Turbines Could Tap the Mississippi's Power

Turbines Could Tap the Mississippi's Power

Underwater turbines could harness a massive amount of energy—but could cause problems for boat navigation.

Tens of thousands of turbines anchored to the bottom of the Mississippi River could someday provide more than a gigawatt of renewable energy, enough to power a quarter of a million homes. That's the vision of Free Flow Power, a startup based in Gloucester, Massachusetts, that recently received preliminary permits from the U.S. Federal Energy Regulatory Commission (FERC) granting it the right to explore energy production at dozens of sites along the lower Mississippi over the next three years.

The proposed development is one of a number of "hydrokinetic" projects in the works. Such projects seek to generate electricity from wave movement, tidal flows, or river currents, without the use of dams.

The ambitious Mississippi project, however, relies on relatively unproven technology. The only commercial hydrokinetic river-power system operating in the U.S. is a single turbine deployed by Hydro Green Energy close to a conventional hydropower dam on the Mississippi River in Hastings, Minnesota.

Free Flow hopes to deploy hydrokinetic power on an unprecedented scale: hundreds of 40-kilowatt turbines, each the size and shape of a large jet engine and attached to pylons jutting out from the riverbed at 88 locations along the Mississippi.

Although most companies developing hydrokinetic technology have focused on tidal or wave energy, Free Flow's chief financial officer, Henry Dormitzer, argues that river power has distinct advantages. "The water flows in one direction, it doesn't have salt in it, and, in the case of the Mississippi, people have spent 100 years tracking water flows and velocities," he says.

But the Mississippi is also one of the world's busiest waterways, and the company will have to demonstrate that its turbines will not interfere with commercial shipping, and that it will have no negative impact on the river's wildlife.

In July 2009, Free Flow began a six-month test of a pilot turbine (a third the size of the planned commercial ones) in the Mississippi, and the company is now testing a commercial-scale prototype in the lab. Free Flow has also received $7.4 million in funding from investors and from the U.S. Department of Energy that will allow it to test its most recent prototype in the Mississippi next year. Free Flow Power is seeking additional funding to test four turbines, each attached to a separate pylon, over a 12-month period, as required by FERC as part of the licensing process.

Free Flow uses a "shrouded turbine" design that channels water through the turbine's blades. Water passes through a rotor with seven blades that are designed for a slow spin rate to minimize fish strikes. The turbines will be sited 10 or more feet off the riverbed. At this depth, water moves, on average, at one to three meters per second.

Nanogenerator Powers Up

Nanogenerator Powers Up

A device containing piezoelectric nanowires can now scavenge enough energy to power small electronic devices.

Devices that harvest wasted mechanical energy could make many new advances possible—including clothing that recharges personal electronics with body movements, or implants that tap the motion of blood or organs. But making energy-harvesting devices that are compact, flexible, and, above all, efficient remains a big challenge. Now researchers at Georgia Tech have made the first nanowire-based generators that can harvest sufficient mechanical energy to power small devices, including light-emitting diodes and a liquid-crystal display.

The generators take advantage of materials that exhibit a property called piezoelectricity. When a piezoelectric material is stressed, it can drive an electrical current (applying a current has the reverse effect, making the material flex). Piezoelectrics are already used in microphones, sensors, clocks, and other devices, but efforts to harvest biomechanical energy using them have been stymied by the fact that they are typically rigid. Piezoelectric polymers do exist, but they aren't very efficient.

Zhong Lin Wang, who directs the Center for Nanostructure Characterization at Georgia Tech, has been working on another approach: embedding tiny piezoelectric nanowires in flexible materials. Wang was the first to demonstrate the piezoelectric effect at the nanoscale in 2005; since then he has developed increasingly sophisticated nanowire generators and used them to harvest all sorts of biomechanical energy, including the movement of a running hamster. But until recently, Wang hadn't developed anything capable of harvesting enough power to actually run a device.

