Abstract:
A reciprocating fluid-energy device to be used in a flowing fluid, having a foil, an upstream support member, a downstream support member, and a frame, where the frame retains the device in place within the flowing fluid, the support members movably attach the foil to the frame, and the foil moves laterally back and forth across the direction of the flow of the fluid, with changes in direction of the movement of the foil occurring automatically and spontaneously without external intervention other than the force of the flowing fluid.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates generally to power producing devices and, more particularly, to a low-speed, low-pressure fluid-energy device that captures kinetic energy from the movement of fluid by the use of one or more reciprocating foils. 
     2. Description of Prior Art 
     Humans have long derived energy from flowing wind or water. The first great human machines were reciprocating air foils or what are commonly known as sailing ships. Water wheels and windmills represent some of the first machines deriving energy from non-animals sources. Hydro-power became a mature technology in the 1900s. Modern hydro-power has relied on high pressure/high head systems often associated with a dam or a reservoir. This technology has been limited to locations where there was a significant vertical drop for the water. These systems cause significant alteration of the landscape and the eco-system. These factors limit the potential growth of conventional hydro-power. 
     Wind power matured into a significant industry more recently. Modern wind power is mechanically different from hydro-power in that the moving fluid is low pressure and free flowing. The advancing technology of ever-larger turbines has made this economically viable but it is still limited to locations where there is sufficient average wind speed. 
     Low-pressure water power or hydrokinetic energy has been largely untapped. Many devices have been tested but they remain economically competitive only where the price of energy is extremely high. Hydrokinetic devices using tides and currents have more reliable and predictable sources of energy than wind power. The energy density of moving water is much higher than moving air, requiring smaller devices. The technology that turns slow moving water into commercial scale power generation will potentially tap one of the largest available sources of mechanical energy on the surface of the earth. 
     Fluid-energy machines operate by the same physical forces whether the fluid is a gas or liquid, high or low velocity, high or low pressure, high or low density. They incorporate some type of structure which is oriented at an angle of attack relative to the direction of flow of the fluid. Such structure may be a symmetrical foil, an asymmetrical foil, an airfoil, a hydrofoil, a turbine blade, a rotor, a sail, or the like. For simplicity and clarity, all such structures shall be denoted a “foil”. The foil has a leading edge and a trailing edge. The angle of attack is the angle formed between the chord line of the foil and the direction of flow of the fluid. 
     Energy transfer occurs either by drag when the foil and the fluid are moving in the same direction or by lift when the foil moves perpendicularly across the flow of moving fluid. Fluid-energy devices can be reciprocating or rotary. Most rotating machines have continuous power transfer while reciprocating machines can only transfer energy during part of their cycles. 
     A wind turbine or propeller has an axis of rotation that is collinear with the flow of the fluid. This provides great mechanical simplicity and continuous energy transfer. One important characteristic of this design is that the linear speed of any point on the blade is proportional to the distance from the axis. Commercial wind turbines have adapted to this relationship with large turbines using long contoured blades and high tips speeds (typically 6 to 7 times the wind speed) that remain efficient given sufficient wind speed. 
     In liquids the length of a rotating blade is limited by this blade speed relationship. Marine propulsion systems increase the width of the blade and its surface area in order to limit blade length. This results in significant turbulence and inefficiency. 
     To efficiently transfer energy from a slow moving fluid, a large slow moving foil is needed. A consistent relationship between the speed of the foil and the speed of the moving fluid minimizes turbulence and increases efficiency. This cannot be achieved with an axial turbine where blade speed varies by radius. 
     It is thus shown that there is a need for a device that can harness energy from a low pressure, low speed moving fluid. 
     It is therefore an objective of the present invention to provide a reciprocating fluid-energy device that can be used with any moving fluid. 
     It is a further objective of the present invention to provide a reciprocating fluid-energy device that extracts power from slow moving fluids. 
     It is yet a further objective of the present invention to provide a reciprocating fluid-energy device that extracts power from low pressure fluids. 
     It is yet a further objective of the present invention to provide a reciprocating fluid-energy device that is simple in design. 
     It is yet a further objective of the present invention to provide a reciprocating fluid-energy device that is efficient. 
     It is yet a further objective of the present invention to provide a reciprocating fluid-energy device that automatically and spontaneously reciprocates using only the force of the moving fluid. 
     It is yet a further objective of the present invention to provide a reciprocating fluid-energy device that is environmentally friendly. 
     Other features and attendant advantages of the present invention will become obvious to the reader and become fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings. It is intended that these objects and advantages are within the scope of the present invention. To the accomplishment of the above and related objects, this invention may be embodied in the form illustrated in the accompanying drawings. Attention being called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated and described within the scope of this application. 
