After submitting a similar idea (see 'Making Money from Noise' in the Docks) this idea was pointed out to me. The only real difference between the two is the level of implementation detail I included. Since this idea made it to the list first I'll just add a few of the possible implementation methods to it so everything will be in one place.
1. The following link describes the auxilliary power units that are
currently being used on 747's (down near the bottom).
Equipment capable of producing electricity similar to the way
dynamic microphones do could be added to these power units to
reduce their energy consumption, saving the airlines money
either by allowing for the usage of smaller/lighter APU's or
reducing the amount of fuel required to run the APU's.
http://members.shaw.ca/translink/ac-7474.htm
2. The following link describes a study that was done to see how
much money people would be willing to pay to get rid of windmill
farms. It turned out that the closer they lived to them the more
they were willing to pay. The reason for this was the amount of
noise produced by the windmills. I'm including the link just to
verify that the windmills do indeed produce noise as a byproduct
and that being the case, the efficiency of the windmills could be
increased by adding a device capable of converting that noise
into electricity (again based on the way dynamic microphones
work).
http://www.akf.dk/eng/wind0.htm
3. As far as implementing it in a factory setting goes, personal
experience has shown me that some of them can be quite noisy.
I don't have a link showing how loud the ones I've been in were
but here's a chart of average noise levels from different
sources. It was taken from the link below it. (According to the
chart it looks like elevated trains might make another candidate
for one of these energy-producing devices.)
Aircraft: Near the Plane 140 dbA
Aircraft: Residence Near Airport 80 to 90 dbA
Elevated Train 120 dbA
Industrial Plant 100 dbA
Auto Horn at 25 ft. 100 dbA
Jazz Band 85 to 100 dbA
Loud Stereo 90 dbA
Person Shouting 80 dbA
Noisy Household 75 dbA and up
Truck at 25 ft. 75 dbA
Busy Street at Curb 70 dbA
Dense Traffic at 200 ft. 65 dbA
Office Conversation 60 dbA
Average Conversation 50 dbA
Quiet Household 40 dbA
Quiet Conversation 25 dbA
http://www.usg.com/Design_Solutions/2_3 ... anning.asp