Salient White Elephant

April 13, 2009

Wind Turbine Stethoscope

Small inexpensive microphones are embedded into the wind turbine tower, blade spar, gearbox, generator, and various other places. Engineers and technicians can download short audio samples (30 seconds?) via the SCADA system in order to listen for abnormalities. The turbine controller occasionally takes samples of the various audio signals, and performs a spectral analysis on the data. The resulting spectra may be compared against several baseline spectra. Possibilities for baseline spectra include:

  • An average of many spectra taken from healthy turbines,
  • An average of many spectra taken from turbines having approximately the same total lifetime number of operating hours as this turbine,
  • the spectra recorded and stored during this turbine’s first day of operation,
  • the spectra recorded and stored after this turbine has been “broken in” (after 30 days of operation?),
  • the spectra of this turbine that was recorded and stored yesterday.

If the controller determines that the current spectrum is significantly different from the baseline spectra, it sends an email to the service department warning of a possible problem.

Microphones may also record aerodynamic noise of the blades. Small, inexpensive cameras (like “web cams”) provide visual feedback on the condition of the surface of turbine blades. These cameras are mounted at the blade root, and look longitudinally down the blade. In addition to damage, accumulation of smashed bugs and other foreign matter may be observed.

I got these ideas from troubleshooting turbines in the field. It always amazed me at how much you could learn about a wind turbine simply by listening to it. Usually I’d just walk around the wind park listening, but sometimes I’d put my head against the tower in order to hear the highly amplified intimate details of the turbine’s inner life. I don’t know why I was surprised by how revealing this audio information was… after all, doctor’s diagnose many illnesses with a stethoscope, so why not turbines? As a matter of fact, this idea might be applied to a wide variety of machinery, from bulldozers to airplanes.

For some strange reason, engineering has long suffered under a trend of making everything “idiot-proof”. You don’t troubleshoot an electronic control system anymore, you read an error code off of a display that tells you which circuit board isn’t working right. Then you replace the bad board with a good one, and send the bad one back to headquarters. This is simply a waste of resources. Sure, if you can speed up troubleshooting in the field, that’s a good thing. But most technicians who do this kind of work have skills that are not being leveraged (and certainly not being developed) by the “idiot-proof” ideology. Some of the technicians I’ve worked with in the wind industry were quite talented. Why waste a resource like that? A better approach is to find the right balance, where on the one hand you wouldn’t be soldering transistors onto a circuit board at the wind park, but you also wouldn’t pay good money for an over-engineered solution when field technicians are perfectly capable of performing a certain amount of on-the-spot troubleshooting. The irony of the idiot-proof ideology is that people who have all of the challenge siphoned out of their jobs eventually lose even the skills they started with.

The problem with idiot-proof technology is that it creates idiots.

A person can acquire an amazing amount of knowledge through troubleshooting, and thus becomes more valuable to the company every day simply by showing up for work. And the feedback and input of an experienced technician like this can greatly improve the design of the next generation wind machine (assuming it is in some way derived from the current generation machine).

Think for a moment of how much you know about your own body on the basis of sound and feeling. For example, when I go to the gym and get on the stairmaster, I know that my right knee will always “tick” when it passes through a certain angle. It always does this, and it’s always at the same angle. Is this normal? No way! It certainly indicates some kind of a problem, even if it’s only an insignificant one. But this example illustrates the need to record audio that is outside of the audibal spectrum, especially the sub-audio. Sub-audio is important because it includes what you might call “vibration”. You can’t hear vibration, but it is obviously very important. I would classify the tick in my right knee as vibration – it is something I feel, not something I can hear.

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March 27, 2009

Wind Turbine With Flow Accelerating Shroud

Everyone knows that a smaller wind turbine rotor may be used if a shroud accelerates flow in its direction. Most people who know much about the design of utility scale wind turbines don’t take this idea seriously, and I confess I never took it seriously either. It just seems like an idea that cannot be applied to a large wind machine. But here are some ideas for light-weight shrouds that can be easily retracted during storm wind conditions:

Wind Turbine With Flow Accelerating Shroud

The idea is to provide a shroud that is made from some flexible material (nylon or whatever… I don’t know anything about materials). The shroud is shaped like a giant cone with its large end opening in the downwind direction (colored aqua in the diagram). But the cone is flexible… how will it be supported? It is supported by attaching its large opening to the small opening of another conically shaped shroud (colored purple in the diagram). The large end of the purple shroud opens in the upwind direction. The two upwind openings of these shrouds are each supported by a rigid ring – sort of like bending a couple of giant coat hangers into circles:

