WO2012098148A1

WO2012098148A1 - Off-shore dam for energy storage, method of building such a dam from a plurality of construction elements and construction element as such

Info

Publication number: WO2012098148A1
Application number: PCT/EP2012/050689
Authority: WO
Inventors: Arild BOLSØ
Original assignee: Statkraft Development As
Priority date: 2011-01-18
Filing date: 2012-01-18
Publication date: 2012-07-26

Abstract

A dam encircles an area of an off -shore sea- or lakebed for storage of energy by changing the water level inside the closed dam in relation to a water level outside the dam by pumping of water. The closed dam is built from elongated construction elements (1) being higher than the water depth and being manufactured on land, brought afloat, transported to the off-shore location, brought aground, and sealed watertight together to encircle an area of the sea- or lakebed. The construction elements (1) comprise a buoyancy chamber (5) which can be flooded, a seal (22) between adjacent construction elements (1) and a sealing cavity (21) having an opening at the front face of the construction elements (1). Creating hydrostatic forces within the sealing cavity (21) pulls the front faces of adjacents elements (1) together, thus improving the sealing between the elements (1).

Description

OFF-SHORE DAM FOR ENERGY STORAGE, METHOD OF BUILDING SUCH A DAM FROM A

PLURALITY OF CONSTRUCTION ELEMENTS AND CONSTRUCTION ELEMENT AS SUCH

FIELD OF THE INVENTION

The present invention relates to off-shore inverted dams for storage of

hydroelectric energy by emptying water in the dam into the surrounding waters.

BACKGROUND OF THE INVENTION

Off-shore power production such as by means of wind, waves, or tidal currents has a large potential as the production facilities are not limited by lack of space or their disturbing effect on neighbouring population or nature. Therefore, they can attain very large scales with proportionally large energy production. For this reason, it is of importance to be able to store the produced energy for periods with high power needs.

Hydroelectric energy storage in inverted dams in relation to off-shore power production facilities has been proposed. The inverted dam principle involves a water basin at an off-shore location which can be emptied for water using pumps driven by the excess power from the off-shore power production facilities. The basin can then later be filled using a hydroelectric power station which produces electric power.

Present off-shore inverted dam concepts take the form of artificial islands such as those envisioned the Green Power Island (www.qreenpowerisland.dk^') or the Energy Island

(www.kema.com/Imaqes/091307%20KEMALievenseEnerqyIslandWebFINALpdn. These artificial islands involve the disadvantages that they must be constructed on location and at positions where suitable building materials can be found, or alternatively they involve expensive transport of very large amounts of building materials. In addition, they are typically prone to erosion and must therefore be protected against strong currents. Also, once constructed, they cannot be moved to another location.

Hence, an improved construction of off-shore inverted dams would be

advantageous. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alternative way to construct off-shore inverted dams.

In particular, it may be seen as an object of the present invention to provide a new method for constructing and a method for storing energy in an off-shore inverted dam that solve the above mentioned problems of the prior art.

In addition, it may be seen as an object of the present invention to provide a construction element for an off-shore inverted dam and a dam build by these elements. Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method for constructing a closed dam enclosing or encircling an area of an off-shore sea- or lakebed for storage of energy by changing the water level inside the closed dam in relation to a water level outside the closed dam, wherein the closed dam is built from elongated construction elements being higher than the water depth and being manufactured on land, brought afloat, transported to the off-shore location, brought aground, and sealed watertight together, wherein they are arranged to encircle or enclose an area of an off-shore sea- or lakebed by itself or by abutting artificial or natural structures rising from the sea- or lakebed.

In preferred embodiments, the method further comprises one or more of:

providing a plurality of elongated construction elements each having a lower base part to be set on a sea- or lakebed, an upper wall part for forming a section of the closed dam, and a total height larger than a water depth at a predetermined off-shore position so that when brought aground the upper wall part will have submerged section and a section above water, the construction elements each comprising :

o a buoyancy chamber which can be flooded;

o a seal arranged at an end face (or an end part or terminal edge) of the construction element, along at least the submerged section of the upper wall part for forming a watertight seal between abutting opposite end faces of adjacent construction elements to form a watertight seal between an inside and an outside of the closed dam;

bringing the construction elements to float, and transporting the construction elements to the predetermined off-shore position;

bringing the construction elements aground by flooding the buoyancy chambers until the construction elements stand on the sea- or lakebed, wherein the construction elements are arranged so that opposite end faces of adjacent construction elements abut; and

- forming watertight seals between the end faces of abutting construction

elements along at least submerged sections of the upper wall parts;

wherein the construction elements are arranged so that they enclose an area of an off-shore sea- or lakebed to form the closed dam. The steps of bringing aground, abutting, and forming watertight seals are preferably carried out sequentially for one construction element at the time.

