GB2479765A

GB2479765A - Floating wave energy device uses overtopping between reservoirs

Info

Publication number: GB2479765A
Application number: GB1006687A
Authority: GB
Inventors: David Vincent Evans; Richard Porter
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-04-22
Filing date: 2010-04-22
Publication date: 2011-10-26
Also published as: GB201006687D0

Abstract

A resonant device for converting energy of ocean waves comprises a tethered buoyant structure with a chamber 15 containing a body of water 11 and an inner reservoir 12. The surfaces 14 of the chamber and inner- reservoir are shaped so that movement of the structure in response to wave action causes water to move from the chamber to the inner reservoir, before being released through a low-head turbine 13 back into the chamber 11. A generally spherical device (figure 3) and cylindrical devices (figures 1 and 2) are described.

Description

A device for converting the energy of water waves into electricity

Field of the invention

This invention relates to the field of converting the energy of water waves into electricity.

Background to the invention

The quadrupling of the price of oil in the mid-1970s led the UK government to initiate a major Research and Development programme aimed at estab-lishing the feasibility of extracting useful electrical energy from ocean waves and to estimate the cost of this energy if used on a large-scale to supply UK needs. This was quickly followed by smaller programmes in Japan, Scandi-navia, Portugal and the USA. The programme ran from 1974 to 1983 and a large number of ideas for capturing wave energy were considered. The most promising of these were tested at small scale in wave tanks with three de-vices being tested in sea conditions at one-tenth scale. Towards the end of the programme eight device teams drawn from universities, government research establishments and industry in the UK had produced and costed reference designs for a 2GW wave power station located off NW Scotland. In 1982 the Department of Energy concluded that the overall economic prospects for wave energy looked poor when compared with other electricity-producing re-newable energy technologies and the programme was terminated apart from some small-scale generic research.

Recently there has beei a revival of interest ii wave-energy with a number of new devices being developed and tested in the UK and elsewhere. It follows from this and the earlier work in the 1970s that there exists a large body of knowledge of the field and a general agreement as to the main challenges faced in developing a wave energy converter having economic viability.

These include: 1. Survival in a hostile environment In seeking to capture an appreciable amount of the wave energy resource a wave energy device needs to be positioned in regions of moderate power levels which will inevitably reach extreme levels from time to time. The primary concern for the design of any device is survival and balancing the needs of both efficiency of wave energy conversion and survivability is a challenge.

The energy in a wave varies as the square of the wave height; ideally a device should continue to function and be rated to deliver a fairly constant power over a range of wave conditions.

2. The frame of reference In order to absorb energy from the waves a device needs a frame of refer-ence against which it reacts. Methods which have been proposed include: mounting several converters on a long common spine floating broadside on to the incident waves and using the relative motion between the converters and the spine; using the snake-like relative motions of each of the segments of a long device heading into the waves; tethering the converter to the sea bed; mounting the moving primary interface of the device with the waves on a fixed mooring.

3. Power conversion Conversion of wave energy to electricity is made difficult by the need to convert the motion at the primary interface between the converter and the waves, having an oscillatory motion of order 0.1Hz into an electrical grid op-erating at 50Hz. Methods which have been proposed include pneumatic and hydraulic systems. In pneumatic systems the motion of the primary interface is used to pump a variable oscillating air flow through a turbine. To avoid rectification of the flow a Wells turbine can be used which rotates in the same direction irrespective of the direction of air flow through it. Problems can occur due to water ingestion into the turbine. In a hydraulic system, the motion of the primary interface is used to pump oil or sea water. In the case of oil, a pump may he connected hydraulically to a motor which would in turn drive a generator. In the case of sea water, a high pressure system using submerged pipes might aggregate power from a large array of devices.

A simple hydraulic system would be to use the primary interface to generate a head of water and use a conventional low-head water turbine to generate electricity.

4. Resonant matching Devices are known which have a natural resonant frequency of motion and work on the principle of matching this frequency to the dominant frequency of the incident wave spectrum in order to transfer the maximum energy from the waves through the primary interface of the device. For floating devices this frequency is derived from balancing the hydrostatic restoring force and inertia forces and for waves of frequency of order 0.1Hz this usually implies large dimensions for the device to maintain resonance nuless some mechanism sneh as phase matching is employed.

Summary of the invention

A first aspect of the invention provides a device having the features set out in claim 1 below. Preferred, but optional, features of the invention are set out in the dependent claims.

Brief description of the drawings

Example embodiments of the invention will now be described, by way of example only, having reference to the accompanying drawings, of which: Fig. 1 is a perspective view of a first example embodiment of the invention; Fig. 2 is a transverse cross-section through the device of Fig. 1; Fig. 3 is a cut-away view of a second embodiment of the invention.

