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NREL and SkyFuel Collaborate on Concentrating Solar Technology

By Joseph B. Verrengia, NREL
January 26, 2009 | 28 Comments

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Colorado, United States [RenewableEnergyWorld.com] It looks like a giant funhouse mirror. But the big new dish atop South Table Mountain could be a renewable energy breakthrough that helps make concentrated solar power more affordable and appealing to utilities and their customers.

"There is a cascade of opportunities to lose some light at every step in the process. That's why we field test the whole unit and get solid data over a number of months. It removes the uncertainty in the final efficiency result."

-- Keith Gawlik, Senior Engineer, NREL

For the next several months, National Renewable Energy Lab (NREL) engineers will be testing the performance of SkyTrough, an innovative parabolic trough that is coated with a gleaming reflective skin instead of mirrored glass.

NREL offers testing and performance analysis for advanced solar technologies and other renewable energy designs.

The SkyTrough was developed by SkyFuel, an Albuquerque, NM-based manufacturer with a research facility near NREL in Arvada, CO. The technology was unveiled at Solar Power International in October 2008 and RenewableEnergyWorld.com reported on it then.

The unit's lightweight glass-free are mirrors made of sheet metal beneath ReflecTech mirror film.

This highly-reflective, silver-metalized film is lighter and less expensive than the breakable glass mirrors that are traditionally used. The film is a joint invention of NREL and ReflecTech and exclusively licensed from NREL. The glossy laminate is comprised of multiple layers of polymer films with an inner layer of pure silver to provide for a highly reflective surface that also protects the silver layer from oxidation

In commercial use, a SkyTrough could measure as large as 375 feet long and 20 feet high. The test model is smaller, but uses the same technologies.

"It's unlike any parabolic trough design used so far," said NREL senior engineer Keith Gawlik. "Our new facility is designed to test the optical efficiency of the unit, which they can't do on their own at SkyFuel."

(Left, see Kathleen Stynes peering at the continuous data provided by analytical instrumentation for field testing of the SkyTrough parabolic trough design.)

How Parabolic-Trough Systems Work

Parabolic-trough systems concentrate the sun's energy through long U-shaped mirrors. The mirrors are tilted toward the sun, focusing sunlight on a vacuum pipe that runs down the center of the trough.

The tube contains heat-transfer oil that absorbs the focused sunlight and reaches temperatures of 400°C. The hot oil then is used to boil water in a conventional steam generator to produce electricity.

As the sun moves across the horizon, the troughs follow its trajectory by rotating along their axes with the help of tracking motors. This keeps the collectors oriented towards the sun to maximize the system's performance throughout the day.

SkyTrough Testing at NREL

The SkyTrough itself is mounted on NREL's Large Payload Solar Tracker. It supports solar components that require 2-axis tracking. The tracker is capable of carrying a maximum vertical load of 9,000 pounds with a tracking accuracy of 1 milliradian. (See rear view of the SkyTrough being tested, below. The unit is attached to NREL's Large Payload Solar Tracker.)

The NREL tests will center on validating the SkyTrough's optical performance. A key step in concentrating solar power (CSP) is making sure the light collected in the parabolic trough is accurately converted and focused on the receiver tube so it can heat the transfer oil efficiently.

"Lots of things come into play when focusing light," Gawlik said. "We have to consider the reflectivity of the surface, the accuracy of the surface and then aiming all of the light into the narrow focal line of the receiver tube."

Typically, a parabolic trough operates at nearly 80 percent optical efficiency, and SkyFuel expects its design to function at least as well, while being less expensive to manufacture, transport and maintain.

The NREL test will span portions of at least three seasons to explore the unit's performance under a variety of weather conditions and sun angles.

"There is a cascade of opportunities to lose some light at every step in the process," Gawlik said.

"That's why we field test the whole unit and get solid data over a number of months. It removes the uncertainty in the final efficiency result."

Future Concentrating Solar Power Plants

The NREL tests will not include actual electricity generation because that step in the process uses conventional steam turbine technology. But it is that hybrid combination of the renewable and the conventional that makes CSP appealing to utilities as a source of cleaner bulk power during peak and intermediate load periods.

