GB2460339A - Water turbine system for capturing tidal energy - Google Patents

Abstract

The invention relates to a marine barrage structure that maintains a small differential hydraulic head for the purpose of extracting energy from a tidal estuary or other body of tidal water. The structure is installed across an estuary and comprises a foundation part 7 in the form of a shallow pre-cast concrete caisson or cellular frame and hydraulic turbine mounting parts 5 and 11 which are installed over the foundation part 7. The caissons may be buoyant and retained in position by piles or anchorages such that the caissons containing the turbines may rise and fall with the tide. An additional crest element 3 may also be incorporated. Island structures (2 figure 3) may be positioned intermediate the caissons on which wind turbines (3 figure 3) may be placed.

Description

DESCRIPTION

Title: Tidal Power Reef.

FIELD OF THE INVENTION

The gravitational forces created by the passage of the moon create the tides within the oceans of the earth. For many hundreds of years man has endeavoured to harness some of this energy, either by constructing a dam across an estuary or sea inlet or by the use of a device within the tidal stream that rotates with the water flow to extract en-ergy.

BACKGROUND TO THE INVENTION

Existing designs for tidal energy systems are on the one hand a very costly fixed barrage that puts the tide out of phase by several hours, producing a differential head of five or six metres to operate conventional low-head water turbines. Alternatively, an array of, tidal stream turbines can be used that cause no delay in the tide but have a lower energy output and are more difficult to maintain, since they are isolated structures requiring ac-cess by boat. The invention here disclosed comprise a novel turbine layout that allows the construction of a low cost and environmentally benign tidal energy system on ac-count of the low differential head employed.

KNOWN AND PRIOR ART

The known art encompasses traditional tide-mills with their stonework dams across es- tuaries or creeks and large sea defence structures and tidal barrages incorporating tradi- tional low-head water turbines. The structures are designed to meet the hydraulic re-quirements of these turbines, in other words, to create a head of water of many metres.

This has the effect of maximising the generation from any particular turbine design.

The large head difference means that the tidal cycle is delayed by many hours in order that the required head of water is created. Such turbines are usually unidirectional, so the water is allowed to flow into the holding pond through sluices and out through the turbines. A significant portion of the civil engineering cost is associated with the sluice gates on these by-pass channels and across the turbine channels. Floating pumps for irrigation systems are known, as are floating turbines and associated pipe work that span inland river weirs.

OBJECT OF AND ADVANTAGES OF THIS INVENTION

The quantity material, concrete and aggregates, required for construction should be sig-nificantly lower than that required for a full height conventional barrage. This is because the head differential and hence the hydraulic pressure exerted on the structure is less.

The hydraulic turbines also operate on a low constant head difference, typically around two metres; a head that gives the prospect of reasonable efficiency, while at the same time reducing the impacts on the environment and commercial shipping.

The generation profile for a low differential tidal barrage as here disclosed will be much flatter than for a convention barrage as it will operated for longer at a lower power output and requiring smaller generator and electrical transmission capacity. This type of tidal power installation will be much easier to integrate into an electrical grid system because the peaks in generation output will be significantly smaller. If operated as a pump stor-age installation or with excess capacity to meet peak power demand, the environmental implications of increasing the head differential by around 25% or 500mm would be minor but with a large storage lagoon this could still give a very significant capacity.

Conventional barrages require variable flow hydraulic turbines on account of the design head only being achieved for a short period during each tidal cycle, so the turbines are almost always operating at less than peak efficiency. The disclosed system is well suited to cheaper fixed flow turbines operating from a constant head differential that can be achieved for the greater portion of the generating period and hence the turbine design can be optimised for those hydraulic conditions.

By limiting the differential head to around two meters, it will not be necessary to incorpo-rate a conventional ship lock. A special channel would allow ships capable of more than knots to pass at all stages of the tide.

By limiting the operating head of the turbines to less than two metres and only causing a short delay in the tidal cycle, the differential pressure exerted on the structure in much lower, making the construction much easier and less sophisticated.

There is scope for using many smaller and more innovative turbine designs because they are physically smaller and more companies have the capability to build them.

By using pre-cast concrete caissons similar to those used in the construction of Mulberry Harbour, the construction can proceed rapidly during the tight windows of opportunity' when the tides and weather are favourable.

Using small units there is no need to have expensive non-revenue generating' technol-ogy such as sluice gates. For maintenance the complete bulb turbine is lifted out and replaced with a serviced unit.

Power generation and revenue earning could start well before completion of the project and would increase as the work progressed and the head increased.

It is envisaged that the visual impact will be reduced, since the structure need not incor-porate a traditional powerhouse with an overhead crane that could extend 30 metres above high water level. It is envisaged that a version of the disclosed barrage might only extend a few metres above sea-level.

The considerably reduced amount of rock fill required will result in a correspondingly lower impact from quarrying activities on the mainland and distributing the construction of caissons and other components will cause less disruption to the local and sensitive environmental areas.

