WO2010102743A2

WO2010102743A2 - Tidal stream energy device alignment control

Info

Publication number: WO2010102743A2
Application number: PCT/EP2010/001297
Authority: WO
Inventors: Aaron Jack Stevens; David Sydney Knott
Original assignee: Rolls-Royce Plc
Priority date: 2009-03-10
Filing date: 2010-03-03
Publication date: 2010-09-16
Also published as: KR20160017136A; GB2480399B; WO2010102743A3; GB201113263D0; KR101772347B1; CA2750782A1; GB2480399A; KR20110125638A; GB0904035D0

Abstract

A method of controlling the alignment of a tidal stream energy device is provided. The method includes the steps of: providing a reference profile for the variation of tidal current velocity with time at the tidal stream energy device; measuring a profile for the present variation of tidal current velocity with time at the tidal stream energy device; correlating the reference and measured profiles to identify a period of slack water; and changing the alignment of the tidal stream energy device during the identified period of slack water so that the device is appropriately aligned to generate energy from a following period of tidal flow.

Description

TIDAL STREAM ENERGY DEVICE ALIGNMENT CONTROL

The present invention relates to a method and a system for controlling the alignment of a tidal stream energy device. Tidal stream energy devices operate most effectively if they are aligned

(i.e. their energy capture elements, such as turbine blades are aligned) with the direction of the tidal flow, which typically changes direction by 180° on the turn of the tide.

One approach is to use bi-directional turbine blades which do not require realignment at the turn of the tide. However, the overall efficiency of such systems tends to be low.

Another approach is to adopt a passive "free-yawing" arrangement in which the drag force of the prevailing tidal current against a tethered, or otherwise movable, tidal stream energy device causes the device to move, or "yaw", on its tether into an appropriately aligned position. However, a problem with a free-yawing arrangement is that it restricts the range of suitable devices. The tethering arrangement can also interfere with power generation at the device.

An approach in which the alignment of a device is actively controlled does not have these disadvantages. However, it is necessary then to determine the appropriate times at which to realign the device. In particular, realignment is preferably accomplished at times of "slack water" between high and low tides when forces on the energy capture element(s) of the device and on the alignment mechanism are at their lowest. The present invention is at least partly based on a realisation that appropriate times for active realignment can be more accurately determined if a profile for the variation of tidal current velocity with time at a device is measured and correlated to a reference profile rather than relying on e.g. just an instantaneous measurement of tidal current at the device. In particular, by measuring a profile, a problem that a single instantaneous measurement may be abnormally high or low, and thereby might activate an alignment mechanism at the wrong time or inappropriately delay activation, can be avoided. A first aspect of the present invention provides a method of controlling the alignment of a tidal stream energy device including the steps of: providing a reference profile for the variation of tidal current velocity with time at the tidal stream energy device, measuring a profile for the present variation of tidal current velocity with time at the tidal stream energy device, correlating the reference and measured profiles to identify a period of slack water, and changing the alignment of the tidal stream energy device during the identified period of slack water so that the device is appropriately aligned to generate energy from a following period of tidal flow. Controlling the alignment of the tidal stream energy device can involve controlling the position of the entire device (e.g. changing the alignment of an entire turbine) or controlling the position of one or more energy capture elements of the device (e.g. changing the alignment of just the turbine blades of a turbine).

The reference and measured profiles extend over respective periods of time, i.e. a single instantaneous value does not constitute such a profile.

Typically, a profile consists of a series of velocity values at different times.

The measured profile can be a profile formed from direct velocity measurements, or can be a profile formed from indirect or proxy velocity measurements. Examples of velocity measurements of varying degrees of directness are the measures of the power generated by the tidal stream energy device, pitot tube pressure induced signals, Acoustic Doppler Current Profile (ADCP) measurements, blade strain measurements, hot wire resistance change measurements, rotational speed measurements etc.

Preferably, the direction of the tidal stream energy device in the changed alignment is determined from the measured profile. This allows the device to be correctly aligned even after a period of shutdown.

Preferably, the reference profile is periodically updated by previous measured profiles, for example the update may be performed at each tidal cycle. In particular, the reference profile can be the corresponding measured profile from the previous tidal cycle. Advantageously, updating the reference profile allows the method to adapt continuously to variations in the tidal cycle, such variations occurring over daily to annual timescales.