     SUMMARY 
     The present invention embodies a device applicable to all fluids, liquids as well as gases. In one embodiment of the present invention energy transfer occurs by using the lifting force. The device incorporates a reciprocating foil. The lift force is not continuous. Energy is not transferred by drag so all positions of the foil have equal potential energy. For illustration purposes the description will be of device deriving power from moving water, though any moving fluid may be used, such as prevailing winds. The invention consists of a foil and the structures that suspend it in the moving fluid. The structures must allow motion of the foil such that it can traverse the fluid in a reciprocating pattern. The orientation of the foil is determined solely by the possible positions of the leading edge and the trailing edge of the foil and the length of the foil. No other force or device is applied to the foil to create, maintain, or adjust the angle of attack. 
     The device is placed into a fluid flow. The axis of the device is substantially parallel to the direction of fluid flow. The device is therefore oriented relative to the “upstream” and “downstream” aspects of the fluid flow. (This could be literally a stream, or a river, or an ocean current, or the like.) The leading edge of the foil is oriented in the upstream direction, while the trailing edge is oriented in the downstream direction. A frame keeps the device positioned in place in the fluid flow. One portion of the frame is located upstream and another portion of the frame is located downstream. An upstream support member movably connects the leading edge of the foil to the upstream portion of the frame, and a downstream support member movably connects the trailing edge of the foil to the downstream portion of the frame. The foil moves laterally across the fluid flow, perpendicular to the direction of the fluid flow, with the support members pivoting laterally as the foil moves. The upstream support member and the downstream support member have a fixed length, as does the foil. The combined lengths of the upstream support member, the downstream support member, and the foil is greater than the distance along the device axis from the upstream portion of the frame to the downstream portion of the frame. In addition, the combined lengths of the upstream support member and the downstream support member is less than the distance along the device axis from the upstream portion of the frame to the downstream portion of the frame. Because of these length relationships, the relative angles between the foil and the support members continuously change as the foil moves laterally across the fluid flow. Moreover, the upstream support member, the foil, and the downstream support member can never be all three simultaneously collinear with each other, thus creating an inherently unstable configuration that \continuously and automatically resets its orientation relative to the fluid flow. 
     Movement of the foil or other moving components of the device is then harnessed, either to translate that movement into direct work (such as operating a pump or a mill) or to convert that movement into electricity (for example, by powering a generator). 
     Automatic reciprocating movement of the foil occurs as follows: 
     When the angle of attack of the foil is greater than zero and oriented to the right of the device axis (that is, the leading edge is oriented to the right of the trailing edge relative to a central longitudinal axis running parallel to the direction of fluid flow), the foil will move to the right due to lift forces. When the angle of attack is greater than zero and oriented to the left of the device axis, the foil will move to the left due to lift forces. When the angle of attack is zero (that is, the leading edge and the trailing edge are parallel to the device axis), lift forces cease acting on the foil and only drag forces act on the foil, biasing the foil in a downstream direction. 
     Because the upstream end of the upstream support member is pivotally connected to the frame of the device at a fixed location, the upstream support member pivots to the left and right as the foil moves laterally, with the end of the upstream support member connected to the leading edge of the foil tracing an arc  72 . Similarly, the downstream support member pivots to the left and right from its connection point with the frame as the foil moves laterally, with its end connected to the trailing edge of the foil also tracing an arc  74 . The two arcs  72 , 74  traced by the upstream and downstream support members are mirror images of each other, and are closest to each other at the midpoint of the lateral traverse of the foil. As the foil moves laterally across the fluid flow, the leading and trailing edges of the foil follow their respective arcs  72 , 74 . 
     As the foil moves rightward, towards the midpoint of its traverse, the leading edge of the foil follows its arc in a downstream direction and the trailing edge of the foil follows its arc in an upstream direction. See  FIG. 2A . This “flattens” the orientation of the foil (tending it towards an orientation perpendicular of the direction of fluid flow), increasing its angle of attack. Once the leading edge of the foil moves to the right of the midpoint of its traverse it moves in an upstream direction, following its arc. See  FIG. 2B . Similarly, once the trailing edge of the foil is to the right of the midpoint of its traverse it moves in a downstream direction, following its arc. Once the entire foil is beyond the midpoint of its traverse, where the leading edge of the foil moves in an upstream direction while the trailing edge of the foil moves in a downstream direction, the foil begins to “straighten up” (tending towards an orientation parallel to the direction of fluid flow), reducing the angle of attack towards zero. 
     However, before the angle of attach reaches zero, the foil become collinearly aligned with the downstream support member. See  FIG. 2C . This represents the furthest rightward extent of the leading edge of the foil—the leading edge of the foil cannot move further to the right nor further upstream along its arc. Since there is still a positive angle of attack, though, the foil continues its overall rightward movement. The trailing edge of the foil continues moving to the right, but the leading edge of the foil is pulled downstream along its arc and thus reverses itself and begins moving to the left. The overall movement of the foil continues rightward—with the leading edge of the foil now moving to the left while the trailing edge of the foil continues to move to the right—until the angle of attack is zero and the foil is aligned parallel to the direction of the fluid flow. See  FIG. 2D . 
