WO2013053575A2

WO2013053575A2 - Arrangement and method for predicting a flow separation at a buoyancy profile of a wave power plant, and method for operating a wave power plant

Info

Publication number: WO2013053575A2
Application number: PCT/EP2012/068483
Authority: WO
Inventors: Nik Scharmann; Benjamin Hagemann; Jasper Behrendt
Original assignee: Robert Bosch Gmbh
Priority date: 2011-10-12
Filing date: 2012-09-20
Publication date: 2013-04-18
Also published as: DE102011115806A1; WO2013053575A3

Abstract

The invention proposes an arrangement for predicting a flow separation at a buoyancy profile (1) of a wave power plant, against which water (2) flows, in which at least two pressure sensors (3) which are arranged behind one another in a profile chord direction on a top side of the buoyancy profile (1) are provided and can be used to determine at least one first pressure and one second pressure exerted on the buoyancy profile (1) by the water (2), and in which evaluation means are provided and are set up to determine a flow state from the at least first and second pressures.

Description

Arrangement and method for predicting a stall on a buoyancy profile of a wave power plant and method of operation for

Wave power station

The present invention relates to an arrangement and a method for predicting a stall on a buoyancy profile of a wave power plant as well as an operating method for a wave power plant.

State of the art

Wave power plants use the energy of ocean waves to generate electricity. Newer design approaches use rotating units that convert the wave motion into a torque. Amongst others, buoyancy profiles are used as coupling bodies, by means of which a buoyant moment is generated from the flowing wave, which can be converted into a rotational movement of a rotor. Corresponding lift rotor are arranged for example on a crank mechanism. Due to a superimposed flow from the orbital flow of the wave motion and the self-rotation of the lift profiles on the crank drive, buoyancy forces on the lift rotors result, whereby a torque is introduced into the crank drive.

The buoyancy of a buoyancy profile can be changed by its angle of attack to the inflowing medium, for example air or water. As explained below in greater detail with respect to FIG. 2, the buoyancy initially increases with increasing angle of attack until the stall occurs and the buoyancy force collapses. Maximum torque is therefore achieved with an angle of attack which is already close to the angle of attack which causes a stall. Reliable control is indispensable, in particular, for such rotating wave power plants with buoyant rotors, since desynchronization of the rotor with the incoming water can lead to complete decoupling from the shaft as a result of a stall. A required (re-) synchronization is complex, since the formation or adjustment of a corresponding buoyancy requires a certain amount of time. As a result, an independent start in the unsteady (non-stationary) flow field is made more difficult. There is a risk that the buoyancy that is developing is not yet sufficient to synchronize the system, ie to start it up, but due to the change of the flow conditions it already comes to a stall again. In a desynchronized system also force combinations can act, which can lead to significantly higher loads on system components and a swinging up of the entire system.

In particular, because of the multichromatic wave states of ocean waves, it is necessary to control and / or regulate a corresponding system in such a way that the buoyancy profile is always optimally flown and operated as close as possible to its conversion optimum. As a result, a maximum energy yield can be achieved. In this case, manipulated variables are, in particular, the generator torque and the adjustment of the angle of attack of the buoyant bodies. This results in corresponding flow angle of the buoyancy bodies and a phase angle between turbine rotation and shaft movement. In the present invention, the system should preferably be operated largely synchronously with the wave motion.

A particular difficulty arises from the fact that the respective flow angles and velocities over the longitudinal extent of a buoyancy profile (ie substantially transversely to the flow direction) are not uniform in real wave situations, but the shape of a buoyancy profile does not change in reverse along its longitudinal extent as a rule. The resulting flow conditions therefore differ fundamentally from those in wind power plants, as explained in greater detail below. It is proposed in US Pat. No. 7,686,583 B2 to determine an angle of attack of a fluid flowing on a buoyancy profile on the basis of a measured buoyancy in connection with the flow velocity. For this purpose, different measuring means, including pressure sensors, are provided. On this basis, a control can then be carried out, which includes, for example, an adjustment or torsion of the lift profiles for adaptation to an oblique incident flow or a corresponding reorientation of the overall system. However, local differences in a buoyancy profile can not be recognized by the method disclosed therein. Furthermore, the method does not permit statements about an impending or already occurring stall.