In a paper published online last week in the journal Nano Letters, Wang's group describes using a nanogenerator containing more nanowires, over a larger area, to drive a small liquid crystal display.

To make the generator, Wang's team dripped a solution containing zinc-oxide nanowires onto a thin metal electrode sitting on a sheet of plastic, creating several layers of the wires. They then covered the material with a polymer and topped it with an electrode. The resulting device is about 1.5 by two centimeters and, when compressed 4 percent every second, it produces about two volts, enough to drive a liquid-crystal display taken from a calculator. "We were generating 50 millivolts in the past, so this is an enhancement of about 20 times," says Wang.

Solar Arrays Do Double-Duty

Solar Arrays Do Double-Duty

A pilot plant at a winery not only generates electricity, it heats the water.

A startup called Cogenra Solar recently installed a bank of solar arrays with a difference at a northern California winery. The arrays combine conventional photovoltaic solar cells with a system for collecting waste heat. This produces electricity for lighting and bottling equipment, and creates hot water that can be used for washing tanks and barrels.

Cogenra plans to install these "hybrid" solar arrays at businesses that use large quantities of electricity and water, and then charge them for supplying both. The company has not released an estimate for the cost-per-watt of its electricity, but it says that the cost of heated water will be considerably less than the norm.

At the winery, owned by Sonoma Wine Company, several parabolic dishes, each 10 meters long and three meters wide and lined with mirrors, concentrate sunlight onto two strips of monocrystalline silicon solar cells suspended above. The parabolic dishes sit on top of mechanical arms that move them to follow the sun. Heat is collected with a mixture of glycol and water that flows through an aluminum pipe behind the solar cells. The glycol solution is fed into a heat exchanger, where it heats up water. The water is then pumped to a storage tank, and the cooled glycol solution is fed back to the solar arrays.

Similar hybrid solar systems have failed in the past because the solar cells have overheated. Cogenra uses sensors to monitor the temperature of its solar cells and an automated control system to draw fluid away more quickly if they need cooling down.

Overheating impairs the performance of a solar cell and is a big problem for hybrid solar systems, says Tim Merrigan, a senior program manager at the National Renewable Energy Laboratory in Colorado. Merrigan notes that more sophisticated equipment for monitoring the buildup of heat and adjusting the flow of the liquid used away from cells can help prevent this, but adds that, "it is certainly not an easy thing to do correctly." With Cogenra's technology, there is also a trade-off between the amount of heat that can be produced and the efficiency of the solar cells—producing more hot water reduces the efficiency of the cells.

The winery installation will serve as an important test bed for Cogenra's technology and for hybrid solar technology in general. The system will generate data showing how efficiently it can produce electricity and heated water under different weather conditions and how well it can meet the fluctuating needs of the winery's operation.

The solar arrays will be able to produce 50 kilowatts of electricity, and the equivalent of 222 kilowatts of thermal energy. Gilad Almogy, the CEO of Cogenra says this will cut the winery's use of natural gas for water heating by 45 to 50 percent, and meet about 10 percent of its electricity needs.

Making the technology cost effective will be another challenge for Cogenra. But a growing number of government programs that dole out rebates for installing solar water heaters could help. One such program was launched in California in October. Lasting through 2017, it will provide $350.8 million in rebates for installing solar water heaters. Most water heaters in the state currently run on natural gas.

Vinod Khosla, whose venture capital company Khosla Ventures has invested $10.5 million in the project, says the technology is remarkably cost-effective. "Other solar companies used hundreds of millions of dollars to go to market," Khosla says.

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Solar Energy in Spain

Last December the company connected the Monte Alto Solar Field to the grid, the largest installation of its kind in Spain, and one of the largest in the world. It consists of a field of standard PV panels on trackers (which leads to 30 percent greater efficiency), spread out over a long disused agricultural field in the southern part of the state of Navarra, about an hour south of Pamplona.