Downwind View Wind Turbine With Flow Accelerating Shroud

In order to help the shrouds hold their proper shapes, some kind of minimally designed light-weight rigid supports may be added:

light-weight ribs may be sewn into fabric of shroud to help maintain proper shape

Now we get to the second big problem with shrouds – their terrible storm wind drag profile. But since the shrouds are nothing but flexible fabric, they can easily be designed to fold into a shape that has a low drag profile! In fact, if the shrouds were well designed, it seems like maybe this machine would have even a much improved storm wind drag profile as compared with a traditional design with the same rated power output.

Giant Spoked Annular Wheel Variation

Here’s an interesting variation on this idea. This shroud sits on the ground or on a small post with bearings for yawing:

Wind Turbine With Spoked Annular Shroud, Downwind View

One interesting feature of this idea is that it makes use of all of the energy in the wind all the way down to the ground!

Wind Turbine With Spoked Annular Shroud, Side View

The following diagram shows how the shroud looks like a doughnut sliced in half (the doughnut slice and the cross section slice are in different planes):

Cross Section of Side View of Fabric Part of Shroud

The next diagram shows how the shroud may be deployed or retracted much like opening and closing drapes:

Wind Turbine With Spoked Flow Accelerating Shroud, Deploying the Shroud

Partial Cross Section of How Shroud is Deployed or Retracted Like Opening and Closing Drapes

Of course, another way to reduce the storm wind drag profile is to just lay the whole turbine down on the ground. Can’t get a much better drag profile than that!

Linear Turbine Variation

Here’s a variation that resembles the Direct Drive Linear Turbine With Yawing Oblong Track:

Shrouded Linear Turbine With Elongated Oblong Track

March 23, 2009

Wobbly Wind Speed and Direction Sensor

Filed under: Wind Turbine Auxilliary Devices — Tags: , , , , — Salient White Elephant @ 5:15 pm

Wobbly Wind Speed and Direction SensorImagine a typical three-cup anemometer. One cup is smaller than the other two, and the other two are the same size. In this case, the rotation speed will vary in a roughly sinusoidal fashion. The speed will be low when the small cup opens up towards the on-coming wind. If, from this position, you rotate the cups 180 degrees, the open end of the small cup will now be in the downwind direction. This will be the position at which the rotation speed is at a maximum. The angular position of the rotating part of the sensor is sampled a number of times per rev. A Fourier Transform is employed to determine the average rotation speed, as well as the phase of the sinusoid relative to some known angular position of the cups. The average rotation speed is proportional to wind speed, and the phase shift reveals its direction. Alternatively,  the amplitude of the sinusoidal variation in rotation speed may be used to calculate the wind speed.

Actuated Variation

In this variation, a small motor drives a rotating paddle at constant rotation speed. Now the instantaneous power consumed by the motor is sinusoidal. The wind speed is a function of the amplitude of the sinusoidal instantaneous power flowing into or out of the motor, and the wind direction is determined from the phase of the sinusoid. Alternatively, the sensor can resemble a miniature Darrieus rotor, with wind speed and direction calculated using similar Fourier techniques.

March 12, 2009

Precision Wind Turbine Yaw Sensor

The following diagrams show the relationship between the tower shadow and the rotational angle of the rotor:

precision-yaw-angle-sensor-yaw-aligned

precision-yaw-angle-sensor-yaw-misaligned

Tower shadow is detected by monitoring the instantaneous power produced by the turbine. Tower shadow produces a very short negative spike (drop) in the turbine’s power output. If this negative spike is sharp (very short in duration), and if it occurs when one blade passes the 6 o’clock position, then the turbine is correctly yawed. If the spike occurs “too early” or “too late”, then the turbine is not correctly aligned with wind direction. The lower diagram above shows that when the yaw angle is not correct, the tower shadow for the blade root occurs at a different time than the tower shadow of the blade tip. In this case the duration of the tower shadow increases. Furthermore, since the blade tip is generating torque when the root is in the shadow, and since the root is generating torque when the tip is in the shadow, the magnitude of the negative spike in power should decrease. So the controller knows that the turbine is misaligned whenever:

  • the duration of the tower shadow is increased,
  • the magnitude of the negative spike in power is reduced, and
  • the tower shadow occurs earlier or later than 6 o’clock.

