GB2460553A

GB2460553A - Wave energy generator with multiple turbines

Info

Publication number: GB2460553A
Application number: GB0909801A
Authority: GB
Inventors: Fraser Johnson; Peter Arnold
Original assignee: Orecon Ltd
Current assignee: Orecon Ltd
Priority date: 2008-06-06
Filing date: 2009-06-08
Publication date: 2009-12-09
Also published as: WO2009147451A2; GB0909801D0; WO2009147451A3; GB0810388D0

Abstract

An energy device 1 includes a first turbine, and one or more additional turbines which can be individually activated in such a manner that both the first turbine and the additional turbine(s) operate at or close to their peak efficiency as mass flow increases. The additional turbine(s) may be in series or in parallel with the first turbine and all turbines can then operate close to their peak efficiency at a mass flow that would be too high for efficient operation of the first turbine. The device can be an Oscillating Water Column (OWC) wave energy device; be tension moored; have multiple chambers 1,2,3 partly submerged in water; each chamber having an air cushion and two or more turbines 4 and be tuned to a different resonance. Air ducted to and from atmosphere can be blocked by a guillotine valve to deactivate the additional turbines. Individual activation of multiple turbines can be used to optimise damping and efficiency for varying sea conditions and wave periods.

Description

I

AN ENERGY GENERATING DEVICE WITH MULTIPLE TURBINES

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a turbine-based energy generating device. In an implementation, it can increase the power generated b. for example, an Oscillating Water Column (OWC) wave energy device.

2. Description of the Prior Art

The simple operation of an OWC is shown in Figure 1. In essence, a fixed open bottom chamber has an upper enclosed air cushion with ducting that allows air to pass between the air cushion and atmosphere. As a wave peak passes the chamber it causes the water in the chamber to rise. As the wave peak progresses and a wave trough passes the column, the water in the column falls. At a specific wave frequency, resonant motion will be induced within the chamber, greatly increasing the amplitude of the internal motion.

Air from the internal air cushion is allowed to pass between the cushion and atmosphere, via a bi-directional air turbine which is located within the air ducting. As the air passes between the air cushion and atmosphere, the mass flow generated imparts rotational energy to the turbine. At tesonant conditions, very considerable power can be generated.

This design of OWC uses a tension mooring system. The term tension mooring' is a term of art. It can he explained as follows: Traditional tension mooring systems are used for vertically moored floating structures. The structure is permanently moored by means of tethers or tendons. The weight of the underwater volume of the structure is greater than the structure's deadweight (underwater volume weight >> deadweight).

Thus the tension-legs are maintained in tension, so that virtually all vertical motion of the platform is eliminated.

This tension mooring design originated as a means of maintaining the relative distance between the seafloor and the production deck on top of the structure. Costly production weliheads were mounted on the structure (connected directly to suhsea wells by rigid risers) instead of the seafloor.

This made for cheaper well completion and gave better control over the production from the oil or gas reservoir.

In this specification, the term tension mooring' therefore requires that the buoyant structure of the \VEC is tethered such that, in still water, it is effectively held substantially below its natural floating draught such that it experiences an upwards thrust; this upwards thrust is counterbalanced by tension in the mooring lines and anchors. A tensioned buoyant platform TBP) can therefore be "a floating structure connected to a seabed foundation by vertical tethers which are kept in tension by excess buoyancy over weight in the platform structure "; see Dynamics of Offshore Structures by Minoo H Patel ISBN: 0-408-01074-6. This detail differentiates the tethering system from, for example, a catenary mooring system where, in still water, a buoyant structure floats at its natural floating draught other than for any additional weight imposed by the tethering lines and any tension in the lines is due to the mooring line weight alone.

Tension mooring dictates that the vertical position of the buoyant structure's centre of buoyancy is substantially below that which would occur for an identical buoyant structure floating at its natural draught. The term substantially below' is taken to mean the vertical position of the buoyant structures centre of buoyancy is at a position which is a substantial portion of the distance between the natural floating draught and the position of the centre of buoyancy when the floating structure is submerged to its operating draught under tension. To achieve this, a substantial portion of the buoyant structure's normally floating volume must be submerged. A tension mooring system typically uses mooring anchors of sufficient holding capacity to counteract the pre-tension that is equal to the underwater displacement minus the structures deadweight. The mooring weights are, in known designs, permanently fixed to the seabed using, for example, piles, suction or embedment anchors or extremely heavy anchors that are designed not to lift from the seabed, irrespective of the sea conditions.

