WO2023083427A1

WO2023083427A1 - Wind turbine fluid tank

Info

Publication number: WO2023083427A1
Application number: PCT/DK2022/050238
Authority: WO
Inventors: Michael STAGHØJ; Poul T. Tietze; Thomas Riis JAKOBSEN
Original assignee: Vestas Wind Systems A/S
Priority date: 2021-11-15
Filing date: 2022-11-10
Publication date: 2023-05-19

Abstract

A fluid tank (12) for a wind turbine (2), the tank (12) comprising: a housing (14) enclosing 5 an interior volume (16) for holding liquid, the housing (14) comprising a vent (36) positioned above a maximum liquid surface level in the interior volume (16), to permit air flow between the interior volume (16) and the surrounding environment, in use; a barrier (38, 138, 238) extending across the interior volume (16) to divide the interior volume (16) into an upper volume (40) and a lower volume (42), wherein the barrier (38, 138, 238) is disposed 10 beneath a minimum liquid surface level in the interior volume (16). At least one passage (62, 68) is provided in the barrier (38, 138, 238) to allow fluids to flow between the upper and lower volumes (40, 42); and at least one opening (28, 32) through which liquid can be communicated into or out of the lower volume.

Description

WIND TURBINE FLUID TANK

Technical Field

The present disclosure generally relates to fluid tanks for wind turbine generators, such as coolant tanks.

Background

Wind turbine generators (WTGs) typically incorporate cooling systems that circulate cooling fluid to regulate temperatures of components of the wind turbine in operation. Such cooling systems often include a tank that holds a reserve of cooling fluid for circulation.

Use of open expansion cooling systems are known in which cooling fluid is pumped between a vented or otherwise unpressurised tank and components requiring thermal control via a heat exchanger. Open expansion systems may be preferred over closed, pressurised systems due to their simplicity, low power requirements and improved safety. By virtue of the open tank, such systems can accommodate expansion of the cooling fluid with temperature as well as providing a buffer against any cooling fluid leakage that may occur throughout the system.

However, due to the unsealed nature of open systems, the cooling fluid is constantly exposed to ambient air, meaning it is inevitable that air will be absorbed by the cooling fluid and travel throughout the system. Coolants containing substances such as glycol (i.e. antifreeze) degrade upon contact with oxygen and so the working life of cooling fluid used within open expansion systems is shortened. Replacing and I or maintaining cooling fluid of a WTG cooling system can be costly and typically requires a period of WTG downtime, thus reducing the WTG’s annual energy production (AEP).

Similar problems may arise elsewhere in a WTG when air is absorbed by a working fluid, such as lubrication oil, when exposed to air in a vented tank.

It is against this background that the invention has been devised. Summary of the Invention

According to one aspect of the invention a fluid tank for a wind turbine is provided. The tank comprises a housing enclosing an interior volume for holding liquid and a barrier which extends across the interior volume to divide the interior volume into an upper volume and a lower volume. The housing comprises a vent positioned above a maximum liquid surface level in the interior volume to permit air flow between the interior volume and the surrounding environment, in use. The barrier is disposed beneath a minimum liquid surface level in the interior volume. At least one passage is provided in or through the barrier to allow fluids to flow between the upper and lower volumes, for example to allow trapped air to flow from the lower volume to the upper volume through the barrier. Additionally, the tank comprises at least one opening through which liquid can be communicated into or out of the lower volume. The at least one opening may comprise an inlet opening through which liquid can be communicated into the lower volume, and an outlet opening through which liquid can be communicated out of the lower volume.

This arrangement beneficially contains any turbulent flow arising from flow through the opening, in use, to the lower volume and minimises the effect of that turbulent flow on the liquid surface level. As a result, the liquid surface level remains relatively undisturbed and aeration of the liquid held within the tank is minimised. Equally, the at least one passage provided in the barrier enables any air that has become trapped within the liquid to rise through the tank in a controlled manner with minimal effect on the liquid surface level. Consequently, the rate at which the liquid degrades due to aeration is reduced and the service life of the liquid is increased, thus reducing costs associated with operating and maintaining the wind turbine.

