Patent Publication Number: US-8123579-B2

Title: Protection of apparatus for capturing wave energy

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to apparatus for converting energy present in surface waves on bodies of water to useful energy, and particularly to means for protecting such apparatus from storm induced surface turbulence by either raising the apparatus above or sinking it below the water surface. 
     Wave energy converters, referred to hereinafter as WECs, are known and described, for example, in co-pending application Ser. No. 10/762,800, filed Jan. 22, 2004, the subject matter of which is incorporated herein by reference. In the co-pending application, there are described two floats, one having an annular or tire-like configuration and floating in generally horizontal orientation. The other float is elongated (referred to hereinafter as a spar) and floats in vertical orientation inside the central opening of the annular float. Both floats bob up-and-down in response to passing surface waves, but generally in an out-of-phase relationship. When the annular float, for example, is rising, the spar generally tends to be sinking. The relative movements between the two floats are used for driving an energy converter, such as a linear electrical generator, for generating useful energy. 
     A problem associated with the use of a WEC disposed near or on the surface of a body of water is the danger that excessively large waves can cause damage to the WEC. A known practice for protecting a WEC in storm conditions is to sink it to a depth below the surface zone of turbulence. While such deliberate sinking of the WEC can be done by flooding a ballast tank, as in a submarine, this requires elaborate and expensive apparatus including a source of pressurized air for blowing the flooded tanks. 
     Another technique for sinking a WEC comprises winding an anchoring cable of the WEC around a motor driven drum on the floor of the water body and forcibly dragging the WEC to a safe depth. A problem here, however, is that for highest energy generating efficiency, the WEC preferably has substantial reserve buoyancy (i.e., is subject to a substantial buoyant force when the instantaneous water surface is elevated relative to the calm condition waterline of the WEC). But the greater the reserve buoyancy of the WEC, the greater is the force required not only to sink the WEC but for controlling its rate of ascent when the WEC is resurfaced. The greater the sinking and elevating forces, the larger must be the overall system including an anchor of sufficient strength for withstanding the applied forces, and the more complex must be the mechanisms to hold the WEC in and release the WEC from a submerged state. 
     An alternative practice for protecting a WEC, usable in situations where the WEC is suspended from a support structure, for example, an ocean platform, is to pull the WEC upwardly out of the zone of influence of the waves. There is a problem in this approach which is analogous to the problem of submerging the WEC: for the WEC to be efficient, it has to displace a substantial weight of water, because this displaced weight is approximately equal to the maximum force experienced by the WEC when the instantaneous water surface drops below the calm condition waterline. The substantial weight required for efficient wave energy conversion however, poses onerous requirements on the mechanisms required to pull the WEC upwardly out of the water and to eventually release the WEC in a controlled manner. 
     The present invention is directed to means for reducing the amount of force required for moving a WEC from its normal surface floating position to a position of safety. 
     SUMMARY OF THE INVENTION 
     A normally highly buoyant float for use in a WEC comprises two vertically stacked components. A first of the components is of fixed buoyancy and the second component comprises a hollow vessel having an outer wall including a number of holes there through admitting flow of water into and out of the vessel. 
     In the instance where the WEC is to be pulled beneath the water surface for storm protection, the apertured component is the upper of the stacked components. As the apertured component is pulled beneath the water surface, it begins to fill with water thereby increasing its weight and reducing the amount of force required to sink it. However, even when the upper vessel is completely filled with water, the buoyancy of the lower vessel is sufficiently high that the WEC remains slightly buoyant. This allows the WEC to automatically resurface when the submerging force is removed. When resurfaced, and under safe operating conditions, the water in the upper vessel gradually drains through the wall openings for returning the WEC to high buoyancy. 
     In the instance where the WEC is to be lifted out of the water for storm protection, the apertured compartment is the lower of the two stacked components and, during normal energy producing usage, is fully submerged and completely full of water. Buoyancy for the WEC is provided by the upper component. As the WEC is pulled upwardly out of the water, the water within the apertured component drains outwardly through the wall openings thus decreasing the weight of the WEC and reducing the amount of force required to raise it. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings are schematic and not to scale. 
