Tidal barrage, France

Turning tides

Marine energy engineers have quietly been developing tidal technology, with the UK leading the way.

Against a global backdrop of rocketing oil prices and concerns over climate change and security of energy supply, interest in renewable energy technologies has never been more intense. Attention has focused primarily on the more mature technologies of wind, solar and hydro, but behind the scenes marine energy engineers have been quietly developing tidal power technology, with the UK leading the way.

Until recently, tidal energy technology has been largely at the prototype stage, but the first half of 2008 has seen several significant developments on the part of UK and Irish tidal energy developers.

In May, the world's first commercial tidal turbine was installed in Strangford Narrows, Northern Ireland. Developed by Marine Current Turbines (MCT), the successful installation of the 1.2MW SeaGen twin rotor turbine represents a leap forward for tidal stream technology. It is now grid connected providing power, on average, for some 1,000 homes.

On 26 May, Irish company OpenHydro became the first tidal energy company to connect a demonstration tidal turbine to the UK national grid. The 250kW open-centre turbine is installed at the European Marine Energy Centre (EMEC) test facility off the island of Eday, in Orkney, Scotland.

In April, government consent was granted for a demonstration 100kW generator to be deployed in the Humber estuary.

Implemented through Yorkshire-based joint venture company Pulse Tidal and supported by the Humber Tidal Power Consortium, the device comprises two 11m hydrofoils which oscillate to generate electricity. It will be connected to the grid via a transformer located at the Millennium Inorganic Chemicals plant on the south bank. The demonstration device is expected to be fully commissioned by September.

In the longer-term, E.ON and Lunar Energy are proposing a joint 8MW tidal stream project off St David's Head on the Pembrokeshire coast, and MCT is joining forces with npower renewables to deliver a proposed 10.5MW tidal array off the cost of Anglesey.

Lunar Energy has also announced a joint venture with the Korean Midland Power Company to build a commercial £500m 300-turbine tidal stream power project off the South Korean coast, and MCT is involved in a joint venture with Canada's BC Tidal Energy Corporation to install at least three 1.2MW SeaGen turbines in Vancouver's Campbell River, subject to gaining consent.

Tidal power

Tides are generated by the gravitational forces operating between the Earth, the Sun and the Moon. The magnitude of the tide is determined by the relative positions of these bodies, by the Earth's rotation, and by the shape of the seafloor and coastline.

Although variable, tidal energy has a distinct advantage over other forms of renewable energy - it is predictable.

Fluctuation in output caused by the twice daily tidal cycle and 14-day spring-neap cycle can be predicted over the lifetime of an installation. There is approximately eight times more tidal stream power during spring tides than at neaps. The energy in a tidal current is proportional to the cube of the water velocity. Thus, small changes in tidal current velocity will lead to large changes in power density.

As water is 800 times denser than air tidal energy, devices can be much smaller than wind turbines and yet still extract significant amounts of energy.

The potential of tidal energy is enormous. However, realising that potential has proved more challenging than for other renewables.

Martin Wright, managing director of MCT, likens the current state of tidal energy technology to the first biplanes in aviation technology.

While commercial wind and photovoltaic devices have been around for some 40 years, modern tidal turbine technology research only started in the UK in the early 1990s. There are a number of reasons for this, not least the challenges associated with erecting and maintaining devices in a tidal marine environment.

Dr Alan Owen of the Centre for Research in Energy and the Environment (CRE+E) at The Robert Gordon University in Aberdeen observes: "Tidal stream environments are hostile and challenging places to operate expensive and complex machinery. Nor do we fully understand the tidal marine environment - the water doesn't just move one way and then back twice a day, there are all sorts of other variables such as shear flows and contra flows and debris. We have no choice but to put equipment in the water and monitor its effects."

The establishment of EMEC's tidal and wave test facility in Orkney has proved a significant driver in UK tidal's recent progress, enabling developers to test their designs in real-world conditions.

Tidal stream energy

At an estimated 18TWh/year, equivalent to around 10-15 per cent of the total known global tidal stream resource, the UK has some of the best tidal stream resources in the world. This resource, combined with its expertise in offshore oil and gas production, wind energy, power generation and ship-building, has meant that UK developers are world leaders in the field of tidal stream technology.

