GB2465485A

GB2465485A - Variable hydraulic transmission for wind turbines

Info

Publication number: GB2465485A
Application number: GB0920282A
Authority: GB
Inventors: Dean Millar; David Parish
Original assignee: University of Exeter
Current assignee: University of Exeter
Priority date: 2008-11-20
Filing date: 2009-11-19
Publication date: 2010-05-26
Also published as: GB0920282D0; GB0821160D0

Abstract

Electricity is generated from wind or water turbines 15, 16 using a hydraulic transmission between the turbines and the shared generator 130. The transmission is controllable so that the blade 40 tip speed ratio (TSR) of the turbine 15, 16 is infinitely adjustable independently of the generator 130 speed, so turbine efficiency can be improved. Adjustment may be achieved by an adjustable pump 50 or motor 120 e.g. using a swash plate. The system could alternatively control the pressure or flow of fluid to or from an accumulator 100, 110.

Description

Electricity generating apparatus The present invention relates to an array of electricity generating apparatus and to a method of generating electricity using said array, wherein the blade tip speed ratio of a turbine blade included in the apparatus is infinitely controllably adjustable.

The use of turbines to produce electricity is well known. Typically the turbines produce torque to drive generators by means of direct mechanical rotation. In the case of wind turbines the generator is typically located in the nacelle at the top of a supporting tower. This arrangement has the consequence that the tower has to be strong enough to support the weight of the generator. Accompanying gear arrangements located between the turbine and the generator exacerbate this situation. It is also known to have the generator located away from the nacelle, possibly at the bottom of the tower, and mechanically connect it to the gear mechanism or turbine via mechanical llnkages such as drive chains. However, these arrangements introduce difficulties such as greater maintenance requirements and further mechanisms to correctiy hnk the motor to the gear or turbine, especially as it is typical for the nacelle to rotate about a substantially vertical axis to allow for changes in the direction of the wind.

Blade tip speed ratio is the ratio of the rotational speed of the blade tip, of the turbine rotor, to the wind speed. The aerodynamic efficiency of a wind turbine rotor varies in relation to the blade tip speed ratio. For instance, generally speaking, it is advantageous to allow the turbine rotor to increase its rotational speed as wind speed increases. However, increasing rotational speed of the turbine rotor beyond a certain threshold often leads to reduced efficiency and is not always desirable for reasons of mechanical/structural competence, noise production and electricity generation.

Accordingly, it is desirable to be able to adjustably control the blade tip speed ratio to maximise efficiency.

The drawback of adjusting the blade tip speed ratio in a direct drive type of turbine is that this affects the electricity generated with regard to, at least, its frequency.

Consequent compllcated mechanisms have been developed to address this issue. For instance, control windings and systems may be used to control rotational speed and the output of a generator. Also, mechanisms allowing the turbine rotor to spill unwanted wind energy, for example at times of relatively high wind speeds, are known. One such

I

system provides means for rotating (feathering) the blades of a turbine rotor about their longitudinal axes (altering their pitch) such that the area they present is altered. Such a system adds to the infrastructure required increasing costs and weight.

Another problem associated with turbines of this type are that they produce turbulence in the wind stream. Accordingly, in an array of turbines each must be located far enough away from the others so that any turbulence produced does not adversely affect any other turbine. It is generally recognised that wind turbines must be located apart by a minimum of 5 times the rotor diameter in a direction substantially perpendicular to the direction of the prevailing wind and 9 times the rotor diameter in a direction substantially parallel to the direction of the prevailing wind. This factor means that relatively small land areas are not typically available for the siting of arrays of wind turbines.

Another way of producing electricity from wind turbines is to use hydraulics.

An example is discussed in \VO 2006/029633 Al in which a wind turbine rotor drives an hydraulic pump to pressurise a fluid which is transported to a separate hydraulic motor which in turn drives a generator. This arrangement is rudimentary and it is therefore an object of the present invention to provide improvements in the production of electricity from the harvesting of wind or other fluid currents.

In a first aspect, there is provided an array of electricity generating apparatus comprising at least two turbines for producing torque from a current of fluid, each turbine comprising a rotor having at least one blade and an hydraulic pump driven by the torque produced by the rotor for producing pressure in an hydraulic fluid, the array further comprising a centralised accumulator for storing said pressurised hydraulic fluid, and a centralised hydraulic motor for driving an electricity generator, the motor being driven by the pressurised fluid stored in said centralised accumulator, wherein each turbine includes blade tip speed ratio control means for infinitely controllably adjusting the blade tip speed ratio of the at least one blade.

It is to be understood that the hydraulic pumps may only create flow rather than pressure in the fluid directly, however, the pressure of the fluid may be indirectly influenced by the pumps.