         FIG. 1  is a side view of a WEC in accordance with this invention floating on a flat surface of a body of water; the WEC being tethered to an anchor assembly on the water body floor for, when necessary, pulling the WEC beneath the water surface; 
         FIG. 2  is a cross-sectional view of the WEC shown in  FIG. 1  and shows water contained within a two-component float of the WEC, the upper of the two components having holes through an outer wall thereof; 
         FIG. 3  is similar to  FIG. 1  but shows the WEC floating within a wave trough; 
         FIG. 4  is a view similar to  FIG. 1  but showing a WEC tethered to an above-water structure for pulling the WEC upwardly out of the water; 
         FIG. 5  is a view in perspective showing a float, similar to that shown in  FIGS. 1 and 2 , but including baffles within the float for reducing sloshing movements of water contained within the float; 
         FIGS. 6 and 7  are plan views of floats similar to that shown in  FIG. 2  but including small tubes for distributing water between internal compartments of the float; 
         FIG. 8  is a cross-sectional view taken along line  8 - 8  in  FIG. 2 ; 
         FIG. 9  is a view of a surface float similar to the surface float shown in  FIG. 1  but identifying certain parameters relevant to the flow of water inwardly and outwardly of the float; 
         FIGS. 9A-9F  are views similar to that of  FIG. 9  but identifying the direction of water flow into or out of the surface float as a function of instantaneous wave amplitude; and 
         FIG. 10  is a graph showing the approximate relationship of amplitude versus time (a sine wave) of a surface wave and identifies, by letter, certain wave amplitudes discussed in the specification. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
       FIGS. 1 and 2  show an illustrative WEC  10  in accordance with the present invention. The WEC includes two floats  12  and  14 . The float  12  comprises two secured together annular members  18  and  20 , and the float  14  (spar) comprises a single elongated member extending through the central opening of the two member (composite) float  12 . The lower end  24  of the spar  14  is weighted to maintain the spar in vertical orientation. In this embodiment, the spar  14  is a closed cylinder having fixed buoyancy. The spar  14  can be hollow or at least partially filled with a ballasting material, for example, water. 
     As previously described, WECs are typically protected against storm damage either by being lifted above the water surface or by being sunk below the surface. The WEC  10  shown in  FIGS. 1 and 2  is of the type designed for protection by sinking and, to this end, the lower member  18  of the composite float  12  is of fixed buoyancy and can be hollow or at least partially filled with water. The upper member  20  of the composite float comprises a normally hollow vessel defined by inner  24  and outer  26  walls and a bottom wall  28 . In  FIGS. 1 and 2 , the upper end  30  of the vessel  20 , which is optionally open or closed, is open. Also, the outer vessel wall  26  includes a plurality of holes  40  therethrough. The diameters of the holes are sufficiently small for allowing only a relatively small amount of water flow into and out of the vessel during the passage of single waves past the WEC. The purpose of the holes is described hereinafter. 
     The WEC is anchored in place by an anchor cable  46  which extends, first, to an auxiliary buoy  47  for supporting the weight of the cable  46 , and then to an anchor assembly  48  on the floor of the water body. (Although not shown herein, the anchor cable  46  preferably extends, along the water surface, from the WEC  10  to an auxiliary buoy which supports the weight of the cable between the water surface and the anchor assembly.) As shown schematically, the cable  46  is wrapped around a drum  50  rotatable in either direction by a motor  52 . The anchor assembly  48  can be embedded in the water body floor or, more simply, is of sufficient weight for remaining stationary against the lifting forces from the WEC. 
     To the extent described, and ignoring the holes  40  in the wall of the vessel  20 , the vessel  20  is simply a part of the float  12  contributing to the buoyancy of the WEC. The buoyancy of the float  12  is such that, when the float is floating on a perfectly flat surface of a body of water, the intercept of the water surface with the float is along a line  44  slightly below the interface  46  between the upper  20  and lower  18  members of the float  12 . As cresting waves pass the float  12 , the rising water level increases the volume of water displaced by the float for increasing the buoyancy of the float for lifting it against the load provided by the energy converter (not shown) connected between the two floats  12  and  14 . 
     The holes  40  through the vessel  20  walls allow entry of water into the vessel. The purpose of the holes  40  is now described. 
     As shown in  FIG. 2 , a cresting wave tends to rise upwardly along the float and to overlap the holes  40  through the vessel wall  26 . How high the wave crest rises along the wall  26  is a function of the wave amplitude and the rate at which the float  12  rises with the cresting waves. Each wave crest, as shown in  FIG. 3 , is followed by a wave trough during which the water surface is below the vessel  20  and below the holes  40 . Water from the passing waves thus flows into the vessel  20  during the wave crests and drains from the vessel during the wave troughs. As noted, the holes  40  are of a relatively small diameter, and taking into account the wave period and the duration of each wave crest, the maximum flow of water into the vessel  20  during the passing of each wave crest is relatively small. While the water is within the vessel, and until the water drains there from, the weight of the vessel is increased and its buoyancy decreased. Under normal wave conditions, the maximum buoyancy decrease is relatively small and with little affect on energy production. 
