GB2460340A

GB2460340A - Low head tidal barrage system

Info

Publication number: GB2460340A
Application number: GB0909166A
Authority: GB
Inventors: Rupert John Armstrong Evans
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-05-29
Filing date: 2009-05-28
Publication date: 2009-12-02
Also published as: GB0909166D0; GB0809745D0

Abstract

A tidal barrage structure 1, 2 is operated to produce a hydraulic head 5 substantially less than the total tidal range 6 and typically less than 2 metres. This reduces the stresses in the structure 1, 2 and turbine 7 and increases the length of time in each tidal cycle that the turbine 7 can be operated. The structure may incorporate buoyant concrete sections (not shown), some of which may contain turbines, that rise and fall with the tide.

Description

DESCRIPTION

Title: Low Differential Head Tidal Power System.

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 energy.

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 access by boat. The invention here disclosed comprise a novel civil engineering structure that requires significantly smaller quantities of material for its construction than a conventional barrage for tidal turbines or sea defences. The costs and environmental impacts are also reduced because the structure operates with a relatively small differential head of water, typically less than two metres.

KNOWN AND PRIOR ART

The known art encompasses traditional tide-mills with their stonework dams across estuaries or creeks and large sea defence structures and tidal barrages incorporating traditional low-head water turbines. The structures are designed to meet the hydraulic requirements 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.

The structures required for such a scheme have to be designed to accommodate the full working head. Likewise the turbines are designed to operate efficiently when a significant head difference has developed between the holding lagoon and the outgoing tide level, in the case of an ebb only generation' project.

OBJECT OF AND ADVANTAGES OF THIS INVENTION

The quantity material, concrete and aggregates, required for construction should be significantly 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 storage' 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 incorporate a conventional ship lock. A special channel would allow ships capable of more than 10 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' technology such as sluice gates. For maintenance the complete bulb turbine is lifted out and replaced with a serviced unit.

Power generation and revenue earning would start well before completion of the project.

As the work progresses the generation will increase.

Maintenance is likely to be easier and cheaper than mono-pile type tidal stream turbines' that will require a maintenance barge.

The environmental advantages The phase change in the tide will be limited to about one hour, resulting in minimal disruption to bird life and the mudflat ecology and fishing interests. The lower head differential will reduce the likelihood of local scouring. The visual impact will be reduced, since the structure need not incorporate 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

An hydraulic turbine system for capturing energy of a tidal flow by means of a barrage type impounding structure disposed across a body of tidal water or estuary so as to produce an hydraulic head across said impounding structure and said hydraulic turbine, that is substantially less than the total tidal range at said structure and typically less than two metres, such as to reduces the stresses in said structure and said turbine and increase the length of time in each tidal cycle that said hydraulic turbine may be operated.

It is envisaged that there would be a plurality of hydraulic turbines that may be of different types, layouts and sizes, disposed along an impounding structure. This structure may be comprised of elements and caissons and additional means to maintain a small head differential across the structure. Such means could include a crest gate, labyrinth, membrane or panel between the turbine containing element and the seabed, each or any being for the purpose of preventing the water flow from going around the turbine part and to maintain the small head differential.

It is envisaged that the structure could incorporate be 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, flexible 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 working head of water.

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 this structure could also be used for the construction of low head hydroelectric installations on large rivers with very low falls.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The disclosed system for operating a tidal power station would typically employ a small differential head of two metres across an impounding structure placed across a tidal estuary. If a tidal range of eight metres is present, the turbines rated at two metres of head would run at full power and full efficiency for a tidal increase of six metres. Allowing the head to even out at high water, the same situation would exist for the falling tide.

Tapering the generation at the top and bottom of each cycle would be possible by reducing the turbine R.P.M. as the working head reduced.

Modules create a difference in head by delaying the tidal flow, but this difference is significantly less than the total tidal range or the head normally required to operate low head turbines. By operating the turbines on a substantially constant head, the power output can be kept very constant and there is no need to have complex turbines capable of operating over a wide range of heads. In other words it is the structure that vanes and not the turbine.

DESCRIPTION OF THE ILLUSTRATIONS

FIGURE 1.

Illustrates, by means of a schematic diagram an impounding structure across a tidal estuary (1) and (2). Although the full tidal range is (6), the hydraulic head used by the turbine (7) is only the head differential indicated (5).

1) Just before high tide the sea level (3) is above the lagoon level (4) by the amount (5).

2) Just after high tide the sea level has fallen to (15),and the lagoon level (16) is held until the head differential (5) is achieved in the reverse direction and the turbines can be restarted.

3) At low water on the seaward side (13) the lagoon level (14) is still at the same head differential above the seaward side.

4) After low water the turbines (7) are stopped until the Sea level (11) has risen to a head differential (5) above the inner lagoon level (12)

Claims

CLAIMS1. An hydraulic turbine system for capturing energy of a tidal flow by means of a barrage type impounding structure disposed across a body of tidal water or estuary so as to produce an hydraulic head across said impounding structure and said hydraulic turbine, that is substantially less than the total tidal range at said structure and typically less than two metres, such as to reduces the stresses in said structure and said turbine and increase the length of time in each tidal cycle that said hydraulic turbine may be operated.
2. An hydraulic turbine system as described in claim (1) in which there is a plurality of hydraulic turbines disposed along an impounding structure.
3. An hydraulic turbine system as described in claim (1) in which said structure is comprised of elements and caissons and means to maintain a small head differential across the structure.
4. An hydraulic turbine system as described in claim (1) in which a turbine containing part is able to float with the rising tide and maintain a small differential head of water.
5. An hydraulic turbine system as described in claim (1) in which there is a submerged or partly submerged base structure of on which there are cast concrete caissons containing hydraulic turbines and above which there is a water flow controlling element attached to the turbine module' for the purpose of directing the water flow through the turbine module.
6. An hydraulic turbine system as substantially described in claim (1) with reference to FIG.1 of the accompanying drawings.