27 Apr 2009: Report

A Potential Breakthrough
in Harnessing the Sun’s Energy

New solar thermal technology overcomes a major challenge facing solar power – how to store the sun’s heat for use at night or on a rainy day. As researchers tout its promise, solar thermal plants are under construction or planned from Spain to Australia to the American Southwest.

by david biello

In the high desert of southern Spain, not far from Granada, the Mediterranean sun bounces off large arrays of precisely curved mirrors that cover an area as large as 70 soccer fields. These parabolic troughs follow the arc of the sun as it moves across the sky, concentrating the sun’s rays onto pipes filled with a synthetic oil that can be heated to 750 degrees Fahrenheit. That super-heated oil is used to boil water to power steam turbines, or to pump excess heat into vats of salts, turning them a molten, lava-like consistency.

The salts are just fertilizers — a mix of sodium and potassium nitrate — but they represent a significant advance in the decades-old technology of solar thermal power production, which has traditionally used mirrors to heat water or oil to generate electricity-producing steam. Now, engineers can use the molten salts to store the heat from solar radiation many hours after the sun goes down and then release it at will to drive turbines. That means solar thermal power can be used to generate electricity nearly round-the-clock.

The plant in southern Spain, known as Andasol 1, began operating last November and now provides 50 megawatts of power, enough electricity to supply 50,000 to 60,000 homes year-round. Andasol 2 will come online later this summer, with Andasol 3 already under construction. When the entire Andasol complex is completed in 2011, it is expected to generate enough electricity to power 150,000 households — about 600,000 people.

In the face of mounting concern about climate change, developing alternatives to coal and natural gas combustion has taken on a new urgency, and the construction of utility-scale solar thermal power plants in
At this 11-megawatt power tower outside Seville, Spain, sunlight reflects off 624 moveable mirrors to heat water pipes atop the 40-story tower, creating steam that drives a turbine.
deserts and arid areas is looking like an increasingly promising option. In the United States alone, solar thermal power projects are now being built near fast-growing centers of electricity consumption, such as Las Vegas, Los Angeles, and Phoenix. The first major solar thermal plant to be completed in decades, dubbed Nevada Solar One, started providing 64 megawatts of power to the neon lights of Las Vegas in 2007, although it lacks the latest molten-salt technology. Across the globe, utilities are currently building or planning solar thermal projects in North Africa, Spain, and Australia, among other regions.

Some of the recent claims for solar thermal power have been stunning. Researchers at the German Aerospace Center have estimated that 16,000 square kilometers of solar thermal power plants in North Africa — paired with a new infrastructure of high-voltage, direct-current transmission lines — could provide enough electricity for all of Europe. And scientists have estimated that constructing solar thermal power plants on less than 1 percent of the world’s deserts — an area roughly the size of Austria — could meet the entire world’s energy needs.

Of course, solar thermal has been here before, experiencing a boom in the late 1970s and early 1980s. Its progress then was stalled by collapsing fossil fuel prices, as well as a lack of government support. Today, some critics of the technology fault it for taking up acreage in fragile deserts.

The case for solar thermal power hinges on economics. The sun bathes the Earth with an average of 6 kilowatt-hours of power per square meter over the course of a day, and a concentrated solar power plant like Andasol is
In the U.S., some 3,100 megawatts of solar thermal power are planned by 2012.
the cheapest way to harvest a portion of that. Photovoltaics — semiconductor panels that convert sunlight to electricity — deliver power at roughly 40 cents per kilowatt-hour, while conventional solar thermal power plants can do so for around 13 cents per kilowatt hour, according to the U.S. National Renewable Energy Laboratory. This is only marginally more expensive than the average U.S. price for coal-generated electricity in 2008 of 11 cents per kilowatt hour. The cutting-edge technology of using molten salts to store solar-generated heat is considerably more expensive, but experts expect that price to fall steadily as the technology improves and is mass-produced.