The apparatus may rely on either, or both, of a hydrostatic and a hydrodvnamic system. The motor may be an axial piston, swash plate motor. The pump may be a radial piston pump, or an axial piston pump.

The term "centralised" with regard to the accumulator may mean that there is only one accumulator, or that there is a main accumulator of a size substantially larger than any other accumulator. The term "centralised" may not relate to its geographical position relative to the other parts of the array. This term may also include the situation where other substantially large accumulators are provided as back-ups during maintenance or repair work of the centralised accumulator.

Similarly, the term "centralised" with regard to the hydraulic motor may mean that there is only one hydraulic motor, or that there is more than one but that only one is in use at any one time, the other(s) being provided as back-ups during maintenance or repair work of the centralised hydraulic motor. The term "centralised" may not relate to its geographical position relative to the other parts of the array.

The underlying concept may be that the accumulator, motor and generator services the array, and that there are many turbines and pumps relative to few accumulators, motors and generators.

By the use of an accumulator the pressure in the hydraulic system may be smoothed to reduce peaks and troughs which naturally occur due to wind speed changes.

The blade tip speed ratio control means may be provided by each hydraulic pump. Each hydraulic pump may be a variable displacement hydraulic pump and the blade tip speed ratio may be infinitely controllably adjustable by means of adjustment of the displacement of the hydraulic pump. Each pump may be of the rotary positive displacement type.

The hydraulic fluid may be mineral-based such as hydraulic oil.

The hydraulic motor may be a positive displacement, variable displacement hydraulic motor and the blade tip speed ratio may be infinitely controllably adjustable by means of adjustment of the displacement of the hydraulic motor.

The hydraulic motor and/or each of the hydraulic pumps may include a swash plate. The hydraulic motor may be of the radial or axial piston type.

The array may include at least one valve for controlling the pressure of the hydraulic fluid stored in the centralised accumulator, and/or for controlling the flow of said stored pressurised hydraulic fluid to the hydraulic motor, wherein the blade tip speed ratio may be infinitely controllably adjustable by means of the control of the pressure and/or flow of the said stored pressurised hydraulic fluid to the hydraulic motor.

The array may include a ow pressure reservoir for receiving and temporarily holding the hydraulic fluid after it has passed through the hydraulic motor. The low pressure nature of this reservoir allows for a pressure gradient to be created across the motor from the higher pressure accumulator.

The current of fluid may be provided by an air stream or by the movement of water such as is found in rivers, estuaries and the open sea. Accordingly, although the invention is primarily described in relation to wind turbines it should be understood that this is merely by way of example and should not be seen as limiting the invention. All of the description relating to wind turbines should be read as applying to water driven turbines or, indeed, driving of turbine rotors by any other fluid movement.

The array may further include an electricity generator, wherein the generator is either synchronous or asynchronous. The generator may have a substantially large angular momentum to assist fault ride-through.

The effective output of each hydraulic pump in the array may be less than or equal to 600kw. Alternatively, the effective output of each hydraulic pump in the array may lie in the range 200kVQT to 600kW. Other ranges are contemplated, such as from 20kW to 600kw. In this regard, the term "effective" may mean that the hydraulic pump may produce more power than the values stated but that due to the losses induced through the system, such as in the conduit network linking the pumps to the accumulator and motor etc., some of this power is lost.

The spacing of the individual turbines in the array may be less than 5 times the diameter of the turbine rotor in a direction substantially perpendicular to the prevailing direction of the current of fluid. The spacing may be substantially or significantly less.

Furthermore, the spacing of the individual turbines in the array may be less than 9 times the diameter of the turbine rotor in a direction substantially parallel to the prevailing direction of the current of fluid. The spacing may be substantially or significantly less.

Alternatively, the spacing of the individual turbines in the array may be less than 3 times the diameter of the turbine rotor in a direction substantially perpendicular to the prevailing direction of the current of fluid.

In one embodiment, the spacing of the individual turbines may be less than 6 times the diameter of the turbine rotor in a direction substantially parallel to the prevailing direction of the current of fluid.

The overall hydraulic coupling between the turbine rotor(s) and the electrical generator aliows greater cushioning from the effects of shock loading of drive trains due to turbulence from an upstream wind turbine rotor or variable and gusting wind speeds.

This aliows for the turbines to be sited closer together in the array than in other known arrays. In addition, the closer positioning of the turbines affords opportunities for optimal control for enhanced wind power capture across the array as a whole.

The array may include a means for driving the electricity generator by means other than the said turbines. For instance, the array may include a hydraulic pump driven by a conventional internal combustion engine powered by petrol, diesel, biogas, syngas or the like. The resultant pressurised hydraulic fluid may be fed into the system (such as the accumulator) to maintain the operation of the hydraulic motor during times when it is not possible to utilise wind, or there is not enough wind, to produce the required pressurised hydraulic fluid. Alternatively, or additionally, the internal combustion engine may be connected directly to the generator to operate it and produce electricity.