This is the latest in these fields, known as ‘huertas,’ or gardens, in Spanish. The 9.5 MW facility at Milagro actually has more than 750 owners, investors, from across Spain, each of whom owns one or two of the panels and trackers and receives payments from the electric utility.

Most Spaniards live in apartment buildings and share rooftops, so the options for investing in solar power are limited. “This way they can have the same opportunities as the rest of the world even if they don’t have their own roof,” says Miguel Arrarás, general director of Acciona Solar. There are ten such fields in Spain, though Milagro is the largest so far, and three more about to enter construction phase.

The region of Navarra (where Milagro is located), with local government support, has become a veritable center of renewable energy, with wind turbines arching over the rolling hills and solar fields stretching across open spaces. The region has more than twenty times the watt peak of PV per inhabitant compared to that of Spain, and nearly double that of Germany, world solar leader. This commitment has led to 70% of Navarra’s electricity generated from wind and solar alone.

Because of this, Navarra – and Acciona’s solar fields – have become a perfect site to evaluate the entire system. “We’re testing 30 different kinds of panels,” says Arrarás. “We also have data on the effects of shadows, fog, everything. We have an agreement with two universities just to analyze this data.” He continues, “This is also the perfect place to evaluate what the effect is on the entire grid when, say, there are clouds, because of the high concentration of solar power here.”

The company’s operations are housed in a zero-emissions building on the outskirts of Pamplona. The building’s design incorporates techniques that reduce energy needs by 52 percent from a typical building, such as natural light and carefully placed shading. The remaining energy is produced with PV cells, solar water heating, and finally a small amount of biodiesel. The investments will pay off in ten years, according to Arrarás. Due to the company’s experience, Acciona Solar is also researching ways to improve and promote these high performance buildings.

Acciona is poised to begin construction on a PV solar field in Portugal that will comprise nearly 50 MW – five times the size of Milagro.

Looking ahead

The Spanish government continues to promote the investment and expansion of both photovoltaic and solar thermal power in the country, with a goal of 400 MW installed power for PV and 500 MW for solar by 2010. This is still only a fraction of the country’s total power use and total renewable production.

The government, however, is committed to advancing the sector. The new building code of 2006 requires increased energy efficiency and an obligation to meet a significant part of the hot water demand with passive solar heating, and the Plan of Renewable Energies sets lofty goals of 5 million square feet of solar collectors by 2010. They new Royal Decree approved in May 2007 improves the feed-in tariffs for both solar thermal and PV facilities. Some experts believe that these developments could lead Spain to become the second largest PV market in the world in 2007. Spanish companies and research institutions plan to continue to be at the forefront of the growing global field.

Says Javier Anta, president of the Spanish Photovoltaic Industry Association, “The solar industry will be a major part of the government’s goal of 20 percent renewable energy by 2020. Despite the fact that solar is only a small percentage of renewable power, it’s grown more than 100 percent a year in the past few years,” and in fact the sector grew 200 percent in 2006. Continues Anta, “We’re facing a grand challenge: consolidating that which we’ve achieved so far, setting the framework for future development, and creating a sector that makes our country proud.

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Solar Energy in Spain

Traditionally, Spanish companies have exported about 80 percent of the cells produced, but with renewed interest within Spain in PV, those numbers are changing. Within only the last two years, nearly 100 MW of PV power have been built in the country. Isofotón expects to sell about 60 percent of its panels within Spain, though the company still exports to Europe, North and South America, and Asia.

Isofotón’s research director says what distinguishes the company is the high quality of the cells. “You can find information in books about how to make solar cells,” says Jesús Alonso, director of Research and Development. “The main difficulty is the know-how – it’s how to make sure that those 400 wafers you put in the furnace are actually good, quality solar cells. That’s the key.”

As with all solar cell producers, Isofotón has been limited lately by the dearth of highly purified silicon necessary both for microelectronics and the solar industry. In response they have begun setting up silicon refining operations in Cadiz, which should begin production in 2008.

