Rotor blade shaped to enhance wake diffusion

1. A blade for a horizontal-axis wind turbine rotor, the blade comprising a radially-outer, energy-extraction portion, a radially-inner, ventilation portion, and a transition portion between the radially-inner portion and the radially-outer portion, wherein the radially-inner ventilation portion is shaped to, in use, ventilate a central area of a wake of the rotor by extracting low levels of energy from the wind,

wherein the blade is a unitary blade,

wherein the blade has a length of at least 50 meters,

wherein the radially-inner portion has an axial length of greater than 15% of the length of the blade,

wherein, in the transition portion, a local blade twist angle of the blade gradually transitions from a local blade twist angle of the radially-outer portion to a local blade twist angle of the radially-inner portion, and

wherein, in the transition portion, an aerodynamic shape of the blade gradually transitions from an aerodynamic shape of the radially-outer portion to an aerodynamic shape of the radially-inner portion.

2. A blade according to claim 1 , wherein the radially-inner portion has an axial length of less than 50% of the length of the blade.

3. A blade according to claim 2 , wherein the radially-inner portion of the blade is shaped so as to, in use, extract less than 2% of the total energy extracted by the blade.

4. A blade according to claim 1 , wherein the radially-inner ventilation portion is shaped such that, in use at rated wind speed, the wind in the central area of the wake immediately downwind of the rotor has a velocity at least 80% of the velocity of the wind immediately upwind of the rotor.

5. A blade according to claim 1 , wherein the radially-inner portion of the blade has a local blade twist angle and/or aerodynamic shape that is not optimal for extracting maximum energy from the wind.

6. A blade according to claim 1 , wherein the radially-inner portion of the blade is shaped such that, when the rotor is operating at its maximum power coefficient, the rotor achieves an average power coefficient of below 15% for the area swept by the radially-inner portion.

7. A blade according to claim 1 , wherein the radially-outer portion is shaped such that a local blade twist angle and local blade shape at each point along the radially-outer portion of the blade is approximately optimal for extracting energy from the wind, when the rotor is in use and operating at its maximum power coefficient.

8. A blade according to claim 1 , wherein the radially-outer portion of the blade is shaped such that, in use, when the rotor is operating at its maximum power coefficient, the rotor achieves an average power coefficient of at least 40% for the area swept by the radially-outer portion.

9. A rotor for a horizontal-axis wind turbine rotor comprising a hub and at least two blades according to claim 1 mounted to the hub.

10. A horizontal-axis wind turbine comprising a tower, a nacelle mounted at the top of the tower, and a rotor according to claim 9 , the rotor being coupled to a generator of the nacelle to generate electrical power.

11. An array of horizontal-axis wind turbines, at least one of the wind turbines being a wind turbine according to claim 10 .

12. A method of manufacturing a rotor blade for a horizontal-axis wind turbine rotor, the method comprising:

creating a design for a rotor blade by a computer-assisted method comprising:

designing a radially-outer portion of the rotor blade so as to extract maximum power from the wind;

designing a radially-inner portion of the rotor blade so as to ventilate a central area of a wake of the rotor by extracting low power from the wind or imparting energy to the wake, and by generating low drag; and

designing a transition portion between the radially-inner portion and the radially-outer portion,

wherein the blade is a unitary blade,

wherein the blade has a length of at least 50 meters,

the radially-inner portion has an axial length of greater than 15% of the length of the blade,

wherein, in the transition portion, a local blade twist angle of the blade gradually transitions from a local blade twist angle of the radially-outer portion to a local blade twist angle of the radially-inner portion, and

wherein, in the transition portion, an aerodynamic shape of the blade gradually transitions from an aerodynamic shape of the radially-outer portion to an aerodynamic shape of the radially-inner portion; and

manufacturing a rotor blade to the design.

13. A method according to claim 12 , wherein the radially-inner portion has an axial length of less than 50% of the length of the blade.

14. A method according to claim 12 , wherein designing the radially-outer portion of the rotor blade comprises at least setting a local blade twist angle at all points along the radially-outer portion to an optimal angle of attack for energy extraction.

15. A method according to claim 12 , wherein designing the radially-inner portion of the blade comprises setting a local blade twist angle at all points along at least part of the radially-inner portion to an optimal angle of attack to minimize energy extraction.

16. A horizontal-axis wind turbine according to claim 10 , wherein the horizontal-axis wind turbine is an offshore horizontal-axis wind turbine.