The barrier may be formed integrally with the housing and/or the opening may be formed in the barrier. In embodiments in which the opening is formed in the barrier, the tank may further comprise a duct that extends through the upper volume and the opening into the lower volume. Where the tank includes multiple openings, each may receive a respective duct. In such embodiments, the or each duct may be configured to convey liquid into and/or out of the lower volume. Also, the or each opening may be larger than the respective duct so as to define a gap that forms one of the passages provided in the barrier. In this way, air can flow through the gap past the duct from the lower volume to the upper volume. In embodiments, the barrier may extend in a horizontal plane when the housing is oriented upright, in use. Also, the barrier may extend from a side wall of the housing.

Alternatively or additionally, the upper volume may be defined by a secondary housing that is nested inside the housing, in which case the barrier is defined by a base of the secondary housing. In such embodiments, the secondary housing may be formed from a continuous wall.

In embodiments, the vent may be defined by an open upper end of the housing.

Furthermore, the housing may be formed by a rotational moulding process.

In embodiments, the tank may further comprise one or more buoyant elements in the upper volume which have a combined cross-sectional area that is at least half of the cross-sectional area of the upper volume so that, in use, the one or more buoyant elements float in liquid held in the upper volume to cover between 50% and 99%, and preferably between 90% and 99%, of the surface of the liquid. This embodiment is technically advantageous in that the buoyant elements further serve to dampen movement of the liquid surface level and thereby minimise aeration of the coolant.

Alternatively or additionally, the at least one opening may comprise an inlet opening through which liquid can be communicated into the lower volume, and an outlet opening through which liquid can be communicated out of the lower volume.

According to another aspect of the invention, a wind turbine system comprises the tank according to the above aspect of the invention. The system may be embodied as a cooling system, in which case the tank is configured to hold coolant in the upper and lower volumes. In embodiments in which the tank comprises a duct, the system may comprise a pump that is connected to the duct to draw liquid from the lower volume of the housing. Alternatively, the system may comprise a return line connected to the duct to deliver liquid to the lower volume. The tank may include multiple ducts, in which case the system may include a pump connected to one duct and a return line connected to another duct. According to a third aspect of the invention, a method of fabricating a fluid tank for a wind turbine is provided. The method comprises moulding a housing to enclose an interior volume and dividing the interior volume of the housing into an upper volume and a lower volume using a barrier that extends across the interior volume beneath a minimum liquid surface level. The housing comprises a vent positioned above a maximum liquid surface level in the interior volume to permit air flow between the interior volume and the surrounding environment when in use. The barrier comprises at least one passage to allow fluids to flow between the upper and lower volumes. The method further comprises providing at least one opening through which liquid can be communicated into or out of the lower volume. The housing may be moulding using a rotational moulding process, for example.

It will be appreciated that preferred and/or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.

Brief Description of the Drawings

So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the following drawings, in which like features are assigned like reference numerals, and in which:

Figure 1 is a front view of a horizontal axis wind turbine generator to which embodiments of the invention may be applied;

Figure 2 is a cross-sectional view of a fluid tank according to an embodiment of the invention;

Figure 3 is a perspective cross-sectional view of the housing of the fluid tank according to the embodiment of the invention shown in Figure 2;

Figure 4 is a rotated perspective cross-sectional view of the housing of the fluid tank according to the embodiment of the invention shown in Figure 2;

Figure 5 is a cross-sectional view of a fluid tank according to another embodiment of the invention; Figure 6 is a perspective cross-sectional view of the housing of the fluid tank according to the embodiment of the invention shown in Figure 5; and

Figure 7 is a schematic view of a fluid tank according to another embodiment of the invention.

Detailed Description

In general terms, embodiments of the invention provide apparatus and corresponding methods for minimising liquid-air mixing within an open or otherwise vented liquid tank. For example, the tank may hold a volume of cooling liquid, or ‘coolant’, for a cooling system for a wind turbine generator (WTG), and the invention is described in this context below. It should be appreciated, however, that in other embodiments a different liquid may be held in the tank, for example oil for a WTG lubrication system.

The approach involves a tank assembly which is configured to reduce circulation of liquid coolant held within by way of an internal barrier which isolates the turbulence caused in the vicinity of the tank inlet(s) and outlet(s) from the surface level of the cooling fluid, thereby reducing movement at the surface. As such, the internal barrier divides the inside of the tank into two volumes; a lower volume beneath the barrier and an upper volume above the barrier. Coolant enters and exits the tank via the lower volume, while the fluid surface level resides within the upper volume. The barrier is therefore positioned beneath an intended minimum surface level of the liquid in the tank.

