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A wave energy converter (100) with a top-section (10) that is tiltable relative to a bottomsection (11), and a primary floater (101) able to carry the weight of the top-section (10) on a sea surface (1). The top-section (10) has rotational symmetry and a wave-driven pump (110) arranged around a reservoir (120). A secondary floater (103) and a secondary ballast tank (104) keep the bottom section (11) vertical. As the top section (1) tilts in the waves, the wavedriven pump (110) lifts water to a design height (h0). Water leaves the reservoir (120) through an eductor section (130), a turbine section (140) and a draft tube (150). Shared infrastructures increase overall efficiency.
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PT Varsel om betaling av første årsavgift (3317) (PT20240128)
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GH Forespørsel
Bjørn Kjetil Mølmann
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Søknadskvittering Patent
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13.02.2024
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Søknadsskjema Patent
Bjørn Kjetil Mølmann
Description, claims and drawings
Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content.Description
3495871BACKGROUNDField of the invention[0001] The present invention concerns a wave energy converter (WEC) that generates electrical power.5 [0002] Real local sea states include swells and local waves with different wave parameters, and are usually and better described by statistical parameters than by one simplified (average) harmonic wave traveling in one dominant azimuthal direction.[0003] Wave heights 0.6 – 12.5 m roughly correspond to wind speeds 4 m/s (gentle breeze) to 25 m/s (storm), which are cut-in and cut-off speeds for typical wind turbines. A turbine’s 10 economic and environmental efficiency grows with the fraction of time the turbine can produce electricity at its rated power. In the following, ‘efficiency’ without prefix means ‘produced energy per cost-unit’.Prior and related art[0004] The Wikipedia article ‘Wave power’ (en.wikipedia.org/wiki/Wave_power) has a 15 good overview of various aspects of wave power. An important point for the following, is that wave power devices tend to be less efficient than wind power devices.[0005] According to the said Wikipedia article, WECs can be classified by their working principle as either: a) oscillating water columns (with air turbine); b) oscillating bodies (with hydroelectric motor, hydraulic turbine, linear electrical generator) or c) overtopping devices 20 (with low-head hydraulic turbine). Our comments:[0006] An oscillating water column (OWC) contains kinetic and potential energy proportional to the column’s volume. Typical diameters of a few metres and slow oscillations severely limit the power (energy per time-unit) available for producing electrical power, for example by a water turbine driven by the OWC.25 [0007] Varieties with an air turbine over an OWC are much less efficient than horizontal axis wind turbines (HAWTs) at the same location. For instance, a HAWT with rotor diameter 4.3 m mounted on a simple steel pole is cheaper to build and maintain than a WEC with a cylinder for a similar rotor. Both devices would yield about 4 kW electrical power.[0008] For comparison, a HAWT for an offshore wind-farm typically has a rotor diameter 30 20 – 65 m and rated power output in the so-called ‘mini range’ 100 kW – 1 MW.[0009] However, some offshore HAWTs need foundation in relatively shallow waters, preferably with a firm seafloor. Relatively shallow waters with sufficient average winds are
3495872typically found om fishing banks, and negative effects on fisheries and marine ecosystems have costs. A seafloor of sands and gravel would require expensive civil works. Other offshore HAWTs need relatively expensive platforms or undercarriages to counteract torques generated by a 20 – 65 m wide rotor catching winds on a tall tower.5 [0010] Devices with working principle b), oscillating bodies, rely on relative motion between solid structures. These include point absorber buoys that move along a rod and has many of the limitations associated with OWC devices, typically caused by a small volume from which energy can be extracted and slow oscillations.[0011] The Pelamis WEC had large semi-submersed tubes connected by hydraulic power 10 take-off (PTO) systems, and extracted energy from wave-shape rather than wave-height. In 2004, a full-scale prototype rated at 750 kW was the first offshore WEC to be connected to the UK grid. The prototype was 120 m long and had four tubes 3.5 m in diameter. A second generation Pelamis P2 was 180 m long, consisted of five tubes with 4 m diameter and weighed more than 1300 tonnes. Two Pelamis P2 were tested offshore the Orkneys 2012 – 15 2016, and are now out of production, probably due to low efficiencies.[0012] A sharp eagle WEC has a wave-absorbing body like the head of an eagle mounted in a semi-submerged barge. 100 kW, 200 kW and 500 kW versions (“Wanshan”, “Xiandao” and “Zhoushan”) were deployed in the South China Sea in 2015, 2017 and 2020 respectively. The 100 kW version is 36 m long, 23 m wide and 16 m high.20 [0013] The Edinburgh Duck, also known as Salter’s duck, has a tear-drop shaped head that ‘nods’ in waves and tilts back and forth around a cylindrical spine. The rotations drive an electric generator either directly or via hydraulic pistons or a hydraulic motor.[0014] An original design had a series of ducks mounted along a slack- moored spine oriented across a dominant wave direction. A duck with a short moment arm exposed to 25 relatively small wave forces generates a relatively small torque to overcome hydraulic or electric resistance from the PTO system. Limited available input power limits produced power, so in this design the total output power depends on the number of ducks.[0015] A 2.2 MW ‘Solo duck’ with foundation on the seafloor had problems with foundation due to large reaction forces. These problems are like those of a HAWT with 30 foundation on the seafloor.