     In the absence of lift forces when the angle of attack is zero, only drag forces act on the foil, moving the foil in a downstream direction. In moving in a downstream direction, the leading edge of the foil moves to the left along its arc, while the trailing edge of the foil moves to the right along its arc. The foil is thus no longer parallel to the direction of fluid flow but rather has established a new angle of attack, in the opposite direction, and thus the overall movement of the foil begins in a leftward direction. The leading and trailing edges of the foil continue to move in opposite directions while the foil moves to the left until the upstream support member and the foil are collinearly aligned. See  FIG. 2E . This represents the furthest rightward extent of the trailing edge of the foil—it cannot move any further to the right nor any further downstream. Since there is a positive angle of attack the foil continues its overall leftward movement. The leading edge of the foil continues to move to the left, and the trailing edge of the foil is pulled upstream and thus reverses itself and begins moving to the left as well. See  FIG. 2F . 
     Now both the leading edge and the trailing edge of the foil move to the left, as does the foil. Movement to the left continues in the same manner as described above, with the foil first “flattening” and then “straightening up”, see  FIGS. 2G and 2H , until the foil reaches the leftmost extent of its traverse and the angle of attack is zero. See  FIG. 2I . The foil reverses in the same manner as described above, with first the leading edge of the foil reversing to the right and then the trailing edge of the foil reversing to the right. See  FIG. 2J . Movement of the foil continues as described above. 
     In summary, the lift force acting on the foil moves the foil across the midline and to the lateral extent of travel (either to right or left) where the angle of attack is reduced to zero degrees and the lift force ceases. In this position the drag/drift force dominates and the downstream motion will always move the leading edge back toward the midline and the trailing edge away from the midline, creating a new angle of attack that is always opposite in direction to the prior angle of attack. This reversal of direction of movement of first the leading edge of the foil and then the trailing edge of the foil, as described above, occurs automatically and spontaneously because of the force of the moving fluid and the relationship of the positions of the upstream and downstream support members to the foil. The foil reciprocates between the two extreme positions where the angle of attack of the foil is zero. This feature is unique in the art. Other devices using reciprocating movement of a foil either require an external mechanism to reorient the foil to allow for movement in the opposite direction, such as a motor or an actuator, or require the foil to strike a surface, with the impact reorienting the foil. Neither of these prior art methods are as simple and efficient as reversal mechanism of the present invention. 
     The present intervention, in its simplest embodiment, requires only 3 moving parts—the foil, the upstream support member, and the downstream support member. These moving parts are connected to the frame at a minimum of two points. The simplest of the connections is no more complex than a pivot around a single axis. This configuration is far simpler than other known devices. 
     The geometry of the invention requires that each of the moving components freely articulates at all of its connections to other components. This means that all forces on the moving parts are either tensile or compression. There is no bending or sheer force except that exerted directly on the foil by the moving fluid. At the end of each lateral traversal the foil (and the upstream and downstream support members) is decelerated—as the angle of attack goes to zero the lift forces decrease and movement slows—and then after reversal the foil (and upstream and downstream support members) is accelerated in the opposite direction. There are no solid or mechanical endpoints to the motion. Shock forces are thereby minimized. 
     Because the device of the present invention may be symmetric in structure, motion, and function, as long as the axis of the device is aligned with the direction of the fluid flow the foil will reciprocate and transfer energy even if the direction of the fluid flow is reversed. This is particularly useful for implementation in tidal regions, where the tidal flow reverses twice a day. 
     Devices with high velocity foils create tremendous turbulence and inefficiency when interfacing with slow moving fluids. Efficiency in this setting requires a large surface area foil moving at a speed not significantly different than the moving fluid itself. The present invention provides just such a low velocity/high surface area solution for settings such as hydro-kinetic power or wind energy. 
     Previous fluid-energy devices have relied either on high velocity fluid such as wind turbines or high pressure fluid such as hydro-electric power. There can be significant kinetic energy in the movement of slow moving fluids if the fluid is of significant volume or density. The current known art has not been able to use this source of power on a commercial scale. The present invention provides simplicity and structural reliability that would allow it to be built on a scale that could be economically successful with low velocity fluids. Moreover, a slower moving foil, even one large enough to transfer economically significant quantities of energy, would cause less environmental disruption than a high velocity foil or a high pressure design. This is most easily appreciated when compared to current hydro-electric systems. The present invention could be placed at multiple locations along a river without the need for a precipitous drop. The invention would require no dam and no reservoir and would have minimal effect on fish. 
