| Topic: |
Science > Physics |
| User: |
"Sam Wormley" |
| Date: |
17 Jan 2008 09:17:02 PM |
| Object: |
Nanowires convert heat to electricity |
Nanowires convert heat to electricity
Phonon effects boost thermoelectric efficiency
http://physicsworld.com/cws/article/news/32446
Silicon --a staple of modern electronics --could soon be used to make
low-cost devices that convert waste heat to electricity. That is the
bold future outlined by two independent teams of scientists in the
US, who have shown that arrays of tiny silicon wires have extremely
good "thermoelectric" properties. The findings could lead to the
development of cheap thermoelectric materials that boost the
efficiency of both massive coal-fired generators and tiny solar
cells.
Many methods of generating electricity --such as burning fossil fuels
and nuclear fission --create vast quantities of waste heat.
Thermoelectric materials that convert heat directly into electricity
could someday be used to recover some of this energy, thereby
boosting the efficiency of conventional power stations.
Thermoelectric materials could also improve the effectiveness of
solar cells and generate electricity from other sources of waste heat
such as computer chips and refrigerators.
Low efficiency
Unfortunately, today's thermoelectric materials are synthetic
nanostructures that are very expensive to make and are nowhere near
efficient enough to be used commercially.
Some researchers believe that silicon-based thermoelectric materials
could overcome these limitations because silicon is easy and
inexpensive to work with and has the right electrical properties to
be a thermoelectric material. However, silicon is also a good
conductor of heat. A thermoelectric material converts a temperature
difference across a material into a voltage and silicon\u2019s high
thermal conductivity means that a large amount of heat is required to
create a small temperature difference --making it a very inefficient
thermoelectric material.
Factor of 100
Now, two independent teams in California have worked out a way to
boost the thermoelectric efficiency of silicon by as much as a factor
of 100. Both results are reported in the journal Nature.
Allon Hochbaum and colleagues at the University of California,
Berkeley, created arrays of tiny silicon nanowires by dipping silicon
wafers into an aqueous solution of silver ions. The nanowires were
20\u2013300 nm in diameter and the team discovered that arrays have a
thermoelectric efficiency about 60 times greater than bulk silicon at
room temperature.
Rough surfaces
They believe that this rise in efficiency occurs because
heat-carrying sound waves called phonons have a very difficult time
moving along the very narrow nanowires, reducing their ability to
conduct heat. One important feature of Berkeley nanowires is that
their surfaces are rough, which the researchers believe contributes
to their high thermoelectric efficiency, although the physics behind
this remains unclear.
Meanwhile, at the California Institute of Technology, Akram Boukai
and colleagues have seen a similar effect in even smaller rectangular
nanowires with cross-sections of 10 × 20 nm and 20 × 20 nm. The team
measured a 40-fold boost in efficiency over bulk silicon at room
temperature. This increased to a 100-fold boost at -73 °C.
Intriguingly, their observations suggest that another phenomenon
--called "phonon drag" --also plays a role in boosting the
thermoelectric efficiency. Phonon drag occurs when phonons moving
through the silicon collide with charge carriers such as electrons
and drag them along.
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| User: "Edward Green" |
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| Title: Re: Nanowires convert heat to electricity |
20 Jan 2008 01:03:21 PM |
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On Jan 17, 10:17 pm, Sam Wormley <sworml...@mchsi.com> wrote:
Nanowires convert heat to electricity
Phonon effects boost thermoelectric efficiency
http://physicsworld.com/cws/article/news/32446
<...>
Many methods of generating electricity --such as burning fossil fuels
and nuclear fission --create vast quantities of waste heat.
I have no doubt there is some good physics behind this, but you
wouldn't know it from reading this description.
Heat engines have a thermodynamically limited efficiency which depends
solely on the temperature of the hot reservoir and the cold
reservoir. I suppose "waste heat" would include the thermodynamically
mandated minimum heat dumped in the cold reservoir for a given input
from the hot reservoir, plus any excess to this resulting from
practical limitations of efficiency.
I may already have put more thought into what "waste heat" ought to
mean than the author of this review, who apparently thinks it is self-
evident, as it would be to a non-techically educated audience, who
might be duped into thinking its meaning was obvious.
Clearly only the second kind of "waste heat" can be recovered in the
sense of this passage.
Thermoelectric materials that convert heat directly into electricity
Let's say they do it in a less mechanically complicated way than
ordinary heat engines coupled to generators -- being solid state, with
no moving parts. Something rankles about "converting heat ... into
electricity", directly or not. That sounds like a second law
violation. Mechanical or solid state, devices generating electricity
from temperature differences skim off some of the energy heading down
the temperature gradient: "direct" here only refers to mechanical
complication.
I see a possible gotcha here in regard to claimed improvements in
efficiency: effective thermoelectric layers are going to act like
thermal insulators. If we place such a layer where a power plant or
refrigerator is radiating "waste heat", the material will not simply
neutrally harness wasted ability to do work. It will also raise the
temperature of the coiling coils, thereby lowering the efficiency of
the remaining part of the generation or refrigeration process! It's
not an obvious slam-dunk win.
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