New solar energy conversion process could revamp solar power production

Aug 02, 2010
A small PETE device made with cesium-coated gallium nitride glows while being tested inside an ultra-high vacuum chamber. The tests proved that the process simultaneously converted light and heat energy into electrical current. Credit: Photo courtesy of Nick Melosh, Stanford University

Stanford engineers have figured out how to simultaneously use the light and heat of the sun to generate electricity in a way that could make solar power production more than twice as efficient as existing methods and potentially cheap enough to compete with oil.

Unlike photovoltaic technology currently used in - which becomes less efficient as the temperature rises - the new process excels at higher temperatures.

Called 'photon enhanced thermionic emission,' or PETE, the process promises to surpass the efficiency of existing photovoltaic and thermal conversion technologies.

"This is really a conceptual breakthrough, a new process, not just a new material or a slightly different tweak," said Nick Melosh, an assistant professor of materials science and engineering, who led the research group. "It is actually something fundamentally different about how you can harvest energy."

And the materials needed to build a device to make the process work are cheap and easily available, meaning the power that comes from it will be affordable.

Melosh is an assistant professor of materials science and engineering, and is senior author of a paper describing the tests the researchers conducted. It was published online August 1, in .

"Just demonstrating that the process worked was a big deal," Melosh said. "And we showed this physical mechanism does exist, it works as advertised."

Most , such as those used in rooftop solar panels, use the silicon to convert the energy from of light to electricity. But the cells can only use a portion of the light spectrum, with the rest just generating heat.

This heat from unused sunlight and inefficiencies in the cells themselves account for a loss of more than 50 percent of the initial solar energy reaching the cell.

If this wasted heat energy could somehow be harvested, solar cells could be much more efficient. The problem has been that high temperatures are necessary to power heat-based conversion systems, yet solar cell efficiency rapidly decreases at higher temperatures.

Until now, no one had come up with a way to wed thermal and solar cell conversion technologies.

Melosh's group figured out that by coating a piece of semiconducting material with a thin layer of the metal cesium, it made the material able to use both light and heat to generate electricity.

"What we've demonstrated is a new physical process that is not based on standard photovoltaic mechanisms, but can give you a photovoltaic-like response at very high temperatures," Melosh said. "In fact, it works better at higher temperatures. The higher the better."

While most silicon have been rendered inert by the time the temperature reaches 100 degrees Celsius, the PETE device doesn't hit peak efficiency until it is well over 200 degrees C.

Because PETE performs best at temperatures well in excess of what a rooftop solar panel would reach, the devices will work best in solar concentrators such as parabolic dishes, which can get as hot as 800 degrees C. Dishes are used in large solar farms similar to those proposed for the Mojave Desert in southern California and usually include a thermal conversion mechanism as part of their design, which offers another opportunity for PETE to help generate electricity, as well as minimizing costs by meshing with existing technology.

"The light would come in and hit our PETE device first, where we would take advantage of both the incident light and the heat that it produces, and then we would dump the waste heat to their existing thermal conversion systems," Melosh said. "So the PETE process has two really big benefits in energy production over normal technology."

Photovoltaic systems never get hot enough for their waste heat to be useful in thermal energy conversion, but the high temperatures at which PETE performs are perfect for generating usable high temperature waste heat. Melosh calculates the PETE process can get to 50 percent efficiency or more under solar concentration, but if combined with a thermal conversion cycle, could reach 55 or even 60 percent - almost triple the efficiency of existing systems.

The team would like to design the devices so they could be easily bolted on to existing systems, making conversion relatively inexpensive.

The researchers used a gallium nitride semiconductor in the 'proof of concept' tests. The efficiency they achieved in their testing was well below what they have calculated PETE's potential efficiency to be, which they had anticipated. But they used gallium nitride because it was the only material that had shown indications of being able to withstand the high temperature range they were interested in and still have the PETE process occur.

