Renewable options 5: Marine Power
Harvesting the power of the sea

It seems ironic that new renewable energy projects such as wind farms or hydro installations often get opposed on environmental grounds; largely because the areas with high human populations which need this energy are precisely those where land space is at a premium.

Fortunately, over 70 per cent of the surface of the planet is covered by seas which are almost empty of human activity and which in themselves represent a huge energy resource. Until recently the seas were regarded as an environment human beings visited at their peril. But, within the last decade or two, technology has developed, largely driven by the wealth to be obtained from exploiting offshore oil and gas reserves, so that for the first time we dare to build large permanent structures intended to survive decades in this hostile environment. Before long, our need for massive quantities of clean energy will drive us in the same direction.

We are, in fact, at the start of exciting new developments in offshore renewable energy engineering, with a variety of practical marine renewable energy options to choose from. These are offshore wind energy, tidal/marine current kinetic energy, wave energy, tidal barrages and Ocean thermal energy conversion (OTEC).

Of these, only the first three seem to have serious prospects for commercial development on a large scale.

Off-shore wind

Off-shore wind is really a land-based technology moved out to sea. The sea only plays its part in offering better average wind speeds than are normally found on land, together with avoiding the conflicts over land-use inherent in setting up large wind farms on-shore.

The high cable costs and extra structural costs to withstand marine conditions make this more costly than land-based wind, but as techniques improve this option can be expected to play an increasingly important role in our future energy supplies. The North Sea in particular is likely to gain several thousand megawatts of offshore wind installations within the next decade, as without this the declared green house gas mitigation targets for Europe are unlikely to be met.

Marine currents

Marine currents are primarily caused by the motion of the tides; the gravitational attractions of the moon and the sun cause the surface of the oceans to bulge by a small amount and hence millions of tonnes of sea water flow continuously, mainly at rather small velocities. At a few places where the seabed and shore topography interact with this flow, these flows get speeded up.

Tidal currents with peak velocities in the range 4 to 8 knots are found in a quite a number of locations. Such high velocities represent a more intense energy resource than almost any other form of renewable energy (4 knots is a power density of about 4kW/m2) and it can be converted to electricity using an underwater turbine (in much the same way that a wind turbines works) with relatively high efficiency. Moreover this is a predictable resource, so its availability can be planned for with some precision in advance.

Work is proceeding on the development of marine current turbines which can be installed in groups under the sea, much like an underwater wind farm. Initial indications suggest that this technology could rapidly become competitive with more conventional methods of energy generation. Such turbines can be mounted on piles that are socketed into holes drilled into the seabed, or alternatively they may in future be mounted underneath moored pontoons.

Wave energy

Wave energy has been studied and experimented with for about 20 years and is proving quite difficult to take through to the stage of commercial viability. The main problem is that the energy is in the surface motion of the sea and it is the surface which is the most hazardous and difficult to place fixed equipment into. Corrosion and storm effects are at their most severe precisely where the wave energy device needs to be located.

There are three main categories of wave energy conversion devices, on-shore, near-shore and off-shore; on-shore devices built into the shoreline have been the main focus of attention so far as it is relatively easier to build a structure which can survive there.

The preferred mechanism is the Oscillating Water Column (OWC) where waves enter a chamber and the up and down movement of the surface pumps air backwards and forwards through a uni-directional turbine known as a Wells Turbine. Unfortunately, the energy availability in waves is least for on-shore and gets greater the further off-shore you can place the device, so the tendency will be to move the technology further out. To do so successfully demands larger and more robust devices than have so far been constructed, but no doubt these will eventually appear.

Other promising wave power devices are so called Point Absorbers, usually some form of buoyant device that moves up and down with the waves and transmits the motion to a suitable generator.

Tidal barrages

Of the remaining options, tidal barrages are limited to a few locations where the topographic features and the tidal range favour impounding of sea water at high tide behind a concrete wall, and to release it through turbines while the tide falls. However tidal barrages are not generally considered as economically viable at present and there is little interest in developing them.

Equally, the last option, OTEC, is only likely to be practical in tropical locations where the greatest temperature differences can be found between the surface of the sea and the deeper waters. It is this temperature difference which is essential to drive a thermodynamic process. Even in the Pacific Ocean where it is being tried, the viability of OTEC remains uncertain.

Nevertheless, we can expect to see energy from off-shore wind, from marine currents and from waves playing an increasingly important role in the forseeable future.

Peter Fraenkel is Technical Director of IT Power Ltd (UK), a research and development company covering the whole field of renewable energy technology. An expert in design and implementation of small-scale energy systems for use in remote regions, he has worked in this field for more than 20 years.

For more details of how you can help, click here.