Working with Antonio Luque’s IES, Isofotón has focused research on developing concentrating PV solar cells. Downstairs in the factory, in a small room on the main factory floor, a machine whirrs as thin sheets of tiny dots of solar cells, only 1 mm large, pass through a machine. They will be attached to gold wires and then serve as the focus of the concentrating lenses.

Outside the building, a panel of concentrating PV cells is mounted on a tracker. Unlike standard PV, which can accept all ambient light, concentrated PV cells are most efficient when tracking the sun to appropriately focus the light through the lenses and onto the dots. As such, concentrating solar will likely be most effective on a large scale, like solar thermal, where fields of trackers can be set up to take advantage of the sun’s angled rays.

The material used in these concentrators is gallium arsenide, 50 times more expensive than silicon. But the cells are concentrated 1000 times, demanding 1000 times less material. 

When it comes to traditional PV panels, most companies focus on marketing to the developed world – where money is available for PV and the process is as simple as creating the product and selling it. But Isofotón has taken the lead in marketing solar power to the developing world, called “rural electrification.” This year they expect it to be nearly a quarter of their market. Even the marketing works differently for this segment of the business, as projects must be researched and appropriate financial models developed for each. Isofotón has rural electrification projects around South America, Morocco, Algeria, Indonesia, and South Africa.

Solar power in these poor, rural regions is not simply used for home electricity, but also for applications such as water pumps and desalination. To maintain a lead in this area, in addition to the decades of experience the company has already built up, Isofotón is focusing research on how best to couple solar power with those types of applications, as much of the existing equipment isn’t appropriately built to work with an intermittent energy source.

“If we look to the really long-term, I think that our main market will be rural electrification, because at the end these are the people who don’t have electricity. Most of the energy increase in the world will be in electricity, and most of that will be in developing countries,” says Alonso.

Acciona Solar, the solar energy arm of Acciona Energía, as with the other major companies involved in this field, has seen phenomenal growth rates. The company’s income exploded from about a half million Euros to more than 96 million Euros in only eight years.

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Solar Energy in Spain

Photovoltaic

The growth of solar in Spain is hardly limited to solar thermal. PV is still the primary source of solar power, and PV has been the central part of the solar power repertoire since the 1970s, when researcher Antonio Luque was sent to the US to share information about microelectronics. He became inspired by American work on PV and returned to Spain, founded the Institute for Solar Research (IES in Spanish) in 1975, and eventually spun off the company Isofotón in 1981. By 1982 the company was already marketing the first Spanish solar cells.

Luque’s first contribution to the solar field was the development of bifacial cells, which take advantage of sunlight from both sides. These cells provided Isofotón’s start, but higher development and maintenance costs prevented their early adoption, and Isofotón reverted to conventional solar cells.

Today, the 60 researchers at IES – one of the oldest solar centers in the world – continue to push ahead with advances in PV technology. The institutes research includes multi-junction cells that utilize a wider bandwidth of solar energy; intermediate band cells that can use photons whose energy is smaller than traditional bands; and concentrated power whereby the cell itself is tiny, and lenses multiply the sun’s energy to reach up to 1000 suns before directing it at the dot-sized material. The last is being developed in partnership with Isofotón.

To further develop this new technology, a new institute called the Institute for Photovoltaic Systems of Concentration is being built in Puertollano, south of Madrid. Companies from Spain, including IES partner Guascor Fotón, will have demonstration sites on the facility, along with companies from the US, Germany and others. The goal is to improve the technology’s efficiency and decrease its cost in an effort to speed commercialization.

Luque believes advances in PV technology will eventually lead to drastically cheaper solar cells, but acknowledges that technological breakthroughs need to occur before the cost drops precipitously. He believes these breakthroughs might be occurring already and that the technological advances in store for PV will allow it to easily overtake solar thermal, even on a power-plant scale.

In a huge, airy, light-filled building near Málaga on Spain’s southern coast, Luque’s spin-off company, Isofotón, hums with the excitement of the exploding PV scene. This factory was completed in 2006, and ground has already been broken next door for an expansion.