With the right material - most likely a semiconductor such as gallium arsenide, which is used in a host of common household electronics - the actual efficiency of the process could reach up to the 50 or 60 percent the researchers have calculated. They are already exploring other materials that might work.

Another advantage of the PETE system is that by using it in solar concentrators, the amount of semiconductor material needed for a device is quite small.

"For each device, we are figuring something like a six-inch wafer of actual material is all that is needed," Melosh said. "So the material cost in this is not really an issue for us, unlike the way it is for large solar panels of silicon."

The cost of materials has been one of the limiting factors in the development of the solar power industry, so reducing the amount of investment capital needed to build a solar farm is a big advance.

"The PETE process could really give the feasibility of solar power a big boost," Melosh said. "Even if we don't achieve perfect efficiency, let's say we give a 10 percent boost to the efficiency of solar conversion, going from 20 percent efficiency to 30 percent, that is still a 50 percent increase overall."

And that is still a big enough increase that it could make solar energy competitive with oil.

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kshultz222_yahoo_com
2.2 / 5 (5) Aug 02, 2010
Is this really as revolutionary as it sounds? How long till this can actually come to market?

It sounds like it could be added to existing large solar farms in a short time. That would be awesome!
fmfbrestel
3.3 / 5 (4) Aug 02, 2010
When they mentioned bolting them to existing thermal conversion systems, i immediately thought of the sterling engine solar systems. But im not sure just how hot they operate and how much heat these would steal from the sterling engine.

You can talk about fusion power all you want, but all we really need are a few decent advances like this one to pan out and we have all the fusion we need 1a.u. away. When (if) solar power becomes cheaper then coal power, we will see major changes in our economy.
fmfbrestel
3.6 / 5 (5) Aug 02, 2010
@kshultz: It reads like it is revolutionary in the sense that it isn't an incremental update (not that those are bad, we need them too) to an existing tech, but a new process to convert energy.

Is it revolutionary in the sense that this tech will single handedly solve our energy problems? Probably not, but it's still nice to see some progress toward that end.
tpb
2.2 / 5 (5) Aug 02, 2010
Gallium is a rare and expensive material.
kaasinees
2 / 5 (4) Aug 02, 2010
How about graphene coated with rubidium??
Parsec
5 / 5 (1) Aug 02, 2010
Gallium is a rare and expensive material.

Really? Last time I checked (about 2 minutes ago) current price bids are $550 US per kg of Gallium. Thats not expensive at all. Current price of Cesium is about $30/gram so thats a lot more expensive, but it really depends on how much is used per solar cell.

Remember... Google is your friend. Just type in 'Gallium prices' and your connected to the gallium commodities market trader slick as you please.
tpb
2.3 / 5 (3) Aug 02, 2010
Parsec, maybe you should Google some more.
Gallium is rare.
Total worldwide Gallium production, 100 tons.
Total worldwide Silicon production for solar cells 1.5 million tons.
Most Gallium is recovered from bauxite ore as a byproduct of aluminum production.
tpb
2.3 / 5 (3) Aug 02, 2010
Less than 50ppm of bauxite is Gallium.
All known reserves of Gallium are less than 1/3 of the amount of silicon used for solar cells in one year.
Gallium is relatively inexpensive because there is almost no demand.
If we actually started making solar cells with Gallium, it would probably become one of the most the most expensive metal produced, at perhaps 50 to 100 times the current price.
MorituriMax
1 / 5 (1) Aug 02, 2010
Just out of curiosity, tpb, when did anyone in the article mention gallium for the final manufacturing material?

"And the materials needed to build a device to make the process work are cheap and easily available, meaning the power that comes from it will be affordable."

I guess the quote above means the people in the article must be thinking of some OTHER materials than gallium to use for the collection hardware.
RealScience
not rated yet Aug 02, 2010
The amount of gallium needed would be much smaller than the amount of silicon currently needed.
First, the Gallium Arsenide the article references is a direct band gap semi copnductor, so only a very thin layer is required (rougnly 50x thinned than typical silicon solar cells).
Second, since the device loves heat, it would be used under high concentration (GaAs-based cells are currently typically used under 500x concentration).
Third, at 3x the efficiency, only 1/3 the material is needed, and gallium arsenide is is only half gallium.