Description

A first example embodiment of a device according to the invention will be described with reference to Figs. 1 and 2. The device comprises a partially or totally submerged buoyant structure in a body of water with a free surface, which is connected to the bed of the body of water, and which encloses a hollow chamber partly filled with water (5) and with an air space (9) above it, and an inner reservoir (6) situated towards the centre of the chamber (5) and (9) housing a low-head turbine (7) suitable for generating electricity. The structure, which in this example embodiment is a circular cylinder having a length greater than its diameter, is held in tension with its axis horizontal and at right angles to the predominant direction in a spectrum of waves on the surface of the body of water which is incident upon the structure, Lv inextensible mooring lines (1) connected rigidly to the bed in such a way as to allow rotatory motion of the structure in response to the incident wave spectrum. During displacement of the structure the tension in the mooring lines (1) due to buoyancy provides a restoring force or moment which opposes the inertia force or moment induced in the structure by the waves thereby producing an oscillatory motion of the structure. The motion of the structure due to the incident wave spectrum creates a sloshing motion in the enclosed water in the chamber (5) causing it to run up the shaped internal sides of the chamber (8) into the air space (9) and overtop into the inner reservoir (6) which in this example embodiment is located towards the centre of the chamber (5) and (9), and extends along its length, and where both chamber and inner reservoir are symmetric about a vertical plane through the axis of the cylinder. The inner reservoir (6) extends above the equilibrium position of the enclosed water (5) so that a differential head of water is built up and maintained by controlling its release into the main body of the water in the chamber through the low-head turbine (5) which is located at the lowest point of the reservoir (6).

In further example embodiments of the device, the structure, in the example embodiment in Fig. 1 being a circular cylinder, is split into separate sections by means of rigid vertical baffles (2) where each section contains its own chamber (5) and (9) partly filled with water (5), inner reservoir (6) and turbine (7), such that no chamber is in water communication with any other.

Another example embodiment of the device is described with reference to Fig. 3. As in the first example embodiment, the device comprises a partially or totally submerged buoyant structure in a body of \vater with a free surface which encloses a hollow chamber partly filled with water (11) and with an air space (15) above it, and an inner reservoir (12) situated towards the centre of the chamber (11) and (15) housing a low-head turbine (13) suitable for generating electricity. In this example embodiment, the structure, together with the chamber (11) and (15) and inner reservoir, (12) is axisymmetric, that is, geometrically symmetric about a vertical axis through its centre, and is secured to the bed of the body of water by a single inextensible mooring line attached rigidly from the centre of its underside to the sea bed (not shown in Fig. 3) in such a way as to allow rotatory motion of the structure in response to a given incident wave spectrum. Because of its axisymmetry, the response of the device will be independent of the direction of incidence of the wave spectrum. During displacement of the structure the tension in the single mooring line due to buoyancy provides a restoring force which opposes the inertia force induced in the structure by the waves thereby producing an oscillatory motion of the structure. The motion of the structure due to the incident wave spectrum creates a sloshing motion in the enclosed water (11) in the chamber causing it to run up the internal curved surface of the chamber (14) into the air space (15) and overtop into the inner reservoir (12). The inner reservoir (12) extends above the equilibrium position of the enclosed water (11) so that a differential head of water is built up and maintained by controlling its release into the main body of the water in the chamber through the low-head turbine (13) which is located at the lowest point of the inner reservoir (12).

In all example embodiments of the device the interaction between the motion of the enclosed water, the motion of the structure and their effect on the incident wave spectrum is complicated. A simple approximate model can he used to illustrate the ideas involved. If the sloshing motion is ignored so that the mass of the enclosed water simply contributes to the overall rigid mass of the structure, then the structure behaves like an inverted pendulum where the force of gravity is replaced by the net upward buoyancy force on the structure, and as such will exhibit resonance at a frequency dependent upon factors which include its buoyancy and the length of the mooring lines.

In turn, any sloshing motion of the enclosed water can be approximated by a sum of natural modes of oscillation of increasing frequencies which depend upon factors which include the mass of water enclosed and the shape of the chamber and the inner reservoir and will exhibit resonance if excited at one of those frequencies. A more sophisticated approximate model which allows for the interaction bet\veen the different elements of the device, shows that the motion of the structure and the enclosed water can each be approximated by sums of natural modes of oscillation of increasing frequencies which are common to both but \vhich are different from those arising in the simpler non-interactive niodel.

