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Using capacity factor as a yard stick for assessing a wind turbines payback and output

I recently attended a Tipperary Energy Agency training course on RETscreen. The excellent trainers referred to and discussed the vestas wind turbines that are used in wind farms in terms of capacity factor. These range  typically in the 30 – 40% bracket as they are placed on well exposed sites.

As explained in the American Wind Energy Association website  capacity factor is one element in measuring the productivity of a wind turbine. This figure compares the wind turbines actual production over a given period of time with the amount of power the turbine would have produced if it had run at full capacity for the same amount of time.

e..g a 1.6 kw(rated at 12 m/s)  turbine generates 3321.792 kwh of electricity in a year.

If 12 m/s wind was available 100 % of the time then it would have generated 14016 kwh of electricity (8760 hrs * 1.6 kw) . The capacity factor is 

amount generated                          =        3321.792
——————–                                       ————–
theoretical amount at full capacity            14016

= 23.7 % capacity     

This does not mean that the turbine is faulty it just implies that wind speed of 12 m/s was not available all of the time.

The community wind power fact sheet reports that wind typically has a capacity factor of 20 – 40 % and hydro (water) has a capacity factor of 30 – 80 %. Water is more predictable and makes for a more dependable source of power.

If Company X makes a claim that a turbine can pay for itself in 7 years a crude measurement would be divide the cost of the turbine by 7 and then calculate the 
number of units that would have to generated and compare this to the cost of the turbine 

e.g. Turbine X sells a 1.6 kw turbine (rated at 12 m/s) for 7000 euros installed, and claims a pay back in 7 yrs. This turbine would have to generate 1000 euros of electricity per year to achieve this.

1000 euros of electricity at 16 cent per unit = 6250 units
The theoretical would be 8760 * 1.6 = 14016 units

Actual production                              = 6250
—————–                                       ————
Theoretical                                            14016

= 44.6 % capacity

This capacity factor is higher than commercial wind farms and would very difficult to achieve.

According to RETscreen awind turbine at 10 meters would n
eed to have wind speeds of 10.9 meters which is 13.2 at 50 meters. Using a shear factor of 0.12 which is open agricultural land with no buildings nearby. These types of wind speed are very difficult to achieve.

Evance who used to be called iskra market their 9000 turbine which they claim will generate 9000 units per year at the uk mean speed of 5 m/s. Their turbine is rated at 5.4 kw at 11 m/s

Capacity factor =                     9000
                                           —————
                                          8760*5.4

= 19% = 9000 * 0.16 = 1440 euro of electricity per year.


So we have one turbine manufacturer who markets at 44% capacity and the other at 19%.

Personally speaking I think 20 – 25 % is a good yard stick for assessing a domestic turbine

e.g. 2 kw turbine at 25% capacity would produce 
(8760 * 2 ) * 0.25 =  4380 units 
4380 at 16 cents = 701 Euros per year.

If you want to get your money back in 10 years it should be priced around 7,000 euros.

Hopefully this will assist you in making some rough calculations on wind turbine claims