Preferably, the method further comprising providing a pump and a turbine and installing these in a duct or another liquid connection between water inside the closed dam and water outside the closed dam.

In a second aspect, the invention provides a construction element for an off-shore closed dam comprising an elongated structure suitable for off-shore construction having a lower base part to be set on a sea- or lakebed, an upper wall part for forming a section of the closed dam, and a total height larger than a water depth at a predetermined off-shore position so that when brought aground the upper wall part will have submerged section and a section above water, the construction element comprising

- a buoyancy chamber which can be flooded; and

- a seal arranged at an end face (or an end part or terminal edge) of the

construction element, along at least the submerged section of the upper wall part for forming a watertight seal between abutting opposite end faces of adjacent construction elements to form a watertight seal between an inside and an outside of the closed dam. In a third aspect, the invention provides a closed dam constructed using the construction elements according to the second aspect.

In a fourth aspect, the invention provides a method for storing energy as a difference in water level between the inside and the outside of the closed dam. Supplied excess energy in the form of electric power is stored pumping water into the dam (normal dam configuration) or out of the dam (inverted dam

configuration). Stored energy is released by letting the water out/in through a hydroelectric turbine to generate electric power.

In a fifth aspect, the invention provides a method for storing energy from differences in water level between the inside and the outside of the closed dam generated by tides, and generating power from this stored energy. The methods according to the fourth and the fifth aspect may be combined and performed by the same closed dam according to the third aspect.

In the present context, the term "closed dam" is intended to mean that the dam encloses an area of an off-shore sea- or lakebed and thereby provide a watertight barrier between water inside the closed dam and water outside the closed dam. Preferably the closed dam encircles an area of an off-shore sea- or lakebed alone and thereby forms a closed loop. In an alternative embodiment, the parts of the dam formed by the constructions elements abuts one or more natural or artificial structures rising from the sea- or lakebed to above the surface to form an enclosed area. Also, the term off-shore is intended to mean "not on land".

However, as is described above, the dam may be closed using naturally occurring structures such as land, islands, reefs, rocks, etc., and hence some of the construction elements may be partly on-shore although the majority of the elements will be grounded off-shore. The construction method and construction elements according to the invention provides the advantage that they can be build on land, such as in a casting basin or dry dock, where construction materials are readily available and land-based construction techniques can be relied upon. A further advantage is that little or no sediment is required on location and that it is less sensitive erosion from currents, and thereby more flexible when it comes to selecting positions for placing. Moreover, the applied principles of constructing elements on land, transporting (e.g. towing) to off-shore locations, sinking by flooding, and interconnecting elements are similar to techniques known from construction of immersed tunnels. Thus, with modifications apparent from the following descriptions, well tested techniques and procedures can be relied upon for the method for construction according to the invention.

In a preferred embodiment, each construction element further comprises:

o a sealing cavity having an opening towards the end face, with the end face defining a brim surrounding the opening;

o wherein the seal arranged along at least the submerged section of the

upper wall part of the end face is also arranged along said brim, to form a watertight seal between abutting opposite end faces of adjacent

construction elements to watertightly close off both the opening of the sealing cavity and an inside of the closed dam ;

For the method of the first aspect, this preferred embodiment further involves creating hydrostatic and/or pneumatic forces pulling the end faces of abutting construction elements together by pumping out water from the sealing cavity.

The construction element according to this embodiment is thus characterized by the sealing cavity defined in its end face and the seal along the submerged section of the upper wall part and the brim surrounding the opening of the sealing cavity. When the element is brought aground, or already when it is brought afloat, the sealing cavity will fill with water through the opening towards the end face.