Although the embodiments described herein refer to only one barrier, and therefore two resulting volumes, arrangements of ‘stacked’ barriers resulting in more than two volumes are possible.

The internal barrier is provided with at least one passage extending therethrough so as to be permeable to air and coolant in such a manner that the upper and lower volumes are in fluid communication and the volume of coolant within the tank can expand and contract as required. Equally, however, the permeability of the internal barrier is such that the rate of air flow between the upper and lower volumes is restricted to minimise flow separation and to control the rate and path of ascent of air bubbles through the coolant, and consequently reduce disturbance at the surface. In principle, the internal barrier may be constructed as a mesh or from a porous material such as open-cell foam that provides the required permeability, for example. In embodiments described below, however, the barrier is defined by one or more solid walls having apertures that act as inlets and outlets for cooling fluid whilst also creating the required permeability.

Since contact with air is known to degrade certain coolant compositions, it is beneficial to minimise contact between the air and the liquid where possible. Air-coolant mixing predominantly occurs at the surface where the air and liquid are in contact. Surface level agitation further increases the contact area, thereby accelerating the rate at which air is absorbed by the coolant. Conversely, minimising surface movement reduces mixing of air with the coolant. As such, embodiments of the invention provide an improved open or vented tank in which turbulence or movement at the liquid surface is reduced. As a result, the invention better preserves the integrity of the coolant stored within and, therefore, reduces the frequency with which it needs replacing. In turn, the costs associated with operating and maintaining the WTG are reduced.

Although the embodiments described herein are provided specifically in the context of WTG cooling systems, as noted above, it is envisaged that they may be useful in other fluid systems where it is beneficial to reduce mixing of liquid with air, such as in oil lubrication systems. Generally, oxygen present in fluid circulation systems can lead to component corrosion and decreased pump efficiency, and so it can be advantageous to minimise air absorption even where the circulated fluid is unaffected by the dissolution of air.

To provide context for the invention, Figure 1 shows a typical horizontal axis WTG, also referred to below as a wind turbine 2, that includes a nacelle 4, mounted atop a tower 6, which supports a front facing rotor 8 comprising a plurality of coplanar blades 10. The rotor 8 is connected to a powertrain or drivetrain housed within the nacelle 4. The drivetrain comprises components required to convert rotation of the rotor 8 into electricity, including a generator, a gear system, transformer(s), converter(s), bearing(s) and brake(s). A cooling system (not shown in Figure 1) is arranged within the nacelle 4 and is configured to cool one or more of the drivetrain components by circulating a cooling fluid to and from the components via one or more cooling devices such as a heat exchanger. Figure 2 shows a cross-sectional view of a fluid tank 12 for use in the cooling system of the wind turbine 2. As shown, the tank 12 acts as a fluid reservoir and comprises a main housing 14 which defines an interior volume 16 configured to hold cooling fluid. The housing 14 is formed as a substantially trapezoidal bucket with a substantially rectangular base 18 at its lower end and side walls 20 extending upwardly from each edge of the base 18, the side walls 20 being joined together to form a continuous wall.

When in use, the volume of cooling fluid in the cooling system is within a certain operational range, which defines a corresponding range for the cooling fluid surface level within the interior volume 16, this range in turn defining a maximum surface level and a minimum surface level.

The tank 12 comprises three vertically-extending tubular pipes projecting downwardly from a top of the tank 12, through which pipes cooling fluid enters and leaves the tank. A first of these pipes, shown furthest to the left in Figure 2, defines an inlet duct 26 configured to direct cooling fluid into the interior volume 16 via an inlet opening 28 of the housing 14. The central and rightmost pipes define a pair of outlet ducts 30a, 30b through which cooling fluid may be drawn out of the interior volume 16 via respective outlet openings 32a, 32b of the housing 14. The outlet pipes are of greater diameter than the inlet pipe in this embodiment.

Two pumps 34 are mounted at the top of the tank 12, each pump 34 being connected to a respective one of the outlet ducts 30. In operation, each pump 34 is arranged to draw cooling fluid out of the interior volume 14 through its respective outlet duct 30, and to drive cooling fluid through a respective portion of a coolant circuit of the cooling system. Returning coolant that has completed a portion of the circuit flows back into the interior volume 14 of the tank 12 through the inlet duct 26. It should be noted that in practice the tank 12 may comprise different numbers of inlet ducts 26 and/or outlet ducts 30 serving a corresponding number of pumps 34, to suit the requirements of the system.

