is first stored and then used compared to just directly taking
the energy. That number is around 93 percent," explains NREL
senior engineer. "[For] things like compressed air and
mechanical type storage, there's more significant losses," an average of
at least 20 percent over all the various technologies.
The Andasol 1 power plant, which cost around $380 million (300 million
euros) to build, is the first to actually use the technology, so it
remains to be seen how it will work in commercial practice. But U.S.
government laboratories—NREL as well as Sandia National Laboratory
in Albuquerque, N.M.—have already proved the technology can work in
demonstration projects that employed it, like the Solar Two power tower
outside Barstow, Calif.
Solar Millennium is so confident the technology will work that a twin solar-thermal power plant (Andasol 2) is already completed. It started operations at the beginning of summer— June 2009
And Arizona Public Service Co. (APS) has contracted with Abengoa Solar to build a 280-megawatt solar thermal power plant—dubbed Solana
or "sunny place"—70 miles (110 kilometers) southwest of Phoenix on
nearly 2,000 acres (800 hectares) of land. "One of the great things
about molten salt technology is that you can get more out of the pure
solar resources, more energy out of the same facility," says Barbara
Lockwood, manager for renewable energy at APS. "It's an alternative that
provides us with additional green energy," as much as 1,680
megawatt-hours when cloudy or after sunset.
But that extra energy comes at a cost. First, the power plant has to be
enlarged so that it is both generating its full electrical capacity as
well as heating up the salts. In the case of Andasol 1 that meant
covering 126 acres (50 hectares) with long rows of troughs and pipe. And
then there is the additional expense of the molten salt storage tanks,
according to Moormann.
All told, that means thermal energy storage at Andasol 1 or power plants
like it costs roughly $50 per kilowatt-hour to install, according to
NREL's Glatzmaier. But it doesn't add much to the cost of the resulting
electricity because it allows the turbines to be generating for longer
periods and those costs can be spread out over more hours of electricity
production. Electricity from a solar-thermal power plant costs roughly 13 cents a kilowatt-hour, according to Glatzmaier, both with and without molten salt storage systems.
That price is still nearly twice as much as electricity from a coal-fired power plant—the
current cheapest generation option if environmental costs are not taken
into account. But Arizona's APS and others can then use solar energy to
meet the maximum electricity demand later in the day. "Our peak demand
[for electricity] is later in the evening, once solar production is
trailing off," Lockwood says. That's "the reason we went that direction
and are so interested in storage technology."
As efficient as solar-thermal power plants
using parabolic troughs with molten salt storage systems like Andasol 1
or Solana are, they don’t capture as much of the sun's heat as is
possible. Above 750 degrees F (400 degrees C), the synthetic oils used
to capture the sun’s heat in the troughs begin to break down, but the
molten salts can take in much more heat than that.
To allow the salts to get hotter, some companies, such as SolarReserve in Santa Monica, Calif., are developing so-called power towers—vast
fields of mirrors that concentrate sunlight onto a central tower.
Because of the centralized design such a structure can operate at much
higher temperatures—up to 1,000 degrees F (535 degrees C)—and use molten
salts directly as the fluid transferring heat in the power plant. "We
are heating the salts to more than 1,000 degrees F and that results in
the same inlet conditions that utilities see today on a coal-fired or nuclear power plant," as said by SolarReserve's president.
But such a power plant—and Murphy says the company has some 50 such
projects in the pipeline and expects at least one (in the U.S. or Spain)
to be operating by 2013—would cost as much as $800 million for a
200-megawatt power tower. "The first molten salt power tower built is going to be a real trial," says Thomas Mancini, manager of Sandia's Concentrating Solar Power Program. "It's going to take someone progressive enough to finance it or take a little more risk."
So researchers are also looking into salts that could be used instead of
the oil in parabolic trough power plants, such as those that melt at
lower temperatures and therefore would not freeze as readily during cold
nights.
Solar Millennium is working on such a salt, according to Moormann, and
Sandia has developed small quantities of a new mixture of salts,
including calcium nitrate and lithium nitrate, that melt below 212
degrees F (100 degrees C). "With the lithium nitrate, it's as expensive
as all the other constituents combined. Though still a lot cheaper than
organic heat-transfer oils," says chemical engineer Bob Bradshaw at
Sandia in California, who is leading the research. "You don't get
something for nothing."
And long-term research projects are looking at other thermal storage
technologies, such as storing heat in sand or creating single-tank
molten salt storage. "The main goal is to find a storage technology that
may reduce the actual capital cost" of adding it to a power plant, says
Phil Smithers, technical services leader for renewable energy at APS,
which is researching those technologies under a U.S. Department of Energy grant.
Ultimately, it will come down to how much value policymakers and
consumers put on electricity that is renewable and emissions-free. "If
we start valuing carbon and force a coal plant to go carbon-free via sequestration
then we're at or over 10 cents per kilowatt-hour from coal," Mancini
says. "Any of these technologies can get to that same 10 cents level
with [molten salt] storage. Then the market will make the call."
And should Andasol 1 spring a leak or otherwise fail to deliver as
expected, the damage would not be confined to a pile of salt fertilizer
on the ground—it could be a setback for the entire effort to store solar energy.
"We had to build the first [commercial] plant [with molten salt
storage] and that's what Andasol is," Mancini says, in order to prove
the technology. "It doesn't have to be perfect, but they've got to make
it work."
It is also estimate that Andasol 1 will generate 178 000 MWh of renewable
electricity per year, whereas the same field of solar collectors and
turbine would turn out just 117 000 MWh sans storage.
