how to use solar energy at night_ scientific american_2008

8/7/2019 How to Use Solar Energy at Night_ Scientific American_2008

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to Use Solar Energy at Night: Scientific American

/www.scientificamerican.com/article.cfm?id=how-to-use-solar-energy-at-night[02/03/2011 07:47:15]

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.

Imag e: COURTESY OF SOLAR MILLENNIUM

The Fu tu re o f A l te rna t ive Energy

Geothermal , so lar thermal , and even nuc lear pow er cou ld prov ide

a l ternat ives to today 's carbon-based fue l sources

bruary 18, 2009

ar Granada, Spain, more than 28,000tric tons of salt is now coursing through

pes at theAndasol 1power plant. That

t will be used to solve a pressing if

vious problem for solar power: What do

u do when the sun is not shining and at

ght?

e answer: store sunlight as heat energy

such a rainy day.

rt of a so-called parabolic trough solar-

ermal power plant, the salts will soon help

e facility light up the nightliterally.

cause most salts only melt at high

mperatures (table salt, for example, melts

around 1472 degrees Fahrenheit, or 800

grees Celsius) and do not turn to vapor

til they get considerably hotterthey can

used to store a lot of the sun's energy as

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at. Simply use the sunlight to heat up the

ts and put those molten salts in

oximity towater via a heat exchanger.

t steam can then be made to turn

bines without losing too much of the

ginal absorbed solar energy.

e saltsa mixture of sodium and

tassium nitrate, otherwise used as

tilizersallow enough of the sun's heat to

stored that the power plant can pump

t electricity for nearly eight hours after

e sun starts to set. "It's enough for 7.5 hours to produce energy with full capacity

50 megawatts," says Sven Moormann, a spokesman for Solar Millennium, AG, the

rman solar company that developed the Andasol plant. "The hours of production

e nearly double [those of a solar-thermal] power plant without storage and we

ve the possibility to plan our electricity production."

ing mirrors to concentrate the sun's energyis an old trickthe ancient

inese and Greeks both used it to start firesand modern power plants employing

might provide a significant source of renewable energy without anygreenhouse

s emissions.

at is a step forward in its own right, but such power plants are limited to

nerating energy only when there is sunshine. So engineers have tried a number of

ferent technologies to store the sun's energy so that such power plants can be

ore broadly employed. They have triedbatteries but too much of the energy that

es in is not returned, and they tend to be too expensive, according to an analysism the National Renewable Energy Laboratory(NREL) in Golden, Colo.

mpressing air or pumping water uphill are more promising, but the opportunities

do that are limited by the number of caverns and the availability of water and

ervoirs.

elting salts at temperatures above 435 degrees Fahrenheit (224 degrees Celsius),

wever, can deliver back as much as 93 percent of the energy, plus the salts are

iquitous because of their application as fertilizers.

here's a term called round-trip efficiency. Basically, it's a measure of how muchctricity is produced if the thermal energy that's generated is first stored and then

ed compared to just directly taking the energy. That number is around 93

rcent," explains NREL senior engineer Greg Glatzmaier. "[For] things like

mpressed air and mechanical type storage, there's more significant losses," an

erage of at least 20 percent over all the various technologies.

e Andasol 1 power plant, which cost around $380 million (300 million euros) to

ild, is the first to actually use the technology, so it remains to be seen how it will

rk in commercial practice. But U.S. government laboratoriesNREL as well as

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University of North Carolina at Chapel Hill, School o

Medicine

105 Mason Farm Rd, Chapel Hill, NC 27599, USA

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ndia National Laboratoryin Albuquerque, N.M.have already proved the

hnology can work in demonstration projects that employed it, like the Solar Two

wer tower outside Barstow, Calif.

lar Millennium is so confident the technology will work that a twin solar-thermal

wer plant (Andasol 2) is already near completion. "It will start operations at the

ginning of summerMay or June," Moormann says.