When correctly abutting another construction element, the sealing cavity can be used to pull, or rather suck, the elements tightly together by hydraulics and/or pneumatics. This is advantageous in that it is an easy way to pull the elements firmly together which does not involve underwater mounting (and subsequent removal) of jacks or similar. Following the compression, the elements may be mechanically locked together to maintain the compression of the seal after equalizing the hydrostatic/pneumatic pressure. The way the seal is mounted to both extend along the submerged section of the element and the brim of the sealing cavity provides the advantage that it serves two functions:

1. forming a watertight seal between adjacent elements, or more precisely

between their abutting opposite end faces of their submerged sections, thereby forming a watertight seal between the inside and the outside of the closed dam

2. forming a watertight seal between the brim of one element and the end face of the adjacent element, thereby watertightly closing off the opening of the sealing cavity from the surroundings. This has the effect of creating

hydrostatic and/or pneumatic forces pulling the elements together when pumping out water from the sealing cavity.

In a preferred embodiment, the seal is compressible, and the pulling together of the end faces of abutting construction elements compresses the seal to

strengthen and improve the watertightness of the seal.

In the remaining part of the description, a number of preferred and/or optional features, elements, examples and implementations will be described in more detail. Features or elements described in relation to one embodiment or aspect may be combined with or applied to the other embodiments or aspects where applicable. For example, features applied in relation to the construction elements or the closed dam may also be used as corresponding features in relation to the method and vice versa. Also, explanations of underlying mechanisms of the invention as realized by the inventors are presented for explanatory purposes, and should not be used in ex post facto analysis for deducing the invention.

BRIEF DESCRIPTION OF THE FIGURES The various aspects of the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set. Figures 1A and B illustrate different views of a construction element according to an embodiment of the invention. The longitudinal channel 5 can also illustrate the sealing cavity 21 or the buoyancy chamber 10 depending on whether it is seen as the end part or a cross section of a central part of the construction element.

Figures 2-4 illustrate different stages in the construction of a closed dam according to an embodiment of the invention.

Figure 5 is a close up of the illustration of Figure 4

Figure 6 illustrates the end face of a construction element according to an embodiment.

Figures 7A-C illustrate different stages in the formation of a seal between two construction elements in accordance with an embodiment of the invention.

Figures 8-12 illustrate different designs of construction elements according to embodiments of the invention. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is a cross-sectional view of an embodiment of a construction element 1 for an off-shore closed dam according to an aspect of the invention. The construction element according to the invention will now be described in more detail in relation to Figure 1, without the additional features of this embodiment suggesting any limitation of the broadest aspects of the invention.

The construction element 1 comprises an elongated structure 2 formed in a material suitable for off-shore construction meaning that it has to be able to withstand standing under water for long periods (many years) without

deteriorating or weakening. For example, the construction element can be cast in concrete using well established techniques known from e.g. bridge pillars, immersed tunnels, and other off-shore constructions such as drilling rigs. As seen in the cross-sectional view, the construction element 1 has a lower base part 4 to be set on/in a sea- or lakebed and an upper wall part 3 for forming a section of the wall of the closed dam (see e.g. Figure 3). At least one end face 6 of the construction element comprises seal (not shown) for forming a watertight seal between the end face 6 of the construction element and an end face 7 of another, abutting construction element (see Figure 7A-C). The seal is arranged on an end face of the constriction elements. To form a watertight closed dam, the seals between elements are formed along at least the entire upper wall parts 3 of the end faces of abutting construction elements. As will be described later, depending on whether and if how deep the base part is set in the sediment, the seal may also be formed along the base parts. However, in preferred

embodiment, no seal is formed between at least lower parts of opposite base parts 4 of abutting construction elements, as the formation of a seal is made difficult surrounding sediment. Construction elements may have only sealing means on one end face, or may have complementary sealing means on its two end faces 6 and 7, so that sealing means in one element is abutted to

complimentary sealing means of the end face of the abutting construction element. Alternatively, both end faces 6 and 7 may have identical, complementary sealing means.

In a preferred embodiment, the seals are formed using hydrostatic and/or pneumatic forces to pull two elements together. This is illustrated in Figures 1, 6 and 7A-C, where an end part of the construction element 1 comprises a sealing cavity 21 with the end face 6 defining a brim surrounding an opening of the sealing cavity towards the end face. The part of the sealing cavity 21 that goes into the construction element is at least temporarily closed and equipped with a duct 23. In the specific embodiment illustrated in Figures 1A and B, the bulkheads (not shown) used to seal off the longitudinal channel 5 to form the buoyancy chamber are positioned a distance into the channel so that end parts of the channel are divided from a central part of the channel, whereby the end parts of the channel forms the sealing cavity 21 and the central part the buoyancy chamber.

