GB2605939A - Improvements in and relating to wind turbines - Google Patents

Abstract

A wind turbine mounted atop a turbine-supporting tower whereby the torque generated through rotation of the blades is split, within the turbine nacelle, into separate outputs. The aim is to lessen the stresses on the nacelle and/or the gearing mechanisms which form a necessarily inherent integer of the turbine output drive train. Preferably included are respectively differentially driven output shafts into the drive train. The differentially driven output may be concentric. A main bearing supporting the turbine blades may be located in the nacelle immediately behind a blade carrying hub of the turbine. Preferably a secondary one of the differentially driven output shafts drives a DC generator.

Description

Improvements in and relating to Wind Turbines

Field of the Invention

The invention relates to wind turbines as a whole and to the power-generating nacelles and nacelle-supporting towers which between them constitute the turbines.

Review of Art known to the Applicant A conventional 8000Kw turbine runs at a rotational speed 10 to 15rpm under medium/high wind speeds. A conventional AC generator needs a speed of 1500rpm or so to match AC 50cycles per second. Therefore there has to be step-up gearing of between 100 and 150:1. This immediately makes it desperately sensitive to fluctuation loads.

The blade shaft "sees-around 5,000,000 Nm of torque, with start-up reversals of the same magnitude.

So a middle number (in Imperial units) is 3,750,000 ft/lbs. If we step this by 100:1 gear ratio we get 37,500 ft/lbs at 1500rpm. In Horse-Power that is 10,700hp, or 8000Kw No account taken of efficiency losses, so the torque's likely to be even higher.

The Problem to be Solved The problem is that any kind of step-up gearing in any application is inherently and indeed disproportionately sensitive to variations in input speed and torque. The very nature of the generator is that it will try to maintain 50cycles/sec by increasing output rather than speed. There is also a significant amount of inertia to overcome.

If some sort of absorbency could be incorporated into the blade shaft drive then all the rest 10 of the system could be down-sized and made very reliable Absorbency does tend to be inefficient, but should be balanced against down-time and repairs.

There are a number of ways of doing this but not all are scalable The Objectives of the Invention To create a constant 50cycles/sec output regardless of wind speed and gusting via an additional sensitive control system as an addition to the known but relatively crude system of blade pitch control To mitigate or remove the massive overloads and reversals caused during start-up and during gusty weather.

To expand the operating range below and above current norms without blade overspeed.

To improve reliability and service life and reduce service costs.

To keep construction costs within a desired current envelope To maintain or Improve the static balance of the generator capsule atop the turbine-supporting tower.

The Inventive Concept In accordance with the inventive concept the objectives listed immediately above are addressed in the following manner.

Currently the first step in power conversion behind the rotor hub is a relatively massive epicyclic step-up gearbox of a fixed ratio to suit the turbine design (but between 50 and 150:1). It is sized to accommodate the torque, it is even bigger to accommodate the sudden loading variations, and thus it has high internal inertia that compounds the loading problems. If this ratio can be reduced, the sudden excess loadings become easier to accommodate.

An epicyclic layout lends itself to incorporate a differential that splits the output into two shafts dependent upon each other for the ratio of torque split vs output speed.

One shaft drives the main AC generator, and it can be seen that its rotational speed can be controlled as a fully variable ratio dependent upon the speed of the rotor vs the speed of the second output shaft. Careful electronic monitoring and control of the latter allows the former to maintain the 50cps generator speed over a greater rotor speed range without resort to expensive phase-adjusting power electronics.

Having, by the incorporation of the differential, converted the majority of the inertial strains into accelerations within the DC generator it becomes possible to stabilise the main AC supply by the addition of a flywheel to the AC generator. Although slower to wind up to speed, there is little energy loss as the DC side works overtime in generation during the wind-up. Once wound up, variability of the required 50cps is much reduced and is much easier to control. Tt is possible that a series of smaller AC units, driven in parallel to the main unit, could be electrically or mechanically switched in or out to cope with very light or overly heavy loadings, the flywheel providing the stability necessary. The design balance between gear ratios, AC/DC splits and inertias can be tailored to suit the working conditions and power requirements of each application. This could result in significant cost reductions over the "one-size-fits-all" approach.

A conventional differential has a simple 50% torque/speed bias, but it is not an issue to create other percentages. Assume 80% to the AC generator and 20% to the second output. This split provides a key area for refinement. As we may not have a limited slip (but we could if it was thought to be an advantage) it is clear that, if the second output is left unbraked, it will spin free at a higher speed and no torque or motion will be fed to the AC generator.