     Other features and advantages of the invention are described below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan top view of one embodiment of the invention. 
         FIGS. 2A through 2J  demonstrate the automatic reciprocating movement of the foil, as follows: 
         FIG. 2A  shows the foil moving laterally to the right due to lift forces acting on the foil; both the leading edge of the foil and the trailing edge of the foil are moving to the right, and both the upstream support member and the downstream support member are pivoting to the right (the upstream support member in a counterclockwise rotation and the downstream support member in a clockwise rotation). In this arbitrary starting position, the upstream support member is oriented substantially parallel to the direction of fluid flow. 
         FIG. 2B  shows the foil continuing to move to the right. Both the upstream support member and the downstream support member continue to pivot to the right. The downstream support member is now oriented substantially parallel to the direction of fluid flow. 
         FIG. 2C  shows the foil continuing to move to the right. In this position, the foil and the downstream support member are in alignment and the leading edge of the foil is at its rightmost position. The leading edge of the foil cannot travel any further to the right. Therefore, the leading edge of the foil stops moving, while the trailing edge of the foil continues to move to the right. The leading edge then reverses itself and begins to move to the left as the foil continues to have an overall movement to the right to arrive at the orientation shown in  FIG. 2D . 
         FIG. 2D  shows the foil in the neutral position; the foil is now oriented substantially parallel to the direction of fluid flow. The leading edge of the foil has moved leftward, while the trailing edge of the foil continues to move to the right. In this orientation no lift forces act on the foil. Drag/drift forces parallel to the fluid flow now act on the foil, biasing it in a downstream direction. 
         FIG. 2E  shows the foil beginning its movement to the left; the leading edge of the foil continues to move to the left, while the trailing edge of the foil stops moving. In this position, the foil and the upstream support member are in alignment and the trailing edge of the foil is at its rightmost position, such that the trailing edge of the foil cannot travel any further to the right. The trailing edge therefore reverses itself and begins to move to the left as the foil moves to the orientation shown in  FIG. 2F . The transition from the orientation of the foil shown in  FIG. 2D  to the orientation of the foil shown in  FIG. 2E  demonstrations the reciprocation of the foil; note that the foil reciprocates automatically, without resort to mechanical reorienting or the foil rebounding off a surface. 
         FIG. 2F  shows the foil moving laterally to the left; both the leading edge of the foil and the trailing edge of the foil are moving to the left, and both the upstream support member and the downstream support member are pivoting to the left (the upstream support member in a clockwise rotation and the downstream support member in a counterclockwise rotation). 
         FIG. 2G  shows the foil continuing to move to the left. Both the upstream support member and the downstream support member continue to pivot to the left. 
         FIG. 2H  shows the foil continuing to move to the left; the leading edge of the foil stops moving, while the trailing edge of the foil continues to move to the left. In this position, the foil and the downstream support member are in alignment and the leading edge of the foil is at its leftmost position. The leading edge of the foil cannot travel any further to the left. The leading edge therefore reverses itself and begins to move to the right as the foil moves to the orientation shown in  FIG. 2I . 
         FIG. 2I  shows the foil in the neutral position; the leading edge of the foil has moved rightward, while the trailing edge of the foil continues to move to the left. In this orientation no lift forces act on the foil. Drag/drift forces parallel to the fluid flow now act on the foil, biasing it in a downstream direction. 
         FIG. 2J  shows the foil beginning its movement to the right; the leading edge of the foil moves to the right, while the trailing edge of the foil stops moving. In this position, the foil and the upstream support member are in alignment and the trailing edge of the foil is at its leftmost position and the trailing edge of the foil cannot travel any further to the left. The trailing edge of the foil therefore reverses itself and begins to move to the right as the foil moves to the orientation shown in  FIG. 2A . The transition from the orientation of the foil shown in  FIG. 2I  to the orientation of the foil shown in  FIG. 2J  demonstrations the reciprocation of the foil in the opposite direction. As should be made clear from the foregoing, the movement of the foil shown in Figures F through J is the mirror image of the movement of the foil shown in Figures A through E. As long as the fluid flow continues, the foil will continue to reciprocate. 
         FIG. 3  is a plan side view of an embodiment of the invention depicting multiple upstream support members and multiple downstream support members. 
         FIG. 4  is a top plan view of an embodiment of the invention depicting multiple foils, a pair of upstream support members, and a pair of downstream support members. 
         FIG. 5  is a top plan view of an embodiment of the invention wherein the foil is a boat and the upstream support member is a chain. 
         FIG. 6A  is a top plan view of an embodiment of the foil, the upstream support member, the downstream support member, and the pivots. 
         FIG. 6B  is an exploded view of the embodiment of the invention shown in  FIG. 6A . 
