Turning the tide

A tidal power generator capable of producing electricity and hydrogen at a low cost is to be tested off the coast of Yorkshire within the next few weeks.

The system, developed by Imperial College spin-off RVCO, has no moving parts below the water surface.

So it is much cheaper to install and maintain than most tidal power devices, said RVCO’s chief executive Dr John Hassard. ‘Huge savings in running costs can be generated because the turbine can be a long way from where the energy is extracted,’ he said.

The Rochester Venturi system concentrates the low-grade energy in the ocean currents into a rapid flow of water through a pipe to the shore.

The water is accelerated through a venturi, or throat, within the pipe, in which the pressure of the water stream is converted into kinetic energy. This is used to drive a turbine above the water surface.

Even slow-moving water contains a great deal of energy. So the system is designed to use some electricity to extract hydrogen from the salt water by electrolysis, said Hassard, who developed the technology with the company’s co-founder Dr Geoff Rochester. ‘Tidal power is ideally placed to contribute to the hydrogen economy,’ he said.

Harnessing power in two different ways means the system is more cost-effective, while even greater economies can be gained by combining the suction output from several venturis to the same generator. The cost of RV generated energy is likely to be below 10p/kWh, and could be as little as 2.5p, said the company.

RVCO will test a 30kW demonstrator off the Grimsby coast within the next three weeks. The company then plans to install a 60-100kW system off the west coast of Scotland, which will be expanded to generate 1MW of power by this time next year. Many megawatts could eventually be produced by a fully operating system, said Hassard.

‘We estimate Scottish waters could provide half the UK’s power needs. ‘

The RV system is flexible, and can be used in coastal sites, fjords and rivers, and even as free-floating installations in open seas. The technology could also be attached to existing and planned offshore windmills. It requires the minimum water speed to be just above 0.3m/s, and can be sited at any depth.

It can also be adjusted for use in water purification, and to remove or control pollution in rivers, said Hassard. The system is likely to have a negligible environmental impact, as it needs to take only a small fraction of the water’s available energy.

Sidebar: Floating turbine cuts cost of wind power

A giant floating ring rotating around a buoy the size of an oil tanker could soon replace conventional wind turbine farms, according to its UK designer.

Ringsail, the brainchild of Clive Coker, head of the business and industry team at the Engineering and Technology Board, has vertical turbine blades mounted on a floating ring, which is rotated by the wind. The ring is connected to an anchored buoy, or hub, containing a gearbox and power generator.

Large-scale Ringsail units, capable of generating 12MW, could be built with diameters of around 400-500m, with a hub the size of a supertanker and blades as tall as an oilrig, said Coker, who is working with Imperial College.

This would give them up to 15 times the swept area of the largest horizontal axis machines, he said. ‘Even though it is not as efficient as a conventional wind turbines, the scale to which it can be built means you can get great economies of scale, so it can generate more kilowatts of energy per capital cost.’

The cost of building Ringsail would be much lower in proportion to conventional wind turbines, as the unit is held in place simply by cables attached to three concrete blocks dropped into the sea. ‘It does away with the sub-sea foundations needed to hold conventional wind turbines in place, which account for one-third of their cost,’ he said.

Ringsail is also much easier and cheaper to decommission, as it can simply be disconnected from its blocks, while the extensive foundations of conventional turbines must be fully removed to avoid becoming a hazard to shipping.

The technology could also be adapted to harness tidal energy simply by mounting vertical blades downward from the floating ring.

A 1m-diameter model Ringsail is undergoing wind tunnel testing, and the team plans to build a medium-sized prototype for trials on a lake.

On the web