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Large Wind Turbines

September 10, 2020
Paul Gipe

Material Intensity of Wind Turbines


A question arises whether large wind turbines are more or less materially intensive than small wind turbines.

The answer, as with most things in life, isn't that simple.

The simple answer is no, large wind turbines are not more materially intensive than small wind turbines. Nor are they less materially intensive.

Both small and large horizontal-axis wind turbines confront the same laws of physics. As their blades increase in length, the forces acting on them increase exponentially at the same rate.

Their material intensity is comparable.

But this is the wrong question.

The better question is what produces relatively more electricity for its size. Does a large wind turbine generate more electricity relative to its size than does a small wind turbine? And the answer is a resounding yes.

Why that is beyond my comprehension of aerodynamics. Suffice it to say that it has to do with Reynolds Number effects. While I can't competently explain this phenomenon, I can digest the historical data from wind turbines in the field and what their manufacturers expect them to produce. I've done this for decades in my books.

Here's Table 11-1 from my most recent book Wind Energy for the Rest of Us.

 

For an average annual wind speed of 7 m/s at hub height, a modern small wind turbine will generate 600 kWh/m2 of rotor swept area per year.

As the turbines increase in size, their efficiency increases. Here's Table 11-2.

For an average annual wind speed of 7 m/s at hub height, a modern household-size wind turbine will generate 770 kWh/m2 of rotor swept area per year.

This effect continues with large wind turbines. Here's Table 11-3.

For an average annual wind speed of 7 m/s at hub height, a large wind turbine will generate 1,060 (from the book) to 1,090 (in this chart) kWh/m2 of rotor swept area per year.

And this assumes that the turbines all encounter the same wind stream. Typically they don't. Large wind turbines are installed on very tall towers. Small turbines are not. For a given location, a taller tower will enable the production of more wind-generated electricity because wind speeds are greater aloft.

Technically, one could argue, that you if you simply install the small turbine on an equivalent height tower, the small wind turbine will experience as much wind as a large wind turbine. While this may be true, we don't do that because it's not economic to do so. In part this is due to the lower aerodynamic efficiency of small wind turbines. It is also because overall small wind turbines are more costly relative to the amount of electricity they generate relative to a large wind turbine.

The principle question for all wind turbines is which generates the most cost-effective electricity in $/kWh. Large wind turbines are much more cost effective than small wind turbines for many reasons, not only because they are more efficient.

On page 452 of my book, I discuss the difference between efficiency and cost effectiveness. In Table 19-4, I use a real-world example of how in essence the same wind turbine with a larger rotor, thus a larger wind turbine, is more cost-effective than its sibling with a slightly smaller rotor that costs less per kW of installed capacity.

In my experience the cost of electricity from small wind turbines is twice that of large wind turbines for the same conditions or, in this case, $1/kWh.

Cost of the resulting electricity from a wind turbine is not the only criteria for choosing what kind of wind turbine to use, but it remains a critical one.

 


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