This is, verison 7, likely my final attempt to turn a PC fan using rising hot air heated using a fresnel lens. It's almost identical to version 6 except that I've added an actual tower in the form of a 5' long, 3" diameter ABS tube.
The result was that the tower caused the paper turbine to turn faster than without the tower but it still couldn't turn the PC fan.
As a sanity test I blew into the paper turbine and the PC fan with three different air flows each (see photos below): gently, medium, hard. Even blowing gently the paper turbine turned faster than it ever did the the solar tower. Only by blowing the hardest did the PC fan turn and in comparing how fast that same blow strength turned the paper turbine with how fast it turned on the solar tower, there's no way this solar tower can ever turn the PC fan.
Video of the mini solar tower
Here's a video of the mini solar tower testing in action, including the blow test comparisons. Enjoy!