Parabolic trough power plants also require relatively large tracts of nearly level open land with strong solar characteristics, as well transmission lines and other infrastructure. These factors make Southwestern states the leading candidates for additional CSP installations; in California, the Solar Energy Generating Systems plants have been operating for two decades.

By 2015, the Western Governors' Association estimates that 4 gigawatts of new concentrating solar power plants could be built in the United States.

Joseph B. Verrengia writes for the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) in Golden, Colorado.

This article originally appeared as a National Renewable Energy Laboratory feature article and was reprinted with permission.

Solar Energy

28 Reader Comments

Comment
1 of 28

anco-blazev-168445

January 26, 2009

"The tests continue..." It appears that the US solar industry is still mostly in test mode, while EU and Asian companies are proactively installing huge solar power fields all over the world. One--the world' largest--300MW CSP plant by Obengoa Solar, Spain is scheduled to start operation in 2010. It uses these same mirror surfaces, which NREL has been testing and writing reports on for the last 30+ years.

Comment
2 of 28

carolyn-luce-54460

January 26, 2009

? You're saying Abengoa is not using glass for their CSP? I assumed that since their mirrors are manufactured at Abengoa Solar's Rioglass Solar factory, that they were made using glass, not this polymer film that NREL is testing.

And Abengoa isn't building a 300 MW CSP plant. They're building several 50 MW CSP plants that should add up to 300 MW. The US already has that, since the 1980s....

http://www.renewableenergyworld.com/rea/news/story?id=48660
"The world's largest parabolic trough facilities, located in the Mojave Desert near Barstow, California, consist of nine plants producing 354 megawatts (MW) of power at peak output."

Comment
3 of 28

anco-blazev-168445

January 26, 2009

Abengoa is using combination of reflective surfaces--including polymers. This is not new technology. We've used polymer surfaces since 1982.

The 300MW Abengoa CSP plant will be located in Gila Bend Arizona. And it is the largest CSP plant in the world thus far. See http://www.azstarnet.com/metro/226345

Comment
4 of 28

alliance-for-responsible-energy-policy-136271

January 26, 2009

More of the same wilderness killing, water wasting, oil circulating, power line necessitating, century old steam generator technology.

In the meantime Germany is on line to add another 1600 MW of building integrated/rooftop PV capacity this year alone (added 1300 MW in 2008) - and with barely half the solar resources we have here in the U.S. No millions of acres of destroyed public lands, no depletion of groundwater resources, no steam, and no need for additional transmission lines. This is not apples to oranges. This is apples to sour grapes.

Why don't we all wise up and leave the 19th century behind us once and for all. Point of use PV is the 21st century answer to decongesting current grid infrastructure (instead of expanding it), creating jobs and revenue for local economies, and adding truly clean renewable peaker power supplies to reduce demand when these reductions are most needed.

Big Energy's enormous lobby and desire to continue their stranglehold over consumers is getting old.

Oh, by the way, the same U.S. Department of Energy concurs with my PV assertions. Over 90% of U.S electricity demands can be met through PV installations on brownfields located within the load centers.

"PV is not "area-impaired" in delivering electricity."

http://www1.eere.energy.gov/solar/myths.html

Comment
5 of 28

kevin-emmerich-168532

January 27, 2009

The parabolic-trough systems are the most wasteful ideas for the southwestern US. How foolish to try to restart an economy in the drought stricken desert southwest with an energy system that requires 6 acre feet of water per megawatt just to keep it operating. There is simply not enough water to keep this over priced system sustainable. There are countless roof tops and many other structures in urban areas that would be great for photovoltaic systems that could provide the energy to the building and use no water at all. Setting up several of these wet cooled systems in the Mojave Desert will use up so much water, that no one will be able to live in the areas that the water will be taken from.

What a shame that these big energy companies are hiding behind the title, "green" when they are really water wasting, habitat destroying nightmares. Do not invest in this kind of solar development. It can not sustain itself. There are simply not enough water resources in the southwest to make this kind of investment worthwhile.

http://www.basinandrangewatch.org/SolarDesert.html

Comment
6 of 28

anco-blazev-168445

January 27, 2009

US DOE, with the help of its technical advisers and other US and world policy makers, made a decision some time ago that CSP (parabolic troughs fall in this category) is the best technology for large scale solar electricity production. They've spent millions if not billions on this technology with some progress to show, but quite limited for the amount of time and money spent, in my opinion.