Preferably, the reference and measured profiles cover a period of at least a quarter of a complete tidal cycle. For example, the profiles can cover a period from maximum current velocity at ebb or flood tide to the next slack tide. A longer correlation period helps to reduce the likelihood that noise or short timescale current velocity fluctuations will detrimentally affect identification of the period of slack water. The reference and measured profiles can be normalised. As the value of the maximum current velocity can vary from tide to tide, normalisation improves the correlation between reference and measured profiles obtained from different tides.

Smoothing algorithms can be applied to either or both of the reference and measured profiles. Smoothing can facilitate a more meaningful comparison of the profile shape, e.g. by counteracting measurement noise.

A second aspect of the present invention provides an alignment control system for controlling the alignment of a tidal stream energy device including: memory for storing a reference profile for the variation of tidal current velocity with time at the tidal stream energy device, a comparator for correlating the reference profile and a measured profile to identify a period of slack water, the measured profile being the present variation of tidal current velocity with time at the tidal stream energy device, and a controller for controlling the actuation of an alignment mechanism of the tidal stream energy device such that during the identified period of slack water the device is appropriately aligned to generate energy from a following period of tidal flow.

Thus the system corresponds to the method of the first aspect. Any one or combination of optional features of that method can provide a corresponding feature(s) in the system. For example, the controller may determine the direction of the tidal stream energy device in the changed alignment from the measured profile.

The reference profile may be periodically updated by previous measured profiles. For example, the update can be performed at each tidal cycle.

The reference and measured profiles may cover a period of at least a quarter of a complete tidal cycle.

The reference and measured profiles may be normalised. The comparator or a separate element of the system may apply a smoothing algorithm to the measured profile. Additionally or alternatively, the reference profile may be smoothed. Preferably, the system further includes one or more sensors for measuring the variation of tidal current velocity with time at the tidal stream energy device to provide the measured profile.

Preferably, the system further includes the alignment mechanism. The memory, comparator and controller may be computer-implemented.

A further aspect of the invention provides a computer program for performing the method of the first aspect. Another aspect of the invention provides a computer program product carrying a computer program for performing the method of the first aspect. Another aspect of the present invention provides a tidal stream energy device controlled by the system of the second aspect.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows schematically a plot of tidal stream current velocity against time over one complete tidal cycle;

Figure 2 is a schematic diagram of a device alignment control system according to an embodiment of the invention;

Figure 3 is a schematic diagram of a device alignment control system according to a further embodiment of the invention; and Figure 4 is a schematic diagram of the actuation control of an alignment mechanism.

Figure 1 shows schematically a plot of tidal stream current velocity against time over one complete tidal cycle. Starting from the left hand null, the velocity rises over time reaching a maxima, returning back to its null value. After a period of null (slack tide or slack water) the tidal current direction changes, and again reaches a maxima (of reverse polarity to the previous maxima) before reducing to a further null (slack tide/water). The cycle then repeats.

The plot is a simple approximation which is distorted in reality by local bathymetry, geographical features etc. Thus the timing and the duration of slack tide can vary on a site by site basis. Furthermore, the temporal position of the tidal cycle shifts each day.

In order for a tidal stream energy device, such as a turbine, to generate power effectively from the tidal stream, it is necessary for the energy capture element(s) (e.g. turbine blades) of the device to appropriately face the oncoming flow. This can be realised for example by providing an alignment mechanism which yaws the device around a vertical axis by approximately 180° at the turn of the tide, or yaws the device by 180° about a horizontal axis. The alignment mechanism, which can include e.g. a yaw motor and yaw gear or a yaw thruster, might align the entire device or just the capture element(s). However, to reduce stresses on the alignment mechanism, it is desirable to perform the alignment during the period of slack tide when current velocity is at a minimum.

