GB2511272A

GB2511272A - A wind turbine

Info

Publication number: GB2511272A
Application number: GB1204415.2A
Authority: GB
Inventors: Torgeir Moan; Made Jaya Muliawan; Madjid Karimirad; Zhen Gao
Original assignee: Norges Teknisk Naturvitenskapelige Forskningsrad
Current assignee: Norges Teknisk Naturvitenskapelige Forskningsrad
Priority date: 2012-03-13
Filing date: 2012-03-13
Publication date: 2014-09-03
Also published as: GB201204415D0

Abstract

A floating wind turbine comprising a wind turbine rotor 1 mounted on a tower 3 of a spar 4 and tower 3 structure comprising a generally vertical buoyant and deep, ballasted spar 4 moored in a mooring 7 with the turbine rotor 1 connected to a first electrical power generator 11. The floating wind turbine comprising the following features, that the spar 4 and tower 3 structure provided with a buoyant torus 62 arranged at mean water level about the spar 4 and tower 3 structure ; that the torus 62 oscillates with sea waves in an axial direction with a motion relative to the spar 4 and tower 3 structure ,and that the torus 62 is provided with a power take-off system coupled to a second electrical power generator 121.Alternatively the first 11 and second 121 electrical generators may be combined into a single generator driven by both the turbine and the torus. The floating wind turbine will then be more efficient, particularly for the lower wind speed range.

Description

A Wind Turbine The present invention relates to a wind turbine.

Tn embodiments, the present invention relates to a wind turbine mounted on top of a tower supported on a floating spar buoy, particularly a ballasted spar buoy moored to the seafloor, wherein a wave energy converter is connected to the same spar buoy and tower structure at mean water level.

Wind turbines mounted on top of a tower supported on a floating spar buoy are known from W02010/122316 which describes a controller for a floating wind turbine.

The controller controls the rotor speed of the turbine by controlling the toique of a load on the rotor so as for the rotor speed to vary in response to wave-induced motion.

Wave energy generators are known from an ni-symmetric vertical floating structure comprising lower and upper coaxially and mutual vertically oscillating bodies of which the upper body is a torus and the lower body is a vertically oriented ballasted bob with almost neutral buoyancy, wherein the bodies have different heave frequencies.

The relative motion is utilized through a hydraulic power transmission for generating electrical energy.

Such a wave energy generator is known from Norwegian patent N0324789 to Wavebob Ltd. It describes a device for converting wave energy. It comprises at least two devices each comprising a surtlice floater, whereas at least one of the surface floaters is fixedly connected to a submerged body. The movement of the two devices as a response to a passing wave may be utilized to achieve an energy conversion.

According to a first aspect of the present invention, there is provided a floating wind turbine comprising a wind turbine rotor mounted on a tower of a spar and tower structure comprising a generally vertical buoyant and deep, ballasted spar moored in a mooring, said turbine rotor connected to a first electrical power generator, characterized in that said spar and tower structure provided with a buoyant torus arranged at mean water level about said spar and tower structure; said torus oscillating with sea waves in an axial direction \vith a motion relative to said spar and tower structure, said torus provided with a power take-off system coupled to a second electrical power generator.

Various embodiments of the invention are defined in the dependent claims.

Examples of embodiments of the present invention and its various embodiments, will now be described in detail with reference to the accompanying drawings, in which Fig. 1 is a simplified perspective view of a wind turbine mounted in a nacelle on top of a tower on a floating spar, with a torus of a wave energy extractor arranged floating in the splash zone of the floating spar and tower structure, the torus arranged to ride on waves in the generally vertical, longitudinal axis of the structure.

In the right lower part of Fig. 1 is illustrated a mooring system for a spar and tower structure used in a modeled embodiment of the invention.

Fig. 2 is a simplified elevation view of the same and shows a section view of the torus with a guidance and stopper mechanism for the torus.

Fig. 3 shows a prior art wave energy converter comprising a torus riding on a vertical part of a deep draught ballasted buoy.

Fig. 4a shows vertical and horizontal sections through the torus part ofthe system, and a detail horizontal section of a wheel between the spar and the torus.

