SYSTEM FOR THE EXPLOITATION OF TIDAL- AND RIVER CURRENT ENERGY

Abstract: The invention concerns a plant for the production of electric energy from ocean- and river currents. The plant is completely submerged below the surface of the water and comprises several turbines (A) with blades (G), a carrying assembly (C), a backstay system (E) and a generator. The shafts of the turbines (A) are oriented perpendicular tot he direction of the water flow and the blades (G) are shaped as wings, such that the turbine (A) rotates in the same direction regardless of the direction of the flow. The turbine shafts are supported in a framework with buoyant vessels (B) secured to the carrying- and bearing system. The plant is built from modules.

Description: System for the exploitation of tidal- and river current energy This application concerns a system for generating energy from water currents. The invention is particularly suited for use in tidal currents, river currents or ocean currents, and particularly where there is a limited cross section that water moves through, for instance rivers or narrow passages or sounds. Compared with other sources of energy, the use of the tidal energy is one of the most environmentally friendly and predictable of all known sources of energy. In sounds that become narrower, the current will converge towards the narrowest cross section. In these places, not only the velocity will be the greatest, but also most evenly distributed and such localization will therefore normally be the ideal site for a tidal plant. In passages the local bathymetry is absolutely decisive, and placing the tidal plant in places there it is exposed for strong currents with differ- ent direction simultaneously should be avoided. In rivers, similar evaluations should be made. Plants for collecting flowing current energy has seen limited use, because the costs will become to prominent as very large plants are required to collect even fairly modest amounts of energy. In the past it has been focused on the use of differences in level, and the use of the energy from the pressure height of the water, as in a low-pressure hydroelectric power plant. In such plants, it is required that the water is dammed up, is exploited through turbines, and will result in interference of the environment similar to those related to the building of tradi- tional hydro electric power plants. The size of the plants has done that they represent environmentally unfavourable changes and at the same time have been obstructing boat traffic etc. Several technical solutions have been suggested to reduce the above-mentioned disadvantages, and the solutions have concerned various turbine wheels or impellers with blades, and turbine/blade combinations that as a whole are placed below the surface of the water. US 3 922 012 shows a patent where a multitude of vertical turbines are placed on a frame that it is to be lower down to be placed on the sea bed. The frame has floating elements enabling the plant to be buoyant and to be towed to a suitable location. The floating elements are then been ballasted, such that the plant sinks and becomes located on the seabed. Patent US 3 912 937, describes a system for the production of electrical energy from ocean currents based on a two-part turbine blade construction. The plant is completely placed underwater and comprises a horizontally placed turbine. Patent US 5440 176 is based on a movable turbine, that is, it can be adjusted up and down in relation to the platform/frame depending of the operating conditions. In connection with the design of the turbine, the previous patent US 2 250 772 shows an impeller. However, with these designs, it is a problem that they are covering a relative small cross section of the flowing water current, or the units becomes so large that installation and maintenance becomes difficult. Accordingly the present invention concerns a plant that can cover a large cross section of a flowing mass of water, but nevertheless with components at a more manageable size, where the maintenance is simplified, and where the components or the entire plant more easily can be raised to the surface for maintenance and repair. This is achieved with a plant for the production of electric energy from ocean or river currents that comprises turbines with blades, shafts and side units, generators for the production of electric current; frame assembly equipped with buoyant vessels, assembled from one or several sub frames, where the plant as a whole is placed below the surface of the water. The shafts of the turbines are oriented substantially perpendicular to the direction of the velocity of the water, and the turbine shafts are supported in the frame assembly. The plant has positive buoyancy adjusted by the buoyant vessel and a backstay system is anchored below surface of the water, such that the plant is kept below the surface of the water by the backstay system. The blades are shaped as wings such that the turbines are rotating in the same direction regardless of the direction of the flow of the water. An embodiment of the plant according to the invention will now be described closer with reference to the enclosed figures where: Figure 1 shows the plant according to the invention as it will be seen from