         FIG. 7  is a top view of an embodiment of the invention wherein the frame components are stretched across a channeled body of water and anchored to the banks of the channel, and a force transfer mechanism is connected to the foil at its first end and to a generator at its second end. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one embodiment, the present invention discloses a reciprocating device  10  for extracting energy from a fluid flow  20 . The fluid flow  20  may be water, air, or any other fluid. The device  10  comprises a foil  100 , an upstream support member  200 , a downstream support member  300 , and a frame  400 . Optionally, it also comprises a force transfer mechanism  500  and a generator  700 . 
     A foil creates lift when fluid passes over its surfaces. The leading edge of the foil splits the fluid, with one component of the split fluid running along one surface of the foil and a second component of the split fluid running along the opposite surface of the foil. When the foil is angled relative to the direction of fluid flow (the “angle of attack”), the component of the fluid passing over the surface of the foil angled away from the direction of fluid flow moves faster than the component of the fluid passing over the surface of the foil angled towards the direction of fluid flow. Because of the Bernoulli Effect, which provides that faster moving fluid exerts less pressure than slower moving fluid, the net pressure differential creates “lift” in the direction of the angle of attack, perpendicular to the direction of fluid flow. 
     The foil  100  of the present invention must be configured to be capable of being oriented at an angle of attack  30  relative to the fluid flow  20 . The foil  100  has a leading edge  112  and a trailing edge  114 . The leading edge  112  of the foil  100  is oriented in the upstream direction of the fluid flow  20  and the trailing edge  114  of the foil  100  is oriented in the downstream direction of the fluid flow  20 . The surfaces of the foil  100  may be curved from the leading edge  112  to the trailing edge  114 , forming a convex profile. The foil  100  may be asymmetric, whereby its profile at its leading edge  112  differs from its profile at its trailing edge  114 , for example, having an elongate teardrop shape. Such a configuration is appropriate where the fluid flow  20  is unidirectional, for example, the current of a stream or river. The foil  100  may also be configured symmetrically, whereby its profile at its leading edge  112  is substantially the same as its profile at its trailing edge  114 . Such a configuration is appropriate where the fluid flow  20  is bidirectional, for example, ocean  50  tidal currents. Preferably, the foil  100  has a hydrofoil configuration when used in liquid fluids and an airfoil configuration when used in gas fluids. Such configurations are well known in the art. 
     The foil  100  of the present invention also is required to have an upstream connection point  122  and a downstream connection point  124 , whereby the foil  100  is connected to the rest of the device  10 . The upstream connection point  122  of the foil  100  is located proximate to the leading edge  112  of the foil  100 , and the downstream connection point  124  of the foil  100  is located proximate to the trailing edge  114  of the foil. 
     The upstream support member  200  of the device  10  is elongate and has a first end  212  and a second end  214 . It may be either substantially rigid or substantially flexible. A flexible upstream support member  200  works because the force of the fluid flow  20  will tend to move the upstream support member  200  away from its attachment to the frame  400 , elongating it to its full length, so that the upstream support member  200  will retain its full length (obviously, a rigid upstream support member  200  retains its full length as well). An example of a flexible upstream support member  200  is one constructed of a chain  250 . An example of a rigid upstream support member  200  is one constructed of an elongate rod  240 . Other configurations are also contemplated. In any configuration the upstream support member  200  has a frame connection point  222  located proximate to the first end  212  of the upstream support member  200  and a foil connection point  224  located proximate to the second end  214  of the upstream support member  200 . The frame connection point  222  of the upstream support member  200  is in pivotal connection with the frame  400  of the device. In one embodiment an upstream pivot  612  is interposed between the frame connection point  222  of the upstream support member  200  and the frame  400  of the device. Similarly, the foil connection point  224  of the upstream support member  200  is in pivotal connection with the upstream connection point  122  of the foil  100 . In one embodiment a leading edge pivot  622  is interposed between the foil connection point  224  of the upstream support member  200  and the upstream connection point  122  of the foil  100 . 
     The downstream support member  300  of the device  10  is elongate and has a first end  312 , a second end  314 , and a length. Unlike the upstream support member  200 , though, the downstream support member  300  must be substantially rigid. This is because the force of the fluid flow  20  will tend to move a flexible downstream support member  300  towards its attachment to the frame, collapsing it, so that a flexible downstream support member  300  will not retain its full length. Only if the downstream support member  300  has sufficient rigidity to offset the force of the fluid flow  20  will it retain its full length. In one embodiment the downstream support member  300  is an elongate rod  340 . Other configurations may also be used. The downstream support member  300  has a foil connection point  324  located proximate to the first end  312  of the downstream support member  300  and a frame connection point  322  located proximate to the second end  314  of the downstream support member  300 . The frame connection point  322  of the downstream support member  300  is in pivotal connection with the frame  400  of the device. In one embodiment a downstream pivot  614  is interposed between the frame connection point  322  of the downstream support member  200  and the frame  400  of the device. The foil connection point  324  of the downstream support member  300  is in pivotal connection with the downstream connection point  124  of the foil  100 . In one embodiment a trailing edge pivot  624  is interposed between the foil connection point  324  of the downstream support member  200  and the downstream connection point  124  of the foil  100 .  FIG. 6  depicts the relationship of the foil  100 , the upstream support member  200 , and the downstream support member  300 . 
