WO2024236269A1

WO2024236269A1 - Wind turbine

Info

Publication number: WO2024236269A1
Application number: PCT/GB2024/051175
Authority: WO
Inventors: Steven William GEE
Original assignee: Fairfield Ip Limited
Priority date: 2023-05-15
Filing date: 2024-05-03
Publication date: 2024-11-21
Also published as: GB202307229D0; GB2630058A

Abstract

The invention relates to a wind turbine for the generation of electricity. The wind turbine has a number of blades carried by a hub which is rotatably mounted to a tower. The wind turbine has a first reservoir and a second reservoir for a working liquid, the working liquid in the second reservoir being above the working liquid in the first reservoir. The wind turbine also has a turbine with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, an output shaft of the turbine being connected to the rotor of an electrical generator. The hub is connected by way of a drive shaft to at least one positive displacement pump. The pump has an inlet and an outlet, the inlet being in communication with the first reservoir. The outlet of the pump has a two-way valve which is changeable between a first condition in which the outlet is in communication with the first reservoir and a second condition in which the outlet is in communication with the second reservoir. The outlet of the pump can be switched between a second condition in which working fluid is pumped into the second reservoir and a first condition in which the working fluid is pumped back into the first reservoir. There can be a plurality of pumps which are independently switchable between their first and second conditions whereby to enable the power consumed by the pumps to be adjusted substantially to balance the power being extracted from the wind. The displacement of the pumps can differ.

Description

WIND TURBINE

FIELD OF THE INVENTION

The invention relates to a wind turbine, and in particular a wind turbine for the generation of electricity.

For the avoidance of doubt, in this specification the term “wind turbine” is used to describe the whole structure including the rotating blades, the tower supporting the blades, and the electrical generating components.

Unless otherwise indicated, directional and orientational terms such as “top”, “bottom”, “lower”, “higher” etc. refer to the orientation of the components of an erected wind turbine as represented in Fig. 1 .

BACKGROUND TO THE INVENTION

Wind turbines are playing an increasing part in generating electricity for the mains electrical networks of many countries. The wind turbine can be located on land (“onshore”) or at sea (“offshore”).

The UK has a relatively developed offshore wind turbine infrastructure with around 10 gigawatts of generating capacity. The UK government has recently published its desire to increase the offshore wind turbine generating capacity to 50 gigawatts by 2030. The number of wind turbines which will be erected around the coast of the UK will therefore increase significantly over the coming years. Also, large numbers of offshore wind turbines are expected to be erected in many other countries around the World, it being recognised that even those countries with the most developed infrastructures are not close to reaching the limits for the available offshore generating capacity.

Whilst the present invention is equally applicable as an onshore or an offshore wind turbine, the specification will refer predominantly to offshore applications as the requirement for offshore wind turbines is expected to exceed the requirement for onshore wind turbines for the foreseeable future. In addition, the desire for increased efficiency generally requires larger wind turbines and it is preferable to locate larger wind turbines offshore where they are less obtrusive and cause less disturbance.

Many offshore wind turbines have megawatt generating capacities and the largest currently available wind turbine is believed to be able to generate 15 megawatts. Offshore wind turbines are often built in groups (or wind farms) comprising many dozens of wind turbines which can together supply gigawatts of electrical energy.

Many of the offshore wind turbines which are presently in use have three blades mounted to a central hub which is in turn mounted to a horizontal drive shaft. The blades and drive shaft may typically rotate at around 15 to 40 rpm for example. A step-up gearbox is provided to increase the rate of rotation as required for the efficient generation of electricity, a suitable rotation rate for an electrical generator being around 1 ,500 rpm for example. The electrical generator of many offshore wind turbines is a doubly-fed induction generator which can provide an AC output with a stable frequency despite short-term variations in the rotation rate of the rotor of the generator (as might occur during wind gusts for example).

The hub, gearbox and electrical generator are typically mounted in a housing (or nacelle) at the top of a fixed tower. The combined weight of the components in the nacelle is significant, for example several hundred tonnes in a large offshore wind turbine.

The electrical generator typically generates an alternating voltage and current which can be fed directly to the shore and then into the mains electrical network. Often, however, the AC output will be rectified and fed to the shore as high voltage DC (HVDC) as that can reduce the transmission losses. The transmission losses for AC can be significant due to the capacitive effects of the seawater around the undersea electrical cable, especially for longer undersea cables.

The electricity which is generated by the wind turbine cannot be connected to the mains electrical network unless it has the required voltage and frequency and is in phase with the mains network. Individual wind turbines in a wind farm can be connected to a base station at the wind farm, and/or onshore, where the voltage, frequency and phase are matched to the mains network by suitable transformers and associated componentry, in known fashion.

The wind turbine will typically have a drive mechanism by which the orientation (yaw) of the nacelle can be adjusted as the wind direction changes so as to maximise the effect of the wind upon the blades. The wind turbine will also typically have a drive mechanism to adjust the pitch of the blades relative to the hub. Adjusting the pitch of the blades is used in many wind turbines to control the rate of rotation of the blades across a range of wind speeds, usually to maintain the rate of rotation within a desired (relatively small) range for the (relatively large) range of wind speeds over which the wind turbine can operate safely and efficiently. Further control over the rotation rate of the rotor of the electrical generator can be provided by a gearbox with a variable step-up ratio. Some large offshore wind turbines can for example generate electricity with sustained wind speeds between approx. 3 m/s and approx. 30 m/s, although the generation efficiency varies across this range of wind speeds.

There are two major problems with the use of wind turbines for the generation of electricity. The first major problem is maintenance of the wind turbines and the second major problem is the requirement to store electrical energy for use during periods in which the electricity consumption exceeds the generating capacity. Neither of these problems is unique to wind turbines but they are significant issues for the reasons set out below.

As regards maintenance, offshore wind turbines in particular operate in a relatively hostile environment where routine maintenance is not straightforward. Also, the variability of the wind speed inevitably causes variability in the strain upon and therefore the durability of components in the nacelle, making it very difficult to determine accurately the optimum maintenance schedules for those components. Maintaining the components in a nacelle which may be approx. 100m above sea level is complex and sometimes hazardous and operators will wish to minimise the number of visits which need to be made to the nacelle by maximising the intervals between maintenance visits. However, insufficient maintenance can result in component failure and expensive shutdowns. Developing accurate maintenance schedules for all of the components is therefore of critical importance, but is inherently difficult because of the variable operating conditions. In this respect it is understood that failures in parts of the gearboxes in particular represent a significant ongoing cost to the operators of the existing offshore wind turbines.

The problem of storage of the electrical energy generated by wind turbines is not unique to wind turbines and is shared by all forms of electricity generation. The requirement to store electrical energy will, however, inevitably increase as the World moves away from generating energy from fossil fuels and increasingly uses renewable sources.

The gross demand for electrical energy from the electrical network can be forecast reasonably accurately and whilst the gross demand will vary throughout each day and at different times of the year those variations are largely predictable. The supply of electrical energy into the electrical network must be balanced to the demand continuously in order to avoid power reductions or ultimately outages or blackouts. The combined output from “thermal” power stations (comprising nuclear and fossil-fuelled power stations) is predictable and can be adjusted to match the demand (the output of gas-fired power stations in particular being rapidly adjustable). However, as the proportion of electricity which is generated from fossil fuels (and in some countries also the proportion generated from nuclear sources) reduces, the ability to balance the supply to the demand by way of these predictable sources will diminish.

The major sources of renewable energy are presently solar and wind, both of which are highly variable and unpredictable. The difference between the gross demand for electrical energy and that which is produced from renewable sources is often called the net demand and this net demand must be supplied from other sources of electrical energy. Because of the variability of renewable energy sources the net demand varies much more significantly than the gross demand and the variations are unpredictable.

Hydroelectric power is another widely-used source of renewable energy. However, the locations which are suitable for the creation of the dams and reservoirs which are required for large-scale hydroelectric power facilities are limited and despite being a relatively mature technology hydroelectric power is (and is likely to remain) a significant source of electrical energy in only a small number of countries. Whilst hydroelectric power is very flexible, is largely predictable, and can be used to supply some of the net demand, it cannot be used to satisfy the net demand in the many countries which do not have significant hydroelectric infrastructure.

A development of hydroelectric power is pumped storage hydropower. Rather than relying upon rainfall to fill a reservoir, electrical energy is used to pump water (up a conduit) into the reservoir. More energy is used to pump the water into the reservoir than is generated by releasing water (down the conduit) from the reservoir but pumped storage hydropower facilities can be valuable in balancing demand and supply (and in particular balancing net demand and supply) by effectively storing electrical energy in the form of gravitational potential energy during periods for which the supply exceeds the demand and converting the stored energy back into electrical energy during periods for which the demand exceeds the supply.

Building a pumped storage hydropower facility typically requires a conduit to be drilled or bored between a lower reservoir and an upper reservoir. This is usually a hugely expensive exercise and can be environmentally damaging. Further environmental damage will be caused if one or two reservoirs need to be created by building dams. Pumped storage hydropower has been in use for many decades and the technology is relatively mature and well-understood. Efficiencies of over 90% are reportedly available at many pumped storage hydropower facilities.

Whilst there are other methods of storing electrical energy, the maturity and efficiency of the technology of pumped storage hydropower, and its ability to store gigawatt-hour amounts of energy, means that pumped storage hydropower is expected to be an important element in the future electrical infrastructure of many countries.

The recent significant increase in the demand for renewable energy has resulted in many developments in the technology and componentry used to exploit the available resources.