the sea bed looking towards the system in the direction of the flowing current. Figure 2 shows the plant seen from 2-2 on figure 1. Figure 3 shows the plant seen from 3-3 on figure 2. Figure 4 and 5 shows turbines in detail. Figure 6 shows an elevated view corresponding to figure 2 where the localization of the system is shown. The turbines (A), comprises wing shaped blades (G), formed such that the turbines (A) will rotate in the same direction, regardless of the direction of the water flow. The blades (G) are supported on each side. Generators (not shown) are connected to the shaft of the turbines (A), and will produce electric energy, that it is transferred through a cable (not shown). The generator is protected against ingress of water by means of a water resistant enclosure and possibly an overpressure in the enclosure. The turbines (A) are supported in frames (C) that comprise a number of vertical and horizontal frames or sub frames that are assembled as modules. The modules are assembled to create a plane with several turbines. This plane can thereby be adjusted to a desired cross section of current in a passage or a river. It is also possible to build the most critical components in such a way that they are reasonably easy to substitute in a running phase. This is particularly relevant in terms of exchanging components having an expected high frequency of repair and maintenance. The framework design gives in general a favourable distribution of the design forces, particularly those that acts on the turbine bearings. The buoyant vessels (B) are also installed on top of each end, and in the middle, to allow easy adjustment of the buoyancy of the plant. Figure 4 and 5 shows a cross section of the impeller (A) where the shape of the wing shaped blades (G) is better shown. The shape ensures that the impellers rotate in the same direction regardless of the direction of the water flow. The blades (G) can furthermore be made with adjustable pitch to better utilize the energy of the current. The blades (G) are secured to circular plates at each end to form the turbines (A). The shafts of the turbines are connected at the centre of each of these circular plates. The shafts of the turbines can be secured on each circular end plate to avoid that the shafts extend through the turbine. This will improve the flow conditions in the turbine. It is shown that the turbine wheels (A), 8 in total, and the 3 buoyant vessels (B) are assembled in the frame (C) that is anchored to the ocean/river bed (D) and is furthermore anchored through anchor wires, anchor lines or backstays (E) to anchoring block (F). From figure 6, the direction of the ocean/river currents (H) in relation to the system (I) is shown. The backstay, or anchoring, comprises tight anchor lines (E) keeps the plant under water and acts against the buoyancy. The anchoring is chosen from a request of the smallest possible play of the plant. Steel ropes or wires of the type "spiral strand" are chosen to achieve a long expected lifespan. The backstays (E) should furthermore be treated to avoid corrosion. The anchor lines are secured to five submerged winches (not shown) installed on the structure. The power to the winches is supplied through hydraulic hoses with quick release couplings placed on top of the plant. The total forces on the plant are relatively high. It is therefore used 10 anchor lines (E) for reducing the load on the anchor lines (E) and to reduce the effect in the event of breaking a line. The anchor lines (E) are secured to the five vertical frames (C) in the carrying assembly. Two anchor lines (E) are secured to each end of the plant. These anchor or backstay lines (E) are loaded with vertical and horizontal forces both in the length axis and transversally of the plant. For the three intermediate, inner frames, the backstays are secured to the bottom or lower part of the frames. All the backstays (E) have an angle 45° in relation to the seabed. Below is an example of a passage with a tidal current as the plant according to the example can be placed in. Water depth: 50 m Width of the passage on the place of installation: 500 m Maximum current velocity: vs = 2.5 m/s (5 knot) Significant wave height : Hs = 0.5 m Maximum wave height : Hm = 1.0 m Wave period range : Tz = 6.0 - 15 s Below are design parameters for the plant according to the described embodiment: Coefficient of drag for the turbine, Cd: 1.2. Drag area is set At = 12 x 12 m 2 x 0.75 = 108 m 2 , where 0.75 is the factor of the permeability for the turbine to accommodate for the flow through the turbine. The carrying assembly is designed as a steal frame. It comprises five vertical frames or sub frames with a height of 14 m and a centre distance 16 m interconnected by means of a horizontal frame. Total centre distance between the outer frames is 64 m. Because of the requirement for 10 m sailing height above the plant, the steel frame (C) will be situated 23 m below the water surface, that is, 27 m above the seabed. The distance to the seabed is thereby 20 m. The frame (C) comprises 500 mm tubular profiles with 20 mm wall thickness. This gives a net dry steel weight of 171 t for the frame (C) and a total buoyancy of 1261. The buoyant vessels (B) are placed on the top of the vertical frames (C) in the carrying assembly, one in each end and one on top of the mid-frame (C). Total buoyancy for the installation is chosen such that the anchor lines will not lose tension in the existing weather conditions. Each of the five vessels are 3.5 m in 5 diameter and 20 m long. This gives 1971 gross buoyancy per vessel. It is assumed steel tanks with a buoyancy to weight ratio of 3, that is, weight for each vessel becomes 66 1. The turbines (A) are arranged in two horizontal levels with four turbines in eacho level and horizontal shaft support. Thereby the centre distance of the turbines (A) becomes 14 m in vertical direction. The turbine consists of five blades that spans 12 m unsupported between two end plates with diameter of 12 m. The blades are of NACA 0016 profile meaning that the greatest blade thickness is 16% of the length of the blade. The length of the blades is set to 3.2 m such thats that the greatest thickness of the impeller blade is 512 mm. Hollow profile blades are chosen to save weight. The end plates comprise circular plates with corrugated core to reduce the weight. Total thickness is 120 mm. Total dry weight per turbine is estimated to 64 t. Similarly total buoyancy per turbine is 81 t. Thereby each turbine will have a net buoyancy of 17 1, which is suf-o ficient to carry the weight of the generator and gear system in connection with the turbine. The greatest spring tide velocity of 2 m/s gives a maximum effect per turbine of 208 kW. It is then assumed a turbine with exposed area of 144 m 2 and an effi-5 ciency of 22%. The plant will, with 8 turbines, have an installed effect of ca. 1 ,66 MW. The duration curve for the measured flow velocity flats out at ca. 1.75 m/s and it should be inquired if it is profitable to dimension for greater velocities than this.o This gives a turbine effect of 139 kW. The contribution from velocities above 1.75 m/s to an annual production is ca. 10%. The plant comprises 8 turbines where each generator shaft is connected to two turbines. This gives a maximum shaft effect of 416 kW per generator. The power cables to the surface can be connected in the atmosphere or under water. When coupled in the atmosphere, the cable is led through the enclosure with a water resistant cable gland or passage, which is a standard product on the market, and is connected to the generator terminals inside the enclosure. The enclosure must then be coupled to the generator before it is installed under water. This requires then that the generator with gear and enclosure is installed and secured to the carrying assembly after the carrying assembly has been installed and anchored or moored. In the invent of a breakdown of the generator, gear or cable, the cable will be pulled up along with the generator/gear module and is disconnected onboard a vessel. This requires that the cable is sufficiently long to, or the cable has sufficient play, so that the module can be elevated. If connecting under water, an electric underwater coupler or connector is used. One part of the connector is then secured on the outside of the enclosure. The other part is then secured to the cable and is connected to the enclosure when the ca- ble is laid. The generator/gear module can then be mounted on the carrying assembly onshore and be installed as a part of the total assembly. In the event of breakdown, the cable can be released from the generator/gear module under water and the generator/gear module or the cable can be hoisted up for repair. The electric under water connectors are expensive units, but this method can prove to reduce installation and maintenance costs more than the added cost for the electric system as a whole. Another critical factor will be the cables. Because of the tidal currents, the cable will be exposed for mechanical loads. It is therefore required that this is secured properly to avoid wear and fracture of the cable. Sea cables are normally flushed down or anchor to the seabed. The critical part becomes the length from the seabed and up to the generators. If it is chosen not to use underwater connectors to terminate the cables to the generators, an additional extra length of approximately 40 meters per cable is necessary to allow for extra play for hoisting up the generator/gear module to disconnect the cable. This extra cable length must be secured to avoid fracture of the cable, but must be simple and quick to release in the event of breakdown of the generator/gear module. When installing the anchoring system, the seabed can be surveyed by video where the plant is to be anchored. Seismic examinations of the seabed should also be performed. In the example it is assumed that a rigid anchoring by means of poles cemented securely in boreholes is used. The poles will typically have a dimension between 30" to 60", and are installed in 10-15 m deep boreholes. The plant is positioned on the place of installation with the short side towards the direction of the current flow, and five anchor lines on one side are secured by means of a smaller craft. Then, forerunners are secured to the five anchor lines on the other side to five under water winches that are installed on the structure. Power supply for the winches comes from hydraulic hoses with quick release couplings on the upper part