Counterweighting a wind turbine hub

1. A counterweight system for a wind turbine comprising a hub mounted to a nacelle such that the hub is rotatable around a rotation axis with respect to the nacelle, the hub comprising a blade root region configured to receive a root of a blade and having a pitch system configured to rotate the blade around a pitch axis; the counterweight system comprising:

a beam mountable to the blade root region at a first point of the beam and a counterweight mass coupled to the beam at a second point of the beam at a distance from the first point of the beam, such that, when the beam is mounted to the blade root region, the beam is arranged substantially perpendicular to the pitch axis and the pitch system can cause the counterweight mass to rotate around the pitch axis.

2. A counterweight system according to claim 1 , wherein the hub is configured to carry a plurality of blades each having a mass; and wherein the counterweight mass is substantially equal to the mass of a blade.

3. A counterweight system according to claim 2 , wherein the beam is a telescopic beam such that the distance between the first and second points of the beam can be varied.

4. A counterweight system according to claim 3 , wherein the wind turbine is a direct drive wind turbine.

5. A counterweight system according to claim 2 , wherein the wind turbine is a direct drive wind turbine.

6. A wind turbine comprising a hub and a counterweight system according to claim 2 , wherein:

the counterweight system is mounted to the blade root region.

7. A counterweight system according to claim 1 , wherein the beam is a telescopic beam such that the distance between the first and second points of the beam can be varied.

8. A counterweight system according to claim 7 , wherein the wind turbine is a direct drive wind turbine.

9. A counterweight system according to claim 1 , wherein the wind turbine is a direct drive wind turbine.

10. A wind turbine comprising a hub and a counterweight system according to claim 1 , wherein the counterweight system is mounted to the blade root region.

11. A method of mounting a blade to a hub of a wind turbine by using a counterweight system according to claim 1 , wherein

the counterweight system is mounted to the blade root region having the pitch system; the method comprising:

operating the pitch system for causing rotation of the counterweight mass, thereby inducing a rotation of the hub to a desired position for mounting the blade to the hub;

mounting the blade to the hub.

12. A method according to claim 11 , further comprising operating a locking system of the wind turbine for locking the hub once the desired position for mounting the blade to the hub has been achieved.

13. A method according to claim 12 , further comprising:

operating the pitch system for causing rotation of the counterweight mass to a position for at least partially compensating for a variation of torque on the hub due to weight loads resulting from mounting the blade to the hub.

14. A method according to claim 13 , wherein operating the pitch system for at least partially compensating for the variation of torque on the hub is performed before mounting the blade to the hub, such that the variation of torque is compensated in advance.

15. A method according to claim 11 , further comprising operating a locking unit of the pitch system for locking the rotation of the counterweight mass once a position for at least partially compensating for a variation of torque on the hub has been achieved.

16. A method according to claim 11 , wherein the desired position of the hub is a position in which the blade can be mounted horizontally.

17. A method of mounting two blades to a hub of a wind turbine by using a counterweight system, the wind turbine comprising a huh mounted to a nacelle such that the hub is rotatable around a rotation axis with respect to the nacelle, the hub comprising a blade root region configured to receive a root of a blade and having a pitch system configured to rotate the blade around a pitch axis; and the counterweight system comprising a beam mountable to the blade root region at a first point of the beam and a counterweight mass coupled to the beam at a second point of the beam at a distance from the first point of the beam, such that, when the beam is mounted to the blade root region, the beam is arranged substantially perpendicular to the pitch axis and the pitch system can cause the counterweight mass to rotate around the pitch axis; the method comprising:

mounting a first blade according to claim 11 ;

operating the pitch system for causing rotation of the counterweight mass, thereby inducing rotation of the huh with the first blade to a desired position for mounting a second blade to the huh; and

mounting the second blade to the hub.

18. A method according to claim 17 , wherein the desired position of the hub for each of the blades is a position in which the blades can be mounted horizontally.

19. A method according to claim 18 , wherein:

the wind turbine further comprises a yaw system for rotating the nacelle around a yaw axis; and the method further comprising

operating the yaw system for rotating the nacelle substantially 180° before mounting the second blade.

20. A method for mounting three blades to a hub of a wind turbine comprising a method of mounting two blades according to claim 17 , and further comprising

operating the pitch system for causing rotation of the counterweight mass, thereby inducing rotation of the hub with the two blades to a desired position for mounting a third blade to the hub;

removing the counterweight system from the hub; and

mounting the third blade to the hub.