Fig. 4b shows vertical and horizontal sections of the torus power take-off system in a hydraulic system embodiment, using several cylinders.

I

Fig. 4c is a very simplified vertical section of the hydraulic cylinders on either sides of the spar in an embodiment corresponding to the one ofFig. 4b.

Fig. 4d shows vertical and horizontal sections of an alternative embodiment of the S torus' power take-off system in another hydraulic system embodiment, using one cylinder.

Fig. 4e is a very simplified vertical section of the hydraulic cylinders on either sides of the spar in the alternative embodiment corresponding to the one of Fig. 4d.

Fig. Sa is a pitch motion spectrum (left) of a "spar only" and an embodiment of the invention modeled for two different situations: without (spar only, continuous line) and with the auxiliary buoyant structure (broken line), an attenuated torus wave energy converter modif3ing the pitch characteristics.

Fig. Sb is a wind power production for a wind velocity example of 8 mIs, which is below the rated velocity for the model embodiment wind turbine. The power production is modeled for the same two different situations: a floating wind turbine without (continuous line) and with (broken line) the torus according to the invention. The time span of Fig. Sb is 100 seconds (from 2000 to 2100 seconds). The dominating wave period Tp is clearly visible in the power production, but more important is that the power production is higher with the invention.

Fig. Sc shows the wind power production as in Fig. Sb, but for a time span from 0 to 3600 seconds; one hour.

Fig. 6 shows a hinged embodiment of a torus which may be retrofitted and / or removed from a spar and to\vcr structure at sea.

Fig. 7 shows, in the upper part: graphs of mean pitch (Es) and standard deviation (EsSTD) ) for a wind turbine generator on a "spar only", and as mean pitch ( c) and standard deviation ( cSTD) for a wind turbine on a spar with the invention, for abscissa values of mean wind velocity and wind power. In the lower part, Fig. 7 shows a graph of mean wind power production (Pc, Ps) and their STD (PcSTD, P5STD) as function of wind velocity. The following parameters for wind and waves according to a JONSWAP spectrum have been used.: Ce no. Vnea:n (inS) I-is (rn)Tp(s) ________ 8. 25 98 Ps P, _________ _____________ ________ Hi 3 14 15 19.2 4 17 4.2. 10.5 Advantages of embodiments of the invention A first advantage of some embodiments of the invention is the utilization of common power cables for the transmission of electrical energy from each combined floating wind turbine and wave energy converter. The length of power cables required for an array of spars according to embodiments ofthe invention is significantly reduced compared to an array of wave energy converters arranged inbetween the spars.

A second advantage of embodiments of the invention is the utilization of a common mooring system already existing for the floating wind turbine.

A third advantage of embodiments of the invention is the significantly reduced combined mass of the wave energy converter as it may ride on the already existing structure of the floating vertical spar and tower.

A fourth and very significant advantage of embodiments of the invention is that the energy output of a device of embodiments of the invention is modeled to be higher than the energy output from the modeled wind generator and from the wave energy converter as counted separately. In this aspect, the power output of the combined device is higher than the sum of the energy output of the components taken separately.

The energy output of the wind turbine part of the invention is higher with a working wave energy generator than without such a wave energy generator, please see Fig. Sb and Fig. 5c, but also Fig. 7. The eurvcs illustratc pitch motion for spar only and for the invcntion indicated as "combined". The wind power in combined mode is higher than wind power for "spar-only". The effect is most pronounced below a rated wind velocity of the wind turbine. This increased power production is a rather surprising effect of the invention.

Embodiments of the invention The invention is illustrated in the attached drawings and a general illustration of the invention is given in Fig. 1 which shows the buoyant torus arranged at mean water level about the vertically floating combined spar and tower which carries a "windmill", i.e. a wind turbine on top of the tower, the wind turbine for generating electrical energy.

In an embodiment, the invention illustrated is a floating wind turbine comprising a wind turbine rotor (1) on a tower (3) of a combined spar and tower structure (4, 3). The spar and tower structure comprises a generaHy vertical buoyant and deep, ballasted spar (4) kept on station by a mooring (7). Due to waves and wind the spar and tower will have mean tilt and dynamic pitch motion, which has to be closely monitored and controllcd.

