Wave energy converter

Abstract

A wave energy converter (100) has a top-section (10) and a column (20) with a turbine section (140) containing a water turbine. The top-section (10) has rotational symmetry about a z-axis and a wave driven displacement pump (110) able to pump water to a design height H over an average sea level into a reservoir (119). A two-axis gimbal (160) connects the top- section (10) with the column (20) such that the top-section becomes tillable relative to the column (20), which is kept vertical during operation. Water leaves the reservoir (119) through the water turbine. An eductor section (130) adds a water flow to the flow from the reservoir (119) due to the Venturi effect. This reduces the need for water from the reservoir (119) required to produce an electrical power P.

Description

Wave Energy ConverterBACKGROUNDField of the invention

[0001] The present invention concerns a wave energy converter (WEC), i.e. a machine that converts wave power to electrical power.

[0002] Local sea states include swells and local wind generated waves, and are described by statistical parameters such as average wave heights and directions with associated variances. On average, swells carry more energy than local wind generated waves.

[0003] The wave climate on a location may be described by a wave energy flux (WEF) computed from measured wave parameters at the location. WEF is measured in kW / m - wave power per metre wavefront. This is a useful design parameter for WECs.

[0004] Global wind systems determine swell-building and thereby WEFs. For example, west coasts of North America and Europe have annual average WEFs > 40 kW / m, while the west coasts of Africa have much lower WEFs due to trade winds from southeast. Land stops swellbuilding. For example, the British Isles limit maximum WEF to 40 kW / m in the North Sea - greatest in winter in the north. Maps of annual average WEFs are available online.

[0005] Average wave heights 0.6 - 12.5 m roughly correspond to wind speeds 4 m / s (gentle breeze) to 25 m / s (storm), which are typical cut-in and cut-off speeds for wind turbines. A turbine’s efficiency grows with the ‘load factor’ (aka ‘capacity factor’), i.e. the fraction of time the turbine produces electricity.

[0006] Here and in the following, ‘efficiency’ without prefix means ‘produced energy per economic and environmental cost-unit’ .Prior and related art

[0007] Today, proposed WECs have been far less efficient than horizontal axis wind turbines (HAWTs) onshore or offshore. A first objective is to produce electrical energy more efficiently than offshore HAWTs, preferably also than onshore HAWTs.

[0008] HAWTs produce power approximately proportional to the area swept by their rotors. The costs of installation, control systems etc. do not grow as fast as rotor areas, so big HAWTs tend to be more efficient than small. However, availability of equipment for transport and cranes able to lift heavy machinery to hub heights > 100 m limit their sizes. In addition, offshore wind parks are usually located or planned in relatively shallow water - often on fishing banks. Required civil works for foundations on a sea floor of gravel, sand andmud raises initial costs significantly. Floating HAWTs are rare due to the costs of platforms able to counteract the torque generated by design wind speeds 60 m / s or more.

[0009] Today, new onshore HAWTs typically have rated powers 3.0 - 4.2 MW, rotor diameters 140 -150 m and hub-heights 105 - 120 m. Offshore HAWTs utilise winds that are not made turbulent by landscape. Compared to onshore HAWTs, they are usually bigger with typical rated powers 8 - 10 MW, and they have greater load factors than onshore HAWTs. Still, offshore HAWTs tend to need large subsidies.

[0010] HAWTs have considerable environmental costs. While the rotors appear to rotate slowly, the tangential speeds at the tip of the blades may exceed 100 m / s. On land, this creates noise that disturbs humans and animals. The blades also kill birds. Offshore, foundations on fishing banks disturb valuable marine eco-systems and thus have large environmental costs.

[0011] WECs can be classified by their working principle as either: a) overtopping devices, b) oscillating water columns (OWCs) or c) oscillating bodies.

[0012] The Wave Dragon is a slack moored overtopping device with a wide pair of wave reflectors that leads water up a ramp into a reservoir. Water leaves the reservoir through a set of low-head water turbines. Such turbines in the ’mini-range’ 100 kW - 1 MW have been developed for small waterways, e.g. with fall (‘head’) < 20 m, during the past few decades.

[0013] The Wave Dragon is large and heavy, and provides a stable platform for a pair of HAWTs. This evens out el-production because waves and winds have peaks at different times. Powerful motors are required to turn the HAWTs toward dominant winds and the overtopping device toward dominant wave fronts.

