GB2298004A - Tidal power generation system - Google Patents

Abstract

A power generating system comprises a turbine 12 installed in a flow channel in an isthmus (man-made or natural) between two unenclosed bodies of water across which there is a tidal gradient, particularly in a "Churchill Barrier" 14 laid to seal off one of the eastern entrances to Scapa Flow in the Orkney Islands. A caisson 10 defines a penstock 11 which feeds the turbine 12. After the caisson has been installed a section 13 of the Barrier 14 is removed to allow tidal flow past the turbine 12.

Description

POWER GENERATION SYSTEM The invention relates to a power generation system.

Hydro-electric generation schemes have been proposed and implemented in various forms, particularly by damming valleys to form elevated reservoirs and by building causeways across tidal basins to hold, at a high level, water brought in by a rising tide so that it can be allowed to flow out after the tide has fallen, thereby driving electrical generating apparatus.

One particular disadvantage of previously proposed hydro-electric schemes is that the body of water created by the scheme inevitably causes environmental changes and such changes on balance have tended to be detrimental. Apart from inevitable ecological changes, the building of a barrier in a flow of water affects the movement of fishes. A barrier to water flow is also likely to be barrier to navigation of shipping and locks may need to be created to avoid this.

According to the invention, a method of generating power by harnessing tidal flow of water comprises creating a flow channel in an isthmus separating two unenclosed bodies of water. across which isthmus there is a tidal gradient, locating a turbine in the flow channel and using rotation of the turbine to effect power generation.

Preferably such power generation is effected by rotation of electricity generating plant.

The isthmus may be naturally or artificially created but preferably is a "Churchill Barrier" laid during the Second World War to seal off the eastern entrances to Scapa Flow in the Orkney Islands.

Thus. for strategic reasons, to protect the Scapa Flow anchorage, four artificial barriers were created in the Sounds of Kirk, Skerry, Weddel and Water which extend respectively between the islands of Mainland and Lamb Holm, Lamb Holm and Glimps Holm, Glimps Holm and Burray and Burray and South Ronaldsay. Prior to creation of the barriers, there was a maximum quoted tidal flow through the Sounds of eight to twelve knots. The flow reverses according to the state of the tide.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 is a map of the southern islands of the Orkney Islands showing the location of the four Churchill barriers indicated I to IV; Figure 2 is a cross-section through a barrier showing the construction thereof; and Figure 3 is a cross section through a barrier of Figure 2 modified to include power generation equipment in accordance with the invention.

The barriers I to IV are essentially identical barriers at present. Installation of power generation plant on each can be carried out independently of the others.

Each site could earn revenue from the time of commissioning, and should currently qualify under the terms of the Scottish Renewables Order for a fixed premium electricity price.

The barriers are currently embedded in minor beaches and all major work would be capable of being accomplished from dry land. The scheme lends itself to modular work and large scale prefabrication with a minimal amount of sea based work.

A large proportion of the cost of the installation of conventional tidal power stations is that of the civil engineering works to form a barrage structure. By definition, effective sites must be subject to extreme tidal conditions and the construction of such barriers in these sites is onerous. In the case of the barriers I to IV the tide has been stemmed and the sites are therefore more passive as a result.

DIMENSIONS OF THE BARRIERS:

Sound H.W. mark Max depth Max depth spacing below L.W. below H.W.

Kirk 2000 ft 46.5 ft 59 ft (666 m) (15.5 m) (19.6 m) Skerry 2100 ft 39 ft 50.5 ft (700 m) (13 m) (16.8 m) Weddel 1400 ft 26 ft 38.5 ft (467 m) (8.6 m) (12.8 m) Water 2050 ft 29 ft 41.5 ft (683.3 m) (9.7 m) (13.8 m) The sounds the barriers currently cross were extensively studied prior to the installation of the barriers and hydraulic measurements taken. Hydraulic models were produced and a report of the work lies in the library of the Institution of Civil Engineers.

