Method and device for monitoring the state of rotor blades

1. Method for monitoring the state of a rotor blade of a wind turbine, comprising:

measuring an acceleration of the rotor blade with a first signal, wherein the acceleration is measured at a first radial position at a predetermined distance from the rotor blade root in at least one direction comprising a first directional component orthogonal to the axis of the rotor blade;

measuring a strain of the rotor blade with a second signal, wherein the strain is measured at a second radial position disposed in the area of the first radial position to the rotor blade root;

determining a first positional change on the basis of the acceleration;

determining a first value corresponding to the rotor blade stiffness by means of calculation on the basis of the first positional change and the strain; and

determining the rotor blade state from the first value.

2. The method of claim 1 , wherein, for determining said first positional change, the signal of the acceleration is integrated over time.

3. The method of claim 1 , wherein the acceleration is measured in three directions, and wherein the strain is measured in order to measure bending moments in two directions.

4. The method of claim 3 , wherein the strain is measured in two orthogonal directions.

5. The method of claim 1 , wherein, for determining the rotor blade state, said first value is averaged.

6. The method of claim 5 , wherein said first value is averaged over a time period of 1 hour or longer.

7. The method of claim 1 , wherein the acceleration is measured by means of a fiber-optic acceleration sensor and/or wherein the strain is measured by means of a fiber-optic strain sensor.

8. The method of claim 1 , wherein the first radial position is located approximately at half the blade radius or in between half the blade radius and a rotor blade (tip and/or wherein the second radial position is located at a distance of 5 meters or less from the blade root.

9. The method of claim 1 , wherein for determining said first value, a coordinate transformation into a coordinate system of said wind turbine and/or into a coordinate system of the rotor hub is performed.

10. Device for monitoring the state of a rotor blade of a wind turbine, comprising:

at least one acceleration sensor adapted to measure an acceleration of the rotor blade, wherein the acceleration is measured in at least one direction comprising a first directional component orthogonal to the axis of the rotor blade;

at least one strain sensor adapted to measure of a strain of the rotor blade with a second signal, wherein the strain is measured at a second radial position disposed in the area of a first radial position of the acceleration sensor to the rotor blade root;

an evaluation unit connected to the at least one acceleration sensor for receiving a first signal from the acceleration sensor and connected to the at least one strain sensor for receiving a second signal from the strain sensor, and wherein the reception of the first signal is performed from the first radial position at a predetermined distance from the rotor blade root;

wherein the evaluation unit is adapted to determine a first positional change at the first radial position on the basis of the first signal of the acceleration sensor;

wherein the evaluation unit is adapted to determine a first value corresponding to the rotor blade stiffness by means of calculation on the basis of the first positional change and the second signal.

11. The device according to claim 10 , wherein the at least one acceleration sensor is a fibre-optic sensor, and/or wherein the at least one strain sensor is a fibre-optic strain sensor.

12. The device of claim 10 , wherein the first radial position is located within the range of half the blade radius to the blade tip and/or wherein the second radial position is located at a distance of 5 meters or less from the blade root.