A solar updraft tower <link> is essentially a large chimney
with a turbine in it. The turbine is powered by hot air
rushing up the chimney. The hot air is made by
constructing a large greenhouse around the base of the
tower and using solar energy to heat the air under it.
A downdraft energy
tower <link> is sort of the opposite.
You have a large tower, and water is sprayed into the top
of it. The water evaporates and cools the air inside the
tower. This increases density and the air falls, through
turbines in the base of the tower to generate power.
Why not do both? The net result of the first tower is a lot
of hot air at the top of the tower. Why not build a second
tower around* the first. Solar heating pushes air up the
inner tower, then water is sprayed around the periphery
causing the air between the first and second tower to fall,
where it will rush down and out over the top of the solar
collector, which thermally & physically isolates the two
countercurrent air streams.
Now you can drive turbines on the way up, and the way
down. A clever addition would be to have the updraft air
entrance facing into the prevailing wind, and the
*or inside, I don't really mind