Copies of the Admiralty charts from 1907 and 1908 give detailed soundings in the areas and show velocities of between 4 and 8 knots.

The sites are of known geology and the sounds themselves are of varying maximum depths from 20 metres to less than 12 metres. The mean depth is less than 10 metres. Before construction of the original barriers the sea bed was scoured by the tide and was bare rock in places. There would therefore be simple foundation conditions for any structures with little or no siltation expected.

Turbine houses could be prefabricated on the mainland and barged into Scapa, itself a natural anchorage, so keeping on-site construction costs to a minimum. Lifts from the barges may well be best carried out by land based cranage so avoiding expensive specialist lift barges.

The site of each turbine house would need to be approximately on the centreline of the barriers as built at present. A temporary access causeway would need to be built to take traffic and to allow a working area. This could be formed from material dredged from the beaches that have accumulated since the original erection of the barriers and would be constructed on the existing beaches. Traffic on the Orkney Islands is essentially light vans and private cars with very few HGV movements.

The site for the turbine houses could be fully prepared in cofferdams driven into the existing barriers. These cofferdams could be driven from land based piling equipment standing on the barriers themselves. Once piled. excavation could also be accomplished from the barrier with excavated spoil being used for construction of other parts of the access ways on adjacent causeways.

The construction of the barriers was well documented and is shown in Figure 2. Each is formed of bolsters 1, 3 and 4 with rubble infill 2 and 5. Five ton blocks of stone are provided at the outer edges 7.

The rubble infill was dropped in wire gabions which will probably have disintegrated by now. leaving the rubble alone. The wave action zone 8 of the barriers is cloaked in ten ton blocks of concrete. The lifting points of the blocks will now probably be unusable, however the concrete appears generally sound. The blocks could be re-used for Rip-Rap in vulnerable areas. A roadway 9 is provided at the top.

It may be anticipated that aggregate supply might well be a problem as several quarries were opened and abandoned during the construction of the barriers due to the unsuitable nature of the stone for concrete production.

Figure 3 shows a possible layout of a power generation station by installation of a caisson 10 the boundaries of which are shown by a heavy black line. A penstock 11 feeds a turbine 12 and it can be seen that a section 13 of the existing barrier, the profile of which is shown at 14, would need to be removed. The existing beach profile is shown at 15 and a temporary roadway at 16. Stop log guide rails are shown at 17 and mean high and low water marks at 18 and 19 respectively.

Features of power consumption and generation in the islands include:1. There is a cable link to the mainland of some 20MW capacity.

2. The electrical power requirement of the Islands ranges between 17MW and 26MW during 24 hours.

3. A gas turbine on Flotta at the ELF oil terminal provides the difference shortfalls of power on a normal basis.

4. In the event of non availability of the gas turbine then a 32MW diesel generator set in Kirkwall is started manually.

Thus the Orkneys have a flexible power supply system at present and the integration of hydroelectric power from the Barriers should be feasible. At present the Islands draw power from the mainland upon demand and supplement the shortfall with diverse supplies.

It therefore follows that the effect of the fluctuating nature of the output from the barrier over the 24 hour period (and 28 day tidal cycle) could be used as a predictable base load, to which power from the mainland could be added. Alternatively the system could operate as at present if the tides were stationary. And finally the possibility exists of exporting up to 20MW of power to the mainland when the Barriers supply more than the islands requirements.

The environment assessment of such a project will be simpler and cheaper than would be the case in other tidal sites. This is principally due to the fact that the changes to the ecosystems have already occurred. The project will be returning a site to nature that has already been altered by man.

Unlike other sites that have been proposed before for tidal power this will not involve altering the ecology of the area. There are no mud flats to affect, fish do not currently pass through the lines of the barriers and the tidal patterns of the rest of Scapa Flow were not dramatically affected by the original closure.

There would be the loss of the beaches that have built up on either side of the barriers, however these are sparsely used and there are many other similar sites nearby. Study into the project will undoubtably show that the environmental benefit of the project will therefore easily exceed the dis-benefit.

