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Clean Power This cross-section micrographs of a tungsten thermal emitter used in the experiment. The top image shows how unprotected tungsten degrades after heating to 1200 degrees Celsius. The bottom image demonstrates how the ceramic-coated tungsten retained structural integrity after being subjected to 1400 C heat for an hour.
Image Credit: Kevin Arpin

Published on October 19th, 2013 | by James Ayre

14

Vastly Improved Solar Cells Possible With Use Of New Heat-Resistant Materials

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October 19th, 2013 by
 
Significant improvements to the efficiency of solar cells could be possible in the near-future thanks to the recent development of a new heat-resistant thermal emitter by researchers at Stanford University.

The new heat-resistant thermal emitter was created as a means of converting the higher-energy portion of light into lower energy waves which can then be absorbed by the solar cells and converted into electricity, along with the lower energy portions that most solar cells convert. Technologies such as this — more broadly known as thermophotovoltaics — have been around for quite some time, but have, until now, possessed a number of important limitations that this new device seems to overcome.

Foremost of which, are the limitations to do with operating temperature ranges — earlier designs have all become nonfunctional at temperatures of around 2200 degrees Fahrenheit, whereas the new one “remains stable at temperatures as high as 2500 F”.

This cross-section micrographs of a tungsten thermal emitter used in the experiment. The top image shows how unprotected tungsten degrades after heating to 1200 degrees Celsius. The bottom image demonstrates how the ceramic-coated tungsten retained structural integrity after being subjected to 1400 C heat for an hour. Image Credit: Kevin Arpin

This cross-section micrographs of a tungsten thermal emitter used in the experiment. The top image shows how unprotected tungsten degrades after heating to 1200 degrees Celsius. The bottom image demonstrates how the ceramic-coated tungsten retained structural integrity after being subjected to 1400 C heat for an hour.
Image Credit: Kevin Arpin

“This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics,” stated Shanhui Fan, a professor of electrical engineering at Stanford University. The new findings are the product of a collaboration between Fan and colleagues at the University of Illinois-Urbana Champaign and North Carolina State University.

The significance of the new device is, of course, that now a potentially much larger portion of the solar spectrum can be harvested by an individual solar cell.

“In theory, conventional single-junction solar cells can only achieve an efficiency level of about 34%, but in practice they don’t achieve that,” stated study co-author Paul Braun, a professor of materials science at Illinois. “That’s because they throw away the majority of the sun’s energy.”


Stanford University explains:

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy. But silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass through the solar panel.

Thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

“Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell,” Fan continued. “That raises the theoretical efficiency of the cell to 80%, which is quite remarkable.”

As of now, though, that 80% theoretical limit is quite a ways off — the peak achieved so far has been an efficiency level of about 8%. The reason for the disparity is primarily because of problems with the intermediate component, which is typically made of tungsten, according to the researchers. To address this issue, the researchers coated the tungsten emitters in a nanolayer of a ceramic material called hafnium dioxide, greatly improving their heat tolerance.

“The results were dramatic. When subjected to temperatures of 1800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2500 F (1400 C), the samples remained thermally stable for at least an hour.”

“These results are unprecedented,” stated lead author Kevin Arpin. “We demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage.”

“We’ve demonstrated that the tailoring of optical properties at high temperatures is possible,” Braun added. “Hafnium and tungsten are abundant, low-cost materials, and the process used to make these heat-resistant emitters is well established. Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven’t been considered.”

The new research was just published in the October 16th edition of the journal Nature Communications.

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About the Author

's background is predominantly in geopolitics and history, but he has an obsessive interest in pretty much everything. After an early life spent in the Imperial Free City of Dortmund, James followed the river Ruhr to Cofbuokheim, where he attended the University of Astnide. And where he also briefly considered entering the coal mining business. He currently writes for a living, on a broad variety of subjects, ranging from science, to politics, to military history, to renewable energy. You can follow his work on Google+.



