The typical US power plant converts only about one-third of the heat energy that comes out of fuels into electricity, according to Fairfield University electrical engineering department Chairman Jerry E. Sergent.
In other words, a huge amount of heat energy simply goes through the smoke stacks.
Just before Earth Day, GMZ Energy announced the commercial availability of a next-generation material that could produce clean electricity from heat and optimize cooling/heating devices. This came less than a month after the groundbreaking method went on express review in the journal Science on March 20.
A team of researchers from Boston College and the Massachusetts Institute of Technology pioneered the surprising simple approach that dramatically boosted the thermoelectric performance of a bulk alloy.
From that team, two researchers co-founded GMZ Energy in Newton, Massachusetts along with CEO Mike Clary.
Thermoelectrics could improve the fuel economy of cars, said Robert Wang, a University of California, Berkley nanoscale energy-transport researcher who was not part of the team.
"The exhaust you throw out of your tailpipe is really hot," said Wang. "You can use that heat to generate electricity with thermoelectric devices."
"The use of thermoelectric materials in clean technology has long been overlooked due to high costs and low efficiency," said Clary in a press release.
"And we've overcome those challenges," he said.
NASA deep space probes, such as Voyager, use thermoelectric generators which convert heat produced by radioactive decay into electricity, said Wang.
As a note, the term "thermoelectric power plant" is not to be confused with thermoelectrics. Conventional thermal power stations operate by another process that first turns heat energy into mechanical energy in order to produce electricity.
In contrast, thermoelectric (TE) generators and coolers are solid-state devices that directly convert between heat and electricity using thermocouples - in other words, they have no moving parts.
A Major Breakthrough
"This paper definitely has changed the way people will think about how to make thermoelectric materials," said Wang.
Until recently, researchers have been unable to come up with practical ways to improve the performance of thermoelectric alloys for 50 years, said Qing Hao, of MIT, one of the co-authors of the paper.
In thermoelectrics, researchers compare the merit of materials using ZT values, which works the same way as mpg (miles per gallon) does for cars. They use ZT as a measurement because efficiency varies depending upon the temperature differences involved.
"A ZT of 1 is about the best you can do in a standard bulk structure," said Wang.
Using their surprisingly low-tech method, researchers achieved a peak of about 1.4 ZT - a 40 percent higher performance rating - with an alloy of telluride with bismuth and antimony.
"It's very promising," said Wang.
"The trick to making any of these things work in the real world is being able to easily scale up... production... and their technique accomplishes that," he said.
"These results open a cost-effective way to improve the performance of thermoelectric materials," said co-authors in their paper.
"Our material may be double the cost of traditional bismuth telluride, but it's still affordable," said Hao.
Investment in TE technology has long been neglected according to researchers.
"For this thing to become widely used... it boils down to how much does it cost," said Wang. "But people tend not invest in things unless they see the return."
Hao gave an example of how the work of scientists can be misunderstood by businesses:
"There is a higher ZT record and it is about 3.5," said Hao. "But it very slowly grew thermoelectric materials atom by atom and is very costly."
Although previous work became too expensive to be practical, researchers cautioned against jumping on the assumption that thermoelectrics could not become affordable eventually.
"First, you demonstrate it in a very careful manner," said Wang on the nature of scientific research.
"After that point you see how much you can relax... so you can easily scale it up," he said.
And the take away point?
"The good thing about that paper is that it demonstrated that nanotechnology can really improve ZT," said Hao.
"There are some people who demonstrate that this will work and then people who try to find a cheaper way," he explained.
Putting it in Perspective
In the United States, thermal power stations supply well over nine-tenths of the nation's electricity - based on statistics from the federal Energy Information Administration.
Given that roughly double the amount of heat energy converted into electricity is pumped straight into the atmosphere, the need for waste heat recovery is evident.
"If you had a ZT of 1 and you could make it cheap, you'd make a huge impact," said Wang.
However, that points out the major barrier to the wide adoption of thermoelectrics: the cost of the substances used to make them.
"Right now you have a ZT of 1.4, but you need a ZT of 2 to make it really commercially interesting," Hao speculated based upon current material costs.
"I think it's possible," he said.
Hao acknowledged that his team has been testing to see whether the method will improve other materials for thermoelectrics.
The Basics behind the Technology
For the sake of simplicity, this article will explain thermoelectrics using concepts from high school science classes.