3495873[0016] In 2019, Zhang et al.1 described a 100 kW Edinburgh Duck with a submerged body that takes up reaction forces and avoids problems with foundation on the seafloor. Reported sea states had wave heights 0.8 – 1.2 m and wave periods about 5 s.[0017] The Wave Dragon is a slack-moored overtopping device with a wave reflector and at 5 least one Kaplan or propeller turbine. The wave reflector leads waves from a limited azimuthal sector up a ramp into a reservoir that evens out variations in incident wave power.[0018] In August 2023 wavedragon.net/specifications/ listed designs for wave climates 24, 36, and 48 kW/m. The 24 kW/m design reportedly had 16 × 250 kW generators, and a reservoir of 5000 m3. The 36 kW/m design was rated at 7 MW, and had an annual energy 10 production that indicated production at the rated power about one third of the time.[0019] The Wave Dragon is large and heavy to minimise motion in waves. Large sizes generally increase manufacturing costs, e.g. for materials and less competition due to fewer potential vendors. Larger sizes also increase costs for deployment and mooring.[0020] DE 102013020209 A1 describes a device that uses varying air pressure over an 15 OWC to push water upwards through a cascade of specially shaped trays to a reservoir-inlet at any height, and may thereby replace a waveguide with ramp. However, this device stops working if too much water flows out of the trays, e.g. if the device tilts in waves.[0021] Due to an increased interest in extracting energy from small rivers and waterways, efficient water turbines for heads less than 15 m and rated power outputs in the mini-range 20 100 kW – 1 MW have been developed during the past few decades. Today, such ‘low-head’ turbines with associated control systems are utilised in overtopping devices, and thereby save significant development costs. These low-head turbines do not work within OWCs.[0022] A general objective of the present invention is to solve or alleviate at least one of the problems above while retaining benefits from prior art. A specific objective is to propose a 25 WEC that has less economic and environmental costs per produced energy unit during the lifetime of the power source than existing HAWTs and WECs.SUMMARY OF THE INVENTION[0023] These objectives are fulfilled by the subject matter in the independent claim 1.30 Further features and benefits appear in the dependent claims. In the claims and elsewhere, articles ‘a’, ‘an’, and ‘the’ mean ‘[the] at least one’, whereas ‘one’ means exactly one.Furthermore, ‘for’ means ‘suitable for’.1 Zhang et al. «Numerical Research and Open Sea Tests of a 100 kW Modified Edinburgh Duck Wave Energy Convertor”, open access online in “Frontiers in Energy Research”.
3495874[0024] More particularly, the invention concerns a wave energy converter with a top-section that is tiltable relative to a bottom-section, and a primary floater able to carry the weight of the top-section on a sea surface. The wave energy converter is distinguished in that the topsection has rotational symmetry and a wave-driven pump configured to lift water a design 5 height into a reservoir within the top section, and that the bottom-section has a turbine section for generating electrical power from a flow of water from the reservoir.[0025] Rotational symmetry makes the top-section sensitive to waves coming from any direction. During operation, roll, pitch and heave of the top section enables the wave-driven pump to lift water into the reservoir, which evens out variation in wave height and wave 10 power. Water leaves the reservoir through a water turbine in the bottom section.[0026] In preferred embodiments, the bottom-section has a secondary floater at an upper end and a secondary ballast tank at a lower end. In use, the secondary floater and ballast tank keep the bottom section nearly vertical. They also provide the bottom-section with near neutral buoyancy.15 [0027] Air-filled ballast tanks facilitate towing and onsite maintenance. Water-filled ballast tanks may enable a ‘survival mode’ where the wave energy converter sinks temporarily to a depth where surface waves have little or no effect.[0028] Preferably, a gimbal connects the top section with the bottom-section. Such a gimbal is probably more robust than other elements that allow the top-section to tilt relative to the 20 bottom section, for instance an elastic connection.[0029] The primary floater preferably has a ring-shaped element around its perimeter. This increases the average length of moment arms that promote roll and pitch of the top-section.[0030] In preferred embodiments, the wave-driven pump is arranged around the reservoir and has several sectors, each with a series of levels and one-way valves arranged to lead water 25 in a zig-zag pattern from a pump-inlet to a pump-outlet.[0031] The sectors ensure that most water flows radially rather than circumferentially. The work required to lift water from one level to the next is the total work required to lift water to the design height divided by the number of levels. Thus, the number of levels determines a low limit of wave sizes able to drive the pump.30 [0032] Preferably, the wav-driven pump has a deflector configured to redirect water flowing along the floor of toward a one-way valve leading to a next level. The deflector reduces energy loss in water that slams perpendicularly into the radially inner and outer walls.[0033] The design of the wave-driven pump just described is inspired by venous valves that allow blood flows toward the heart, but not in the opposite direction. The pump design does
