It is well known that so-called horizontal axis wind turbines are widely used in wind power generation systems. An ordinary horizontal axis wind turbine is comprised by a rotor whereon at least two or more blades are radially attached from a hub, a nacelle that is connected to the hub and axially supports this rotor through a main shaft that is extended in an approximate horizontal direction, and a tower that is installed in an approximate vertical direction and supports the nacelle in a yaw rotatable state.
Recently progress has been made in increasing the size of horizontal axis wind turbines in order to improve the electrical generation capabilities. Lightweight and strong, FRP (fiber-reinforced plastics) have been the choice for the materials for blades.
Moreover, in order to facilitate ease of transport, separable blades have been proposed wherein part blades, which are partitioned in the longitudinal direction, are connected in series to form the whole. There is, for example, the technology such as described in Japanese Unexamined Patent Application Publication 2004-11616, wherein a connecting member is provided in the vicinity of a connecting section between part blades that are connected in series together, where the part blades and the connecting member are secured together with bolts.
In this case, a connecting member is required when the part blades are connected together, which is undesirable not only because of the precision required in the machining thereof, but because of the additional cost and weight.
Furthermore, there is another technology wherein bolts and straps are used to connect together the part blades such as described in Japanese Examined Patent Application Publication 2003-518586. Connecting in this way forms a large number of protruding parts on the surface of the blade, causing problems in terms of aerodynamic performance and noise. Additionally, this ties to increased weight and cost, the same as with the technology described in Japanese Unexamined Patent Application Publication 2004-11616.
Additionally, when the connecting members between part blades are secured by a metal frame, as described in Japanese Unexamined Patent Application Publication 2003-214322, this not only leads to increased weight and cost, but also to concerns about problems such as resonant frequencies and abrupt changes in stiffness (concentration of stresses).
On the other hand, for wind-power generation, horizontal axis wind turbines are installed in wide open spaces, and the blades thereof are located in environments prone to lightning strikes.
Hereupon, blades comprising a non-conductive material such as FRP differ from metallic blades and if struck by lightning, there is danger that the lightning bolt current will not be able to dissipate to ground resulting in damage. Recently, the increasing sizes of wind turbines which use FRP has resulted in increasing occurrences of damage due to lightning. Because of this, a conductive material is attached to an FRP blade body as a countermeasure against lightning damage in a conventional wind power generation system that uses FRP blades.
In consideration of the various problems described above, in the technology described in Japanese Unexamined Patent Application Publication 2005-220805, two part blades that are connected together, namely an inner blade part (on the blade root side) and an outer blade part (on the blade tip side) are connected together by a connecting member (a barrel nut and a tension bolt) with a metal plate interposed there between. The periphery of this metal plate is exposed at the surface of the blade, and when a lightning strikes, the electrical current from the lightning that is conducted in the metal plate is carried to the outside of the part blade through lightning discharge conductors within the blade, to escape to ground. Although this metal plate functions as a lightning receptor, there is still the problem of the provision of the metal plate leading to increased cost and weight.
The present invention was the result of contemplation on the problems with the prior art described above, and the object thereof is to provide a light and economical separable blade for a wind turbine that is also equipped with a lightning protection function.
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