[0014] Today, there are five designs for WEFs 12 - 60 kW / m with rated power 3 - 38 MW divided 50 / 50 between the overtopping device and the HAWTs. From annual energy productions (in GWh / y) on wavedragon.com / technology / , one can estimate expected load factors 0.38 - 0.40 for all five designs.

[0015] The proposed WEC should be much smaller per rated MW, and thus have lower initial costs (manufacture, transport, installation) than Wave Dragons. It should preferably also have a load factor > 0.4, and thereby produce more energy per year and per rated MW than a Wave Dragon. Lower initial costs and a higher load factor increase efficiency.

[0016] 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. One may also consider WEF times diameter, for example 30 kW / m- 4 m = 120 kW, as a. maximum available wave power.

[0017] Varieties with an air turbine over an OWC are much less efficient than HAWTs at the same location. For instance, a HAWT with rotor diameter 4.3 m mounted on a steel pole is cheaper to build and maintain than a WEC with a cylinder for a similar rotor. Both devices would yield a rated power < 10 kW - much less than the 120 kW in a previous example.

[0018] Devices with oscillating bodies rely on relative motion between solid structures. Specifically, water in waves moves up and down causing wave energy to change continuously between kinetic energy and potential energy due to gravity. On average, the wave energy per m2sea surface is * pgA1where A is an amplitude half the average wave height. This energy per m2is evenly distributed between potential and kinetic energy, *4 pgA1each. Here and in the following, p is the density of water, e.g. 1025 kg / m3for seawater, and g is the acceleration of gravity, typically 9.81 m / s2,

[0019] Point absorber buoys move vertically along a rod and / or relative to a heavy platform. Many proposed varieties utilise potential energy only, and thereby misses half the wave energy available for production of electricity. Some varieties also make use of kinetic energy.

[0020] In general, water tends to pass by bodies that resist motion due to power take-off (PTO) systems. This limits the wave power converted to electrical power. Some examples:

[0021] The Pelamis WEC had large semi-submersed tubes connected by hydraulic PTO- systems, and extracted energy from wave-shape rather than wave-height. A prototype rated at 750 kW 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. Pelamis P2 is out of production, probably due to low efficiency

[0022] 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.

[0023] 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 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.

[0024] 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. The rather large dimensions are due to relatively low WEFs in the South China Sea.

[0025] The Triton WEC (pscillapower.com / ) has a point absorber buoy connected to a heavy submerged platform by three flexible tendons. Each tendon drives a rotating electrical PTO- system.

[0026] 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 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

[0027] These objectives are fulfilled by the subject matter in the independent claim 1. 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’.

[0028] More particularly, the invention concerns a wave energy converter with a top-section and a column with a turbine section containing a water turbine. The wave energy converter is distinguished in that the top-section has rotational symmetry about a z-axis and a wave driven displacement pump able to pump water to a design height H over an average sea level into a reservoir, and in that a two-axis gimbal connects the top-section with the column.

[0029] Rotational symmetry makes the top-section equally sensitive to waves from all directions, so there is no need to ‘turn the wave energy converter toward dominant waves’. The displacement pump displaces incompressible and inviscid water, so losses related to compressing air and friction are insignificant. The reservoir evens out variations in water flow from the displacement pump. It is understood that a two-axis gimbal is rotatable about mutually perpendicular x- and y-axes, both of which are perpendicular to the z-axis. This makes the top-section tiltable relative to the column.

[0030] In preferred embodiments, the top-section has a lid with an opening for letting excess water escape. The lid enhances structural integrity. The opening provides a maximum height, and thereby enables ‘cheap’ propeller turbines without means to accommodate variable flows.

[0031] In further preferred embodiments, the top-section has a ring-shaped first floater and ballasting means for keeping the top-section afloat during operation, and the column has a second floater and a ballast for keeping the column neutrally buoyant and vertical during operation.

[0032] When the top-section and column both are neutrally buoyant, loads and wear on the gimbal are minimised. A vertical column minimises loads and wear on bearings in rotary machines within the turbine section.

[0033] In embodiments with a ring-shaped first floater, the second floater preferably has a bowl-shaped bottom for leading water toward a one-way valve that opens towards the wave driven displacement pump and is located radially inwards from the ring-shaped first floater.

