It seems to me a 1 km hight for a solar tower is a bit excessive. <http://www.wired.com/news/technology/0,1282,66694,00.html?tw=wn_tophead_2>
The tower has to be insulated to be effective, otherwise the air density returns to that outside outside the tower and no additional bouyancy is achieved. The tower runs on the bouyancy of hot air. Also, a minor point, but air does get thinner as the tower gets higher, so there are diminishing returns with height. The density is about 10 percent lighter at 1 km than at sea level. It might be more effective to build a lower altitude but better insulated tower. It may also be effective to use a shorter insulated tower but "amplify" the stored heat using heat pumps or auxiliary energy of some kind. If the heat pumps are located inside the base of the tower none of the waste heat for doing so would actually be wasted, except that which is lost due to the inefficiency of the tower itself. Heat pumps, oddly enough, exhibit a COP greater than 1. In appropriate areas a solar tower might be coupled with a wind farm which merely feeds energy in the form of heat into the tower. This would avoid the need for complex circuitry to manage the power from each windmill and get it onto the grid, and help keep the solar tower runnning at night and on dark days. Regards, Horace Heffner