dArizona Public Service Co. (APS) has contracted with Abengoa Solar to build a0-megawatt solar thermal power plantdubbed Solanaor "sunny place"70

les (110 kilometers) southwest of Phoenix on nearly 2,000 acres (800 hectares) of

nd. "One of the great things about molten salt technology is that you can get more

t of the pure solar resources, more energy out of the same facility," says Barbara

ckwood, manager for renewable energy at APS. "It's an alternative that provides

with additional green energy," as much as 1,680 megawatt-hours when cloudy or

er sunset.

t that extra energy comes at a cost. First, the power plant has to be enlarged so

at it is both generating its full electrical capacity as well as heating up the salts. In

e case of Andasol 1 that meant covering 126 acres (50 hectares) with long rows of

ughs and pipe. And then there is the additional expense of the molten salt storage

nks, according to Moormann.

told, that means thermal energy storage at Andasol 1 or power plants like it costs

ughly $50 per kilowatt-hour to install, according to NREL's Glatzmaier. But it

esn't add much to the cost of the resulting electricity because it allows the turbines

be generating for longer periods and those costs can be spread out over more

urs of electricity production. Electricity from a solar-thermal power plantcosts

ughly 13 cents a kilowatt-hour, according to Glatzmaier, both with and without

olten salt storage systems.

at price is still nearly twice as much as electricity from a coal-fired power plant

e current cheapest generation option if environmental costs are not taken into

count. But Arizona's APS and others can then use solar energy to meet the

aximum electricity demand later in the day. "Our peak demand [for electricity] is

er in the evening, once solar production is trailing off," Lockwood says. That's

e reason we went that direction and are so interested in storage technology."

efficient a s solar-therm al power plants using parabolic troughs with

olten salt storage systems like Andasol 1 or Solana are, they dont capture as muchthe sun's heat as is possible. Above 750 degrees F (400 degrees C), the synthetic

s used to capture the suns heat in the troughs begin to break down, but the

olten salts can take in much more heat than that.

allow the salts to get hotter, some companies, such as SolarReserve in Santa

onica, Calif., are developing so-called power towersvast fields of mirrors that

ncentrate sunlight onto a central tower. Because of the centralized design such a

ucture can operate at much higher temperaturesup to 1,000 degrees F (535

grees C)and use molten salts directly as the fluid transferring heat in the power
http://energylan.sandia.gov/sunlab/http://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.aps.com/http://www.aps.com/solanahttp://www.aps.com/solanahttp://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.sciam.com/blog/60-second-science/post.cfm?id=coal-war-can-the-fossil-fuel-be-cle-2008-12-23http://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.solar-reserve.com/http://www.sciam.com/article.cfm?id=is-the-sun-setting-on-solar-power-in-spainhttp://oascentral.scientificamerican.com/RealMedia/ads/click_lx.ads/sciam.com/energy-and-sustainability/L9/33543817/x41/sciam.com/i_2010-09_IDR-Email/i_2010-09_IDR-Email_120x90.html/76524a2b486b317546734d41426b7131http://oascentral.scientificamerican.com/RealMedia/ads/click_lx.ads/sciam.com/energy-and-sustainability/L9/1007808525/x40/sciam.com/i_2010-09_twitter/i_2010-09_twitter_120x90.html/76524a2b486b317546734d41426b7131http://www.sciam.com/article.cfm?id=is-the-sun-setting-on-solar-power-in-spainhttp://www.solar-reserve.com/http://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.sciam.com/blog/60-second-science/post.cfm?id=coal-war-can-the-fossil-fuel-be-cle-2008-12-23http://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.aps.com/solanahttp://www.aps.com/http://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://www.sciam.com/article.cfm?id=sunny-outlook-sunshine-provide-electricityhttp://energylan.sandia.gov/sunlab/


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2 Share 26 2

ant. "We are heating the salts to more than 1,000 degrees F and that results in the

me inlet conditions that utilities see today on a coal-fired or nuclear power plant,"

ys Terry Murphy, SolarReserve's president.