3495875not depend on air-filled volumes. However, air-filled volumes over water surfaces in the proposed wave-driven pump have known technological advantages to be explained in the description of Figs. 4a and 4b.[0034] The reservoir preferably has a top-lid with a central opening. The top-lid keeps water 5 within the reservoir when the top-section tilts. The central opening determines when the reservoir is full, and thereby the design height.[0035] The wave energy converter advantageously has an eductor section between the reservoir and the turbine section. During operation, such an eductor section adds a secondary flowrate to a primary flowrate from the reservoir, and regulates the ratio of kinetic energy to 10 potential energy in the flow of water that enters the turbine section.[0036] Drainage valves enable release of water from the top-section. This facilitates inspection and maintenance.[0037] The wave energy converter preferably has a frustoconical draft tube downstream from the turbine section. If possible, a draft tube shaped like a frustum with its smaller diameter at 15 the turbine section is preferred. Draft tubes on land usually have other shapes, mainly due to costs of civil works.BRIEF DESCRIPTION OF THE DRAWINGSThe invention will be described in greater detail with reference to the accompanying20 drawings, in which:Fig. 1 illustrates use of the proposed WEC,Fig. 2 is a schematic partial cross section of the proposed WEC viewed from a side, Fig. 3 is a schematic partial cross section of the proposed WEC viewed from above, Fig. 4a illustrates a pump segment tilting clockwise,25 Fig. 4b illustrates the pump segment in Fig. 4a tilting counter-clockwise,Fig. 5 is a schematic top-view of a gimbal,Fig. 6 illustrates damping,Fig. 7 illustrates feeding sections for a water turbine,Fig. 8 shows a generic propeller turbine, and30 Fig. 9 illustrates turbine efficiency as function of rotation speed,[0038] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS[0039] The drawings are schematic, and details known to the skilled person are omitted. We will use the following symbols, terms, and conventions:
3495876Symbol SI-unit DescriptionA m2 Area perpendicular to a flow of waterB N Buoyancy. Boldface means a vector, i.e. a quantity with direction g m/s2 Acceleration of gravity. About 9.81 m/s2 directed downward h m Height of a water columnm kg Massp Pa Pressure = force/area = energy/volume-unitP W Electrical output powerQ m3/s Flowrate = volume of water per time-unitv m/s Velocity = speed with directionη Turbine efficiency = (electrical power out)/(hydraulic power in) ρ kg/m3 Density, e.g. 1025 kg/m3 for seawaterTable 1[0040] Fig. 1 shows three WECs 100a – 100c on a wavy sea surface 1. As illustrated by the WEC 100a, each WEC 100 has a top-section 10 with rotational symmetry about a z-axis and a bottom section 11 with rotational symmetry about a Z-axis. The rotational symmetries make 5 the WEC 100 equally sensitive to waves coming from any direction.[0041] The top-section 10 moves in waves like a boat. We borrow traditional terms ‘heave’ for translation along the z-axis, and ‘roll’ and ‘pitch’ for rotations about mutually perpendicular x- and y-axes that are perpendicular to the z-axis. The x-, y- and z-axes are fixed to the top-section 10 and preferably pass through its centre of mass.10 [0042] A primary floater 101 carries the weight of the top-section 10. Roll, pitch and heave of the top-section 10 enables a wave-driven pump 110 to lift water to a design height h0 over the sea surface 1 into a reservoir 120. The reservoir 120 evens out variations in wave height and wave power.[0043] A cardan suspension or gimbal 160 connects the bottom section 11 with the top-15 section 10. In use, the Z-axis remains nearly vertical, and the gimbal 160 enables the topsection 10 to roll and pitch easily relative to the bottom-section 11. The gimbal 160 does not contain hydraulic or electrical PTO-systems that impede roll and pitch.[0044] A secondary floater 103 and a secondary ballast tank 104 keep the bottom section 11 nearly vertical, and provide it with neutral buoyancy. This reduces loads on the gimbal 160.20 [0045] The bottom section 11 has, from top to bottom, an eductor section 130, a turbine section 140, and a draft tube 150.[0046] The eductor section 130 utilises the Venturi effect to add a secondary flowrate to a primary flowrate from the reservoir 120. This reduces the primary flowrate required to produce a rated power. The description of Fig. 7 has a more detailed explanation.
