Sunday, February 24, 2019

Wind power

The wind is blowing at sustained speeds in excess of thirty miles per hour today.

We had to drive around an elm tree that had fallen across one lane of M-50 on the way home from church.

Today is a good day to talk briefly about wind-power.

A windmill is a device that converts some of the kinetic energy of the wind that blows through its swept area and turns it to electrical power.

Consequently, it is possible to estimate the absolute, top-end of the power a given windmill can produce knowing only the diameter of the blade(s) and the wind speed. Note: the actual number can be much, much less than this.

Key things to be aware of:
The kinetic energy of wind goes up with velocity cubed (^3). The kinetic energy of each unit volume goes up by V^2 but you have to remember that the number of unit volumes also goes up by V^1.

The swept area goes up by Diameter^2.

Would you care to guess how much more energy a windmill with an 8' diameter blade in a 12.5mph wind will produce compared to a 4' diameter blade in a 7.5mph wind?

The answer is 18X as much. That is why you want to erect your windmill at the top of the hill or crest of a ridge. It is also why you want a large diameter. You might be able to squeeze 400W from a 4' diameter windmill but you might need 30mph winds to achieve those numbers.

If I were handy, I would strongly consider a 10' diameter wheel and construct the blades from 8' long, 5/4", Western Yellow Pine decking planks shaped to be air foils and cut to 4' lengths. The 2' diameter sans blades has negligible area.


  1. Well, Joe...

    Considering that the thermodynamics of the matter dictate that energy can be neither created nor destroyed...if you remove millions/billions of joules of energy from the wind in a region, what are the ecological/geological impacts of this loss of wind energy downrange? How might local climates be affected? Rain? Temperatures? Insect hatches?

    1. I am going to totally not answer your question.

      Part of my intent in some of my posts is to prep readers for Fat Cow installments. I may or may not take a loop through alternative energy.

      Based on a couple of emails I received, that is a burning question with some readers.

      As a person who delights in poking around the edges of things, one of my interests is in the value of tiny amounts of power.

      Thirty kW-hr per day is worth about thirteen cents a kW-hr. But what is the incremental value at the margin if 10% of that amount was available? How about 1% or 0.1%. Surely that power would not be used to toast bread.

      An essay here:

  2. Max theoretical energy extraction is 60% of wind kinetic energy...Betz limit.


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