of the carrying structure. In the change of tide, tugs turn the entire plant 90 degrees such that the long side is perpendicular to the direction of the current. When the forces from the current are tightening the five anchor lines that are secured to the structure, the under water winches are run until the plant is in position. In this position the next five anchor lines are locked, and divers or a ROV releases the hydraulic hoses during the change of tide. How the anchor lines are to be locked in the structure has not been evaluated in detail, but will be important during the design of the details. The method requires somewhat longer anchor lines than those that are anticipated for the construction of the carrying structure. Furthermore the use of fibre ropes with a protective layer (against fouling and sand intrusion) is recommended to ease the installation work. Any increase of costs with this change, is in this connection assumed to be of a smaller magnitude. The tidal plant will be submerged 10 meters below the ocean surface and will therefore not be visible. Where the cable is to be landed on shore, houses for frequency converters and transformers will be installed and will be visible, as before the installation. The plant will reduce the tidal current with approximately 20% in the area the plant will be placed. The water mills will rotate the same direction regardless of the direction of the water flow. A tidal power plant will have two opposite effects on the tidal current: A reduction of the velocity due to the increased resistance of the flow given by the plant, and an increase of the velocity because the plant blocks some of the cross section and chokes or throttles the current. If the water mills are distributed in the passage as windmills are distributed in large parks, the mills will only increase the resistance and reduce the current. But if the watermills are placed tightly together in the one and same cross-section, the velocity of the current in this cross section will increase even though it is reduced elsewhere in the passage. Thereby the watermills can drain more energy when they are placed in the same cross section and block the passage than they would if they were distributed. The given example is only described to help understanding the invention, and the invention is only defined by the appended claims.

Claims: C L A I M S 1. A plant for the production of electric energy from ocean- or river currents comprising turbines (A) with blades (G), shafts and side pieces, generators for the production of electric current, frames (C) equipped with floating vessels (B), assembled of one or several sub frames, where the plant as a whole is placed below the surface of the water, where the shafts of the turbines (A) are orientated substantially perpendicular to the direction of the flowing water, and where the turbine shafts are supported in the frame assembly (C), characterised in that the plant has a positive buoyancy controlled by the floating vessels and a backstay system (E) anchored below the surface of the water, such that the plant is held below the surface of the water by the backstay system (E), and where the blades (G) are shaped as wing profiles in a way, such that the turbines (A) rotate in the same direction regardless of the direction of the water current. 2. The plant according to claim 1 , characterized in that the shafts of the turbines (A) are substantially horizontal. 3. The plant according to claim 1 , characterized in that the turbine shafts are only secured to the side pieces such that the water current are not unnecessarily restricted by the shafts. 4. The plant according to claim 1 , characterized in that the turbines (A) are placed side by side and above each other in a plane to create a cross section where as little water as possible can flow on the outside of the turbines (A). 5. The plant according to claim 1 , characterized in that the backstays (E) are secured to winches such that the plant can be raised to the surface or lowered down as needed. 6. The plant according to claim 1 , characterized in that each sub frame creates a module. 7. The plant according to claim 6, characterized in that it is assembled from several modules and that each module can be raised to the surface, replaced or repaired independent of the others. 8. The plant according to claim 6, characterized in that the modules can be assembled in a plane with a configuration to cover a required cross section of a sound or a river. 9. The plant according to one of the preceding claims, characterized in that the angle of the turbine blades (G) are adjustable. 10. Use of a plant according to claim 1 in a part of a sound or a river with a current, where flow cross section is substantially unidirectional. 11. Use according to claim 10 where the plant covers the entire flowing cross section. 12. Use according to claim 10 where the plant covers the entire flowing current cross section up to a given level to allow ships to pass unhindered over the plant.

Inventors: FREDRIKSEN SVEIN, JOHANSEN HARALD, FREDRIKSEN, SVEIN, JOHANSEN, HARALD
Applicants: FREDRIKSEN SVEIN, HAMMERFEST STROEM AS, JOHANSEN HARALD, FREDRIKSEN, SVEIN, JOHANSEN, HARALD
Application year: 2000
Main classification: F03B17
IPC codes: F03B17