Wind Turbines

We have been asked many times why Hartzell doesn’t get into making blades for the wind turbine industry.  The simple answer is that blades designed to move the air don’t work very well when trying to capture the air.

In the old days, we used to call these devices windmills.  They have been used for centuries to convert wind energy into useful work.  Most of us have seen pictures of the old Dutch windmills, which have a romantic connotation in older movies, slowly turning but quite inefficient. 

The amount of energy in the wind is tremendous, and people have tried to capture it in various ways.

Making use of wind energy using a wind turbine has several challenges:

  1. Only a small portion of the energy can be captured – typically about 30%.  The total energy available varies as the square of the turbine diameter, and the cube of the windThis is the primary reason why today’s wind turbines are so large.  They have to be huge in order to capture enough energy to make them economically feasible for use on the public power grid.
  2. Wind energy is veryUnlike running a conventional power plant, the wind varies all the time, so matching the source to the load is difficult.  Energy storage technologies are not capable of making much of a dent in this mis-match.
  3. Large wind turbines, and the capability to withstand high thrust loads from wind gusts, forces the designer to create a strong tower structure, and a massive underground anchorage.

Today, wind turbines have recaptured the imagination.  The old farm windmills did a good job of pumping water because the water was stored in a large basin.  These machines seldom created more than about 75 watts of power in a good breeze. 

Here are some other statistics to help you understand the difficulty in capturing the wind.  A 125 foot diameter three-bladed machine can capture about 100 kW in an 18 mph wind.  In 2011, the average U.S. residential utility customer used 940 kilowatt-hours per month.  If the wind blew at 18 mph for 24 hours continuously, this is enough energy for about 7 typical American households.

In Dayton, Ohio, the wind velocity is below 7.5 mph 32% of the time, below 15 mph 77% of the time and below 20 mph over 98% of the time.  When you do the math, using the cube rule on wind velocity, you find that 90% of all the wind energy available comes from 12 mph and higher wind velocities.

The total available energy in the airstream is given by the formula:

            AHP = 2 *pi * Rho * R^2 * V^3           where:   Rho is the air density in lbs./ft^3

                                                                                    R = radius of turbine in feet

                                                                                    V = air velocity in mph

Using standard values for air density, this formula becomes:

            AHP = 5.26 * D^2 * V^3 * 10^-6         where:  D = diameter of turbine in feet

                                                                                    V = air velocity in mph

The average rotor is not a highly efficient machine, although they are approaching 75% with the newer designs.  They also cannot capture all of the energy in the airstream, because there are not enough blades.  This factor is about 55%.  So, combining these factors reduces the captured energy to 41% of the total available.  Then include the mechanical efficiency of the gearbox needed to drive the generator, the efficiency of the mechanical couplings, and the efficiency of the generator, and you are down to the 30% effectiveness number mentioned earlier.

One other challenge for wind energy is the wind velocity varies by the height above the ground.  In ‘normal’ wind velocities, the formula is:

            Vh = Vo * (Hh / Ho)^0.2                      where: Vh = velocity at height

                                                                                    Vo = velocity at the observation height

                                                                                    Hh = height in feet

                                                                                    Ho = height at the observation

Most weather stations take wind velocities at 25 to 30 feet elevation.  Using this formula tells you that having the wind turbine at 150 feet will provide 43% more energy than what the reported wind velocity would indicate.  Of course, all of these variables are based on theoretical installations without nearby interference or ground effects.  The best approach is to do a site survey.

Recently, it was reported that wind farms are not producing the amount of energy that the mathematical models indicate.  A major university did a study, and found that the downstream velocity distortions from an upstream wind turbine were reducing the efficiency of the downstream turbine by almost 30%.  The results of the study are being used to place wind turbines almost twice as far apart as the earlier wind farms.

This whole discussion has been about circular wind turbines.  There are many other designs now being introduced, with many of them being vertical shafted.  Some of these newer designs are quite creative, and appear to have addressed some of the limitations of a circular turbine.  For one thing, you can place the gear box and generator at the ground, rather than at the top of the tower, greatly reducing the installation and maintenance costs.  

Although many of the things discussed in this article might sound like wind turbines are not a good solution to our energy needs, they do have their place.  We just have to be careful that we rely on more consistent energy producing devices for our base load, and use wind turbine energy for reducing the base load requirements when available.