Wind Energy Handbook. Michael Barton Graham

Wind Energy Handbook - Michael Barton Graham


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uco = 4.17 in this case is well above the highest value of u shown in Figure 3.67 for which there is any significant energy.

      The annual energy capture would be upper E equals 4.8138 dot 10 Superscript 8 Baseline k upper W h which is a 6% increase in energy capture compared with the fixed‐speed, stall‐regulated machine. Variable‐speed operation has a number of other advantages that are discussed in Section 6.9.4; it is increasingly being implemented.

      3.17.1 Introduction

      For many years the wind turbine industry relied on aeronautical experience for the aerodynamic design of turbine blades, but it became clear that aerofoil sections that were optimum for aircraft wings were not necessarily optimum for wind turbine blades.

      A major problem for modern wind turbines in the early years of development was sensitivity to insect deposition on the leading edge regions of the blades. There were reports of turbines in the 1970s having to be regularly hosed with water to clear accumulated debris on the blades to restore power levels that had fallen dramatically. An aerofoil that was tolerant to leading edge roughness was required.

      Because of data availability, a popular range of aerofoil sections for wind turbine blades was, but less so now, the NACA six‐digit series, an example of which is discussed in Section 3.9. Although more tolerant to leading edge roughness, the NACA six‐digit series is no better overall than the NACA four‐digit series described in Appendix A3. The main reason for the popularity of the NACA aerofoils is because high quality experimental data is available from tests that were carried out in the 1930s in the pressurised wind tunnel built by NACA (superseded by NASA in 1959). The NACA technical reports are available free on the NASA website, and much of the force data is given in Theory of Wing Sections by Abbott and von Doenhoff (1959).

      3.17.2 The NREL aerofoils


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