GB2382381A

GB2382381A - Improvements in wind turbines

Info

Publication number: GB2382381A
Application number: GB0127897A
Authority: GB
Inventors: John Freer Green
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-11-21
Filing date: 2001-11-21
Publication date: 2003-05-28
Also published as: AU2002343017A1; GB0127897D0; WO2003046376A1

Abstract

A wind turbine comprising more than one set of bladed rotors mounted concentrically, the outer rotor rotating at a slower speed than the inner rotor(s). The outer set of rotor blades may rotate on bearings supported on a wheel comprising a ring girder suspended by cables attaching it to a central cylinder. The support wheel may rotate on a vertical axis to allow the rotor blades to face the wind, and may be mounted on a fixed tower stayed to the ground by cables and anchor points that also rotate with the support wheel. The turbine may also be mounted on a floating raft at sea or on a lake. Electrical power may be extracted by stator windings on the support wheel using magnetic fields rotating with the blades.

Description

Improvements in wind Turbines The invention relates to a method of increasing the output of wind turbines by employing multiple concentric rotors The output of single rotor wind turbines is limited by the fact that there is an optimum velocity at which an element of the rotor blade, of given chord, should move transversely to the wind for maximum torque. When moving below optimum speed the aerodynamic force on the element has a component in the direction which accelerates the element. As the speed increases the magnitude of the force increases, but the direction changes so that the component in the direction of rotation falls, Thus at speeds above the optimum the force will become smaller and eventually become a retarding force. In a single rotor the velocity of an element transverse to the wind is the product of the angular velocity and the radius of the element. Thus only at one radius is the aerodynamic force optimum. Outboard of this radius the blade is moving too fast, inboard too slow. In optimising the blade design the chord will vary with radius. Similar amounts of power can be extracted from a narrow blade moving fast (low solidity) or a broader blade moving more slowly (higher solidity).

Structural and cost considerations favour low solidity designs for electricity generation.

As the length of the blade is increased to collect energy from a larger disc the angular velocity must be reduced to prevent the tip torque becoming negative. The energy collection from the central parts of the disc would therefore become less, and the optimum chord in the centre will increase.

By dividing the rotor disc into a number of annular rings, and allowing the inner rings to rotate at a higher angular velocity than the outer, each ring can be optimised separately, thus leading to greater output from a given disc area.

There is also the possibility of increasing the swept area of the disc by using several rings of blades of similar length to those currently in use, As an example of this invention a 3-rotor wind turbine designed to give an output of 25MW is described on page 2 and in figures 1 to 7 Figure 1 shows the rotors Figure 2 shows the support wheel in front and side view Figure 3 shows the lattice ring girder in more detail.

Figure 4 shows the tower with part of the wheel and the anchor carriage Figure 5 is a front view of the complete wind turbine Figure 6 is a plan view of the complete wind turbine Figure 7 is a side view of the complete wind turbine

25 Megawatt wind turbine

Rotors See Figure 1 The inner rotor (1) has three blades each 35 metres long and it rotates at 16 rpm. This is the same as is used in current wind turbine designs and will give an output of 1 5 MW Outboard of this the second rotor (2) has nine blades, each 45 metres long rotating at 7rpm and giving an output of 6 6MW These blades are mounted on a ring of 35 metres radius so that the inside of this rotor has the same radius as the tips of the inner rotor. The third annular rotor (3) has 21 blades 60 metres long mounted on a ring or radius 80 metres. It rotates at 4rpm and produces 17MW Each blade is mounted on a wheeled bogie that runs on rails on the outside of the ring girder Support wheel See Figures 2 & 3 The rotors are supported on a wheel (4) made up of a ring lattice girder (4a) which is connected to a central horizontal cylinder (4b) by a number of steel cables (4c). The horizontal cylinder is rigidly attached to a yaw bearing (4d) so that the whole structure can be turned into the wind. The outer rotor revolves on rails on the periphery of the ring lattice girder (4a) whilst the middle rotor revolves on a similar, but smaller, ring girder (4e) that is secured to the cables (4c) on the upwind side. The bearing for the inner rotor is on the upwind end of the cylinder (4c).

Tower See Figure 4 The support wheel assembly is mounted on a slim tower 150 metres high (5). If land based, the tower may be stayed by steel cables in order to carry the wind loads down to ground.

As the rotors and their supports must rotate about a vertical axis to face the wind the stay cables will also have to rotate so the ground end of the cables will be anchored to a carriage (6) that moves on a circular track with the rotor support as it yaws into the wind..

If based offshore in shallow water the tower can be mounted on the sea bed and the stay cables can be secured to a heavy floating raft that can revolve around the tower so that the rotors face the wind.

In deeper water the whole assembly can be mounted on a raft so that it can turn to face the wind Power extraction The power from the inner rotor is extracted via a gearbox and alternator as in current wind turbine designs. For the middle and outer rotors there are several possibilities Since there is relative movement between the inner ring of the rotor and the support wheel, a ring of multipol magnets on the rotor will induce alternating voltages in conductors mounted on the support wheel, the stator For an 80 metre radius rotor at 4rpm, 750 poles at a pitch of 67cm are required to give 50 hertz. This would be the simplest system as no gearbox is required, but the stator windings are spread out and access is difficult.

If the rotor ring carries a toothed rack then a pinion in a cabin at he lowest point of the support ring girder can engage the rack and drive an alternator in the cabin, Alternatively the alternator could be driven by a chain and sprocket, rather than a rack and pinion

Claims

Claims 1 A Wind Turbine comprising more than on set of blades mounted concentrically, with the outer set rotating more slowly than those in the centre.
2 A turbine as claimed in Claim 1 where the outer sets of rotor blades rotate on bearings supported on a wheel comprising a ring girder and cables attaching it to central cylinder.
3 A turbine as claimed in Claim 1 where the support wheel can turn on a vertical axis so that the rotor blades face into the wind.
4 A Turbine as claimed in Claims 1 to 3 where the support wheel is mounted on a fixed tower which is stayed by cables attached at the ground end to anchor carriages that rotate with the support wheel.
5 A Turbine as claimed in Claims 1 and 2 where the support wheel is mounted on a raft floating at sea or in a lake.
6 A turbine as claimed in Claim 1 where the electrical power is extracted from stator windings on the support wheel in which alternating voltages are induced by multipol magnetic fields rotating with the blades.