My wife was using one durring the winter and bad weather, nice warm sunny days we hang out also when we can. Sometimes with the humidity though it seems like the clothes actually get wetter around here!

Anyway, when the heating element breaks in our dryer (as it has several times) the dryer tumbles normally but of course no heat. I just got a really nice GAS dryer for about $10 at a yard sale. Was so cheap because it is set up for natural gas and all  we have available in this area is propane and nobody wanted to mess with changing it over. I think the gas companies do it free though so go figure.

Anyway again, with either dryer they will tumble fine without heat, and the duct for air intake is easy to get to really. The gas one points to the front so I have a little box I have to take off and turn to the rear, on this one it was easy. Thats where the burner was. On the electric one there was a nice little duct going up the back that also has the heat element in it. Take the wires off the element and insulate them well so they can't short out, then just run a duct to the bottom of that duct.

For a heat source anything will work that heats air as it passes by :)
Build a solar collecter box for those hot sunny days? Build a small trash burner/ wood burner. Got garbage? Take out the trash to burn when drying clothes, use the heat produced to heat a seperate chamber of fresh clean air. As mentioned you could also use the heat from the gennie exhaust if it's running, but drying on 110v you may not need it then.

Another thought if you can, build one of those producer gas plants to run a gennie on, then when you burn wood for fuel for the gennie you can heat water for washing and hot air for drying too, and still run the gennie on the cooled gas. Get all three in one shot :)

In all dyrers I have looked at myself, the blower runs anytime the drum is tumbling. The heater makes no difference. As a matter of fact I think they are almost all built onto the motor shaft that turns the drum, so if the motor runs the air moves. No need to worry about it only working like when heat is present as a friend thought, they move air always. So however you do it, all you need is 110v for the motor and timer normally and some heat from any source, plus a small duct pipe/hose to connect to the heat source.

The only things I would even worry about is if you use some sort of fire to heat the air make sure it can't get too hot! You want to heat the clothes, not bake at 550 for an hour or till melted :(
As mentioned by another, dyrer lint is flamable, but that is on the exhaust side not intake, and you won't be ducting that to the heat source.
An you don't want smoke or fumes, but that's mostly easy, just don't lay the open end of the pipe into the fire :)

We burn our trash here and I am building a producer gas plant, so I am planning to use the heat from those too sources for alot of things.
I would compost the trash, but I get alot of it that's not well composted so I incenerate it (burn baby burn) cleanly. Darn packing peanuts!

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nothing to lose

What I did with my old electric dryer was plumb it off my wood burning furnace. Seal up the inlet vents, holes etc. cut a six inch diameter hole init. Put a starter duct collar on, then pipe it to the burner's hot air plenum. Only drawback the furnace is design around a closed filtered force air system had to put a damper inline of ductwork to prevent hot air blowing through the dryer when not in use. Wife called me cheap I've called it practicable. She slowly coming around.:>)
Next goal is to make something like nothing to lose's a solar flat plate collector for when the solar clothes line isn't fast enough.