Recycling waste heat into energy: Researchers take a step toward more efficient conversion

Dec 22, 2010

(PhysOrg.com) -- The right material wrapped around your car's exhaust system could one day scavenge heat that would otherwise be wasted, turning it into energy to warm the cabin or recharge the battery.

Engineers and physicists at the University of Michigan have taken a step toward improving the efficiency of a promising candidate for this burgeoning .

The researchers studied skutterudites, a class of mechanically strong thermoelectric materials that, when combined with certain such as the metal barium, has the right mix of properties to effectively make this energy conversion: The material conducts electricity well, and conducts heat poorly. The researchers identified certain configurations of the in the compound that drastically increase the materials' efficiency.

Their work is published in the current edition of .

"We knew that skutterudites are promising materials. But we did not know what features we could manipulate to maximize the conversion of heat into ," said Ctirad Uher, (pronounced STEERad YOUher). My surname is Uher professor in the Department of Physics. "In this paper, we propose that certain configurations of the filler element barium will be very effective in lowering the materials' and thus increasing their .

"This is an important advancement in the sense that it provides guidance for the experimentalists to focus as they try to synthesize highly efficient thermoelectric materials."

Today's state-of-the-art thermoelectric materials are only five percent efficient. Skutterudites, and this new knowledge about how best to arrange their atoms, could help improve their performance to 15- or 20-percent, at which point they become useful in many practical applications, said Massoud Kaviany, professor in the Department of Mechanical Engineering.

"We explained the physics of these materials for the first time. This will help to advance the development of these materials. If you are designing them based on fundamental physics and materials and not just by trial and error, then you need to know how they work so you can avoid haphazard experimentation," Kaviany said.

Car companies are extremely interested in this technology, Uher said. The ideal environments for these materials are spots where large differences in temperatures exist. One such place is the pipe system of a car between the motor and the catalytic converter.

"That's a big source of heat that you paid for already," Uher said.

The title of the paper is "Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites." Also contributing to the research is Anton van der Ven, an assistant professor in the Department of Science and Engineering.

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Eikka
not rated yet Dec 22, 2010
Would't the temperature difference also influence how effient a TEC is, or are we talking about relative efficiency to the Carnot limit?
lexington
not rated yet Dec 23, 2010
Isn't converting heat into useful energy a violation of thermodynamics?
Quantum_Conundrum
not rated yet Dec 26, 2010
Isn't converting heat into useful energy a violation of thermodynamics?


Uh, no, not at all. Every engine that has ever been built converts 1 form of energy to another. Most engines still in use are converting some form of thermal energy into mechanical work or electricity in either 1 or 2 steps.

Steam Engine
ICE
Coal power
Nuclear power

Solar converts photons to electricity
Wind and water convert mechanical energy to electricity

===

Thermodynamics only requires total entropy to increase in universe, but doesn't specify by how much. So all laws of thermodynamics implies in this situation is that you probably cannot ever re-capture energy of ALL of the waste heat. It says nothing about recapturing "some" or even "most" of the waste heat and making some other useful form of energy out of it.
Quantum_Conundrum
not rated yet Dec 26, 2010
Skutterudites, and this new knowledge about how best to arrange their atoms, could help improve their performance to 15- or 20-percent.


This would be completely amazing in computers and all hand held electronic gadgets.

Converting 15 to 20 percent of waste heat back to electricity would make a PC run so much cooler, and reduce the total power consumption considerably. Moreover, this would also seem to be useful in any refrigerator or air conditioner by absorbing some of the waste heat and converting it back to electricity.
Data centers might save quite a lot on their energy bills if all their computers and cooling units were equipped with something like this.

Your smart phone might gain an extra hour or two worth of battery life.
Quantum_Conundrum
not rated yet Dec 26, 2010
To put this in perspective, if you had a pure electric engine and lets' say it is 60% efficient, such that 40% of energy is released as waste heat.

If you had the thing coated in this material, and it is 15% efficient, then you convert 15% of 40% back to electricity to re-charge batteries, which is 6% of the original energy.

So then when the battery would have been dead, it instead still has 6% of maximum charge.

Now by the time you use that 6% at 60% efficiency, you have waste heat of 2.4% of the original battery worth of energy.

Then the 15% of the 2.4% can be saved again, giving back 0.36% of the original charge.

So by the time you use 106% of original battery charge, it will still have 0.36% (about one third of a percent) of original charge remaining.

so it looks like if you can convert 15% of waste heat to electricity then an electric engine could increase it's total efficiency by about 5% to 7%, depending on the original efficiency. Huge gain.
BuddyEbsen
not rated yet Dec 31, 2010
I keep reading about this effect applied to cars, and they always talk about attaching to the exhaust. Wouldn't it be much better to get the heat at its source, grab the coolant as it leaves the engine jacket? Otherwise you are losing heat in the radiator and exhaust system.
Eikka
not rated yet Jan 01, 2011
Wouldn't it be much better to get the heat at its source


Most of the waste heat in an engine is contained in the exhaust. When the engine is operating at 20 kW, it is producing 40 kW of heat, of which about 7-8 kW is lost in the engine and the rest gets blown out through the tailpipe. The heat is "diluted" into a large volume of exhaust gas.

If all of the heat was contained in the engine block, you would need a massive radiator to keep it from melting.
Eikka
not rated yet Jan 01, 2011

so it looks like if you can convert 15% of waste heat to electricity then an electric engine could increase it's total efficiency by about 5% to 7%, depending on the original efficiency. Huge gain.


You would need to find a really bad electric motor to have it less than 80% efficient to begin with. The only ones that would gain that much are small motors in the 1-10W range where it doesn't really pay to optimize design because the efficiency gain results in neglible real savings.
trekgeek1
not rated yet Jan 05, 2011
Isn't converting heat into useful energy a violation of thermodynamics?


No, as long as you have a temperature difference. Heat is relative ( well, not really, but for this example). If you have 150 degree exhaust and a source of 100 degree air, you can extract work. Second law of thermodynamics states that you cannot heat flow from a cold region to a hot region, therefore you need the temp. differential.
Eikka
not rated yet Jan 18, 2011

Converting 15 to 20 percent of waste heat back to electricity would make a PC run so much cooler


Actually, it would have to run hotter, because the cooler it runs, the smaller a percentage of the waste heat you can recover.

A theoretically perfect heat recovery system would be able to get 15% of the energy back IF the processor was operating at 200 F, at which point most ordinary computers would fail.