Technical overview of wave and tidal stream energy

How can marine energy be harnessed?
A wide variety of concepts have been proposed for the extraction and conversion of marine energy to electricity. Some of these are introduced below:

• Tidal stream turbines – These work on a similar principle to wind turbines and indeed may look quite similar. Both horizontal- and vertical-axis machines are being investigated, some with ducting/cowling around the rotor. The turbine may be coupled directly to a standard generator via a gearbox, or use an alternative power train design;
• Reciprocating tidal stream devices – These have hydrofoils which move back and forth in a plane normal to the tidal stream, instead of rotating blades. One design uses hydraulic pistons to feed a hydraulic circuit, which turns a hydraulic motor and generator to produce power; and
• Venturi effect tidal stream devices – In these, the tidal flow is directed through a duct, which concentrates the flow and produces a pressure difference. This causes a secondary fluid flow through a turbine.

Alongside these options, there is a debate about the relative merits of fixing devices to the seabed for stability and deploying floating devices to allow retrieval for maintenance. Some tidal stream devices are being demonstrated in the UK and the MEC has a work package to look at all the designs mentioned above.

Tidal power can also be extracted from tidal barrage and tidal lagoon systems. These are outside the scope of the MEC but are mentioned here for completeness. The tidal barrage is a long-established, technically-proven concept which essentially involves a structure with gated sluices and low-head hydro turbines. Bridging two sides of an estuary, the principle of operation is to allow water to flow into the area behind the barrage with the flood tide and out during the ebb tide. As water flows out, the collected head of water turns the turbines to generate power. A tidal barrage has been in operation at La Rance on the northern French coast for more than 40 years, and schemes have previously been proposed in the UK, notably at the River Severn. The tidal lagoon operates in a similar way to the barrage, but uses an impoundment structure rather than a barrage.

Wave power

The problem of wave energy extraction is complex and many designs of device have been proposed. It is helpful to introduce these in terms of their physical arrangements and energy conversion mechanisms.

Physical arrangements

• Placement – Devices may convert wave power at the shoreline, near to the shore or offshore. The distinction between near-shore and offshore is often related to design requirements for water depth (this generally increasing with distance from shore), the energy content of waves (this being greater offshore), and access for deployment, retrieval, operation and maintenance.
• Fixing – Near-shore and offshore devices may be either bottom-mounted or floating, the former being fixed to the seabed by a static member and the latter moored to hold on station.
• Reaction – Wave energy devices need a system of reacting forces in order to extract energy and this is one of the biggest design challenges. To create such a system, two or more bodies need to move relative to each other, while at least one body interacts with the waves. There are numerous approaches towards this, some of which are being considered in the MEC. One approach is to allow one body to move freely with the waves, while another is held static (as in the case of a floating buoy reacting against the seabed). Alternatively, all of the bodies may be dynamic.
• End stop – Within the reaction system, a common requirement is to avoid situations where the relative motion is so large that destructively high forces occur between the bodies, (e.g. a hydraulic piston being forced beyond the end of its stroke).

Wave energy devices can be classified by means of their reaction system, but it is often more instructive to discuss how they interact with the wave field. In this context, each moving body may labelled as either a displacer or reactor .

• Displacer – This is the body moved by the waves. It might be a buoyant vessel, or, as in the case of Oscillating Water Column (OWC) devices (see below), a mass of water. If buoyant, the displacer may pierce the surface of the waves or be submerged.
• Reactor – This is the body that provides reaction to the displacer. As suggested above, it could be a body fixed to the seabed, or the seabed itself. It could also be another structure or mass that is not fixed, but moves in such a way that reaction forces are created (e.g. by moving by a different amount or at different times). A degree of control over the forces acting on each body and/or acting between the bodies (particularly stiffness and damping characteristics) is often required to optimise the amount of energy captured.

In some designs, the reactor is actually inside the displacer, while in others it is an external body. Internal reactors are not subject to wave forces, but external ones may experience loads that cause them to move in ways similar to a displacer. This can be extended to the view that some devices do not have dedicated reactors at all, but rather a system of displacers whose relative motion creates a reaction system.

The generality of the above might suggest there are many ways in which a wave energy device can be configured and indeed this is the case. Some of the most well-known device concepts are introduced below.

• Oscillating Water Column (OWC)– This comprises a partly submerged structure (‘collector’) which is open to the sea below the water surface so that it contains a column of water. Air is trapped above the surface of the water column. As waves enter and exit the collector, the water column moves up and down and acts like a piston on the air, pushing it back and forth. The air is channelled towards a turbine and forces it to turn. The turbine is coupled to a generator to produce electricity.
• Overtopping– This consists of a structure over which the waves topple, a reservoir to collect the water and hydro turbines installed at the bottom of the reservoir. The head of collected water turns the turbines as it flows back out to sea and the turbines are coupled to generators to produce electricity.
• Point absorber– This is a floating structure that absorbs energy in all directions by virtue of its movements at or near the water surface. It may be designed so as to resonate – that is, move with larger amplitudes than the waves themselves. This feature is useful to maximise the amount of power that is available for capture. The power take-off system may take a number of forms, depending on the configuration of displacers/reactors.
• Terminator – This is also a floating structure that moves at or near the water surface, but it absorbs energy in only a single direction. The device extends in the direction normal to the predominant wave direction, so that as waves arrive, the device restrains them. Again, resonance may be employed and the power take-off system may take a variety of forms.
• Attenuator – This device is a long floating structure like the terminator, but is orientated parallel to the waves rather than normal to them. It rides the waves like a ship and movements of the device at its bow and along its length can be restrained so as to extract energy. A theoretical advantage of the attenuator over the terminator is that its area normal to the waves is small and therefore the forces it experiences are much lower.

There are many further different design concepts.

• One other device being considered in the MEC is the Wave Rotor , which is a form of turbine turned directly by the waves. It is coupled to a generator in order to generate electricity.
• Other devices use contained volumes of water, or exploit differences in water pressure. Flexible structures have also been suggested, whereby a structure that changes shape/volume is part of the power take-off system.