Roughly 612,000 megawatt-hours of electricity from the sun were produced in 2007, according to the Energy Information Administration (EIA), and solar thermal collectors sufficient to cover more than 15 million square feet were shipped and ready for installation that year — more than double the amount in 1998.

In the United States, some 3,100 megawatts of solar thermal power are planned by 2012, and capacity worldwide is expected to reach 6,400 megawatts within 3 years — roughly 14 times the current amount. Still, electricity from the sun contributes just 1 percent of the renewable energy generated in the U.S., and all renewables taken together only provide 7 percent of U.S. energy needs.

Traditionally, solar thermal power plants have been built two ways — using trough-like mirrors to focus the sun’s heat on water or oil in nearby pipes, or using mirrors to focus solar radiation on a central spot, such as a liquid-filled “power tower.”

Since 1984, vast arrays of curved mirrors have concentrated the sun’s rays on regimented lines of pipe filled with a synthetic oil at the Solar Energy Generating Systems (SEGS) power plant in the Mojave Desert of
Using molten salts overcomes a drawback of solar energy generation — that when the sun sets, the lights go out.
California. The SEGS plant was part of a brief boom in alternative energy projects in the late 1970s and early 1980s, when enthusiasm for the technology ran high in the wake of the first energy crisis. But the Reagan administration phased out research and development funding for solar thermal technology, as well as delaying tax credits that had driven the creation of projects like SEGS. Coupled with cheap fossil fuel prices in the late 1980s and 1990s, solar thermal power could not compete — although it still grew by roughly 4 percent a year from 1986 to 2000, according to the EIA.

Given its long operation record, developers are now copying SEGS and its parabolic trough collectors. In the U.S. alone, nearly 1,800 megawatts of such power plants are scheduled to be completed — mainly in the desert Southwest — by 2011, according to investment research from Friedman, Billings, Ramsey & Co.

Palo Alto, Calif.-based Ausra will employ compact linear Fresnel reflectors — flat mirrors that deliver the same focus — to heat water at a planned 177-megawatt solar thermal plant named Carrizo Plains in central California by next year. Already, the company opened a 5-megawatt demonstration plant last October near Bakersfield, Calif.

Nor is the technology confined to the southwestern U.S.: Florida Power & Light will build a 75-megawatt solar thermal trough plant north of Miami. Hurricanes are not a major concern; Ausra’s chief scientist and founder, David Mills, notes that in testing their mirrors have withstood winds of more than 91 miles per hour.

But the most promising technology is one using molten salts, as it overcomes one of the chief traditional drawbacks of solar energy generation — that when the sun sets, the lights go out. The Andasol power plant uses more than 28,000 metric tons of sodium and potassium nitrates to store some of the sun’s heat for use at night or on a rainy day. The molten salts are stored in enormous hot and cold vats, able to be employed on command to soak up extra heat or drive the generation of electricity.

“The turbine is running more hours every day because we have storage and we have the possibility to plan our electricity production,” said Sven Moormann, a spokesman for Solar Millennium, the German company building Andasol.

Abengoa Solar and Arizona Public Services are now using the molten salts technology in portions of the Solana — or “sunny place” — power plant, located 70 miles southwest of Phoenix on nearly 2,000 acres of land. The plant will ultimately produce enough electricity to power 70,000 Arizona homes.

“One of the great things about molten salt technology is that you can get more energy out of the same facility,” says Barbara Lockwood, manager for renewable energy at Arizona Public Services.

But molten salts don’t have to be just used for storage, as they are at
“We’re not going to solve the problem without putting large-scale, concentrated solar facilities in the American Southwest.”
Andasol and will be at Solana. They can also be used directly as a fluid in solar thermal power plants that operate at a much higher temperature, replacing the synthetic oil or water used in power towers. In this variation of the solar thermal technology, large fields of mirrors concentrate the sun’s heat on a central tower that glows with intense light.