As above stated, most of the larger wind turbines in use today utilise a mechanical gearbox to increase the rotation rate of the blades to drive an electrical generator. US 2010/0320770 proposes replacing the mechanical gearbox with a closed loop hydraulic system comprising a hydraulic pump and hydraulic motor interconnected by a conduit. The rotating blades of the wind turbine drive the pump which forces water along the conduit to rotate the hydraulic motor, the output shaft of the motor being connected to the rotor of an electrical generator. The displacements of the pump and motor are significantly different so that the output shaft of the motor is driven to rotate significantly faster than the input shaft of the pump. The pump and/or motor can have a variable displacement so that the rate of rotation of the rotor can for example be controlled somewhat independently of the rate of rotation of the blades. JP 199928718 discloses a similar arrangement with a variable displacement motor. CN 201884215 discloses another similar arrangement but with multiple pumps which operate together.

In WO 2006/029633 the output of the electrical generator is decoupled from the rotation of the blades by storing the pumped water in a reservoir. This document also discloses the use of multiple pumps having different pumping capacities which can be selectively operated according to the wind speed. Other arrangements with a reservoir to store a pumped fluid are disclosed in CN 201 884 215 U, GB 2 370 614 and WO 2010/140038.

CN 107 327 368 has a reservoir to compensate for short-term fluctuations in hydraulic pressure delivered to a turbine connected to the rotor of an electrical generator. The device of this document utilises wind power and tidal power and has a first variable displacement pump driven by the wind blades and a second variable displacement pump driven by the tidal blades. The variable displacement pumps are connected in series to the turbine by way of a diverter valve. Longer-term fluctuations in the hydraulic pressure due to changes in wind power and tidal power are countered by adjusting the outputs of the variable displacement pumps and also the position of the diverter valve.

CN 101 033 731 discloses an arrangement in which one or more wind turbines pump water from a lower reservoir to an upper reservoir, the water subsequently being used to generate electricity as it flows from the upper reservoir back to the lower reservoir. This document is therefore a direct development of the pumped storage hydropower systems described above but with a wind turbine pumping the water directly. Another development of the pumped storage hydropower systems is disclosed in DE 4 301 659.

US 7 183 664 describes a wind turbine in which the blades drive a water pump with the pumped water being stored in a reservoir within the tower. Again, the water flows from the reservoir through a turbine connected to the rotor of an electrical generator. A similar arrangement is described in KR 1011 45323 in which the pumped water can be sea water or fresh water. KR 101 1 45323 also discloses the use of a clutch between the wind blades and the pump. Both of these documents disclose a valve between the reservoir and the turbine to adjust the water flow and thereby to control the rate of rotation of the turbine and the rotor of the generator.

CN 2861529Y discloses a wind turbine with a number of electrically-operated pumps for pumping water from a lower reservoir to an upper reservoir. The water flows back from the upper reservoir to the lower reservoir through a water turbine which is connected to an electrical generator. A portion of the electrical energy which is generated is used to drive the pumps and a power controller switches the pumps on or off depending upon the power which is being generated.

J PS 57-68574 describes a method for pumping a fluid (such as water) with a wind turbine. The blades of the wind turbine are connected to a pump which pumps fluid into a header or manifold. A number of outlet conduits are connected to the header and valves control the flow of fluid along each of the conduits. As the wind speed changes the rate at which fluid is pumped into the header changes. The control system opens or closes the valves sequentially so as to increase or decrease the flow rate from the header and maintain a desired pressure in the header across a range of wind speeds.

Notwithstanding the many patent documents which utilise a wind turbine to pump water and subsequently use the pumped water to generate electricity, none have been put into widespread use. On the contrary, most onshore and offshore wind turbines use a mechanical gearbox to drive an electrical generator, despite the drawbacks of such systems.

Most offshore wind turbines are built on the sea bed and are therefore limited to relatively shallow waters. The UK is fortunate in that large regions of its coastal waters are sufficiently shallow for the present wind turbines. As part of its plans to increase the offshore generating capacity the UK government is seeking to incentivise the development of floating offshore wind generation. Floating facilities are not restricted to shallow waters and the successful development of such facilities will greatly expand the sea area which can be utilised for electricity generation. As stated above, the larger offshore wind turbines in use today have very heavy components in the nacelle and are therefore extremely top heavy; it is not expected that such top-heavy structures will be the most suitable for use on floating platforms.

The inertia of the rotating blades of larger wind turbines is sufficiently large that the blades can continue to rotate for a significant period of time after the wind has stopped blowing. The wind turbine can therefore continue to generate electricity as the blades slow down. If, however, the wind speed falls below a minimum operating wind speed for more than a minute or so the blades will stop rotating and the wind turbine will shut down. The wind turbine can also be shut down deliberately for maintenance for example. It is necessary to have means for restarting the wind turbine after a shutdown. This is commonly known as a “black start” procedure and for an offshore wind turbine is usually automated.

During operation of the wind turbine its various drive mechanisms and the control systems for those drive mechanisms will typically draw their power from the output of the electrical generator. Such power will not be available during a shutdown. Whilst some power can be provided by a back-up battery this is not usually sufficient for all of the operations of a black start procedure and larger wind turbines typically consume electrical power from the mains network during the black start procedure.

In a wind farm comprising many dozens of wind turbines it is known to use mains network power for the black start procedure of one or more of the wind turbines and then to use the power generated by those wind turbines for the black start procedures of other wind turbines until all of the wind turbines in the wind farm are operating again.

US 7 183 664 discloses a black start procedure. As in many of the prior art documents identified above, during normal operation the blades of the wind turbine drive a pump to lift water into a reservoir. The water can continue to flow from the reservoir through the turbine, and electricity can continue to be generated, after the wind has stopped blowing and the blades of the wind turbine have stopped rotating. It is arranged that the water level in the reservoir does not drop below a minimum level required for a black start procedure. When it is determined that the wind speed is sufficient to resume normal operation the hydraulic circuit is reconfigured so that the remaining stored water flows from the reservoir back to the pump and drives the pump as a motor to drive the blades of the wind turbine to rotate. When the blades have been restarted and their rotation is sustained by the wind the hydraulic circuit reverts to normal and the pump delivers fluid to recharge the reservoir and subsequently to generate electricity.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wind turbine which avoids the use of a mechanical gearbox to provide the required rotation rate for an electrical generator.

It is another object of the invention to provide a wind turbine which decouples the rotation of the blades from the rotation of the electrical generator.

It is another object of the invention to provide a wind turbine which enables the storage of energy and thereby can temporarily provide more electrical power than is being extracted from the wind.

It is another object of the invention to provide a wind turbine which is mechanically robust and reliable and which can enable effective maintenance schedules to be determined.

It is another object of the invention to provide a wind turbine which can operate over a large range of wind speeds. In particular, it is an object to allow the blades of the wind turbine to continue to rotate at relatively low wind speeds.

It is another object of the invention to provide a wind turbine with a black start procedure, ideally fully autonomously (i.e. without requiring an external power supply).

It is another object of the present invention to provide a wind turbine which is suitable for use with a floating platform or as a fixed structure built upon the ground or sea bed.

According to a first aspect of the invention there is provided a wind turbine having: a number of blades, the blades being carried by a hub, the hub being rotatably mounted to a tower, the hub being connected to a drive shaft; a first reservoir and a second reservoir for a working liquid, the working liquid in the second reservoir being above the working liquid in the first reservoir; the drive shaft being connected to at least one positive displacement pump, the pump having an inlet and an outlet, the inlet being in communication with the first reservoir, the outlet having a two-way valve, the two-way valve being changeable between a first condition in which the outlet is in communication with the first reservoir and a second condition in which the outlet is in communication with the second reservoir; a turbine with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, the turbine having an output shaft; and an electrical generator having a rotor connected to the output shaft.

The turbine is driven by the flow of working liquid and could be referred to as a working fluid turbine if necessary to distinguish it from the wind turbine as a whole.

When the two-way valve is in the second condition the wind turbine utilises the rotation of the blades to pump the working liquid from the first (lower) reservoir to the second (higher) reservoir, similarly to some of the prior art arrangements. Working liquid flows from the second reservoir to the first reservoir through the turbine to rotate the output shaft and rotor in order to generate electricity.

Unlike the prior art, the two-way valve in the first condition pumps the working liquid back to the first reservoir. The pump can therefore be driven continuously and there can be a permanent connection between the drive shaft and the pump. Accordingly, as long as the blades are rotating the pump will be operating to pump working liquid from its inlet to its outlet. The position of the two-way valve determines whether that working liquid is pumped into the second reservoir or into the first reservoir. Enabling the pump to operate continuously avoids the complication of controls and actuators to stop and start the pump. It also avoids the problems which can occur when starting a positive displacement pump after a period of non-use.

The two-way valve could be referred to as a switching valve because it switches the outlet of the pump between the first reservoir and the second reservoir.

It will be understood that with the two-way valve in the second condition the pressure differential between the inlet and the outlet of the pump is determined by the pressure head or height difference between the working liquid in the second and first reservoirs. With the two-way valve in the first condition, however, the pressure differential is minimal and arises primarily from pumping losses. The power required to drive the pump is proportional to the pressure differential and will therefore be directly affected by the operating condition of the two-way valve.

