SUNSHINE STORAGE IN A SALT: 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.
Image: COURTESY OF SOLAR MILLENNIUM
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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 Greg Glatzmaier. "[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 near completion. "It will start operations at the beginning of summer—May or June," Moormann says.
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," says Terry Murphy, 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, according to Hank Price, a vice president for technology development at Abengoa Solar.
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."
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18 Comments
Add CommentHas anyone sent this article to President Obama?
Reply | Report Abuse | Link to thisI'm all in favor of renewable energy sources, but not at any cost or efficiency. Both of the aforementioned items (cost and efficiency)identified in this article are suspicious, and I will explain why.
Reply | Report Abuse | Link to this1. Cost: Earlier articles on this same technology (IEEE Spectrum Oct 2008) quoted a cost of 27 cents/Kwhr, which was a subsidized cost. And, to me, this is acceptable for a "new" renewable technology in it's formative stage. However, in this article, the proponents now quote a price of 13 cents/Kwhr! That's 50% less in only a few months. I think I would like to see some cost justification on how the price decreased 50% in 6 months before jumping on this band wagon.
2. Efficiency: The round trip efficiency of 93% quoted in the article is identified in a very misleading manner, which leads one to believe that the "round trip" efficiency of the plant is 93%. The efficiency of 93%pertains only to the thermal storage part of the plant. Having said that, I would like to state that this is a "very" good efficiency for an energy storage mechanism that far exceeds any type of battery storage. However, the actual round trip efficiency of the entire plant (solar collector to steam to turbine to generator to electricity) is less than 20%; (Reference technical article "Central REceiver System (CRS) Solar Power Plant Using Molten Salt As Heat Transfer Fluid" by J. Ortega, J. Burgaleta, F. Tellez, Oct 2008) and this is not mentioned anywhere in this article.
I would like to see our government have a balanced approach in its implementation of renewable energy generation that includes all types of renewable sources (hydro, wind, solar, geothermal, pumped storage, etc.). However, a well balance plan cannot be developed until the respective proponents of various schemes tell the true stories about their strengths and weakenesses. The above mentioned article by Mr. Biello is not accurate or truthful.
Great article! Its clear that the days of cheap energy are over, and we need to concentrate on finding ways like with this solar-thermal power to use the renewable resources we have at hand. Super Eco is pointing to your piece tomorrow: http://ecoworldly.com/2008/04/12/mega-solar-the-worlds-13-biggest-solar-thermal-energy-projects/ Thanks!
Reply | Report Abuse | Link to thisThis technology is no new russian use to made almost 30 years ago.
Reply | Report Abuse | Link to thisOf course, solar power plants already produce electricity when it is needed most (during the day). This is a cool way to turn solar into baseload (sort of) but it might be unnecessary, no? On the other hand, this could be a solution to the question of 'what do we use if we stop building more coal-fired plants?'
Reply | Report Abuse | Link to thissee here:
http://climateprogress.org/2009/02/18/obama-replace-dirty-coal-nuclear-efficency-cogeneration-wind-solar-csp-biomass-cofiring/
To avoid the considerable transmission and other losses, generate electricity only there where it is needed?
Reply | Report Abuse | Link to thiseco steve
Reply | Report Abuse | Link to thisI believe we've had this discussion before. Transmission lines are designed for 10% losses. Fossil fuel plants are 40 to 50% efficient (coal to steam to electricity); hydroelectric plants are 85% efficient (potential energy of the water to electricity); wind generators are 50% efficient (85 to 90% of the Betz coefficient).
Solar panels(solar to electrical) have only a 15 to 20% efficiency. Therefore, for a comparison of efficiencies, if you delete the 10% transmission losses from the lowest plant efficiency (the fossil fueled plant), you have a 30 to 40% overall efficiency. This is still more efficient than solar panels at 15 - 20% efficiency. Moreover, the hydro plant and wind plant have a 75% to 40% overall efficiency.
What other losses are you talking about? The above comparisons show why centralized power plants are utilized vs home generation.
The most important is how to use solar energy better.People need cheaper and clean energy.Who stop it to develop fast.