     The frame  400  of the device  10  is a structure suitably configured to be at least partially placed into the fluid flow  20  and to remain in place therein. It may be a unitary structure or a collection of multiple disconnected components. It may be substantially rigid, such as a post or a stanchion, or it may be flexible, such as a chain  440 , or a combination of both. In one embodiment the frame  400  is configured to be anchored to the ocean floor  52 . See  FIG. 3 . In another embodiment the frame  400  is configured to be anchored to a riverbed. In yet another embodiment the frame  400  is configured to span a channeled body of water  40 , such as a stream, and to be anchored on the banks  44  of the stream, with a portion of the frame  400  extending into the stream. See  FIG. 7 . The frame  400  may be anchored by simply setting it onto a surface, where its weight holds it in place; alternatively, a portion of the frame may be embedded into a surface; or both. Any other suitable configuration of the frame  400  is also contemplated, provided the frame  400  is capable of retaining the device  10  in place relative to the fluid flow  20 . 
     The frame  400  has an upstream portion  412  and a downstream portion  414 . The upstream portion  412  of the frame  400  is oriented in the upstream direction of the fluid flow  20  relative to the foil  100 , and is located upstream of the leading edge  112  of the foil  100  relative to the fluid flow  20 . The downstream portion  414  of the frame  400  is oriented in the downstream direction of the fluid flow  20  relative to the foil  100  and is located downstream of the trailing edge  114  of the foil  100  relative to the fluid flow  20 . Where the frame  400  is a multi-component structure, at least one component of the frame  400  comprises the upstream portion  412  of the frame  400  and at least one other, different component of the frame  400  comprises the downstream portion  414  of the frame  400 . 
     The frame  400  has an upstream connection point  422  located on the upstream portion  412  of the frame  400 , and a downstream connection point  424  located on the downstream portion  414  of the frame  400 . The upstream connection point  422  of the frame  400  is in pivotal connection with the frame connection point  222  of the upstream support member  200 . In one embodiment the upstream pivot  612  is interposed between the upstream connection point  422  of the frame  400  and the frame connection point  222  of the upstream support member  200 . The downstream connection point  424  of the frame  400  is in pivotal connection with the frame connection point  322  of the downstream support member  300 . In one embodiment the downstream pivot  614  is interposed between the downstream connection point  424  of the frame  400  and the frame connection point  322  of the downstream support member  300 . The frame  400  has a frame axis running from the upstream connection point  422  of the frame  400  to the downstream connection point  424  of the frame  400  in a direction substantially parallel to the direction of the fluid flow  20 . One configuration of the device  10  disclosing the foil  100 , the upstream support member  200 , the downstream support member  300 , and the frame  400  is shown in  FIG. 1 . 
     In order for the device  10  to work properly, each of the lengths of the upstream support member  200 , the foil  100 , and the downstream support member  300  must be fixed, and combined must be greater than the distance from the upstream connection point  422  of the frame  400  to the downstream connection point  424  of the frame  400  along the frame axis. Also, the combined lengths of the upstream support member  200  and the downstream support member  300  must be less than the distance from the upstream connection point  422  of the frame  400  to the downstream connection point  424  of the frame  400  along the frame axis (i.e., their ends cannot overlap each other). These relationships ensure that the upstream support member  200  and the downstream support member  300  can never be simultaneously collinear with each other, although the foil  100  may be collinear with either the upstream support member  200  or the downstream support member  300 . 
     As such, the foil  100  will always have a non-zero angle of attack  30  relative to the fluid flow  20 , allowing for lift forces to act upon it causing it to move laterally  80  relative to the direction of the fluid flow  20 , except at the far extremes of the lateral travel of the foil  100 . That is, the foil  100  has a neutral position wherein it is oriented substantially parallel to the direction of the fluid flow  20  at its leftmost and the rightmost extents of travel; at the neutral positions, the angle of attack  30  is essentially zero and there are no significant lift forces operating on the foil  100 . However, the neutral position is not stable, and drift forces pushing against the foil  100  tend to cause it to “tip” out of the neutral position, thereby creating an angle of attack  30  and the resulting lift forces to cause lateral movement  80  of the foil  100 . See  FIGS. 2D and 2I . It is the relationship of the upstream support member  200  and the downstream support member  300  to the foil  100  that causes the foil  100  to “tip” in the correct direction to cause the correct lateral movement  80 . The movement  80  of the foil  100  towards and then into a neutral position, the subsequent “tipping” of the foil  100  out of the neutral position, and the ensuing movement  80  of the foil  100  towards and then into the other neutral position represents the reciprocating movement defining the device  10 . None of this reciprocation requires a force other than the fluid flow  20  to act on the foil  100 , nor does it require the foil  100  to rebound off any object in order to reorient itself. Rather, the foil  100  pivots against the upstream support member  200  and the downstream support member  300  as it travels laterally  80 , resulting in a smooth transition of the reorientation of the foil  100 . 
