GB2544724A - Extendable and stowable volume for wave energy converters and other applications - Google Patents

Abstract

A wave energy converter has a hull volume and means to vary the hull volume on command in response to prevailing wave conditions. The means to vary the hull volume may comprise inflatable modular structures 2 located around a rigid, load-bearing core 1. The inflatable structures may comprise a plurality of axially aligned compartments 3 arranged in an annular structure around the core, and may have internal fabric webs (4, fig 3) and internal elastic elements (5). There may be a pair of inflatable structures (10, fig 4) that each inflate to a convex shape and are stowed in a concave shape. The rigid core may comprise a pair of tubes (11) with a faired shear web between them, or a trefoil frame arrangement with three longitudinal structures connected together by truss members (12). A pair of rigid casings (13, fig 5) may each have a first end connected to one of a pair of inflatable structures (14) and a second end articulably connected to the core, where an elastic member (17) connected between the inflatable structures controls the fabric of the inflatable structures.

Description

Extendable and Stowable Volume for Wave Energy Converters and Other Applications

Wave energy converters must be both robustly survivable and produce sufficient average yield to deliver attractive economic performance. Many different wave energy converter concepts, forms and engineering solutions have been proposed to deliver against these two commonly conflicting requirements, but it remains a major barrier to improved cost of energy. The invention described here uses systems to give a wave energy converter the ability to substantially and cost effectively change its volume on command in response to the prevailing wave conditions it is operating in at the time to provide a much better balance between performance and survivability. Also, other engineered structures may benefit from similar capabilities improving their viability or cost effectiveness.

This document provides a brief functional specification to describe the proposed operating concept followed by descriptions of preliminary engineering embodiments. Some examples are also provided of existing materials, technology, and applications that may be drawn on to enable practical engineering solutions.

There have been many different configurations of wave energy converter proposed and/or developed. The majority of these have had a rigid, fixed volume hull or hulls, the motion of which in the presence of waves is restrained to absorb power. Some proposals have been included to allow the level of flotation of the hull itself to be changed or the system to be sunk below the waves to reduce loading in extreme conditions, but such systems have proposed filling and emptying trimming tanks within the hull structure. A number of other wave energy converters have been proposed and/or developed that use volume change as a primary means of absorbing power from the wave. Inflatable or movable volumes are mounted on a stable fixed or floating reference hull. Passing waves cause the inflatable or movable volumes themselves to grow or shrink to displace water and absorb power rather than the vertical or horizontal motion of the hull itself.

In addition, the materials and construction methods required to realise the invention described here are already well developed in the prior art. Other example applications of inflatable/deflatable structures for other purposes are shown in Figure 6. However, these systems do not include means to manage the inflation and deflation process in a controlled way, and stow the delated structure in a conformal and protected way. In addition, the invention proposed here is new and uses volume change means to directly vary a wave energy converter hull volume with the slowly changing hour by hour prevailing wave conditions, to deliver substantially more power in small seas and substantially lower loads and motions in storm conditions increasing the economic attractiveness of the wave energy converter concept. Similar capabilities in other engineering application can be expected to confer analogous operating advantages.

Summary of invention

Wave energy converters must be both robustly survivable and produce sufficient average yield to deliver attractive economic performance. Many different wave energy converter concepts, forms and engineering solutions have been proposed to try to deliver against these two commonly conflicting requirements, but it remains a major barrier to improved cost of energy.

Wave energy converters generally use their volume to displace water as they move in response to waves, thereby absorbing power. It follows that the amount of power that can be absorbed from an ocean is strongly related to the size and volume of the system, the greater the volume the greater the absorption potential. A major issue is that the extreme loads and motions experienced in storms are also strongly related to machine size and volume, the greater the volume the higher the loads and or motions in extreme conditions. This means that fixed volume wave energy converters must inherently be a compromise between these two competing design requirements which also compromises the economics of the system. In addition to the pure size and volume of the system, to maximise absorption of power many wave energy converters have a tuned hydrodynamic response so that the incoming waves excite the wave energy converter at its resonant frequency. While this behaviour can markedly increases absorption from small seas, having a system which is inherently tuned to incoming waves is extremely dangerous in larger storm conditions. The extendable/stowable volume system described here also offers the ability to significantly change the dynamic response of the system to remove unwanted resonances in the stowed condition when the waves are large. Further to this, partial inflation/extension of the system also offers the ability to tune the system to different resonant frequencies to match incoming waves to further enhance economics.

