|MadSci Network: Engineering|
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.
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