The company’s production and sales have shot up in the past few years, despite rough patches since its inception in 1981. Isofotón nearly went bankrupt twice in the company’s history, as the international solar power scene languished. But in the late 1990s, Germany decided to invest heavily in solar power. Isofotón was able to take advantage of this, supplying 15 percent of the German market. Isofotón grew to become the seventh largest producer of solar cells in the world – but the global market has grown so rapidly, and a handful of new companies have jumped in to fill the need, that Isofotón’s status has dropped slightly even as its business has dramatically expanded.

Spain has been one of the top world producers of solar cells for the past decade; the two main companies producing those cells are Isofotón and BP Solar, which has been present in Spain for more than 20 years and is now planning a major production expansion. In addition, the Spanish company Atersa builds solar panels and provides full solar power installations. At their new Valencia factory, the company has grown to 14 MW of annual capacity and will soon expand to 30 MW of capacity.

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Solar Energy in Spain

Research has focused on technologies to increase the efficiency and decrease the cost of these concentrating solar systems. They’ve refined reflectors and absorber pipes, and worked on improving the coupling between the solar and conventional systems. One technology, the use of molten salt for heat storage, was tested on-site before SENER went ahead with plans to install such a system in the new Andasol facility. Researchers also continue partnering with European companies to develop alternative and even more effective storage systems, which could greatly increase solar thermal’s viability in the marketplace.

The center is currently investigating replacing heating oil in absorber pipes with water, so the steam turbine could be linked to the solar field directly, bypassing a heat exchanger. “Conceptually this seems so simple,” says Zarza, “but that’s not actually the case. Water boils and then turns to steam, and during the transition phase there could be very high temperature differences between the top and bottom of the glass tube, which could cause it to break.” Heating oil, unlike water, remains in liquid form throughout the process.

Scientists have tinkered with tubes to develop one that can withstand these temperature changes, and soon a new 3 MW facility will be built at PSA to test these new tubes.

Fernández of Abengoa’s Solúcar, one of the companies participating in the research project, looks forward to replacing heating oil with water. “Oil is expensive, and in theory you can go to higher temperatures with water and pressurized steam because oil has a heat limit. It’s also more efficient if you can do away with the heat exchanger,” he says.

A significant challenge facing developers of solar concentration plants remains cost – in large part because these plants haven’t been built before. The technology depends on parabolic mirrors made to exacting specifications, and tubes for the oil that consist of a double glass tube with a vacuum between the two layers. There’s currently one mirror manufacturer in Europe and two manufacturers of the glass tubes – one in Israel and another in Germany. “So when there are more manufacturers producing those tubes, and when there’s a larger production in general, you’re going to get more competition and a scale advantage,” says Peter Duprey, director of Acciona North America.

He adds,” I think this is at a fairly early stage in its evolution, and with more money and more people focusing on this energy alternative, I think you’re going to drive costs down, just like what happened with wind. In the 1980s it was 30 cents per kilowatt hour, now it’s down to about 7 cents. I think you’ll see the same thing with concentrating solar.”

Both Abengoa and SENER are working with other Spanish companies to jumpstart the production of parabolic mirrors and glass tubes in Spain, to increase production, competition, and local access to the necessary parts. At least two local companies will soon begin producing mirrors within the year, and another few are investigating developing new absorber pipes.

“Electricity costs are going up – and solar thermal costs are going down. We think they will meet somewhere in the middle,” says Zarza. 

In the US

The first solar thermal power plants in the world, nine in total, were built in Kramer Junction, in dry, sunny southern California, in the 1980s and still harness 350 MW of solar heat. Since the last of those plants was built, however, the technology in the US – as in the rest of the world – halted, with research continuing at American research centers such as the National Renewable Energy Lab (NREL).

This summer, the first new plant, built by Spanish company Acciona with technology from the US’s Solargenix, came on-line outside of Las Vegas in the abundantly sunny Nevada desert.