The invention described in the document is designed to break the fundamental conflicts and compromises that a fixed volume wave energy converter has between performance and survivability by introducing means to substantially change the wave energy converter hull volume on command in response to the prevailing wave conditions at the time. In the same way that sailing ships reef to deliver both speed and survivability, this would allow a wave energy converter to grow substantially and tune its response to absorb much more power during most of the year when waves are small or moderate, while reverting to a smaller robustly survivable form when the wave conditions are extreme. The volume change technology proposed here will thereby provide the economics of an otherwise unsurvivable machine with the survivability of an otherwise uneconomic machine. This is the inventive step and main claim set out in this patent application.

To illustrate viable methods by which this could be effected, we have considered and evaluated a number of ways of engineering such a system. The most promising of these use inflatable membranes or other flexible structures to cost effectively provide a gross increase in volume around a rigid load bearing core. The well-researched articulated line-absorber configuration has been used in this description as a robust case study for the proposed embodiments but the ideas can of course be readily adapted to other machine designs and the example presented below are not meant to prejudice the generality of the invention.

The benefit of the invention is therefore general across wave energy and has the double benefit of not only delivering a major reduction in the starting cost of energy of many wave energy converter forms but also in the long term floor cost when the technology is mature, markedly improving the attractiveness of wave energy as a competitive energy source.

Accordingly, the invention described here is to use inflatable and extendable structures to provide substantial additional volume and/or improved dynamic response to rigid bodied wave energy converters in small seas to increase power absorption, that are debatable and stowable on command to remove the additional volume and/or detune the dynamic response to minimise loads and motions in large seas, or to use such systems to substantially change the volume of other engineered structures or systems to achieve some other major operating benefit. This forms the inventive step and main claim of this patent application, other dependent claims related to the embodiments described below and other applications will be detailed during the patent process.

The necessary characteristics of the inflatable/stowable volume system are as follows: • The volume increase arising from the inflated system should be a substantial proportion of the core volume. The concepts proposed here are capable of doubling the volume, giving the potential to commensurately increase yield in normal seas without proportionate increases in peak loads or motions and therefore costs. • The volume must be automatically stowable on command in a time-frame corresponding to changes in wave conditions and associated forecasts. In practice this comfortably means tens of minutes if not a few hours. • The stowed configuration must be robust to extreme conditions without damage or risk to the wider WEC. This implies no loose or flapping material. • The volume must be stowable with an extremely high level of reliability to avoid compromising the survivability of the WEC in extreme conditions. This may readily be provided with redundancy in the deflation system and in the modularity of the volumes themselves. • The inflatable volume system must be robust to contact with vessels, lines, quayside and general wear and tear. Confidence is gained from existing marine inflatable applications in this regard. • The system must be readily maintainable, installable, and replicable with minimal quayside equipment. This can be achieved with multiple manageably sized units, also serving the requirement above. Minimising or avoiding fastenings below the waterline will also help in this regard.

The example engineering implementations described below aim to meet these criteria, but they are only examples and many other different configurations would be possible within the general inventive step set out in this application. ·.. j-·^ p- j ρ j ' J ρ pr j p"jj ρ | pp ·;\ "S' 'ip ^ p p j·'; p>"\ p >·> p

Three main concepts have been selected for implementation using Automatically Inflatable and Stowable Volume (AISV): • A modular inflatable annular tube structure, with concertinaed internal membranes to divide the tube into axially aligned compartments to provide stability, and to allow neat and passive folding into the stowed configuration. • A pair of larger single volume membranes that can inflate to a high volume convex shape and then on deflation conform into a specially shaped concave rigid hull of equal perimeter to the inflated form, stowing the membrane without the need for folding. • An additional embodiment is a hinged section of hull structure with a sealed bellows style gusset linking it to the main rigid hull such that the hinged portion is held out in a high volume rigid position when the gusset is inflated and sucked flat to the rigid core when the gusset is deflated.

Any of these embodiments may be based on thin fabric membranes or sheets alone or in combination with an alternative known as drop-stitch membrane constructions as shown in Figure 7. It is implicit that other embodiments are possible using a combination of these features and this application is intended to protect any such means operating on one or a mixture of the principles above, it is expected that the embodiments will be refined and expanded through further development work.