[0034] During operation, the bowl- shaped bottom leads upward-moving water up under a skirt formed by the ring-shaped first floater. This inhibits water from flowing past the topsection, and thereby increases the wave power available for production of electrical power.

[0035] In preferred embodiments, the wave driven displacement pump is divided into several sectors limited radially by an outer wall and an inner wall, each sector having a displacement chamber at a lower end.

[0036] Displacement chambers distributed along the circumference of the wave-driven pump make the wave-driven pump independent of wave direction and instantaneous tilt of the top-section. During operation, some displacement chambers become more or less compressed depending on how the top-section tilts. The compressed chambers must collectively provide a flow of water required by the water turbine.

[0037] The wave driven displacement pump preferably also has an inclined plate with a series of one-way valves, each opening in an upward direction. The inclined plate may be contained in a sector, or it may be a helical plate.

[0038] The column preferably contains an air-filled machine room. This may conveniently be part of the second floater, and may contain pumps and compressors for ballasting, electrical components etc., thereby eliminating a need for water-proofing all components.

[0039] Preferred embodiments further comprise an eductor section in the column upstream from the turbine section. The eductor section uses the Venturi-effect to add a flow of water to the flow from the reservoir. This reduces the required flow from the reservoir.

[0040] In preferred embodiments, the turbine section contains sound insulation for dampening noise that disturbs sea mammals. For instance, the noise might otherwise disturb communication between whales.

[0041] Preferably, the wave energy converter has a frustoconical draft tube downstream from the turbine section. Its purpose is to convert kinetic energy at the turbine runner to static pressure at a lower, wider end. This increases the hydraulic power available for production of electricity and eliminates a risk for harmful backflow of water through the turbine. It is well known that the most effective draft tube geometry is a cut-off cone with rotational symmetry.BRIEF DESCRIPTION OF THE DRAWINGSThe invention will be described in greater detail with reference to the accompanying drawings, in which:Fig. 1 illustrates possible uses of the proposed WEC,Fig. 2 illustrates a possible cluster of proposed WECs moored at arbitrary depth,Fig. 3 is a schematic cross section of the proposed WEC,Fig. 4 is a schematic cross section through a top-section of the WEC viewed from above,Fig. 5 illustrates geometries and some further details,Fig. 6 illustrates a turbine section with a generic, commercially available water turbine,Fig. 7 shows a typical turbine efficiency as function of angular speed,Fig. 8 is a schematic cross section of an alternative wave driven pump, andFig. 9 shows the pump in Fig. 8 viewed from above.

[0042] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] The drawings are schematic, and details known to the skilled person are omitted.

[0044] Figs. 1 and 2 illustrate use of the proposed invention. Items with reference numerals > 200 illustrate known technology for context. They are not part of the invention.

[0045] Fig. 1 depicts a WEC 100 according to the invention on a wavy sea surface 1 in a windpark with offshore wind turbines 300. The WEC 100 may share an area already approved for energy production, a local grid and a power cable from the windpark. Thus, WECs 100 with lower economic and environmental cost than wind turbines 300 would improve the overall efficiency of the park.

[0046] The WEC 100 has a top-section 10 with a cylindrical outer wall 11 and a ring-shaped first floater 12 around the lower part of the outer wall 11. The first floater 12 contains enough air to keep the top-section 10 afloat. Rotational symmetry makes the WEC 100 equally responsive to waves from all directions.

[0047] The proposed WEC 100 may also so be deployed outside wind parks and ‘close to’, e.g. 10 - 20 km from, a town or an industrial facility. This reduces a need for long, high- power cables to the town or industrial facility, and thus improves economic efficiency.

[0048] Fig. 2 shows that the top-section 10 is tiltable relative to a column 20. The topsection 10 moves on the sea surface 1 like a boat and pumps water upwards into a reservoir. Here, the traditional roll, pitch, yaw, surge, sway and heave correspond to rotations about andmovements along mutually perpendicular x-, y- and z-axes affixed to the top-section 10. The column 20 is kept vertical to minimise wear on bearings in a water turbine with associated generator.

