Peter Thiel, a favorite venture capitalist of mine, just paid $300,000 to Louis Michaud, a Canadian inventor on the picture below who plans to build artificial tornadoes – the so-called [atmospheric] vortex engines: Wikipedia, Michaud's web – that may supply us with lots of energy.
This idea surely sounds provoking at first – way too close to a description of a perpetual motion machine – but I am already in a different stage in which I tend to think that this most elementary criticism is unjustified. However, it is still not clear how ambitious a change in the energy sector is being promised here.
The basic underlying mechanism is said to be the same as for a solar chimney. In the troposphere – atmospheric layer between the surface and the tropopause 10 km or so higher – the temperature generally decreases with the altitude. This gradient isn't far from the "adiabatic lapse rate". What is it?
The warm air heated from the Earth's surface at the bottom wants to go up because at the same pressure, it has a lower density than the cooler air. As it goes up, it expands, its pressure decreases, and so does the temperature. The calculation of these changes is enough to see that the temperature decreases by 6.5 °C per kilometer of height. Note that it's possible, albeit a bit counterintuitive, that the atmosphere may sustain a quasi-equilibrium with non-uniform temperatures. There's no paradox here, however: these non-uniformities are powered by the constantly added heat from the Sun and, indirectly, from the Earth's surface.
This behavior stops above the tropopause, in the stratosphere. The stratosphere is stratified – "separated" into horizontal layers that keep their altitude and don't mix. It's possible because the air temperature is actually increasing with the altitude in the stratosphere – because it's being heated by the Sun – so the cooler (and therefore denser, heavier) air at the bottom has no reason to go up.
Back to the troposphere. The mixing of the air that enforces the lapse rate also brings some circulation which is responsible for many kinds of weather phenomena and winds – and tornadoes are actually the most typical ones. The processes and gradients occurring in a tornado are pretty complicated, however, and your humble correspondent isn't able to evaluate all these things.
It's conceivable that the artificial tornadoes could replaces chimneys and increasing their efficiency by dozens of percent. When it comes to this "evolutionary" improvement, I am willing to bet that the existing chimneys are far from optimal, so some tornado-inspired improvement is likely to exist, whatever it is. However, it's plausible that one could do better (and it seems like the folks actually claim that this is the ambition here): the modestly elevated temperatures on the surface could be enough to play the role of the "hot burning coal" in a conventional power plant that powers the flow so if one could extract the mechanical energy from the man-made tornado, it could be energy obtained "almost for free".
It would be nice if someone told me whether this is physically possible at all. And if it is, whether there's an upper limit on the number of tornadoes or the energy per unit time that could be extracted from this truly attractive hypothetical source. ;-) For example, the restriction may arise because the man-made tornado would lower the lapse rate and would stop working when the lapse rate decreased beneath a certain threshold.
I have asked the same question at Physics Stack Exchange and there's already an interesting answer over there.