Salient White Elephant

April 25, 2009

Very High Capacity Factor Wind Turbine

Filed under: AeroArchitecture, Wind Turbine Capacity Factor — Tags: , — Salient White Elephant @ 10:18 pm

This post describes a means for greatly increasing the capacity factor of an AeroArchitecture Wind Turbine, such as the Circular Wind Dam. The concept is very simple. The fixed structure of the wind turbine is extremely large, yet very flimsy. For example, the walls of the Circular Wind Dam are made to withstand winds that blow no faster than 20 miles per hour! Because the walls are so flimsy, they are very inexpensive. What happens when the wind blows faster than 20 miles per hour? Vents in the wall are opened to let some of the wind pass through. The turbine hits rated power when the wind speed reaches 20 miles per hour. When the wind speed exceeds 20 miles per hour, power is regulated using exactly the same mechanism – vents are opened just enough to prevent the generators from overpowering.

If you have a million dollars to spend on the fixed structure of an AeroArchitecture Wind Turbine, you spread that money over the largest possible geographical area. Every square foot added to the size of the fixed structure provides an increase in the capacity factor of the machine.

One very interesting twist to this design technique is whether to spread the cost of the fixed part of the turbine over a single very large wind turbine, or over multiple flimsy structures that are located relatively far apart. I suppose the wind is even more likely to reach cut in speed if only the average wind speed at all the different wind turbine locations needs to be considered. Of course, this also spreads the transmission lines over a larger area. So you can see that some sophisticated computer modeling would be required to develop the optimum design.

Another intriquing question is whether to use the capital cost of the fixed structure to expand in the horizontal or vertical direction. It would seem that you could increase the size of the structure more if the increased dimension is in the horizontal direction (the structure is wider) rather than in the vertical direction (the structure is taller). But of course the extra wind harvested has more energy if you expand in the vertical direction. So again, the best design choice is not obvious, and a sophisticated means for developing this aspect of the design is desirable.

And finally, I wish I could see come kind of mathematical analysis that validates or refutes the fundamental principle behind this post – that the use of flow concentrators can increase capacity factor. My reasoning is only intuitive; it is not scientific or quantitative. It is known that a flow accelerating (upwind) shroud or a flow deccelerating (downwind) shroud increases the speed of wind that passes through the rotor disk. This must be so, of course, to maintain a constant volume rate of flow. Say for example that the cut-in speed for the rotor is 10 miles per hour (mph). If a shroud multiplies velocity such that a 5 mph wind becomes a 10 mph wind at the rotor, then isn’t it true that the cut-in speed has been reduced from 10 mph to 5 mph? And if so, doesn’t this mean that a flow manipulating shroud is capable of increasing capacity factor? This may seem like a minor point when you consider that there isn’t much power in a 5 mph wind or a 10 mph wind, but doesn’t the same mechanism apply for any windspeed? For example, doesn’t this reasoning mean that all of the time that the wind blows 10 mph means that the rotor spends the same amount of time harvesting a 15 mph wind? (I realize that the flow accelerating effect probably isn’t linear, but that’s just a detail.)  In fact, it seems to me that one of the challenging aspects of designing this type of machine is how you would bleed energy away from the rotor at higher wind speeds, while still allowing the rotor to safely and reliably harvest an amount of energy equal to its rated output. (This would be necessary because, if it is our desire to increase capacity factor, then we wouldn’t want to lose the productive hours at higher wind speed simply because those winds don’t occur very often. Aiming for high capacity factor, we want the machine to be producing power in the highest number of different possible conditions. As a matter of fact, seems to me that it may also be possible to add a little bit to the capacity factor on the high end of the wind speed spectrum. This is true because the rotor is small and therefore, from a mechanical perspective, it is very robust and very strong, and also because wind velocity may be regulated somewhat by opening vents in the shroud.)

One final point. If the flow manipulating shroud is very large and very expensive, then it may be desireable to add a few rotors and generators so that some of the energy that would have been allowed to pass through the shroud during high winds could be harvested by the extra rotors or generators. This would do much for the capacity factor of the machine, but since the energy is there, and since you only need a small rotor and supporting equipment to harvest it, then it may be cost effective to do so. I’m glad I included this paragraph because, if nothing else, it illustrates how much of the complexity of the design of a very high capacity wind turbine has been shifted from areas like blade resonance and tower resonance, to a much more hazy problem of optimization, such as might be solved with linear programming.


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