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Over the years we've commented on new developments in renewable energy in this forum. One I don't recall seeing before is the 'bladeless wind turbine', described in this article in the Independent from a couple of months ago:
This machine harvests wind energy using an array of oscillating vertical sticks. Advantages of the bladeless wind turbine are that the vertical sticks can be closer together than the normal wind farm and it avoids the expensive propeller-type feature (obviously inspired by old-fashioned windmills) of the conventional wind turbine. I suppose another advantage, not mentioned in the article, is that the bladeless wind turbine is unlikely to kill any birds and bats.
I've got a feeling that in engineering people are keen on avoiding flow-induced vibration of arrays of tubes, so this might be another 'enginaster' project.
Wobbly sticks idea can be discounted on first principles.
Energy is force × distance.
Wind power works by converting the kinetic energy from a mass of air into electrical energy.
Wobbly sticks in the ground don't individually have an effect on a large massflow of air; they can be an odd shape to maximise that effect but in essence, the worst drag coefficient is less than about 3; meaning that their effective cross-section is about 3 times that of their physical area exposed to the wind. That doesn't change even when they're oscillating, even though the transverse motion through air could extract more energy, the displacement and thus the nett energy available will be less; exacerbated that the structure will be moving through turbulent eddies with localised air motion in many directions.
Let's throw some numbers at the concept and see how they pan out:
So your 100 metre tall tower with an average diameter of 2 metres has an effective coss-sectional area of 600 m². It can only plausibly displace air over that cross-section, extracting kinetic energy from the airflow. If one assumes 100% efficiency for converting ALL the kinetic energy in the wind (i.e. it could magically stop behind the stick without violation flow continuity), then at a moderate wind speed of 10 m/s (6 on the Beaufort scale) the wind pressure of around 100 Pa (not kPa) gives a total force of 60 kN; which at 10 m/s gives 600 kW as the absolutely perfect, ideal, boundary value for power output.
Applying Betz' Law (technically inappropriate because it's not a turbine) 16/27 is the theoretical limit so 355 kW is a realistic upper bound for "generation" at 10 m/s. My gut feel is that such an engine would produce barely 10% of that potential.
Dialling up the wind speed to 25 m/2 (10 Bft), takes the wind pressure to around 500 Pa; resulting in about 5 times the power being extracted from the wind; ideally 1.8 MW. Nothing to sneeze at; but also something to treat with respect to try to convert to electrical power from a mechanical motion. Reliably. Over hundreds of millions of cycles. It would entail a vast amount of underground technology to operate efficiently.
Maybe they could fabricate it out of Piezo stacks and deal with the billions of volts at milliamperes directly. ;-)
It "could be done" somehow; but it's a dumb way to get 1.8 MW (sometimes - usually much less) when there are other types of proven, reliable and much, much cheaper generators available that don't look like they're about to topple over when they're working and disrupt the natural environment by sapping it of (laminar) wind.
Anything that takes energy out of the wind will generate noise because there will be a change in air pressure. Audibility isn't always a good measure. People and other animals can sense changes in pressure at low frequency; not through hearing as such.
When designing a wind "farm" are the developers required to model the local and distant effects of disrupting natural airflow?
It occurs to me that if one can generate electricity from wobbling "sticks" then surely one could generate electricity from waving tree branches. Somewhat unpredictable of course but we must be used to that by now.
So the mechanism (the tree) is also a carbon sink, a pollutant scrubber and air purifier and at some future point a source of bio-mass.
It supports wildlife too. Much better than killing it with a turbine or simply offering no support at all, so far as one can see, with a stick.
There you go. Planet saved.
This is based on a misconception that turns up every few years, that you can use resonant systems as a means of efficiently transferring energy. What the enginasters ignore is the fact that withdrawing energy from a resonant system destroys the Q factor of the system, so that it eventually ceases to become resonant. Even the giant brains at MIT fell into this trap (see Trivial Pursuit, Numberwatch, June 2007).
What you never see is the subsequent announcement that it did not work.
We don't see the subsequent announcements of success either.
Or we see premature announcements of success..... whatever happened to Steorn?...... still going strong it seems.
I liked the expression "perpetual Commotion" in one blog reporting on the "ORBO Power cube"