Thus IF this process pans out (and that is a big if), 100 tons of gallium would be equivalent to 15 million tons of silicon.

Gallium averages almost 20 ppm in the earth's crust, so silicon is only 15,000 times more plentiful, not 150,000. (Arsenic is 10x rarer than gallium).
Graeme
not rated yet Aug 03, 2010
Well if you could use aluminium nitride, that would be much lower cost, but does this work as a suitable semiconductor?
cld
5 / 5 (1) Aug 03, 2010
there was recent article on STEP ("The new process, called Solar Thermal Electrochemical Photo (STEP) carbon capture, was recently suggested theoretically by a team of scientists from George Washington University and Howard University, both in Washington, DC.") how does this relate?
StandingBear
1 / 5 (2) Aug 03, 2010
the revolutionary part of solar lies in the fact that once built, solar seems virtually maintenance free compared to the competition. They just....run...and run...and run. Built a lot of them. No matter what the cost per kilowatt...if it runs long enough, it will pay the cost down to make its energy eventually to seem a LOT cheaper than oil. Now this one seemingly will do for large scale systems a huge benefit. Use it!
Doug_Huffman
1 / 5 (4) Aug 03, 2010
Solar Constant is still 1350 Watts per square meter. Energy renewal at a greater rate indebts the future, just as fossil fuel, a bio-renewable at a geologic time rate, has done.

Nuclear power is secure power.
DozerIAm
not rated yet Aug 03, 2010
I read this as more evolutionary than revolutionary - they are adding a process to an existing technology.

That said, I hope this proof of concept translates readily to production levels. Solar power is currently mostly a boutique source of energy, I'd be very happy if it were cost competitive with coal or oil products.
fmfbrestel
5 / 5 (5) Aug 03, 2010
@Doug 1350 watts/m^2 sure, but when you can concentrate 30m^2 that is 40,000 Watts. Now you may only get 35% efficiency, so thats 14000 watts per 6 inch wafer (plus a few mirrors).

Also, its not possible to "indebt the future" with solar. You CANT take more then 1350 W/m^2 and no one is trying. Nuclear power will be needed for a long time anyway to even out peaks and valleys in solar collection. At least until we perfect grid scale batteries (and there is good work being done on this problem too).
Nuclear isn't secure, it is a giant bulls eye mocking terrorists and other enemies of the state into attempting an attack. Someone blows up a few solar chips and mirrors -- who cares? worst case senario, temporary black outs till power is rerouted. Worst case scenario for nuclear is Chernobyl. Or, god forbid, they steal some of the waste and disperse it on LA or New York. You cant poison an entire city with a solar chip.
fmfbrestel
3.7 / 5 (3) Aug 03, 2010
Can solar deliver right now -- No. Is it easily the most promising source of power we have -- Yes.

We dont need to burn anything to get it. We dont need giant reactors filled with insanely dangerous (and expensive) components. We dont need to dam any rivers, and we dont need to provide constant upkeep to wind turbines (which are constantly falling apart because of the huge mechanical stresses involved).

We only need to harvest .02% of available solar resources to entirely power the entire world. Now forcing solar down peoples throats is just stupid, but once grid parity or better occurs (will happen long before they see first light at ITER) The economic forces will stop the building or rebuilding of coal and nuclear plants. It will just happen, it wont need to be legislated. If it becomes cheaper, it will be used.
danman5000
4 / 5 (1) Aug 03, 2010
Nuclear isn't secure, it is a giant bulls eye mocking terrorists and other enemies of the state into attempting an attack. Someone blows up a few solar chips and mirrors -- who cares? worst case senario, temporary black outs till power is rerouted. Worst case scenario for nuclear is Chernobyl. Or, god forbid, they steal some of the waste and disperse it on LA or New York. You cant poison an entire city with a solar chip.