There is therefore still a need for improved possibilities for operating a wave power plant, in particular for predicting a stall.

Disclosure of the invention

Against this background, the present application proposes an arrangement and a method for predicting a stall on a lift profile of a wave power plant and an operating method for a wave power plant with the features of the independent claims. Preferred embodiments are subject of the dependent claims and the following description.

Advantages of the invention

The invention proposes an arrangement with at least two pressure sensors arranged successively on an upper side of a lift profile in a chordwise direction. The respective buoyancy behavior can be determined by suitable evaluation means (for example a computer or computer) and an emerging stall can be predicted. In this case, statements without knowledge of the respective flow velocity can be made in comparison with the prior art, so that corresponding (additional) sensors can be dispensed with. In particular, pressure sensors arranged at or near the front and rear edges of the lift profile are advantageous here.

A necessary criterion for a stall is a positive pressure gradient along the chordwise direction. The pressure increases in the direction of the rear profile edge, which corresponds to an at least partial reversal of the flow direction at the rear profile edge due to a turbulent flow. With the help of the at least two pressure sensors, the development of such a pressure distribution can be reliably sensed.

A corresponding arrangement also advantageously has at least two sets of pressure sensors arranged side by side in a profile longitudinal direction, each set comprising at least two pressure sensors arranged in the chordwise direction. This makes possible statements about a local distribution of the inflow patterns in the profile longitudinal direction. Overall, therefore, a network of pressure measuring points can be created on an upper side of a corresponding lift profile, which enables a local resolution with regard to the respectively prevailing wave states and flow conditions. Using a suitable control, it is therefore also possible to react locally to the respective conditions.

Such a reaction to the local conditions can be carried out in particular when using axially segmented lift profiles. Such buoyancy profiles comprise individual profiles arranged next to one another, each of which can be adjusted independently of one another. The respective buoyancy can thus be adjusted individually. A corresponding adaptation is also possible through the use of so-called flaps, ie lift flaps, which are subdivided over the length of a corresponding profile and thus can be set locally differently.

The corresponding pressure values are advantageously detected as pressure gradients or converted into pressure gradients. From this can be, for example, the Determine the position of a turnaround or a trailing edge between laminar and turbulent flow on an upper surface of the lift profile, as explained in more detail below in connection with Figure 2. Corresponding wave power plants are typically operated in a Reynolds number range of 10 ⁶ to 10 ⁷ . The transition point or the separation edge between laminar and turbulent flow is therefore in the front region of a corresponding lift profile, so that the probability of occurrence of a so-called laminar flow stall is low. A laminar stall is present when a flowing medium separates laminar from the buoyancy profile. If, on the other hand, the transition point or the trailing edge, for example on aircraft wings, is closer to the trailing edge of the profile, so-called "laminar separation bubbles" may occur, whereby the flow first breaks off from the profile, but later (ie in the direction of the profile trailing edge) again (turbulently ) Get in touch with this.

Due to the conditions mentioned in wave power plants, therefore, a reliable determination of an (imminent) stall can be made via the determination of the position of the transfer point or the spoiler edge.

By means of the proposed pressure measurement, it is also possible to detect the longitudinal alignment of a profile with respect to a wave motion and, if appropriate, to react thereto. Another measure which may be advantageous in this context is the measurement of the surface friction on a buoyancy profile (so-called skin friction). At the transfer point, as mentioned, an envelope of the flow velocity takes the form of a reverse flow (reverse flow). At this point, therefore, the flow velocity gradient reaches zero and the surface friction disappears. By measuring the change in the surface friction in the profile depth direction, it is therefore possible to estimate the position of the transition point and to influence it by a regulation. For measuring the surface friction For example, appropriately trained shear force sensors or so-called bending trabeculae can be used.