The sealing means preferably comprises a seal such as a packing, a gasket, a molding, or similar formed in one or more flexible and/or compressible materials capable of blocking passage of water at least when compressed. Such materials include rubber, plastics, putty, jointing compound or paste and other known sealing materials. The seal can be mounted on the end face of the construction element. In a preferred embodiment, the sealing means comprises a rubber strip for underwater use, such as the seal described in WO 98/33988.

The construction element has a buoyancy chamber 10. The buoyancy chamber can be flooded meaning that it is either open or only temporarily closed. Examples of open buoyancy chambers are upwardly open compartments similar to a ship hull, or air-filled downwardly open compartments. The buoyancy chamber preferably has a volume capable of keeping the construction element afloat when filled by air. In the specific embodiment illustrated in Figures 1A and B, the element comprises a through-going longitudinal channel 5 which is sealed off in its ends using bulkheads to form the buoyancy chamber 10. In a preferred

embodiment, the construction element is a modified element for the construction of an immersed tunnel.

Construction elements will normally be constructed for a specific closed dam at a previously determined, well known off-shore position, as it is important to know the depth, and potentially the character of the sediment, at this position when deciding the dimension of the element. The total height of the construction element is chosen to be larger than a water depth at the predetermined off-shore position, or to be larger than the water depth plus an estimated immersion of the base part of the construction element in the sea- or lakebed. However, it is also possible to increase the height of construction elements on location, prior to or after connecting the elements. This may especially be relevant if the water level changes or if the closed dam is moved to a new location.

Also, depending on the desired shape (as seen from above) of the closed dam, the various concrete structures forming the construction elements can be give appropriate shapes. In one embodiment, the construction elements have an arc shape in a horizontal plane so that end-to-end coupling of a plurality of

construction elements forms a round, closed loop. In another embodiment, the closed loop is a polygon and formed by a number of straight elements and a number of corner elements. Also, with regard to both the height and horizontal shape of the construction elements, if the closed dam is formed in connection with artificial or natural structures rising from the sea- or lakebed, the elements closer to such structure will typically have a customised shape and height which is different from the elements further away.

Figures 2-4 illustrate one possible embodiment of a method for constructing a closed dam encircling an area of an off-shore sea- or lakebed for storage of (hydroelectric) energy according to an aspect of the invention. The method according to the invention will now be described in more detail in relation to these figures, without the additional features of this embodiment suggesting any limitation of the broadest aspects of the invention. Construction elements 1 as described above in relation to Figure 1 are provided and brought to float, typically by flooding of a casting basin or a dry dock. The construction elements may float by means of the buoyancy chamber alone or may be equipped with additional buoyancy means for transportation. As will be described in relation to preferred embodiments later on, openings of the buoyancy chambers may be temporarily sealed to bring the elements afloat. The floating construction elements are then transported to the predetermined off-shore position, typically by towing as illustrated in Figure 2.

When at the predetermined positions, the construction elements are brought aground. By controllably flooding the floating chambers and releasing any additional buoyancy means, the construction elements are lowered until they stand on the sea- or lakebed. During the lowering, the position and orientation of the elements are controlled using e.g. towing vessels and wires to fixed anchors and possibly already grounded constructions elements. Thereby, each

construction element can be positioned as close to the intended position as possible. Abutting constriction elements are seen in Figure 3. After or during the grounding of construction elements, the sealing means of each construction element is abutted to an end face of an adjacent construction element, and a watertight seal is formed between the end faces of abutting construction elements. As the seal should form a watertight separation of water inside/outside the closed dam, the seal must be formed at least along those parts of the upper wall parts that will be below the water level on either side.

The last construction element to be placed which will close the dam may in some arrangements be specially adapted so that a seal can be formed at both its end faces without having to move the other, already grounded and sealed construction elements. In one embodiment, the last element forms a corner or part of a sharp bend in the overall arrangement of construction elements so that it can be drawn sideways into abutment with its two adjacent construction elements

simultaneously without having to move these. Alternatively, the end faces of the last construction element can be tilted or skewed to also allow it to abut adjacent construction elements at both its end by drawing it sideways. In another embodiment, at gap between one end face of the last grounded elements may be filled by a concrete element cast on-site with a lathing frame, or by a pre- manufactures element in suitable material. In a referred embodiment, a special last element is also special in that it involves a duct for pumping and/or power generation as well as the placing of a hydroelectric power station. Figure 4 illustrates a closed dam 12. Figures 4 and 5 illustrate one possible embodiment of a closed dam 12

constructed from a plurality of construction elements being arranged to enclose or encircle an area of an off-shore sea- or lakebed according to an aspect of the invention. The closed dam according to the invention will now be described in more detail in relation to these figures, without the additional features of this embodiment suggesting any limitation of the broadest aspects of the invention.

