A novel way to concentrate sun's heat

Dec 02, 2011
A novel way to concentrate sun?s heat

Most technologies for harnessing the sun’s energy capture the light itself, which is turned into electricity using photovoltaic materials. Others use the sun’s thermal energy, usually concentrating the sunlight with mirrors to generate enough heat to boil water and turn a generating turbine. A third, less common approach is to use the sun’s heat — also concentrated by mirrors — to generate electricity directly, using solid-state devices called thermophotovoltaics, which have their roots at MIT dating back to the 1950s.

Now, researchers at MIT have found a way to use thermophotovoltaic devices without mirrors to concentrate the sunlight, potentially making the system much simpler and less expensive. The key is to prevent the heat from escaping the thermoelectric material, something the MIT team achieved by using a photonic crystal: essentially, an array of precisely spaced microscopic holes in a top layer of the material.

The approach mimics Earth’s greenhouse effect: Infrared radiation from the can enter the chip through the holes on the surface, but the reflected rays are blocked when they try to escape. This blockage is achieved by a precisely designed geometry that only allows rays that fall within a very tiny range of angles to escape, while the rest stay in the material and heat it up.

The new device was described in a paper by Research Laboratory of Electronics research scientist Peter Bermel and other MIT researchers, published in October in the journal Nanoscale Research Letters.

Bermel explains that if you put an ordinary, dark-colored, light- and heat-absorbing material in direct sunlight, “it can’t get much hotter than boiling water,” because the object will reradiate heat almost as fast as it absorbs it. But to generate power efficiently, you need much higher temperatures than that. By concentrating sunlight with parabolic mirrors or a large array of flat mirrors, it’s possible to get much higher temperatures — but at the expense of a much larger and more complex system.

“What I’m looking at is an alternative to that paradigm,” Bermel says, by “concentrating the sunlight thermally”: capturing it and reflecting it back into the material. The result, he says, is that the device can absorb as much heat as a standard black object, but “in practice, we can get it extremely hot, and not reradiate much of that .”

Such a system, he says, “at large scale, is efficient enough to compete with more conventional forms of power. This is an alternative to concentrators.”

In addition, the system is simple to manufacture using standard chip-fabrication technology. By contrast, the mirrors used for traditional concentrating systems, he says, require “extremely good optics, which are expensive.”

The next step in the research, Bermel says, is to test different materials in this configuration to find those that produce power most efficiently. With existing solar thermophotovoltaic systems, he says, “the highest efficiency [in converting solar energy to electricity] is 10 percent, but with this angular-selective approach, maybe it could be 35 to 36 percent.” That, in turn, is higher than the theoretical maximum that could ever be achieved by traditional photovoltaic solar cells.

In the solar-cell business, Bermel points out, “even small differences of 1 percent or so are considered important.” At this point, however, his research has been “mainly theory,” so the next step is building and testing more actual devices. So far, he says, “we have some preliminary results” that validate the theory.

Jason Fleischer, an associate professor of electrical engineering at Princeton University who was not involved in this work, says that for thermophotovoltaic systems to work well, “sunlight typically needs to be concentrated, and re-emission back into space is a problem.” The advance made by Bermel and his co-authors, he says, is to use existing light-absorbing material and create a photonic structure in it, “so that it preferentially emits light in a direction and wavelength range that is optimal for photovoltaic conversion.” By doing so, this “increases the efficiency significantly beyond classical predictions based on unconcentrated sunlight, enabling a small device to generate as much electricity as a conventional one that is much larger.”

This research, Fleischer says, was of “exceedingly high” caliber.

The paper was co-authored by MIT’s John Joannopoulos, the Francis Wright Davis Professor of Physics; professor of physics Marin Soljačić; and four students. It was funded by the National Science Foundation, the MIT S3TEC Energy Research Frontier Center of the Department of Energy, and the Institute for Soldier Nanotechnologies.

Explore further: Bloomberg invests $5M in solar-powered lamp

More information: Nanoscale Research Letters 2011, 6:549 doi:10.1186/1556-276X-6-549

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User comments : 31

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mrCalvin
5 / 5 (5) Dec 02, 2011
Could the same principle be economically (And beneficially) applied to domestic solar panels for hot water ?
plasticpower
3.5 / 5 (2) Dec 02, 2011
For some reason I am having trouble imagining this generating much electricity on a cloudy winter day.. Solar panels, on the other hand, should still work to an extent. Not trying to knock this discovery by any means though, sounds like a novel idea definitely worth exploring, especially in areas where it will exceed the solar panel limitations.
hjbasutu
1.2 / 5 (20) Dec 02, 2011
these guys are always running away from cold fusion...what a pity!
Vendicar_Decarian
3.4 / 5 (5) Dec 02, 2011
"Could the same principle be economically (And beneficially) applied to domestic solar panels for hot water ?" - MrCalvin

Yes of course, but it isn't needed. Solar hot water heaters are readily available, reasonably inexpensive, and able to generate enough hot water for daily use, although not in winter for the northern American states.