The measures proposed according to the invention preferably also include a control strategy that regulates a machine behavior corresponding to the present flow field. In the event that an announcing stall is detected via the sensors, appropriate countermeasures can be initiated. These include an adaptation of the generator torque and / or a change in the angle of attack of a lift profile. Further possible countermeasures include a change in the lever arm and / or a change in the blade geometry, for example by adjusting corresponding flaps. In addition, the generator can also be operated as a drive to prevent stalling or to resynchronize a desynchronized rotor with the oncoming wave or to prevent desynchronization in the first place. Other drive mechanisms, such as assistive motors, are also usable.

As already mentioned, the flow conditions at a wave power plant are significantly different to the (aerodynamic) conditions in a wind power plant. Rotor blades of wind turbines generally have a profile that changes in the longitudinal direction of the wing (so-called skewed wings), since the flow conditions along a wing change greatly due to the large radial extent of the wings. A wing is usually optimized for a high-speed number (ie a ratio of wind speed to rotor peripheral speed), which can not be achieved at high wind speeds. The wing is thus limited due to its structural conditions with respect to the maximum speed. In rotors of wind turbines, therefore, there is the potential risk of stalling at high wind speeds, as the angle of attack at the blade root (on the hub side of a rotor blade) is very large. In wind turbines, a measurement of pressures at most on a blade tip in the longitudinal direction, to determine the formation of a so-called wake turbulence at the blade tip and optimize. Unlike the wind turbine are in the wave energy conversion system, the buoyancy body preferably oriented substantially parallel to the rotor axis. As a result, with uniform wave characteristic, a uniform energization over the entire blade strain occurs. If the axis of the wave power plant is not oriented substantially perpendicular to the wave propagation direction or the current is not uniform for other reasons, then different angles of incidence arise along the wing extension, which can lead to a different detachment behavior. With several rows of sensors, such a misalignment of the machine can be detected and suitable countermeasures for adjusting the alignment can be initiated.

Ideally, in order to achieve maximum energy yield in a wave power plant, a large part of the energy of a corresponding wave should be absorbed via at least one buoyancy profile. On the basis of the proposed measures, in particular it can be prevented that a corresponding regulation of a wave power station requires unattainably high lift values for absorption of energy from the sea wave, which would lead to a stall. In order to ensure a synchronicity, for example, the generator torque could instead be reduced for such an operating case.

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation, without departing from the scope of the present invention.

The invention is illustrated schematically with reference to an embodiment in the drawing and will be described in detail below with reference to the drawing. figure description

FIG. 1 shows a schematic representation of pressure distribution patterns on a buoyancy profile impinged by a medium.

FIG. 2 is a schematic representation of a developing stall on a buoyancy profile surrounded by a medium.

Figure 3 shows arrangements according to particularly preferred embodiments of the invention in a schematic representation.

FIG. 1 shows, as part of a wave power plant (not shown), a buoyancy profile 1 which is impinged in the direction of the arrow (flow direction H) by a flowing medium 2, in the case of a wave power station water. The medium strikes the buoyancy profile 1 at a front edge 11 of the buoyancy profile 1. The point of impact is also referred to as stagnation point and is explained in more detail in connection with FIG. The medium 2 flows around the buoyancy profile 1, whereby due to the pressure forces building up on the underside of the buoyancy profile and a suction that builds up on the upper side of the buoyancy profile 1, a buoyancy L is produced. In a preferred wave power plant, at least one buoyancy profile 1 is arranged on a crank drive, which is arranged by a caused by the lift L moment in rotation (rotation axis largely horizontal or largely vertical and transverse to the wave propagation direction) is added. The illustration of Figure 1 corresponds to a steady flow without stall. Each of the total buoyancy L resulting pressure contributions / Saugbeiträge I are symbolized in the form of force vectors. The buoyancy profile 1 is inclined with its chord C by an angle α against a flow direction H. A trailing edge of the lift profile 1 is designated by 12. The resulting flow on the profile results from a superposition of the orbital flow of the water particles in a wave motion and the proper motion of the buoyancy profile. FIG. 2 shows in the views 201 to 204 buoyancy profiles at different angles of attack α of a buoyancy profile 1. As in FIG. 1, a medium 2 flows around the buoyancy profile 1. The direction of flow is indicated by an arrow 2 '. At a stagnation point S, the medium 2 is accumulated. With a sufficiently low setting angle α, the medium 2 flows around the buoyancy profile 1 on the top and bottom sides completely laminar. As mentioned, due to the Reynolds numbers present on a buoyancy profile of a wave power plant, specific flow conditions result.