Figure 2 is a schematic diagram of a device alignment control system according to an embodiment of the invention. In this embodiment, the slack period is identified by identifying a predetermined number, N, (e.g. 1000) of consecutive measurements of current velocity that are near identical. That is, the reference profile is a period of unvarying current velocity, such as that experienced at slack tide. A timer instructs performance of the consecutive measurements at a rate that is determined as a function of the expected duration of slack tide, allowing also for the amount of time the tidal stream energy device requires to realign its capture elements by (typically) 180°. A first-in, first-out (FIFO) buffer stores the last N measurements, and the comparator correlates the stored measurements for consistency with an invariant current velocity. When a high degree of correlation is determined (e.g. a correlation coefficient determined by the comparator reaches or exceeds a threshold value), the period of slack water is taken to have been entered and the comparator actuates the alignment mechanism to actively realign the device.

For example, in a hypothetical case where slack water lasts 30 minutes, assuming a buffer storing 1000 consecutive measurements and a timebase of 1 second (i.e. measurements occur once a second), a period of 1000/60 = 16.7 minutes is required for the comparator to determine that the slack water period has been entered, leaving 13.3 minutes of slack water to realign the device.

The velocity measurements are provided by a sensor or sensors that measure more or less directly the current velocity. A variant system, which uses largely the same configuration as shown in

Figure 2, compares the measured profile against a reference profile that requires a maximum in tidal stream current velocity. As shown in Figure 1 , such a maximum occurs at the peaks of the ebb and flood tides. As the period from a peak to the next slack water period is generally reliably predictable, the comparator, having identified the time of a velocity maximum actuates the alignment mechanism a predetermined period after that time.

An advantage of identifying a velocity maximum rather than a period of invariant velocity is that the system is less likely to falsely diagnose slack tide in the event of the failure of a velocity sensor. Also the duration of slack tide varies throughout the year and local turbulence can cause velocity sensors to fail to recognise the slack water period, whereas identification of the velocity maximum is usually less affected by local conditions.

Figure 3 is a schematic diagram of a device alignment control system according to a further embodiment of the invention, and illustrates a more sophisticated approach to device realignment. In particular, in this embodiment the measured profile is correlated with a reference profile that more closely represents features of actual current behaviour. Indeed, the reference profile can be a stored profile measured from a previous tidal cycle. Velocity measurements are provided by a sensor or sensors, and are sampled in time (prompted by a timer) to generate a profile at a FIFO buffer for the variation of velocity with time. A FIFO buffer moves the profile forward in time with each new measurement sample. Typically, the profile extends over a period of at least a quarter of a complete tidal cycle (e.g. from a velocity maximum to slack water).

The measured profile is sent to a comparator. A stored reference profile is also sent to the comparator. The reference profile extends over the same time duration as the measured profile, and represents the expected variation in velocity leading up to (and optionally including the start of) slack water. When the comparator determines that a high degree of correlation exists between the measured and reference profiles, the beginning of the period of slack tide (arrowed) is taken to have commenced and the comparator signals for actuation of the alignment mechanism.

In this approach, failure of a velocity sensor will not necessarily cause misdiagnosis of slack tide. Further, the use of a more detailed stored profile helps the system to take better account of local conditions.

Optionally, the stored profile can be replaced periodically by a more recent measured velocity. This allows the system to adapt to changes in local conditions. Indeed, if the stored profile is updated every tidal cycle, the slight shifts in tide characteristic which occur on daily to annual timescales are automatically accommodated by the system.

A further optional addition to the system, is an override function. The override allows, for example, the capture element(s) to be turned away from the current as a safety precaution in the event of e.g. shutdown of the device.

An optional position encoder (describing the present alignment direction of the device) can be used to tell the system which way the device should be aligned in preparation for the next flood or ebb tide. Additionally or alternatively, the measured profile when slack water is identified can be used to tell the system which way the device should next be aligned. For example, Figure 4 is a schematic diagram for the actuation control of an alignment mechanism which uses both position information and the measured profile. Upon confirmation of slack water period, it is first determined if the measured profile is consistent with the next tide being a flood tide or an ebb tide. A check is then performed, using information from the position encoder, to confirm that the device is presently positioned for the opposite tide. If that confirmation is positive, the yaw clamp is unlocked, the yaw motor yaws 180°, and, when yaw is complete, the yaw clamp is relocked to prevent unintended yaw caused by tidal turbulence. Advantageously, because up to date current velocity and position information is used, the device will adopt the correct alignment even if it has been held without realignment for a number of tidal cycles.