     However, under storm conditions when it is desired to submerge the WEC for safety purposes, the motor  52  ( FIG. 1 ) is activated to begin winding the WEC anchor cable  46  onto the drum  50 . As the volume of the WEC being pulled beneath the water surface increases, the force required to sink the WEC also increases. However, once vessel holes  40  sink beneath the water surface, water flows into the vessel  26  without subsequent draining, as with passing wave crests, and the weight of the water within the vessel decreases the force necessary to further submerge the WEC. The overall buoyancy of the WEC remains positive even as the vessel  26  completely fills with water. Accordingly, some force must be applied to completely submerge the WEC. However, the total force required to sink the WEC is considerably reduced in comparison with the sinking force required absent the holes. 
     Specifically, if the vessel  26  contained no through holes  40 , the force required to completely submerge the WEC is equal to the weight of water corresponding to the volume of the WEC between the flat surface intercept line  44  ( FIG. 1 ) and the upper end  30  of the vessel  26 . Such volume is the amount of water to be displaced for completely submerging the float  12  from its normal floating depth. With the holes  40 , and allowing the vessel  26  to fill completely with water during the sinking process, the force required to submerge the WEC is reduced to being equal only to the weight of water corresponding to the volume of the WEC between the water intercept line  44  and the interface  46  between the two members  18  and  20 . Such force reduction is because the weight of the water filling the vessel  26  provides the force necessary to sink that volume of the float  12  corresponding to the volume of the water filled vessel  26 . 
     As noted, the buoyancy of the WEC is such that even with the vessel  20  completely filled with water, positive buoyancy remains. Thus, when the storm conditions have abated and it is safe to resurface the WEC, the cable  46  is unwound from the drum  50  to allow the buoyant WEC to float to the surface. The WEC positive buoyancy is sufficiently high that an upper portion of the water filled vessel  20 , including some through holes  40 , extends above the water surface. Draining of the vessel through the holes then begins and continues until normal buoyancy of the WEC is reached. 
     Another advantage of filling the submerged vessel  20  with water is that, during the re-surfacing of the WEC, its buoyancy remains reduced thereby reducing the risk of the WEC escaping from its anchoring restraint and racing at an uncontrolled and dangerous speed to the surface. 
     As shown in  FIG. 2 , the upper end  30  of the float  12  is open. An advantage of this is that, during approaching storm conditions, once the wave crests become so high as to reach over the top end of the float, the vessel  20  immediately fills with water for immediately reducing the WEC buoyancy. Accordingly, even prior to protectively submerging the WEC, the decreased buoyancy WEC is less responsive to wave action and less likely to be damaged by waves of excessive amplitude. Also, less force is required to submerge the WEC. 
     A disadvantage of an open top end is that complete filling of the vessel  20  can occur even under safe operating conditions in response to the passage of a random wave crest of extra high amplitude. While the WEC would not sink, decreased efficiency operation results until the water drains from the vessel. 
     A compromise arrangement is to close the upper end  30  of the vessel  20 , but to provide larger diameter holes  40  through the vessel wall  26  towards the upper end  30 . Thus, as the wave amplitudes begin to build in response to an approaching storm, the rate of water flow into and out of the vessel  20  increases in proportion to the increased wave amplitudes. But, if only an occasional large amplitude wave completely enveloping the vessel  20  arrives during otherwise normal conditions, the closed upper end  30  of the vessel  20  prevents complete filling of the vessel  20 , and less time is required for draining the extra water from the vessel. 
       FIG. 4  is a view of a WEC  70  designed for protection against storm damage by being lifted upwardly out of the water by means of a cable  72  attached, for example, to a motor-driven pulley  74  mounted on an above-surface structure, for example, an ocean platform  76  (indicated only schematically). 
     In this embodiment, the WEC  100  is similar to the WEC  10  shown in  FIGS. 1-3  in that it comprises an elongated spar float  78  extending through a central aperture of an annular float  80  comprising two secured together annular members  82  and  84 . The two members are similar to the two members  18  and  20  shown in  FIG. 1  in that the member  82  is a closed container while the member  84  includes a plurality of openings  40  through the outer wall thereof. A difference between the float  12  shown in  FIG. 1  and the float  80  shown in  FIG. 4 , however, is that in  FIG. 4  the apertured member  80  is disposed below the closed member  82 . 
