I think the only study that modeled this stated that the height...

I think the only study that modeled this stated that the height for 200MW would need to be 1000m with a diameter of 7km.

From 2005 they mention enviromission in the first paragraph trying to build one in Australia. If I have time i'll check to see if I can spot some Cahoots from back then as well haha.

I can access it via uni but you'll have to pay to read it.
http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JSEEDO000127000001000117000001&idtype=cvips&gifs=yes&ref=no

Wiki has the details though http://en.wikipedia.org/wiki/Solar_updraft_tower.