And while the mighty US DOE is still in research and test mode with the CSP technology, see above article, a come-from-nowhere Spanish company, which no body even heard of until a year ago, is building the largest CSP (troughs) plant in the world right here in DOE' back yard. That says something...

Comment
7 of 28

carolyn-luce-54460

January 27, 2009

Two years ago
http://www.renewableenergyworld.com/rea/news/story?id=47054

Comment
8 of 28

john-sotack-156579

January 27, 2009

Comment
9 of 28

jb1010

January 28, 2009

The sun is already distributed, why convert the photons to electricity far away from the point of use? There are CSP/CPV systems that can be put on industrial/retail flat roofs.

The main advantages:

1) The left over heat energy can be used for cooling/heating purposes

2) No need to build grids, which is more than half the cost of energy.

3) The owner of the buildings/plots can make their own energy, rather than depending on varying power prices from the utilities.

4) No need to disturb wild life or pristine eco systems.

5) Easier access to water for cleaning the mirrors (use rainwater)

Rooftop CPV - what is not to like about that?

Comment
10 of 28

natalie-villella-18262

January 28, 2009

Government funds large corporations co-operating usually with large Universities, due in large part to the fact we (in the U.S. of A.) allow top down structure to exist. Largess breeds largess. Until you keep the money and control out of the hands of Government no real change will take place. When people wake-up to the fact that money spent on or by Government is money wasted they may finally open their eyes to the fact that real and sustainable change only happens when the free unresticted man on his own unrestricted land owns the benifits of his own industry and may co-operate w/ others IF he wishes. Energy, Currency, and Government must be kept "of the people, by the people, for the people" or you'll always have SSDD (Same S*^t Different Day). Put up a Wind generator on you house - Solar thermal on your roof - Post your land w/ a One Billion Dollar infringment damages NOTICE and- Sue the first trespassing/infringing B%^*&*d beauracrat that mutters a word. (See: Palozzio v. Rhode Island , Summa v. California, Title18USC241,242 and Title 42USC1983 et seq)
Or RECALL Fed/STATE/COUNTY/LOCAL Government and start over erecting new safegaurds for the Protection of each mans rights to Life Liberty and Property.
(Constitutional /common law - Free Speech rights hereby claimed and Bill of Rights -1st Amendment protections hereby invoked)

Comment
11 of 28

xiaohong-chen-153313

January 28, 2009

One question,
Condensing steam back to water requires energy. What is the overall energy efficency?

Comment
12 of 28

lmagic007-lmagic007-58442

January 28, 2009

We need all major types of solar for different reasons. There is no single solution. CSP is required for off peak storage. PV for daytime use. Solar hot water for domestic and industrial use. We need it all.

Comment
13 of 28

roger-lay-145666

January 28, 2009

I agree that we need all types of solar for a complete solution. PV is good for roof tops but has 2 down sides: it stops producing power with passing clouds, and the production of silicon has worrisome environmental issues.

CSP troughs have no environmental issues in production, and can store thermal energy so passing clouds have no impact and can even store enough energy to continue producing power for several hours after sunset. But CSP is not suitable for residential roof-top power generation.

The good news about CSP is the new MicroCSP that allows for the production of power for the "distribution grid" instead of the traditional "transmission grid". Instead of being installed in huge mega-plants in the middle of the desert where the power must be sent via transmission lines to the demand centers, these smaller power plants can be sprinkled around major urban centers and tied directly into the local distribution grid.

With regards to Abengoa building several 50MW plants around Spain, there is actually an American company (Sopogy) who is building some of these plants in Spain using their MicroCSP technology:

http://sopogy.com/blog/2008/12/

Comment
14 of 28

brian-jeffries-149117

January 28, 2009

Answer to Xiaohong Chen:

Converting steam back to water does not take energy, it releases energy. Imagine this, you are heating water to make coffee or tea. You can tell that energy is being used because the gas is being burned by the stove and the water is getting warmer. But once the water has reached 100 degrees C, the temperature stays the same, even though the gas is still being burned. Where does the energy go? It goes into converting the water from its liquid state into its gaseous state. Once all of the water has been converted, the steam can then be heated even further.

