THAWT energy harvester

Successful tests of a new type of tidal turbine have boosted hopes that a full-size prototype can undergo sea trials in 2011. Its developers believe the transverse horizontal axis water turbine (THAWT) could harvest energy from tidal flows more efficiently and at a lower cost than other systems.

THAWT is a horizontal rotor with the key advantage that it is scalable. The conventional tidal turbines that it could supersede are similar to the familiar three-bladed wind turbines, each of which intercepts just part of the available flow. The main advantage of THAWT, according to its inventors from Oxford University, is that it can be extended to capture energy from across the entire flow.

’It is effectively a horizontal truss on two bearings,’ said Prof Martin Oldfield. ’This means it can be scaled up, across the tidal flow.’ Outline descriptions suggest a single turbine may be 60m long and that dozens can be coupled together across a tidal flow, into a line several kilometres long.

A 1km array of THAWT rotors could generate up to 60MW. A tidal site up to 20km across would generate more than a gigawatt, the output of a small coal-fired power station.

The turbine is mechanically far less complicated than anything available today, and requires fewer generators and foundations, so will cost less to build and maintain. The manufacturing costs are about 60 per cent lower and the maintenance costs are 40 per cent lower than current tidal devices, said its developers, Oldfield, Prof Guy Houlsby and Dr Malcolm McCulloch.

They have taken a Darreius vertical axis wind turbine and laid it on its side, then enhanced it by adding more blades.

A key improvement has been to triangulate the blades, a modification that gives the structure sufficient strength to be able to be ’stretched’. ’A couple of years ago we had a brainstorming session and, over half an hour we evolved the truss structure,’ said Oldfield. ’The design process is not supposed to happen like that but on this occasion it worked out very well.’ Another benefit of the Darreius turbine is that the rotor always turns in the same direction, no matter which way the tide is flowing.

Tests of a 0.5m diameter turbine in a flume at Newcastle University over two weeks in April have proved the principle of the mechanically-simple idea. Now there are plans to trial a full-scale prototype, possibly at the European Marine Energy Centre in the Orkneys where tidal flow can reach 4m/sec (7.8 knots) at the Fall of Warness, west of the island of Eday. If they go ahead next year, as hoped, and are successful the inventors said there could be a commercial version by 2013.

’The blades will be made from a mixture of glass and carbon composite and the structure would probably have neutral buoyancy so it could be more easily manoeuvred,’ said Oldfield. The bearings will be large journal bearings or pre-greased roller bearings within the concrete or steel foundations.

Each turbine end would be fixed to a foundation and these could support the ends of two adjacent turbines. This layout, say the inventors, would reduce construction costs compared with conventional tidal turbines, each of which requires its own discrete foundation.

At farm scale, the THAWT devices could be installed at about £1.7m per MW, according to the inventors. That compares with about £3m per MW for modern marine turbine technology and just over £2m per MW for wind power.

Darreius turbines are not used for generating power from the wind because they are unstable in strong gusts and storms. However, the slow rate and predictability of tidal flow means the general principle will be effective in marine applications. They will turn relatively slowly, at about five seconds per revolution.

The turbines would best be sited in the centre of the tidal flow, with the axis halfway between the ocean floor and the surface. Oldfield points out that wing structures work when swept up to 30º, so he believes it may not be necessary to align the tidal turbines exactly at a right-angle to the flow.

Isis Innovation, Oxford University’s technology transfer company, is promoting THAWT and has already had enquiries from companies that could help to exploit and develop the tidal turbine.

Dr Stuart Wilkinson is managing the project for Isis. A patent application has been made.

Max Glaskin