SUMMARY OF THE INVENTION

In a water turbine system for capturing tidal energy, a water impounding structure to pro-duce an hydraulic head of water in which there is in combination a submerged or partly submerged foundation part incorporating pre-cast concrete supporting elements, to-gether with at least one pre-formed turbine carrying part, able to accommodate different designs of turbine and forming a complete hydraulic power unit with inlet and outlet pas-sages and being capable of being mounted and demounted from the foundation part and may have their buoyancy increased by the incorporation or addition of watertight voids or tanks.

The purpose of this system is to reduce the materials cost and allow rapid construction.

It also reduces the length/mass of the draft tubes from the turbine modules because they move with the tide and maintain a substantially constant operating head.

The base/causeway structure provides a means of access for construction and mainte-nance without the complication and cost of providing an all weather roadway high above the maximum tide and storm level. Islands can be provided at intervals along the struc-ture to act as inter tidal' refuges for plant and machinery, and to provide anchorage points or service areas for electrical services or the foundations for wind turbines. When the turbine modules are in place, access can be gained along their top surface whatever the state of the tide.

Precast concrete modules house the water turbines that are typically low specific speed-machines designed for a small differential head (small phase change of the tidal cycle) and have wide clearances so migratory fish can pass unharmed between the blades (smooth surfaces and no pinch points').

Because the head differential' is small and large storm waves can pass over the struc-ture, it is also more economic to build than a conventional barrage designed to counter the waves and support higher differential pressure. A conventional barrage is more of a hard engineering structure' with an adequate freeboard to protect an access or conven-tional roadway from flooding or being damaged. This invention is a soft engineered structure' that is much lower. It is a hybrid of conventional systems in that it gives a bet- ter energy capture than a free-stream tidal current turbine' and allows access for main- tenance with track-laying machinery (as opposed to the use of very expensive crane-barges).

The causeway structure or reef like structure is created with armour stone' and a core of smaller rubble. This rock may be placed by barge or from the landfalls with earthmoving machinery at low tide. It is envisaged that concrete performs may be incorporated into the structure or used to convey some of the fill material which is them dropped to the seabed or the performs are submerged or partly submerged as part of the construction process.

It is envisaged that the structure could incorporate concrete caissons that are buoyant that would be restrained and located with piles or anchorages such that the caissons, some of which could contain turbines, would rise and fall with the tide. A labyrinth, flexi-ble membrane or rigid movable element would prevent the flow from passing under the caisson as the water levels change, thus maintaining a roughly constant differential It is envisaged that the structure could incorporate concrete caissons that are part of the causeway structure or sit directly on the causeway structure and may contain turbines. A flexible membrane or rigid movable element supported by a floating part, would prevent the water flowing over the caissons as the water levels change, thus maintaining a roughly constant differential working head of water.

It is envisaged that the caisson structures will be built remotely from the construction site and towed into place where they will be fixed permanently with piles, anchorages or rock, or fixed in such a way that they can be removed for servicing.

It is envisaged that the caisson structures may contain submersible turbine units that can individually be removed for servicing, the system would be described as being modular.

It is envisaged that tanks or voids could be incorporated into the turbine modules' in such a way that the buoyancy could be altered to change the depth of submergence.

This feature could be used to shutoff turbines by raising them and breaking the vacuum in the water passages, or to allow excess water to flow over the module by blowing the tanks'.

It is envisaged that this structure could also be used for the construction of low head hy-droelectric installations on large rivers with very low falls.

The purpose of the causeway structure is to regulanse' the seabed without the need for extensive sub-sea excavation. This part of the structure also provides a means of access during construction without the complication and cost of providing an all weather road-way high above the maximum tide and storm levels. Islands can be provided at intervals along the structure to act as inter-tidal' refuges for plant and machinery.

Because the head differential' is small and large storm waves can pass over the struc-ture, it is also more economic to build than a conventional barrage designed to counter the waves and support a higher differential pressure. A crucial feature of this barrage system is that it is the civil engineering structure and low environmental impact dictate the turbine layout and design which is the subject of this invention here disclosed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A low head-water turbine system comprising a causeway' type foundation structure, onto which are installed a large number of floating turbine modules. The foundation structure would be constructed from precast concrete caissons that would be floated-into place, and anchored with piles or rock armour. The surmounting turbine modules would be also be pre-.cast and floated or lifted into place so that they were captive' over the foundation. Additional restraining features could be incorporated to guard against storm surges and bad weather.

DESCRIPTION OF THE ILLUSTRATIONS

FIGURE 1.

Illustrates a structure according to this invention in which a foundation element (7) con-structed as a pre-cast concrete caisson is affixed to the seabed with armour-stone (8) and/or piles (14). A turbine containing element (5) with turbine (11) is mounted on the foundation element (7) and together with additional crest gate element (3), forms the dis-closed structure. Said structure is designed to withstand the small differential head (1) between the sea and the lagoon.

FIGURE 2.

Illustrates a structure with a plurality of turbine containing elements (6) disposed be-tween a high head (1) and a low head (2) such as to produce a small differential head typically less than two metres. The foundation element (7) is shown on the seabed (10) and retained by armour-stone (8) FIGURE 3.

Illustrates an impounding structure comprised of turbine containing elements (1) and in-termediate Island structures that are also used as a support structure for additional wind turbines.