Then the steam, in its superheated state, travels to the turbine, and in moving the turbine creates electricity. But how does it give up the energy to move the turbine? By losing temperature, and by at least partially condensing. What returns to the boiler is now a mixture of hot water and steam. That hot water ad steam actually requires less energy to turn back into steam because it's already so hot. And the process repeats itself continuously.

Of course, some water will be lost to leakage, and some heat lost to imperfect insulation of the system. But it's in the utility company's best interest to not let hot water out of the system because they have a given amount of solar energy. So the amount of sellable electricity is dependent on not wasting the energy or the water.

That all said, distributed energy generation is better. I wonder how these systems would compare to PV's on large buildings in cities. The comparison would need to consider total energy generation as well as cost per watt.

Comment
15 of 28

chbrosz

January 28, 2009

The reason centrallized CSP is the answer - ENERGY STORAGE. plain and simple.

Otherwise, if this weren't the case, (and in a perfect world where PV wasn't more expensive than CSP), distributed roof-top PV would be far better as you obviate the need for new transmission in addition to the highly debated land-use footprint.

PV has no CHEAP, efficient, environmentally friendly way to store energy (and Lead-acid batteries that last 5 years are not environmentally friendly). And UNTIL plug-in hybrids are a thing of reality, PV does nothing to reduce peak load for utilities. Sure, some days, when the sun is shining brightly, they may not need to turn on that peaking plant. But other days, when the clouds come out to play and the output drops off, on come the fossil-fuel thermal plants. And why, i ask, with most utilities seeing the continued growth and need for NEW capacity (both baseload and peak), would you want to invest in a technology that you cannot count on? CSP is dependable. CSP is dispatchable and firm. CSP has cheap, effective energy storage, easilly implementable and well-proven. THIS IS WHY CSP WILL BE BETTER THAN PV. We are spoiled as people, being able to turn on a light bulb on command, every day, every year. And until cheap electricity energy storage is available to help out PV, CSP and geothermal and biomass and biogas will be the technologies that utilities will pursue procurement.

We need energy storage, and we need a smart grid to reduce the amount of expensive energy storage we need. DG PV is ideally the best solution, but not realistic if you look at the whole picture - at least for the time being.

Comment
16 of 28

kevin-emmerich-168532

January 28, 2009

Some of the comments above talk about "wasting water".

The wikipedia article on Thermal Power at "http://en.wikipedia.org/wiki/Thermal_power_station" contains the text:
"After it passes through the turbine, the steam is condensed in a condenser; this is known as a Rankine cycle."

This implies steam from turbine generators is condensed back into water. If the same water is used over and over again, then how is it wasting water?

Also, in one comment the phrase "habitat destroying nightmares" is used. How is a power plant a habitat destroying nightmare more than any other land use? It does not require coal, oil or other mined fuel, so it does not indirectly destroy the environment. It also does not emit carbon dioxide which may be warming the earth. I might describe a trough plant in the desert more as environment saving.

It appears that trough plants do not consume finite resources and that they use land that is amoung the least inhabited.
John"

The steam has to be released to turn the turbine and the surplus of lost water is used to cool the system in the cooling towers. Becuase of the need for immediate cooled water to cool the system, this water is lost. To dry cool would be better, but that produces much les energy. The main use is the cooling effect and it does escape in spite of how much water is attempted to be recycled. This is why Solar One near Boulder City, Nevada uses 400 acre feet a year. An acre foot is about 320,000 gallons. Multiply this by 400 and you have no water left in the arid lands. From an investor's perspective, this is money down the toilet because it has no sustainable future.

Green energy requires a lot of land to sustain itself. Power from rooftop photovoltaic can be stored in a battery.You can live off the grid with personal solar. But there are very few affordable roof top photovoltaic systems. Why? We have to buy it from the big "green" guys! There is absoultly NO reson to trash all the pristine land they want to.