That's assuming we don't figure out fusion power in the near future, which I think we will. Fusion produces almost no radioactive waste, and fusion plants can't melt down and so are inherently very safe.
A_Paradox
not rated yet Aug 03, 2010
fmfbrestel, I agree with much you are saying although I think the use of smaller scale nuclear [eg Adams Atomic engine type thing] will come to be very useful in 'topical' applications such as where sea water desalination while other systems are under construction.

Meanwhile, I see Stirling engines as essential in facilitating solar heat concentration for cooking, home heating, drying the laundry, and providing 12v/24v [whatever] power for electronics around the house/farm/caravan/boat.
fmfbrestel
not rated yet Aug 03, 2010
near future? we are at least 20 years from the first experimental fusion reactor that will create more power then it consumes. And then another 20 years or so before one creates enough excess power to provide grid competitive prices.

Current expectations for solar is that solar will achieve grid parity for 60% of the US (varies based on region) by 2015. lets see 50 years out VS 5 years out. 6 inch wafer and a few mirrors VS 20 Billion Euros (ITER), another 20 Billion (DEMO), and then 5-10 Billion per plant thereafter.
fmfbrestel
5 / 5 (1) Aug 03, 2010
Nuclear will always have a place. Subs, severe northern latitudes, processing decommissioned nuclear warheads, ect. And really, im not trying to be a cheer leader for solar, im just saying cut the subsidies (including those for oil and coal), and let the cheapest solution win.
A_Paradox
2 / 5 (1) Aug 03, 2010
As I see it, I have to construct a solar heat concentrator to heat air in a pipe to 250deg C. Then use Stirling engine to pump/blow the hot air along copper pipes [underground will be safest] to a heat store [bricks]. I want a 100deg+ store to keep and urn of hot water for tea & coffee, a 230deg ['Hot oven' temp] store for cooking [above 230degC/451Far baking paper etc will burn] and a 45deg room for drying clothes in winter.

I am yet to read of anyone taller than me pointing out a truly obvious bit of basic physics which is that you can't store light, storing electricity is REALLY expensive, but storing heat is the easiest thing in the world!

What every house needs on its roof [or discretely away in the back yard for the snobs] is a set of parabolic trough shaped solar collectors. Focused on static collector pipe, trough mirror rotates around the pipe following the sun. Stirling engine uses same heat source, pumps air, and so forth. 19C technology ... cheap, anybody can build it!
A_Paradox
4 / 5 (3) Aug 03, 2010
Fusion power? That's "about twenty years away" just as it was when I first read about it in the '60s. :-)

Thermonuclear from Sol on the other hand; that's been powering life on Earth for a billion years or more.
MaxwellsDemon
5 / 5 (1) Aug 03, 2010
Nice. Every day I look at all these hot black rooftops around the city, just beckoning to be used for some kind of energy harvesting system like this. Combined with ever more efficient technologies like LED light bulbs, we’re inching closer to independence from the grid, which would be good for both the economy and national security.

But I’m surprised there isn’t more work being done on tapping the most powerful nuclear energy source we’ve got: the Earth. We’re sitting on top of a radioactivity-driven thermal energy magma battery that could power our civilization for billions of years, and the only thing standing I the way is some drilling, plumbing, and geothermal power stations (safe, simple steam engines).
elvenchakra
not rated yet Aug 03, 2010
steam engines? The use of steam engines for anything stationary is over and done with. Modern geothermal stations use sterling engines, not steam ones.

