A New Way to Harvest Heat

September 27, 2010 | 5 Comments

Catching the energy lost from waste heat has been on the wish list of engineers for as long energy has been understood.  Yet the functioning concept for replacing existing techniques that is both more efficient and economically competitive hasn’t arrived.

Now University of Arizona physicists are announcing they have discovered a new way of harvesting waste heat and turning it into electrical power.

The UA physicists take advantage of the laws of quantum physics, a realm not typically tapped into when engineering power-generating technology. To the uninitiated, the laws of quantum physics appear to fly in the face of how things are “supposed” to behave.

Thinking cap time: The key to the concept is using the laws of quantum physics area physicists call wave-particle duality: Tiny objects such as electrons can behave either as a wave or as a particle.

Justin Bergfield, a doctoral candidate in the UA College of Optical Sciences explains, “In a sense, an electron is like a red sports car. The sports car is both a car and it’s red, just as the electron is both a particle and a wave. The two are properties of the same thing. Electrons are just less obvious to us than sports cars.”

The research group led by Charles Stafford, Associate Professor of Physics, Bergfield lead author and Michelle Solis a student in an independent study published their paper in the September issue of the scientific journal, ACS Nano.

Bergfield adds by reporting, “Thermoelectricity makes it possible to cleanly convert heat directly into electrical energy in a device with no moving parts.  Our colleagues in the field tell us they are pretty confident that the devices we have designed on the computer can be built with the characteristics that we see in our simulations.”

Stafford backs that up with, “We anticipate the thermoelectric voltage using our design to be about 100 times larger than what others have achieved in the lab.”  These are major claims.  Using a theoretical model of a so-called molecular thermoelectric device, the technology holds great promise for making cars, power plants, factories and solar panels more efficient, to name a few possible applications.

The UA group’s concept device requires no mechanical activity and no ozone-depleting chemicals.  Instead, a rubber-like polymer sandwiched between two metals acting as electrodes can accomplish the harvesting. Different from existing heat-conversion devices such as refrigerators and steam turbines – car or factory exhaust pipes could be coated with the material, less than 1 millionth of an inch thick, to harvest energy otherwise lost as heat and generate electricity.  One could collect the heat from the refrigerator compression coil and use that energy again.  The potential from recovering the energy in lost heat is always astounding.

Bergfield and Stafford discovered the potential for converting heat into electricity when they studied polyphenyl ethers, molecules that spontaneously aggregate into polymers, long chains of repeating units. The backbone of each polyphenyl ether molecule consists of a chain of benzene rings (yes, benzene is a component of gasoline), which in turn are built from carbon atoms. The chain link structure of each molecule acts as a “molecular wire” through which electrons can travel.

Quantum Sandwich to Harvest Heat. Click image for the largestview.

“We had both worked with these molecules before and thought about using them for a thermoelectric device,” Bergfield said, “but we hadn’t really found anything special about them until Michelle Solis, an undergrad who worked on independent study in the lab, discovered that, low and behold, these things had a special feature.”

The secret to the molecules’ capability to turn heat into power lies in their structure: Like water reaching a fork in a river, the flow of electrons along the molecule is split in two once it encounters a benzene ring, with one flow of electrons following along each arm of the ring.

Using computer simulations, Bergfield then “grew” a forest of molecules sandwiched between two electrodes and exposed the array to a simulated heat source.  “As you increase the number of benzene rings in each molecule, you increase the power generated,” Bergfield said.

Quantum Effect Used to Harvest Voltage From Heat Graphic. Click image for more info.

Bergfield designed the benzene ring circuit in such a way that in one path the electron is forced to travel a longer distance around the ring than the other. This causes the two electron waves to be out of phase once they reunite upon reaching the far side of the benzene ring. When the waves meet, they cancel each other out in a process known as quantum interference. When a temperature difference is placed across the circuit, this interruption in the flow of electric charge leads to the buildup of an electric potential – voltage – between the two electrodes.
Presto! Voltage can be harvested.

Stafford said, “We are the first to harness the wave nature of the electron and develop a concept to turn it into usable energy.”

“You could just take a pair of metal electrodes and paint them with a single layer of these molecules,” Bergfield said. “That would give you a little sandwich that would act as your thermoelectric device. With a solid-state device you don’t need cooling agents, you don’t need liquid nitrogen shipments, and you don’t need to do a lot of maintenance.”

“You could say, instead of Freon gas, we use electron gas,” Stafford added.

“The effects we see are not unique to the molecules we used in our simulation,” Bergfield said. “Any quantum-scale device where you have a cancellation of electric charge will do the trick, as long as there is a temperature difference. The greater the temperature difference, the more power you can generate.”

These guys are more than a little excited.  The simulations seem to have them quite energized.  If the fabrication colleagues can hit the construction targets from the simulations and the concept proves up – well that would be quite something new and sort of a revolution inside several alternative energy fields.

The UA leader’s favorite examples are molecular thermoelectric devices that could help solve an issue currently plaguing photovoltaic cells harvesting energy from sunlight.  Stafford explains, “Solar panels get very hot and their efficiency goes down. You could harvest some of that heat and use it to generate additional electricity while simultaneously cooling the panel and making its own photovoltaic process more efficient.”

And, “With a very efficient thermoelectric device based on our design, you could power about 200 100-Watt light bulbs using the waste heat of an automobile,” he said. “Put another way, one could increase the car’s efficiency by well over 25 percent, which would be ideal for a hybrid since it already uses an electrical motor.”  This is a significant calculation.

Perhaps the most interesting part of the story is Ms. Solis in the physics department connecting to Bergfield and Stafford in the college of optical sciences, plus the hard fact that the funding was local – provided by the UA physics department – no big grant, major support or other oversight stepping on the connection of intellect and innovation.

That added with the simulation up for some proving is making cause for a cheering squad. We’ll be watching to see how this works out.  Good enough and engineering could skip an engine energy conversion part entirely.


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