Comment
17 of 28

chbrosz

January 28, 2009

One more quick comment:

CSP will never work in distributed (roof-top) situations FOR ELECTRICAL GENERATION for many reasons - too costly being at the top of the list. For respectable RAnkine cycle thermodynamic efficiencies, the larger the delta T of your steam your system can create, the higher efficiency (the more electricity per photon striking your mirrors) you can obtain. The heat sink will always be the ambient temperature, so we want the mineral oil, water, whatever to reach as high a temp as possible. large scale parabolic plants get temps around 700 deg F. Power-tower (aka central reciever) has proven temps in upwards of 1000 deg F. Coal can get water (well, steam) to 3000. HOWEVER, microCSP, as it's called - Sopogy being one of these companies - can only promise temps in the 400's. This leads to inefficiencies in the thermodynamic cycle, which means less electrical generation, which means...you guessed it - an unattractive business model. Hey - if you don't care about a ROI, then fine - you can buy a 400 deg F steam power block and go for it.

And forget about cheap energy storage... you couldn't even use molten salts (can't reach sufficient temps to melt it), so you'd have to use thermal mass (concrete, thermocline...) or steam storage, but steam storage is expensive as it requires many pressure vessels, so typically the storage is reduced and used for a buffer during short-term cloud transients in order to maintain operation (for power quality) and to reduce thermal cycling for your expensive power island componetry.

micro CSP does have it's place in a niche market for PROCESS STEAM generation, as many processes need steam conditions that a product like Sopogy's yields. However, microCSP is even overkill for DHW loads, as the temps are too high.

As "Solar 1" noted earlier...there is no ONE solution. CSP for baseload. PV for peak load. PHEV (or another effective - yet to be proven economical - energy storage) for peak shaving.

Comment
18 of 28

david-sofio-67759

January 28, 2009

Sopogy? I once interviewed with them...or was supposed to. The interviewer couldn't manage to make it into the office for the appointment, though I was asked to wait and did so for 45 minutes. That idle time let me get familiar with the office: a rabbit-warren, with multiple generations of rooms tacked on to a warehouse, all giving the feel more of a mechanic's shop waiting room, rather than the ballyhooed hotbed of local genius being actively applied. There was no technology in sight other than a PC and a telephone, but the walls were plastered with press-releases and such about the accomplishments of the "CEO." I'd been excited about the prospects with them, but left with the impression that it might all be ham-handed hype, and that what they were really doing was trying to sell some very basic efficiency devices to commercial institutions - things like CFLs and improved A/C thermostats, from what little the office manager was able to tell me. Worse, it turned out they'd advertised with an incorrect job description, and wouldn't even cop to that when they called later -- to tell me I shouldn't have applied (yeah, thanks for the feedback).

So with that bad taste left in my mouth, I have to wonder why this Honolulu-based company would have to go as far as Spain to install a micro CSP plant, when certainly their own backyard would do amply well. Yes, we have sun in Honolulu, too. If only we could run our industries on hype.

Comment
19 of 28

-24125

January 28, 2009

CSP that use steam water cycles and wet cooling use 6 acre feet of water/ MW year. The water evaporates in the cooling process, when steam is condensed--there is also make-up and blow down water. This requires Zero discharge systems. The Condensate, boiler feed water needs chemical treatment and make up water needs demineralization.The boiler needs qualified and certified boiler operators. Dry cooling can be applied ( does not totally elimate water use) This adds cost and impacts performance of the Rankine cycle.
CSP using Cascading Closed Loop Organic Rankine cycle-CCLC Patented by WOW Energies Inc, totally eliminates water use, can be operated remotely, un-attended, can be built as small as 5.0 MWe up to 25 MWe or multiples there of. Smaller systems can be loacted closer to load centers, eliminate transmission losses--why fuss with steam water cycles--this should keep Solar boosters happy and save water for the thirsty planet.

Comment
20 of 28

tennessee-cornstoves-168750

January 28, 2009

My company would like to purchase solar pv wholesale and install at retail.
What company in the US manufacturers and sells solar PV generating cells at wholesale?

