Some friends were discussing the recent visit of Department of Energy Secretary Steven Chiu to Clemson University to deliver a check for $45 million to start a test facility for horizontal wind turbine gearboxes. It seems that there have been a number of gearbox failures in recent years that suggest a wider problem that will need to be solved in order for wind energy to become as reliable and cost effective as promised. Gear boxes are failing in the range of 10 years of operation, and occasionally, sooner than that.
This is not difficult to understand. The forces acting on the gearbox are huge. On the input side you have 3 blade propeller with blades that are now approaching 200 feet in length. I don’t care how light weight they are, carbon fiber epoxy or Kevlar or whatever, the forces are tremendous. In addition the blades have to rotate to take them out of the wind when the wind is too fast for the system to operate. So there are actuators at the base of the blades adding to the weight and mechanical complexity.
Then there is the intermittency of the wind itself. This can manifest itself as bursts of wind or winds of different speeds hitting the same rotor. Which can lead to all kinds of impulse loads on the gearbox. Gear teeth becoming momentarily unloaded and loaded in response to the wind. This is actually one of my favorite “Stump the Band” questions for mechanical engineers; what is the formula for the shock load of gear tooth reversal? It’s big, whatever it is. And the shock load of the propellers is driving the gearbox against a high inertia load, the generator. So there is a lot of resistance to overcome.
But the really scary part is that the gear systems are often in the range of 30,000 pounds in weight. And they are mounted on metal masts at heights of 1.5 times the blade length. So that would be 300 feet up in the air in the case of a system with a 200 foot blade. Making the replacement of a failed gearbox a bit more complex than dropping the transmission out of a car, for example. Especially since most wind farms are in very remote locations where the land is cheap and the wind blows some of the time.
This lead the Department of Energy to put out requests for proposals to address the technical question of providing the industry with a resource to help in the design of gearbox systems with much higher reliability than the current designs. Total cost of this effort, approximately $100 million dollars. The proposed test facility is targeting 20 megawatt power handling capability, or approximatley 27,000 horsepower depending on the exact rpm of the system. This is an incredibly big piece of machinery.
Clearly, gearbox technology has to get better for the wind industry to continue to prosper. I wonder if we are putting a band aid on a technology that is fundamentally flawed. Maybe we need to be concentrating on the next generation of the technology and improving the cost performance by an order of magnitude. Surely we can do better.