[0001] This invention relates to devices for extracting energy from waves.
[0002] An example of a device adapted to generate electricity from energy extracted from
waves is provided by the buoy device invented by Yoshio Masuda and described in British
Patent Specification No. 1,014,196, in which the oscillation of a column of water
in a chamber is arranged to drive air through an air turbine.
[0003] A breakwater is another example of a device intended to extract energy from waves
but so as to provide calmer regions on its leeward side.
[0004] According to the present invention, there is provided a device adapted to be partially
submerged in a liquid and extract energy from waves thereon, wherein that portion
of the device adapted to be below the surface of the liquid is of asymmetric shape
in a vertical plane aligned in a direction of propagation of the incoming waves, and
the device is adapted to be held by means for inhibiting movement of the device in
the liquid so that in operation waves generated by the device itself are substantially
unidirectional and in a direction towards the incoming waves.
[0005] The device may be of buoyant construction, and adapted to be held below its natural
level of floatation in the liquid by mooring means so as to inhibit said movement
of the device. Alternatively, the device may be of non-buoyant construction and adapted
to be held by a submerged support means which may include the sea bed.
[0006] In yet a further alternative, the device may be held by being adapted for incorporation
in a barrier means arranged to be incident to incoming waves, for example a coastal
sea wall.
[0007] The asymmetric shape may be provided by having the front of the device upon which
the waves are intended to be incident, arranged so as to be immersed less deeply in
the liquid than the rear of the device. In a device having a chamber in which a column
of water is arranged to oscillate, the asymmetric shape may be provided by closing
the bottom of the chamber, and providing a port means at the incoming wave side of
the chamber positioned so as to be immersed in the liquid.
[0008] A stationary partially submerged device which has a symmetrical underwater shape
in a vertical plane aligned in the direction of incoming waves will have a theoretical
maximum power absorption of about 50% of the energy of the incoming waves, the remaining
wave energy being lost and distributed equally between reflected and transmitted waves.
By adopting an asymmetric underwater shape as required by the invention, by which
waves generated by the device itself in extracting energy from the incoming waves
are unidirectional and towards the incoming waves, the energy absorption efficiency
of the device is increased. This is because a device which generates and radiates
waves preferentially in a certain direction when in forced oscillation will also absorb
that frequency preferentially from the same direction, so that theoretically in an
idealised condition there will be no reflected and transmitted waves through which
energy may be lost. In practice, however, this condition is unlikely to be realised
but energy extraction efficiencies above 50% should be attainable by devices incorporating
the invention.
[0009] To enable the present invention to be more readily understood attention is directed
by way of example only to the accompanying drawings, in which:-
Figure 1 shows a diagrammatic sectional representation of a buoyant device having
one asymmetric shape and for generating electrical energy from wavepower;
Figure 1a shows a view in the direction of arrow 'A' in Figure 1;
Figure 1b shows a view in the direction of arrow 'C' in Figure la;
Figure 2 shows a diagrammatic sectional representation of a device similar to that
shown in Figure 1 but having an alternative asymmetric underwater shape;
Figure 3 shows a diagrammatic sectional representation of a device similar to that
shown in Figure 2 but of non-buoyant construction;
Figure 3a shows a fragmentary sectional view to an enlarged scale on the line IIIa-IIIa
of Figure 3;
Figure 4 shows a diagrammatic sectional representation of a device similar to those
shown in Figure 2 and Figure 3 but incorporated in a sea barrier;
Figure 5 shows a fragmentary view to a reduced scale in the direction of arrow X of
Figure 4;
Figure 6 shows a diagrammatic model of part of a device similar to that of Figure
1; and
Figure 7 shows a diagrammatic model of part of a device similar to that of Figure
2.
[0010] In the above Figures, like parts have like numerals.
[0011] Referring now to Figure 1, the device shown has a forward buoyancy tank 10 and a
rear buoyancy tank 11 for supporting the device in a liquid 13 (e.g. seawater), the
forward buoyancy tank 10 being immersed less deeply in the liquid 13 than the rear
buoyancy tank 11. The forward and rear buoyancy tanks 10 and 11 respectively define
between them the front and rear walls of a chamber 14 in which a column of the liquid
13 oscillates as indicated by the arrows as a result of wave motion of the incoming
waves in the direction of arrow 'B'. The chamber 14 has a roof 15 with an air outlet
16 having a non-return valve 17
., and an air inlet 18 having a non-return valve 19. The air inlet 18 is connected
at its other end to the rear wall 20 of the device where it is protected against spray
by a shroud 21.