Concertinaed wraparound lube structure

This resembles a compartmentalised inflatable ΊϊΙο' wrapped into a hoop around the rigid core. The axially aligned sub-volumes formed by a triangular concertina pattern, which may be inflated with air to expand the volume. The triangular webbing provides rigidity against buoyancy induced shearing or shape change of the inflatable structure. Individual compartments may be water filled through porous compartments with air or partial air inflation of others providing the expansion and structural rigidity as required whilst achieving the required ballast level. Several views of this embodiment are shown in Figures 2 & 3. The embodiment shown has a circular cylinder form rigid central hull (1) with a circular cylinder form of inflatable section hull (2) mounted on this. It is important to note that any form and shape of hull can be used using the same invention. It could be axisymmetric about any axis or indeed completely general in form and shape. The inflatable sections can have a completely different form and shape to the rigid internal hull. In the embodiment shown in Figure 2, the flexible structure takes the form of an 'arm-band' or lilo with a segmented concertina type inflatable structure (2) wrapped around the inner rigid hull (1). Each of the section is a separate section (3) with internal fabric elements to control its shape when inflated, deflated and during the transition, these are shown in more detail in Figure 3. On the right of Figure 2 it is shown flat prior to attachment to the hull and on the left it is shown in the inflated when wrapped around the inner hull. Figure 3 shows the system at several stages of inflation, and shows how the inflation and deflation of the individual cells of the segmented structure (3) is controlled by internal fabric webs (4) and internal elastic elements (5) to mean the shape and stability of the structure is well controlled and behaved at all states of inflation. Through careful design of the system this embodiment offers the potential to have the volume of the structure continuously variable between the fully inflated and stowed states. Annotations in Figure 3 describe how this process occurs. Individual segments (3) may be inflated individually or they may be linked. They may be inflated with gas or water or a mixture of the two.

While air inflation would provide the expansion into the inflated position, on stowing the compartmentalised construction would require a mechanism to collapse the membrane into a flat core-hugging configuration not subject to flapping. This could be achieved with internal elastic members (5) pulling the membrane panels (4) into a covered folded shape during deflation and passively holding that stowed configuration until reinflation. The effectiveness of this mechanism depends on the controlled and progressive collapsing and folding behaviour during deflation as the elastic members (5) take up the slack left by the vacating air.

In one proposed implementation, strong elastic tendons at the top (6) would pull the outer cylindrical layer (8) of the inflatable structure tangentially relative to the inner layer (9), while the internal tendons or an elastic fabric sheets (5) attached to every second web membrane provide the folding action to progressively collapse the concertinas at equilibrium pressure into a neat overlap secured underneath the smooth outer tube later, passively stowing the complete structure. The ends of the triangular compartments (7) may fold flat in concert with the wider structure by shaping them as tetrahedrons that continue the wider folding pattern to a rounded point. This point presents an unavoidable double-angle fold but it is understood this may be accommodated with some appropriate material reinforcement and bend limiting in that area, given that the number of folding cycles is limited. A separate elasticated outer wear sheath may be included over the entire structure, and rubber fender strips may be included as familiar on rigid inflatable boats and other applications.

Splitting the hoop at the top allows the outer skin (8) to shear relative to the inner layer (9) and hence lay smooth and flat over the folded concertinas when in the stowed configuration. As a further advantage, splitting the hoop at the top allows it to lay flat for initial manufacture and servicing. For installation and servicing, the whole deflated membrane structure would be pulled around the underside of the machine rigid tube with messenger lines and then attached above the water line once in position, much like the 'habitat' membrane was routinely in the Pelamis application.

The hoop structure gives well distributed bulk buoyancy loading of the rigid hull, without concentrated load points or edges as the whole inflatable structure is essentially fitted on to the core like an arm band. Relative rotation can be made fast above the waterline, along with the tangential elastic tendons (6). The cross section of the tubular inflatable may be non-circular, as may the rigid core, with adaptations in the design. • Retention of a simple round core tube structure may be an advantage as a well understood and structurally efficient form. • Rigidity in the inflated configuration is likely to be substantially greater than for a single non-compartmentalised inflating volume • Compartmentalisation allows water to be taken on in some compartments - compensation with internal pumping of ballast water may not even be necessary with associated cost savings. • It is anticipated that the system would also be compartmentalised along the length of a cylindrical hull to create redundancy in service and reduce cost of repair in the event of damage.

Larger conforming membrane volumes

An alternative embodiment is to attach larger single inflatable volumes (10) against a specially designed conformal area of the rigid hull (11) as shown in Figure 4. Two examples are shown but it is understood that a large number of configurations are possible within the context of the invention itself. The additional beam, size and volume achievable through inflation is a function of the conformal hull shape, since the deflated membrane surface area preferably substantially matches it to minimise wrinkling or flapping. Again, the examples in Figure 4 are presented as cross section of a long cylindrical form hull but the concepts are readily adaptable to other forms of axisymmetric or general form hull shapes.

This approach may offer greater flexibility in the shape and construction of the hull structure to provide a better balance of load carrying capacity against material requirements, while also offering the potential for further reduced core volume for improved survivability characteristics and hence reduced structural and bearing system requirements in extreme conditions. As an example, separating the volume (wave radiation) function from the load carrying function means that a deep space-frame arrangement (12) might more cost effectively transmit the necessary bending moments (as they do in crane and bridge structures where volume is not a requirement). Such a truss need only displace sufficient volume to remain floating when the inflatable volume is stowed. This might also allow for a lower CoG and associated roll stiffness and much lower volume and hydrodynamic loading in the deflated condition giving much improved survivability in storms.

