Air at ambient temperature is pulled through a counter-current system in a tower, receiving heat from a source of waste heat, so that almost all the waste heat is passed into the tower's air, with the air at the bottom of the tower being the hottest (and the original "fluid" - whatever it was, is coolest at the same point).
Tethered air filled nylon bags, similar to wwii parachutes, are receiving the heated air, and pulling upwards on a tethered line, which raises a weight.
Once the weight reaches a certain height, it is released running a pump or electric generator.
It is assumed here that concentrating the energy along one single tethered line is a process efficient enough to get a good amount of useful work from waste heat at 55-75C.-- pashute, Dec 11 2012 The original Hullaballoon http://bz.pair.com/fun/hullaballoo.htmlFrom the finest halfbaker ever, remembered on the web [pashute, Dec 12 2012] As long as it is possible for heat to flow from a higher temperature toward a lower, it is possible to extract some energy from that flow. Don't expect to capture a lot, though.
In this case, the rate at which air inside the balloon cools, through contact with the gas bag and the cooler air outside, will affect how much lift you can get.
Most hot-air balloons dump a lot of heat quickly into the gas bag, to overcome that sort of heat-loss. You might have trouble getting any lift at all.-- Vernon, Dec 11 2012 The bags volume must be huge for this to work-- piluso, Dec 11 2012 This is essentially a batched, or packeted, version of a thermal updraught generator. With those, the air moves while the balloon (canopy and tower) remains stationary.
I would suggest that the problems of efficiency and scale would be similar (or worse), and the manufacturing costs greater, so you are unlikely to break even.-- spidermother, Dec 12 2012 [+] different principle of course, but it would resemble a sideways hullaballoon.
Rather a lightweight Stirling engine.
The weight is non-sequitur as long as you have at least two balloons equally spaced.-- FlyingToaster, Dec 12 2012 random, halfbakery