In operation, the temperature of the cooling fluid is liable to increase as components of the drivetrain reach their normal operating temperatures. As a result, the volume of cooling fluid held within the interior volume 16 expands correspondingly. It will therefore be understood that the fluid surface level varies between a minimum and maximum level within the interior volume 16. In order to maintain an appropriate system pressure as the cooling fluid expands, the tank 12 comprises an opening to the atmosphere 36 which acts as a vent to allow air to enter or exit the interior volume 16 as required. That means to say, the opening to the atmosphere 36 is configured to equalise the air pressure between the interior volume 16 and an ambient environment outside of the housing 14. Accordingly, the tank 12 is configured as an open tank and correspondingly the cooling system is an open system.

Figures 3 and 4 show perspective cross-sectional views of an embodiment of the housing 14. As can be seen, the housing 14 is formed as an open topped container in which the opening to the atmosphere 36 is integral to the structure of the housing 14. That is to say, when the housing 14 is oriented upright in its operating position as shown in Figure 2, the upper end is open, thus allowing air to freely enter and exit the interior volume 16. Accordingly, the open end of the housing acts as a vent 36 in this embodiment. In other embodiments, the upper end of the housing 14 is at least partially closed to enclose the interior volume 16 more fully, and the housing 14 is provided with an opening to the atmosphere 36 in the form of a vent positioned above the maximum cooling fluid surface level.

At this juncture it should be recognised that the delivery and extraction of cooling fluid from the internal volume 16 results in fluid circulation and turbulence within the internal volume 16. As mentioned above and as described further below, the tank 12 is configured to prevent such turbulence from reaching the fluid surface level so as to minimise movement at the surface and consequential mixing of air with the cooling fluid.

In this respect, the tank 12 further comprises a barrier 38, shown most clearly in Figure 2, which extends across the interior volume 16, to divide the interior volume into an upper volume 40 and a lower volume 42, the upper volume 40 being immediately above the lower volume 42. More specifically, the barrier 38 extends in a horizontal plane (according to the orientation of the figures) across the interior volume 16.

The barrier 38 is positioned at a vertical level such that, when in use, the minimum cooling fluid surface level is above the barrier 38 (i.e., in the upper volume 40). Equally, the arrangement of the inlet 26 and outlet ducts 30 in relation to the barrier 38 is such that, when in use, cooling fluid flows into and out of the interior volume 16 below the barrier 16 (i.e. via the lower volume 42). As such, the barrier 16 serves to separate the cooling fluid surface level from the inlet and outlet openings 28, 32. Accordingly, turbulence caused by fluid entering and exiting the lower volume 42 is kept separate from the fluid in the upper volume 40, so that movement of cooling fluid in the upper volume 40 is substantially less than in the lower volume 42.

The barrier 38 is permeable by means of at least one passage extending through the barrier 38 to allow cooling fluid to flow between the upper and lower volumes 40, 42. Accordingly, the volume of cooling fluid can expand and contract within the interior volume 16 with the fluid surface level adjusting above the barrier 38.

Similarly, the permeability of the barrier 38 allows air to flow between the upper and lower volumes 40, 42. Accordingly, any air present in the cooling fluid in the lower volume 42, for example air carried by coolant conveyed through the inlet duct 26, can rise as bubbles through the barrier 38 and then ultimately be released to the external ambient environment via the vent 36.

According to the embodiment of Figures 2 to 4, the barrier 38 is formed integrally as part of the housing 14 and the two are constructed from the same material as a moulded part. Any moulding material with suitable mechanical properties may be used to form the housing 14 and the barrier 38, such as plastics materials and composite materials for example. This one-part construction beneficially reduces manufacturing complexity and shortens the bill of materials required.

As seen more clearly in Figures 3 and 4, the barrier 38 comprises a pair of solid planar walls extending between opposed side walls 20 of the housing in vertically-spaced horizontal planes to define an upper surface 48 and a lower surface 50 separated by a void 52. This two-layer construction advantageously improves the stiffness of the barrier and, in turn, confers stiffness to the housing 14.