Acciona acquired 55 percent of Solargenix early 2006 and then began plans to build Nevada Solar One, as the plant is known. The parabolic troughs supply 64 MW, enough to power about 14,000 homes annually. Acciona is also in the permitting stage for two 50 MW concentrating solar plants in Spain.

Duprey, director of Acciona North America, says, “In the southwest of the US we have plenty of land that effectively is unused, and is near grid connection points. That can be developed, and I think we can get gigawatts worth of concentrating solar power over the next ten years.”

Nevada has a renewable portfolio standard that requires its utilities to generate a percentage of their electricity from renewable sources. The wind is weak in southern Nevada, but the sun burns hot – plus the state provided an investment tax credit –– so Acciona took on the project.

This type of technology demands vast amount of land for the parabolic troughs, and the plant is most efficient close to the demand. Conditions in the western US, particularly the southwest, meet both those requirements. The Western Governors’ Association has stated its commitment to increasing the use of solar thermal power in the region.

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Solar Energy in Spain

One of SENER’s innovations in this field was the development of new simulation software, called SENSOL, that takes into account all the parameters necessary to build a solar plant, determining the production costs and the appropriate dimension for that plant. This technology has also been used outside the country; the Japanese Institute of Technology purchased SENER’s services to determine the best dimension of a solar plant they wanted to develop.

Andasol is SENER’s first solar thermal site, though they’ve already broken ground on another site nearby, and a third is in the planning process for a location in the northern part of country.

The company has run into hurdles in building this facility, the first major parabolic trough system in Spain. “There have been a lot of challenges,” says Nora Castañeda, engineer in charge of the site’s construction, laughing. “We can begin with the design itself. It was difficult to find the right manufacturers, because there are so few suppliers of the parts. We had to learn how to assemble a solar field like this in a short time. Once we solved one problem, another appeared.”

But as quickly as problems have appeared, she says, the staff worked hard to find solutions. They built an assembly plant on-site and worked with Spanish construction companies to create appropriate jigs with laser trackers for building the extremely precise structure for the parabolic mirrors and then transporting the system to the field without altering the precision. Castañeda says she expects that the lessons learned from Andasol 1 will help drive down the cost of constructing future systems. Other companies are part of this rising trend: the Spanish utility giant Iberdrola recently announced plans for 10 parabolic trough systems across the country.

Advancing the Field 

Eduardo Zarza is having a great day. In fact, he’s having a great year. With a barely suppressed grin, the director of concentrating solar research tells the story of how the Solar Research Center in Almería (PSA in Spanish) has gone from a research outpost, as he and the other researchers toiled away on solar thermal power for 25 years, to an international superstar – at least in certain circles – with nearly daily visits from companies and scientists from around the world.

Says Zarza, “Every week we have several companies coming to see the facilities to get information because they’re interested in investing in solar thermal plants. The situation has changed dramatically in only two years.”

The center, surrounded by dusty rose-colored mountains dotted with green, lies in a particularly dry area of the region, with only twenty percent of Andalusia’s average rainfall. Back in the 1970s, with the pressure of lack of access to oil at the heels of western countries, a consortium of nine countries - eight European and the US - signed an agreement to investigate two solar concentrating technologies: one of a parabolic trough, and the other with a central receiver (such as Solúcar’s tower receiver).

In 1985, the test results were in: both technologies were commercially feasible, but costs were too high.

Since then, the center has continued testing and refining the technology, working with universities and countries around the world. Though there are other research centers with departments dedicated to concentrated solar power,  PSA is the largest such research center in the world.

The center is one of two Spanish research centers that operate as part of what’s known as CIEMAT (the other, near Madrid, focuses on wind and biomass). Sixty percent of the budget comes from the government, while the other forty comes from grants and industry partnership. Lack of funds threatened the center’s operations several times, and it nearly closed.

