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Comment by monsterix | Hacker News Reader

Okay so it starts with normal wind at as little as 2mph (influx) and then with series of venturi tubes and processes the wind is sped up to 45 miles per hour. Then the blast is shoved through a turbine generating electricity and an exit wind at 15 miles per hour (efflux). All right.

So why not take this efflux wind at 15mph back to the top again, or even half way with some loss of course, and achieve a perpetual motion machine [1]?

Is something missing in the introduction video about this concept?

[1] http://en.wikipedia.org/wiki/Perpetual_motion

[Okay got it: The blade area/size of a fan to turn at winds as low as 2mph is quite high (and expensive) as compared to rotor blade size that would work with winds at speed 45 miles per hour. This one looks like a play on lever action of wind on fan blade area/angle w.r.t achievable influx velocity. Not too bad it's a kind of optimization, but for the introduction video that wasn't clear enough.]

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Fluids move faster in a smaller tube.

http://en.wikipedia.org/wiki/Bernoulli's_principle (Search for [18] in the Wikipedia article. Note that if z and p decrease from the air being lower and in a smaller pipe, then v increases.)

The reason to make it go faster is that you can't spin a turbine to generate electricity if the wind is 2mph, unless the turbine is really, really big.

Yep, driving wind faster is the key to lowering the size of the turbine's rotor blades.

It's an interesting optimization, but the broad design of the tophat might face challenges from adverse weather. During hurricanes or say something like 100mph wind, the upper flower has to recede to save itself. This is not the case with standard wind turbines that just rotate only faster. I am sure they must have thought through all these situations, simply considering only 600% improvement over standard installation doesn't seem like a good way to compare.

> This is not the case with standard wind turbines that just rotate only faster.

Standard turbines are not left to rotate in high winds -- if they would be, they would destroy themselves. There's a link to a video of this happening in this here: http://www.youtube.com/watch?v=_e9uRSVun30

Instead, they are feathered and locked in place. That is, the angle of attack of the blades is reduced to zero so that wind hitting them won't make them turn, and then there is a huge brake in the system that keeps them in place.

Yes, I am aware of the extreme torsional scenarios -- did mechanical/flu-mech course in engineering -- thanks for the video. In case of Sheerwind's funnelling technology a normal windy day could also become a challenge though.

They do look a bit like old horn style microwave antenna towers - and they seem to survive well enough.

Thank you for making that comparison.

There are severe economic issues with paying for a giant funnel using the wind it generates.

No one has discussed if you basically turn the funnels into cell tower antennas then you piggy back onto the existing cell phone ecosystem.

Or piggy back onto land mobile radio trunking system ecosystem, or legacy broadcast media ecosystem.

You're building a giant horn antenna. Why not make it conductive and collect some "free" cash?

Interestingly enough their pilot installation is right next door to a municipal water tower which is bristling with cell antennae.

Luckily, as the cross sectional area shrinks, the price of valves shrinks. As for pressure inside the horn theoretically bursting it, well, I think you'll be OK but if you insist, protection gear would be little more than hinges along one edge of a panel and a spring on the other, not much different than a very large turbocharger wastegate (very large as in stationary multimegawatt class diesel size)

The speedup comes at the cost of the decreasing the cross-sectional area of the flow. The input is 2mph x [big cross section], the output is 15 mph x [small cross section].

So if you take the cross-sectional area into account, the speedup does not violate conservation of energy.

There's less energy in the wind after it leaves the turbine (by necessity, energy is transferred into the turbine to make it turn). If you were to expand the air moving at 15mph to the same size as the inlet its' velocity would decrease in the same way that funneling it down increases that velocity. Plus losses from viscosity and turbulence.

Technically speaking, you could add a second stage turbine to it but you'd be getting a lot less energy out of it and it's probably not worth it. Something similar is done in the natural gas turbine world with combined cycle turbines, where a natural gas turbine is operated at very high temperatures and pressures, producing a lot of energy, and the exhaust (for comparison: the air/gases coming out of it) is still hotter and higher-pressure than atmospheric air. This exhaust is fed to a steam turbine which manages to capture some more energy out of it on its' way back into the atmosphere, where it leaves lower-temperature and lower-pressure than it did from the gas turbine.

Also, for any real-life physical system involving airflow, even if you had perfect turbines that cost zero dollars, everything creates viscous drag with the air that lowers velocity/energy/etc.

You could do this without having to "move" the exhaust back up to the top I think.

Just stack them in a line at different elevations so that one's exhaust turns into the fuel for the next one. If speed of the exhaust increases after a cycle you'd be creating stronger "breezes" as you go down each step.

It wouldn't be 'perpetual' but pods of 3-4 like this might be able to increase the yield substantially.

Then again I don't know much about this and I'm just trying to comprehend it all.

if you output one into another, the resistance would back up into the first one and reduce its output, or else the pressure would push the air backward out one of the other tubes of the 2nd device and not spin the rotor.

Well the "pod" could be set up with multiple levels so that every subsequent level receives a mixed flux of natural wind and the ejecta of the previous level, no?

Couldn't you space them out a little and rather than backing up it would expand and flow like regular wind? (serious, just curious)

When you say space them out do you mean open to the air?

If so then the wind will just escape into the easy air rather than push a turbine.

If in a pipe then spacing makes not difference.

This is doable, and it's done in the natural gas turbine world (look up combined cycle turbine, essentially you hook up a lower-power steam turbine to the exhaust of a natural gas turbine).

The main problem is that turbines are expensive and lossy, and when the main turbine is already going off of a relatively small amount of wind energy, the amount of energy you could produce from its' output is probably very small.

The Venturi effect makes speed inversely proportional to pressure - I'm assuming there isn't enough pressure [as the turbine generator harvests most of it] to take the wind back up into the intakes.

Also I don't know how much gravitational forces are at play with relation to winds - but bear in mind wind is nothing more than the movement of gasses and as such can be treated as a fluid, so I'm assuming gravity aids the Venturi effect, but must be fought against if you want the wind to go back to the intake.

Two reasons:

1) Your 15mph wind would slow down the 45mph wind going in.

2) Retarding the exhaust in any way will reduce the power output of your turbine.

take this efflux wind at 15mph back to the top again

you need energy to do that, so perhaps by the time 15mph wind gets all the way up to the tower influx, it's less than 2mph

This is not the reason why it won't work. The 15mph flow will be much thinner than the original input flow, so feeding it back into the top will not have the same effect.

There's also the factor that pushing the turbine blades takes energy. Otherwise, you could just take any turbine based generator system, and put hundreds of turbines in a row!

I'm no expert, but the area subject to the wind seems quite different, the total amount of area moving is the same, it just goes through a smaller hole.

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