     The device  10  may further comprise a force transfer mechanism  500 . The force transfer mechanism  500  is any apparatus that is suitably configured to capture at least part of the motion of the device  10  created by the fluid flow  20 , and transferring that motion to a machine to do useful work. At the least, the force transfer mechanism  500  has a first end  512  and a second end  514 , with the first end  512  in connection with at least one of the foil  100 , the upstream support member  200 , and the downstream support member  300 . The second end  514  of the force transfer mechanism  500  is in connection with the machine doing useful work. The force transfer member  500  may be substantially rigid, such as a rod, or substantially flexible, such as a belt. It may have gearing associated with it. It may translate movement in an oscillating fashion or as rotary movement. However the force transfer mechanism  500  is configured, movement of the foil  100  causes movement of the upstream support member  200  and the downstream support member  300 , thereby resulting in movement of the first end  512  of the force transfer mechanism  500 . In another embodiment, the first end  512  of the force transfer mechanism  500  may be in connection with at least one of the foil  100 , the upstream support member  200 , the downstream support member  300 , the upstream pivot  612 , the downstream pivot  614 , the leading edge pivot  622 , and the trailing edge pivot  624 . Movement of the foil  100  causes movement of the upstream support member  200 , the downstream support member  300 , the upstream pivot  612 , the downstream pivot  614 , the leading edge pivot  622 , and the trailing edge pivot  624 , thereby resulting in movement of the first end  512  of the force transfer mechanism  500 . 
     The machine to do useful work may be a pump or a grinding mill or some other such machine that directly receives the kinetic energy transferred by the force transfer mechanism  500 . In the preferred embodiment it is a generator  700 . In such embodiment the second end  514  of the force transfer mechanism  500  is in connection with the generator  700 . The generator  700  is suitably configured to convert movement of the force transfer mechanism  500  into power. Movement of the force transfer mechanism  500  therefore causes the generator  700  to generate power. Generators  700  which are capable of converting reciprocating motion into power are well known in the art and are not herewith further described. 
     As described above, and as shown in  FIGS. 1, 2A through 2J, 3, 5, and 7 , the device  10  may be oriented within the fluid flow  20  such that the lateral movement of the foil  100  is from side to side (i.e., from river bank  44  to river bank  44 ). However, there is nothing preventing the device from being oriented within the fluid flow  20  such that the lateral movement of the foil  100  is from top to bottom (i.e., from ocean surface to ocean floor  52 ). The lateral movement of the foil  100  could also be a diagonal path. Essentially, the lateral movement of the foil  100  can take place in any linear direction across a plane perpendicular to the direction of the fluid flow  20 . 
     In another embodiment of the invention, where the fluid medium is water, the foil  100  may be buoyant. As such, the foil  100  floats on or near the surface of the water. In this embodiment the foil  100  may be a boat  60 . The leading edge  112  of the foil  100  is the bow portion  62  of the boat  60 , and the trailing edge  114  of the foil  100  is the stern portion  64  of the boat  60 . The upstream support structure  200  may be a chain  250  or a rope, attached to the bow  62  and anchored onto a piling or mooring ball, which constitutes the upstream portion  412  of the frame  400 . Similarly, a traditional boat anchor may constitute the upstream portion  412  of the frame  400  and the boat&#39;s anchor rode may constitute the upstream support member  200 . A rigid downstream support member  300  is pivotally connected to the stern  64  and pivotally connected to the downstream portion  414  of the frame  400 . See  FIG. 5 . The boat  60  moves laterally across the surface of the water. A force transfer mechanism  500  may be attached to the boat  60  and connected to a generator  700 , so that the reciprocating motion of the boat  60  can be used to generator electricity. 