Enabling the pumping of the working liquid back to the first reservoir has benefits when the wind speed is very low and the power which is extracted by the blades is small. In particular, the minimum wind speed necessary to maintain the rotation of the blades can be relatively low and it is expected that the minimum wind speed is significantly lower for the present invention than for a corresponding wind turbine with a mechanical gearbox. The continued rotation of the blades at very low wind speeds can avoid a shutdown and can thereby avoid the requirement for a black start procedure. Accordingly, reducing the minimum wind speed necessary to keep the blades rotating has practical benefits even if the working liquid is not being pumped into the second reservoir at the lowest wind speeds. It will be appreciated that movement of the two-way valve between its first and second conditions is effectively equivalent to switching the pump on and off, which might otherwise be achieved with the use of a mechanical clutch between the drive shaft and pump. As above stated, it is preferable not to switch the pump on and off. Also, the two-way valve is preferable to utilising a mechanical clutch for two additional reasons. Firstly, the maintenance of a two-way valve will depend primarily upon the duration of use rather than upon the number of changes between the valve’s first and second conditions. An optimum maintenance schedule can therefore readily be determined for the two-way valve. On the contrary, the maintenance of a mechanical clutch will depend primarily upon the number of cycles of operation which in a wind turbine will depend upon changes in the wind speed above and below an operating threshold, and which cannot be predicted nor therefore determined in advance.

Secondly, the mechanical clutch must be very robust as during normal operation it is required to communicate the maximum power of the wind turbine from the blades to the pump, which might be several megawatts. Such a mechanical clutch is necessarily complex and expensive and the power required to actuate the clutch to disconnect the drive to the pump is large. On the contrary, whilst the two-way valve must be large and robust enough to accommodate the required flow rate it can be mechanically simple.

Furthermore, the reliability of the valve is increased because it can be designed for use in only two operating conditions, i.e. either fully open to the first reservoir or fully open to the second reservoir. The valve will be in an intermediate position for only relatively short periods during the changeovers between those conditions.

Preferably, a check valve is located between the two-way valve and the second reservoir. It will be understood that during a changeover between the first and second conditions the pump outlet may be temporarily connected to both reservoirs and a check valve will prevent working fluid passing from the second reservoir to the first reservoir by way of the two-way valve.

It will be understood that positive displacement pumps are particularly suited to relatively slow actuation, i.e. being driven by a drive shaft which is rotating relatively slowly. Such pumps are highly suitable for use at the rotation rate of the blades of a wind turbine without requiring a step- up gearbox for the drive shaft.

The use of a positive displacement pump also avoids the requirement to control the pumping rate. With the present invention the pumping rate is directly proportional to the rate of rotation of the drive shaft. A positive displacement pump can be mechanically simpler and more robust than a variable displacement pump.

The positive displacement pump may be of the rotary or reciprocating type, the former including gear pumps, screw pumps, lobe pumps, vane pumps and cam pumps for example, and the latter including piston pumps, plunger pumps and diaphragm pumps for example.

The working liquid is preferably water. The working liquid may be fresh water or sea water. Fresh water has the advantage that it is less corrosive and erosive than sea water but sea water may be preferred for offshore applications. The appropriate protection of the components, and an appropriate maintenance schedule, can take account of the erosion and corrosion which will be caused by sea water in particular. The working liquid can include anti-freeze and/or a corrosion inhibitor.

If the invention is utilised as an offshore wind turbine and the working liquid is sea water, the first reservoir can be open to the sea, i.e. it is not necessary that the first and second reservoirs are parts of a closed system. However, a closed system is preferred so as not inadvertently to allow contaminants or debris to be introduced into the working liquid.

In any event, the invention is not limited to a water-based working liquid. An oil-based working liquid could, for example, be used if desired.

Preferably, the drive shaft is connected to more than one positive displacement pump and the outlet of each pump has a two-way valve.

Desirably, the two-way valve of each pump is independently changeable between its first and second conditions whereby the outlets of chosen pumps can be put into communication with the second reservoir and the outlets of the remaining pumps can be put into communication with the first reservoir.

Preferably, all of the positive displacement pumps are connected directly to the drive shaft and rotate with the blades without any intervening clutches, actuators or the like. All of the pumps therefore operate to pump working liquid from the inlet to the outlet whenever the blades and the drive shaft are rotating.

The wind turbine preferably includes a control system which is connected to the two-way valve(s) and is configured to switch each of the two-way valves between its first and second conditions independently. The outlet of each pump can thereby be independently switched between the first reservoir and the second reservoir.

Desirably, the control system is configured to balance the power driving the pumps with the power being extracted from the wind so that the blades can rotate at a consistent rate (or within a predetermined range) at which the blades can operate at or close to their optimum efficiently. Accordingly, the control system can determine the required condition of each two-way valve according to the instantaneous power which is being extracted from the wind. The control system can for example monitor the rotation rate of the blades and/or drive shaft and open/close the two- way valves in response to increases/decreases in the rate of rotation in order to maintain the rotation rate at a predetermined rate or within a predetermined optimum range. Alternatively or additionally, the control system can measure the rate of rotation and the torque in the drive shaft (for example by way of a strain gauge) and can use the measured torque and rotation rate to determine the power being delivered by the drive shaft. The control system can include a memory having data upon the power required to drive each of the pumps in both of their first and second conditions in order to determine which pumps should be in their second condition in order to balance the power being extracted from the wind.

The use of multiple separate pumps, each of which has its own two-way valve, and the independent control of those two-way valves, enables the combined power of the pumps to be adjusted (substantially continuously) to match the power being extracted from the wind. As above stated, the power required to drive a pump with its two-way valve in the first condition is minimal and the combined power required to drive all of the pumps is therefore dependent almost totally upon the pumps with their outlets in communication with the second reservoir. The combined power to drive the pumps is therefore almost directly proportional to the flow rate at which the working liquid is being pumped into the second reservoir. That flow rate can be adjusted to match the power being extracted from the wind.

Balancing the power being extracted from the wind by adjusting the outputs of multiple pumps is preferable to allowing the blades to speed up and slow down with changes in the wind speed. In common with many large wind turbines it is preferable to restrict the rate of rotation of the blades to a range at which the wind turbine can operate efficiently without for example the tips of the blades moving so fast that excessive noise and turbulence is generated. For many large wind turbines the desired rate of rotation of the blades is between approx. 15 rpm and approx. 40 rpm.

The present invention can include means to control the pitch of the blades in order to respond to changes in wind speed, but the primary means of responding to changes in wind speed is to adjust the power required to drive the pumps. Thus, as the wind speed increases the torque of the drive shaft will increase and more of the two-way valves can be switched to their second conditions to seek to balance that torque, the additional load upon the drive shaft opposing an increase in the rate of rotation of the blades. The pitch of the blades can nevertheless be adjusted so as to maximise the efficiency of the blades at a given wind speed and at the particular rate of rotation.

In a known wind turbine with a mechanical gearbox, it is known to adjust the pitch of the blades to maintain a desired rate of rotation of the blades. It is also known to adjust the ratio of the gearbox and to adjust the electrical output to change the load on the gearbox output shaft. Each of these adjustments is effective for gradual changes in wind speed but are usually too slow to react to rapid changes in wind speed caused by wind gusts. The inertia of the blades goes some way to moderating changes in rotation rate but a mechanical brake can be applied to slow the blades if the wind speed increases sufficiently quickly, and is sustained for a sufficiently long period, to cause the blade speed to increase. The mechanical brake can be applied until one of the longer-term adjustments can be made. A mechanical brake is not expected to be required for the same purpose with the present invention, however, since changes in the rate of rotation of the blades will affect the rate of rotation of the drive shaft and therefore the pumping rate, but since the pumping rate is decoupled from the rotation of the turbine such changes will not affect the generation of electricity. Changes in the rotation rate of the blades can therefore be better accommodated with the present invention, but it is nevertheless desirable for the rotation rate to remain within a range for which the wind turbine operates most efficiently. It is also desirable to have an upper limit for the rate of rotation of the blades and to avoid that limit being breached in very strong winds by adjusting the pitch of the blades and/or applying a mechanical brake, if required.

It will be understood that a single pump with a variable displacement could be provided, with the pumping rate into the second reservoir being matched to the power being extracted from the wind by varying the pump displacement. It is, however, preferable to provide multiple pumps and to switch the two-way valves for those pumps between their first and second conditions as that avoids the additional complication (and consequently the potential unreliability) of a variable displacement pump.

It will also be understood that a single pump with a flow control valve could be provided, the flow control valve delivering a chosen (and varying) proportion of the pump output to the second reservoir so as to match the power being extracted from the wind. It is, however, preferred to avoid the additional complexity (and consequently potential unreliability) of the flow control valve and actuator. Also, in a practical wind turbine a flow control valve would need to operate almost continuously in a partially open state (i.e. corresponding to wind speeds between the operational minimum and maximum) and is likely to suffer greater wear and degradation in a partially open state than a two-way valve which can operate almost always in one of its two fully open states. Furthermore, the number of adjustments in the position of the flow control valve, and the degrees of those adjustments, depend upon changes in the wind speed and cannot be known in advance. The use of a flow control valve would therefore make it more difficult to determine an accurate maintenance schedule.

Preferably, the multiple separate positive displacement pumps have differing displacements, i.e. they each pump a different volume of working liquid for each rotation of the drive shaft. The first pump therefore has a first displacement, the second pump has a second displacement, and if present the third pump has a third displacement (and so on depending upon the number of pumps in a particular wind turbine). Desirably, the second displacement is double the first displacement and if present the third displacement is double the second displacement (and so on depending upon the number of pumps in a particular wind turbine). Such an arrangement will minimise the number of pumps required for a desired range of flow rates to the second reservoir.