TE devices fall into two categories: generators and heat pumps (coolers and heaters).
To start, TE devices depend upon a heat imbalance across the module for electricity generation.
"Heating excites electrons," said Hao. "In certain materials, heat allows them to break free from their atoms."
When there is uneven heating, it frees more electrons on some parts of a TE module than in others. This creates a build-up of electron "pressure" in those parts.
Since other areas have a lower electron "pressure," free-electrons will flow towards them - producing an electric current.
Although the full process is more complex, this works similarly to how convection drives ocean currents (or how hot coffee can stir itself).
While TE generators can passively cool electronics or engines by recycling waste heat energy into electricity, TE devices can also become active coolers as well.
Depending on the material and direction of the applied current, it "will carry heat from one side to the other," said Hao. "You just pass electricity through it."
Not So Easy
Thermoelectric efficiency is only as high as the difference in temperature between the hot and cold sides of a TE module. (By the way, this also means that they cannot function as perpetual motion machines that produce electricity from heat in the atmosphere.)
Both Wang and Hao said that thermoelectrics research could be frustrating because electrically conductive materials generally conduct heat very well too.
This is problematic for TE devices since efficiency declines to zero as the temperature difference evens out.
For heating and cooling applications, they lose the ability to pump heat as soon as the temperature levels out on both sides. Furthermore, it would be counter-productive for heat to flow back to the side it was just removed.
"Thermal conductivity limits the maximum ZT you can get," said Hao.
However, many ways to reduce heat flow may have unwanted effects.
"You can't really independently tune thermopower, electrical conductivity, and thermal conductivity that well," said Wang on the three properties used to calculate ZT.
For non-engineers, think of "thermopower" as how easily atoms can release electrons when exposed to heat.
"You need to have a balance between all three properties," said Hao. "You cannot sacrifice one and still get a good ZT."
(To avoid confusion, Hao meant that low thermal conductivity cannot be sacrificed.)
Greatness through Simplicity
While this method may sound complex described as "nanotechnology," the term nano
simply describes the size of particles.
In fact, the concept behind what researchers did is not much harder to understand than turning a raw material like wheat into flour to make bread.
First, researchers crushed the thermoelectrical substance into a fine powder. Next, these nanosized specks were hot-pressed back together into a material with a randomly arranged internal structure - fresh from the bakery: a new nanocomposite.
"Using this approach, you can dramatically reduce thermal conductivity," Hao said on ball-milling followed by pressing.
By scrambling the composition of the alloy, researchers created what amounted to a "speed bump" for the flow of heat as it must jump from grain to grain at every few nanometers (or billionth of a meter - hence nanotechnology).
Because they operate by a different mechanism, a random structure greatly scatters heat flow while hardly affecting the flow of electricity, according to Hao.
More TE Applications
Although not yet ready for large-scale power generation, TE devices have already found a place in many markets.
Hao said that thermoelectrics could come in handy in situations where heating is required sometimes and cooling for other times.
"You have a double function for heating and cooling," he said. "Pass the current from one direction to the other and you can easily switch."
While thermoelectrics has been used for temperature control in laboratories, according to Hao, many new TE-based innovations have been surfacing recently.
Earlier in the year, Electronic Products magazine gave Nextreme Thermal Solutions(TM) a 2007 Product of the Year award for their compact, thin-film embedded thermoelectric cooler (eTEC) for precision cooling built directly on microchips.
According to its website, Nextreme eTECs "can pump 10-20x more power than conventional technology."
Another device, announced early April 2008, is a battery-less brainwave-monitoring headset. Powered by the warmth of the human head and solar panels, the device also comes with built-in wireless to communicate with doctors.
Researchers from the Interuniversity Microelectronics Centre in Belgium designed the wearable medical instrument to increase patient quality of life, according to the press release.
What Does the Future Hold?
Hao suggested that the method might make researchers reconsider more-common materials that previously had too much thermal conductivity for thermoelectric applications. If possible, it would mean lower costs in making TE devices.
Future innovations to remove heat from the cool side of TE modules might also increase their performance, suggested Sergent, mentioned at the start of this article.
"I think thermoelectric devices are going to keep on improving," said Wang. "It's still very fast moving... nano hasn't really reached its peak yet."
Special thanks to researchers, experts, my editor, and other contacts not mentioned in this article for all their help in creating this.