WO1991011614A1

WO1991011614A1 - Electrical power generation using tidal power

Info

Publication number: WO1991011614A1
Application number: PCT/GB1991/000138
Authority: WO
Inventors: Geoffrey Edward Lewis
Original assignee: Geoffrey Edward Lewis
Priority date: 1990-02-01
Filing date: 1991-01-30
Publication date: 1991-08-08
Also published as: GB2235252B; AU7232091A; GB2235252A; GB9002209D0

Abstract

Figs. (1 and 2) show a power generating installation constructed either within a river, an estuary or off-shore between an artificial island (3) and the coast line (2). Both the main structure of the barrier and the turbine ducts have venturi shaped profiles to maximise the water velocity. The turbines are positioned within profiled ducts in the barrage (1), linking these two structures. The profiles being designed to act as primary and secondary venturis, to increase the flow rate of the water through the turbines. The cases for both bi-directional and uni-directional water flow being covered. To avoid the periodic complete loss of power generation, that would occur with estuarine and coastal installations at times around tidal flow changes, several such generating stations can be built to make use of the tidal phasing. If three such stations are suitably positioned and their outputs linked by a central distribution station, power generation will be continuous.

Description

ELECTRICAL POWER GENERATION USIN6 TIDAL POWER.

Specification.

This invention relates not to the use of water turbine driven electrical generators for creating electrical power, but to the siting, positioning and design of the necessary

infrastructure required to overcome the problems associated with other installations. It is generally accepted that the most effective way of generating electrical power from a river or the sea is by the use a barrage or barrier that houses the necessary turbine/generator equipment. However, current technology does not allow energy to be generated during periods close to the times of high and low water levels. In addition, the full barrier systems that use the Potential Energy of a head of water retained within a

reservoir, have some serious environmental problems

associated with access, flooding and siltation. The system proposed in this invention, utilises the efficient generation equipment of such schemes, but resolves the above problems by dividing the full barrier (1) into two sections (2) and separating them as shown in Fig.1. Alternatively, the barrier can be moved off-shore and into deeper water as indicated in Fig.2. The barrier (1) is formed between the coast-line (2) and an artificial off-shore island (3), constructed from concrete. The proposed system is therefore driven by the Kinetic Energy of the water velocity. It can be shown that a velocity of 11 metres per second can produce as much energy as a 6 metres head of water. To obtain such a velocity from an average flow of 1 to 2 metres per second requires a gain by a factor of approximately 10. This is achieved by using venturi actions in two stages as indicated in Fig.2, where the profiles of island and shore-line and turbine ducts provide the primary and secondary gains. This proposal avoids the above environmental problems, allows power generation to be continous and at the same time, can in off-shore cases, help to reduce coastal erosion. Observation of the tidal flow along a coast line will show that most of the wave energy approaches land obliquely. The forces due to this flow can be resolved into two components. One directly on-shore which is responsible for coastal errosion and the other parallel to the shore-1 ine and responsible for the tidal phasing along the coast-line. These forces are shown in Fig.3, together with the resolved components. If the angle of obliqueness A lies in the range 30° to 45°, then since the available power is proportional to the sine of angle A, 50% to 70% of the energy in the tidal flow is available to drive the generation system. Fig.2. shows the juxtaposition of the turbine/generator barrage (1), the coastline/river bank (2) and the artificial island (3), in relationship to the tide levels and flow. Since the system is scaleable to meet local power and other demands, the dimensions have all been normalised. Fig.3. shows the relationship between the direction of tidal flow (T), the coastal errosion force (E) and the resolved power component P = TsinA.

Descr i p t i on .

Two specific cases can be indentified. In a river

application, the water flow will be unidirectional and the profiles to maximise the venturi gains can be derived from Bernoulli's Equation. Statistically these diameters follow an approximate Rayleigh distribution. For coastal or estuarine applications, the tidal flow is bidirectional and this requires that the Venturis should have a symmetrical profile. Fig.2 shows that the island and coastal outline has been given an elliptical shape. This is to produce a primary venturi effect that will increase the flow rate of sea water through the turbines/generators. These are built into the barrage and housed within similarly profiled secondary venturi ducts. In particular, this helps to maximise the water flow rate at periods close to high and low water levels and under conditions of Neap tides.

The elliptical shape is a good compromise under

bi-directional flow conditions. The profile is calculated from the formula (x²/a² + y²/b² ) = 1, where the

constants a and b are the semi-major and semi-minor axes of the ellipse respectively. The choice of the ratio a:b

determines the curvature of the profile and hence the throat restriction of the venturi. Typically a 20% reduction

provides a good compromise for the primary venturi and this is given by using a ratio of 1.9: 1. The dynamics of the venturi is quite simple; as the cross sectional area of the duct reduces, the flow velocity increases in direction proportion. Fig.4 shows the modifications necessary for an application where the water depth is insufficient. The maximum height of the open end of the duct can be reduced by making this elliptical. Provided that the cross sectional area πab = πr² the velocity gain will be maintained. The circular throat section that houses the turbine must be retained. Since the proposed concept is scaleable, design starts with the required number of generator sets that are needed to provide the desired power output. This then fixes the length of the barrage and from this the mouth dimension and island length can be calculated.

To avoid the periodic complete loss of power generation during tide reversals, several such generating stations can be situated along a coast-line to make use of the tidal phasing. If three such stations are suitably positioned and their outputs linked by a central distribution station, power generation will be continuous.

Claims

Claims.

1. A power generating station is established either wiithin a river, an estuary or off-shore, to produce electricity from the Kinetic Energy of the water velocity using a barrage between the river bank or shore and an artificial island. The water velocity being magnified by the application of the venturi effect.

2. As claimed in Claim 1, a chain of such coastal stations, if judiciously located can continuously produce electricity.

3. As claimed in Claim 1, the half barrier does not have to be high enough to retain a reservoir of water. Therefore the structure is smaller, can be built faster at lower cost and be brought into commission earlier than the equivalent full barrier.

4. Whereas the full length barrier schemes cannot easily be built in series to provide continuous generation, the claim in Claim 2, effectively places generating stations in

parallel. This allows the chain of stations to provide continuous generation due to the overlapping nature of the tidal phases.

5. As claimed in Claim 1, the magnitude of power generation is completely scaleable. The simplest case consisting of a single turbine/generator set.

6. If judiciously placed, the off-shore structure claimed in Claim 2, will have a significant impact on the reduction of coastal errosion.

7. As claimed in Claim 1, dividing the estuar ine/river barrage into two sections, avoids the silting and

environmental problems associated with a full barrage.

8. As claimed in Claim 7, this method of construction allows full access to any up-river port without the need to provide access locks.

9. As claimed in Claim 2, electrical power can be generated continuously from the non-polluting power source of the tides and rivers.

10. As claimed in Claim 1, some of the islands need not be completely passive. In suitable places, the outer walls can be adapted to perform the duties of harbour or marina type installations.

11. As claimed in Claim 1, to avoid using smaller turbines that tend to be relatively more costly than larger ones, the velocity gain of the secondary Venturis can be maintained in shallow water applications, by adopting an elliptical aperture for the open end of the duct.