Clean, renewable energy from the sea could power up to 20 million homes in Europe. However, a number of technical challenges have to be overcome first

Up to 20 million homes in Europe could be powered by clean renewable energy from the sea, according to ocean energy expert Teresa Pontes of Portugal, who was speaking at the recent EurOCEAN marine science and policy event in Galway, Ireland.

Pontes went on to estimate that by harnessing energy from waves and marine currents, Europe would produce around 200 TerraWatt (200 million megawatt) hours per year of electrical power.

“The oceans contain a huge energy resource with different origins,” she said. “The most developed conversion technologies are for tidal energy, thermal energy, marine currents and ocean waves. The last two technologies are those which have received most attention in that last 15 years, as they show the best prospects of becoming competitive in the short to medium term,” she added.

Europe is characterised by particularly high ocean energy, with the highest offshore power levels off Ireland and Scotland. Going eastwards and southwards, energy levels drop from the 70 kiloWatts per hour off Ireland and Scotland to 30 kiloWatts per hour off Norway and the southern archipelagos of Madeira and the Canaries.

The technologies

In terms of wave energy technology, the first prototypes were the oscillating water column (OWC) type. These consist of partly submerged concrete or steel structures, open below the water surface, inside of which air is trapped above the free water surface. Oscillation of the internal free surface due to incident waves pushes air through a turbine that drives an electric generator. Norway, Japan, UK, India, China and Portugal have all tried such devices, with powers ranging from 20-500kW. Often built on the shoreline, these are generally classified as first generation devices.

The second generation uses similar technology, but are fixed to the seabed near the shore. Portugal and Australia are leading the way here.

According to Pontes, offshore systems – the third generation of device – are more appropriate for extensive exploitation of the wave energy resource. Their deployment is less constrained by geomorphologic and environmental aspects and they harness a higher resource because waves lose part of their energy in shallow waters due to breaking and, generally to a much less extent, bottom friction.

The majority of prototypes under development are floating types, although a bottom mounted system is being tested too. Their size can vary from the 130 metre long articulated Pelamis to relatively small buoys only five metres in diameter.

When it comes to exploiting marine currents, most techniques rely on a turbine rotor that is either mounted on the seabed or suspended from a floating platform. However, an alternative concept comprising a reciprocating wing design whose attitude to the water is controlled in such a way that the arms reciprocate against a power take-off restraint, such as a hydraulic ram, has been proposed.

The greatest technical problems are likely to arise from the need for adequate operational life and low maintenance costs from machinery operating in a hard environment, although as Pontes pointed out the offshore industry has solved similar problems. Tests are currently underway with prototypes in Italy, England and Scotland.

Pontes concluded by highlighting the key R&D priorities for the EU. For OWCs, this involves improving the efficiency and control of wave-to-air systems, refining the design of the main structures for survivability, energy capture enhancement and lower material costs.

The engineering challenges for floating devices include gaining a better understanding of different types of power take-off systems and how these can be tailored to wave energy, control, array configuration, moorings and flexible electrical connections.

The required generic research includes a wide range of topics such as tank-testing facilities improvement, computer modelling techniques, electrical conversion and storage, improved bearings and seals. Fabrication, transportation, operation and maintenance, monitoring, testing, proving and certification, and power smoothing are other areas where improvement is relevant.

For marine current devices similar topics are important, as is a better understanding of cavitation, corrosion, sealing and wakes.

EurOCEAN is the four-yearly gathering of the ESF Marine Board, an association of 25 national marine funding agencies and institutes from 17 European countries. The Board provides a forum to facilitate integration of marine research activities and strategic policy, promoting synergies and centres of excellence in marine science. This latest meeting was hosted by the Irish Marine Institute.