The barrier’s 38 permeability is achieved by way of one or more aperture arrangements 54 formed within the barrier 38 such that the upper volume 40 and the lower volume 42 are in fluid communication. For each aperture arrangement 54 an upper opening 56 is formed in the upper surface 48 of the barrier 38 and a corresponding lower opening 58 is formed in the lower surface 50 of the barrier 38 in vertical alignment with the upper opening 56, the upper and lower openings 56, 58 being joined by a tubular wall defining a conduit 60 through which fluid can flow between the lower volume 42 and the upper volume 40. In other words, continuity between the upper and lower volumes 40, 42 is achieved by means of the conduit 60.

In the embodiment shown, the barrier 38 comprises three such aperture arrangements 54a, 54b, 54c, each comprising an upper opening 56a, 56b, 56c and a corresponding lower opening 58a, 57b, 58c joined by a conduit 60a, 60b, 60c. Each aperture arrangement 54 is configured to receive a respective one of the inlet ducts 26 and the outlet ducts 30 shown in Figure 2. Specifically, the first aperture arrangement 54a, which is shown furthest to the left in Figure 3, is configured to receive the inlet duct 26, see Figure 2, so that the inlet duct 26 extends through the upper opening 56a, the conduit 60a and the lower opening 58a of the first aperture arrangement 54a. The inlet duct 26 therefore extends through the upper volume 40, through the barrier 38 and into the lower volume 42. Accordingly, the first aperture arrangement 54a in the barrier 38 may be regarded as an inlet opening 28 of the housing 14 through which the cooling fluid is directed into the interior volume 16, noting that the fluid is isolated from the interior volume 16 until it exits the inlet duct 26.

Similarly, each of the second and third aperture arrangements 54b, 54c, shown centrally and to the right in Figure 3, is configured to receive a respective one of the two outlet ducts 30b, 30c, see Figure 2, through the respective upper openings 56b, 56c, conduits 60b, 60c and lower openings 58b, 58c, such that each outlet duct 30 similarly extends through the upper volume 40, through the barrier 38 and into the lower volume 42. Accordingly, the second and third aperture arrangements 54b, 54c may be regarded as outlet openings 32 of the housing 14 through which the cooling fluid is drawn out of the interior volume 16.

Each aperture arrangement 54 is dimensioned to create a continuous clearance around the duct 26, 30 received within. In the embodiment shown, the ducts 26, 30 and their corresponding conduits 60 are all of a circular cross-section. As such, the diameter of each opening 56, 58 and conduit 60 is sized to exceed the outer diameter of the duct 26, 30 to be received within. In this way, and as seen in Figure 2, an annular gap defining an annular passage 62 is defined between the walls of the duct 26, 30 and the aperture arrangement 54 so that continuity between the upper and lower volumes 40, 42 is retained, albeit restricted by the presence of the duct 26, 30. Accordingly, fluid and air can flow through the annular passage 62 and past the exterior of the duct 26, 30 between the lower and upper volumes 42, 40. Additionally, the barrier 38 is formed such that it covers the full depth of the internal volume 16 but does not extend across the full width of the internal volume 16.

Accordingly, the upper and lower surfaces 48, 50 of the barrier 38 are connected at each end by an end wall 66 which is offset from the side walls 20 of the tank 12. In this way, the geometry of the housing 14 provides further gaps defining side passages 68 around the ends of the barrier 38 through which fluid can travel between the upper and lower volumes 40, 42.

Each aperture arrangement 54 and its associated duct 26, 30 are specifically dimensioned such that the resulting annular passage 62 between them is of a width that creates a constriction that restricts the rate at which coolant and air moves between the upper and lower volumes 40, 42. The side passages 68 defined between the side walls 20 of the housing 14 and the ends 66 of the barrier 38 are similarly configured to restrict the rate of fluid flow between the two volumes 40, 42. In this way, the barrier 38 serves to regulate fluid exchange between the upper and lower volumes 40, 42, thereby containing the turbulent flow within the lower volume 42, while still allowing air to escape the system in a controlled manner.

As can be seen in Figures 3 and 4, the edges 70 where the conduit 60 intersects the upper opening 56 and the lower opening 58 are filleted to create a curved transition between the walls of the conduit 60 and the upper and lower surfaces 48, 50 of the barrier 38. The edges 72 where the end walls 66 of the barrier 38 intersect the upper and lower surfaces 48, 50 of the barrier 38 are similarly filleted to create a curved transition between the surfaces. These radiused edges 70 serve to guide air bubbles gently to minimise flow separation as air travels between the upper and lower volumes 40, 42 via the annular and side passages 62, 68. In addition the tank is designed such that pockets, without a passage 62 between the upper and lower volumes 40, 42, where air may collect are avoided to the extent possible.