No surplus. No rejects. No one-of-a-kinds. No e-bay. No TVA resale. No used. No one-batch. No hot air. No hot water. No resale. No manufacturing machinery. No gimmicks. No tricks.
We are looking for just one solar PV manufacturer that produces new, warrantied, solar PV at wholesale for retail installation inside the US. The 100 watt size would be just great.
Please post response where the retail demand for local, dedicated, individual solar generation is healthy www.groups.yahoo.com/group/cornstoves or www.groups.yahoo.com/group/cornplace

Comment
21 of 28

kevin-emmerich-168532

January 28, 2009

"CSP will never work in distributed (roof-top) situations FOR ELECTRICAL GENERATION for many reasons - too costly being at the top of the list. "

The reason that a personal roof top system is too expensive is because the fat cats in DC (yes, that includes Mr. Obama) are not putting any stimulus money in it. Instead, they are subsidizing big "green" industrial energy, which normally, in the case of CSP plants cost almost three times to build as conventional energy plants. To decentralize the grid, would not help the big lobby contributers that are now expecting a pay back from the new administration.

But it is just false and uneducated to suggest that a family that produces their energy on the roof is not sustainable. It is really the only good way to use renewables in an economically sustainable way and really the best way to save energy because it requires no fossil fuel backup like most big wind and solar farms.

whatever you opinion is on the environmental impacts of these projects, it is an obvious stock market money trap. If you care about your assets, DO NOT invest them in the green economy.

Comment
22 of 28

matt-karber-163571

January 29, 2009

For those who haven't heard, there will be CSP installed, not just tested, in the US. Our local electric utility, APS, will be working with Abengoa Solar to build a CSP plant with thermal storage (molten salt) just west of Phoenix, Arizona. The project is called Solana Generating station.

Comment
23 of 28

kevin-emmerich-168532

January 30, 2009

"Furthermore,
I do not believe that CSP would require so much water that it would become a problem. If it did, that water would be recycled. In the "as much as all the roads" scenario, that water would easily be piped in from the oceans. The result, distilled water for CSP use and possibly left over for some very important stuff."

You obviously have no clue as to the dire sitruation of the over allocated water resources in the Mojave Desert or how significant 400 acre feet is to the area. .What is funny about your flawed point is that you say roof top photovoltaic is too expensive, yet you have no clue how many billions it would cost to pipe sea water to the eastern Mojave Desert! Not to mention the osmosis process needed to desalinize it. Hey, what's another billion? Again proving the point that it is a waste of money to invest in CSP technology. It just can not pay for itself!

Comment
24 of 28

evert-ervasti-155671

January 30, 2009

Here seems to be many experts of concentrating solar power (solar thermal).
What do you think of this, would it work:

http://openfarmtech.org/solar.html

"Herein we propose a method for manufacturing solar thermal concentrator (STC) electric systems – the Solar Turbine - at $1/watt – for a system with a 50 year lifetime. We invoke a novel Community Supported Manufacturing (CSM) model."

Comment
25 of 28

tennessee-cornstoves-168750

January 31, 2009

Is there a solar PV factory anywhere in the US that sells wholesale to small business?

Comment
26 of 28

roberto-petracca-147314

January 31, 2009

Dear Xiaohong Chen,
Condensing steam back to water do not requires any energy.
The problem is the opposite one: how to avoid to loose the heat when condensing steam back to water.

Comment
27 of 28

-15350

February 7, 2009

Abenoga has the Barcelona area of Spain on the fast track to electric generations from the sun only; agree that DOE is the old DOD in disguise with it's research perpetually repurchasing the $500 toilet seat concepts....

Comment
28 of 28

jimmiller5417

May 19, 2009

CHP closed loop units use refrigerants which must be cooled from the gas state to the liquid state. The cooling takes energy. The partial solution would be to run the system night and use a forced air evaporative cooler, powered by the electricity generated by the system. At night the lower air temperature will "help" in cooling the gas to liquid. A water-based cooling tower could be used and is commonly used in the desert areas for cooling air conditioning units. Brackish water could be used for cooling but would require more maintenance.

Heat can be parsed to the CHP unit from a thermal energy storage unit during the night. During the day, a CSP unit would replenish the heat in the TES.

So, why has this solution sitting on the inventor's shelves?

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