The turbine rotor (I) is connected to a first electrical power generator (I I). Tn an embodiment the rotor maybe connected through a mechanical gearbox to the axle of the first electrical powcr generator (11) which is then arranged in a nacelle at the top of thc tower, but other solutions such as hydraulic transmission to the generator may be used, and then the electrical power generator (II) may be arranged anywhere in the tower or spar, below the nacelle.

The novel fcaturcs of cmbodiments of the invention comprises that the spar and tower structure (4, 3)is provided with a buoyant torus (62) arranged at mean water level about the spar and tower structure (4, 3), please see Figs. 1 and 2.

The torus (62) is arranged for oscillating with sea waves in an axial direction, i.e. in the nearly vertical direction, in a motion relative to the motion ofthe spar and tower structure (4, 3). The torus (62) is providcd with a power takc-off system (120) which converts part of the energy of the relative motion and is coupled to a second electrical power generator (121).

In an embodiment of the invention the power output is transmitted via common a conmion power cable for the transmission of electrical energy from the combined floating wind turbine and wave energy converter of the invention. The common power cable is connected to a power grid.

The turbine rotor (1) is mountcd on an axle supported in axlc bearings (131) in a nacelle (13) on said tower (3). In an embodiment of the invention the nacelle (13) is arranged on a rotating bearing (132) with an axial rotation on said tower (3), which is necessary if the tower and spar structure is not allowed to turn azimuthally in its moorings.

As will be seen from Figs. I and 2, there is no sharp transition from the top of the spar to the bottom of the tower; it may be smooth or slightly conic. A tower base (31) of said tower (3) and a spar top portion (41) of said spar structure (4) form a generally smooth transition near said splash zone between said tower (3) and said spar (4). Please notice that T have used the term "combined spar and tower structure (4, 3)" in order to describe an axially aligncd spar buoy cylinder supporting a tower.

In the modelling a sea water density (1025 kg/m3) is used. We have considered the spar (4) as the part of the spar and tower structure below the design water level, the "watcrlinc', plcasc scc Fig. 2.

The embodiment of the combined spar and tower with the torus according to the invention is modelled as a two-body system. The modelled two-body system is modelled using features available in a computer program called STMO developed by Marintek for simulating motions and station-keeping behaviour of complex systems of floating vessels and suspended loads. SIMOs cssential feature for the prcscnt purpose is its flexible modelling of multi-body systems that accommodate the introduction of both mechanical and hydrodynamical coupling between the spar and tower structure and the torus.

the power take-off system is the coupling between the two main components of the system and is modelled in a simplificd manner as an ideal linear damper (Bpto) and spring (Kpto) that couple the motion between the two bodies. The parametei have been set to Bpto = 8000 kN s/m and Kpto = 50 kM /m, please see the table below.

The total displaced volume of the floating wind turbine with the spar and tower structure is 8015 nf, with a total mass of 8216 tons (including tower, turbine, ete), disregarding the floating torus (62).

In the dynamically modeled embodiment of the invention, the following parameters have been used as an example embodiment: Property Value Unit Torus (62) Outer diameter 20 m Inner diameter 10 in Draught 2 m Height 8 m Displacement 408 Mass 418 t(I000kg) Centre of mass below sea level 0.9 in Moment of inertia I,,,. about a horizontal axis 10760 tin2 Moment of inertia 1, about a vertical axis 20560 tnY Vertical stroke length of torus (62) 6 in Stiffness of upper end stop spring 1 0 kM/rn Stiffness of lower end stop spring 10° Spar and tower structure: height of Tower (3) above waterline 90 m diameter at waterline of Spar (4) 6.5 in diameter at boftom of spar (4) 9.4 m Draught of spar (4) below waterline I 20 m (combined draught and height I 20m+90m=2 I Om) Displacement f5par (4) 8016 ni' Total mass of spar (4) 8216 t(l 000 kg) Centre of mass, below waterline 78.5 m Moment of inertia about a horizontal (x) axis of spar and 69840000 t tower (4,3) Moment of inertia i, about the long (z) axis of spar and tower 167800 t in' Fairlead (mooring attachment) elevation below waterline 70 in Other parameters Wind turbine power output 5 MW Power take-off damper property 8000 kN s/rn Powertake-off stiffness 50 kN/m Water depth on site 320 m As is seen from the table above, the torus (62) contributes insignificantly with an additional 10760 t m2 to the moment of inertia 69840000 t m2 ofthe combined spar and tower structure. However we shall see that the inventionsignificantly affects the dynamic behaviour of the spar and tower structure so as to provide more produced power to a degree not expected.