3495877[0047] The turbine section 140 has a water turbine that generates an electrical power P. The rotary machines in the turbine section 140 advantageously rotate about the Z-axis, which is always nearly vertical during operation. This reduces wear on bearings within the turbine section 140. Furthermore, the water flow through the turbine has small variations, so there is 5 no need for adjustable gates and/or adjustable propeller blades with associated control systems. Simpler control systems and mechanisms reduce investment and maintenance costs.[0048] The draft tube 150 is a standard component, and often considered part of the design of a water turbine. It reduces a risk for harmful backflow into the turbine’s runner, and increases the turbine’s ability to produce electrical output power P. If possible, as here, a 10 frustum with a smallest diameter at the turbine runner is preferred. Costs of civil works often dictate other shapes of draft tubes on land.[0049] The wave-driven pump 110 can pump water into the reservoir 120 at quite low waves. Also, the WEC 100 is essentially a closed structure that is nearly unsinkable even if overtopped by ‘large’ waves. For example, wave heights 0.6 – 1.2 m correspond to gentle 15 breeze and wind speeds 3.4 – 5.4 m/s that includes the cut-in speed 4 m/s for HAWTs. Wave heights > 14 m correspond to hurricanes and wind speeds > 33 m/s, which are well above the cut-off speed 25 m/s of many HAWTs. Thus, the flowrate (Q) though the turbine section 140 varies little over a broad range of wave-conditions, and the WEC 100 is expected to produce more electrical energy per year than a HAWT with the same rated power at the same location.20 [0050] The WEC 100 may have a ‘survival mode’ in which it sinks temporarily to a depth where surface waves have little or no effect. Real embodiments of the WEC 100 probably have one or more small pump(s) and/or flasks with pressurised air for moving water into and out of ballast tanks. Accordingly, we expect the costs of implementing and maintaining such a survival mode to be much less than the corresponding costa of securing a HAWT to a 25 designed ‘survival speed’ at wind speeds 60 m/s or more.[0051] Real embodiments of the WEC 100 should obviously have nets or similar over inlets and outlets to prevent that fish or sea-mammals enter the WEC 100 by accident.[0052] Elements with reference numerals 199 – 302 are outside the scope of the invention, and shown to provide context.30 [0053] A ‘slack’ anchor cable 200, for example a chain, a steel wire, or a nylon rope, is attached to the WEC 100 at one end and an anchor 201 at the other. The anchor 201 is on a seafloor 2 at arbitrary depth, and hardly affects the marine environment. The length of the anchor cable 200 is greater than the depth to allow for tides and varying wave heights. On the anchor cable 200, a weight 202 provides a force G directed downward, and a buoy 203
3495878provides a buoyancy B directed upward. The forces G and B tend to pull the WEC 100 toward a position directly above the anchor 201. The principles behind so-called slack-mooring are well known, and devices for slack-mooring are not part of the invention.[0054] An offshore and sub-sea control centre 300 is deployed at a depth greater than L/2 5 where waves with wavelengths L or less have no effect. The control centre 300 may have positive buoyancy and a tight anchor cable 301 connecting it to the seafloor 2.[0055] The control centre 300 controls, and receives power P from, several WECs 100a – 100c through umbilicals 199a – 199c. A shared control centre 300 saves costs relative to a control system in every WEC 100, and thereby increases the overall efficiency of a wave-10 farm. A power line 302 delivers AC or DC to a local grid or an onshore terminal. Electrical devices and circuits as such are not part of the invention.[0056] An offshore wave-farm with WECs 100 may share a local grid and/or a power line with a wind-farm at the same location. In general., shared infrastructures save costs and thereby increases an overall efficiency. Alternatively, a single WEC 100 with output power in 15 the range 100 kW – 1 MW might cover the needs of a small village or factory.[0057] Fig. 2 is a partially cut-away side view of the WEC 100. The primary floater 101 has a ring-shaped element 101a around its perimeter, a primary ballast tank 102 at its bottom and an air-filled space between a concave top face 105 and a bowl-shaped bottom 106. Channels 107 around the primary floater 101 extend through the concave top face 105 and the bowl-20 shaped bottom 106.[0058] In production, the primary floater 101 provides a buoyancy approximately equal to the weight of the top-section 10 with water. The ring-shaped element101a tilts with the waves and provides relatively long moment arms for torques acting on the gimbal 160, here represented by a tiltable gimbal ring 161 and a vertical gimbal cylinder 164. The water-filled 25 primary ballast tank 102 is closer to the z-axis than the ring-shaped element 101a, which implies shorter average moment arms for torques that counteract roll and pitch. The concave top face 105 effectively scoops water into a pump-inlet 111. The bowl-shaped bottom 106 offers minimal resistance to roll and pitch. The channels 107 prevent that water becomes trapped by the ring-shaped element 101a and provide hydraulic damping. The primary floater 30 101 should scoop as much water as possible into the pump-inlet 111.[0059] Primary and secondary floaters and ballast tanks 101 – 104 should be sectioned to provide redundancy and minimise circumferential waterflows according to common practice.[0060] If water is removed from the ballast tanks 102, 104, the wave-driven pump 110 and/or the reservoir 120, the WEC 100 will float higher in the water. Components floating