A tension-moored OWC is also heave resistant'. By this, we mean that the natural vertical motion of the buoy associated with each passing wave (known as heave') is deliberately resisted by the tension mooring lines or tethers. There will typically be some degree of heave (due for example to the inherent elasticity of tension mooring lines, whether synthetic or steel, but the amount of stretch is very small in relation to the overall length of the mooring lines). And heaving is not required for the correct operation of a heave-resistant tension moored OWC. Instead, the lower end of each chamber in the buoy is open to water thove the wave base; there is therefore a pressure head at the base of a tube when a wave is passing the device. When a wave peak passes the device, the buoy remains at substantially the same vertical distance from the seabed. But the water column in each chamber of the buoy does rise in absolute terms because there is a pressure head at the chamber base. Because the buo' remains essentially stationary relative to the seabed, the water column in the tube moves up relative to the buoy. It is this relative movement of the water column in each tube that drives the air movement in each tube. It requires the buoy to be heave-resistant.

This stands in contrast to other designs of OWC, which must heave in order to operate and are hence not tension moored. In heaving OWC systems, an open-bottomed tube extends to be/ow the wave base; there is therefore no pressure head at the base of a tube when a wave is passing the device. When a wave passes the device, the buoy heaves upwards, but the water column in a tube does not rise in absolute terms because there is no pressure head at the tube base. Because the buy heaves upward, the water column in the tube moves dowii relative to the buoy. It is this relative movement of the water column in each tube that drives the air movement in each tube. But it requires the buoy to heave.

As detailed above, power is extracted from the OWC via an air turbine being located within the air ducting between the internal air chamber and the atmosphere. A range of air turbines exist that can be used for this type of energy conversion including both mono and hi-directional radial or axial airflow turbines. Regardless of the pe of air turbine used the rotational torque generated is converted into electricity with the use of a typical generator system.

Extracted electrical power is then conditioned and fed to a local load bank, shore power grid or appropriate resistive component.

The operation of an OWC is such that at a wave period corresponding to the resonant period of the chamber, the mass of water within the chamber is caused to tesoflate. In this condition the water surface motion within the chamber may be far greater than that of the external water surface. This effect greatly increases the pumping action of the internal water surface producing elevated levels of air mass flow and as a consequence elevated levels of positive and negative air pressure. In wave periods shorter and longer than this resonant wave period the vertical motions of the internal water surface are reduced.

Figure 2 shows the trend of differential air pressure generated within an OWC across a range of wave periods (the v axis is differential pressure, Pa; the x-axis is wave period, S). In low wave periods, such as 0 -1.5. seconds, oscillations within the chamber are low, resulting in a small range of positive and negative air pressure, + or -50 Pa at a 1,5 second wave period. As the wave periods are increased, so the vertical motion of the internal water surface within a chamber increases and as a result the range of differential air pressure increases. The peak internal differential air pressure is generated when the OWC is in resonance. This occurs at about 1.9 seconds in the system shown in Figure 2. As the wave periods are increased further, past resonance, the elevation of the internal water surface falls towards a one to one relationship with that of the external water surface. This leads to a reduction in the range of differential air pressure generated within the buoy. As a real wave climate varies in time in terms of the wave period and wave height, so the range of mass flow and the resulting differential pressure will vary over time.

Consider the operation of the air turbine in a real sea with regards to the range of mass flow rates and resulting differential pressures generated by the incident wave train. An air turbine has an optimum mass flow rate to turbine rotational speed (flow ratio) which is related to the optimum angle at which the air flow hits the blades of the turbine. If the air flow to rotational speed is too low or too high, the efficiency of the turbine can drop dramatically. This is shown in Figure 3 where the x axis represents the ratio of mass flow to turbine speed and the Y axis represents the turbines efficiency. Theta is the setting angle of the guide vanes. The source of the graph is "Performance of an impulse turbine with fixed guide vanes for wave power conversion" H. Maeda et a! , Renewable Energy, Vol 17, 1999, pp 533 547.

