Some of you might have heard that in Australia they are planning to build a solar thermal wind tower which is a greenhouse with a radius of 3.5 kilometers with a stack that is a kilometer high. Why? Because the heat collected under the greenhouse roof will create a draft that goes up the stack, and on the way out turns a turbine to generate couple hundred MW. Here is the problem, besides the construction costs and problems, the thing is a maintenance nightmare, and has to be far way from people.
Biggest problem with this new “clean” energy source is that it will have to run at full output 24/7/365 for ~6,600 years just to make back the energy it took to build it. Concrete requires a massive amount of energy to make! Also, isn’t it a bad idea to pump huge amount of particulate matter high into the atmosphere?
If you took less then half the money, and drilled a couple holes in the ground say 15,000 feet down and pumped water into them you can extract a lot more energy. (The Earth is hot 24/7, but the sun is only up about half the day.) If you go that deep you don’t need a geothermal hotspot and because wells for natural gas routinely go below 15K feet the technology is slightly more practical than chimney a kilometer high (the tallest free standing building is only 553 meters tall). If 15,000 feet deep seems like too much you can drill the wells only a couple of thousand feet and use a closed loop system which boils something other than water and turn a small turbine. Even better do a little horizontal drilling a couple of hundred feet down and you could tap into the fact that close to the surface it is cool and use that to condense the working fluid before you inject it back in to the ground.
If you are dead set on induced draft thermal wind generation then I suggest instead of a giant greenhouse and stack, a smaller stack with a thermal mass underneath it, and a mirror array focusing the light on the mass. Unlike direct solar thermal generation, which melts salt or something to boil water this wouldn’t require that level of precision engineering, so cheaper mirrors can be used and the material wouldn’t be subjected to as intense heat. The greater heat from the focused mirror array hitting a thermal mass would reduce the need for a really tall stack since the temperature differential would be much greater. Plus, the thermal mass would carry production through the evening peak energy demand. The production of energy would also be much more responsive to demand since as solar irradiation increases, load on generators increase (mostly A/C loads), but more irradiation means more heat into the thermal mass so more temperature differential. Also greenhouse with a surface area of nearly 40 square kilometers makes it hard to not lose a lot of heat, so cold but sunny days will see a significant reduction in energy production. A mirror array and a thermal mass would remain efficient during the winter since the heated area is much smaller and cooling the thermal mass is the point. Even better my design allows the heating to occur higher in the stack, so the bottom of the stack can be built stronger and serve as a place to preheat the incoming air, where as in the greenhouse version the bottom of the stack is open since that is where the draft is induced. I still think solar thermal wind thing is a bad idea, but I think my idea better then what they are building in Oz.
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