3495879near or at the sea surface are relatively easy to tow, e.g. for deployment, and they are relatively easy to access during maintenance onsite. In particular, the WEC 100 most likely has much lesser costs of deployment and maintenance than a tall HAWT offshore.[0061] The wave-driven pump 110 in Fig.2 has a cylindrical outer wall 112 with pump-5 inlet(s) 111 at the concave top-face 105. A cylindrical inner wall 113 is also an upper wall of the reservoir 120. Small triangles and hinged flaps represent one-way valves. Small rectangles represent normally closed release valves for releasing water from the pump 110 and/or the reservoir 120, e.g. for maintenance.[0062] As the top-section 10 tilts, water moves upward in a zig-zag pattern through a series 10 of levels and one-way valves from the pump-inlet 111 to a pump-outlet 119.[0063] The reservoir 120 has a top-lid 121 that keeps water within the reservoir 120 as the top-section 10 tilts. The pump 110 has one or more pump-outlet(s) 119 above the top-lid 121. Scuppers 123 pass through the inner wall 113 and the outer wall 112 above the top-lid 121.[0064] During normal operation, water from the pump-outlet(s) 119 enters the reservoir 120 15 through a central reservoir opening 122 in the top-lid 121, and a full reservoir 120 has a water height h from the sea surface to the central reservoir opening 122. The pump-outlet(s) 119 open(s) to the atmosphere such that water in the reservoir 120 does not inhibit water flows from the pump-outlet(s) 119. Any excess water from the wave-driven pump 110 or from an overtopping wave leave through the scuppers 123, so the height h from the sea surface varies 20 insignificantly and/or temporarily. If a pump-outlet 119 temporarily becomes fully or partially submerged, a one-way valve in its top level prevents water from flowing in.[0065] The reservoir 120 has a cylindrical lower reservoir wall 124, which advantageously has a diameter large enough to pull the turbine section 140 through during onsite maintenance. To contain a desired volume of water, the upper wall 113 of the reservoir 120 may 25 have a greater diameter than the lower reservoir wall 124.[0066] The description of Fig. 1 has brief explanations of reference numerals 130 – 150.[0067] A flexible seal 165 connects the lower reservoir wall 124 with the gimbal cylinder 164, which is connected to the bottom section 11. In use, the flexible seal 165 prevents that water from the reservoir 120 escapes between the lower reservoir wall 124 and the gimbal 30 cylinder 164.[0068] Fig. 3 is a schematic section along the plane III – III in Fig. 2. It shows the floater 101 with its ring-shaped part 101a and the channels 107 viewed from above.[0069] Fig. 3 also illustrates the top level of the wave-driven pump 110 with its outer wall 112 and inner wall 113. The pump 110 is divided into 16 equal sectors, each with a pump
34958710segment 110a that may receive waves from a 360°/16 = 22.5° wide azimuthal sector. Narrow sectors lead water radially toward the pump-outlet 119 of each pump segment 110a, and limit water motion in the circumferential directions. Real embodiments may have more than or fewer than 16 sectors, each with a pump segment 110a.5 [0070] One-way top-valves 117 leading from a level below correspond to the small triangles in Fig. 2. Small rectangles represent normally closed release valves 118.[0071] Water from the pump-outlets 119 may flow over the top-lid 121, through the central reservoir opening 122 and into the reservoir 120.[0072] Fig. 4a illustrates a pump segment 101a tilting clockwise through an angle θ1. The 10 pump-inlet 111 leads to a level 1, and a one-way top-valve 117 leads to a level 2 above.Levels 1 and 3 have one-way inward-opening valves 114 that open toward the inner wall 113 and open top-valves 117 at the inner wall 113. Level 2 has a one-way outward-opening valve 116 that opens toward the outer wall 112 and a closed top-valve 117 at the outer wall 112. The release valves 118 are described previously.15 [0073] There may be more levels where even-numbered levels are like level 2 and oddnumbered levels are like level 3. The top level is like level 3, except that the pump-outlet 119 in the inner wall 113 replaces the top-valve 117 which is superfluous in the top level.[0074] In the following, we will use indices ix where the first digit i = 1, 2, 3 is a level number in the pump-section 110a.20 [0075] In Fig. 4a, water flows into the pump-inlet 111 with a flowrate Q10. A deflector 115 redirects the water flow through the open top-valve 117 between levels 1 and 2 such that the height of water becomes approximately equal in level 1 and in level 2 between the inner wall 113 and the closed valve 116. Measured from a reference level 0 at level 1, the common height is h10. Measured from a reference level at level 2, the common water height is h20. 25 [0076] Water with mass m21 and height h21 > h20 between the outer wall 112 and the outward-opening valve 116 cause a net force F21 that keeps the valve 116 closed.[0077] In level 2, the influx of water traps air with increasing pressure p20 in a decreasing volume V20 over the water surface with height h20. Such an air-cushion evens out (sudden) changes in water pressure and prevents undesired water hammering.30 [0078] In level 3, the height of water h30 pushes water to an equal height in level 4. An air pressure p30 in a volume V30 increases the flowrate of water to level 4 at any angle θ1.[0079] Fig. 4b shows the pump segment 101a tilting through an angle θ2 counter-clockwise. Counter-clockwise tilts θ2 cause the valves 114 to close and the valves 116 to open.