The transition between inflated and deflated states could be problematic with a loose membrane subject to flapping and movement of air pockets when not fully pressurised. This could be controlled through segmentation/compartmentalisation along the length and indeed within the membrane compartment to form a hybrid between this and the previous embodiment. Alternatively it could be stabilised and controlled in all states controlled using a system functioning in a similar manner to the drop-stitch approach shown in Figure 7.

Hinged or articulated structures with bellows A further embodiment would be using a combination of hinged or articulated rigid elements (13) in conjunction with a bellows or gusset seal of segmented inflatable structures (14) as described previously to create a hybrid structure with the large volume change of the previous embodiments but better protected flexible elements and a more fully defined transition behaviour. One example of such a system is shown in Figure 6, but many are possible within the principle described here. Conformal casings (13) are hinged or articulated (15) on the main hull and inflated bellows or compartments (14) drive them (16) between a high-volume and stowed states. Elastic members (17) or conformal structures or other means similar to those described in other embodiments would be used to control the flexible fabric between the two states.

Claims (24)

CLAIMS:

1. A wave energy converter having a hull volume and comprising means to vary that hull volume on command in response to prevailing wave conditions.

2. The wave energy converter of claim 1 further comprising a rigid, load-bearing core and wherein the means to vary the hull volume comprises inflatable structures located around the core.

3. The wave energy converter of claim 2, wherein the core has a first volume and the inflatable structures collectively have a second maximum volume which is substantially equal to the first volume.

4. The wave energy converter of claim 2 or claim 3, wherein the inflatable structures are automatically stowable on command in response to wave conditions.

5. The wave energy converter of any of claims 2 to 4, wherein the inflatable structures are modular.

6. The wave energy converter of any of claims 2 to 5, wherein the inflatable structures comprise a plurality of axially aligned compartments arranged in an annular structure about the core.

7. The wave energy converter of claim 6, wherein each compartment comprises internal fabric webs and internal elastic elements which pull the webs into a stowed configuration and passively hold the webs in that stowed configuration until inflation.

8. The wave energy converter of claim 7, wherein the inflatable structures comprise an inner layer, a cylindrical outer layer and a plurality of elastic tendons which pull on the outer layer tangentially relative to the inner layer, whereby the internal elastic elements and elastic tendons stow the inflatable structures.

9. The wave energy converter of any of claims 6 to 8, further comprising an elasticated outer sheath covering the entire converter structure.

10. The wave energy converter of any of claims 6 to 9, wherein each compartment is inflated individually.

11. The wave energy converter of any of claims 6 to 9, wherein each compartment is fluidly linked to the other compartments.

12. The wave energy converter of any of claims 6 to 11, wherein the compartments are inflated with water and/or gas.

13. The wave energy converter of any of claims 6 to 12, wherein the annular structure of the aligned compartments is split at the top so that it may lie flat during manufacture and servicing.

14. The wave energy converter of any of claims 6 to 13, further comprising an automated inflation system housed inside the rigid core.

15. The wave energy converter of any of claims 2 to 5, comprising a pair of inflatable structures which can each inflate to a convex shape and are stowed in a concave shape.

16. The wave energy converter of claim 15, wherein the rigid core comprises a pair of tubes with a faired shear web between the tubes, the tubes and web defining a concave shape into which the deflated inflatable structures can be stowed.

17. The wave energy converter of claim 15, wherein the rigid core comprises a trefoil frame arrangement including three longitudinal structures connected to one another by truss members, and wherein the inflatable structures can be stowed between respective pairs of the longitudinal structures.

18. The wave energy converter of any of claims 15 to 17, wherein each of the pair of inflatable structures is compartmentalised to control the structures during inflation and deflation.

19. The wave energy converter of any of claims 2 to 5, comprising a pair of inflatable structures and further comprising: a respective pair of rigid casings, where each casing has a first end connected to one of the pair of inflatable structures and a second end articulatably connected to the core; and an elastic member connected between the pair of inflatable members to control the fabric of the inflatable structures when moving between inflated and deflated states.

20. A method of operating a wave energy converter having a hull volume, the method comprising the step of varying the hull volume on command in response to prevailing wave conditions.

21. The method of claim 20, wherein the step of varying the hull volume comprises inflating or deflating inflatable structures located around a rigid core.

22. The method of claim 21, further comprising the step of automatically stowing the inflatable structures on command in response to wave conditions.

23. The method of claim 21 or claim 22, wherein the inflatable structures comprise a plurality of axially aligned compartments arranged in an annular structure about the core, and wherein the step of inflating the inflatable structures comprising inflating each compartment individually.

24. The method of claim 23, wherein the compartments are inflated with water and/or gas.