Figure 4 illustrates a closed dam 12 with a cut-out section illustrating a duct 15 for pumping and/or power generation and a hydroelectric power station 13. As mentioned previously. Figure 5 shows and enlarged illustration from where a lock 16 allowing a floating vessel to pass through the closed dam can also be seen.

A closed dam according to the invention is envisioned to have a diameter/diagonal between 300 - 5.000 meters, using constructions elements according to the invention having a length between 25 - 150 meters and a height between 10-75 meters. However, smaller and larger sizes can also be made. In the following, a number of specific embodiments of the construction elements and the method for constructing a closed dam using these elements are described in more detail.

Sealing

A watertight seal can be formed between the end faces of abutting construction elements along at least their upper wall parts. These seals between elements serve to make the dam watertight. Watertight seals can be formed using e.g. techniques know from e.g. immersed tunnels.

The sealing means arranged on the end face here comprises a seal 22 positioned to abut at least the entire brim-part of the end face 6 and the opposite end face of an abutting construction element - these forming the sealing faces (see e.g.

Figure 7B). In addition to thereto, the end face 6 and the seal 22 can involve a spurred extension 24 along the upper wall part, so that a seal is also formed along the upper wall part when a seal is formed around the brim of the sealing cavity. The part below the sealing cavity can involve a similar spurred extension 25. The process of abutting and forming a watertight seal with this embodiment is described in relation to Figures 7A-C. First, in Figure 7A, opposite end faces 6 and 7 of adjacent construction elements are brought together, e.g. by pulling using winches or jacks. At this stage, the seal 22 does not abut the end face 7 of the opposite element and the sealing cavity is filled with water. In Figure 7B, the opposite end faces 6 and 7 are brought together so that the seal 22 also abuts the end face 7 of the opposite element. At this stage, the part of the seal 22 around the brim of the sealing cavity 21 ensures that the sealing cavity is closed off. Water is then pumped out of the sealing cavity 21 through the duct 23, e.g. by connecting a pipe 26 to the surface. The emptying of the sealing cavity creates a large hydrostatic force which pulls the adjacent construction elements close together and thereby compresses the seal 22, as illustrated in Figure 7C.

Following the sealing compression, the elements may be mechanically locked together to maintain the same contact force even after equalizing the hydrostatic pressure. In an alternative embodiment, the duct 23 leads into another closed compartment in the construction element, e.g. a buoyancy chamber which is not flooded when setting the element aground, which will thereby also have lower pressure than the surroundings. Opening of the duct into such closed compartment will equalize the pressure in the sealing cavity and the closed compartment, and thereby form an effectively lower pressure in the sealing cavity which will pull the adjacent construction elements together. Hence, the pumping action in the step of emptying the sealing cavity may also be a sucking action or simply pressure equalization.

Several steps can be performed to make the construction elements stand more stable on the sea- or lakebed, some of these will be described in the following.

Suction anchor

Hydrostatic forces can also be employed to settling the closed dam in the sea- or lakebed using the principle known from suction anchors.

For this purpose construction elements can comprise a downwardly open compartment 8 in the base part 4, please refer to Figures 1A and B. When placing the construction element on the sea- or lakebed, the brim of the compartment 8 will be close off by the sea- or lakebed and the compartment 8 will be closed. By pumping out water from the closed compartment 8, the construction element will be sucked into the sediment of the sea- or lakebed and the base part 4 will become at least partly buried.