The Orkneys derives a substantial proportion of its revenue from the scuba diving community who come to visit the scuttled German fleet.

The blockships sunk in the First World War which partially blocked the Eastern entrances before construction of the barriers would need to be removed to prevent damage to the turbines from pieces breaking off and being pushed by the tide into the generating structure. However they have now substantially disintegrated and are not of historic interest.

None of the vessels affected by re-opening of the barriers are part of the commonly dived wrecks.

There would be no need to provide fish ladders or passes on the barriers as there are currently no means of passage.

The provision of the original barriers was an enormous boost to the Orcadians as previously cut off communities gained access to the main island. It would therefore be essential that there would be no restriction to access across the sites once completed. This would not be a problem as other tidal power sites also provide access causeways for the public.

No navigational considerations apply. Thus there will be no need to provide ship access locks as the barriers already seal the Sounds, so there would be no presumption in favour of providing locks.

The site is different from main tidal generation schemes in that it is not a basin that needs to fill before it can be used. Other tidal power station sites normally allow water rapidly into their basins and impound it there. They then only generate on the ebb tide, even though they may be designed for bi-directional generation. This permits maximisation of the hydraulic head available at mid tide. Some sites also make use of pumping of water into the basin during periods of cheap electricity and as such are able to act as pumped water storage stations.

Since in this scheme the other entrances into Scapa Flow will continue to operate, the site will therefore not be a closed basin. It would therefore be impossible to attempt to hold back and impound water so as to create a greater head. Similarly there would be no opportunity for the use of the site for pumped water storage.

Generation therefore needs to be 'opportunistic' and simply a case of letting the water pass when the differential tidal forces cause water movement.

In the case of the Churchill Barriers, compared with other tidal schemes. the cost of the civil works would be substantially less significant an element due to the ready existence of the barriers themselves in that the entrances are currently not affected by current, they will only be so once the scheme is 'turned on' and the majority of work will not involve working over water, it will simply be a matter of dry work which is later flooded.

It may be prudent to install the turbine houses without the turbines in place at first to ensure no damage to them by the high sediment load that might be expected during re-stabilisation of sea bed profile.

It is absolutely essential that the integrity of the barriers is maintained. Once breached the whole project may become unstable and consequently unviable. In addition the barriers would inevitably be required to be re-instated at considerable cost. It took five years to build the barriers by dropping rocks from aerial ropeways into the enormous tide races. Careless scheduling of work on the site could prove catastrophic.

Claims (8)

1. A method of generating power by harnessing tidal flow of water comprising creating a flow channel in an isthmus separating two unenclosed bodies of water, across which isthmus there is a tidal gradient, locating a turbine in the flow channel and using rotation of the turbine to effect power generation.

2. A method according to claim 1, wherein power generation is effected by rotation of electricity generating plant.

3. A method according to claim 1 or claim 2, wherein the isthmus is a "Churchill Barrier" laid during the Second World War to seal off one of the eastern entrances to Scapa Flow in the Orkney Islands.

4. A method according to claim 3, wherein the flow channel is created in the "Churchill Barrier" by installing cofferdams in the barrier. excavating between the cofferdams, installing generating equipment in the excavated region and subsequently removing the cofferdams and marginal portions of the "Churchill Barrier" outside the cofferdams.

5. A method of generating power as claimed in claim 1 and substantially as hereinbefore described.

6. A power generation system comprising a turbine installed in a flow channel located in an isthmus separating two unenclosed bodies of water, across which isthmus there is a tidal gradient and power generation equipment coupled to the turbine.

7. A power generation system according to claim 6, wherein the isthmus is a "Churchill Barrier" laid during the Second World War to seal off one of the eastern entrances to Scapa Flow in the Orkney Islands.

8. A power generation system substantially as hereinbefore described and illustrated with reference to the accompanying drawings.