It is hereby achieved to minimise the likelihood of bubble collapse and the transfer of turbulence to the upper volume 40, and therefore decrease disturbance to the fluid in the upper volume 40. The arrangement promotes steady, vertical movement of air bubbles from the lower volume 42, through the passages 62, 68 and to the fluid surface, meaning that the air can be released from the cooling fluid with minimal disturbance to the cooling fluid surface. Figures 5 and 6 show another embodiment of the tank housing 14, in which the internal barrier 138 comprises only a single skin having apertures 54 (or aperture arrangements) suitable for receiving ducts 26, 30. This embodiment is otherwise similar to the first embodiment described above and so the below description focusses on the differences. In particular, the positions and geometry of the inlet duct 26 and the outlet ducts 30 are the same as for the first embodiment.

As in the previously described embodiment, the apertures 54 are larger than the ducts 26, 30 received therethrough so as to create annular gaps defining annular passages 62 between each duct 26, 30 and the receiving aperture 54, each passage 62 being suitable for air and fluid to travel between the lower and upper volumes 40, 42.

Furthermore, the barrier 138 is formed integrally as part of the housing 14. More specifically, each end of the barrier 138 includes an upwardly extending portion 76 that connects with a top edge 78 of a respective housing side wall 20, so that the barrier 138 and the side walls 20 are formed from a continuous wall. More generally, the barrier 138 may be considered to be defined by a base 80 of a secondary housing 82 that is nested inside the housing 14 of the tank 12, the secondary housing 82 also enclosing the upper volume 40 in this embodiment. Therefore, it is contemplated that the housing 14 and secondary housing 82 may be formed as separate parts to be assembled together in other embodiments.

Side cavities 84 defined between the side walls 20 of the housing and the upwardly extending portions 76 of the barrier 138 are provided with vent openings at the top, to prevent accumulation of air rising into the cavities 84.

It is contemplated that the housing 14 embodiments described above are formed through means of a rotational moulding process. Rotational moulding processes involve rotating molten material within a hollow mould such that the material disperses and builds up on the inside walls of the mould. Accordingly, such processes are typically only capable of producing fully closed surfaces which correspond to internal geometry of the mould. Therefore, open designs may require post-processing in which material is removed to create open features such as apertures. As such, the embodiment shown in Figures 5 and 6 is advantageous in that, by virtue of the continuity between the housing 14 and the secondary housing 82, no post processing is required to open the tank 12 (i.e. by removing the top of the housing 14, for example). Only the apertures 54 of the barrier 138 need to be removed in this embodiment. As a result, material waste during manufacture is minimised.

Figure 7 shows another embodiment of the tank 12, in which the internal barrier 238 is formed from a sheet of permeable material extending horizontally across the housing 14 and attached to the side walls 20 of the housing 14, thereby dividing the interior volume 16 of the housing 14 into upper and lower volumes 40, 42 in the same manner as the integral barrier 38, 138 of the earlier embodiments. As shown, inlet and outlet ducts 26, 30 connect with the internal volume 16 at respective inlet and outlet openings 28 formed in opposing sides 20 of the housing 14. More specifically, the inlet and outlet openings 28 are positioned below the internal barrier 238 so that fluid from the inlet and outlet ducts 26, 30 flows into and out of the lower volume 42 in a similar manner to the earlier embodiments. Unlike the earlier embodiments, however, the inlet and outlet openings 28 are not formed in the barrier 238 itself, and thus permeability in the barrier 238 is provided for by the permeability of the material from which the barrier 238 is formed in this embodiment.

Additional or alternative means for reducing contact between the cooling fluid and air are contemplated. For example, a flexible, permeable layer may be laid on the fluid surface to physically prevent contact between the liquid and air while permitting air to be released from the cooling fluid. In the example shown in Figure 7, the permeable layer is composed of a set of discrete buoyant elements in the form of shade balls 86 that cover the surface of the coolant to a desired extent. Shade balls are typically floated on top of reservoirs to block sunlight and minimise evaporation, but may be used to similarly reduce contact with air at the surface in the present embodiments. Covers such as those used for swimming pools may be used to similar effect. More generally, one or more shade balls 86, covers or other buoyant elements may be floated on the fluid surface in the upper volume such that the buoyant elements cover a majority of the surface of the liquid. That is to say, the buoyant elements preferably have a cross-sectional area that is at least half of a cross-sectional area of the upper volume 40. As such, when in use, the one or more buoyant elements float in liquid held in the upper volume 40 to cover between 50% and 99%, and preferably between 90% and 99%, of the surface of the liquid.