Such plants operate at more than 1,000 degrees Fahrenheit — closer to the temperatures employed at a coal-fired power plant — and therefore can use the salts directly as a heating medium. At night, when temperatures begin to drop, the cooling salts that have already transferred their heat to drive a turbine simply drain to the bottom of the tower, where they are stored in tanks, ready to be heated again the next sunny day.

Cheaper power towers that do not employ the molten salts are also being built: an 11-megawatt power tower that employs steam directly — built by Abengoa — now operates outside Seville, Spain. Southern California Edison has contracted for 1,300 megawatts of such direct steam solar thermal power towers with developer BrightSource.

Some utility-scale solar thermal projects have provoked opposition due to the large land area occupied by the arrays in Bakersfield, Los Angeles, San Diego, and elsewhere in California. But proponents of solar thermal power argue that the benefits of the carbon-free technology far outweigh any local impact.

“We’re not going to solve the [climate change] problem without putting large-scale concentrated solar facilities in the American Southwest,” SolarReserve’s Murphy says. “It doesn’t take that big a footprint to make a pretty big impact.”

In addition, there is another way to use this technology for capturing the sun’s heat — cleaning up existing fossil fuel-fired power plants or other operations that burn a lot of CO2-emitting fossil fuels.

By employing the mirrors of a solar thermal array to pre-heat steam, the amount of natural gas, oil, or coal that must be burned can be reduced. In fact, Ausra’s first installation boosted the efficiency of a coal-fired power plant in Australia by providing 9 megawatts of steam to the 2,000-megawatt Liddell Power Station. The company also hopes to work with some California oil producers that currently inject steam — generated by burning natural gas — into the old reservoirs to enable more oil to be pumped to the surface. Ausra argues that it can generate the same steam without any CO2 emissions by employing its solar thermal technology.

The Electric Power Research Institute, a utility-funded consortium, will study the potential of the technology to reduce fossil fuel-burning at power plants in Arizona, New Mexico, Nevada, and North Carolina.

“People need to look at this as a hedge against fossil fuel prices,” says Murphy. “You could start deploying a new type of power plant. We used to burn coal and natural gas — now we can use the sun to make steam.”

POSTED ON 27 Apr 2009 IN Climate Energy Policy & Politics Sustainability Europe Europe North America North America 


These are viable technologies that will continue to develop into affordable sources of baseload electric generation. Along with continuing efficiency and value improvements of distributed solid state solar i.e. photovoltaics, windpower, run-of-river hydro, biomass cogeneration and other clean energy electric generation, as well as a commitment to end-use efficiency and conservation, it is possible to imagine phasing out coal combustion within the next several decades.
Posted by James Newberry on 27 Apr 2009

It's all very nice, but with current levels of energy consumption per capita, which are ever increasing, how much time will this technology buy us — a decade? Maybe two? This will still not solve our energy problem, because the problem lies in the fact we are consuming too much.
Posted by DamirB on 28 Apr 2009

What about the water? I've heard the opposition against these plants in California has more to do with water than the land they'd take up. If I'm not mistaken, solar thermal arrays require the same or more water per kWh as thermoelectric production, which is responsible for 48 percent of all water withdrawals in the U.S. While i am a big fan of solar, a viable clean energy solution — especially something in the SW or other arid regions — cannot add to water demands.
Posted by Bevan on 28 Apr 2009

Without huge increases in energy efficiency none of this will matter. I was just reading the comment field on a New York Times blog article where some guy says he cut his electricity bill in half with just two things. He replaced all of his incandescent bulbs with curly bulbs and the brushed motor on his furnace with a brushless motor and controller. Now, of course he was just looking at a bill in the winter when his furnace motor runs a lot but he has the right idea.