A rapidly growing interest in renewables, government incentives to promote energy alternatives, and the rising cost of oil and gas placed PSA in the perfect position to take a leading role in the development of renewable energy technologies. After decades of working in the literal and figurative desert, Zarza finds himself at the center of a renaissance, as the technology is finally, once again, entering the marketplace – and the center’s activities appears secure and flourishing.

“We’re very happy with the situation now,” says Zarza. “In the past few people wanted to learn about our systems – now everybody wants to.

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Solar Energy in Spain

Technologies

The most common technology so far, and the one in use at Andasol 1, is based on a series of parabolic troughs, huge curved mirrors about 18 feet wide that collect the sun’s energy and focus it at a point in the middle of the trough. Glass tubes filled with oil stream through that focus point along a long loop of troughs. The mirrors slowly track the sun from east to west during daytime hours, and the oil reaches about 400 degrees Celsius (about 750 degrees F).

The heat transfer fluid then travels to the steam generator, where the heat from the oil is transferred to water, immediately turning the water into steam. That steam powers a turbine, the same technology used in conventional power plants

.

The tower technology works on the same principle as the troughs – the sun’s heat – but uses curved mirrors called heliostats, mounted on trackers that shift position with a slight mechanical groan every few seconds, that direct the sun’s light to a central receiver at the top of the tower. Testing towers exist in Spain, the US, and Israel, but the Solúcar PS10 site is the first commercial application of the technology.

At PS10, 624 heliostats, 120 square meters each (nearly 1300 square feet), concentrate solar radiation at the top of a 115 meter tower (about 377 feet). A receiver at the top transfers the heat directly to water, and the pressurized steam reaches 250 degrees Celsius.

The engineering aspects of building such a plant take into account both the need to heat up the receiver – and also to moderate the energy directed at that receiver. “At this plant, we’re working with the potential of about 3000 suns, but the absorption panels can only handle 600 suns. We have to control the aiming to protect the solar panels. So it has to be very well designed and operated to provide the best results,” says Valerio Fernández, head of engineering and commissioning for Solúcar.

Fernández says that so far the facility is operating as intended, but that improvements will be incorporated into future towers. “This isn’t the best temperature for the highest efficiency,” says Fernández “but we wanted to test the safety and security of the design for this first installation. We’ll do the remaining research necessary in order to use higher temperatures in future plants.” He explains that the cooling system for the boiler is more complicated as temperatures increase, but that once those changes are implemented, the tower’s efficiency could improve by 20 percent.

The tower is also supported by a small amount of natural gas, used when a stretch of rainy or overcast days prevent the plant’s full operation and the stored energy cannot stretch far enough through the end of the rainy phase. “It’s good to be able to maintain stability, not be stopping and starting up the turbines more than once a day, as they’re designed to do,” says Fernández.

When completed in 2012, the entire Solúcar facility called the Sanlúcar La Mayor Solar Platform will house more than 300 MW of solar power, utilizing tower and trough technologies along with PV installations. Abengoa, owner of Solúcar, has also recently signed plans to build combined cycle power plants in Algeria and Morocco using parabolic troughs in conjunction with natural gas power plants.

One of the main advantages of solar thermal power, in addition to the cost benefit, is the potential for power storage. The Solúcar tower uses a system of heat storage system based on pressurized water. SENER’s Andasol site will use a more advanced storage taking advantage of the specific properties of molten salts, tested in Spain but not yet implemented commercially.

Located about an hour outside Granda, home to the world-famous Alhambra, Andasol 1 will provide power well into the evening hours. SENER, constructing the plant with a company called COBRA, has built extra troughs that will direct their heated oil to 28,000 tons of molten salt (the salts are being imported from Chile). The salts must reach a high enough temperature to liquefy – and then they must be maintained in a liquid stage to prevent them from causing blockages. Tubes with heated oil will flow into the molten salts, raising the temperature even higher, and the salts retain the heat energy. As evening falls, the thermal energy will be transferred back to heating oil, which will continue on to the heat exchanger and power the steam turbine