For example, in a wind turbine having four pumps and with the drive shaft rotating at 30 rpm it can be arranged that a first pump has a displacement of 1 m³/second, a second pump has a displacement of 2 m³/second, a third pump has a displacement of 4 m³/second and a fourth pump has a displacement of 8 m³/second. With the two-way valve of the first pump in its second position and the other two-way valves in their first positions the flow rate at which working liquid is pumped into the second reservoir will be 1 m³/second whereas with all four two-way valves in their second positions the flow rate into the second reservoir will be 15 m³/second. With such an arrangement it is possible to have a flow rate into the second reservoir of 1 , 2, 3, 4, 5 ... 15 m³/second. Such a wind turbine could thereby operate over a large range of wind speeds with the flow rate of liquid being pumped into the second reservoir closely matched to the power extracted from the wind. Clearly, more or fewer than four pumps can be provided, and with different actual and relative displacements, to match the desired range of flow rates into the second reservoir (and therefore the pumping power).

It will be understood that changes in the rate of rotation of the blades will also affect the pumping rate into the second reservoir and therefore the power required to rotate the pumps. Relatively small changes in the rotation rate of the blades together with the switching of the two-way valves can be combined to balance the power extracted from the wind substantially continuously across a large range of wind speeds.

The positive displacement pumps, two-way valves, turbine and electrical generator are preferably located at or adjacent to the bottom of the tower. The first reservoir is also preferably located at or adjacent to the bottom of the tower. The second reservoir is necessarily located above the first reservoir, and ideally is located in the tower. It is thereby arranged that the minimum of components, and a minimum of mass, is located at the top of the tower. This directly avoids the top-heavy arrangement of the known offshore wind turbines and therefore significantly increases the applicability of the present wind turbine to use with a floating platform. This also avoids the requirement to access the top of the tower to conduct routine maintenance for much of the componentry. Preferably, the second reservoir occupies a large proportion of the tower (e.g. the tower can be filled with working liquid). Ideally the second reservoir occupies the tower up to a predetermined maximum level. The predetermined maximum level can be very close to the top of the tower in order to maximise the volume of the second reservoir and to maximise the pressure head acting upon the turbine.

Desirably, the predetermined maximum level is determined by an overflow conduit through which working liquid can flow from the top of the second reservoir to the first reservoir, bypassing the turbine. Accordingly, during periods for which the supply of electricity to the mains network exceeds the demand, the generation of electricity by the wind turbine can be reduced or stopped without stopping or slowing the blades; working liquid can continue to be pumped into the second reservoir to balance the power being extracted from the wind, with the working liquid passing back to the first reservoir by way of the overflow conduit.

The flow of working liquid through the turbine can be controlled electrically, i.e. by adjusting the characteristics of the electrical generator in order to vary the torque required to rotate the rotor and thereby to control the turbine speed. Alternatively, a turbine valve can be located at the turbine inlet by which the flow of working liquid through the turbine (and thereby the generation of electricity) can be adjusted. A turbine valve can also be used to stop the turbine when required, example for maintenance.

If present, the turbine valve is preferably a flow control valve so that the rate of flow of working liquid through the turbine can be adjusted. The use of a flow control valve enables the flow rate of working liquid through the turbine to be matched to the flow rate of working liquid into the second reservoir, whereby the level of working liquid in the second reservoir can be maintained at or close to the predetermined maximum level. It is desirable to maintain the maximum level of working liquid in the second reservoir during normal operation so that the pressure head acting upon the turbine is consistent. The electrical output of the generator can be adjusted to change the load acting upon the turbine in order to maintain the turbine rotation at or close to its most efficient operating rate despite changes in the flow rate of working liquid through the turbine.

It is expected to be preferred to operate the wind turbine to generate electricity continuously and to adjust the electrical output (by way of the flow rate through the turbine) in order closely to match the power being extracted from the wind.

It will be understood, however, that the generation of electricity is decoupled from the power being extracted from the wind and the turbine valve can be used to adjust the electrical output independently of the instantaneous power being extracted from the wind. At one extreme, it is possible to generate electricity cyclically, with the wind turbine undergoing cycles during which the second reservoir is alternately emptied and replenished. Thus, the working liquid can flow to the turbine at a greater rate than it is being pumped into the second reservoir, causing the level of working liquid in the second reservoir to fall. When the working liquid reaches a predetermined minimum level in the second reservoir the turbine valve is closed to stop the generation of electricity. This allows the second reservoir to fill back up to the predetermined maximum level, whereupon the turbine valve is reopened and the generation of electricity is recommenced. Such a cyclical generation of electricity is most likely to be utilised in a wind farm comprising many wind turbines for which the cycles of generation can be staggered and the overall generation of electricity from the wind farm can be substantially consistent. A large wind turbine can for example have a tower which rises approx. 100 m above the ground. Using the tower as the second reservoir can provide a pressure head of perhaps 90 m which is similar to many dams and pumped storage hydropower facilities. An offshore wind turbine can, however, additionally utilise the depth of the sea with the first reservoir being located below sea level. The pressure head between the first and second reservoirs can thereby be increased, in some cases significantly. Locating the first reservoir below sea level will also be valuable in a floating wind turbine, with the mass of all of the parts below sea level aiding stability and counteracting the mass of the parts which are above sea level. The pressure head can also be increased in an onshore wind turbine by locating the first reservoir below ground.

It will be understood that it is necessary to convert the rotation of the hub around a substantially horizontal axis into rotation of the drive shaft about a substantially vertical axis so that the torque can be communicated to the pumps at the bottom of the tower. Preferably, a bevel gear set is provided at the top of the tower, desirably with a bevel drive gear connected to the hub and a bevel driven gear connected to the drive shaft. Ideally, the drive shaft and bevel driven gear are located centrally of the tower.

Preferably, the blades and hub are carried by a top housing which is mounted to the tower and which can rotate (yaw) relative to the tower about a substantially vertical axis. Desirably the wind turbine has a yaw drive mechanism for the top housing so that the orientation of the top housing can be adjusted depending upon to the direction of the wind. With the drive shaft and bevel driven gear located at the central axis of the tower, and with the top housing rotating about that central axis, the orientation of the top housing can be adjusted with the bevel drive gear maintaining engagement with the bevel driven gear.

The gear ratio of the bevel gear set may be 1 :1 or the bevel gear set may step-up or step-down the rate of rotation as desired. As above stated, positive displacement pumps are suited to operation at the relatively slow rotational rate of the blades of a wind turbine but it may nevertheless be desirable for the drive shaft to rotate faster or slower than the blades in order to maximise the efficiency of the pumps. Providing a bevel gear set with a (small) step-up or stepdown ratio is not likely to impair the mechanical simplicity and reliability of the bevel gear set.

In addition to the yaw drive mechanism for the top housing, and in common with many wind turbines, the top housing will preferably include a mechanical brake for the hub and a pitch drive mechanism for the blades. The mechanical brake is provided to lock the hub and blades against rotation when that is required (for example during maintenance of the wind turbine). The pitch drive mechanism for the blades is provided to adjust the pitch of the blades relative to the hub in accordance with changes in wind speed.

In the present invention the mechanical brake can be made mechanically simple since it may not be required to slow down and stop the rotation of the blades. Thus, when it is desired to stop the blades rotating whilst the wind continues to blow (for routine maintenance, for example), one or more of the pumps may be changed to their second conditions in order to overpower the blades. If the power required to drive the pumps exceeds the power being extracted from the wind the pumps will act as a hydraulic brake causing the blades to slow down and stop. The mechanical brake can be used to lock the blades once they have stopped rotating and is not also required to slow the blades’ rotation. In addition, the wind turbine may have a hub drive mechanism, specifically to initiate the rotation of the hub and blades after a shutdown. It will be understood that a hub drive mechanism avoids the requirement for the wind alone to initiate rotation of the blades.

Access to the top housing will be required for periodic maintenance of the components which are carried by or mounted to the housing. However, it is expected that visits to the top of the tower will be required significantly less frequently than are the visits to the nacelle of the known offshore wind turbines, primarily because fewer of the components are located at the top of the tower and secondly because the components of the present wind turbine are relatively mechanically simple. An access lift or shaft by which an operator can access the top housing can be provided inside the tower or can be mounted to the outside of the wall of the tower, the latter arrangement avoiding any reduction in the volume of the second reservoir. Alternatively or additionally, the top housing can include a landing platform for a helicopter by which access can be gained to the top housing.

As above stated, it is expected to be preferred that the electrical power which is outputted by the wind turbine closely and continuously matches the wind power being extracted by the blades (minus the efficiency losses in the system). In this way, the level of working liquid in the second reservoir can be maintained at a substantially constant level and the pressure head at the turbine is substantially consistent. In a typical wind turbine the pressure head may be maintained at close to 90 m for example. The form of the turbine can be suited to this pressure head, for example a Francis turbine or a Pelton turbine. A Kaplan turbine could alternatively be used but the adjustable blades of such a turbine make it more complex and usually beneficial only with a variable pressure head. It is understood that a Francis turbine can usually provide the greatest peak efficiency but the efficiency of a Pelton turbine is more uniform across a range of rotation rates.

A consistent level of working liquid in the second reservoir is also beneficial for the positive displacement pumps since the power required to rotate the pumps is proportional to the pressure head; maintaining the pressure head will assist the control system in balancing the power to rotate the pumps to the power being extracted from the wind.

Preferably, the control system has sensors to monitor the operation of the wind turbine. For example, the wind turbine can have one or more of the following sensors: a rotation sensor at the hub to measure the rate of rotation of the hub, a pitch sensor for measuring the pitch of the blades, a torque sensor for measuring the torque in the drive shaft, sensors to determine the condition of each of the two-way valves, a sensor to determine the position of the turbine flow control valve, a rotation sensor at the output shaft of the turbine to measure the rate of rotation of the output shaft, and electrical sensors to measure the voltage and current generated by the electrical generator.

