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An alternative performance curve can be produced for a turbine controlled at constant speed. The C_P –λ curve shows, non‐dimensionally, how the power would vary with rotational speed if the wind speed was held constant. The K_P –1/λ curve describes, again non‐dimensionally, how the power would change with wind speed when constant rotational speed is enforced. K_P is defined as

(3.95)

The C_P – λ and K_P – 1/λ curves for a typical fixed‐pitch wind turbine are shown in Figure 3.55. The K_P – 1/λ curve, as stated above, has the same form as the power–wind speed characteristic of the turbine. The efficiency of the turbine (given by the C_P – λ curve) varies greatly with wind speed, a disadvantage of constant‐speed operation, but it should be designed such that the maximum efficiencies are achieved at wind speeds where there is the most energy available.

Figure 3.55 Non‐dimensional performance curves for constant‐speed operation.