[0012] The rear wall 20 and a cover 25 define a duct 26 for air between the aft buoyancy
tank 11 and the roof 15 as described in Offenlegungsschrift 27 37 143, to interconnect
the air discharged from adjacent chambers 14.
[0013] An orifice 27 in the cover 25 directs air from the duct 26 through an air turbine
28 which is arranged to drive an electric generator 29.
[0014] As shown in Figure la several chambers 14 having side walls 24 are connected in parallel
by the duct 26 to drive the single air turbine 28, the generator 29 of Figure 1 being
omitted for clarity. The device is held as shown in Figure 1b, to which reference
is made by mooring means in the form of cables 22 each attached at one end to a respective
winding gear 23 (only two are shown) on the device, and to a respective anchorage
24 in the sea bed to inhibit movement of the device. The cables 22 are kept taut by
constructing the device to have excess buoyancy and hauling on the cables 22 with
the winding gear 23 to pull the device downwardly below its natural floatation level
in the liquid 13 to a required mean depth of immersion. Such cables 22 may be supplied
by British Ropes Limited, Doncaster, England.
[0015] In operation, energy is extracted from the incoming waves by the oscillation of the
liquid 13 in the chamber 14 drawing air into the chamber 14 through inlet 18 and expelling
air from the chamber 14 through the outlet 16 and into the duct 26. The pressure of
the air in the duct 26 depresses the liquid level at the rear of the rear buoyancy
tank 11 to provide a restoring force acting on the air and tending to maintain a substantially
uniform air pressure in the duct 26. Air from the duct 26 escapes through the orifice
27 and drives the air turbine 28 which drives the electric generator 29. Because of
the asymmetric underwater shape of the device, the energy extracted from the waves
is greater than that extracted by a similar device but having a symmetric underwater
shape.
[0016] In the alternative asymmetric shape shown in Figure 2 to which reference is now made,
the chamber 14 is closed at the bottom by a baseplate 30 which defines a front entry
port 31 for the chamber 14 below the buoyancy tank 10. In other respects the device
shown in Figure 2 is identical to that shown in Figures 1 and la, and its asymmetric
underwater shape increases its energy absorption efficiency in the same way compared
with a device having a symmetric underwater shape.
[0017] The device of Figure 2 may be held by mooring means (not shown) similar to those
described in relation to Figure 1b.
[0018] As an alternative to the buoyant devices shown in Figures 1, la, 16 and 2, a non-buoyant
device may be used as shown in Figure 3, to which reference is now made, for applications
where tidal movement is relatively limited.
[0019] The device of Figure 3 is similar in most respects to the device of Figure 2 except
that the use of buoyancy tanks has been dispensed with to provide the non-buoyant
construction required. The device is provided with a solid forward wall 40, a solid
rear partition 41, and a solid base portion 43, which define the corresponding portions
of a chamber 14 similar to that of Figure 2. An air inlet 48 having a non-return valve
49 extends from the chamber 14 through the rear partition 41, a boss 50 extending
in the duct 26, and the rear wall 20 where the air inlet 48 is protected by a shroud
51. The boss 50 is also shown in fragmentary sectional view in Figure 3a to which
reference can be made.
[0020] The base portion 43 has a rounded corner 52 at a front entry port 31 to the chamber
14, and is shown resting on a submerged surface 53, such as the sea bed, to present
the device at the required partially submerged depth in the liquid 13. In all other
respects, the device of Figure 3 is essentially the same as the device of Figure 2
and extracts energy from waves on the liquid 13 in an identical manner.
[0021] It will be understood that cables (not shown) may be attached to the device of Figure
3 to retain it in its required position on the submerged surface 53, and weights (not
shown) provided to increase the inertia of the device.
[0022] The invention may also be incorporated in a barrier as shown in Figure 4, to which
reference is now made, for installations subject to relatively limited tidal movement.
[0023] In Figure 4 a portion of a coastal sea wall 59 of ferro-concrete construction is
shown, and is shaped to provide the essential features of a device similar to that
of Figure 3 for extracting energy from incoming waves on the sea 13. In more detail
the device of Figure 4 has a forward wall 60, a rear wall 61, and a base portion 63,
which define the corresponding portions of a chamber 14 having a front entry port
31 similar to that of Figures 2 and 3.