In a second aspect, the invention provides a method of generating electricity from an array of turbines, as described and/or as claimed herein, by infinitely controllably adjusting the blade tip speed ratio of the at least one blade in each individual turbine rotor independently from the other turbine rotor(s).

The blade tip speed ratio of the at least one blade in each individual turbine rotor may be infinitely controllably adjustable by means of adjusting the displacement of each hydraulic pump independently from the other hydraulic pump(s).

The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing, which illustrates, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawing.

Figure 1 is a schematic of an array of electrical generating apparatus according to one embodiment of the invention.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawing described is only schematic and is non-limiting. In the drawings, the si2e of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Similarly, it is to be noticed that the term "connected", used in the description, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression "a device A connected to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Connected" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may refer to different embodiments.

Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this

disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth.

However, it is understood that embodiments of the invention may be practised without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of

this description.

The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims.

In Figure 1 the general arrangement of the array is referenced 10. It comprises two turbines 15, 16. Each turbine 15, 16 includes a turbine rotor 40 comprising three blades, although other numbers of blades are possible. Each turbine rotor 40 is mounted for rotation on a nacelle 30. Each nacelle 30 is mounted at the top of each turbine 15, 16 supported by a tower 20.

Although not shown, means for directing the rotors 40 into the wind are contemplated.

Within each nacelle 30 an hydraulic pump 30 is mounted for operation by means of the torque generated by each rotor 40. It is, of course, possible that the hydraulic motors are mounted in places other than the nacelles 30, such as in the tower, but still mechanically connected to the rotors 40.

Each hydraulic pump 50 produces pressure in an hydraulic fluid which is directed to and from the pumps 50 by means of conduits 60, 70. These conduits 60 direct the hydraulic fluid to an accumulator 150 located away from the devices 15, 16.

A hydraulic motor 120 is driven by the pressurised fluid stored in the accumulator 150. This in turn produces torque in the form of a rotating drive shaft 125 which in turn drives an electrical generator 130. The generator 130 produces electricity 140 which is passed to a local and/or national electricity supply grid. The motor 120 may be chosen such that its efficiency is greatest when the rotational speed of the drive shaft 125 lies in the range 1000 to 1500 rpm. The motor 120 is rotated due to the flow of hydraulic fluid. This flow is created by higher pressure fluid coming from the accumulator 150 and passing to a lower pressure reservoir 160. The hydraulic fluid is returned to the hydraulic pumps 50 from this lower pressure reservoir by conduits 70.

The flow of fluid and the fluid pressure within the various components of the array 10 may be controlled at least by means of valves (not shown) arranged throughout.

These valves may be used to isolate individual turbines 15, 16. Additionally, the valves may be used to regulate the flow and return of pressurised fluid to and from accumulator 150 and low pressure reservoir 160.

Monitoring and control equipment, not shown, may be used to monitor and control all aspects of the array 10 whether of the parts of the array individually, in groups, or collectively. For instance, the rotational speed of a turbine rotor 40 may be monitored and if it reaches a preset threshold value its speed may be controlled in one or more of a number of ways. For instance, if the hydraulic pump 50 within the nacelle is of the variable displacement type the displacement may be varied to increase the mechanical resistance to rotation. Furthermore, if the pump is of the radial piston, variable displacement type, the displacement may be varied by adjusting the settings of an eccentric cam. If the pump is of an axial piston, variable displacement type, the displacement may be varied by altering the swash plate angle. Another way is to introduce a pressure gradient in the hydraulic fluid across that pump 50 such that, again, the mechanical resistance to rotation is increased. This may be achieved by increasing the pressure in the hydraulic fluid on the outlet side of the pump 50. This increase in pressure may be supplied from the centralised 150 accumulator. Alternatively, or additionally, a mechanical gear box and/or a braking system may be included for use with the hydraulic pump and/or turbine rotor. In other words, tile rotational speed of the rotor 40 may be varied.

It will be understood that these methods may he adapted to reduce mechanical resistance in the hydraulic pumps and thus allow a turbine to rotate more quickly, or to increase resistance to such an extent that it is possible to completely stop one or more rotors 40 from rotating if, for instance, the peak electrical power output of the array 10 can be met with on'y a few of the turbines in the array.

The blade tip speed ratio may, in the ways described, be continually, or intermittently, infinitely controllably adjustable from zero to a maximum and allow it to be maintained, as necessary, at an optimum level. This allows for a more flexible operating methodology. Furthermore, the invention described herein may allow for a closer grouping of turbines than is typical. The present limit is typically a minimum spacing between wind turbines of five times the diameter of the turbine rotors 40. This is to mitigate for turbulence created by each turbine from affecting other turbines.