Preferably also, the control system is connected to control actuators or mechanisms for one or more of the following: a pitch drive mechanism for adjusting the pitch of the blades (the pitch sensor may be connected to the pitch drive mechanism if desired), an actuator for changing each of the two-way valves between its first and second conditions, an actuator for the turbine flow control valve (the sensor to determine the position of the turbine flow control valve may be connected to the actuator if desired). The control system is preferably also connected to the electrical generator and can adjust the amount and/or relative phase of the AC power passing to the stator and/or to the rotor of the electrical generator, whereby to adjust the electrical power output.

According to a second aspect of the invention there is provided a wind turbine having: a number of blades, the blades being carried by a hub, the hub being rotatably supported by a tower, the hub being connected to a drive shaft; a first reservoir and a second reservoir for a working liquid; the drive shaft being connected to at least one pump configured to pump working liquid from the first reservoir to the second reservoir; a turbine with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, the turbine being connected to a primary electrical generator; a hydraulic motor with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, the motor being connected to a secondary electrical generator, the secondary electrical generator being a permanent magnet generator; and a black start valve at the inlet of the hydraulic motor.

During normal operation of the wind turbine the black start valve is closed and working liquid flows from the second reservoir to the primary turbine and the wind turbine outputs electricity generated by way of the primary electrical generator. In common with many large wind turbines, the primary electrical generator may be an induction generator. Such a generator requires a supply of AC power to create the stator and/or rotor magnetic fields. If the wind stops blowing the inertia of the blades will cause them to continue to rotate for a short period of time. In the known wind turbines the mechanical gearbox can continue to rotate and electricity can continue to be generated until the blades slow down to a minimum operating rotation rate. With the present invention, however, the rotation of the turbine is decoupled from the rotation of the blades and the turbine can continue to be driven after the blades have stopped rotating, using the working liquid stored in the second reservoir.

If, however, the wind stops blowing for an extended period of time, the present wind turbine, like the known wind turbines, will cease to generate electricity and will shut down.

Whilst the wind turbine is not generating electricity the AC power which is used to create the magnetic field(s) for the induction generator cannot be generated by the wind turbine itself. That power can instead be supplied from an external source such as the mains network. The present invention according to the second aspect seeks to avoid the requirement for (any) external source of AC power so that the wind turbine can undertake a black start procedure, preferably autonomously. In particular, the provision of a permanent magnet generator avoids a requirement for an external power supply to the secondary electrical generator and the secondary generator will produce electrical power when its rotor starts spinning.

The rotor of the secondary generator is driven by the hydraulic motor. The hydraulic motor may be a positive displacement hydraulic motor or a second turbine, as desired. A positive displacement hydraulic motor is likely to be less efficient than a turbine but is suitable for a black start procedure because it will typically require a smaller pressure head to commence and sustain its rotation. A positive displacement hydraulic motor will also typically rotate more slowly than a turbine and a slower rotation usually impacts adversely upon the efficiency of the electrical generator. However, since the secondary generator is only used during a black start procedure its efficiency is not of primary importance. The secondary generator may generate DC at a relatively low voltage, for example 12V or 24V. Whilst such low DC voltages are not usually appropriate for the electrical generation of a wind turbine they can be suitable for powering the components of the wind turbine during a black start procedure.

It will be understood that a black start procedure requires the black start valve to be opened to allow working liquid to flow from the second reservoir to the hydraulic motor. Some control system power and some operational power are therefore required to open the black start valve but the amount of power for these operations is relatively small and it is expected that a back-up battery can provide sufficient power.

A continuous supply of electrical power is also required to operate the control system and other components during a shutdown. Thus, it is necessary that the control system monitors the wind speed during a shutdown and can initiate the black start procedure when the wind speed rises sufficiently to sustain the rotation of the blades.

Preferably, the wind turbine has an anemometer by which the wind speed (and the wind direction) can be monitored continuously (or periodically), even during a shutdown. Electrical power is required for the anemometer to communicate the wind speed and wind direction to the control system.

It is expected that the electrical power required during the shutdown can be provided by the backup battery, with the back-up battery preferably being recharged by way of a solar panel during the shutdown.

Preferably, the secondary electrical generator is connected to the back-up battery. Accordingly, once the secondary generator is creating electricity the back-up battery can be continuously recharged so that the control system can complete the black start procedure.

Desirably, the control system monitors the rotation of the blades and can thereby prepare for a shutdown. It will be understood that since the primary electrical generator is decoupled from the rotation of the blades the present wind turbine can continue to generate electricity for some time after the blades have stopped rotating. The wind turbine will only be shut down if the wind speed falls too low to rotate the blades for an extended period of time.

Preferably, the wind turbine has one or more sensors to detect the level of working liquid in the second reservoir. The sensor(s) may be a pressure sensor at the bottom of the second reservoir, or may be multiple level sensors at different heights of the second reservoir. The control system can be configured to cause a shutdown when the level of working liquid in the second reservoir reaches a predetermined minimum. It is arranged that the pressure head at the predetermined minimum level exceeds that required to drive the hydraulic motor and secondary generator. It is also arranged that the volume of working liquid remaining in the second reservoir at the predetermined minimum level is sufficient to drive the secondary generator for longer than is required to complete all of the stages of a black start procedure, as explained below.

The control system can be configured to apply the mechanical brake as part of the shutdown procedure. Thus, it is not desired that the blades should automatically commence rotating when the wind speed rises and it is instead desired to initiate the rotation of the blades only in a fully controlled manner by releasing the mechanical brake as and when desired.

Accordingly, the secondary generator is connected to an actuator for the mechanical brake, whereby the mechanical brake can be released as part of the black start procedure.

It will be understood that the wind turbine can be put into a shutdown in order for maintenance to be undertaken. Additionally, the wind turbine can shut down because of a sustained period of little or no wind. If the shutdown is caused by a fall in wind speed it is expected that all of the two- way valves would have been changed to their first conditions to match the falling wind speed before the shutdown commences. If, however, the shutdown is for maintenance, one or more of the two-way valves may be in their second condition at the start of the shutdown. The control system is preferably configured to change all of the two-way valves to their first conditions as part of the black start procedure so as to minimise the power required to rotate the blades.

Preferably, the secondary generator is connected to the yaw drive mechanism. It may be, for example, that the wind direction has changed during the shutdown and it is therefore necessary to reorient the housing as part of the black start procedure. Desirably, the housing is reoriented before the mechanical brake (if present) is released.

Desirably, the secondary generator is connected to the pitch drive mechanism. The secondary generator can thereby be used to adjust the pitch of the blades to match the wind speed as part of the black start operation.

Preferably, the secondary generator is connected to the hub drive mechanism. Accordingly, the secondary generator can initiate the rotation of the blades in the event that the wind alone cannot initiate the blades’ rotation. In common with the known wind turbines, the drive mechanism may be required to accelerate the blades to a threshold speed at which the rotation can be sustained by the wind.

It will be understood that when the blades start rotating, all of the pumps start to pump working liquid, albeit back to the first reservoir.

When the blades are rotating under the force of the wind the control system can change the conditions of one or more of the two-way valves to balance the pumping power to the power being extracted from the wind. The pumps will thereby re-fill the second reservoir, it being understood that the flow rate of working liquid out of the second reservoir through the hydraulic motor is relatively small compared to the pumping rate, even at the slowest wind speeds.

Preferably, when the working liquid reaches a first predetermined level in the second reservoir the black start valve can be closed to terminate the electrical generation by the secondary generator. Desirably, when the working liquid reaches a second predetermined level in the second reservoir the turbine valve can be opened to recommence electrical generation from the primary generator. Preferably, the first predetermined level and the second predetermined level are the same so that the primary generator commences the generation of electricity at approximately the same time as the secondary generator ceases the generation of electricity. Alternatively, the first predetermined level is below the second predetermined level and there is a (known) interruption in the generation of electricity. Alternatively again, the first predetermined level is above the second predetermined level and there is an overlap in the generation of electricity by the primary and secondary generators. The first predetermined level and/or the second predetermined level may be the maximum level of the second reservoir, for example.

There is also provided a method of operating a wind turbine according to the second aspect, the wind turbine having: a number of blades, the blades being carried by a hub, the hub being rotatably supported by a tower, the hub being connected to a drive shaft; a first reservoir and a second reservoir for a working liquid; the drive shaft being connected to at least one pump configured to pump working fluid from the first reservoir to the second reservoir; a turbine with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, the turbine being connected to a primary electrical generator; a hydraulic motor with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, the motor being connected to a secondary electrical generator, the secondary electrical generator being a permanent magnet generator; and a black start valve at the inlet of the hydraulic motor; the method comprising the following steps:

{i} monitoring the rate of rotation of the blades and the level of working liquid in the second reservoir;

{ii} undertaking a shutdown procedure when the blades have stopped rotating and the level of working liquid in the second reservoir has dropped to a predetermined lower level, the shutdown procedure including stopping the flow of working liquid to the turbine;

{iii} monitoring the wind speed and commencing a black start procedure when the wind speed exceeds a predetermined wind speed;

{iv} opening the black start valve to enable working liquid to drive the hydraulic motor and for the secondary electrical generator to generate electricity;

{v} using at least a portion of the electricity generated by the secondary electrical generator to provide at least some of the power to complete the black start procedure.

Preferably, the completion of the black start procedure includes initiating the rotation of the blades. Desirably, the completion of the black start procedure includes actuation of the at least one pump.

Preferably, the wind turbine has a mechanical brake and the shutdown procedure of step {ii} includes applying the mechanical brake to prevent the rotation of the blades, and the black start procedure includes the releasing of the mechanical brake before step {v}.

