Turning the tide

Wave-power technology able to adapt to a wide variety of tidal conditions could be supplying power to south-west England within two years.

Following several years of laboratory and prototype trials, Orecon – a spin-out company of the University of Plymouth – aims to install a full-sized wave energy conversion unit off the coast of north Cornwall and connect it to the local electricity grid by 2005.

Each unit would generate around 1MW, with any eventual commercial deployments coming in arrays of 10 devices upwards.

Building on work first carried out at Plymouth’s department of mechanical and marine engineering, Orecon has developed a multiple oscillating water column (MOWC) wave-power device. This is a refinement of single oscillating water column (OWC) systems used in other wave-power systems, allowing the unit to deliver a constant flow of energy from a variety of wave bandwidths.

In OWC-based systems waves flow in and out of a cylindrical chamber suspended vertically in the water. When a wave enters the column, it compresses the air inside the system and passes it across an enclosed turbine. As the wave recedes and the water leaves the chamber, the air is sucked backwards past the turbine for a second time.

The turbine is self-rectifying, ensuring that it turns in the same direction whether the air is flowing in or out of the device. But single OWC units have to be ‘tuned’ to operate within the frequency characteristics of a particular type of wave. Outside a small frequency bandwidth either side of this their efficiency drops sharply.

Orecon’s MWOC harvests waves via six columns, each of which can be tuned to different wave frequencies, and feeds the energy created to a single turbine.Fraser Johnson, technical development director at Orecon, said: ‘The wave climate is continuously changing. The first column would be tuned to work with the longest frequency of wave. As the waves get shorter and shorter the efficiency of one column would fall away and the next would take over.’

Johnson said the Plymouth team had incorporated a highly efficient type of turbine that had never before been applied to a wave energy system. When combined with custom-developed technology that Orecon plans to patent, this allows the unit to vent each individual column to a single energy collector component.

According to Orecon, one of the advantages of its system is that most of the technology involved is standard offshore equipment used in the oil and gas industry. This should reduce the cost of commercial deployment and ensure that the units are robust enough to operate with minimal maintenance for long periods.

The Orecon system is designed to work up to eight miles at sea, minimising the environmental impact, while single OWC wave- power units are built to operate on or near the shoreline.

‘Efficiency alone is not enough, because everything we do has to take account of the environmental impact,’ said Johnson.