Technical Comparators: Swept Area preferred over Rated Power

By Geoff Henderson

Windflow Technology Newsletter Nº 31, June 2011

Some people use capacity factor as the basis for comparing wind farms and turbines.

Capacity factor is the average kilowatts produced over a period, divided by the rated output power of a wind turbine (kWa/kW). It has appeal as a technical comparator because it is dimensionless, however performance based on swept area is used in the international wind industry for technical and economic comparisons.

Reasons being:

For economic comparisons, swept area is a more significant driver of turbine weight and cost than rated power. For purely technical comparisons, swept area is a more significant driver of average power output than is rated power. Swept area determines the amount of wind energy being intercepted by the turbines and thus provides a better comparison between turbines operating in similar wind regimes.

Specific average power (SAP) is the average power divided by the swept area of a turbine or wind farm. It has units of average watts per square metre (Wa/sq.m). All else being equal, differences in SAP will reflect differences in wind conditions or turbine availability, whereas Capacity Factor is a misleading comparator because identical wind conditions can result in significantly different values of capacity factor even at 100% availability.

The following graph shows the output in 2010 of the Te Rere Hau wind farm expressed as SAP and compared with the other wind farms in the lower North Island. It shows that all the wind farms produced similar amounts of power per unit of rotor swept area. By contrast a comparison of capacity factor would show large, but meaningless, differences.


Geoff Henderson Windflow Specific Power.

Windflow has deliberately optimised the turbine for high wind conditions, with a very high value of specific power rating (SPR, W/sq.m). Thus a capacity factor comparison is entirely predictable and completely irrelevant without further information about $/kW (or $/sq.m or $/kg).