     In normal, energy producing usage, the lower, apertured member  80  is completely submerged and full of water. Buoyancy for the WEC is provided by the upper, closed member  82 . 
     Under approaching storm conditions, the WEC  70  is lifted upwardly out of the water by known means, such as above-described. As the apertured member  80  is lifted out of the water (whereby its weight would normally increase) the water contained in the lower member  80  drains there from the member  200  through the wall openings  40 , thereby decreasing the weight of the WEC and reducing the amount of force required to lift it. 
     As described, a feature of the invention is that the WEC&#39;s include hollow vessels intended, under certain circumstances, to be partially or completely filled with water. A problem, however, is that when water is introduced into a compartment in any non-fixed maritime structure, tilting motions of the structure in response to wave action can induce rapid motions of the water, or “sloshing”. This sloshing can have a detrimental effect on stability and can impede desired dynamic behavior. Additionally, the water, if unrestrained, flows to the lower side of the compartment in response to the tilting motions of the structure. This tends to enhance the tilting movements and further jeopardize structural stability. 
     A known solution in similar situations is the use of impervious vertical walls or barriers within liquid containing compartments to stop internal water flows. However, this solution is inadequate in conjunction with WECs used in accordance with the present invention because wave conditions may exist which cause water to flow preferentially into one of the compartments, accumulate therein in excess of the mass of water in other compartments, and thus accentuate tilting of the structure. 
     In accordance with this invention, porous baffles are disposed within a WEC float sub-dividing the float interior into multiple compartments. The compartments are individually small enough to minimize sloshing effects, but are interconnected such that uniform distribution of the water among the compartments occurs regardless of any particular direction of arrival of surface waves. 
     In  FIG. 5 , for example, four plates  90  are disposed, in vertical orientation, within the interior of the upper compartment  20  of a float identical to the float  10  shown in  FIGS. 1 and 2 . The plates  90  sub-divide the float interior space into four separate compartments  92 ,  94 ,  96  and  98 , each isolated from the others to the extent that sloshing movements in one compartment are substantially isolated from, and do not contribute towards sloshing movements in other compartments. However, while the plates inhibit free flow of water between compartments, the plates are pervious, e.g., by including a pattern of small openings  90  there through, to allow water flow between compartments for obtaining uniform distribution of the water over time. 
     In an alternative arrangement, the compartment forming plates are impervious to water, but each compartment is connected to a spaced apart compartment via a tube through which water can flow in moderate volume for obtaining uniform distribution of the water. In  FIG. 5 , for example, two spaced apart compartments  104  and  108  are interconnected by a tube  116   a  which passes through compartment  110 . Likewise, compartment  106  is connected to compartment  110  via a tube  116   b  which passes through compartment  108 . 
     This concept can be applied to any symmetrical disposition of compartments. If there are eight compartments, such as shown in  FIG. 7 , for example, labeled  1 A,  2 A, . . .  8 A, then compartment  1 A can be connected to compartments  3 A,  5 A and  7 A by respective tubes  116   c, d  and  e . Likewise, compartment  2 A can be connected to compartments  4 A,  6 A and  8 A. 
     In the embodiment of the invention shown in  FIGS. 1-3 , the apertured member  20  floats above the water surface. Still, during normal use, some water is always present in the member  20 . This occurs because water flows in when a wave rises, and flows out when the wave crest recedes. In most practical applications of the invention, some equilibrium will be reached in steady waves with a relatively constant amount of water in the upper chamber. It is desirable to have this amount of water be minimal, since the presence of this water does not benefit the wave energy conversion process. A preferred way to minimize the amount of water inside the upper chamber in operational wave conditions is by providing at least some of the wall holes with valves so that fluid flow is preferentially outward. Thus, it would be possible to arrange, say, a ratio of 5 valves which only allow outward flow to 1 hole which allows bi-directional flow. This assures that almost all water which comes in during a wave crest flows out during the subsequent wave trough. 