[0024] An air inlet 65 having a non-return valve 66 extends upwardly from the chamber 14
to the top of the device where it is protected from spray by a shroud 67. The device
has a duct 26 into which air from the chamber 14 enters through the outlet 16 and
non-return valve 17-in a similar manner to that described in relation to Figures 1
to 3. The duct 26 is shown unpressurised, but may be arranged to be pressurised as
described in Offenlegungsschrift 27 37 143. In all other respects, the device of Figure
4 is essentially the same as the devices of Figures 2 and 3 and extracts energy from
the incoming waves in an identical manner.
[0025] The device of Figure 4 may be of considerable length broadside to the incident waves,
and arranged as a multiplicity of chambers 14, as shown in Figure 5 to which reference
is now made. In Figure 5 sets of three chambers 14a, 14b, 14c are shown connected
by a common duct 26 to a single orifice 27 and air turbine 28 along the length of
the device, the generator 29 of Figure 4 being omitted for clarity. The chambers 14a
and 14b, and 14b and 14c are separated by side walls 70 below the duct 26 but the
chambers 14c and 14a are separated by side walls 71 which extend into the duct 26
to separate the air flow in one set of chambers 14a, 14b, 14c from that in another
set. If desired a similar arrangement of sets of chambers 14 may be used in relation
to the devices of Figures 1 to 3, and it will be understood that a set of chambers
14 may comprise more than three chambers 14, or two such chambers 14.
[0026] Referring now to Figure 6 the essential features of a device similar to that of Figure
1 are shown in a vertical plane aligned with the incoming wave for the purpose of
establishing the dimensions thereof for maximum power absorption efficiency. These
features are represented by:
L = distance in the chamber 14 between the forward buoyancy tank 10 and the rear buoyancy
tank 11
x = width at the bottom and maximum width of the forward buoyancy tank 10
y = distance between the bottom of the forward buoyancy tank 10 and mean sea level
s = vertical distance between the bottom of the forward buoyancy tank 10 and the bottom
of the rear buoyancy tank 11
w = width at the bottom of the rear buoyancy tank 11
fo = frequency of the incoming waves.
[0028] The essential features of a device similar to that of Figure 2 in a vertical plane
aligned in the direction of propagation of the waves are shown in Figure 7 to which
reference is now made.
[0029] The features are represented by:
L = distance in the chamber 14 between the forward buoyancy tank 10 and the rear buoyancy
tank 11
x = width at the bottom and maximum width of the forward buoyancy tank 10
y = distance between the bottom of the forward buoyancy tank 10 and mean sea level
s = height of the front entry port 31.
[0030] For maximum power efficiency:
where

= wavelength of the incoming waves corresponding to the natural frequency, fo, of
the water column in the chamber 14.
[0031] a = 0.67 L ) y = 0.4 L ) approximately x = 0.25 L )
[0032] The devices of Figures 1 to 5 may be provided with means for changing the resonant
frequency of the liquid 13 in the chamber 14, for example as described in UK Application
No. 19199/77, to optimise the device for maximum energy absorption efficiency over
a range of incoming wave frequencies.
[0033] It will be appreciated that other asymmetric underwater shapes may be used in accordance
with the invention, and asymmetric underwater shapes may be incorporated in alternative
devices for extracting energy from wavepower to those shown in the Figures, for example
a breakwater. Such a breakwater might be provided by the devices shown in the Figures
but with the air turbine 28 and electric generator 29 dispensed with and the dimensions
of the orifice 27 selected to maximise the energy loss as air is driven therethrough.
[0034] The devices shown in Figures 1 to 3 may be made from materials conventionally used
by those skilled in the art of designing devices for extracting energy from wavepower
depending on the chemical properties of the liquid and the forces to which the device
is likely to be subjected in use (e.g. metals, or ferro-concrete). Although alternative
materials can be used for the device of Figure 4, ferro-concrete is likely to be the
preferred material.
[0035] It will be understood that although the invention has been described in relation
to a device having several chambers in which.the oscillation of a column of liquid
in each chamber is used to discharge air into a common duct, the invention may also
be incorporated in a device having at least one such chamber from which air is discharged
directly to an air turbine.
[0036] The invention may also be incorporated in devices having alternative means for rectifying
the air discharged from adjacent chambers before the air is directed to an air turbine,
and also in devices having alternative means of deriving mechanical power from the
oscillations of a column of liquid in a chamber of the device.