34958711[0080] In Fig. 4b, a preceding clockwise rotation has transferred a mass m10 of water from level 1 to level 2. The top-valves 117 close quickly to minimise the mass of water that flows back to a lower level when the rotation changes direction.[0081] In level 1, a right-hand side water height h12 results from the closing time of the top-5 valve 117 between levels 1 and 2. A longer closing time of the inward-opening valve 114 leads to a left-hand side water height h11 < h12. The difference h12 – h11 causes a net force F12 that keeps the inward-opening valve 114 in level 1 closed.[0082] In level 2, a flow rate Q22 has until the angle θ2 moved a mass m22 of water from level 2 to level 3. A pressure p21 from a volume V21 of air exerts an extra force on a water 10 surface at height h21, and thereby increases the flowrate Q22 of a waterflow to level 3.[0083] In level 3, a resulting height difference h32 – h31 causes a net force F32 that keeps the inward-opening valve 114 in level 3 closed.[0084] The description of Figs. 4a and 4b may be extended to levels 4 and above with minor modifications. A major point is that the work required to lift water to the design height h0 is 15 divided by the number of levels, such that ‘low’ waves can lift water to a next level.[0085] The one-way valves 114, 116 and 117 inhibit water from flowing back toward the pump-inlet 111, but they also increase the water’s traveling time through the pump 110.However, in a stable state the average flow from each pump-outlet 119 is nearly equal to the average flow through the corresponding pump-inlet 111.20 [0086] Fig. 5 shows the gimbal 160 mounted in the cylindrical lower reservoir wall 124. The gimbal ring 161 has two outer axles 162 mounted coaxially and rotatably about an x-axis. The outer axles 162 extend through the reservoir wall 124. The gimbal ring 161 also has two inner axles 163 mounted coaxially and rotatably about a y-axis perpendicular to the x-axis. The inner axles 163 extend through the wall of the gimbal cylinder 164, which has a bottom 168 25 with a gimbal-outlet 169. The flexible seal 165 is described with reference to Fig. 2.[0087] Fig. 6 illustrates a support 170 for the axles 162 and 163 that are rotatable about the x- and y-axes, respectively. A bearing 171 reduces friction to the axles 162 or 163, e.g. by water lubrication. An elastic element 172 dampens vibrations and, to some extent, sudden motions, or shocks. An outer bushing 173 contains the elastic element 172. Stoppers, perhaps 30 hydraulic dampers, may define a maximum free tilt angle 174.[0088] Large waves are steep and tall. In such waves, large and abrupt forces (large shocks) will probably affect the WEC 100, for example if it slides down a crest and hits the bottom of a wave. Reference numeral 175 depicts a generic shock-absorber with a spring that provide a
34958712force proportional to displacement and a piston in a fluid filled cylinder that provides a force proportional to speed.[0089] The floater 101 provides a buoyancy that increases with the volume of floater that is under the sea surface. Drag and the channels 107 provide forces that are proportional to water 5 speed. Thus, a real shock absorber should include the floater 101, the channels 107 and the shape of the bottom section 11. The ‘survival mode’ described with reference to Fig. 1 provides a limit to the required damping during extreme wave conditions.[0090] The remainder of this description specifies generally known technology, and is provided to avoid possible misunderstandings.10 [0091] Fig. 7 is a schematic cross section that shows details in the eductor section 130 and the lower part of the reservoir 120. The turbine section 140 may be connected directly to the gimbal cylinder 164, so the eductor section 130 is optional.[0092] A reservoir floor 125 extends radially from the lower reservoir wall 124 to form the floor in level 1 of the wave-driven pump 110. Reservoir release valves 126 are shown in the 15 lower reservoir wall 124 just over the flexible seal 165. Thus, all water in the reservoir 120 can leave through the valves 126 for maintenance.[0093] Reference numerals 161 – 165 relate to the gimbal 160, and are explained previously. The flexible seal 165 is shown as a bellow. Bellows are generally known to withstand a long series of expansions and contraction.20 [0094] The gimbal cylinder 164 is open to the reservoir 120. Additional gimbal-inlets 166 through the wall of the gimbal cylinder 164 enable radial waterflows to the gimbal-outlet 169.[0095] The main parts of the eductor section 130 have traditional names: Eductor nozzle 131, suction port(s) 132, throat 133, diffuser 134 and discharge 139.[0096] The optional eductor section 130 may be connected to the gimbal cylinder 164 by a 25 first flange connection 135, and/or to the turbine section 140 by a second flange connection 136. Flange connections are relatively easy to open for inspection or maintenance.[0097] In the following, ‘static pressure’ ps = ρgh is potential energy per volume unit, and ‘dynamic pressure’ pd = ½ ρv2 is kinetic energy per volume-unit. We avoid unspecified terms ‘pressure’ and ‘head’ = ‘pressure’/ρg.30 [0098] Conservation of energy implies that a total pressure ptot = ps + pd = constant. An effective head he = ptot/ρg.[0099] Conservation of mass implies that a mass flowrate ρAv remains constant in a fluid flow. In water, the density ρ is constant, so a flowrate Q = A1v1 = A2v2 = constant through cross sectional areas A1 and A2 in a flow of water. Hence, an area A2 < A1 implies v2 > v1 and
