GB2317923A - Walls for ocean wave control - Google Patents

Abstract

Wave training surfaces in horizontal and vertical curved planes are utilised to control the passage of waves, thus protecting installations from ocean wave action by reducing the wave impact force and in eliminating or reducing a so-called Mach Stem effect which reduces available wave energy in wave energy electrical power generation installations. The surfaces may form part of a breakwater (fig. 1) or walls that channel waves towards a power-generating turbine (figs. 2-4).

Description

Wave Training Walls in Horizontal and Vertical Curved Planes The invention relates to ocean wave energy control and to electrical energy generation from ocean waves.

Training walls, breakwaters and harbour walls have been constructed for hundreds of years.

in recent years emphasis has been given to electrical energy generation using the inherent energy in ocean waves.

In particular the Energy Support Unit of the Department of Trade and Industry at Harwell has carried out a review of wave energy (Review volume 1, December 1992) and subsequently came to the conclusion that "Wave energy has no significant benefit to the United Kingdom." (Magazine "New and Renewable Energy" issue of 27 December 1995, page 8).

This application introduces a phenomenon, not widely known, which materially affects the performance and cost structure of sea defences and is particularly applicable to ocean wave energy production. The applicant has witnessed this phenomenon in wave action on a low sea wall. Additionally, Professor Weigel of the United States has carried out laboratory experiments and observed the effect (Oceanographic Engineering, published by Prentice Hall) - the Mach Stem effect (which may be better known to acoustic engineers).

Accordingly the present inventions consider the phenomenon described as follows: An ocean wave whose direction is normal to the wave crest line striking a wall at 45 degrees or less gives rise to the "Mach Stem" effect in which part of the wave - the larger part - is reflected at 90 degrees to the wall with the remainder continuing along the line of the wall (drawing No. 1).

If the angle of incidence is greater than 45 degrees total reflection of the wave occurs (the applicant considers that 40 degrees angle of incidence may be a more reliable upper limit).

The converging channels do not cause a loss of energy apart from the Mach Stem effect. Friction between water molecules is assumed to be zero and molecules of liquid adhere to the channel walls forming, effectively, a frictionless skin. If the Mach Stem effect is eliminated the height of the wave at exit, which is a measure of the electrical power to be generated less turbine/generator losses, can be calculated by equating the total wave energy of the length of crest at the entrance, to the wave energy of the length of crest at the exit.

Thus, if the Mach Stem effect is eliminated or reduced significantly by the curve in the vertical plane of each wall, the total wave energy is available for electrical generation.

It is pointed out that waves reaching the western shores of the United Kingdom have travelled several thousand miles from the major storm centre which originated them, augmented by prevailing winds. Any loss of energy in the length of the channel of the device will be minimal.

Case 1. Breakwaters for harbours If a curved training wall is so constructed that the angle of incidence of worst case storm waves is less than 40 degrees throughout the length of the wall much of the wave will be reflected at 90 degrees to the line of the wall into the path of following incidence waves reducing their energy. A small component of the incidence waves will continue along the line of the wall (sketch 1).

Two such breakwaters could be so constructed as to permit sheltered water behind the breakwater with an entrance at the apex of the two walls.

Many breakwaters are provided with armour, i.e. large rocks, concrete blocks, tetrapods, stabbits and dolos etc., which increase the cost of the breakwater construction. The breakwater described above reduces the requirements for such armour.

Case 2. Wave energy electrical generation.

Many of the wave energy electrical generation schemes outlined by the Department of Trade and Industry's review do not consider the total energy of a wave and cannot do so. The wave energy extends from the surface wave form to approximately a depth of half the wave length of a particular wave regime. Thus part of the energy passes under or over the proposed devices examined. (Although the wave energy does decrease from a maximum at the surface to zero at a depth of approximately half wave length).

Other of these reviewed structures, to be founded on the ocean bed, form an obstruction from which part of the wave would be reflected, destroying its energy. Above all the cost of design and construction appears to be prohibitively high.

The one example which attempts to avoid this was the Norwegian "Tapchan," which was given little attention in the Review as the electrical output was low. Very little detail of design for this was provided. Basically two training walls converged towards a reservoir which is filled by wave action spilling over the reservoir entrance wall. Electrical generation was achieved by draining the reservoir through a turbine generator. The reservoir emptied in 12 minutes. The claimant assumes the design suffered from the Mach Stem effect. Tapchan, presumably tapered channels, could however be constructed in a manner which would achieve economic viability by reducing the Mach Stem effect. The claimant's proposals are as follows: 1) Two converging training walls are provided each in a curved form in horizontal and vertical planes to reduce the Mach Stem effect (sketches No. 2 and 3). Each wall is normal to the line of ;he wave crest at the entrance, curving to 20 degrees at the exit to he turbine generators. This 20 degrees is assumed from Professor eigel's statement that no Mach Stem effect was visible n his work at this angle of incidence. The Mach Stem effect zill still exist to a significant extent.