t such a power plantand Murphy says the company has some 50 such projects

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

olten salt power tower built is going to be a real trial," says Thomas Mancini,

anager of Sandia's Concentrating Solar Power Program. "It's going to takemeone progressive enough to finance it or take a little more risk."

researchers are also looking into salts that could be used instead of the oil in

rabolic trough power plants, such as those that melt at lower temperatures and

erefore would not freeze as readily during cold nights, according to Hank Price, a

e president for technology development atAbengoa Solar.

lar Millennium is working on such a salt, according to Moormann, and Sandia has

veloped small quantities of a new mixture of salts, including calcium nitrate and

hium nitrate, that melt below 212 degrees F (100 degrees C). "With the lithium

rate, it's as expensive as all the other constituents combined. Though still a lot

eaper than organic heat-transfer oils," says chemical engineer Bob Bradshaw at

ndia in California, who is leading the research. "You don't get something for

thing."

d long-term research projects are looking at other thermal storage technologies,

ch as storing heat in sand or creating single-tank molten salt storage. "The main

al is to find a storage technology that may reduce the actual capital cost" of adding

o a power plant, says Phil Smithers, technical services leader for renewable

ergy at APS, which is researching those technologies under a U.S. Department of

ergy grant.

timately, it will come down to how much value policymakers and consumers put

electricity that is renewable and emissions-free. "If we start valuing carbon and

ce a coal plant to go carbon-free via sequestration then we're at or over 10 cents

r kilowatt-hour from coal," Mancini says. "Any of these technologies can get to

at same 10 cents level with [molten salt] storage. Then the market will make the

l."

d should Andasol 1 spring a leak or otherwise fail to deliver as expected, the

mage would not be confined to a pile of salt fertilizer on the groundit could be aback for the entire effort to store solar energy. "We had to build the first

mmercial] plant [with molten salt storage] and that's what Andasol is," Mancini

ys, in order to prove the technology. "It doesn't have to be perfect, but they've got

make it work."

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5/8



he Sun Setting

Solar Power in

in?

Solar Power

Lightens Up with

Thin-Film

Technology

Sunny Outlook:

Can Sunshine

Provide All U.S.

Electricity?

Solar utility:

electricity from

sunshine on a

massive scale in

California

Next Generation

Nuclear Power

The Gre

How Ca

Adapt t

Change

t i c les You Migh t A lso L ike

15 Comment s Add Comment

ow All | Jump To: 1-10 | 11-20 | Next View Oldest to Newest

. JamesDavis

1:22 PM 2/18/09

Has anyone sent this article to President Obama?

Reply | Report Abuse | Link to this

. David M. Clemen

2:15 PM 2/18/09

I'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.

1. 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 thatthis 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

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6/8



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.


. Karen Murphy

4:09 PM 2/18/09

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!


. v ibora

4:15 PM 2/18/09

This technology is no new russian use to made almost 30 years

ago.


. M ims

6:26 PM 2/18/09

Of 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?'

see here:

http://climateprogress.org/2009/02/18/obama-replace-dirty-coal-nuclear-efficency-cogeneration-wind-solar-csp-

biomass-cofiring/


. eco -s teve

3:57 PM 2/25/09

To avoid the considerable transmission and other losses,

generate electricity only there where it is needed?


. David M. Clemen5:35 PM 2/25/09

eco steve

I 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


7/8



s by Google

newable Energy Info

d Out About Eco Energy Sources &w To Be Green. Get A Free Report!w.The-Green-Guide.info/

terprise Green IT

Add a Commentou must log in or registeras a ScientificAmerican.com member to submit a comment.

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.


. J a c k i e

1:29 AM 4/13/09

The most important is how to use solar energy better.People

need cheaper and clean energy.Who stop it to develop fast.

We 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.


. J a c k i e

1:36 AM 4/13/09

I'm from a solar energy factory in China.If you want know

more about solar energy you can E-mail :

[email protected] I hope get some imformaition about

the market you are living there.

Hope the enenrgy can be cheaper.


0. th ink f i rs t

0:18 PM 4/23/09

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).


mp To: 1-10 | 11-20 |
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