However, if the turbine rotors can be effectively operated, as described herein, at slower rotational speeds, or be individually controllable then this minimum spacing may be reduced thus allowing smaller land areas to be utilised.

In one embodiment, the present invention allows for the blade tip speed ratio to be in the range 1:5 to 1:15.

The control may be provided in the form of signals sent via electrical and/or optical and/or radio frequency and/or mechanical means. It is also possible that control signals may be sent as pressure pulses in the hydraulic fluid. Other means are also contemplated. Although only two turbines 15, 16 have been shown, other turbines may be included in the array 10.

Although not shown, the array 10 may further include a reservoir of hydraulic fluid from/to \vhlch additional fluid may be added/removed to/from the conduit system.

As already mentioned, the array 10 described herein could be adapted for use \vith water currents as opposed to air currents.

The array 10 lends itself particularly to one in which the effective output of each hydraulic pump is less than 600kw, such as a range of 200 to 600kw, or from 20kW to 600kW.

Since it is possible for eaci turbine to be controlled individually or collectively with one or more other turbine, an array of turbines may be managed in a more efficient manner.

Claims

Claims 1. An array of electricity generating apparatus comprising at least two turbines for producing torque from a current of fluid, each turbine comprising a rotor having at least one blade and an hydraulic pump driven by the torque produced by the rotor for producing pressure in an hydraulic fluid, the array further comprising a centralised accumulator for storing said pressurised hydraulic fluid, and a centralised hydraulic motor for driving an electricity generator, the motor being driven by the pressurised fluid stored in said centralised accumulator, wherein each turbine includes blade tip speed ratio control means for infinitely controllably adjusting the blade tip speed ratio of the at least one blade.
2. The array according to claim 1, wherein the blade tip speed ratio control means is provided by each hydraulic pump.
3. The array according to claim 2, wherein each hydraulic pump is a variable displacement hydraulic pump and the blade tip speed ratio is infinitely controllably adjustable by means of adjustment of the displacement of the hydraulic pump.
4. The array according to any preceding claim, \vherein the hydraulic motor is a positive displacement, variable displacement hydraulic motor and the blade tip speed ratio is infinitely controllably adjustable by means of adjustment of the displacement of the hydraulic motor.
5. The array according to any preceding claim, wherein the hydraulic motor and/or each of the hydraulic pumps include a swash plate.
6. The array according to any preceding claim, including at least one valve for controlling the pressure of the hydraulic fluid stored in the centralised accumulator, and/or for controffing the flow of said stored pressurised hydraulic fluid to the hydraulic motor, wherein the blade tip speed ratio is infinitely controllably adjustable by means of the control of the pressure and/or flow of the said stored pressurised hydraulic fluid to the hydraulic motor.
7. The array according to any preceding claim, wherein the current of fluid is provided by an air stream.
8. The array according to any one of claims I to 6, wherein the current of fluid is provided by the movement of water.
9. The array according to any preceding claim, further including an electricity generator, wherein the generator is either synchronous or asynchronous.
10. The array according to any preceding claim, wherein the effective output of each hydraulic pump is less than or equal to 600kw.
11. The array according to any preceding claim, wherein the effective output of each hydraulic pump is in the range 200kW to 600kW.
12. The array according to any preceding claim, wherein the spacing of the individual turbines is less than 5 times the diameter of the turbine rotor in a direction substantially perpendicular to the prevailing direction of the current of fluid.
13. The array according to any preceding claim, wherein the spacing of the individual turbines is less than 9 times the diameter of the turbine rotor in a direction substantially parallel to the prevailing direction of the current of fluid.
14. The array according to any preceding claim, wherein the spacing of the individual turbines is less than 3 times the diameter of the turbine rotor in a direction substantially perpendicular to the prevailing direction of the current of fluid.
15. The array according to any preceding claim, wherein the spacing of the individual turbines is less than 6 times the diameter of the turbine rotor in a direction substantially parallel to the prevailing direction of the current of fluid.
16. The array according to any preceding claim including a means for driving the electricity generator by means other than the said turbines.
17. A method of generating electricity from an array of turbines, according to any one of claims 1 to 16, by infinitely controllably adjusting the blade tip speed ratio of the at least one blade in each individual turbine rotor independently from the other turbine rotor(s).
18. The method of claim 17, wherein the blade tip speed ratio of the at least one blade in each individual turbine rotor is infinitely controllably adjustable by means of adjusting the displacement of each hydraulic pump independently from the other hydraulic pump(s).
19. An array of electricity generating apparatus substantially as hereinbefore described with reference to the accompanying drawings.
20. A method of generating electricity from an array of turbines substantially as hereinbefore described with reference to the accompanying drawings.