A device alignment control system as described above can be dedicated to the control of a single tidal stream energy device, or can control a plurality of such devices (e.g. in a sea bed array). However, if one system controls a plurality of devices, the velocity measurements and references profile used by the system should be applicable to the local conditions of all the controlled devices. Further, if a system controls only one device it is preferred (although not essential) that the velocity measurements are taken at the location of the device. Physically, the system can be mounted as a unit to a particular device to control that device, and optionally a plurality of other devices. Alternatively, the system can be a stand alone unit operatively connected to the device(s).

To take account for the variation in tidal current minimum and maximum velocities throughout the year (most markedly between spring and neap tides), the measured profiles and reference profiles can be normalised, to allow easier comparison between profiles. For example, when the profiles include the maximum velocity at flood or ebb tide, normalisation can ensure that that maximum is always expressed by a value of 1 in the profiles.

Furthermore, smoothing algorithms could be applied to the measured and reference profiles, to minimise the impact of noise and of minor discrepancies between profiles.

While a device alignment control system as described above is predominantly intended for application to control of tidal stream energy devices, the identified periods of slack water can be usefully made available to other devices, operators etc. For example, device maintenance can be scheduled to take place during the identified periods. In particular, the measured profiles can be communicated along device control cables (or even superimposed onto grid export cables) to a shoreside control centre for maintenance schedule planning. The control centre can then, if necessary, liaise with maintenance vessels, or relay the profile data directly using for example radio or satellite transmission.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

Claims

1. A method of controlling the alignment of a tidal stream energy device including the steps of: providing a reference profile for the variation of tidal current velocity with time at the tidal stream energy device, measuring a profile for the present variation of tidal current velocity with time at the tidal stream energy device, correlating the reference and measured profiles to identify a period of slack water, and changing the alignment of the tidal stream energy device during the identified period of slack water so that the device is appropriately aligned to generate energy from a following period of tidal flow.

2. A method according to claim 1 , wherein the direction of the tidal stream energy device in the changed alignment is determined from the measured profile.

3. A method according to claim 1 or 2, wherein the reference profile is periodically updated by previous measured profiles.

4. A method according to claim 3, wherein the update is performed at each tidal cycle.

5. A method according to any one of the previous claims, wherein the reference and measured profiles cover a period of at least a quarter of a complete tidal cycle.

6. A method according to any one of the previous claims, wherein the reference and measured profiles are normalised.

7. An alignment control system for controlling the alignment of a tidal stream energy device including: memory for storing a reference profile for the variation of tidal current velocity with time at the tidal stream energy device, a comparator for correlating the reference profile and a measured profile to identify a period of slack water, the measured profile being the present variation of tidal current velocity with time at the tidal stream energy device, and a controller for controlling the actuation of an alignment mechanism of the tidal stream energy device such that during the identified period of slack water the device is appropriately aligned to generate energy from a following period of tidal flow.

8. A system according to claim 7, wherein the controller determines the direction of the tidal stream energy device in the changed alignment from the measured profile.

9. A system according to claim 7 or 8, wherein the reference profile is periodically updated by previous measured profiles.

10. A system according to claim 9, wherein the update is performed at each tidal cycle.

11. A system according to any one of claims 7 to 10, wherein the reference and measured profiles cover a period of at least a quarter of a complete tidal cycle.

12. A system according to any one of claims 7 to 11 , wherein the reference and measured profiles are normalised.

13. A system according to any one of claims 7 to 12, further including one or more sensors for measuring the variation of tidal current velocity with time at the tidal stream energy device to provide the measured profile.

14. A system according to any one of claims 7 to 13, further including the alignment mechanism.

15. A tidal stream energy device controlled by the system of any one of claims

7 to 14.

16. A data carrier comprising machine-readable instructions for the control of one or more processors to: determine or access a reference profile for the variation of tidal current velocity with time at the tidal stream energy device, correlate the reference profile with a measured profile for the present variation of tidal current velocity with time at the tidal stream energy device, identify a period of slack water, and determine a change to the alignment of the tidal stream energy device during the identified period of slack water such that the device is appropriately aligned to generate energy from a following period of tidal flow.