

MIT develops Sun-less solar cells that uses heat to generate electricity - pgatzke
http://bostinnovation.com/2011/08/01/mit-develops-sun-less-solar-cells-that-uses-heat-to-generate-electricity/

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frankus
They were studying these sorts of devices at Western Washington University
when I was there back in the late 90's. They were using simple black-body
radiators and the efficiency was pretty poor, even with a recuperator pre-
heating the combustion air. They also had to run coolant behind the solar
cells to keep them operating, so some of the energy was lost through that
route.

One thing I was never able to quite pin down is what the theoretical
efficiency limit would be for a device like this, specifically whether it was
analogous to a heat engine or could do better than the Carnot limit. Any
physicists/physical chemists care to chime in?

EDIT: Wikipedia says they're limited to Carnot efficiency.

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iwwr
Apparently, these panels work on the thermoelectric effect (different from the
photoelectric effect), they're Peltier piles working in reverse.
Thermoelectrics are Carnot-limited, but photovoltaics are not.

~~~
thirdhaf
The article suffers a bit from the fog that covers most popular science
articles but I think I get the picture. As others have said this particular
technology is based on photovoltaics, in particular narrow-bandgap
photovoltaics typically referred to as Thermophotovoltaics [1][2].

Bad news first, unfortunately the Carnot limit to efficiency still applies [3]
and as an aside the limit for simple, single-junction photovoltaics under one
sun illumination is about 31%. Since we're not dealing with either the sun nor
necessarily single junction TPVs there's more to say on the topic.

The scheme proposed here seems to be very similar to a design proposed by one
of the awards of the DARPA RIMS [4] (later MIPS [5]) program. Specifically
they're proposing a combustion cell for the butane, a tungsten photonic
crystal [6] emitter to reduce long wavelength "waste" emissions and THEN a
TPV. A TPV consists of one or more pn junctions consisting of narrow bandgap
semiconductors, typically horrendously complicated structures of ternary or
quaternary materials like InGaAsP. [8] Please note that they don't even try to
address the structure of the TPV in the article.

Some practical engineering difficulties include the fact that the
nanosctructured tungsten emitter is quite reactive in oxygen at elevated
temperatures (probably still true for their proposed structure). I should
know, I reduced an incredibly expensive photonic crystal emitter to a very
pretty layer of rust while trying to measure its emission spectrum while it
was under argon. It turns out that it's incredibly difficult to purge oxygen
from this structure after it's been exposed even ONCE. If you're using this as
a battery replacement for soldiers you have to ensure that it can really take
a beating and since the TPV is quite brittle (yep, killed a few of those too)
you have to have an enclosing structure that is quite rigid, adding to your
weight. The combustion cell is also going to be an engineering challenge since
you have to maintain pretty good temperature control to keep your conversion
efficiency up. How do you control your fuel-air mixture? What happens at high
altitudes? How are you minimizing parasitic heat loss?

The reason I think a lot of people are enamored with this fairly niche piece
of engineering is due to an erroneous Sandia Labs announcement that they had
excess emission in a particularly favorable wavelength region. [7] That and
the fact that DARPA is throwing an increasing quantity of money at a problem
space with relatively few solutions.

Despite all the problems I listed this sort of TPV structure is still a very
attractive option in at least one market namely powering deep space missions
like JUNO or New Horizons [9]. Currently these are powered by something called
an RTG [10] using thermoelectrics, but more on thermoelectrics another day.
Below I have linked to a very good article on the subject of TPVs for space
written by some of my former co-workers. [11]

Sorry for the lengthy reply I got excited that I have something to contribute
to one of these discussions!

[1] <http://en.wikipedia.org/wiki/Thermophotovoltaic> [2]
[http://gcep.stanford.edu/research/factsheets/ultrahigh_therm...](http://gcep.stanford.edu/research/factsheets/ultrahigh_thermosolar.html)
[3] eprints.soton.ac.uk/69617/1/chapter_2_with_header.pdf [4]
[https://www.fbo.gov/index?id=c57c017b04923ce367344c9fdd369ee...](https://www.fbo.gov/index?id=c57c017b04923ce367344c9fdd369ee1)
[5]
[http://www.darpa.mil/Our_Work/MTO/Programs/Micro_Isotope_Pow...](http://www.darpa.mil/Our_Work/MTO/Programs/Micro_Isotope_Power_Sources_%28MIPS%29.aspx)
[6]
[http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/Nature/nature417-05...](http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/Nature/nature417-052.pdf)
[7][http://www.sandia.gov/news-center/news-
releases/2003/other/p...](http://www.sandia.gov/news-center/news-
releases/2003/other/planck-lin.html) [8]
<http://iopscience.iop.org/0268-1242/18/5/309> [9] <http://pluto.jhuapl.edu/>
[10]
[http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen...](http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator)
[11]
[http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/selected%20papers/j...](http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/selected%20papers/journal%20of%20physical%20chemistry%20c/112-7841.pdf)

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bradleyland
This could be used like a turbocharger for your electronics: Scavenge heat
energy that would normally be wasted in order to perform additional work.

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politician
The wikipedia article (<http://en.wikipedia.org/wiki/Thermophotovoltaic>) has
some excellent applications for this technology including off-the-grid power
generation from camp fires or earthen heat stores.

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teilo
How much battery power is lost to heat?

Imagine a CPU which integrated this technology into the package so that it
could re-capture a portion of its generated heat as power. Current designs
could be made more efficient, and lower battery usage.

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protomyth
original article link: [http://web.mit.edu/newsoffice/2011/sun-free-
photovoltaics-07...](http://web.mit.edu/newsoffice/2011/sun-free-
photovoltaics-0728.html)

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matmann2001
Then they're not really solar cells anymore. "Thermal cells?"

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AretNCarlsen
My impression is that they utilize a second conversion stage that is
essentially a conventional solar cell. So the sequence is: heat => light =>
electricity. The novelty is the design of the first conversion stage, which is
specifically tuned to produce wavelengths that can be efficiently converted by
the second stage solar cell. As opposed to just burning the butane and running
a solar panel off of the firelight.

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protomyth
from the article: "Imagine using the heat generated from your laptop to charge
it as well? Or charging your car’s hybrid battery by using the heat off of the
engine?"

Wish the author had rephrased these in view of The Law of Conservation of
Energy. Don't want to give the impression of developing a perpetual motion
machine.

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kmccarth
hey protomyth, author here. Haha, I see your point abt the conservation of
energy. I assumed that everyone would know that law, so I didn't address it
specifically. Obviously, the return energy would be less than the expended.

I guess to assume really does make an ass out of u and me.

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protomyth
It's not so much for the HN audience, but the next generation of article down
the chain will use that paragraph as a basis for their headline. It is by far
the catchiest and I don't think they will be explicit about conservation of
energy with their readers.

Nice article though.

~~~
kmccarth
great point about the generation of articles down the chain. I will be more
mindful of that in the future. thanks for reading

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napierzaza
Sunless solar cells? Can we think of some other term for this? Thermal cells?
Heat Cells? Something that makes sense?

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joejohnson
Did you read the article? How about "thermal photovoltaics".