Uhe entrance width is assumed in this case to be 50 metres converging to 5 metres exit width but these dimensions can be raried to suit wave length and energy.

.ne depth of channel so formed is assumed to be half the wave iength (assuming energy of the wave extends to a depth of lpproximately half the wave length). The two walls so formed ioin at mid channel throughout their length. Thus the system overcomes many of the shortcomings of the Department of Trade and industry Review, i.e. full wave energy depth employed and a much greater entrance than exit width provided and use of turbines of proves design and efficiency (e.g. the Rance tidal scheme).

'otal wave energy is assumed by theory to be composed of 50% ?otential energy and 50% kinetic energy. It has been assumed that potential energy only contributes to the increase in wave weight and kinetic energy remains unchanged in order that the wave period remains unchanged.

En the context (curving converging walls) the Mach Stem effect, although reduced by the horizontal curved walls, is detrimental in that the Mach Stem waves from each wall will meet at mid channel, bliminating the energy from these Mach Stem waves completely.

Uhis invention overcomes this by the introducing the vertical urve to each wall, of circular and/or ellipsoidal form. The Mach ;tem wave is now reflected towards the surface of the wave, increasing the wave height (sketch 3).

rhe wave of increased height at the exit falls to Kaplan type, Low head turbine/generators.

[t is pointed out that the training walls also raise the trough zf the wave as the channel is approximately one wave length in Length.

'he construction of the training walls, turbine/generator housing nd electrical control room could be in the form of concrete aissons, partly constructed ashore, launched and completed afloat, and thence towed to site and sunk in place by flooding.

;iting would be limited to wave depth of the order of 50 metres.

nhe site chosen should be adjacent to an existing shore line ?ower cable connected to the National Grid to reduce the cost of ?roviding new power cables to the shore, (e.g. at Dounreay or 'hurso).

3ecause of the high cost of preparation of the sea bed :oundations for the above scheme, an alternative proposal would be ;o site the wave energy generator afloat. This would provide a much greater choice of sites at improved wave regimes. The converging doubly curved walls (horizontally and vertically curved) would be maintained. (Note that the construction proposed requires the design to be floated to site in both inshore and floating systems).

An additional advantage of the floating structure is that it could be aligned by adjusting the mooring lines so that the entrance always faced into the prevailing uind/wave direction.

Mooring lines would pass out through the bottom of the caissons to reduce the length of mooring line to reduce the possibility of fouling by attendant vessels.

The preceding designs each with two training walls would provide a fluctuating electrical output as the crest of each wave is followed by the trough. This could be reduced partly by adjustment of the turbine blade pitch and by electrical rectification. An alternative solution to this is proposed as follows: A second system identical to the first but sited one half wave length down wind of the first (sketch No.4). Thus the crest of the second system wave would discharge into the trough of the first and the first system wave into the trough of the second, ensuring a much more regular flow but of much increased volume of water passing to the turbine and thus increasing electrical generation. This system would be much more costly than the single system proposed, particularly the foundation construction on the ocean bed.

The double system outlined could be constructed also as a floating generation system as earlier described, with its attendant advantages.

No knowledge is claimed for turbines, draft tubes and generator design.

Attention is drawn to the extensive bibliography of the Department of Trade and Industry ETSU publication.

Claims (5)

1) A training wall or walls, curved in the horizontal plane and

in the vertical plane so as to form a breakwater or breakwaters if so required, constructed in concrete or steel utilising the effect of a wave phenomenon described as a Mach Stem wave, reducing the need for breakwater armour. (Note that patent EO2B who82/00840 18.3.1982. is in error in that the velocity of water may increase initially at point of impact but much of the energy is reflected at 90 degrees. Thus the initial velocity may decrease. In any event the decreased energy at any point would not ennanc nratin nHrmti;n in this pat-nt 2) Two converging training walls curved in horizontal and vertical planes as described previously and forming a device for raising the height of the crest and trough of a wave entering the device and which allows for the volume of the water to fall to Kaplan type low head turbines, with coupled generators, at the exit.

The total construction of training walls, turbine/generator housing and control housing constructed as concrete caissons and founded on the ocean bed.

3) As for claim 2) but designed as a floating structure with suitably fitted mooring lines permitting the entrance to face into oncoming waves from an arc of directions and with lower cost of foundations and operating in improved wave regime siting.

4) As for claim 2) but arranged as a double structure of four training walls of which each pair forms a device as in 2) but with the second device sited one half a wave length dow wind from the first pair, so that the crest of the wave in the second pair discharges effectively into the trough of the first pair and the crest of the wave in the first pair discharges into the trough of the second pair, increasing the flow to the turbine generators.

5) As in claim 3) and 4), a floating double structure improving the flow to the turbine generators but with reduced cost of construction by eliminating costly foundations and permit installation offshore in the most favourable wave regime.