What we need to do is find ways to motivate people to seek energy efficiency:

Posted by Russ Finley on 29 Apr 2009

If 50 megawatts will power 50,000 homes, then one megawatt will power a thousand homes. You wrote that roughly 612,000 megawatt-hours of electricity from the sun were produced in 2007. That implies 612 million homes or close to one quarter of all the homes in the world. There has to be a mistake in these figures somewhere, else we have already solved the world's energy problem almost.
Posted by Luke Lea on 29 Apr 2009

There are a number of challenges to actually getting these projects done, especially in the US: water (as noted above), access to transmission, uncertain total costs, large minimum efficient scales (optimum size is ~250 MW, which at ~10 acres/MW means 2500 acres, or ~ 4 square miles), long construction times, the highly corrosive nature and problematic failure modes of molten salts, and in some cases endangered species habitat concerns.

Concentrated photovoltaics (CPV) represent an alternative to solar thermal with great potential. CPV systems use ultra-high efficiency multi-junction PV cells, require fewer acres per MW, can be built at project sizes ranging from 100s of kW to 100s of MWs, and don't consume water. The theoretical efficiency of multi-junction cells is 66% and they share the same process technology as LED lights (MOCVD), so there is considerable room for cost and efficiency improvements. Amonix, a portfolio company of my former employer, is commercializing CPV systems with 37% efficient cells that are 25% efficient at the system level on an AC-basis. I think we will soon see that there are many sub-segments in the solar market and that each technology will find its niche. Solar thermal and CPV, as well as traditional silicon and thin-film PV, will all have their place.
Posted by Ramsay Ravenel on 30 Apr 2009

What about the land consumption?
Posted by Pete Geddes on 30 Apr 2009

Without discussing consumption or efficiency (which are more involved topics) some quick responses:

Luke - Unfortunately, it's not that easy. The Andasol 1 plant is 50 megawatts of capacity, which is enough to power about 50-60,000 Spanish homes. The 612,000 megawatt-hours is actual generation over a year for both solar thermal and PV. But the actual total installed capacity is just under 500 MW. So that's only 500,000 homes or so. Plus, you've got to remember the commercial and industrial users. It's not all going to run air conditioners in Las Vegas houses.

Loads more info (capacity and generation numbers, etc.) is available here:


Bevan - You are right to look at water use as a key metric for any thermoelectric power plant, particularly in the arid Southwest (where the sunshine is). The question is whether future solar thermal plants will use cooling towers (and their massive water needs) or innovative air-cooling technologies. That remains to be seen but certainly BrightSource, among others, is at least claiming to use air cooling.
Posted by David Biello on 30 Apr 2009

Luke, there is another step or two needed in your calculations.

If 1 megawatt powers 1000 homes, then 1000 watts is needed for 1 home (ten 100 watt lamps seems a little low for a home power consumption to me, however, perhaps that is the average rate over a whole day including night with lights, TV and electric oven off).

But that is 1,000 watt-hours per hour, or 24,000 watt-hours per day, or 8760,000 watt-hours per year

So each home would use about 9 megawatt-hours per year, and the generated electricity would be enough for almost 100,000 such homes — just a large town.

That does not include business or industrial use. Nor recharging your new electric car.
Posted by Simon on 30 Apr 2009

I have some doubts about the 11 cent vs. 13 cent cost comparison. The 11 cent figure must be for the retail rate, that is, including the cost of transmission, distribution and assorted utility vice presidents. On the other hand, when cost is given for technologies like solar thermal, it usually is the cost of the generation alone. (A quick search did not turn up the NREL reference so I cannot confirm that this is the case for the 13-cent figure.)
Posted by Robert Charles Means on 30 Apr 2009

Re: DamirB ..."This will still not solve our energy problem, because the problem lies in the fact we are consuming too much."

Then the answer is to stop consuming so much, combined with the use of these technologies!
Posted by Paul on 01 May 2009

On retail versus generation, you're right: average cost of generation in the U.S. is 10¢ / kwh according to EIA (and much lower in some locales).


As for where 13¢ / kwh came from? It's a figure from the National Renewable Energy Lab and some of the researchers there.
Posted by David Biello on 01 May 2009

From the article above: "Some of the recent claims for solar thermal power have been stunning."