The standard "Chinese" hot water heater consists of a flat panel consisting of glass tubes contained within another glass tube about 6 feet long, inclined and turned to face the sun.

The inner tube is coated black, and the space between the inner and outer tube is evacuated. The inner tube is filled directly with water or in some cases a circulating fluid like glycol.

cont...
Vendicar_Decarian
2.7 / 5 (7) Dec 02, 2011
Sometimes a copper bar or some other metal is placed into the inner tube to conduct heat.

In the case of simple water, a tube exposed to direct sunlight can in short order raise the water temperature within the tube to boiling.

No concentration of sunlight is needed due to the fact that the vacuum between the inner and outer tubes prevents conductive thermal loses. IR radiative thermal osses can be limited by a thin layer of gold on the inner surface of the outer tube.

In any case a rectangular array of 4 feet by 6 feet can supply all of the hot water needed by an average home, and will operate from at least mid spring to mid fall. Longer depending on your local climate.

Units are typically sold with a well insulated water tank sitting on top of the panel. Upward thermal conduction heats the water in the tank. There are no moving parts. Units are completely passive.

Search for "Chinese Water Heater".

http://www.youtub...lFpgR6LY

RETT
3 / 5 (2) Dec 02, 2011
I cannot imagine why a cloudy day would have more effect on such a device than on regular solar cells. Also, since heat transmits through material much more readily and inexpensively than light, the volume of electricity producing material can be much higher, thicker in this case, leading to more spacial efficiency. Also, thermal solutions trap a much higher percentage of the sun's energy than straight photovoltaic devices, where heat is often a problem that reduces efficiency. With the narrow angles of re-emission, it should be easy to capture the light that is emitted for other purposes.
antialias_physorg
3 / 5 (2) Dec 02, 2011
The approach mimics Earths greenhouse effect: Infrared radiation from the sun can enter the chip through the holes on the surface, but the reflected rays are blocked when they try to escape.

Wouldn't this be the prfect application for some metamaterials which let no heat escape? Or would these be too limited in the wavelengths they could capture (i.e. too narrow a spectrum so as not to get all of the infrared photons)?

But the whole idea is exciting, because it could open the way for simple energy generation from areas which are normally not used for photovoltaics (north sides of buildings etc. ) - generating electricity out of reradiated ambient infrared radiation.
Isaacsname
5 / 5 (1) Dec 02, 2011
Light sponges. I wonder if you could design a material like this with something that uses the Porphyrin that's being tested for DSC's ( dye-sensitized cells ), they are adept at harvesting diffused light and light in low-light situations.

I had a science project in grade-school where I tried to design a surface/material could be made to have cavities to trap light, similar to an eyeball, they'd have small adjustable apertures and large inner cavities to allow light to bounce until harvested.

Of course, ...we still used rotary phones and manual typewriters in skuul, meta-materials were purely science fiction at that point o,O
Jeddy_Mctedder
1 / 5 (5) Dec 02, 2011
this will be huge. converting photons to electrons is in herently inefficient for generating power--- rathern than transporting it, relative to thermal concentration----

this is one of the big secrets of solar thermal
Malio
1 / 5 (1) Dec 02, 2011
What about a combination of both, photovoltaics and thermophotovoltaics?
Isaacsname
not rated yet Dec 02, 2011
What about a combination of both, photovoltaics and thermophotovoltaics?


http://www.freepa...0783.pdf
that_guy
not rated yet Dec 02, 2011
I cannot imagine why a cloudy day would have more effect on such a device than on regular solar cells. Also, since heat transmits through material much more readily and inexpensively than light, the volume of electricity producing material can be much higher, thicker in this case, leading to more spacial efficiency. Also, thermal solutions trap a much higher percentage of the sun's energy than straight photovoltaic devices, where heat is often a problem that reduces efficiency. With the narrow angles of re-emission, it should be easy to capture the light that is emitted for other purposes.


I can imagine why. Physics.

Water vapor tends to reflect the incoming infrared away but lets a significant amount of UV pass through. A PV system can still harvest the remaining UV, but a thermal system would not be able to gather enough heat potential to function very well.

