Harnessing the tides

Lunar power could play a significant part in the UK's energy mix. The Moon's gravitational pull on our oceans means the British Isles are surrounded by strong tidal flows and engineers are aiming to tap them with undersea turbines. But maintaining underwater tidal mills could be difficult.

One group thinks it has the answer. Swanturbines has deliberately simplified its design to minimise the risk of problems. The Swansea-based team recognises that the harsh sub-sea environment can take its toll on moving parts and control sub-systems so it has eliminated those elements more likely to require regular maintenance or repair. The result, it claims, is a cheaper and more versatile solution than its rivals'.

'We've gone for a direct drive, low-speed permanent magnet generator,' said Swanturbines' co-founder Dr Ian Masters. 'It gives a single moving part, which is an advantage. We looked at all the things that one could have, such as pitch control, pitch motors, the whole drive train. We said to ourselves, "Let's leave all those things out and do something else to reduce the risks."'

The team has spent five years developing its answer. The key is fault-tolerant electronics which generate DC. The electric current is fed to units on dry land, which are easily accessible by maintenance and repair crews, and where it can be converted to more useful flows.

The one remaining moving part, the rotor itself, can be maintained by raising it to the surface. A patent granted to the Swanturbines designers reveals that the pylon on which the rotor sits could be telescopic and raised above the waves simply by pneumatic or other means.

One of the Welsh turbine designs could generate 350kW when fitted with a rotor up to 15m in diameter. 'Tidal turbines are much slower moving than wind turbines but the sea is 800 times denser than the air,' said Masters. 'Ours will turn at about 12rpm.'

A one metre diameter prototype has been towed by Swansea University's research vessel for tests at a range of flow speeds and rotational speeds. Swanturbines is now seeking investment for the first full scale demonstrator, planned to be installed off the Swansea shore. 'It will be in the order of a small number of millions of pounds,' said Masters.

He believes the time is right for tidal turbine funding. 'Denmark spent 20 years investing in wind turbine design,' he said. 'Now it has 25 per cent of the global industry. We are at the beginning of tidal turbines and there's a realisation that wind can play only a part of the need for renewable energy sources. There has to be some diversity and tidal can provide that.'

Masters is not alone with that belief. A recent report by Black & Veatch for the Carbon Trust says that the UK resource of tidal stream energy accounts for about 15 per cent of the planet's total and half of Europe's. If this is correct, the country would be in a good position to develop a strong tidal energy expertise and capability.

However, the authors also report that, apart from two areas within UK waters, suitable sites are dispersed around our coast. The exceptions are the Pentland Firth and the Channel Isles, which account for 40 and 25 per cent of the UK resource respectively. If tidal turbines are to contribute significantly to the national grid these two areas could host undersea mill farms.

A one metre diameter prototype has been tested for a range of flow speeds and rotational speeds

It is early days for tidal turbines. There are only two in place in the world, the first was installed off Lynmouth, North Devon, in 2003. A few months later a much larger system was built in the Kvalsun channel at Hammerfest, Norway, where the 10m rotor turns at 7rpm to generate 300kW and produces enough power for 30 local homes. The same amount of energy would supply up to 80 homes in the temperate climate of the UK.

Environmental concerns have driven the demand for renewable energy, and the impact of tidal turbines has come under scrutiny. For instance, it may be necessary to install protective shields around the turbines to protect curious sea creatures. Otherwise, Masters is positive about the ecological footprint of the Swanturbine.

'One potential advantage is for the areas around the turbines to be designated no-take zones for fishing,' he said. Apart from preventing nets tangling in powerful, slow-moving rotors, such restrictions would also provide a haven for fish. 'They could become spawning grounds that would help fish stocks to recover,' said Masters.