I think you're right, the only way this could really generate net energy is if they had a continual supply of water. After a bit more research, the original concept called for these to be placed on a hot dry coast, and use a constant supply of seawater. This comes with it's own problems, but also with some added bonuses. You could reliably recover a small fraction of the water from the moist air at the bottom for relatively cheap, so it would act as a desalination facility as well.
One issue that I can see as a big problem though is the downdraft itself. The turbulence at the top of the tower would be murder to any birds that got to close. A windmill might take out a few birds here and there, this thing could suck in a whole flock all at once.
Kingfisher: "The temperature gradient that is being exploited here is between the hot air and the cool water, but in order for the water to stay cool they would need some sort of heat sink."
What bugs me is that this is an artificially created temperature difference. I think that, whatever cooling you achieve at the top of the chimney, you give up at the bottom when you try to reverse the process.
All other mechanical energy generators exploit some naturally occuring energy difference and shrink that difference in the process. That shrinkage is equivalent to the energy extracted by the generator. I don't see this here because you've got hot air all around with no difference to level out, if I understand their idea correctly.
Unless they have a permanent water supply and generate energy by evaporating it. But then they can't reuse the water.
[ - You pump up water. Costs energy. ]
[ - You evaporate the water. Costs energy.]
The energy comes from the hot dry air.
[- Air takes on the water. Now, in what way is this better than just letting the water run down the tower? What role does the air play here?]
The water takes heat from the air in the evaporation process, causing the air to cool.
[ - Heavier air sinks down. What is significant here? Does it heat up on the way down?]
As the air cools it creates a pressure gradient, the hot air above the tower pushes down on the cool air inside the tower, creating a downdraft.
[ - Some process is supposed to extract the water from the air. How does this work? How is the energy balance between spraying the water on top and, like, freezing it out on the bottom?]
This is the tricky part, and they don't really explain it. There are a number of ways to extract water from air, but all of them would require some energy, and none of them could get all of the water out.
[ - After all the air has been processed for the water, enough kinetic energy is supposed to be left that some wind turbines can produce a net gain.]
[ Air and water on top, Air and water at the bottom - which part is different from a perpetuum mobile? What energy gradient gets used? ]
The temperature gradient that is being exploited here is between the hot air and the cool water, but in order for the water to stay cool they would need some sort of heat sink. There are a number of heat sinks they could potentially use, but none of them are freely accessible in the dessert.
In short, there are some unanswered questions, but the process does comply with thermodynamic principles. My concern is that this relies too much on having an adequate water supply, and I question their ability to recover the water from the air at the bottom.
Call me dense but I don't understand how this is supposed to work.
- You pump up water. Costs energy.
- You evaporate the water. Costs energy.
- Air takes on the water. Now, in what way is this better than just letting the water run down the tower? What role does the air play here?
- Heavier air sinks down. What is significant here? Does it heat up on the way down?
- Some process is supposed to extract the water from the air. How does this work? How is the energy balance between spraying the water on top and, like, freezing it out on the bottom?
- After all the air has been processed for the water, enough kinetic energy is supposed to be left that some wind turbines can produce a net gain.
Air and water on top, Air and water at the bottom - which part is different from a perpetuum mobile? What energy gradient gets used?
An interesting idea. I'm not sure how it compares to a solar updraft tower, but I could see applications of using this for providing cooled air for buildings.
This is effectively a cooling tower, like those used to remove heat from cooling water in power plants. Here heat is being removed from the air, but the exact same principles apply. Only here they are also extracting useful energy from that heat, which is neat.
The biggest apprehension I have about this is the use of water. Yes, most of the water can be recovered and reused, but you are not going to be able to recover it all, so the tower would consume some water. Given that these things will work best in places where there is very little water, I could see this as a potential problem.
Something's counter-intuitive about this concept, but I can't put my finger on it.
A hot air balloon rises because the air it contains is less dense than the surrounding air. A cold air balloon released at altitude would sink for the opposite reason.
When either of these balloons move, the surrounding air has to move around the balloon to replace its volume or there would be a vacuum. It seems like air movement in this proposed tower would be a lot like the dynamics of a lava lamp. You would have cool air bubbling down and warmer air bubbling up, but I don't see any reason the cold air would accelerate out the bottom. Rather my mental simulation has it deccelerating as it approaches the bottom and mixing until equilibrium is achieved -- or until more water is pumped to the top.
After seeing so many "break through" energy generation schemes come and go, this one does not pass the sniff test for me. Given the hyperbolic language on the site describing the process and that their tower design appears to be upside down (hyperboloid towers like this are designed to increase wind speed at the top), I predict scam. Still would like to see them build it to see what happens, just not with my money.
"...at speeds up to and in excess of 50 mph..." "...the average hourly output per day..."
I hate bad writing.
I think it is a great way to diversify energy production. Given that it works best in an area (hot, dry) where it's primary input resource (water) is scarce, I'm curious how the production cost per KWh compares to other technologies.
It will be interesting to see, since it appears to be funded. But I have serious doubts. 50 mph winds in sufficient volume to drive a dozen wind turbines to produce over 1000 MWH per day? By mist-cooling dry, hot air? It's not beyond the realm of plausibility, but their 3D graphic suggests that this is entirely hypothetical, and I have doubts as to how much engineering and science is behind that theory.
And of course, the obvious problem is that it requires vast amounts of water in places where there is little. It also seems that it would take vast amounts of energy to pump vast amounts of water to a significant height.