34958713thereby a greater dynamic pressure, and a correspondingly smaller static pressure at the smaller area A2. This is the Venturi effect.[0100] In use, water leaves through the eductor nozzle 131 with a primary flowrate Q1. A reduced static pressure at the nozzle tip draws water with a secondary flowrate Q2 through one 5 or more suction port(s) 132. The throat 133 has a reduced diameter that keeps the static pressure ‘low’. The diffuser 134 is frustoconical, reduces water speed and increases the static pressure. Accordingly, the design of the eductor section 130 determines the rate of static to dynamic pressure.[0101] Real eductors have Q2/Q1 < 0.4, for example 0.3. Then Q1 + Q2 = 1.3Q1, and a 10 turbine that requires a flowrate Q would need a flowrate Q1 = Q/1.3 from the reservoir 120.[0102] Fig. 8 illustrates a turbine section 140 with a water turbine that converts the kinetic and potential energy in a flow of water to electrical energy. Any suitable low-head water turbine may be used with the present invention. These include propeller turbines, Kaplan- and Francis-type turbines, and other reaction turbines, preferably with a vertical axis of rotation.15 [0103] Inlet gates 141 represent devices for directing water toward a runner 142, here with propeller blades 143. Real turbines have different devices 141 upstream from the runner142, and different runners 142.[0104] A vertical shaft 144 in a bearing 145 represents devices that transmit rotation of the runner 142 to a rotation suitable for a generator 146. An axis of rotation that is kept nearly 20 vertical reduces loads, and thereby wear, on bearings 145.[0105] A hood 147 represents a bulb-turbine, which is one of many turbine varieties.Electrical power P leaves the turbine section 140 through a powerline 149.[0106] Commercially available low-head turbines have small environmental footprints. For example, water lubricated bearings eliminates oil pollution. These turbines may also have low 25 costs of operation and maintenance. For example, an operation schedule may have a check by radio communication once a week and planned maintenance once a decade.[0107] Developing and testing a turbine with associated transmissions and generator takes time and is expensive. It is probably faster and cheaper to design and build a WEC 100 around a commercially available turbine than to develop a new turbine for a WEC 100.30 [0108] Immediately downstream from a spinning runner 142, the dynamic pressure is large compared to the static pressure. The draft tube 150 gradually replaces dynamic pressure with static pressure to avoid that ambient static pressure pushes water back into the turbine.Specifically, the ambient static pressure to withstand is ρgd for a draft tube ending at depth d. Accordingly, the draft tune 150 must be adapted to the dynamic pressure at the runner 142.
34958714[0109] Fig. 9 illustrates turbine efficiency η as function of rotation speed ω for a typical propeller turbine. The turbine efficiency η is greater than 0.8 in a range extending on both sides of a rotation speed ωm that corresponds to a maximum turbine efficiency ≈ 0.9.[0110] In the WEC 100, the height of the reservoir 120 is constant, and the effective head he 5 over, and flowrate Q through, the runner 142 have small variations. Also, a propeller turbine with fixed inlet gates 141 and fixed propeller blades 143 would have a good turbine efficiency, e.g. η ≥ 0.8, with small variations in he and Q. For comparison, a Kaplan-type turbine has adjustable inlet gates and/or adjustable propeller blades to accommodate a wider range of flow conditions. In the WEC 100, the investment and maintenance costs of the 10 additional control systems and movable parts in a Kaplan-type turbine would probably not yield a significantly greater average turbine efficiency.[0111] A final example: The electrical power P from a water turbine is P = ρgheQη. Using a rated power P = 100 kW = 100000 W, he = 7.0 m and numbers above, we compute a required flowrate Q = 100000/(1025∙9.81∙7.0∙ 0.8) m3/s = 1.78 m3/s through the turbine. With the 15 eductor section 130 from a previous example, the wave-driven pump 110 must supply a flowrate Q1 ≥ 1.78/1.3 = 1.37 m3/s to maintain the required flowrate Q through the turbine.[0112] While the invention has been described by means of examples, the scope of the invention is defined in the accompanying claims.
