Heat engines gain quantum afterburner

By Eric Smalley, Technology Research News

Considering the heat a car engine gives off, it's pretty clear that a lot of the energy it produces goes to waste. Even the hot gases produced by the most efficient engine possible are a significant source of wasted energy.

The trick to tapping these hot gases is using them for something other than mechanical work.

A physicist at Texas A&M University has figured out how to use the hot gases produced by an Otto engine, a piston engine similar to those in automobiles, to drive a laser. The laser produces energy from the hot gas and therefore increases the total amount of energy derived from the engine. The theoretical work can be applied to any engine that produces hot gases, said Marlan Scully, a professor of physics and electrical engineering at Texas A&M.

Heat engines use heat to expand a gas by causing its atoms to move faster. The expanding gas, contained within a chamber, pushes a piston, which, in turn, rotates a shaft.

This classic setup ignores the quantum nature of the atoms, however. "Atoms are not just little hard balls, but actually have internal structure," said Scully. "The internal energy levels that the electron occupies inside the atom are also a source of energy. The electron can fall from a high state to a lower state and emit light. That's how we have light. It's going on in the sun... all the time," he said.

In a laser, the photons emitted by the atoms of the laser material are trapped between two parallel mirrors and bounce back and forth, which causes them to hit other atoms that in turn release additional photons. This amplifies the light and produces the familiar intense, monochromatic beam.

Ordinarily, starting a laser requires pumping energy, usually electricity, into the laser to charge up, or excite, more than half of its atoms. "You've got to have more atoms in the excited state than the ground state because atoms in the excited state emit light but atoms in the ground state absorb light," said Scully. "You've got to have more emitters than absorbers on average in order to get a net laser action."

Scully and other researchers previously found a way to start laser action without first exciting more than half of the atoms in the laser. The method, lasing without inversion, uses a second laser to block the majority of atoms from absorbing light, which allows laser action to occur more readily in the remaining atoms.

That work led him to look at the atoms in a hot gas as a potential laser source. The atoms in a hot gas like the gas in a heat engine are in the lowest energy, or ground state. Exciting more than half of the gas atoms would take more energy than would be produced by the laser. Using lasing without inversion, however, requires less energy, allowing for a net gain.

The quantum afterburner follows the laws of thermodynamics, which dictate that you can't get any more energy out of a perfect heat engine, according to Scully. When he applied his theory to the Carnot engine, which is a mathematical representation of a perfect heat engine, it did not extract any additional energy, he said.

But even the most efficient real-world heat engine is not perfectly efficient, and so can produce at least a little more energy using the quantum afterburner. The question is how to develop practical applications.

"Maybe it would be effective to have a different kind of mechanism for powering a nano engine," said Scully. For example, the laser could trigger chlorophyll molecules in cells, causing the cells to do useful work, he said.

Lasing without inversion could be put to use within the next couple of years, said Scully. Using the lasers to power nano engines is "way off in the future," he said.

The research was funded by the National Science Foundation (NSF), the Office of Naval Research (ONR), the Air Force Office of Scientific Research, the state of Texas and the Robert A. Welch Foundation.

Timeline:   <2 years; Unknown
Funding:   Government; Private
TRN Categories:   Applied Technology; Physics
Story Type:   News
Related Elements:   Technical paper "Quantum Afterburner: Improving the Efficiency of an Ideal Heat Engine," Physical Review Letters, 4, 2002


March 6, 2002

Page One

Interactive robot has character

Atomic cascade broadens laser

Biology harbors hidden complexity

Heat engines gain quantum afterburner

Nanotubes branch out


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