I like the three-tube concept, with the lifts on the outer two. Possibly showing lifts moving up and down within the cages, so it's not just one big platform lift, but more like a high-rise with multiple elevators. They're all running at different positions to show people constantly moving around. Like you guys, I'm a little stumped about the fan. On a planet with little or no atmosphere, the fan would still be understandable. It's a means to dissipate excess heat. You could also go with some form of radiators. On hot planets, it would still make sense because you're removing the heat from the colony, not the planet. Even on a hot planet this would still work. Not as efficiently, but it would work. As long as you output temperature is less than the surrounding area, you will get a cooling effect. It's planets that have extremely high atmospheric pressure AND high temperatures that you lose.