Taylor air compressor - free air!

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Pumping water is endemic to operating deep rock mines, and pumping out a little extra is not a big problem. But it costs more energy to pump out the extra water than is obtained from the compressed air.

Just think about the drawings of "perpetual motion machinesa", in which a waterwheel operates a pump, which pumps water back to the top of the waterwheel, which operates the waterwheel, etc.

I'll agree, there is no such thing as a free lunch. The compressed air is "free" in this case only if one neglects to consider the initial construction cost of the facility along with any maintenance costs throughout it's useful life. These are significant investments that cannot be ignored in evaluating the "cost" of the compressed air. However, I see no pumps involved in this facility, so any costs associated with pumps in the mine itself would appear to be not relevant to the cost of the compressed air.

Rod

I grew up in Houghton, Michigan. This is right in the middle of the area where copper was mined extensively. With the possible exceptions of the Cliff mine, and maybe the Quincy mine, there were no readily available waterfalls or other convenient combinations of flowing water and geographical height differences to exploit in this way. So that's why I figured the air produced by such an arrangement, at least in Michigan, was not "free", since they'd have to dump water down a mine shaft to get the energy.

The deepest copper mines ended up at 5000+ feet below the surface. Since the area is surrounded by Lake Superior, you can easily imagine how much water they had to pump out of these mines, and how much energy they had to spend to keep them de-watered. The cost of energy to run air compressors was quite small compared to the cost to run the pumps.