That said, PV systems don't function especially well on cloudy days either - but they do a tolerable job.
Callippo
1 / 5 (12) Dec 02, 2011
Cold fusion finding makes the research of power photovoltaic obsolete, abstract research without practical usage and suffering with high consumption of raw materials. Such research is merely serving as a salary generator of physicists involved, which should be relocated to another research. Thermophotovoltaic can still find some usage in bolometers, i.e. sensitive detectors, but these detectors are routinely produces in different ways.
antialias_physorg
4.6 / 5 (10) Dec 02, 2011
Give it a rest rawa/calippo/Zephyr/and whatever other sockpuppets you have created to push cold fusion.

You're like omatur. Haven't you noticed that no one buys into Rossi's (and your) scam here? Repeating the same old statements on a thousand threads isn't going to change that. Go out. Buy an Ecat. Feel smug in the knowledge that you now have unlimited power. Why don't you?
Tseihta
1 / 5 (1) Dec 02, 2011
Would the use of carbon nanotubes to capture 'all' the light be something worth pursuing in this application?

http://www.physor...deo.html
that_guy
not rated yet Dec 02, 2011
Would the use of carbon nanotubes to capture 'all' the light be something worth pursuing in this application?

http://www.physor...deo.html


I believe the main issue at question here is the re-emitting of the already absorbed heat.

commercially available black paint (designed to absorb heat) can reach an absorbtion rate of 95%. Converting to a much more expensive nanotube pain would increase efficiency by about 5%.

So a 10% efficient panel would reach 10.5% efficiency (because an additional 5% of 10% = .5%)

Capturing and keeping the heat might increase the efficiency by over three fold.

"the highest efficiency [in converting solar energy to electricity] is 10 percent, but with this angular-selective approach, maybe it could be 35 to 36 percent."


Any more questions?
ArtflDgr
2.3 / 5 (6) Dec 02, 2011
plasticpower the real problem with solar cells

i am going to be gracious and rather than try to build our energy in a more comon place, i will use the sahara

2.650kWh/m2/year in the Sahara

1,000-MWe reactor at 90% capacity factor in one year
7.9 billion KWhenough to supply electricity for 740,000 households = 13.7M Brls oil = 3.4M short tons coal

2,981,132,075 sq meters or one 54,599 meter square 33 miles or so on a side.

we can look at this as a power density vs natural landscape used.... or just a resource problem (all that raw material needed to make either. or how about a labor and Windex issue?

numbers from nuclear energy institute, and sahara from looking it up. try plugging in 10% of what the US in total uses and see...

ArtflDgr
2.3 / 5 (6) Dec 02, 2011
Palo verde is 3,942 MW 4 times the example above
Ft. Calhoun is just under 500MW..its the smallest

19.6% or 807.0 billion kilowatt-hours (bkWh) nuclear power in the US 2010. well, lets say thats 20% or 1/5 of 4035.

so imagine that solar got 20%, they all get sahara like light and 100% conversion, like above.

304528301886sq meters = 551,840 meter sq 342 miles on a side... 116,964 sq miles... about the size of Massachusetts or Hawaii.

any reality plugged into the above would make it much worse if you want to keep its output the same.

London Array (1000 MW)and will cover 90 square miles according to wiki...

phase I 2.2 billion 630MW

1/6th the power of palo verde, which is 4,000 acres (16 km2)
they bought the land for $2 million and the total cost to build the plant was $5.9 billion US

at the above rate it will be 13 billion EU (17+ billion US)

boggles the mind... no?

Isaacsname
not rated yet Dec 02, 2011
Anybody working on anything related to solar power systems utilizing thermoacoustics ?
Newbeak
not rated yet Dec 02, 2011
Give it a rest rawa/calippo/Zephyr/and whatever other sockpuppets you have created to push cold fusion.

You're like omatur. Haven't you noticed that no one buys into Rossi's (and your) scam here? Repeating the same old statements on a thousand threads isn't going to change that. Go out. Buy an Ecat. Feel smug in the knowledge that you now have unlimited power. Why don't you?

Sadly,as P.T.Barnum once said,there's one born every minute..
Vendicar_Decarian
2.6 / 5 (5) Dec 02, 2011
"my co-worker's half-sister made $199945 so far just working on the internet for a few hours" - Sonias

She must type quick because the going rate of pay for posting Conservative/Pro Coroprate propaganda to the web is 30 to 50 cents per substantive post.
JohnMoser
2.3 / 5 (6) Dec 03, 2011
"She must type quick because the going rate of pay for posting Conservative/Pro Coroprate propaganda to the web is 30 to 50 cents per substantive post."

