THAWT energy harvester | the Engineer

Originally Published: 17 September 2008 08:40 AM
Source: The Engineer

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

http://www.theengineer.co.uk/Articles/307958/THAWT+energy+harvester.htm

Second generation tidal turbines promise cheaper power | the guardian

Alok Jha, green technology correspondent guardian.co.uk, Thursday September 04 2008 17.07 BST

Harnessing the vast energy of the UK’s coastal tides could become much simpler and cheaper with a new design for the next generation of underwater turbines. The device, unveiled by a team of engineers from Oxford University, re-thinks the way power is generated underwater and the inventors believe it will be more robust, more efficient and cheaper to build and maintain than anything in operation today.

There is an immense potential resource of clean energy from the tidal flows around the UK: conservative estimates suggest there is at least five gigawatts of power, but there could be as much as 15GW, equivalent to 15 million average family homes. Tidal generators can harvest the energy of these moving streams, with the added advantage that the resource is, unlike wind, predictable.

There are only a few underwater turbines in operation today and they all operate like underwater windmills, with their blades turning at right angles to the flow of the water. In contrast, the Oxford team’s device is built around a cylindrical rotor, which rolls around its long axis as the tide ebbs and flows. As a result, it can use more of the incoming water than a standard underwater windmill.

At full size, a Transverse Horizontal Axis Water Turbine (Thawt) rotor would be 10m in diameter and 60m long. Connecting two of these together with a generator in the middle could produce around 12MW of power, enough for 12,000 average family homes.

“To do that, you only need three foundations and one generator,” said Martin Oldfield, senior research fellow of engineering science at Oxford University. “To do that with a [windmill] would require five foundations and 10 generators.”

The Thawt device is mechanically far less complicated than anything available today, meaning it would cost less to build and maintain. “The manufacturing costs are about 60% lower, the maintenance costs are about 40% lower,” said Malcolm McCulloch, head of the electrical power group at Oxford’s engineering department.

So far, the researchers have successfully tested a version of Thawt that is 1m in diameter and 6m long. They are now planning to build a 5m-diameter test device that could generate electricity for the grid. By 2009 the team wants to carry out sea trials to test the device’s durability in open water.

Scaling up the power at a coastal site would involve connecting together a series of Thawt rotors across the sea floor. The engineers said that, if all went well, farms of Thawt devices could be built starting around 2013. “If you have a tidal site of 20km, you could build 20km of these turbines going across [the sea floor] and then you would be into the gigawatt class,” said Oldfield. This would make the farm equivalent to a small coal-fired power station.

McCulloch said that their economic analysis of the Thawt device showed that, at farm scale, the Thawt devices could be installed at around £1.7m per MW. That compares with around £3m per MW for modern marine turbine technology and just over £2m per MW for wind power.

Doug Parr, chief scientist at Greenpeace said the UK is a potential global leader in wave power. But he noted: “Many good ideas for wave power generation suffer from a lack of finance, lack of assured market and lack of access to business expertise.

“Some of these bottlenecks need to be addressed by the industry – others need government to play a boosting role rather than hoping that the rules and organisations that got us into the climate problem are going to be the ones that get us out.”

In July, Bristol-based company Marine Current Turbines installed the SeaGen device, an underwater windmill device, at Strangford Lough in Northern Ireland. It is the first commercial-scale tidal device to generate power for the grid. When it is eventually running at full power, MCT said it will have an output of 1,200 kW, enough for about 1,000 homes.

“There are presently tidal devices undergoing testing – we regard those as first generation device and we regard ours as a second generation,” said Oldfield. “To some extent we admire them for being pioneers of the technology but we think what we’ve got will end up being better.”

Steph Merry, head of marine renewable energy at the Renewable Energy Association welcomed the Oxford team’s work but said that, in terms of backing a technology to harness tide power, nothing can be ruled in or out. “We’ve got this 15% renewables target for 2020 to achieve, which equates to 40% electricity, so you have to look at all possible options of generating it.”

Merry added that, technology aside, there were other stumbling blocks in building tidal projects around the UK, including what she sees as an excessive need to monitor the environmental impact of turbines. “We have to get it in proportion, you can’t have an unlimited budget for environmental monitoring when every engineering company has to work to a budget for any project. At the moment, there is no limit to the monitoring that can be imposed.”

She said that the industry had to sit down with environmental groups and government to find a balance between the need to tackle climate change and the requirements to safeguard the ecology of tidal areas