Claims
34958715CLAIMS1. A wave energy converter (100) with a top-section (10) that is tiltable relative to a bottom-section (11), and a primary floater (101) able to carry the weight of the top-5 section (10) on a sea surface (1), characterised in that the top-section (10) has rotational symmetry and a wave-driven pump (110) configured to lift water a design height (h0) into a reservoir (120) within the top section (10), and that the bottom-section (11) has a turbine section (140) for generating electrical power (P) from a flow of water from the reservoir (120).102. The wave energy converter (100) according to claim 1, wherein the bottom-section (11) has a secondary floater (103) at an upper end and a secondary ballast tank (104) at a lower end.15 3. The wave energy converter (100) according to claim 1 or 2, wherein a gimbal (160) connects the top section (10) with the bottom-section (11).4. The wave energy converter (100) according to claim 1 or 2, wherein the primary floater (101) has a ring-shaped element (101a) around its perimeter.205. The wave energy converter (100) according to any preceding claim, wherein the wavedriven pump (110) is arranged around the reservoir (120) and has several sectors (110a), each with a series of levels and one-way valves (114, 116, 117) arranged to lead water in a zig-zag pattern from a pump-inlet (111) to a pump-outlet (119).256. The wave energy converter (100) according to claim 5, wherein a deflector (115) is configured to redirect water flowing along the floor of a level toward a one-way valve (117) leading to a next level.30 7. The wave energy converter (100) according to any preceding claim, wherein the reservoir (120) has a top-lid (121) with a central opening (122).8. The wave energy converter (100) according to any preceding claim, further comprising an eductor section (130) between the reservoir (120) and the turbine section (140).349587169. The wave energy converter (100) according to any preceding claim, wherein drainage valves (118, 123) enable release of water from the top-section (10).5 10. The wave energy converter (100) according to any preceding claim, further comprising a frustoconical draft tube (150) downstream from the water turbine section (140).34958717PATENTKRAV1. Bølgeenergiomformer (100) med en toppseksjon (10) som er vippbar i forhold til en bunnseksjon (11) og en første flyter (101) i stand til å bære vekten av toppseksjonen 5 (10) på en havflate (1), karakterisert ved at toppseksjonen (10) har rotasjonssymmetri og en bølgedrevet pumpe (110) innrettet til å løfte vann en designhøyde (h0) inn til et reservoar (120) inne i toppseksjonen (10), og at bunn-seksjonen (11) har en turbinseksjon (140) til å generere elektrisk effekt (P) fra en vannstrøm fra reservoaret (120).10 2. Bølgeenergiomformer (100) ifølge krav 1, hvor bunnseksjonen (11) har en andre flyter (103) ved en øvre ende og en andre ballast tank (104) ved en nedre ende.3. Bølgeenergiomformer (100) ifølge krav 1 eller 2, hvor en slingrebøyle (160) forbinder toppseksjonen (10) med bunnseksjonen (11).154. Bølgeenergiomformer (100) ifølge et hvilket som helst foregående krav, hvor den første flyteren (101) har et ringformet element (101a) omkring seg.5. Bølgeenergiomformer (100) ifølge krav 1, hvor den bølgedrevne pumpen (110) er 20 anbrakt omkring reservoaret (120) og har flere sektorer (110a), hver med en rekke nivåer og enveisventiler (114, 116, 117) tilpasset til å lede vann i et sikk-sakk-mønster fra et pumpeinnløp (111) til et pumpeutløp (119).6. Bølgeenergiomformer (100) ifølge krav 5, hvor en deflektor (115) er tilpasset til å 25 omdirigere vann som strømmer langs gulvet i et nivå mot en enveisventil (117) som leder til et neste nivå.7. Bølgeenergiomformer (100) ifølge et hvilket som helst foregående krav, hvor reservoaret (120) har et topplokk (121) med en sentral åpning (122).308. Bølgeenergiomformer (100) ifølge et hvilket som helst foregående krav, videre omfattende en eduktorseksjon (130) mellom reservoaret (120) og turbinseksjonen (140).349587189. Bølgeenergiomformer (100) ifølge et hvilket som helst foregående krav, hvor dreneringsventiler (118, 123) muliggjør utløp for vann fra toppseksjonen (10) 10. Bølgeenergiomformer (100) ifølge et hvilket som helst foregående krav, videre 5 omfattende et stumpkonisk sugerør (150) nedstrøms fra vannturbinseksjonen (140).