1. A device for extracting energy from waves on a liquid in which the device is adapted
to be partially submerged, the device having a chamber with an opening for the flow
of the liquid into and out of the chamber to provide a quantity of the liquid which
is arranged to oscillate in the chamber from the motion of the waves, and means arranged
to use or dissipate at least some of the energy of the oscillating liquid in the chamber,
characterised by,
a) an asymmetric shape of that portion of the device adapted to be submerged and lie
in a vertical plane aligned in the direction of propagation of the waves; and
b) means (22, 23, 43, 59) for holding the device in said partially submerged position
and inhibiting movement of the device in response to said waves, so that in operation
waves generated by the device itself are substantially unidireo- tional and in a direction
towards the incoming waves.
2. A device as claimed in Claim 1, further characterised by the asymmetric shape being
provided by,
a) a forward wall (10, 40, 60) of the chamber (14) with respect to waves incoming
towards the device;
b) a rear wall (11, 41, 61) of the chamber (14), said forward wall (10, 40, 60) being
shorter than the rear wall (11, 41, 61) so as to be immersed less deeply in the liquid
than the rear wall (11, 41, 61); and
c) an open lower end of the chamber (14) to provide said opening.
3. A device claimed in Claim 1, further characterised by the asymmetric shape being
provided by,
a) a closed lower end (30, 43, 63) of the chamber (14); and
b) a forward wall (10. 40, 60) of the chamber (14), with respect to waves incoming
towards the device, and shaped to define at least in part said opening (31).
4. A device as claimed in Claim 2, further characterised by the device being arranged
so that in a vertical plane aligned in the direction of propagation of the waves:
a) the difference (S)between the mean depth of immersion of the forward wall (10,
40, 60) and the rear wall (11, 41, 61) of the chamber (14) with respect to incoming
waves is not less than the distance (L) in the chamber (1.4) between said walls (10,
40, 60 - 11, 41, 61) and not greater than twice said distance (L) between said walls
(10, 40, 60 - 11, 4l, 6l);
b) the mean depth (y) of immersion of said forward wall (10, 40, 60) of the chamber
(14) is not less than one quarter of the distance (L) in the chamber (14) between
said forward wall (10, 40, 60) and said rear wall (11, 41, 61) of the chamber (14)
and not greater than one half of said distance (L) between said walls (10, 40, 60
- 11, 41, 61);
c) said rear wall (11, 41, 61) of the chamber (14) has a thickness at the lower end
thereof in said vertical plane not less than the distance (L) in the chamber between
said forward wall (10, 40, 60) and said rear wall (11, 41, 61) of the chamber (14)
and not greater than three times said distance (L) between said walls (10, 40, 60
- 11, 41, 61); and
d) said forward wall (10, 40, 60) of the chamber (14) has a thickness (x) at the lower
end thereof in said vertical plane not less than one half the mean depth (y) of immersion
of said forward wall (10, 40, 60) and not greater than said mean depth (y) of immersion
of said forward wall (10, 40, 60).
5. A device as claimed in Claim 3, further characterised by the device being arranged
so that in a vertical plane aligned in the direction of propagation of the waves:
a) the height (S)of the opening (31) is about 0.67 of the distance (L) in the chamber
(14) between the forward wall (10, 40, 60) and the rear wall (11, 41, 61) with respect
to incoming waves;
b) the mean depth (y) of immersion of the forward wall (10, 40, 60) is about 0.4 of
the distance (L) in the chamber (14) between the forward wall and the rear wall (11,
41, 61); and
c) said forward wall (10, 40, 60) of the chamber (14) has a thickness (x) at the lower
end thereof in said vertical plane about 0.25 of the distance (L) between said forward
wall (10, 40, 60) and said rear wall (11, 41, 61).
6. A device as claimed in any one of Claims 1 to 5, further characterised by the device
being of buoyant construction, and the holding means (22, 23) being adapted for holding
the device below the natural level of floatation of the device in the liquid so as
to inhibit said vertical movement of the device.
7. A device as claimed in any one of Claims 1 to 5, further characterised by the device
being of non-buoyant construction, and the means comprising a submerged support (43,
53).
8. A device as claimed in any one of Claims 1 to 5, further characterised by the device
being adapted for incorporation in a barrier means (59) arranged to be incident to
incoming waves.