The principle of the invention is to make nse of these shared natnral frequen-cies in designing the overall device so as to maximise the quantity of water which is overtopped into the inner reservoir when the structure is excited by a given incident wave spectrum. This is done by matching the lowest shared frequency with the frequency having largest energy component in the mci-dent wave spectrum and by shaping both the chamber arid the inner reservoir to facilitate the overtopping and retention at that frequency. This involves numerical modelling and model testing in experimental wave tanks. The concept is similar to the principle of operation of a Tuned Liquid Damper (see, for example, Vandiver & Mitome, Applied Ocean Research, 1979, Vol 1, No 2, pp. 67-74) where the sloshing of liquids in storage tanks attached to a structure can be designed to suppress the dynamic response of the structure at its fundamental flexural natural frequency In contrast, here the aim is not to reduce the motion of the structure hut to maximise the transfer of energy from the incident wave spectrum through the structure and into the enclosed water, so as to cause the maximum overtopping and hence energy extraction from the incident wave spectrum.

In all embodiments the electricity generated is carried by cables (3) from the generators through the core of the mooring lines to the bed of the body of water (j) and thence to the shore.

Advantages of the example embodiments include: 1. Because the device is partially or totally submerged, it is protected from the effect of severe seas and corresponding wave forces.

2. Securing the structure to the sea bed provides a fixed frame of reference against which it can operate whilst allowing it to move with the waves, again reducing excessive wave forces.

3. A careful choice of design parameters to achieve resonance ensures an optimal transfer of energy from the waves first to the sides of the struc-ture and then into the enclosed water.

4. The resonance can be achieved with smaller dimensions of the struc-ture than would he the case for a vertically oscillating device which depended on the hydrostatic restoring force to achieve resonance.

5. Theory suggests and experiments confirm that a device absorbing en-ergy through a horizontal motion in the direction of the incident waves has the potential for absorbing more energy than a device absorbing energy from nioving vertically.

6. The axisymmetric embodiment will operate equally regardless of the direction of the incident waves.

7. The device is robust, having few moving mechanical parts and uses a well-established power take-off technology.

Claims

Claims 1. A device for converting wave energy into electricity, comprising: a buoyant structure connectable to the bed of a body of water such that, when connected to the bed, the structure is free to rotate, and comprising a chamber for holding water, wherein the chamber com-prises: (i) an inner reservoir attached to the chamber, the inner reservoir and the chamber, when partially filled with water, being shaped to facilitate the maximum overtopping and retention of water from the chamber into the reservoir as a result of oscillations of the en-closed water in the chamber at a frequency equal to the frequency of waves of largest energy in a spectrum of waves on the surface of the body of water incident on the structure, so as to form a dif-ferential head of water between the inner reservoir and the water in the chamber; and (ii) a turbine within the inner reservoir arranged, when in operation, to generate electricity from the release of the differential head of water.
2. A device as claimed in claim 1 in which the structure is tethered to the bed of the body of water.
3. A device as claimed in claim 2 wherein the structure remains totally submerged.
4. A device as claimed in claim 2 wherein the structure is partially sub-merged.
5. A device as claimed in either claim 3 or claim 4 wherein the structure is a circular cylinder closed at both ends having length greater than its diameter.
6. A device as claimed in claim 5 wherein the cylinder is tethered to the bed of the body of water by a mooring line attached to the centre of each end of the cylinder.
7. A device as clainied in claim 5 wherein the cylinder is tethered to the bed of the body of water by a pair of mooring lines attached to each end of the cylinder so as to allow rotatory motion of the cylinder about the bed.
8. A device as claimed in claim 6 or claim 7 wherein the longitudinal axis of the cylinder is horizontal.
9. A device as claimed in either claim 6 or claim 7 or claim 8 wherein the longitudinal axis of the cylinder is aligned at right angles to the predominant direction of the incident waves.
10. A device as in any of the claims 6, 7, 8, or 9 wherein the chamber is divided by rigid baffles at right angles to its length into two or more separate chambers such that said chambers are not in water communi-cation with each other and wherein each chamber comprises: (i) an inner reservoir attached to each chamber, each inner reservoir and chanther, when partially filled with water, being shaped to facilitate the maximum overtopping and retention of water from each chamber into its reservoir as a result of oscillations of the enclosed water in that chamber at a frequency equal to the fre-quency of waves of largest energy in a spectrum of waves on the surface of the body of water incident on the structure, so as to form a differential head of water between each inner reservoir and the water in the corresponding chamber; and (ii) a turbine within each inner reservoir arranged, when in operation, to generate electricity from the release of the differential head of water between each inner reservoir and the water in the corre-sponding chamber.
11. A device as claimed in any one of the claims 1 to 4 wherein the struc-ture, including the chamber and internal reservoir, are geometrically symmetric about a vertical axis.
12. A device as in claim 11 wherein the structure is secured by a single mooring line attached from the centre of its underside to the bed of the body of water so as to allow rotation of the structure in any direction.
13. A device as in claim 11 wherein the structure is secured by multiple mooring lines attached to the structure and to the bed of the body of water so as to allow rotation of the structure in any direction.
14. A method of converting wave energy into electricity, comprising moor-ing a device according to any preceding claim to the bed of a body of water and generating electricity from the release of the differential head of water.