For those who don't know, several different fusion prototypes have been made that show that a large enough scale device can produce much more energy than it consumes. The problem is the same as it is for building new fission plants: everyone is afraid of the danger (this is a mute point, a fusion plant failure isn't nearly as dangerous as people make it out to be).
passemoilesel
not rated yet Aug 04, 2010
So many great news about solar energy conversion, solar cell efficiency improvement and solar cell cost reduction over the last few years however none translated into real and available products. This is just one more. Seems we have difficulties industrializing all these great bright ideas.
ForFreeMinds
1 / 5 (3) Aug 04, 2010
Given the cost of photovoltaics is over 30 cents per KWhr as compared to 6 cents for coal, they'll need to improve efficiency and cost. I'm not sure they're there yet, but it's big step in the right direction.
Modernmystic
1 / 5 (6) Aug 06, 2010
Solar power is idiotic...you'll never meet the eventual demands of our civilization even if you cover the entire planet and put up so many satellites you can't breathe in Earth orbit.

Fusion or Fission are our ONLY long term options...no matter what the kooks say.
fmfbrestel
3 / 5 (2) Aug 06, 2010
Um, sorry Modernmystic, but you're just wrong. unless by eventual demands you mean 2000+ years into the future at current growth rates.
Modernmystic
1 / 5 (5) Aug 06, 2010
Um, sorry but I'm dead on, in probably less than a hundred years. The world will need twice the energy we use now in just twenty years.
rnn
not rated yet Aug 06, 2010
How big is this first prototype PETE device? About a quarter of an Inch square? Its not out of the Lab and yet the headlines make it sound like the entire rest of the solar industry can close up shop and shutter their businesses, as being too old fashioned, expensive, and inefficient! Get real!
watergroove
5 / 5 (1) Aug 07, 2010
@Doug
Nuclear isn't secure, it is a giant bulls eye mocking terrorists and other enemies of the state into attempting an attack. Someone blows up a few solar chips and mirrors -- who cares? worst case senario, temporary black outs till power is rerouted. Worst case scenario for nuclear is Chernobyl. Or, god forbid, they steal some of the waste and disperse it on LA or New York. You cant poison an entire city with a solar chip.


Thorium is non proliferation, has the same levels of power conversion efficiency can be easily controlled and does not suffer from thermal runaway. Google FUJI reactor. Solar can never compete with coal. Impossible. Scientists from Japan and China estimate we will need 10,000 10GW reactors within 50 years to sustain the planets appetite for electricity. We simply cannot keep denying 3rd world countries power, it is unconscionable. So everyone who talks about efficiency how do you do that with someone who has nothing. You are the hubris to the poor.
Starblade_Enkai
not rated yet Aug 07, 2010
Too bad some patent troll is going to steal the idea within the year and make us wait 20 years or more for this invention to reach the public.

Really, I think if some process is published and not even attempted to be patented immediately (think in a matter of seconds) it should be public domain, End Of Story!
Modernmystic
1 / 5 (2) Aug 08, 2010
Too bad some patent troll is going to steal the idea within the year and make us wait 20 years or more for this invention to reach the public.

Really, I think if some process is published and not even attempted to be patented immediately (think in a matter of seconds) it should be public domain, End Of Story!


Yeah because making sure people can't benefit from their hard work will really make sure those neat innovations keep rolling....

Newbeak
not rated yet Aug 08, 2010
Combine this development with solar roadways,and energy independence may be just around the corner.See: http://www.solarr...ain.html
Starblade_Enkai
5 / 5 (1) Aug 08, 2010
My point is the actual people who MADE it should patent it. If someone just comes across it and goes "Aha! They didn't patent it so that means I can!" the onlooker, who had NOTHING to do with its creation, should have no right to do so. THAT'S what I'm saying.
Modernmystic
1 / 5 (2) Aug 09, 2010
My point is the actual people who MADE it should patent it. If someone just comes across it and goes "Aha! They didn't patent it so that means I can!" the onlooker, who had NOTHING to do with its creation, should have no right to do so. THAT'S what I'm saying.


Funny I seem to remember something about "public domain" in your post. Which is hardly letting the people who MADE it benefit from it...

Perhaps you should be more clear.
Starblade_Enkai
5 / 5 (1) Aug 10, 2010
I said IF the creator relinquishes it without attempting to patent it THEN it becomes public domain.

I hate to sound rude, but you need to read what I said a lot better.

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