Salient White Elephant

March 18, 2009

Flow Accelerating Ring Generator for Horizontal Axis Wind Turbine

Flow accelerators don’t seem appropriate for utility scale horizontal axis machines. It’s probably much easier to just make the blades longer, especially given that blades will probably generate far less drag in storm winds. But what if the real purpose of the flow accelerator is to hide a ring generator? In this case, the size of the flow accelerator is the minimum required to shroud and contain the generator. Any increase in power output due to accelerated flow is “icing on the cake”, since otherwise the accelerator is designed to have minimum storm wind drag profile.

Flow Accelerating Ring Generator for Horizontal Axis Wind Turbine
Though not shown in the diagrams, the end of the airfoil goes through a slot in the accelerator, so that the blade tip is actually inside the accelerator. Alternatively, some kind of metal extension can extend from the tip of the airfoil through the slot in the accelerator. The part of the blade that is inside the accelerator is attached to an arc shaped housing for the permanent magnets of the ring generator.

Given modern wind machines are approaching a scale that leaves even the engineers pointing and goggling, it makes little sense to keep taking the torque off the wrong end of the airfoil.

There are two major challenges to this design. The first is how to accommodate for bending and vibration of the end of the rotor blade. The second is how to maintain close mechanical tolerances in such a large ring generator. I have not given thought to the specifics of how these problems might be solved, but I do have a philosophical approach I’d like to share with you. When a pilot turns the steering wheel of his state of the art modern airplane, the ailerons are electronically actuated. As expected, electric motors and microelectronics realize the actuating system. This system has a very cool trick for moving the ailerons to just the right angle. The trick does not involve impossibly accurate and brittle (in the sense of non-robust) design and manufacturing processes. The electric motors are not precision Swiss watches. Instead, a feedback loop is employed. The challenge in designing such a system mostly involves control theory – a well understood science. I am suggesting that the successful implementation of a “mega-ring generator” may possibly employ the same approach.

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