Scale Effect – Part 2

While on the subject of Scale Effect in the design of Wind Turbines, I came across some interesting aspects of the problem that are making the headlines.  Though it may not be high profile in the US, there are disturbing reports coming in from wind project in England and Denmark.

The United Kingdom is pursing a major shift in energy supply to make wind 30% source of electricity .  In pursuit of this goal, a number of wind farm projects in the UK that have been approved and are under construction.  Some recent inspections of turbine foundations have revealed that parts of the foundation have moved since their recent installation.  The movement has been significant and is requiring engineering evaluation to determine what steps may be necessary to correct the problem.  Translated, the suppliers don’t know for sure what happened or how to fix it.  And don’t even get started on how much it will cost to fix.

This should come as no surprise.  Even after you get rid of the gearbox and convert the system to direct drive, the generator portion of the system is 90 tons in the new 4 megwatt GE design.  Not a small number.

The larger blades designed for these systems are expected to be greater than 20 tons each in spite of the fact use of carbon fiber which is very light weight.  Add to this hydraulic actuators used for pivoting the blade to optimize the angle for the wind speed.

With housing, power conversion and everything else, the turbines must be in the range of 200 tons.  The mast for the turbine is a concrete cylinder that uses 1100 tons of cement.  Add to this a steel tapered cylinder that weighs around 3000 tons.  So there is a 4200 ton load sitting on a 20 foot diameter circular path.

That’s a lot of load.  And it’s all resting on 113 square feet of surface area.  (The base is a circle of cement 2 feet wide with an outside diameter of 20 feet, check my math just in case I made a mistake)

We should not be surprised that there are unanticipated problems with the foundation.  It works out to 74,000 pounds per square foot of direct load, not including dynamic stresses.   That is an incredible load situation.

So as we seek to “scale up” and apply new wind power designs, subtle problems begin to appear. Good engineering practices that would work under known conditions may exceed limits as a design is scaled up.  And systems that have predictable costs, suddenly will become much more expensive because of unanticipated issues.  In this case, increasing the cost per megawatt instead of decreasing it.

But the idea behind “scale effect” is that the bigger you make something the less it costs per unit of output.  Only no supplier is claiming to reduce the price per megawatt paid for wind power.  At least, not so far.  After all, these are new designs.  And there is bound to be some de-bugging needed.

There are reasons to be concerned about the fresh crop of wind harvesting systems.  And reasons to be concerned about the hype that surrounds the industry.  Scale Up is just one of them.