For the mooring illustrated in Fig. 1, lower part, the following parameters have been used: I0 The properties of mooring system components Property I Upper Lower Clump End Line Line mass Line ___________ UL) (LL) ______ (EL) Length (m) 300 600 2 370 Diameter(m) 009 0.09 1.67 0.09 Mass/Length 42.5 42.5 17253 42.5 (kgm) _______ _______ _______ ______ Axial stifthess 384243 384243 384243 384243 Other dimensions of the spar, the tower, the maximum wind turbine power output, the tons, or the power take-off system and the mooring are envisaged, and the given parameters by no way shall limit the scope of the invention, as the inventors believe that the effect of the invention is not only present for the values of the parameters used in the modeling.

The modeling using the parameters of the example embodiment wihout a torus, indicated as spar only' and for the invention indicated as "combined concept' may be summarized with the following power outputs as follows: ______ ___________ _____ ____ Power_(kW) _____________________________ Spar only Combined concept wave wind power1 power2 wind power wave power Case Hs Tp no. Vmean(nVs) (m) (s) Mean STD Mean STD Mean STD Mean STD 1 8 2.5 9.8 1727.1 244.35 --1820.07 233.2 275.4 379.9 2 11.2 3 10 4573.044 327.95 --4874.972 296.46 420.9 576.2 3 14 3.6 10.2 4937.625 12.39 --4947.001 13.13 622.62 852.79 4 17 42 10.5 4929.498 8.167 --4939.5 8.713 883.45 1211 According to the modeling, the energy output of a device according to the invention is modeled to be higher than the energy output from the modeled wind IS generator and from the wave energy converter as counted separately. Details will be discussed below.

Please notice that the term "wave power" in the table above should be understood as "absorbed power" for the power output from the torus' wave power take-off system: is this the power absorbed by the dampened spring system and not necessarily the power output from the generator due to loss in the system, and should be multiplied throughout S by some efficiency factor less than I. As an example well below a rated wind velocity for the modeled wind turbine, in which the turbine control system does not cut the power output, please see "case no. 1 above: For a mean wind speed of 8 mIs, and a wave state Hs of 2.5 m and a wave period of 9.8 s, the mean wind power output of the wind turbine driven generator alone, without the invention, is 1727 kW and its standard deviation is 244.35 kW, please see particularly Fig. Sb which shows the power production of 100 seconds, but also Fig. Se which spans a time period of 1 hour. For the invention for the same wind and wave conditions, the wind power generator mean wind power output is 1820.07 kW, please also refer to Figs. Sb and Sc, and with a lower standard deviation: 233.2 kW, which is both more power and more stable power output than without a torus. In addition the potential wave-generated power output from the torus is calculated to be 275.4kW with a standard deviation of 379.9 kW, please refer to the table above.

For higher wind velocities the wind turbine control system will prevent an over rate rotor speed or over rate energy production (Pc, Ps) by controlling the pitch and / or the gear ratio, please see Fig. 7, lower graphs.

As an example near below a rated wind velocity for a wind turbine, near the wind velocity for which above the turbine control system starts cutting the power output, please see "case no. 2 above: For a mean wind speed of 11.2 mIs, and a wave state Hs of 3 m and a wave period of 10 s, the mean wind power output of the wind turbine driven generator alone, without the invention, is 4573 kW and its standard deviation is 328 kW.

For the invention for the same wind and wave conditions, the wind power generator mean wind power output is 4875 kW with a lower standard deviation: 296.5 kW, which is both more power and more stable power output than without a torus. In addition the potential wave-generated power output from the torus is calculated to be 420.9 kW with a standard deviation of 576.2 kW.

