Downwind Faster Than The Wind

The speed of wind seems like a plausible upper bound on the speed of a vehicle powered by that wind, and David Mackay thought so even though he knew of wind-powered vessels which can sail directly upwind. But he is delighted to discover that he is wrong (here is proof and explanation).

His reaction and reflection on his mental processes is interesting:
What intrigues me philosophically about the wind-powered-travel expositions is that it reveals how fragile and weak "understanding" can be: I thought I understood wind-powered travel, and I already knew about wind-powered vessels that can sail directly upwind (eg, Revelation II, pictured). But I got the answer to the question "is DWFTTW possible?" wrong! - even though the principle by which upwind travel works is just the same as the principle of DWFTTW travel. So it seems that when I "understood" upwind travel, what I really did was append to my stack of physics heuristics another heuristic, permitting upwind travel; I didn't add a piece of knowledge that was capable of working in new situations.
I find this encouraging not because the specific discovery is cool (although it is), but rather because seeing “obvious” truths overturned bodes well for the prospect of technological energy advances that we have not imagined today. A lot of economic analyses of renewable energy potential work from back-of-the-envelope uppers bounds on the resources available given “reasonable” tech advancement; while they conclude, I think rightly given that starting point, that renewables won’t make up a major part of energy production for decades, this gives reason to believe that “unreasonable” tech advances could help us get closer.

Now, I wonder if we could solve the intermittency problem with wind power by engineering the equivalent of a wind turbine atop a giant treadmill?

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