The impulse turbine shows a relatively rapid increase in efficiency as the flow ratio increases to its optimum value at and a relatively slow decrease in efficiency as the flow ratio increase past this point. Therefore it is advantageous to run an impulse turbine at or above its optimum mass flow ratio. Since the turbines rotational speed is in reality limited by mechanical constraints, once maxed out, the flow ratio becomes purely a function of mass flow rate.

SUMMARY OF THE INVENTION

The invention is an energy generating device including a first turbine, and one or more additional turbines, in which the additional turbine(s) can be individually activated in such a manner that both the first turbine and the additional turbine(s) operate at or close to their peak efficiency as mass flow increases.

Specific implementation features include the following: The additional turbine(s) may be positioned in series or in parallel with the first turbine and all turbines can then operate at or close to their peak efficiency at a mass flow that would be too high for the first turbine to operate at efficiently. When we say close to their peak efficiency', we mean substantially doser to the peak efficiency than would be achieved without the distribution of excessive flow between multiple turbines, e.g. if the first turbine alone were activated, or the additional turbine(s) alone were activated. A combination of series and parallel configured turbines would also be possible.

Each turbine can be individually activated in such a maimer as to optimize their efficiency as mass flow increases and can be individually deactivated in such a manner as to optimize their efficiency as mass flow decreases. Each turbine can be activated in such a manner as to optimize the damping applied by that turbine.

The energy generating device can be a wave energy device, although in its broadest form it is not limited to that kind of device.

The main implementation is a tension-moored oscillating water column (OWC) device, in which the OWC device has multiple chambers, each tuned for different resonance conditions, each chamber being part submerged in water and hence including an air cushion sitting above the water level, and in which there are two or more air turbines for each chamber through which air is ducted to pass from the air cushion to atmosphere (and reversd and in which, in a relatively calm sea state, only a first turbine per air chamber is open to allow air to pass between the air chamber and atmosphere, with the or each additional turbine being mechanically blocked to the passage of airflow. The first turbine reaches its peak efficiency at a relatively low operating mass flow rate and the damping applied by the first turbine to the water column's motion reaches the optimum value for the water column at the same relatively low operating mass flow rate.

This OWC cai however operate efficiently over a broad range of sea states or within a sea state.

Multiple turbines are located within (e.g. are placed in the air outlet of) each air chamber in order to extract the maximum amount of energy. There can be two or more air outlets for each chamber, each with their own turbine or turbines. One or more of the air turbines can be made active at a given time in response to the wave climate varying in order to alter the air flow mass through each turbine so that they each operate more efficiently. Hence, efficient operation is possible over a much broader range of sea states than was previously possible.

The invention can be applied to all the turbine types shown in Figure 3, including the Wells turbine, or indeed any other design of turbine.. In addition, the Dresser Rand HydroAirTM impulse turbine is used in a preferred implementation.

Considering the operation of an OWC, as a wave climate develops from short to long wave periods, the range of differential air pressure generated within the chamber will vary from low', to a maximum and back to a low differential pressure as shown in Figure 2.

The varying air pressure and associated air mass flow rates may be such that at times it is greater than the optimum value for a single air turbine. Therefore in order to keep the turbine operating at a high efficiency level and thus extract a greater level of energy from the passage of the mass flow through the turbine, the present invention distribute excessive flow rates between two or more turbines for each chamber.

A second but equally important consideration concerns the level of damping which the turbine applies to the water column's motion. Essentially, the turbine acts as a restriction to the entry and exit of water from the surrounding sea into the air chamber. The extent of the restriction is known as the damping level. For optimum power transfer from wave to the turbine, there exits an optimum damping or restriction level. For an impulse turbine, the damping level the turbine applies increases as the air mass flow rate and differential air pressure increase. As the applied damping level increases there comes some point when the damping level will exceed the optimum value. In this situation it is again advantageous at some point to re-distribute the mass flow between two or more turbines so as to lower the applied damping level back towards the optimum level.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanying Figures, in which S Figure 1 shows a schematic, side view of the operation of an oscillating water colunrn (OWC) wave energy device; Figure 2 is a graph that shows how the differential pressure generated within an OWC device varies with the wave period; Figure 3 is a graph that shows how turbine efficiency is a function of mass flow, dropping off as mass flow increases; Figure 4 is a plan view of an OWC implementing the present invention, in which there are three air chambers, each with a primary turbine and an additional turbine.