In one embodiment, the downwardly open compartment 8 of each construction element is open at the end faces so that it will be connected to the downwardly open compartment of adjacent construction elements. Thereby, the entire dam will form one long downwardly open compartment 8. Using such elements, the entire dam will be sucked into the sea- or lakebed when the compartment is pumped empty after connecting and sealing all elements. In another embodiment, each construction element involves a downwardly open compartment 8 which is closed at each end face (not shown in Figure 1A). By this means, each element can be sucked into the sea- or lakebed after abutting adjacent elements but prior to forming a watertight seal. Water may be pumped out of the downwardly open compartment 8 using a pipe going into the sediment and under the brim of the compartment. Alternatively, the construction elements comprise a duct with a lower end opening in an upper wall section of the compartment 8 and an upper end opening accessible from the outside of the construction element, to where a pipe to the surface or a pump can be attached. In an alternative embodiment, the duct leads into another closed compartment in the construction element, e.g. a buoyancy chamber which is not flooded when setting the element aground, which will thereby also have lower pressure than the surroundings. Opening of the duct into such closed

compartment will equalize the pressure in the compartment 8 and the closed compartment, and thereby form an effectively lower pressure in the compartment 8 which will pull the base part of the construction elements into the sediment of the sea- or lakebed. Hence, the pumping action in the step of emptying the compartment 8 may also be a sucking action or simply pressure equalization.

Burying and/or weighing down

The base parts 4 can be buried or partly buried to make them stand more stable as illustrated in Figure 3. For this purpose, the base parts of construction element comprise structuring 30 for receiving and holding material along its sides. The structuring may be in the form of upwardly exposed surface parts or flanges along its sides onto which sediment from the sea- or lakebed can be deposited. Different examples of such structuring are illustrated in the various embodiments shown in Figures 1 and 8-11. After or during construction of the dam, material is deposited onto the structuring to settle the base part in the sea- or lakebed. As this will not cause significant movement of the construction elements, it can be carried out during the construction for those elements already set aground and sealed together. Also, material can be filled into the buoyancy chambers of the construction elements to increase their weight and thereby.

The deposited material serves to weigh down the construction elements and must therefore be heavier than water. In one embodiment can be sediment from the sea- or lakebed, preferably from inside the closed dam. Alternatively, it can be material found at nearby off-shore locations and transported to the dam, e.g. drilling waste/slurry. Further, well known use of bentonite in the sediments below or adjacent to the dam can reduce the permeability and prevent water to pass under the dam structure.

Construction element designs

The construction elements may have a number of different designs as illustrated in Figures 1 and 8-12 which shows end faces of the construction elements. In the design illustrated in Figures 1A and B, the construction element has a cross-sectional shape with the lower base part 4 being wider than the upper wall part 3. Preferably, the lower part of the channel 5 is also wider than the upper part of the channel, so that it has an upwardly tapering cross-sectional shape. This serves to make the construction element stand more stable if the channel is later filled up with material for weighing the element down. The Buoyancy chamber preferably has an opening in its upper part equipped with a cap which can be opened to let water in/air out when flooding the buoyancy chamber.

The construction element 31 illustrated in Figure 8 is similar to that described in relation to Figures 1A and B above, except that the sealing cavity 21 (and preferably the longitudinal channel 5 and thereby the buoyancy chamber 10, neither shown here), extends all the way to the top of the construction element 1, i.e. all the way through the upper wall part 3. Thereby, the spurred extension 24 of the seal illustrated in Figure 6 is not necessary. Figures 9 and 10 illustrate similar construction elements 32 and 33 with different shapes and with less structuring 30 of the base part to receive and hold material. The designs in Figures 8-12 have the advantage that they can float stably in an upright position without additional buoyancy means. If the wall part above the channel 5 becomes too long and heavy, there is a risk that the construction element will lie on its side or upside down when brought afloat.

It is preferred that the opening of the sealing cavity is positioned to cover a middle or a centre of the entire height of the construction element as is the case for the elements of Figures 1, 8, 9, and 10. In other words, this means that the opening is placed to overlap the middle or centre point of the total height of the element. This provides the advantage that the hydrostatic pulling force acts close to the centre of the element, thereby ensuring that the pull is somewhat homogeneous over the vertical extent of the element, i.e. not skewed. For the purpose of illustration, consider pulling two elements together by only pulling at their upper corners, this is likely to cause them to pivot so that the base parts are properly not pulled together. Instead, pulling half way between top and bottom ensures a more homogenous pull.

In another preferred embodiment, the size of the opening has a vertical dimension equal to or larger than half the total height of the construction elements. This is the case for the elements of Figures 1, 8, 9, and 10. This provides the advantage that the force can be made large. For the purpose of illustration, consider pulling two elements together by hydrostatic forces generated by pumping water out of the sealed cavity with an very small opening (e.g. the size of a teacup) versus with a very large opening (e.g. as in Figure 8). In general, the larger the opening of the sealing cavity, the more efficient will the hydrostatic pull be (assuming the same pump, pumping ducts, size of the cavity, etc.).