  • Gerald Katz

    Flat plate silicon cells proven long life ( I have modules made in 1980 that still work fine) Modules now getting so affordable that some new homes have them integrsted with the roof at a cost not much more than high end stsndard roofing. Utility systems with tracking pv provides maybe 30% more energy, if there are going to use concentrating collectors go thermal. Turbines produce Ac no need for inverters, thermal can be stored. Low temp solar thermal organic rankine turbines can also be run on with industrial waste heat or combined with geothermal energy for 24/7 with afternoon peaking. High temp concentrating towers and mirror fields have been able to store enough energy to run 24/7 and the high temperature energy might also be used to power a wide variety of chemical and industrial processes.

  • JamesWimberley

    I don’t get the relevance of this. The temperatures cited (1200-1400 deg C) are miles above anything a solar cell could experience. They could just be reached in solar tower collectors, but the article doesn’t mention CSP. So the connection to improved silicon cells is handwaving. What am I missing?

    BTW, science runs everywhere in the world on SI units, including degrees Celsius or Kelvin. You can use Fahrenheit if you insist to talk about air-conditioning to an American audience.

    • J_JamesM

      I agree. Where exactly is 1400° C going to be relevant? Have they started putting solar cells in foundries?

      • Teddy

        The Idea is to try to test out long term heating issues in as small a time frame as is possible.

        It’s not a great real world test, but waiting 30-40 years for results is also not so feasible.

        • JamesWimberley

          Does a material’s standing up to 1400 deg C for an hour tell you anything useful about whether it will stand up to 60 deg C for 30 years?

          • Ronald Brakels

            Let’s see what Materials Science And Engineering An Introduction, Eighth Edition has to say…

            Crikey! There are a lot of words in this book!

            Oh look! There’s a whole section on “Generalized Creep Behaviour”. That could be handy to know.

            Let’s see, Stress and Temperature Effects, Data Extrapolation Methods… Well, the short answer is, yes, materials that can withstand high temperatures are generally good at withstanding not so high temperatures.

          • JamesWimberley

            You mean 1200 deg C isn’t enough to ensure long-term stability at 60 C, and 1400 C is?

          • Ronald Brakels

            Well, I’ll give you a commonly used extrapolation procedure and you can use it to see for yourself how much difference 200 degrees makes. The Larson-Miller parameter is:

            T(C + log tr)

            Where T is the temperature in Kelvin. C is a constant which is usually about 20 and tr is the rupture lifetime in hours (How long before things go bad.) Note that this may not be appropriate for the material mentioned in the article, this is for demonstration purposes only.

          • Sean

            Let’s not be too smug when someone is asking from ignorance. You were once ignorant.

    • Bill Kalahurka

      There is a huge misunderstanding here. A thermophotovoltaic device is very different from a PV cell. All I know is what I’ve read on Wikipedia, but in a tpv device, the immediate source of photons is not the sun, but an “emitter” which has the property that it releases photons of a desirable wavelength, whenever it is heated to a certain temperature. This article is basically reporting that some electrical engineers have built an emitter that remains stable at higher temps.
      The emitter itself can be heated by sunlight. So, yeah presumably you’d need to concentrate sunlight on the emitter in order to make the tpv work.

    • Omega Centauri

      I can only imagine this in some sort of CPV context, where the light is focused of this new special material, and the emitted infrared is then directed to the (much cooler) PV cell. Hard to imagine it being practical in any way -CPV is already dying because of cheap panels.

      • JamesWimberley

        Thanks, that makes sense. and so does your scepticism. CPV is just too complicated to pay at scale.

        If you could deposit a *transparent* layer of the emitter on top of a silicon cell, that might work, But the photos are of a layer micrometers thick, not nanometers. The perovskite cells (330nm= 0.3 µm) reported on earlier on this site are semi-transparent so the compound cell looks possible. See phys.org/news/2013-09-team-physicists-perovskite-conventional-solar.html

    • KennyDude

      I’m no expert but I believe it just means that more of the energy thats wasted in heating up the solar cell is used towards electricity generation. 2nd law of thermal dynamics states the more heat you generate the less efficient the conversion of energy to useful work ( I think). So I believe this filters out the spectrum thats not convertible into one that is to get more efficiency. So for consumers it should translate into better efficiency. I wish they stated the efficiency numbers after using the ceramic coating.

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