[0049] In this example, three anchor lines 201 connect the WEC 100 to anchors 202 at arbitrary depths. Each anchor line 201, e.g. a nylon rope, has a buoy 203 that provides a force B directed upwards and a weight 204 that provides a force IF directed downwards. Viewed from above, the three anchors 202 form a triangle. The force pairs B, IF pull the WEC 100 towards a position over this triangle. Thus, the WEC 100 neither needs shallow water nor an expensive foundation such as those required for offshore wind turbines 300. The anchors 202 hardly affects the marine environment, thereby reducing environmental costs relative to those of HAWTs founded on fishing banks.

[0050] Umbilicals 210, 210a connect some WECs 100 to a node 211. Nodes containing electronic control and support systems for 8 low-head turbines are commercially available. In this example, the node 211 contains Li-ion batteries able to receive power P from the WECs and deliver a consistent power to a local grid through a cable 299. Furthermore, the node 211 is waterproof, has positive buoyancy and is deployed at a depth where surface waves have no effect. In deep water, this depth is Lm / 2 where Lmis a longest design wavelength. A typical depth might be about 30 m from the average sea surface level.

[0051] Fig. 3 is a schematic cross-section through the WEC 100. A general feature is a sturdy outer shell for structural integrity and for resisting collisions with floating debris. We note that steels have greater ductility and greater salvage values than fiberglass, and that internal elements may be less sturdy and cheaper per area unit.

[0052] The top-section 10 has a lid 13 to improve structural integrity. The lid 13 has an opening 14 to let excess water escape. A one-way inlet valve 15, for example a flexible flap, is mounted in a gap between the first floater 12 and the column 20. Reference numeral 16 represents equipment, e.g. ballast pumps and air compressors, for adjusting the buoyancies of the top-section 10 and the column 20. This includes an ability to submerge the WEC 100 temporarily during extreme wave conditions.

[0053] The top-section 10 contains a wave driven pump 110, a reservoir 119 and a two-axis gimbal 160 that connects the top-section 10 with the column 20. The column 20 ha a bowlshaped second floater 120, an eductor section 130, a turbine section 140 and a draft tube 150. The second floater 120 and a ballast 158 at the opposite end of the column 20 are designed to keep the column 20 neutrally buoyant and vertical during operation. Neutral buoyancy minimises loads on the gimbal 160 because the column 20 will move up and down in waves.

[0054] As shown in Fig. 4, the wave driven pump 110 is divided into N, for example 16 or 32, sectors that are limited radially by the outer wall 11 and an inner wall 111. Fig. 3 shows a vertical cross-section through two of these sectors. The inner wall 111 has a design height H over an estimated mean (average) sea surface 1.

[0055] Each sector contains a displacement chamber 112 at its lower end, specifically between the first floater 12, the inlet valve 15, a pressure plate 114 extending radially inwards from the outer wall 11, an upper face 124 on the second floater 120 and a flexible seal 113 that separates the displacement chamber 112 from the reservoir 119.

[0056] According to best practice, bulk heads divide the first floater 12 and other relevant elements into secondary sectors that not necessarily correspond to the sectors in Fig. 4. The secondary sectors provide redundancy: If one secondary sector leaks, the remaining secondary sectors uphold functionality.

[0057] During production, the displacement chambers 112 works as bellows for water: When the top-section 10 tilts, some displacement chambers 112 are compressed and their inlet valves 15 are closed. This forces water up a series of inclined plates 115 through a series of one-way valves 116. On the diametral opposite side of the tilting top-section 10, open inlet valves 15 let water into the associated displacement chambers 112. These will displace water when the top-section 10 tilts in the opposite direction.

[0058] The force required to push a mass m up a frictionless plane inclined an angle a from a horizontal plane isF = mgsina (1)Here, the inclined plane is folded, the angle a is measured from an x, y-plane defined by rotational axes of the two-axis gimbal 160, m is the mass of water carried by the inclined plates 115, and the force F is averaged over several wave cycles. A small force for opening the one-way valves 116 must be added.

[0059] In other words, the inclined plates 115 carry most of the water’s weight, so the force required to push water upward becomes much less than the weight mg. Compare with a car driving up a winding road. In terms of energy, a potential energy mgH = mgsina- ( / 7 / sin<z) (2)

[0060] In words: a force much smaller than the weight mg times a path much longer than a height H equals a potential energy mgH as long as the inclination angle a is small.