Claims

1. A fluid tank (12) for a wind turbine (2), the tank (12) comprising: a housing (14) enclosing an interior volume (16) for holding liquid, the housing (14) comprising a vent (36) positioned above a maximum liquid surface level in the interior volume (16), to permit air flow between the interior volume (16) and the surrounding environment, in use; a barrier (38, 138, 238) extending across the interior volume (16) to divide the interior volume (16) into an upper volume (40) and a lower volume (42), wherein the barrier (38, 138, 238) is disposed beneath a minimum liquid surface level in the interior volume (16) and wherein at least one passage (62, 68) is provided in the barrier (38, 138, 238) to allow fluids to flow between the upper and lower volumes (40, 42); and at least one opening (28, 32) through which liquid can be communicated into or out of the lower volume.

2. The tank (12) of claim 1 , wherein the barrier (38, 138, 238) is formed integrally with the housing (14).

3. The tank (12) of any preceding claim, wherein the opening (28, 32) is formed in the barrier (38, 138, 238).

4. The tank (12) of claim 3, comprising a duct (26, 30) that extends through the upper volume (40) and the opening (28, 32) into the lower volume (42), the duct (26, 30) being configured to convey liquid into or out of the lower volume (42).

5. The tank (12) of claim 4, wherein the opening (28, 32) is larger than the duct (26, 30), defining a gap that forms one of the at least one passages (62).

6. The tank (12) of any preceding claim, wherein the barrier (38, 138, 238) extends in a horizontal plane when the housing (14) is oriented upright, in use.

7. The tank (12) of any preceding claim, wherein the barrier (38, 138, 238) extends from a side wall (20) of the housing (14).

8. The tank (12) of any preceding claim, wherein the upper volume (40) is defined by a secondary housing (82) that is nested inside the housing (14), and wherein the barrier (38, 138, 238) is defined by a base (80) of the secondary housing (82).

9. The tank (12) of claim 8, wherein the housing (14) and the secondary housing (82) are formed from a continuous wall.

10. The tank (12) of any preceding claim, wherein the vent (36) is defined by an open upper end of the housing (14).

11. The tank (12) of any preceding claim, wherein the housing (14) is formed by a rotational moulding process.

12. The tank (12) of any preceding claim, comprising one or more buoyant elements (86) in the upper volume (40), wherein the one or more buoyant elements (86) have a combined cross-sectional area that is at least half of a cross-sectional area of the upper volume (40) so that, in use, the one or more buoyant elements (86) float in liquid held in the upper volume (40) to cover between 50% and 99%, and preferably between 90% and 99%, of the surface of the liquid.

13. The tank (12) of any preceding claim, wherein the at least one opening (28, 32) comprises: an inlet opening (28) through which liquid can be communicated into the lower volume (42); and an outlet opening (32) through which liquid can be communicated out of the lower volume (42).

14. A wind turbine system comprising the tank (12) of any preceding claim.

15. The system of claim 14 when dependent on claim 4, comprising a pump (34) that is connected to the duct (30) to draw liquid from the lower volume (42) of the housing (14).

16. The system of claim 14 when dependent on claim 4, comprising a return line connected to the duct (26) to deliver liquid to the lower volume (42). 16

17. The system of any of claims 14 to 16, embodied as a cooling system, wherein the tank (12) is configured to hold coolant in the upper and lower volumes (40, 42).

18. A method of fabricating a fluid tank (12) for a wind turbine (2), the method comprising: moulding a housing (14) to enclose an interior volume (16) for holding liquid, the housing (14) comprising a vent (36) positioned above a maximum liquid surface level in the interior volume (16), to permit air flow between the interior volume (16) and the surrounding environment, in use; dividing the interior volume (16) of the housing (14) into an upper volume (40) and a lower volume (42) using a barrier (38, 138, 238) that extends across the interior volume (16), beneath a minimum liquid surface level in the interior volume (16), wherein the barrier (38, 138, 238) comprises at least one passage (62, 68) to allow fluids to flow between the upper and lower volumes (40, 42); and providing at least one opening (28, 34) through which liquid can be communicated into or out of the lower volume (42).