Learn how to spell Mr Science. Jesus you're dumb.
MNIce
1 / 5 (2) Dec 03, 2011
"my co-worker's half-sister made $199945 so far just working on the internet for a few hours" - Sonias

She must type quick because the going rate of pay for posting Conservative/Pro Coroprate propaganda to the web is 30 to 50 cents per substantive post.

Vendicar, I'm sure you're being paid full value for your anti-Constitutionalist, anti-corporate rants - exactly nothing.

Sonhouse
not rated yet Dec 03, 2011
plasticpower the real problem with solar cells

i am going to be gracious and rather than try to build our energy in a more comon place, i will use the sahara

2.650kWh/m2/year in the Sahara

1,000-MWe reactor at 90% capacity factor in one year
7.9 billion KWhenough to supply electricity for 740,000 households = 13.7M Brls oil = 3.4M short tons coal

2,981,132,075 sq meters or one 54,599 meter square 33 miles or so on a side.

we can look at this as a power density vs natural landscape used.... or just a resource problem (all that raw material needed to make either. or how about a labor and Windex issue?

numbers from nuclear energy institute, and sahara from looking it up. try plugging in 10% of what the US in total uses and see...


There is about 1200 watts/meter squared on top of the atmosphere, and maybe half that in the desert so call it 600w/m squared. With 20% PV cells, that is 200 w/m squared. So assuming you get max sun for 8 hours a day, that is 1.6 Kwhr per DAY.
Sonhouse
5 / 5 (1) Dec 03, 2011
So to continue, 365 times 1.6 Kwhr= 584 KWr/yr per square meter.If you multiply that times your approx. 3 billion square meters you get more like 1500 billion kwhr/year. Your math is off a bit.
tarheelchief
not rated yet Dec 04, 2011
If this works successfully,individuals will be able to greatly reduce home utility costs. Gas,oil,and electric heating can be replaced by radiant heating in flooring.
It would also help those in the greenhouses or dairies or feedlots where large scale panels could replace expensive heating apparatus.
The potential for greenhouse construction are enormous and helpful in far colder climes.
unknownorgin
1 / 5 (3) Dec 06, 2011
For those that like to expirement here is something to try; On a flat piece of styrofoam at least one inch thick place several clear glass jars that will fit inside of each other upside down on the flat panel in sunlight and notice that the inner most jar can obtain several hundred degrees temperature elevation.
antialias_physorg
3 / 5 (2) Dec 07, 2011
There is about 1200 watts/meter squared on top of the atmosphere, and maybe half that in the desert so call it 600w/m squared.

Average values for northern latitdes (e.g. germany) are 600-1000W per meter squared (summer) and 300-500 (winter) - assuming clear skies.

According to Wikipedia the average yearly radiated energy onto the surface is 1.000 kWh per meter squared in central europe and about 2.350 kWh in the Sahara desert.

brizzadizza
not rated yet Dec 09, 2011
ArtflDgr's math is incorrect. Sonhouse is much closer. He made a mistake with respect to wattage at the surface, but a subsequent mistake wrt 20% of 600= 200 led him to roughly the correct of energy/day.

An averaged 2GW solar PV powerplant needs 1mi^2 of solar panels to produce 6GW peak. Figure add another 25% for access and infrastructure and solar PV has a reasonable footprint/GW.

In the real world, La FLorida Solar Farm in Spain produces 50MW on a .2 sq mi facility (not .2 sq mi of solar panels/reflecting surfaces) which squares pretty closely with my own estimate (give or take a ten, but who's counting?)
brizzadizza
not rated yet Dec 09, 2011
Oops, let me amend the above, 1 sq mi of solar panels @ 1kW/m^2 will give an average 172MW power output. My new estimate squares better with reality.
Vendicar_Decarian
not rated yet Dec 09, 2011
In comparison a single nuclear reactor will produce something like 1,000 MW of power and do so continuously.

Improving consumptive efficiency is the first order of the day.

Directive 1: All new homes must be constructed so that the largest area of their roof faces south.

Directive 2: All roofs must be constructed so that they are PV generation and Passive Solar hot water friendly.

Directive 3: All new homes must be wired for 12V lighting and other low power devices.

Directive 4: All new homes must be constructed with high efficiency insulation and low E windows.

Directive 5: All new home plumbing must be insulated on the hit water side.

Directive 6: All new home plumbing must be heat-exchanger friendly on the drain side so that hot drain water heats incoming cold water.

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