DETAiLED DESCRiPTiON

Orecon is developing an Oscillating Water Column (OWfl wave energy device termed the MRC (multi-resonant chamber). The system incorporates the following features: * A buoy sub divided into 3 internal chambers; The buoy is tension-moored (see earlier for a fuller explanation of this term) * The buoy is partly submerged and extends into the water by a set \vater draught below the still water line; * The buoy extends above the water by a set air draught above the still water line; * The water and air draughts are design inputs that dictate the power output; * A weather deck encases the upper ends of the chambers at the maximum level of their air draughts, thus encasing the three individual air cushions (i.e. the large volume of air in each of the three internal chambers); * The individual water chambers (i.e. the volume of water in each of the three internal chambers) are acted upon by the incident wave train, being induced to pump like pistons up and down within the buoy; * The three chambers are given different water draughts, which enclose different masses of water. As a consequence each water column's natural period of vertical oscillation is different such that they resonate in response to different wave periods in the incident wave climate. The system is therefore tuned' to the incident wave climate by varying the three draughts chosen; * The system requires that the water and air draughts are maintained in order that operating range of the system he maintained; * The pumping action of the water pistons' generate positive and negative air pressures within enclosed air cushions; * Each section of weather deck includes a duct (or tow or more ducts) that allows air to pass between an air chamber and atmosphere in both directions; * Located within each duct is a bi-directional air turbine used to convert the air flow into rotational torque and subsequently electrical power; Further details are provided in WO 20051085631, the contents of which are incorporated by reference.

The system operates as follows: An OWC has two, or more, air turbines per air chamber; air is ducted through the turbines to pass from the OWC air cushion to atmosphere and reverse.

Considering a relatively calm sea state, only one turbine per air chamber would be open to allow air to pass between the air chamber and atmosphere. Additional turbines would be mechanically blocked to the passage of airflow. Turbine no. 1 would he designed to teach its peak efficiency at a relatively low operating mass flow rate. In addition the damping applied by turbine 1 to the water column's motion would be designed to reach the optimum value for the water column at the same relatively low operating mass flow rate. As the sea state begins to become more energetic and wave periods become longer the motions of the internal water surface would generate an increase in the mass flow rate and air pressure between the chamber and atmosphere.

As the air pressure rises it would eventually reach a level where the mass flow rate and air pressure exceeds the peak efficiency operating point of turbine no.1 and also the damping applied to the water column would rise above its peak level. At a set level of air pressure above the optimum of turbine no.1 the blockage inhibiting airflow through and identical turbine no.2 would be removed.

The removal of this blockage would split the air flow between the two turbines and have the dual effects of reducing the mass flow rate back to a level closet to the peak operating point of both turbines and also to reduce the damping applied to the water column back towards its optimum level, hence ensuring that resonance conditions continue to be met. A key to high performance for a OWC is to maximize the range of sea states at which resonance is occurring.

In the primary embodiment Turbine no.2 would be identical to Turbine no. 1. In a second embodiment Turbine no.2 has an operating range with a peak efficiency at a lower flow rate than that of Turbine no.1 and with an applied damping level higher than turbine I. In the latter case, as both turbines became operational, more air flow would pass through turbine I and less through turbine no.2 such that they would each be operating close to their optimum mass flow rates and applied damping values As the energy in the sea state then begins to decrease the average mass flow rate would fall and the turbine process would then operate in reverse i.e. as the air pressure fell below a set point turbine no.2 would again be blocked, thus leaving air to pass solely through turbine no.1.

Activation of individual turbines can be implemented using, for example, an annular guillotine valve actuated either with a ball screw or electromechanically.

The present invention is applicable to any system that benefits from having multiple turbines that can be individually activated in such a manner to optimize their efficiency as mass flow varies; typically, replacing a single turbine whose efficiency drops off as mass flow increases with two or more series or parallel turbines that can each operate at or closer to their peak efficiency. It can hence be used in any kind of OWC device, whether or not tension-moored, and in other kinds of power generating systems.

Multiple turbines are located within, or define the exit from, each air chamber in order to extract the maximum amount of energy. There can be two or more air outlets for each chamber, each with their own turbine or turbines. Or there could he just one air outlet per chamber, with several turbines in parallel or in series, but all being fed from this single air outlet.