The construction element 34 illustrated in Figure 11 has no longitudinal channel, and instead has an upwardly closed, downwardly open compartment 35. The thin lines illustrate stays for stiffening the element. The part of the compartment 35 in the upper wall part 3 forms the sealing cavity 21 (and preferably buoyancy chamber 10), and having this part at least partly filled with air, the construction element can stably float in an upright position. The part of the compartment 35 in the lower base part 4 forms the downwardly open compartment 8 for suction anchoring of the element as previously described. When the construction element 34 has been set aground by flooding the buoyancy chamber 10, pumping out water will such the base part 4 into the sediment, this will work best if the buoyancy chamber is completely flooded so that the compartment 35 is

completely filled with water.

The construction element 36 illustrated in Figure 12 has C-shape, with a downwardly open compartment 8 in its base part for suction anchoring of the element as previously described. The lower portion of the C shape forms a structuring 30 that can be used to hold large amounts of material for stabilizing the construction element. When rotated 90 degrees counter clockwise, the C- shape forms an upwardly open, downwardly closed compartment 37 functioning as a hull and thereby forms the buoyancy chamber. The construction element 36 can preferably be cast lying on its side in a dry dock, so that when the basin is filled, it will automatically float and can be towed to the off-shore position.

By adding a few walls in the end parts, sealing cavities 21 can be provided in the construction element 36 in Figure 12. Many of the design features described in relation to Figures 1 and 8-12 can be combined to form other construction elements falling within the scope of the invention.

Power station and duct

After sealing all elements to form the closed dam 12, a pump for pumping water out of (or into) the closed dam a turbine for generating hydroelectric power are installed. These are preferably controlled by a hydroelectric power station 13. According to the method for storing energy according to an aspect of the invention, the power station is connected to receive electric power and store this energy as a difference in water level between the inside and the outside of the closed dam. The difference is created by pumping water into the dam (normal dam configuration) or out of the dam (inverted dam configuration). When the stored energy is to be retrieved, the difference in water level is equalized through a turbine generating hydroelectric power.

The closed dam is preferably used in the inverted dam configuration. Figure 5 shows an embodiment of a power station 13 in the inverted dam configuration. Here, the bottom of the closed dam is connected to the outside of the closed dam through a duct 15 comprising a penstock 17 and a horizontal pipe 18. A turbine (not shown) for hydroelectric power production is positioned in the horizontal pipe 18. During energy storage, the same machinery may be used in reverse mode to pump water out of the dam. The power station is preferably positioned inside the closed dam, and one of the construction elements is specially adapted to hold this. Storing of tidal energy

In addition to the storage of energy, the closed dam may also be used to generate tidal energy according to another aspect of the invention. Tides at the off-shore position will naturally incur differences in water level between the inside and the outside of the closed dam. By exploiting the timing of the tides, water can be pumped into or out of the dam using less electric power than would otherwise be used. For example, in the inverted dam configuration, storage of energy by pumping water out of the closed dam is preferably carried out during low tide, as the amount of work necessary for pumping water out is smaller at smaller differences in water levels inside/outside the dam. Since the difference in water levels inside/outside the dam is smaller during low tide, more water can be pumped out (and thus more energy stored) using the same amount of supplied energy. Similarly, in the normal dam configuration, storage of energy by pumping water into the closed dam is preferably carried out during high tide. Operation of the closed dam according to these principles thereby stores additional energy (more water is pumped using the same energy). This additional energy is supplied by the changes in water level outside the closed dam at different tides as compared to the average water level outside the dam. The corresponding scenario can be exploited when generating energy by filling water into the dam through the turbine for generating hydroelectric power from the stored energy (difference in water levels). In the inverted dam configuration, more hydroelectric power can be generated during periods of high tide where there is a larger pressure difference as the difference in water levels

inside/outside the dam is higher.

