Monday, October 11, 2010

Converting Waste Heat into Electricity

    Converting Waste Heat into Electricity
  • A new device that captures waste heat and converts it into electricity could increase the efficiency of solar panels, car engines and power plants—all while rendering CFCs and complicated turbines obsolete.
  • A new device that captures waste heat and converts it into electricity could increase the efficiency of solar panels, car engines and power plants—all while rendering CFCs and complicated turbines obsolete.A few weeks ago, I wrote about a new technique by which energy can be harvested from the air. Now scientists at the University of Arizona (UA) are looking to harness power from waste heat with a theoretical device that could reduce fossil fuel use, increase factory efficiencies and make chlorofluorocarbons (CFCs) obsolete.
    Current methods of heat conversion, such as refrigeration and steam turbines, require complex mechanics and ozone-depleting chemicals. In the new theoretical model, called a molecular thermoelectric device, a rubberlike polymer is placed between two metals, which serve as electrodes. The material could prove to be an inexpensive and environmentally friendly alternative to traditional heat-conversion devices.
    The ringlike structure of the molecules causes electrons to interfere with one another and build up voltage (source: Justin Bergfield, University of Arizona).
    "Thermoelectricity makes it possible to cleanly convert heat directly into electrical energy in a device with no moving parts," says Justin Bergfield, a doctoral candidate in the UA College of Optical Sciences. "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."
    The scientists' work relies on basic laws of quantum physics, particularly wave-particle duality. In this law, tiny particles, such as electrons, can exist as both particles and waves.
    "In a sense, an electron is like a red sports car," explains Bergfield. "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 team applied the laws of quantum physics to their work with polyphenyl ethers, molecules that spontaneously amass into polymers. Polyphenyl ether molecules have backbones of benzene rings, which form from carbon atoms. The unique chain-link structure of these molecules allows electrons to easily travel.
    "We had both worked with these molecules before and thought about using them for a thermoelectric device," Bergfield says, "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. As you increase the number of benzene rings in each molecule, you increase the power generated."

    In the specially designed molecules, electron waves—passing through the benzene rings at different intervals—cancel each other out through a process known as quantum interference. Interrupting the electric flow by placing a temperature difference across the circuit leads to a build-up of voltage between the two electrodes.
    "We are the first to harness the wave nature of the electron and develop a concept to turn it into usable energy," states Charles Stafford, an associate professor of physics at UA.
    Charles Stafford (left) and Justin Bergfield demonstrate how electrons flow around a benzene ring (Daniel Stolte, University of Arizona). 
    The new device requires no moving parts and is self-contained, easy to maintain and inexpensive to manufacture compared with current technology.
    "You could just take a pair of metal electrodes and paint them with a single layer of these molecules," Bergfield explains. "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.
    "The effects we see are not unique to the molecules we used in our simulation," Bergfield continues. "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."
    The scientists envision many possible applications for their device, from solar panels to car engines. "Solar panels get very hot and their efficiency goes down," Stafford states. "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.
    "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 says. "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."
    The scientists' findings were published in the September issue of the journal ACS Nano. Funding was provided by the UA physics department.

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