     In yet another embodiment of the invention, the upstream support member  200  is comprised of a plurality of elongate rods  240 , and the downstream support member  300  is comprised of a plurality of elongate rods  340 . See  FIG. 3 . In this configuration the foil  100  is substantially planar, having an appropriately curved cross section. The foil  100  moreover has a plurality of upstream connection points  122  corresponding to the plurality of elongate rods  240  of the upstream support member  200 , and has a plurality of downstream connection points  124  corresponding to the plurality of elongate rods  340  of the downstream support member  300 . Similarly, the frame  400  has a plurality of upstream connection points  422  corresponding to the plurality of elongate rods  240  of the upstream support member  200 , and has a plurality of downstream connection points  424  corresponding to the plurality of elongate rods  340  of the downstream support member  300 . Each elongate rod  240  of the upstream support member  200  has a first end, a second end, a frame connection point, and a foil connection point. These elements are located and function as described above. Each elongate rod  340  of the downstream support member  300  also has a first end, a second end, a frame connection point, and a foil connection point. These elements are located and function as described above. 
     In the foregoing configuration, the force transfer mechanism  500  (if present) may be in connection with at least one of the foil  100 , one or more of the elongate rods  240  of the upstream support member  200 , and one or more of the elongate rods  340  of the downstream support member  300 . Movement of the foil  100  causes movement of the elongate rods  240  of the upstream support member  200  and movement of the elongate rods  340  of the downstream support member  300 , thereby resulting in movement of the force transfer mechanism  500 . Where a plurality of upstream pivots  612 , downstream pivots  614 , leading edge pivots  622 , and trailing edge pivots  624  are used, the force transfer mechanism  500  may be in connection with one or more of these as well. In a variation of this embodiment, the upstream support member  200  may constitute a plurality of chains  250  rather than elongate rods  240 . In all other respects the device  10  is configured the same. 
     In yet another embodiment of the invention, the device  10  comprises a plurality of foils  100 . See  FIG. 4 . Each of the plurality of foils  100  is configured as described above. However, instead of the foils  100  being attached directly to the upstream support member  200  and the downstream support member  300 , they are connected to a first foil support member  800  and a second foil support member  900 . The first foil support member  800  is elongate and oriented substantially perpendicular to the direction of fluid flow  20 . It has a first end, a second end, a pair of upstream support connection points, and a plurality of foil connection points. One of the upstream support connection points of the first foil support member  800  is located proximate to the first end of the first foil support member  800 , while the other of the upstream support connection points of the first foil support member  800  is located proximate to the second end of the first foil support member  800 . The plurality of foil connection points of the first foil support member  800  are distributed substantially evenly along the length of the first foil support member  800  and correspond in number to the plurality of foils  100 . Each foil connection point of the first foil support member  800  is in pivotal connection with the upstream connection point  122  of a corresponding foil  100  by one of the leading edge pivots  622 . 
     Similarly, the second foil support member  900  has substantially the same size and shape as the first foil support member  800  and is oriented substantially perpendicular to the direction of fluid flow  20 . It has a first end, a second end, a pair of downstream support connection points, and a plurality of foil connection points. One of the downstream support connection points of the second foil support member  900  is located proximate to the first end of the second foil support member  900 , while the other of the downstream support connection points of the second foil support member  900  is located proximate to the second end of the second foil support member  900 . The plurality of foil connection points of the second foil support member  900  are distributed substantially evenly along the length of the second foil support member  900  and correspond in number to the plurality of foils  100 . Each foil connection point of the second foil support member  900  is in pivotal connection with the downstream connection point  124  of a corresponding foil  100  by one of the trailing edge pivots  624 . 
     A pair of upstream support members  200  connects the first foil support member  800  to the frame  400 , and a pair of downstream support members  300  connects the second foil support member  900  to the frame  400 . Each upstream support member  200  has a foil support member connection point, which is in pivotal connection with one of the upstream support connection points of the first foil support member  800  by a first foil support pivot  632 . Each downstream support member  300  has a foil support member connection point, which is in pivotal connection with one of the downstream support connection points of the second foil support member  900  by a second foil support pivot  634 . 
     The frame  400  has a pair of upstream portions  412  and a pair of downstream portions  414 . Each upstream support member  200  is in pivotal connection with one of the upstream portions  412  of the frame  400  by an upstream pivot  612 . Each downstream support member  300  is in pivotal connection with one of the downstream portions  414  of the frame  400  by a downstream pivot  614 . In this embodiment, the fluid flow  20  acts upon the plurality of foils  100 , causing them to move together substantially perpendicular to the direction of the fluid flow  20  in a reciprocating motion. A force transfer mechanism  500  may be attached to one or more of any of the moving components of the device  10 , as described above. 
     What has been described and illustrated herein is a preferred embodiment of the invention along with some it its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention in which all terms are meant in their broadest, reasonable sense unless otherwise indicated. Any headings utilized within the description are for convenience only and have no legal or limiting effect. Other embodiments not specifically set forth herein are also within the scope of the following claims, whereby modifications and variations can be made to the disclosed embodiments of the present invention without departing from the subject of the invention as defined in the following claims.