Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 94

The theory described earlier in this chapter gives the wind turbine designer a means of examining how the power developed by a turbine is governed by the various design parameters. The usual method of presenting power performance is the non‐dimensional C_P – λ curve, and the performance curve under fixed‐pitch conditions for a typical three bladed turbine of the type used for large‐scale generation of electrical power is shown in Figure 3.50.

The first point to notice is that the maximum value of C_P is only 0.47, achieved at a tip speed ratio of 7, which is much less than the Betz limit for that tip speed ratio. The discrepancy is caused, in this case, by drag and tip‐losses, but the stall also reduces the C_P at low values of the tip speed ratio.

Even with no losses included in the analysis, the Betz limit is not reached because the blade design is not perfect; see Figure 3.51.

Figure 3.50 C_P ‐ λ performance curve for a modern three blade turbine.

Figure 3.51 C_P ‐ λ performance curve for a modern three blade turbine showing losses.