Higher speeds equal smaller scale and better controllability. If the second shaft is connected to a DC generator, the load upon that generator (and therefore the torque and speed supplied to the AC unit) can be very finely controlled by simple low-voltage control of the input to the field windings. The more energy we put into the field the more DC output we get from the generator and the greater the torque applied back through the system to spin up the big AC unit. Thus we can maintain the critical speed for the AC unit without resort to expensive phasing power electronics, and balance its output against a significant infinitely variable and easily controllable DC supply from the other side.

Whilst the field control electronics are simple and cheap, some of the more expensive electronics then shift to the DC side to invert the supply to AC and phase it into the grid alongside the output from the main AC generator. But this technology is well proven, at least on a smaller scale.

The efficiency of an unbraked differential is in the high 90's in terms of percentage.

Therefore almost any wind speed sufficient to rotate the hub below the phasing speed of the main generator would be sufficient to generate power through the DC side, thus broadening the supply range. In addition, the principle could be scaled at least once more, with a further differential added downstream on the DC side driving ever smaller generators to broaden the range further. Relatively simple cost/benefit calculations would dictate just how far to go for advantage.

A further significant advantage of generating DC electricity is that, when the electrical demand of the grid is below that of the supply, it can be used to generate hydrogen, which could then be pumped ashore to mn emission-free combustion engines, further broadening the usage window of the turbine.

That covers the first five of the objectives listed above; the sixth being slightly more esoteric, namely that of the static balance of the entire assembly. Research shows that this is important for the tower construction cost.

The main hub shaft must be kept as short as possible and brings the first epicyclic gearbox close up behind the turbine, offsetting the CofG undesirably. The shaft is also of a massive diameter with the centre core doing nothing for torque transmission and strength. It can therefore be hollow. An epicyclic gearbox and epicyclic differential lend themselves to through-shaft design. The AC generator could allow a through-shaft running to the rear of the combined unit to drive the secondary DC system described, thus significantly improving the static balance.

The Accompanying Drawing Reference is now made to the accompanying single sheet of drawings, each and all of whose contents are hereby incorporated into a disclosure of this specification The drawing illustrates schematically a wind turbine generator embodying the present invention and its interpretation will be apparent to those who are appropriately skilled in the field to which the invention relates and who have read and reflected on the text above.

Aspects resulting from the Inventive Concept 1. Concentric shafts exiting the differential enable better structural balance of the entire 30 turbine.

2. Since a differential is a torque-splitting device, if the DC motor is made to run slowly, the AC generator will run fast. Thus the troublesome step-up gears of normally (say) 100:1 ratio could reduce to (say) 75:1, thus improving their ability to withstand shock loads and improving the mechanical efficiency.

3. The above arrangement allows the blade pitch to be used to maximise efficiency (torque) rather than being used to control blade speed, thus broadening the wind-speed range within which the turbine can operate.

4. The rapid and sometimes almost instantaneous speed control achievable by the use of a feedback loop to manipulate the field winding resistance in the DC generator allows the AC unit to run at its 50cycles/second control target regardless of rotor speed and without the need for complex re-phasing electronics.

5. The feedback loop and speed control system could also be used rapidly as a fluctuation damper to cancel input shock loads, thus further reducing gearbox stresses.

6. If the AC circuit is controlled to 50c/s by virtue of the DC circuit acting as a speed control, the DC circuit simply generates a varying amount of power that makes up the difference. As DC volts this can be run through a relatively simple sine-wave inverter and phased into the existing AC output, or it can be used as DC volts to charge a storage system, or it can be used to manufacture Hydroxy, thus broadening the versatility and the operating range of the turbine.

Scope of the Invention The scope of the invention is defined in the numbered claims which now follow and which are to be interpreted in the manner currently established by statute and by relevant currently authoritative case law precedent

Claims (1)

1. Claims 1 A wind turbine, intended and adapted to be mounted in use atop a turbine-supporting tower, characterised by the provision of means by which the torque generated by the rotation of the blades of the turbine in use is split, within the turbine nacelle, into separate outputs whose overall effect is to lessen the stresses on the nacelle and/or the gearing mechanisms which form a necessarily inherent integer of the turbine output drive train 2. A turbine according to claim 1 and characterised by the incorporation of respectively differentially driven output shafts into the drive train.3. A turbine according to claim 2 and characterised in that the differentially driven output shafts are concentric.4. A turbine according to any preceding claim and characterised in that the main bearing supporting the turbine blades in use is located in the nacelle immediately behind the blade-carrying hub of the turbine.5. A turbine according to any preceding claim and characterised in that a secondary one of the differentially driven output shafts drives a DC generator in use.