In view 201 results due to the flat angle of attack α almost completely laminar flow. Only at the trailing edge of the buoyancy profile may there be slight turbulence. The point at the top of a lift profile where the flow transitions from a laminar to a turbulent state is also referred to as a transition point U.

As the buoyancy profile 1 increases, as illustrated in views 202 through 204, the point of transition U increasingly shifts from a trailing edge of the buoyancy profile 1 to a leading edge. To a certain extent, a steeper adjustment of the lift profile 1 increases the total lift L despite a further forward shifting point U and thus a reduced proportion of a laminar flow on the upper side of the lift profile 1. From a certain angle of attack a, as illustrated in view 204, a so-called stall occurs, as a result of which the lift L abruptly decreases sharply. For a person skilled in the art is obvious that in addition to a change in the employment of a corresponding buoyancy profile 1 and the change of flow velocities of a medium 2 relative to a speed of a buoyancy profile 1 or a moment, which is an incoming medium 2 counteracted by a buoyancy profile, significant influences on the Position of a Umschlagpunkt U and / or the time of a stall may have. At lower flow rates, an even steeper Adjustment angle α of the lift profile 1 are tolerated, whereas at low flow velocities a flatter profile position is required.

As explained, multichromatic wave states are frequently to be observed in the actual operation of wave power plants which, in addition to a temporal change, can also change over the longitudinal extent of a buoyancy profile, ie perpendicular to the plane of the paper of FIG. In addition to different flow velocities, different effective angles of attack α can also be observed here. Along the longitudinal extent of a buoyancy profile 1, therefore, the turbulence situation and thus the position of the transition points U are not uniform in a wave power plant. Thus, at some longitudinal position of the lift profile 1, a turbulent flow may occur, while at other locations the flow of the medium 2 remains laminar, which to a certain extent may even be acceptable. In the subfigures 3a and 3b of Figure 3 respectively buoyancy profiles 1 according to particularly preferred embodiments of the invention are shown schematically in partial perspective view. A medium 2 flows from a direction of flow 2 'the profile and meets at a front edge 1 1 on this. By means of arrow 2, the profile 1 flowing around the medium is illustrated. Although arrow 2 symbolizes a laminar flowing medium, it will be understood that the invention is particularly suitable for detecting turbulent flows. On the upper side of the lift profile 1, two rows 31, 32 of pressure sensors 3 are arranged in FIG. By means of the pressure sensors 3, a pressure gradient can be determined along the chordwise direction, that is to say along a direction which corresponds to a chord of the lift profile 1. If, for example, a first pressure gradient is determined in a first angle of attack α between a pressure sensor 3 arranged on the front edge side and a pressure sensor 3 arranged on the rear edge side, and this changes significantly or exceeds a threshold value at a second angle of attack a, for example at a steeper angle of attack, (in particular in FIG a suitable evaluation means, such as a computer unit 4 or a computer) be concluded that possibly the Umschlagspunkt U has moved accordingly and an impending stall is to be feared. In addition, additional sensors in the middle region of the lift profile can be used to achieve an improved measurement resolution.

If, as shown in FIG. 3a, a plurality of rows 31, 32 are provided on pressure sensors 3 along a longitudinal extent of the lift profile 1, consideration can also be given to local conditions, as may be mentioned, in real shaft situations.

In one embodiment, in each case at least two pressure sensors are arranged in chordwise direction at several points of the lift profile 1 in order to determine the pressure distribution and thus the flow behavior locally resolved. This is particularly advantageous in the case of locally adjustable buoyancy profiles, since an optimum flow without stall can thus be ensured for each buoyancy profile section.

Overall, therefore, three-dimensional pressure gradients can thus be detected, and therefore improved statements regarding a stall situation can be made.

3b illustrates a buoyancy profile 1 in which two sets 3 ', 3 "are provided by pressure sensors 3 and arranged one after the other in a chordwise direction on the buoyancy profile Corresponding pressure sensor sets can, for example, be arranged at locations on a profile top at which a point of transition U is expected.The pressure sensor sets 3 ', 3 "can also be arranged in several rows on the buoyancy profile 1, so that Here, too, can achieve a three-dimensional stall detection.

