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Isothermal Compressed Air

 
gardener
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In this video (http://www.youtube.com/watch?v=u0dpguSoSxE) Bill Mollison talks about compressing air with water and gravity and using it for a number of different things. Has anybody here tried this?
 
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Isaac Hill wrote:In this video (http://www.youtube.com/watch?v=u0dpguSoSxE) Bill Mollison talks about compressing air with water and gravity and using it for a number of different things. Has anybody here tried this?



I never tried it and I had not heard of it until now. I watched the video, then I cobbled together a test unit with flex tubing, valve, and a venturi. There's no reason to bother with this as the principle is simple (I was just bored), but it does work. The pressure will be determined by the water head as described in the video. The video is not clear on a couple things, and seems to get it wrong with the suggestion that the water leaving the trompe system can be at the same level as the water entering (well, at least it can't be at the same level while actually tromping, but once the air is compressed the water level in the two columns can equalize). The water has to leave at a level lower than the venturi (while tromping) and then some to compensate for the lower hydraulic head of the aerated water column, then let the water columns fill and equalize to fully pressurize the air in the reservoir. Also, I expect a modern hydroelectric plant to be more efficient... even a small one would be more efficient. For example, the speaker in the video mentions the very high efficiency of an overshot water wheel properly designed. However, if the terrain is ideal, then a system of large trompes seems very appealing (and I think a trompe could be highly efficient as well).

About compressed air for direct cooling, yes that is very convenient but not particularly efficient. It would be preferable to expand the compressed air through an expander (i.e. "engine") to drive a refrigerant compressor as this would get the cooling effect of the air expansion plus the cooling effect of the vapor compression cycle. Total cooling for the same mass of compressed air could be about 4-5 times as high, and it could also recover most of the energy in the compressed air in the form of heat at the vapor compression cycle condenser for water heating. It's also possible to devise a very efficient heat engine using compressed air by heating the air with a furnace and expanding it in stages (with incremental reheating) through the expander. This might be used to generate electricity or for direct mechanical power for whatever use is desired. The hot air exhausted from the system can then be used to support combustion thereby recycling a lot of heat into the system, and there can be further heat recovery by using the hot air exhaust for heating applications. Since the compression of the air is done elsewhere in this case, then the expander has no need of a compression cycle, and the heat energy provided by the fuel (let's say biomass) will be used much more efficiently. Quite literally most of the heat energy provided by a biomass furnace could be converted to electricity in this case, and it's easier to find ways to use the "waste" heat when there is less of it to recover. This could be a very efficient way to use limited energy resources.
 
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