Ed, just a thought about the warm air. I can't be certain, but I think you may need to keep the air blowing through the tube as close as you can to the outside air temp so you don't end up with a fogged lens. Or, maybe warm air but very dry warm air. I don't know if this is possible but I think any humidity would be a detriment.
I think you're right - that's why we would not use any heat source. The air would only be "warm" in that it would be warmer than the outside temperature - it would not actually be heated in any way. In theory, we would draw it from inside the top of the tower - this air is still pretty cold and dry in the winter. The top of the tower is not heated. It would be difficult to use outside air in that this would include the moisture that we're trying to avoid in the first place.
Hope that helps explain things better.