If the tidal differences are very high, it may even be possible to empty the closed dam (inverted dam configuration) without using electric power at very low tides, and thus store tidal energy. Similarly, it may be possible to fill water into the closed dam (normal dam configuration) without using electric power at very high tides. For this purpose, one way valves, such as valves operated by pressure differences on its sides, may be provided in the closed dam. The additional storage of tidal energy depends of the timing with peak production (energy to be stored in closed dam) and peak consumption (energy to be extracted from closed dam) periods, and will not always be possible. However, as tide are very regular and can be predicted with a high accuracy, it is possible to make a protocol for the additional storage of tidal energy.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method for constructing a closed dam enclosing an area of an off-shore sea- or lakebed for storage of energy by changing the water level inside the closed dam in relation to a water level outside the closed dam, the method comprising : providing a plurality of elongated construction elements each having a lower base part to be set on a sea- or lakebed, an upper wall part for forming a section of the closed dam, and a total height larger than a water depth at a predetermined off- shore position so that when brought aground the upper wall part will have submerged section and a section above water, the construction elements each comprising :

- a buoyancy chamber which can be flooded;

- a sealing cavity having an opening towards an end face of the construction element, with the end face defining a brim surrounding the opening ;

- a seal arranged at the end face along at least the submerged section of the upper wall part and said brim for forming a watertight seal between abutting opposite end faces of adjacent construction elements to watertightly close off both the opening of the sealing cavity and an inside of the closed dam; bringing the construction elements to float, and transporting the construction elements to the predetermined off-shore position; bringing the construction elements aground by flooding the buoyancy chambers until the construction elements stand on the sea- or lakebed, wherein the construction elements are arranged so that opposite end faces of adjacent construction elements abut; and creating hydrostatic forces pulling the end faces of abutting construction elements together by pumping out water from the sealing cavity; wherein the construction elements are arranged so that they enclose an area of an off-shore sea- or lakebed to form the closed dam.

2. The method according to any of the preceding claims, further comprising moving sediment from the sea- or lakebed encircled by the closed dam onto the base parts of the construction elements to settle the base part in the sea- or lakebed.

3. The method according to any of the preceding claims, wherein construction elements further comprises a downwardly open compartment in its base part, and wherein the method further comprises pumping out water from the compartment to cause the base part to be sucked into the sea- or lakebed.

4. The method according to any of the preceding claims, further comprising moving sediment into the buoyancy chambers of the construction elements to increase the weight of the construction elements.

5. The method according to any of the preceding claims, further comprising providing a pump and a turbine and installing these in a duct between water inside the closed dam and water outside the closed dam.

6. The method according to any of the preceding claims, wherein seal is compressible, and wherein the pulling together of the end faces of abutting construction elements compresses the seal to strengthens and improves the watertightness of the seal.

7. A construction element for an off-shore closed dam comprising an elongated structure suitable for off-shore construction having a lower base part to be set on a sea- or lakebed, an upper wall part for forming a section of the closed dam, and a total height larger than a water depth at a predetermined off-shore position so that when brought aground the upper wall part will have submerged section and a section above water, the construction element comprising

- a buoyancy chamber which can be flooded;

- a sealing cavity having an opening towards an end face of the construction element, with the end face defining a brim surrounding the opening; and

- a seal arranged at the end face along at least the submerged section of the upper wall part and said brim for forming a watertight seal between abutting opposite end faces of adjacent construction elements to watertightly close off both the opening of the sealing cavity and an inside of the closed dam.

8. The construction element according to claim 7, wherein the elongated concrete structure has an arc shape in a horizontal plane so that end-to-end coupling of a plurality of construction elements forms a closed loop.

9. The construction element according to any of claims 7 - 8, wherein the base part comprises structure elements or flanges for receiving and holding sediment from the sea- or lakebed along its sides.

10. The construction element according to any of claims 7 - 9, wherein the construction element further comprises a downwardly open compartment in its base part.

11. The construction element according to claim 10, further comprising a duct with a lower end opening in an upper wall section of the compartment and an upper end opening accessible from the outside of the construction element.

12. The construction element according to any of claims 7 - 11, wherein the opening of the sealing cavity is positioned to cover a middle of the entire height of the construction element.

13. The construction element according to claim 12, further comprising a longitudinal channel and bulkheads dividing end parts of the channel from a central part of the channel so that at least one of the end parts of the channel forms the sealing cavity and the central part of the channel forms the buoyancy chamber.

14. The construction element according to claims 12 or 13, wherein the end face and the seal involve a part surrounding the opening of the sealing cavity and a spurred extension along the upper wall part of the end face of the construction element, so that a seal is also formed along the upper wall part when a seal is formed around the opening of the sealing cavity.

15. A closed dam constructed from a plurality of construction elements according to any of claims 7 - 14 being arranged to enclose an area of an off-shore sea- or lakebed.