[0061] The gimbal 160 has a gimbal ring 161 with two outer axles connected to the topsection 10 and two inner axles connected to the column 20. The two outer axles are rotatable about an x-axis, and the two inner axles are rotatable about a y-axis pointing into the paperplane. The x- and y-axes are perpendicular to each other and to the z-axis - the axis of rotational symmetry for the top-section 10. Preferably, the x-, y- and z-axes pass through the centre of mass of the top-section 10.

[0062] For clarity of illustration, Fig. 3 shows the inner axles connected to a pipe. The skilled person will notice that this is an unnecessarily weak design for a real embodiment, and accordingly propose a stronger mechanical design.

[0063] In Fig. 3, the Z-axis (capital Z) is the axis of rotational symmetry for the column 20. During operation, the Z-axis is opposite and parallel to the direction of gravity.

[0064] The second floater 120 has a bowl-shaped bottom 121. In use, the bottom 121 leads water flowing upwards toward the inlet valves 15 rather than letting water flow past the topsection 10. This increases the wave power available for production of electricity.

[0065] Under some wave conditions, fast waterflows directed upwards may overcome the resistance of one or more inlet valves 15 to the displacement chamber(s) 112. This would simply add water displacement to the displacement provided by the tilting top-section 10.

[0066] Preferably, the second floater 120 has a machine room 125 with equipment for adjusting the buoyancy of the top-section 10 and the column 20, e.g. by pumping water into or out of the first floater 12 and ballast tanks 126 in the second floatier 120. The machine room 125 may also contain equipment for adapting produced electrical power to a grid.

[0067] During operation, water flows from the reservoir 119 through one or more column inlets 127 into a conduit 128. A nozzle 129 at the end of the conduit 128 increases water speed. Specifically, since the water flow Q = Av is constant through the nozzle 129, water speed at the tip of nozzle 129 is V2 =Aivi / A where Ai and Ai are cross-sectional areas of the inlet and outlet of the nozzle 129, and vi is the water speed through the inlet to the nozzle 129.

[0068] 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.

[0069] We may neglect small energy losses to friction etc. such that the energy in a flow of water becomes a sum of potential and kinetic energy. Per volume unit: pgh + Yipv2= pgH (3) where p, g, v and H are described previously.

[0070] In equation (1), the first term (potential energy per volume unit) is called ‘static pressure’ and is often assigned the letter p. The second term (kinetic energy per volume unit) is known as ‘dynamic pressure’ . For clarity, we avoid the traditional term ‘head’ here.

[0071] Equation (1) implies that the static pressure pgH in the reservoir 119 becomes a combination of static and dynamic pressures as water flows down the column 20. At the tip ofthe nozzle 129, the increased water speed v causes a drop in static pressure. This is known as the ‘Venturi effect’, and was first described in 1797.

[0072] The eductor section 130 has a cylindrical outer shell 131 for structural integrity and one or more eductor inlet(s) 132 under the second floater 120. An inner surface 133 has a reduced diameter at the tip of the nozzle 129. There is an eductor volume 134 between the outer shell 131 and the inner surface 133.

[0073] In use, water is sucked through the eductor inlet(s) 132 due to the Venturi effect. This adds a volume flow Qi to a volume flow Qi from the reservoir 119 for a total Q = Qi + Q through a water turbine to be described. The eductor volume 134 may contain variable amounts of water for ballasting.

[0074] The turbine section 140 has an outer shell 14 for structural integrity. Its main components are a commercially available water turbine with an associated generator. Fig. 3 shows an arrow representing electrical power P from the generator. A real embodiment would preferably have a cable from the generator to equipment in the machine room 125 - a mechanical shaft between the turbine and its generator though the eductor section 130 would be impractical or counterproductive.

[0075] A sound insulation 148 dampens acoustic frequencies that disturb sea mammals, e.g. frequencies whales use for communication.

[0076] The draft tube 150 has an outer shell 151 for structural integrity. Its main purpose is to convert dynamic pressure just downstream from the turbine to static pressure at a discharge 159. (A relatively low static pressure just downstream from the turbine increases power available for el-production and increases the risk for harmful backflow through the turbine) It is well known that a frustoconical draft tube 150 with its smaller diameter at the turbine and a cone angle about 6° is ideal.