Desirably, the wind turbine includes a yaw drive mechanism for the hub and means to measure the wind direction, and the black start procedure includes the step of comparing the orientation of the hub and the wind direction. In the event that the orientation of the hub differs significantly from the wind direction, the black start procedure can include the step of actuating the yaw drive mechanism to reorient the hub.

Preferably, the wind turbine includes a pitch drive mechanism for adjusting the pitch of the blades relative to the hub and the black start procedure includes the step of comparing the actual pitch of the blades with the desired pitch for the wind speed. In the event that the actual pitch of the blades does not equal the desired pitch, the black start procedure can include the step of actuating the pitch drive mechanism to adjust the pitch of the blades. Desirably, the wind turbine has means to monitor the electrical output of the secondary electrical generator and the black start procedure is only commenced when the output reaches a predetermined threshold.

Desirably, the final step of the black start procedure is to close the black start valve and this final step is undertaken only when the working liquid is flowing through the turbine and the primary electrical generator is delivering a predetermined minimum electrical output.

Effectively therefore, the black start procedure can include all of the steps required to initiate the rotation of the blades and the supply of electricity to the mains network autonomously, with most of the control and power required being provided by the secondary electrical generator. A backup battery is required only to provide power to the control system during a shutdown, and to open the black start valve when the wind speed is sufficient to sustain the blades’ rotation and the recommencing of normal operation.

In a wind park with many wind turbines it can be arranged that one (or more than one) primary wind turbine has a black start valve and secondary electrical generator, and that the primary wind turbine is electrically connected to the other wind turbines in the wind park; once the primary electrical generator of the primary wind turbine is generating electricity that electricity can be used to power the black start procedures of other wind turbines.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in more detail, by way of example, with reference to the accompanying schematic drawings, in which:

Fig .1 shows a wind turbine according to the present invention, in an offshore location;

Fig.2 shows an arrangement of four positive displacement pumps and two-way valves as used in an embodiment of wind turbine;

Fig.3 shows the turbine valve, turbine and electrical generator of the wind turbine; and

Fig.4 shows the black start valve, hydraulic motor and secondary electrical generator.

DETAILED DESCRIPTION

Fig.1 shows an offshore wind turbine 10 in accordance with the present invention. The wind turbine 10 is built on the seabed 12 but it will be understood could alternatively be mounted on a floating platform which is suitably anchored to the seabed 12.

The wind turbine 10 has three blades 14 which are mounted to a hub 16 and only two of which are visible in Fig.1 . The hub 16 is rotatably mounted to a top housing 18 mounted at the top of a tower 20. The hub 16 is connected to a hub drive shaft 22 which is also connected to the drive gear 24 of a bevel gear set. The bevel gear set also has a driven gear 26 which is connected at one end of a main drive shaft 30. Rotation of the blades 14 thereby causes rotation of the drive shaft 30. In this embodiment the drive bevel gear 24 is slightly smaller than the driven bevel gear 26 so that the drive shaft 30 rotates somewhat slower than the blades 14. The bevel gear set can alternatively be a step-up gear set or can provide a 1 :1 ratio, as desired to maximise the efficiency of operation of the wind turbine 10 at the desired rotation rate (or desired range of rotation rates) of the blades 14.

It will be understood that the invention is not limited to the number of blades 14 and more or fewer than three blades can be provided if desired. Also, whereas the blades 14 rotate about a substantially horizontal axis, the invention is also suitable for a wind turbine in which the blades rotate about a substantially vertical axis.

In known fashion, the top housing 18 is rotatably mounted to the tower 20 and a yaw drive mechanism (not shown) is provided to enable the orientation of the top housing 18 (and therefore the blades 14) to be adjusted depending upon the wind direction. It is arranged that the driven bevel gear 26 and the drive shaft 30 are centred at the axis of rotation A of the top housing 18 so that changes of orientation of the top housing 18 do not affect the communication of rotation from the hub drive shaft 22 to the main drive shaft 30.

Also in common with known wind turbines, the hub 16 carries a pitch drive mechanism (not shown) which can adjust the pitch of the blades 14 relative to the hub (i.e. rotating each blade about its longitudinal axis). The pitch of the blades can thereby be adjusted according to the wind speed to provide one of the ways to maintain the rotation of the blades 14 at a desired rate (or preferably within a desired range of rates) at which the efficiency of the blades in extracting power from the wind is maximised.

Also in common with known wind turbines, the wind turbine 10 has a mechanical brake (not shown) for the hub 16. The mechanical brake is provided to lock the hub 16 and blades 14 against rotation when that is required (for example during a shutdown). The shutdown may be initiated by an operator so that maintenance can be undertaken, or the wind may have stopped blowing. In both cases it is desired that the blades should only start rotating again as part of a controlled procedure and the mechanical brake can be released as part of that procedure.

The wind turbine 10 has a first reservoir 32 and a second reservoir 34 for a working liquid 36. In this embodiment the working liquid 36 is sea water. In this embodiment also, the first reservoir 32 and the second reservoir 34 are parts of a closed system and the working liquid 36 is isolated from the sea water surrounding the wind turbine 10. It may nevertheless be possible to fill (or refill) the first reservoir 32 from the sea water surrounding the wind turbine (with suitable filtration) as and when required.

The upper surface of the working liquid 36 in the second reservoir 34 is above the upper surface of the working liquid in the first reservoir 32 and this height difference H creates the pressure head to drive the turbine 76 as set out below. It will be understood that in this embodiment the first reservoir 32 is located below sea level so as to maximise the pressure head H.

The drive shaft 30 continues into a chamber 38 which contains four positive displacement pumps 40a-d and four two-way valves 42a-d as shown in Fig.2 and described below. The chamber 38 has an inlet 44 in communication with the first reservoir 32. The chamber 38 also has two outlets for working liquid, a first chamber outlet 46 in communication with the first reservoir and a second chamber outlet 48 in communication with the second reservoir 34.

As seen in Fig.2, the four positive displacement pumps 40a, b,c and d each have their own inlet 50a, b,c and d and outlet 52a, b,c and d respectively. In this embodiment all of the inlets 50a-d are connected to the chamber inlet 44, but in an alternative embodiment each of the inlets 50a-d is in direct communication with the first reservoir 32.

The outlets 52a-d of each of the pumps 40a-d are connected to respective two-way valves 42a, b,c and d respectively. Each of the two-way valves 42a-d has two valve outlets 56a-d and 58a-d respectively. Each of the valves 42a-d is each independently switchable between two operational conditions in which the respective pump outlet 52a-d is put into communication with the first valve outlet 56a-d or with the second valve outlet 58a-d respectively.

The first valve outlets 56a-d are all connected to the first chamber outlet 46 (but in a different embodiment each of the outlets 56a-d is in direct communication with the first reservoir 32). The second valve outlets 58a-d are all connected to the second chamber outlet 48 (but in a different embodiment all of the outlets 56a-d are in direct communication with the second reservoir 34).

In the first operational condition of a two-way valve 42a-d the outlet of a pump 40a-d is put into communication with the first reservoir 32 and this is the condition shown for the two-way valve 42d in Fig.2. In this first operational condition the working liquid 36 passes from the first reservoir 32, through the pump 40d and valve 42d and back to the first reservoir 32 as shown by the arrow.

In the second operational condition of a two-way valve 42a-d the outlet of a pump 40a-d is put into communication with the second reservoir 34 and this is the condition shown for the two-way valves 54a, b and c in Fig.2. In this second operational condition the working liquid 36 passes from the first reservoir 32, through the respective pump 40a-c and valve 42a-c and into the second reservoir 34 as shown by the arrows.

The valve members 60 of each of the valves 42a-d are mounted to rotate through an angle of approximately 30° in this embodiment in order to change between their first and second operational conditions. The actuators for the valve members (which are preferably electrically controlled) are not shown in Fig.2, but are each controlled, independently, by the controller 62 (Figs. 3 and 4). Accordingly, the controller 62 can determine the operating condition for each of the two-way valves 42a-d substantially continuously and can switch each of the valves between its two operating conditions independently as required.

It will be understood that the pressure at the second valve outlets 58a-d is significantly higher than the pressure at the first valve outlets 56a-d. Each of the second valve outlets 58a-d contains a check valve 64a-d respectively in order to ensure that working liquid 36 does not pass from the second reservoir 34 to the first reservoir 32 during the transitions at which the valve outlets 56 and 58 are temporarily put into communication by way of the valve member 60. In this respect it will be understood that the valves 42a-d cannot have an intermediate position in which they are disconnected from both of the valve outlets 56, 58 since the pumps 40a-d are positive displacement pumps and the valves 54a-d must continue to allow liquid flow during the transitions between their first and second operational conditions. It will be understood that there is a permanent driving connection between the blades 14 and the drive shaft 30. In addition, the positive displacement pumps 40a-d are all in permanent driving connection with the drive shaft 30. Accordingly, when the blades 14 are driven to rotate by the wind, working liquid is pumped at a corresponding flow rate from the pump inlets 50a-d to the pump outlets 52a-d. The operating condition of each of the two-way valves 42a-d determines the proportion of the pumped working liquid 36 which passes to the first reservoir 32 (at low pressure) and the proportion which passes to the second reservoir 34 (at high pressure). Since the power required to operate a pump is dependent upon the difference in pressure between the pump inlet and the pump outlet, the power required to drive each of the pumps 40a-d is directly dependent upon the operational condition of the valve for that pump. Accordingly, the combined power required to drive all of the pumps 40a-d is directly dependent upon the operating conditions of all of the valves 42a-d.

