[londons_explore]

So by this same reasoning, adding a little water spray to the input of my air compressor, and a water seperator on the output, would reduce energy consumption since the water would absorb heat during the compression stroke making the compression closer to isothermal rather than adiabatic?

Do any commercial products do this?

[jandrese]

You would have to demineralize the water first or you are going to get deposits building up on the warm parts of the compressor (most of it) where the water evaporates on contact.

It would seem to be better to have a closed loop water cooling system around the compressor head, but this is a very rare feature. So many home compressors treat cooling like "LOL 5% duty cycle for you", it's pretty annoying. Most of the time the cooling fans aren't even running unless the compressor is squeezing air, so the whole thing just soaks in its own heat once the tank is full.

[londons_explore]

I'm more thinking cooling should theoretically ~halve the energy required to compress air...

For some industrial applications, electricity used to compress air is substantial, so there might be big cost savings to be had.

Even if you are pumping up bicycle tyres by hand there are benefits to cooling - by keeping the air cool, the same human muscle power can pump up twice as many tyres.