In designing the yaw angle control system, the traditional vane sensor might provide an estimated value for yaw angle while the precision yaw angle sensor provides for fine tuning this value.

March 2, 2009

Wind Turbine Flow Accelerator for Downwind Rotor

Filed under: Wind Turbine Auxilliary Devices — Tags: , , , , — Salient White Elephant @ 10:46 am

wind-turbine-flow-accelerator-for-downwind-rotor

The shroud and the cone are connected. They form a teardrop shaped unit called the “flow accelerator”. The accelerator is supported by rods that are not seen in the diagram because they are inside the accelerator. These supporting rods are connected to the extended rotor shaft in a way that allows the accelerator to rotate about the extended rotor shaft. This allows the rotor to turn while the accelerator remains more or less stationary. Wind accelerates as it veers to avoid the flow accelerator. This causes the wind that passes through the outer portions of the rotor blades (the portions furthest from the rotor shaft) to be traveling faster than it would have been traveling had the accelerator not been present. In this way, the accelerator prevents air from “escaping” through the “aerodynamic doughnut hole” in the center of the rotor, and directs it instead to the outer sections of the blades where its energy can be extracted.  The flow accelerator must be carefully designed not to produce turbulence. This is so because turbulence can rob the rotor of some of its energy harvesting capabilities.

The size of the annular airfoil like ring is exaggerated in the diagram above. A practical design will minimize the size of this component, and will maximize the size of the windsock. This allows the sock to open up so that it has a near zero drag profile when the turbine is shut down for high wind conditions.

The flow accelerator might be a good combination with a rotor that has variable length blades. As the blades are extended to sweep more area, the section near the hub where there is no root will be in front of the flow accelerator, so it won’t matter that there’s no root section there.

February 24, 2009

Automatic Wind Turbine Blade Washer

The spinning brush moves up and down the tower so that it washes the entire length of each blade. The strands of the brush are thick, and they are made of soft material such as cotton. The spinning brush is similar to a spinning brush you’d expect to see in an automatic car wash. The strands are not stiff, but they are held in an extended radial position by centrifugal force. Soapy water is continuously applied to the brush as the blades are washed so that the strands stay wet and soapy.

The turbine blades rotate at a slow to moderate rpm while they are being cleaned. The strands of the spinning brush slap against the sides of the blades that face the tower, and then drag across these surfaces. This cleans the tower sides of the blades. But what about the sides that face away from the tower? The length of each strand is equal to the distance required to reach the blades, plus the length of the chord near the root (where the chord is a maximum). Whenever a blade collides with the spinning brush, some of the strands will wrap around the side of the blade that faces away from the tower. These strands will subsequently drag across the far surface of the blade as it continues to move through the spinning brush. Thus, both sides of the blade are thoroughly cleaned.

An Alternative Method

The spinning brush idea is simple. I see no reason why it shouldn’t work. But in case it doesn’t, here’s an alternative approach.

Imagine a tube that carries soapy water through the inside of a blade, exiting the trailing edge of the airfoil at a point exactly halfway between the root rib and the tip. The diagram above depicts only the water ejected from the top blade as it travels from the 6 o’clock position to the 12 o’clock position. The water pressure and rotor rpm are adjusted so that the water that the top blade ejected as it passed through 6 o’clock has just enough time to fall half a blade length before the next blade passes through 6 o’clock. You can see the result in the diagram – the water hits the tip of the bottom blade. Given this state of affairs, it follows that the water that the top blade is ejecting in the 12 o’clock position depicted in the diagram will also fall half a blade length before the bottom blade passes through 12 o’clock, and that water will hit the blade at its root. Since the stream of water is continuous, and since it hits the tip of a 6 o’clock blade and the root of a 12 o’clock blade, then it must hit every other point on a blade as it passes from 6 o’clock to 12 o’clock. So the entire length of each blade is washed.

Of course, either of these blade washing techniques will work on upwind or downwind turbines, regardless of how may blades they have.

Spoiling Wind Turbine Yaw Control

Simply fit one turbine blade with the light-weight, reliable, inexpensive, and fast-acting spoiler of your choice. Now if the spoiler is deployed at 2 o’clock and retracted at 4, the turbine will yaw in one direction. If the spoiler is deployed at 8 o’clock and retracted at 10, the turbine will yaw in the opposite direction. That’s all there is to it!

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