The controller 62 is configured to balance the rate at which power is being extracted from the wind to the power which is being consumed by the pumps 40a-d (taking account of the intervening power losses) substantially continuously (or at least sufficiently frequently as to be effectively continuous). As the wind speed increases and the power which is extracted by the blades 14 increases, the valves 42a-d are re-configured to increase the pumping power, and correspondingly to increase the rate at which working liquid 36 is pumped into the second reservoir 34 (and vice versa).

The number of pumps 40, and the displacement of each of the pumps 40, can be chosen to suit the desired range of pumping power, and thereby to suit the desired range of power which can be extracted from the wind. In this embodiment the displacement of the pump 40d is double the displacement of the pump 40c, which in turn is double the displacement of the pump 40b, and which in turn is double the displacement of the pump 40a. The different displacements reduces the number of pumps required for a given range of pumping power.

It will be appreciated that changes of the conditions of the valves 42a-d, and thereby changes of the pumping power, can be used to match changing wind speeds without any change in the rotation rate of the blades 14. The blades will typically have a peak efficiency at which they can extract power from the wind, which peak efficiency is dependent upon wind speed and the rate of rotation. Blade designers will typically seek to maximise the blade efficiency across a range of wind speeds and across a range of rotation rates. The present invention can readily accommodate changes in the rate of rotation of the blades as might be caused by short-term gusts or longer term changes in wind speed. The controller 62 can (substantially continuously) determine if the change in rotation rate of the blades 14 is sufficient to justify a change in the operating conditions of the valves 42a-d. The controller 62 can, for example, ignore short-term changes in the rotation rate such as those caused by gusts which last less than a few seconds. The controller 62 can also ignore longer-term changes in the rotation rate provided that the rotation rate remains within a predetermined range for which the wind turbine is at or close to its peak efficiency.

It will be understood that changes in rotation rate of the blades 14 will cause a corresponding change in the rotation rate of the pumps 40a-d and thereby a change in the pumping rate of working liquid 36. Only small changes in the rotation rate of the blades 14 are required to allow the wind turbine 10 to provide an effectively continuous range of pumping power and thereby an effectively continuous range of pumping rates into the second reservoir 34 between a predetermined minimum and maximum. It will also be appreciated that the power consumed by a pump is proportional to the product of: {i} the flow rate, {ii} the density of the working liquid, {iii} the force of gravity and {iv} the pressure head H. With the pumps 40a-d moving water at 1 m³/sec, 2 m³/sec, 3 m3/sec and 4 m3/sec according to the previous example, and with a pressure head H of 100 m, it can be calculated that the power consumed by the pumps 40a-d can be adjusted (substantially continuously) across a range between approx. 1 megawatt and approx. 15 megawatts, which range is suitable for a large wind turbine.

It will be understood that the chamber 38 is optional and the pumps 40a-d, valves 42a-d and the valve actuators could all be submerged in the first reservoir 32. Submerging the pumps in particular would have the advantage of avoiding cooling measures which might otherwise be required. However, it is desired to provide an (air-filled) chamber 38, and also to provide operator access to that chamber, in order for routine maintenance to be carried out without requiring the emptying of the first reservoir 32.

The second reservoir 34 has an outlet 68 which passes to a chamber 70 in the first reservoir. The chamber 70 has an outlet 72 in communication with the first reservoir 32. The components inside the chamber 70 are shown in Fig.3 and include a flow control valve 74 and a turbine 76. The turbine 76 has an output shaft 78 which is connected to the rotor (not shown) of an electrical generator 80. The detail of the flow control valve 74, the turbine 76 and the electrical generator 80 are not shown in Fig.3 since that detail is incidental to the present invention, and many different components can be used, suited to the wind turbine 10. For example, the flow control valve 74 can be a needle valve, a butterfly valve or another form of valve suited to the working liquid and the range of flow rates required. The turbine 76 can be of the Francis, Pelton or Kaplan types, for example. The turbine 76 can be a single turbine or a stack of interconnected turbines with flow control valves controlling the flow of working liquid 36 to each turbine in the stack. The electrical generator 80 can be an induction generator, for example a doubly-fed induction generator.

As shown in Fig.3, the flow control valve 74 and the electrical generator 80 are connected to the controller 62. In a continuous mode of operation, the controller 62 is configured to adjust the flow rate of working liquid 36 though the turbine 76 to the flow rate at which the working liquid is being pumped into the second reservoir 34. In such a mode of operation the level of working liquid in the second reservoir can be maintained substantially at the maximum level Max. as represented in Fig .1 . This mode of operation will ensure a substantially constant pressure head H acts across the turbine 76, and also across the pumps 40a-d, and the flow rate will result in a variable output from the generator 80 along the electricity supply line 54, which output is directly dependent upon the instantaneous wind speed.

In the continuous mode of operation the flow rate of working liquid 36 though the turbine 76 can be adjusted directly by opening or closing the flow control valve 74. Alternatively, the flow rate can be adjusted indirectly by increasing or decreasing the electrical load at the generator 80 which will act to slow or speed up the output shaft 78. Both of these modes of adjustment are preferably used together to seek to maximise the efficiency of the wind turbine 10 across a range of wind speeds.

In a cyclical mode of operation the controller 62 is configured to allow the level of working liquid

36 in the second reservoir 34 to fall to a minimum operational level Min. as indicated in Fig .1 . In this mode the electrical power output along the supply line 54 is not directly dependent upon the instantaneous wind speed and electrical energy is generated from the stored potential energy of the working liquid in the second reservoir 34. When the level of working liquid 36 in the second reservoir 34 drops to the Min. level the flow control valve 74 is partially or fully closed to allow the second reservoir to refill. The flow control valve 74 is subsequently re-opened fully when the working liquid reaches the Max. level again. The flow control valve 74 can be adjusted in the cyclical mode in order to vary the electrical output to match the requirements of the mains network.

The minimum operational level Min. is close to the minimum level of working liquid at which the pressure head can sustain the rotation of the turbine 76 and the rotor of the electrical generator 80. In a practical wind turbine the level Max. might provide a pressure head H of 100 m for example, whereas the level Min. might provide a pressure head of 15 m.

It will be understood that the chamber 70 is optional and the flow control valve 74 and turbine 76 could be submerged in the first reservoir 32. Submerging the turbine 76 in particular would have the advantage of avoiding cooling measures which might otherwise be required. However, it is desired to provide an (air-filled) chamber 70, and also to provide operator access to that chamber, in order for routine maintenance to be carried out without requiring the emptying of the first reservoir.

In any event, it is not desirable to submerge the electrical generator 80, especially if the working liquid is sea water, as that would likely cause capacitive and inductive reactances to the alternating electrical current(s) and adversely affect the electrical generation. Advantage can, however, be taken of the surrounding working liquid 36 to cool the electrical generator 80, for example by including thermally-conductive bridges between the first reservoir 32 and the generator 80.

It will be understood from Fig.1 that in this embodiment the second reservoir 34 occupies a large proportion of the volume of the tower 30 and in particular fills the tower 30 up to the predetermined level Max. An overflow conduit (not shown) is located in the tower 30 through which working liquid 36 can flow from the top of the second reservoir 34 to the first reservoir 32, bypassing the turbine 76.

Though not shown in the drawings, the top housing 18 can include a landing pad for a helicopter, and/or the tower 20 can have an access lift or shaft by which an operator can access the top housing 18, for periodic maintenance of the blades, hub, bevel gear set etc. The access lift or shaft can be provided inside the tower 20 or can be mounted to the outside of the wall of the tower, as desired.

Though also not shown in the drawings, the controller 62 is connected to various sensors to monitor the operation of the wind turbine 10. For example, the wind turbine can have sensors by which the controller can monitor one or more of: {i} the rate of rotation of the hub 16, {ii} the pitch of the blades 14 relative to the hub 16, {iii} the torque in the drive shaft 30, {iv} the condition of each of the two-way valves 42a-d, {v} the position of the flow control valve 74, {vi} the rate of rotation of the output shaft 78, {vii} the voltage and current generated by the electrical generator, and {viii} the level of working liquid 36 in the second reservoir 34.

Though also not shown in the drawings, the controller 62 is connected to control actuators and/or mechanisms for controlling one or more of: {i} the pitch of the blades 14 relative to the hub 16, {ii} the operating condition of each of the two-way valves 42a-d, {iii} the turbine flow control valve 74, {iv} the amount and/or relative phase of AC power passing to the stator and/or to the rotor of the electrical generator 80.

In order to restart the wind turbine 10 after a shutdown, the wind turbine 10 additionally has a secondary electrical generator 82 which is used in a black start procedure. As shown in Fig.1 , the second reservoir 34 has a secondary outlet 84 which passes to a chamber 86 in the first reservoir 32. The chamber 86 has an outlet 88 in communication with the first reservoir 32. The components inside the chamber 86 are shown in Fig.4 and include a black start valve 90 and a hydraulic motor 92. The hydraulic motor 92 has an output shaft 94 which is connected to the rotor (not shown) of the secondary electrical generator 82.

The detail of the black start valve 90, the hydraulic motor 92 and the electrical generator 82 are not shown in Fig.4 since that detail is incidental to the present invention and many different components can be used, suited to the wind turbine 10. For example, the flow control valve 90 can be a ball valve, a butterfly valve or another form suitable for the working liquid. The hydraulic motor 92 can be a turbine or a positive displacement motor. To avoid the requirement for an electrical supply to generate the magnetic field, the electrical generator 82 is a permanent magnet motor, but the detailed form of the motor (including whether its output is DC or AC for example) can be determined for a particular wind turbine.

It will be understood that a permanent magnet motor can generate electricity with a relatively slow rate of rotation of the shaft 94. Nevertheless, if desired a step-up gearbox can be provided to increase the rate of rotation of the rotor of the secondary electrical generator 82. Increasing the rate of rotation of the rotor will typically increase the efficiency of the secondary electrical generator, but will also increase the load upon the hydraulic motor and thereby the pressure head required to rotate the hydraulic motor. It will also be understood that the efficiency of the secondary electrical generator is not critical as it is only operated infrequently.