Although, as illustrated in FIGS. 3 a and 3 b, pressure sensors can be arranged over the entire width (ie in the chordwise direction) of a lift profile, a particular advantage results if corresponding sensors are arranged in the rear area (essentially between the center line and the rear edge) of a lift Profiles 1 are arranged, since here, as mentioned, the probability of a stall is highest.

Is in the context of this application of a "chord direction" the speech, hereunder the direction between the so-called profile nose, with which it is usually oriented in the direction of the inflowing medium, understood in the direction of the profile trailing edge. When sensors are shown arranged "one behind the other" along the chordwise direction, this does not necessarily require a strictly linear arrangement along the chordal direction. Of this formulation, all arrangements are moreover included in which at least one of the corresponding

Sensors closer to a profile leading edge and at least one further sensor is arranged closer to the profile trailing edge. The "profile longitudinal direction" designates the direction of the greatest longitudinal extent and is usually parallel to the leading and trailing edge and perpendicular to the chord direction. The "centerline" runs in the middle between the leading and trailing edges.

Claims

claims

1 . Arrangement for predicting a stall on a buoyancy profile (1) of a wave power plant which is streamed by water (2), characterized by at least two pressure sensors (3) arranged successively in a chordwise direction on an upper side of the buoyancy profile (1), by means of which at least one first and at least one second pressure exerted by the water (2) on the buoyancy profile (1) can be determined, and evaluation means (4) which are set up to determine a flow state from a difference between the at least one first and the at least one second pressure.

2. Arrangement according to claim 1, which has at least two in a profile longitudinal direction juxtaposed rows (31, 32) of pressure sensors each having at least two in the chordwise direction successively arranged pressure sensors (3).

3. Arrangement according to claim 1 or 2, wherein at least one pressure sensor (3) in the vicinity of a front edge (1 1) of the lift profile (1) is arranged and / or at least one pressure sensor (3) in the vicinity of a trailing edge ( 12) of the buoyancy profile (1) is arranged and / or at least two pressure sensors (3) in the vicinity of the trailing edge (12) of the buoyancy profile (1) are arranged.

4. Arrangement according to one of the preceding claims, wherein the evaluation means (4) are arranged for determining at least one pressure gradient from the at least one first and the at least one second pressure.

5. Arrangement according to one of the preceding claims, comprising means for measuring a surface friction on a surface of the buoyancy profile (1).

6. A method for predicting a stall on one of water (2) flowed up buoyancy profile (1) of a wave power plant comprising, at least two pressures exerted by the water (2) on an upper side of the lift profile (1), at different points along a chord direction, in particular by means of at least two pressure sensors (3) of an arrangement according to any one of the preceding claims, and on the basis of at least two pressures, in particular by means of the evaluation means of an arrangement according to one of the preceding claims, to determine an existing or forthcoming flow outline.

7. The method of claim 6, including determining at least one pressure gradient between the at least two pressures, comparing the at least one pressure gradient with a predetermined pressure gradient threshold, and determining that a stall is imminent if the at least one pressure gradient exceeds the pressure gradient threshold.

8. The method according to claim 6 or 7, comprising determining the position of at least one turnover point (U) on an upper side of the lift profile (1) and / or an alignment of the lift profile (1) with respect to a wave motion.

9. The method according to claim 8, which comprises in each case to determine pressure differences between pressure sensors (3) arranged in at least one row on the upper side of the lift profile in the chordwise direction, wherein the position of the turnup point (U) is determined to lie between the pressure sensors (3) between which the greatest pressure difference is determined.

10. The method of claim 8 or 9, wherein the surface friction of the buoyancy profile is measured and from the transition point is determined.

1 1. A method of operating a wave power plant comprising predicting a stall by a method according to any one of claims 6 to 10 and wherein, when a stall is predicted, an angle of attack (a) of at least one lift profile (1) and / or generator torque of the shaft - changed power plant and / or the wave power plant by means of a generator at least temporarily operated by a motor.