For an O\VC with say three different chambers, each sized to resonate at a different wave period which is different from being sized to resonate at different wave sizes), each chamber could have its own dedicated turbines, sharing none with any other chamber. This is shown in Figure 4, which is a plan view of the OWC. There are three air chambers 1, 2 and 3, each with a pair of dedicated air turbines, 4, each connected to an air chamber via its own air outlet.

Also, it is possible for some, or indeed all, of the chambers to displace air into one or more shared turbines. There are many possible permutations. For example, each chamber could have its own dedicated primary turbine, for use in relatively calm sea states. That turbine would not be shared with any other chamber. A single additional turbine could then connected to and shared across each chamber, for use in higher sea states. Or (as shown in Figure 4) each chamber could have its own dedicated additional turbine, not shared with any other chamber. That could be connected in parallel with the main turbine, or in series; either option could use a single air outlet, or a pair of outlets. Another arrangement is for all chambers to feed just a single main turbine, and for there to be an additional turbine placed either in series or in parallel with that main turbine. The exact arrangement chosen will depend on many different factors, but they are all within the broadest scope of this invention.

Claims

CLAIMS1. An energy generating device including a first turbine, and one or more additional turbines, in which the additional turbine(s) can be individually activated in such a manner that both the first turbine and the additional turbine(s) operate at or close to their peak efficiency as mass flow increases.
2. The energy generating device of Claim 1 in which the additional turbine(s) are positioned in series with the first turbine and all turbines can then operate at or close to their peak efficiency at a mass flow that would be too high for the first turbine to operate at efficiently.
3. The energy generating device of claim 1 in which the additional turbine(s) are positioned in parallel with the first turbine and all turbines can then operate at or close to their peak efficiency at a mass flow that would be too high for the first turbine to operate at efficiently.
4. The energy generating device of any preceding Claim in which each turbine is activated in such a manner as to optimize the damping applied by that turbine.
5. The energy generating device of any preceding Claim in which each turbine can be individually activated in such a manner as to optimize their efficiency as mass flow increases and can be individually deactivated in such a manner as to optimize their efficiency as mass flow decreases.
6. The energy generating device of any preceding Claim in which the device is a wave energy device.
7. The energy generating device of Claim 6 in which the wave energy device is an oscillating water column (OWC) device.
8. The energy generating device of Claim 7 in which the OWC device is a tension moored 0WC.
9. The energy generating device of Claim 8 in which the OWC device has multiple chambers, each tuned for different resonance conditions, each chamber being part submerged in water and hence including an air cushion sitting above the water level, and in which there are two or more air turbines for each chamber through which air is ducted to pass from the air cushion to atmosphere and reverse and in which, in a relatively calm sea state, only a first turbine per air chamber is open to allow air to pass between the air chamber and atmosphere, with the or each additional turbine being mechanically blocked to the passage of airflow.
10. The energy generating device of Claim 9 in which the first turbine reaches its peak efficiency at a relatively low operating mass flow rate and the damping applied by the first turbine to the water column's motion reaches the optimum value for the water column at the same relatively low operating mass flow rate.
11. The energy generating device of Claim 9 or 10 in which, when the mass flow rate and air pressure exceeds the peak efficiency operating point of the first turbine and the damping applied to the water column rises above its peak level, then a blockage inhibiting airflow through the or each additional turbine is removed, the removal of this blockage splitting or passing the air flow between the fit-st turbine and the or each additional turbine.
12. The energy generating device of Claim 9 -I tin which the additional turbine is identical to the fit-st turbine.
13. The energy generating device of Claim 9 -Ii in which the additional turbine has an operating range with a peak efficiency at a lower flow rate than that of the first turbine and with an applied damping level higher than the first turbine so that, as both turbines became operational, more air flow passes through the first turbine and less through the additional turbine such that they would each be operating close to their optimum mass flow rates and applied damping values.
14. The energy generating device of any preceding claim in which activation of individual turbines is implemented using an annular guillotine valve actuated either with a ball screw or electromechanically.
15. The energy generating device of any preceding Claim in which each turbine is an impulse turbine, or a Wells turbine or a HydroAir turbine.