[0077] Fig. 4 is a schematic cross section through the top-section 10 with N = 32 sectors. The first floater 12 preferably extends around the lower part of the outer wall 11.

[0078] In use, about half of the displacement chambers 112 (oner per sector) will be more or less compressed. The mass of water in each sector depends on the number of sectors, the radii of the outer and inner walls 11, 111 and the design height H of the inner wall 111.

[0079] In Fig. 4, two concentric outer axles 162 extend from the gimbal ring 161 to respective supports 163 affixed to the top-section 10. The outer axles 162 enable the topsection 10 to tilt about the x-axis. Two concentric inner axles 164 connect the gimbal ring 161 to the column 20, and enable rotation about the y-axis. It is understood that the column 20 may have sturdy supports similar to the supports 163 rather than a ‘flimsy’ pipe as shown.

[0080] Fig. 5 shows a point 19 through which the x- and z- axes of the top-section 10 as well as X-, and Z-axes bound to the column 20 passes. An extension of the inclined top-face 124 as well as y- and F-axes (not shown explicitly) pointing into the paper plane also pass through the point 19. In a real embodiment, the point 19 should be near the centre of mass of the topsection 10.

[0081] In Fig. 5, the top-section 10 tilts an angle 0(f) relative to the vertical column 20. As time t increases, 0(f) increases and the pressure plate 114 approaches the inclined top-face 124. Water in the displacement chamber 112 can only escape through the one-way valve 116 onto the inclined plates 115 (not shown in Fig. 5).

[0082] A maximum allowable tilt angle for the top- section 10 depends on a fixed angle fl between the inclined top-face 124 and an AT-plane perpendicular to the Z-axis. The maximum allowable tilt angle also depends on the orientation of the pressure plate 114, and must be less than 45°, preferably less than 40°.

[0083] A spring 121 at the outer end of the inclined top-face 124 between two adjacent displacement chambers 112 provides a spring force for damping if the pressure plate 114 comes close to the inclined top-face 114. Specifically, damping depends on the spring force, a damping coefficient provided through the one-way valve 116 and the moment of inertia of the top-section 10. The actual damping should be close to critical damping. Theory is available online and in textbooks.

[0084] A tilt sensor 122 may trigger mechanisms to submerge the WEC 100 if 0(f) approaches the maximum allowable tilt angle. The tilt sensor 122 may replace or be an addition to the spring 121.

[0085] Fig. 6 is a schematic vertical cross-section through a turbine section 140 between the wide end of the eductor section 130 and the narrow end of the draft tube 150. The machines within represents any suitable low-head turbine, e.g. for H < 15 m and 100 kW < P < 1 MW.

[0086] As noted, the turbine shell 14 provides structural integrity. Here, the turbine shell 14 is connected to the eductor section 130 and the draft tube 150 by nuts and bolts. Thus, the machinery within becomes available for planned maintenance or repairs.

[0087] The turbine in Fig. 6 has fixed inlet gates 141 and a runner 142 with fixed rotor blades 143. Such turbines are known as ‘propeller turbines’, and are relatively simple and cheap varieties of full-fledged Kaplan turbines, which have adjustable inlet gates and rotor blades with associated control systems. A WEC 100 with a fixed design height / / has does not need adjustable inlet gates and / or adjustable rotor blades.

[0088] Varieties of the WEC 100 with adjustable heads H due to variable ballasting and other Kaplan turbines are anticipated. ‘Low-head’ Francis turbines may also be employed.

[0089] A turbine shaft 144 extends through a bearing 145 to a generator 146 within a bulb 147. A generator cable 149 leads electrical power P from the generator 146 to electrical equipment in the machine room 125 shown in Fig. 3.

[0090] As indicated, the turbine section 140 is kept vertical during operation to minimise wear on bearings in rotary machines, here exemplified by the bearing 145. Also, the generator cable 149 conveys power to the machine room 125 in a more practical manner than a turbine shaft 144 extending through the narrow part of the eductor section 130.

[0091] Commercially available low-head turbines may have low 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. Life expectancies often exceed a typical design lifetime 20 y for wind turbines. Longer life implies lower financial costs.

[0092] Sound insulation 148 surrounds the machinery 141 - 147. The purpose is to dampen acoustic frequencies disturbing sea mammals, e.g. frequencies whales use for communication.

