i have thougth of similar concepts.  the problem is the deeper the water the higher the water pressure and the more difficult to bring air down.
nice idea though.
keep trying

@timewarp,

You are a genius, but not with air, with water.

In the above ground world there are solar chimney power generation plants but the efficiencies are low unless they have very high chimneys.

With your idea modified we could put solar power down an evacuated mirrored wall tube.

Then we have a bit of ocean 5,000 meters deep and put in a big bore tube with a water turbine in the tube. The sun heats the water with a starting temp of say 4C (maximum density) which expands and rises through the tube and does work through the turbine.

Now I have not done the maths here but it seems offhand that the conversion efficiency of the solar energy could be quite high, perhaps 20% which is way better than solar chimneys on the ground (just 4% for 1,000m high). When the water get to the top excess heat could be extracted for other puposes such as greenhouses given that you would also have the ability to partially evaporate sea water and condense it as fresh water in a green house. So you could have fresh water and electricity from a simple relaible device.

Mind you the number of 5000m deep areas of water near to shore is probably limited but it could lead to a floating island with water, power and food. Ocean front land would sell for at least a million a block!

What is interesting is that ideas lead to ideas.

Regards

Phil