The mean energy output of the wind turbine part of the invention is thus higher S while using a working wave energy generator than without such a wave energy generator, please also see particularly Fig. 5b and Fig. 5c showing modeled wind turbine power production for a wind velocity of 8 mis which is significantly below the rated velocity for the wind turbine.

The wind power in combined mode is higher than wind power for "spar-only".

This is a rather surprising effect of the invention.

Moreover, we see that the standard deviation for cases No. 1 and 2, wherein the standard deviation of the produced wind power is reduced from 244.35 and 327.95 to 233.2 and 296.46 respectively, this means that the power production stability from the wind generator is improved for lower wind speeds. The power production stability for the cases no. 3 and 4 for higher wind speeds is very high both without and with the invention due to the effect of the control system, with negligible RMS values of 12.39 kW and 8,167kw without a torus, and 13.13kw and 8.713 kW with a torus. Anyway, the increased wind power production stability for lower wind speeds is another advantage of the inention.

The tension force on the mooring lines has been calculated and shows an insignificant increase for all sea states.

In an embodiment of the invention the torus (62) is provided with guide wheels (64) for running along said spar and tower structure (4, 3) in its axial, generally vertical direction. The dimension and the number of the guide wheels (64) will depend on the interface force between said spar and tower structure (4,3) and torus (62).

Tn an embodiment of the invention, the power take-off system (120) comprises a hydraulic power take-off system (63) fbr converting the relative motion into hydraulic energy for running the second electrical powcr generator (121), please see below.

Fig. 4b shows vertical and horizontal sections of the tows' power take-off system in a hydraulic system embodiment, using several cylinders, and in a fUrther embodiment sharing support structure with the end stop structures extending out radially from the lower part of the tower.

Fig. 4c is a very simplified vertical section of the hydraulic cylinders on either sides of the spar in an embodiment corresponding to the one of Fig. 4b, wherein each hydraulic cylinder is connected via a control manifold to a closed hydraulic circuit with a hydraulic motor, preferably with displacement control, driving an electrical generator.

Fig. 4d shows vertical and horizontal sections of an alternative embodiment of the torus' power take-off system in another hydraulic system embodiment, using one cylinder, with the one cylinder having a separate support structure extending radially out from the lower part of the tower, independent ofthe end stop structures.

Fig. 4e is a very simplified vertical section of the hydraulic cylinders on either sides of the spar in the alternative embodiment corresponding to the one of Fig. id, wherein the single hydraulic cylinder is connected via a control manifold to a closed hydraulic circuit with a hydraulic motor, preferably with displacement control, driving an electrical generator.

According to an alternative embodiment of the power take-off system (120) it may comprise an electromagnetic inductive system (123) utilizing part of the relative motion between said auxiliary buoyant structure (61) and said spar and tower structure (4, 3). The electromagnetic inductive system may comprise a static inducing coil (122) in the spar, and a reactive moving coil (123) in the tows, so as fUr generating electric energy in the static coil which may thus constitute a part of the second electric generator (121) from which energy may be exported to the grid via the power cable otherwise used for transporting away the energy from the wind turbine.

In an embodiment of the invention the floating wind turbine's power take-off system (120) comprises said guide wheels (64) coupled directly or indirectly to said second electrical power generator (121).

In an embodiment of the invention, heave motion characteristics of the buoyant structure (61) are tuned relative to heave motion characteristics of the tower and spar structure (3, 4) so as for improving the energy output of said second electrical power generator (12).

The torus (62) may comprise a ballast tank (611). The ballast tank (611) may be constituted as the torus (62) as such, and provided with a valve and pump system (612) arranged for ad Lusting its ballast by pumping sea water in or outThe ballasting may be part of the tuning. Other parameters for tuning the torus' relative motion with regard to the spar and tower structure comprise ad usting the power takwff systems force characteristics and the upper and lower end springs' characteristics.