[0093] Fig. 7 shows turbine efficiency / (eta) vs. angular speed co (omega) of a water turbine. A curve 40 illustrates that a typical turbine has a peak / between 0.8 and 1.0 at an optimal com. The curve 40 also shows that the turbine efficiency / exceeds 0.8 in a range on both sides of the optimal angular speed com.

[0094] The required volume flow Q (in m3 / s) through a water turbine isQ = P / (pgHt]) (4) whereP is electrical power in W, p is the mass density of water, e.g. 1025 kg / m3for sea water, g is the acceleration of gravity, typically near 9.81 m / s2, H is the design height / head in m described previously, and / is the turbine efficiency, conservatively set to 0.8.

[0095] Recall that the volume flow Q = Qi + Qi where Qi is from the reservoir 119 and Q is added in the eductor section 130 due to the Venturi effect. Real eductors have Q2 / Q1 < 0.4, for example 0.3. Then Qi + Q2 = 1.3 , and a turbine that requires a flowrate Q would need a flowrate Qi = 01.3 from the reservoir 119.

[0096] Fig. 8 illustrates a version of the wave-driven pump 110 with four helical plates 115a - 115d arranged between the outer wall 11 and the inner wall 111. Each plate is inclined an angle a from the xv-planc defined by the two-axis gimbal 160, cf. equations (1) and (2).

[0097] During operation, water passes from a displacement chamber 112a through a oneway valve 116a onto the helical path 115a. Similarly, water passes from a displacement chamber 112b through a one-way valve 116b onto the helical path 115b. It is understood that there are similar displacement chambers under the helical plates 115c and 115d, and that the number of helical plates with associated displacement chambers may be different from four.

[0098] For clarity of illustration, Fig. 8 does not show further mandatory one-way valves 116 along the inclined plates 115a - 115d. Instead, small arrows indicate the upward flow of water during operation.

[0099] Fig. 9 shows the embodiment in Fig.8 viewed from above. Here, the mandatory one- way valves 116 are illustrated by small triangle pointing in the allowed direction of flow.

[0100] While the invention has been described by means of examples, the scope of the invention is defined in the accompanying claims.

Claims

CLAIMS1. A wave energy converter (100) with a top-section (10) and a column (20) with a turbine section (140) containing a water turbine (141 - 147), characterised in that the topsection (10) has rotational symmetry about a z-axis and a wave driven displacement pump (110) able to pump water to a design height H over an average sea level into a reservoir (119), and in that a two-axis gimbal (160) connects the top-section (10) with the column (20).

2. The wave energy converter (100) according to claim 1, wherein the top- section (10) has a lid (13) with an opening (14) for letting excess water escape.

3. The wave energy converter (100) according to claim 1 or 2, wherein the top-section (10) has a ring-shaped first floater (12) and ballasting means (16) for keeping the top- section (10) afloat during operation, and the column (20) has a second floater (120) and a ballast (158) for keeping the column (20) neutrally buoyant and vertical during operation.

4. The wave energy converter (100) according to claim 3, wherein the second floater (120) has a bowl-shaped bottom (121) for leading water toward a one-way valve (15) that opens towards the wave driven displacement pump (110) and is located radially inwards from the ring-shaped first floater (12).

5. The wave energy converter (100) according to any preceding claim, wherein the wave driven displacement pump (110) is divided into several sectors limited radially by an outer wall (11) and an inner wall (111), each sector having a displacement chamber(112; 112a, 112b) at a lower end.

6. The wave energy converter (100) according to any preceding claim, wherein the wave driven displacement pump (110) has an inclined plate (115; 115a - 115d) with a series of one-way valves (116; 116a, 116b), each opening in an upward direction.

7. The wave energy converter (100) according to any preceding claim, wherein the column (20) contains an air-filled machine room (125).

8. The wave energy converter (100) according to any preceding claim, further comprising an eductor section (130) in the column (20) upstream from the turbine section (140).

9. The wave energy converter (100) according to any preceding claim, wherein the turbine section (140) contains sound insulation (148) for dampening noise that disturbs sea mammals.

10. The wave energy converter (100) according to any preceding claim, further comprising a frustoconical draft tube (150) downstream from the turbine section (140).

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