MadSci Network: Engineering

Re: Is there a generator which uses CO2 phases to make electricity?

Date: Tue Aug 31 08:45:35 1999
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Engineering
ID: 935875569.Eg

Is there a generator which uses CO2 phases to make electricity?

My sister and her husband have been (in the wake of Y2K-mania) asked to invest in a company that produces what she said they call "CO2 generators" that produce electricity from the energy in "phase shifts" of CO2 (ie. liquid-gas). I didn't think there was much energy in CO2 bonds. Are they being led down the primrose path? The company claims that these will be sold for $200 each and can be used to run an entire household. As a biologist, I am also a skeptic. Should they zip their purses and run?

Yes. They should run (not walk) to the nearest office of the Better Business Bureau and report these frauds.

You are correct; there are not very strong forces between molecules of carbon dioxide; in fact it is perfectly non-polar and can't liquify at pressures below about 4.1 atmospheres. It is possible to obtain energy from the condensing or freezing process; melting or boiling sucks up energy. But the catch is that pesky old second law of thermodynamics, which I usually teach to my students this way:

The First Law says you can only break even, you can't win. The Second Law says you can't even break even.
To make a long story short, if you could obtain net energy from a forced phase change, why isn't your home refrigerator powered by the energy released by water freezing in your freezer? Forced phase changes take energy, and even if the phase change releases energy (as does the change from gas to liquid, or liquid to solid) it takes more energy to force the phase change.

Their idea sounds to me like that old bugbear (which thermodynamicists call "a perpetual motion machine of the second type"), a reverse heat engine. That's one which obtains energy by taking heat from something cold and putting it into something hot, as if the flame on your stove got hotter while the water in the pan froze. To accomplish something like that you need a useful little sprite called Maxwell's Demon, who picks and chooses among molecules and only allows the hot ones to pass in one direction. Unfortunately if such a thing existed, it would use more energy than could be obtained by the selection process!

As a matter of fact, your refrigerator is a version of Maxwell's Demon. It removes heat from a cold place and dumps it into a warmer place.

  Bluffton College
Maxwell's demon:
A hypothetical intelligent being (or a functionally equivalent device) capable of detecting and reacting to the motions of individual molecules. It was imagined by James Clerk Maxwell in 1871, to illustrate the possibility of violating the second law of thermodynamics. Essentially, this law states that heat does not naturally flow from a cool body to a warmer; work must be expended to make it do so. Maxwell envisioned two vessels containing gas at equal temperatures and joined by a small hole. The hole could be opened or closed at will by "a being" to allow individual molecules of gas to pass through. By passing only fast-moving molecules from vessel A to vessel B and only slow-moving ones from B to A, the demon would bring about an effective flow from A to B of molecular kinetic energy. This excess energy in B would be usable to perform work (e.g., by generating steam), and the system could be a working perpetual motion machine. By allowing all molecules to pass only from A to B, an even more readily useful difference in pressure would be created between the two vessels. About 1950 the French physicist Lon Brillouin exorcised the demon by demonstrating that the decrease in entropy resulting from the demon's actions would be exceeded by the increase in entropy in choosing between the fast and slow molecules.

This information was obtained from (now defunct).
See's_demon as an updated link.

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