In an embodiment of the invention the first and the second electrical power generators (I I, 12) are combined into one generator. This is particularly useful if using a hydraulic transmission to the generator and the generator is arranged in the tower and spar structure. Such a combination may thus further increase the power output of the combined device according to the invention without significantly increasing the weight and cost of the generator system.

A broader aspect of the thventiim As seen in a broader perspective aspect the buoyant structure (61) can be considered as a pitch attenuator (12) which modifies the dynamic motion of the spar; at least the pitch dynamic motion of the spar and tower structure (4, 3). Thus the invention maybe defined as a floating wind turbine with a turbine rotor (I) in a tower (3) of a combined spar and tower structure (4, 3) comprising generally vertical buoyant and deep, ballasted spar (4) moored in a mooring (7), said turbine rotor (1) connected to a first electrical power generator (II), wherein said spar and tower structure (4, 3) is provided with an auxiliary buoyant structure (61) arranged in the splash zone of said spar and tower structure (4, 3); the said buoyant structure (61) oscillating with sea waves in a relative motion in an axial direction of said spar and tower structure (4,3), said buoyant structure (61) provided with a an attenuator (12) modifying at least a pitch dynamic motion of said spar and tower structure (4, 3). In this very broad, general definition of the invention, the buoyant structure actually is not necessarily yet used to generate electrical energy; the wind turbine alone produces more energy even without regard to any power output of the wave energy converter part (6) of the buoyant structure (61), and is an advantage in itself lease see Fig. 5 and Fig. 7 which shows that the energy output from the wind generator alone is higher with an operating torus than without. Please notice that the attenuator' (12) in this definition is not merely a mechanical damper, but which converts the otherwise pitch energy through the power take-off system to electrical energy. The aftenuator (12) in this context is provided with the wave energy converter (6) comprising the power take-off system (120) coupled to the second electrical power generator (121).

Embodiments ofthe present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the prnscnt invention.

Claims

Claims: A floating wind turbine comprising -a wind turbine rotor mounted on a tower of a spar and tower structure comprising a generally vertical buoyant and deep, ballasted spar moored in a mooring, -said turbine rotor connected to a first electrical power generator, characterized in that * said spar and tower structure provided with a buoyant torus arranged at mean water level about said spar and tower structure; * said torus oscillating with sea waves in an axial direction with a motion relative to said spar and tower structure, * said torus provided with a power take-off system coupled to a second electrical power generator.
2. The floating wind turbine of claim 1, said torus provided with guide wheels for running along said spar and tower structure in its axial generally vertical direction.
3. The floating wind turbine of claims 1 or 2, said power take-off system comprising a hydraulic power take-off system for converting said relative motion into hydraulic energy for running said second electrical power generator.
4. The floating wind turbine of any of the preceding claims, said power take-off system comprising an electromagnetic inductive system utilizing the relative motion between said auxiliary buoyant structure and said spar and tower structure.
5. The floating wind turbine of claim 2, said power take-off system comprising said guide wheels coupled directly or indirectly to said second electrical power generator.
6. The floating wind turbine of any of the preceding claims, wherein heave motion characteristics of said buoyant structure being tuned relative to heave motion characteristics of said tower and spar structure so as for improving the energy output of said second electrical power generator.
7. The floating wind turbine of any of the preceding claims, said torus comprising a ballast tank.
8. The floating wind turbine of claim 7, said ballast tank constituted as said torus as such, and provided with a valve and pump system arranged for adjusting its ballast by pumping sea water in or out
9. The floating wind turbine of any of the preceding claims, a tower base of said tower and a spar top portion of said spar structure fbnning a generally smooth transition near said mean water level between said tower and said spar.
10. The floating wind turbine of any of the preceding claims, wherein said turbine rotor is mounted on an axle supported in axle bearings in a nacelle on said tower.
11. The floating wind turbine of claim 10, wherein said said nacelle is arranged on a rotating bearing with an axial rotation on said towert
12. The floating wind turbine of claim any of the preceding claims, wherein said first and said second electrical power generators being combined into one generator driven by said wind turbine and said buoyant torus.
13. A floating wind turbine substantially as shown in and/or described with reference to any of Figures 1 to 7 of the accompanying drawings.