Reply | Report Abuse | Link to thisWe know it can do more thing than we looked.I am working a solar energy factory in China. If you use it ,must through factory-trader-nativetrader-distributor and you.In factory we made it must spend 2.3?/w.But i don't know how much somebody buy it in us and euro.In fact the most expencive of systerm is cells,in China maybe just 10 company control the markets.
I'm from a solar energy factory in China.If you want know more about solar energy you can E-mail : jackie@skysolarpv.com I hope get some imformaition about the market you are living there.
Reply | Report Abuse | Link to thisHope the enenrgy can be cheaper.
I don't think this will be able to compete with coal or nuclear baseload generation without subsidies, but can compete with natural gas. The thermal storage component provides a buffer for problems of intermittent solar radiation. This also allows for short-term energy storage to deal with the time lag between daily peak solar radiation (~noon) and peak electrical demand (~3-4 pm).
Reply | Report Abuse | Link to thisWhen I saw the phrase "it costs roughly $50 a kilowatt-hour to install," I began to question the validity and the accuracy of the whole article. I had to think that either Biello or Glatzmaier does not understand the difference between energy and power.
Reply | Report Abuse | Link to thisBRILLIANT
Reply | Report Abuse | Link to thisWe are reinventing the wheel again
Was it not the Aztecs or Russians that used salt to keep there houses warm and dry?
The scientific community needs to have some new thinking in its dissemination of information, and break down the restrictive practices in data handling especially from state research centers and universities across the globe.
We need to move fast and quickly to be able to employ more brains on the future energy supply.
The Glasshouse horticultural industry is already employing water storage to provide heating day and night employing buffer tanks.
Reply | Report Abuse | Link to thisThis technology should be employed in factory ,office design employing heat pumps and storage to save energy.
Offices generate heat from passive solar through windows, from computers,servers,air conditioning units, surley all this wasted energy could be stored and reused in electrical or hydrogen generation.
We need to see the heating and cooling industries use the "new" technologies in there products NOW, so we need the scientific community communicate far more effectively.
Perhaps a SCIENCE IDEAS TV CHANNEL RUN BY THE UN WORLDWIDE linked with comprehensive databases for global access free of charge .
The world does not need more information, it needs better quality information. Half truths and massaged facts are the real problem. If you don't already know many of the facts sighted, how do you know you are being lied to? Articles like this one are a PR song to a friendly writer and not a real objective piece of facts, warts and all. It is too bad so many opinions are based on drive by information from articles like this one.
Reply | Report Abuse | Link to this(Reference technical article "Central REceiver System (CRS) Solar Power Plant Using Molten Salt As Heat Transfer Fluid" by J. Ortega, J. Burgaleta, F. Tellez, Oct 2008) and this is not mentioned anywhere in this article.
Reply | Report Abuse | Link to thisWD REALLY APPRECIATE GETTING THIS ARTICLE FROM DAVID CLEMEN
Has anyone tried compressed air. You run compressors and fill tanks during the day and use it to run generators at night. The compressed air engine to power the generator already exists and has run a car for some years now. We have had compressors for more than 100 yeas and solar panels for 50 or more years. Actually any steam engine will run on compressed air so I know someone has tried this. How did it not work and why have I not seen one article about it?
Reply | Report Abuse | Link to thisI assume the corrosive nature of salt was considered in the design and choice of piping material by the engineers of these plants.
Reply | Report Abuse | Link to thisIt clearly explain what round trip efficiency means. "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"
Reply | Report Abuse | Link to thisI don't get how you read that clear cut explanation and came to the conclusion that it is misleading. No one here thought it was a plant efficiency.
Secondly If I write an article now about a 5 year old plant and say it has 30c/KWH and write another one the next day of a current plant being build that is 10c/KWH. How is this deceiving? Are you going to ask me how did it drop from 30 to 10 in just day? The c/KWH depends on the plant where it was build and using which technology. The numbers change from plant to plant. You have to also look at who is giving the numbers. Different analyst give different numbers. There are actual numbers and projected numbers.