The chamber 86 is represented in Fig.1 as being smaller than the chamber 70, and similarly the secondary outlet 84 is shown to be smaller than the outlet 68. The components of Fig.4 are similarly represented as somewhat smaller than the components of Fig.3. This is because the electrical power which is required to be generated by the secondary electrical generator 82 will be significantly less than that required to be generated by the primary electrical generator 80, and consequently the components and the flow rate of working liquid can be much smaller in practice.

During normal operation of the wind turbine 10 the black start valve 90 is closed and no working liquid 36 flows through the secondary outlet 84 or hydraulic motor 92. If the wind stops blowing for a very short period of time the inertia of the blades will be sufficient to continue to pump working liquid into the second reservoir 34 and the turbine 76 and electrical generator 80 can continue to operate.

If the wind stops blowing for a longer period of time and the blades 14 slow down sufficiently to be outside their efficient range, the controller 62 can react by changing all of the valves 42a-d to their first operating condition whereby to minimise the load upon the blades 14 and maintain their rotation for as long as possible. If the wind does not recommence blowing before the blades have stopped rotating, the wind turbine 10 can nevertheless continue to generate electricity by allowing the level of working liquid 36 in the second reservoir 34 to fall. If the wind stops blowing for a sufficient period of time for the level of working liquid to fall to the level Min. the flow control valve 74 is closed. The turbine 76 stops rotating and the wind turbine 10 stops generating electricity and goes into a shutdown. A black start procedure is required to restart the wind turbine 10 after a shutdown. With the present invention the black start procedure can be carried out autonomously, i.e. without requiring any external controls or power.

The wind turbine 10 has a battery back-up (not shown) and an array of solar panels (also not shown) which feed electrical energy to the battery. The wind turbine also has an anemometer (not shown) by which the controller can monitor the wind speed and the wind direction. During normal operation of the wind turbine the controller 62 receives a supply of electrical power from the primary generator 80 and all of the control functions and operations are powered by the primary generator. During a shutdown the controller 62 receives electrical power from the battery back-up, supplemented by the solar panels. It can also be arranged that electrical power is diverted from the electrical supply line 54 to charge the battery before the turbine 76 stops rotating, for example as the level of working liquid 36 approaches the Min. level.

It is arranged that all unnecessary control functions are stopped during a shutdown in order to save power. For example, it is not necessary to monitor the pitch of the blades 14 or the condition of the valves 42a-d during a shutdown. The controller 62 does, however, monitor the wind speed during a shutdown, either continuously or periodically. When the wind speed has returned to a level exceeding the minimum operating wind speed (i.e. the lowest wind speed which can sustain the rotation of the blades 14) the controller can initiate the black start procedure.

The first step of the black start procedure is to open the black start valve 90. The power to open the valve is provided by the battery.

Opening the black start valve allows working liquid to pass through the hydraulic motor 92, causing the output shaft 94 to rotate and to generate electrical power from the secondary electrical generator 82. It is arranged that the level Min. in the second reservoir 34 provides a pressure head which is sufficient to initiate operation of the hydraulic motor 92 and to rotate the rotor of the secondary electrical generator 82. It is also arranged that sufficient working liquid 36 remains in the second reservoir 34 at the level Min. to continue to drive the secondary electrical generator 82 for sufficient time to provide the electrical power required to complete the black start procedure (or perhaps several black start procedures in case the wind stops blowing again before the wind turbine 10 can resume normal operation).

The controller 62 receives a signal from the secondary electrical generator 82 to confirm that the generator is operating. The controller can then cause the secondary electrical generator 82 to send electrical power along a supply line 96 to recharge the battery if required and also to provide the power required by the components to re-start the wind turbine.

Firstly, the controller 62 can compare the wind direction with the orientation of the top housing 18 and if required actuate the yaw drive mechanism to reorient the blades 14 with the new wind direction. Secondly, the controller 62 can compare the pitch of the blades 14 with the wind speed and if necessary adjust the pitch drive mechanism. Thirdly, the controller can release the brake mechanism to allow the hub 16 and blades 14 to rotate. Fourthly, if necessary the controller 16 can actuate the hub drive mechanism to initiate the rotation of the hub 16 and blades 14. It will be understood that not all of these operations is necessarily sequential and some can be carried out together (and/or in a different order) if desired. When the blades 14 are rotating again, and that rotation is being sustained by the wind, the blades accelerate until their rotation reaches a minimum operating rate, which usually equates to a minimum efficiency for extracting energy from the wind. The pitch of the blades is adjusted as required to achieve the efficient rotation rate. The controller 62 then actuates one or more of the valves 42a-d so that working liquid 36 is pumped into the second reservoir.

It is expected that the power required by the controller 62 and the other components during the black start procedure is sufficiently small that the flow rate of working liquid 36 through the hydraulic motor 92 will be smaller than the pumping rate of the smallest pump 40a so that the level of working liquid 36 in the second reservoir will increase despite the continued flow of working liquid through the outlet 84.

When the level of working liquid reaches the level Max. the flow control valve 74 is opened to recommence the operation of the primary electrical generator 80. When the controller receives a signal indicating that electrical power is being generated by the primary electrical generator 80 the black start valve 90 can be closed and normal operation can resume.

Claims

1. A wind turbine having: a number of blades, the blades being carried by a hub, the hub being rotatably mounted to a tower, the hub being connected to a drive shaft; a first reservoir and a second reservoir for a working liquid, working liquid in the second reservoir being above working liquid in the first reservoir; the drive shaft being connected to at least one positive displacement pump, the pump having an inlet and an outlet, the inlet being in communication with the first reservoir, the outlet having a two-way valve, the two-way valve being changeable between a first condition in which the outlet is in communication with the first reservoir and a second condition in which the outlet is in communication with the second reservoir; a turbine with an inlet in communication with the second reservoir and an outlet in communication with the first reservoir, the turbine having an output shaft; and an electrical generator having a rotor connected to the output shaft.

2. The wind turbine according to claim 1 in which the first and second reservoirs are parts of a closed system for the working liquid.

3. The wind turbine according to claim 1 or claim 2 in which the drive shaft is connected to a plurality of positive displacement pumps and the outlet of each of the plurality of positive displacement pumps has a two-way valve.

4. The wind turbine according to claim 3 in which all of the plurality of positive displacement pumps are in permanent driving connection with hub.

5. The wind turbine according to claim 3 or claim 4 having a control system which is connected to the two-way valves and which is configured to switch each of the two-way valves between its first and second conditions independently.

6. The wind turbine according to claim 5 having means to determine the power driving the blades, the control system being configured to substantially balance the power driving the pumps with the power driving the blades.

7. The wind turbine according to claim 6 in which the means to determine the power driving the blades comprises measuring the rate of rotation of the drive shaft and the torque in the drive shaft.

8. The wind turbine according to any one of claims 5-7 having means to measure the level of working liquid in the second reservoir.

9. The wind turbine according to any one of claims 5-7 in which the control system has a memory with data upon the power required to drive each of the pumps in their second condition.

10. The wind turbine according to claim 8 and claim 9 in which the memory has data upon the power required to drive each of the pumps in their second condition with different levels of working fluid in the second reservoir.

11 . The wind turbine according to any of claims 3-10 in which the plurality of positive displacement pumps have differing displacements.

12. The wind turbine according to claim 11 having a first positive displacement pump and a second positive displacement pump, in which the displacement of the second positive displacement pump is double the displacement of the first positive displacement pump.

13. The wind turbine according to claim 12 having a third positive displacement pump, in which the displacement of the third positive displacement pump is double the displacement of the second positive displacement pump.

14. The wind turbine according to any one of claims 1-13 in which the positive displacement pumps, the two-way valve(s), the turbine and the electrical generator are located at or adjacent to the bottom of the tower.

15. The wind turbine according to any one of claims 1 -14 in which the first reservoir is located at or adjacent to the bottom of the tower.

16. The wind turbine according to claim 14 or claim 15 adapted for an offshore location with a submergable part which will be below sea level in use, and in which the positive displacement pumps, the two-way valve(s), the turbine, the electrical generator and the first reservoir are located in the submergable part.

17. The wind turbine according to any one of claims 1 -16 in which the second reservoir occupies a large proportion of the tower.

18. The wind turbine according to any one of claims 1 -17 having an overflow conduit through which the working liquid can flow from the second reservoir to the first reservoir.

19. The wind turbine according to any one of claims 1 -18 having a turbine valve at the turbine inlet.

20. The wind turbine according to claim 19 in which the turbine valve is a variable flow control valve.

21 . The wind turbine according to any one of claims 1 -20 in which the hub is connected to the drive shaft by way of a bevel gear set.

22. The wind turbine according to any one of claims 1-21 in which the blades and hub are carried by a top housing which is mounted to the tower and which can rotate relative to the tower about a substantially vertical axis.

23. The wind turbine according to any one of claims 1 -22 having a mechanical brake for the hub.

24. The wind turbine according to any one of claims 1-23 in which the electrical generator is a primary electrical generator, and in which the wind turbine has a secondary electrical generator, the secondary electrical generator being a permanent magnet generator.

25. The wind turbine according to claim 24 in which the secondary electrical generator has a rotor which is connected to a hydraulic motor, the hydraulic motor having an inlet in communication with the second reservoir and an outlet in communication with the first reservoir.

26. The wind turbine according to claim 25 having a stop valve at the inlet of the hydraulic motor.