       FIG. 8  shows an example of one of numerous types of known valves that can provide directional flow. Shown in the drawing is a hole  40  through an outer wall  26  of a float  12  such as shown in  FIG. 2 . The interior of the float is to the left of the wall segment shown. Disposed within the hole is a ball  130  which is movable in either direction in response to water flow through the hole  40 . When water is flowing out of the float, i.e. from left to right, the ball is moved into contact with a mesh  132  overlying the hole which, while blocking escape of the ball, allows flow of water past the ball and through the mesh. Conversely, when water tends to flow through the hole  40  from right to left, the ball moves into sealing engagement with a gasket  134  for sealing an opening  136  through the gasket. 
     Other, suitable valves are known. 
     Now described is a method of determining the amount of water in the apertured upper vessel  20  of the float  12  shown in  FIG. 1 . For ease of illustration, the float  12  is shown in  FIG. 9  on a slightly larger scale than that of  FIG. 1 . As previously described, the float  12  comprises two components  18  and  20  in vertically stacked relationship. The interface between the two components is identified by the reference numeral  46 . A schematic of the drawing is shown in  FIG. 9 . Quantities displayed include:
     y Vertical displacement of device from mean waterline   n Vertical elevation of water surface from mean waterline   h Height from waterline to draining orifices in upper chamber   d Amount of water remaining in the upper chamber in the steady state   

     When the wave elevation n is sufficiently high that n&gt;y+h+d, then water flows into the upper chamber. Otherwise, water flows out of the upper chamber. 
     When the inflow condition occurs, the rate of inflow is proportional to the square root of the differential pressure across the valves, multiplied by some constant relating to the orifices. 
     For simplicity, the following assumptions are made:
         All valves are located just above the interface between the upper and lower chambers.   The incident wave is sinusoidal, with an amplitude n 0      The WEC does not move.   The amount of inflow/outflow is sufficiently small with each passing wave that the height d of the water in the upper chamber is assumed to be constant.       

       FIG. 10  shows a single wave cycle, and indicates 6 points of interest labeled A, B, C, d, E, F, which correspond to distinct regimes of inflow/outflow. These points are shown in  FIGS. 9A  to F, respectively, and are described below.
     A: The wave elevation is right at the mean free surface. There is outflow, and the rate of outflow is governed by some orifice-specific constants multiplied by the square root of the pressure, which is given by pg(d).   B: The wave elevation is at the interface between upper and lower chambers. There is outflow, and constants multiplied by the square root of the pressure, which is given by pg(d).   C: The wave elevation is less than h above the interface between upper and lower chambers. There is outflow, and the rate of outflow is governed by some orifice specific constants multiplied by the square root of the pressure, which is given by pg(n-h).   D: The wave elevation is at the same height as the surface of the water inside the upper chamber. There is no net flow into or out of the upper chamber.   E: The wave elevation is at a greater height than the surface of the water inside the upper chamber. There is a net flow into the chamber. The rate of inflow is governed by some orifice specific constants multiplied by the square root of the pressure, which is given by pg(n-h-d).   F: The wave elevation is below the waterline of the WEC. There is outflow, and the rate of outflow is governed by some orifice specific constants multiplied by the square root of the pressure, which is given by pgd.   

     Analysis of this simplified case shows the following:
         1) That an equilibrium of the amount of water inside the upper chamber will be reached in typical conditions (i.e., where the wave amplitudes are greater than h, and not substantially greater than the height of the device).   2) That this equilibrium is affected by the height h of the interface between upper and lower chamber.   3) That it is desirable to have a different set of orifice-specific constants governing inflow and outflow.       1—Equilibrium is reached. Consider  FIG. 10 . The time where water flows out of the upper chamber is limited to the interval when the wave elevation is greater than the dotted line indicated by h+d. Suppose that the level of water is rising in the chamber with each cycle. Equilibrium will eventually be attained because the amount of water flowing in on each cycle will decrease as the duration of said interval decreases.   2—Equilibrium is affected by the height h. As height h is increased, the interval over which water flows into the upper chamber decreases in duration, which affects the equilibrium.   3—It is desirable to have a different set of orifice-specific constants governing inflow and outflow. It is desirable in practice to have the height h and the height d both be relatively small. If both are small, then the interval of time over which water is free to flow into the chamber is almost a full half-cycle. However, since the rate of inflow is proportional to the square root of the pressure differential, there will be much more water flowing in than out. Equilibrium will be reached, as described above. However, equilibrium won&#39;t be reached until the level d of water inside the upper chamber has grown relatively large. Thus, if inflow and outflow are not symmetric, it is possible to design the flow rates so that the equilibrium levels have desired properties.