EDIT: This was responding to a post that was later deleted. I've edited it to reflect that.
My entire point is that using the solar updraft approach - which is where much of this discussion has focused - has several challenges. The first of these is due to the variable power reaching the surface from solar radiation throughout the course of the day. You will need to find way to mitigate this effect (which are, incidentally, the same issues facing many renewable energy technologies). That is not a cheap thing to do, and generally people cite improved battery technology as the solution to this dilemma. That isn't really available yet on a large scale, and what is available is extraordinarily expensive. If you want to avoid using fans, I suppose those batteries will need to power something like heaters under the collectors to make up for the lower heat from the sun at various times during the day. If you have several consecutive cloudy days (as is certainly common in London), then I suppose you could just run those heaters off of power from the normal electric grid. That wouldn't be terribly efficient.
The second issue is the massive structure that would be required. Solar updraft towers are generally huge, and to create a high velocity, you will need a very large collector. That translates to a very large cost both in construction and in land acquisition, and you certainly aren't going to fit it in a major city when the collector likely takes up multiple city blocks. This means your potential market has now shrunk to include only businesses who are willing to be located in suburbs or further out. This also doesn't account for building codes, as many cities and towns likely don't want a giant tower with a huge collector surrounding it to be nearby. It is an eyesore.
Finally, you haven't really stated why you think current technology is insufficient. Believe it or not, the photo posted above of the tunnel under construction in King of Prussia looks very efficient to me. Wind tunnels very frequently include 90-degree turns like that (look up closed-circuit wind tunnels), and that can be accomplished very efficiently using turning vanes. The fans are always placed downstream of the test section (or in this case the place where people will be) because the flow you get by "sucking" air through the test section is much more smooth than what you get by blowing it through. If you placed the fans below and blew air up, you would end up with a large amount of vorticity and turbulence and you would throw the "skydivers" all over the place in the tunnel.
So, why do you think the current state of the art needs to be replaced? You may have some altruistic motivation to try and make something new and more efficient, but businesses don't think that way. They are going to see a phenomenal up-front cost with effectively zero difference from the customers' perspective (i.e. they can't charge more than a conventional tunnel), and they will just pass and build a conventional tunnel instead. Why do you think this will not be the case? Do you have any other ideas that overcome the above challenges?