You can say that again! Or you might say that advocates of solar and wind power are prone to make huge exagerations, and/or provide outright disinformation. For instance the article makes the following claim:

"Still, electricity from the sun contributes just 1 percent of the renewable energy generated in the U.S., and all renewables taken together only provide 7 percent of U.S. energy needs."

This may be true, but if it is, it includes all types of power that most of us would not consider green at all. For instance the burning of wood and trash. It is this disinformation campaign that groups like E360 should seek to stop. In order for us to make informed choices and intelligent decisions, Americans need facts, not fantasy projections, or "pie in the sky" predictions.

Posted by f1fan on 04 May 2009

Taking the most inefficient, hugely expensive source of electricity, solar power and further adding cost and inefficiently by producing the electricity, using it to melt salt, later extracting the heat from the molten salt and regenerating electricity takes an extremely poor source of electricity into a horrendous source.
Posted by Dahun on 06 May 2009

I'd like to piggyback on the comment from Mr. Means about your figures for the cost per kwh. Your 11 cent figure for coal is certainly far too high. It's arguably the cheapest source except for hydro power. I'd put the wholesale cost at about 5 cents, although I've seen some figures lower that that. And the 13 cent figure for solar thermal sounds a little low. Frequently people quoting costs for wind and solar power include the subsidies they enjoy. I suspect the real figure should be about 15 cents.

So in reality, using wholesale prices, thermal solar is probably about 3 times the cost of coal. But that's not the whole story since it doesn't include the cost of storage. Would that double the cost? Who knows. Finding reliable data on costs is very difficult. Furthermore, a massive storage system that could store enough energy to generate electricity through the night would be sub par. Cloudy days even happen in the desert. Some day, solar power will play a big role, but that day is still a long ways off.
Posted by Jim Ogden on 20 May 2009

The future of the world supply and demand of electricl power looks promising as these new technologies evolved.

Here in Jamaica, our yearly consumption of electrical energy increases at a rapidly alarming rate while we rely heavily on imported oil, with just littly over 1 percent dependence on renewable energy. Last year we have surpassed the 1 billion u.s. dollar mark for imported oil, which burdens our economy.
Posted by Dwight Walsh on 25 May 2009

Jim, if your estimates are true, how do they alter given the costs of carbon-scrubbing the output gasses? I've heard a (very speculative) price for that of about the mid 2008 eu carbon price per ton.

That shifts the balance seriously in favour of solar obviously! It's better if the price triples than if it multiplies by five.
Posted by Josh W on 20 Jun 2009

Minimizing our dependency on nonrenewable fuel sources may be a lot more achievable than eliminating our need for fossil fuels, especially when wind and solar power are the alternative, but what does that mean for cost-efficiency and for the environment?

* Off-shore drilling destroys natural habitats and increases the risk for hazardous spills and water, as well as air, contamination.
* Coal, while it may be abundant at the present time, is the dirtiest option. Clean coal technology is in the works, but an expensive process that isn’t sustainable – especially with the rising demand for coal from developing countries.
* The obvious need for fossil fuel-based energy, alongside renewable options like wind and solar, means an even greater cost to the public for those receiving grid-supplied power.

There are many other possibilities for renewable energy options beyond wind and solar power, or in combination with wind and solar. Off-grid homes have more potential than solar and wind based grid production – energy storage solutions are more feasible on a small scale and there are also more cost-effective methods of generating small scale power. It’s worth it saving our planet.
Posted by home solar powr on 14 Sep 2009

Comments have been closed on this feature.
david bielloABOUT THE AUTHOR
David Biello has been covering energy and the environment for nearly a decade, the last three years as an associate editor at Scientific American. He also hosts 60-Second Earth, a Scientific American podcast covering environmental news. In previous articles for Yale Environment 360, Biello wrote about how geothermal technology can contribute to the world’s energy needs and the the resurgence in dam construction worldwide.



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