Turbine Power


When comparing the power output of different wind turbines, there are three variables which you should consider.  They are:


This is not an overstatement.  If we grant the assumption that most modern turbines, which generate electricity, have roughly equivalent efficiencies, then the power which any particular turbine is capable of generating will be directly proportional to the swept area of its rotor.  For a horizontal axis turbine, the swept area is directly proportional to the rotor diameter.  It’s essentially as simple as that!  A turbine with a larger diameter will generate more power than a turbine with a smaller diameter.



Nameplate ratings tell you nothing about how much power a turbine will produce.  Some small turbines have nameplate ratings.  For example, a turbine might be called 10 kW, 5 kW or 25 kW.  The nameplate rating describes the rated capacity of the turbine at a particular wind speed.  A 10 kW turbine might produce 10 kW at a 30 MPH wind speed.  A 25 kW turbine might produce 25 kW at a 26 MPH windspeed.  These numbers do not tell you how much power the turbine will produce at the average wind speed at your location – or any location for that matter.  There are few, if any, locations where a small wind turbine will be installed where the average wind speed is 25 or 30 MPH.  Most locations where small turbines will be installed have average wind speeds in the 10-14 MPH range.

Consider the example of two 5 kW turbines.  One has a 10 foot diameter rotor and the other a 14 foot diameter rotor.  Both have 5 kW generators.  The 14 foot turbine has twice the swept area of the 10 foot turbine.  It will therefore produce twice the power of the smaller turbine.  The 5 kW nameplate rating obviously tells the prospective customer nothing about the turbine’s capacity to produce electrical power.

Unfortunately, some states and funding agencies persist in using nameplate ratings as a basis for grants or awards for the installation of wind turbines.  As the above example demonstrates, this practice can unfairly prejudice a particular turbine.  Both turbines would receive the same amount of funding even though the turbine with the larger rotor would produce twice the power of the smaller one.  Because the turbine with the larger rotor would cost more than the smaller one, the owner of the larger turbine would not receive their fair share of funding.  The result of such misguided policies is to encourage manufacturers to attach large (less efficient) generators to small diameter rotors in order to secure a funding advantage.  This doesn’t do anyone any good, particularly the consumer.

No wind turbine can extract more than 59.6 % of the energy from the wind.  In 1919, a German physicist named Albert Betz determined that a wind turbine cannot convert more than 59.6% of the energy in the wind to mechanical or electrical energy.  The reason for this is that wind turbines extract energy from the  wind by slowing the wind.  For a turbine to be 100% efficient it would have to reduce the wind velocity to zero.  If you were to stop the wind there would be no wind to turn the rotor.


In reality, small wind turbines currently in production are not capable of delivering anywhere near 59.6% of the energy in the wind.  All turbines have losses such as aerodynamic losses, generator losses, gearbox losses, etc.  It would probably be overgenerous to say that an extremely efficient small wind turbine might have an overall efficiency of about 35%, roughly half the theoretical limit.

The table shows how many killowatt hours per month such an extremely efficient (Good) small turbine might produce at a given wind speed.  For example, at 12 MPH, a good turbine would produce about 3.2 kWH per month per square foot of swept area.  A turbine with a 22 foot diameter rotor would produce about 1,200 kWH per month

kWH/Month =  (22/2)2  * PI * 3.2
A turbine with a 12 foot rotor would produce about 360 kWH per month.  These figures are probably on the high side, at least by about 10%.  If a manufacturer claims figures higher than these, it is more than likely too good to be true and you may want to consider talking to some owners to see for yourself exactly how many kilowatt hours their turbine is producing.

Energy per Month
Wind Speed Good Turbine Good Turbine
mph -(m/s) per Sq. M. per Sq. Ft.
5 – (2.24) 2.65 0.246
6 – (2.68) 4.74 0.440
7 – (3.13) 7.70 0.715
8 – (3.58) 11.66 1.083
9 – (4.02) 16.63 1.545
10 – (4.47) 22.36 2.078
11 – (4.92) 28.45 2.643
12 – (5.36) 34.38 3.194
13 – (5.81) 39.75 3.693
14 – (6.26) 44.30 4.116
Data Courtesy Michael A. Klemen
Energy per Month at 12 MPH
Diameter Ft. Diameter m kWH
6 1.83 90
8 2.44 160
10 3.05 251
12 3.66 361
14 4.27 491
16 4.88 641
18 5.49 812
20 6.10 1002
22 6.71 1213
24 7.32 1443
26 7.92 1694
28 8.53 1964
30 9.14 2255
32 9.75 2566
34 10.36 2896
36 10.97 3247
38 11.58 3618
40 12.19 4009
42 12.80 4420
44 13.41 4850
46 14.02 5301
48 14.63 5772
50 15.24 6264

The table shows the expected kilowatt hours per month at a 12 MPH average wind speed from very efficient wind turbines.  The left column gives the rotor diameter in feet and the second column in meters.  The right column is the number of kilowatt hours per month.  As in the previous energy calculation, an efficiency of 35% has been used.  These figures are probably too optimistic in that they represent the high end of what might reasonably be expected from small turbines.

Aerostar® Wind Turbines www.aerostarwind.com

RLS ENERGY 517.930.7290

7290 Island Hwy • Eaton Rapids, MI 48827

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