According to the Sustainable Development Commission (SDC): "The UK is currently leading the world in the development of a wide range of tidal stream devices. The long-term potential for this new industry - both in terms of its contribution to UK electricity supply, and its export potential - is considerable. However, this nascent industry has a long way to go."

The tidal stream energy industry in the UK is characterised by a number of small teams based in start-up companies, specialist equipment manufacturers, university engineering departments or a combination of the three. However, some large utilities are starting to look at tidal stream as a potential investment area.

npower renewables and MCT have formed a development company, SeaGen Wales, to deliver a 10.5MW tidal stream farm comprising seven 1.5MW SeaGen turbines, off the coast of Anglesey in a 25m deep area known as the Skerries. Subject to successful planning consents and financing, the scheme could be commissioned by 2011-2012. 

In March 2007, E.ON announced a joint project with Lunar Energy to build an 8MW tidal stream farm off the Pembrokeshire coast.

Ultimately powering some 5,000 homes, the multi-million pound project is at a very early stage. A scoping report was presented to the Welsh Assembly and Department for Business Enterprise and Regulatory Reform in October 2007 and E.ON is planning to start the environmental impact assessment this summer.

This will coincide with dry testing Lunar Energy's tidal stream turbine followed by in-water tests on a 1/3 scale model at EMEC next year. Construction is not expected to start before 2011. Lunar Energy plans to re-use the scale model as an in-situ demonstration device for future projects.

Device design

According to Entec there are some two dozen tidal stream device designs at various stages of development and these can be broadly divided into three categories - horizontal axis cross flow rotor, vertical axis axial flow rotor, and reciprocating hydrofoil. The cross-sectional entry area of a ducted device or the rotor swept area presented to the current determines the energy capture of the device; the larger the swept area or entry area the larger the energy capture.

Analysis by MCT indicates that approximately 300m2 swept area is the minimum necessary to produce commercially competitive energy due to the high fixed overheads of offshore projects.

Axial flow turbines are the most common design, extracting energy from the water flowing along their axis of rotation. Cross flow rotors extract energy from water flowing perpendicular to their axis. Hydrofoils do not rotate but move up and down in the tidal stream. Rotors can be free stream or ducted, with the duct channelling water through the turbine to enhance velocity.

MCT's SeaGen is a non-ducted axial flow rotor device which drives a generator via a mechanical planetary gearbox, similar to that used in hydroelectric and wind power technology. Peter Fraenkel, MCT's technical director, explains: "We are using the technology of choice from the wind and hydro sectors. At 45-50 per cent efficiency, axial flow rotors are the most efficient and cost-effective kinetic energy conversion mechanism.

"The rotor blades can be pitched through 180° to enable them to operate bi-directionally in both the ebb and flow tides. Pitch control gives a high degree of controllability - we can maximise power at low current velocities and limit the power or even stop the rotor at high velocities using pitch control."

The design is based on Seaflow, an earlier 300kW demonstration model, which has been operating off the Devon coast near Lynmouth since May 2003.

Other developers have opted for ducted designs to improve efficiency. OpenHydro's open-centre turbine comprises an enclosed single rotor turning within a fixed housing with a shaped inlet duct.

Based on existing technology used in the oil and gas industry, the Rotech Tidal Turbine (RTT), a bi-directional horizontal axis turbine housed in a symmetrical venturi duct, is being championed by Lunar Energy. The venturi duct removes the need for blade pitch control as it straightens the tidal flow as it approaches the turbine blades ensuring maximum extraction of the available energy.

An alternative to turbines are hydrofoils which oscillate rather than rotate and so can operate in shallower water than turbines. The Pulse Tidal device in the Humber estuary will be sited in just 5m of water (mean low water spring). Being close to shore reduces installation and O&M costs, and makes grid connection easier.

All tidal stream devices need to be carefully sited to ensure they don't disrupt navigation routes or interfere with other marine users, and this is particularly true of those positioned in shallow water.

The scope of the £2m demonstration project is to demonstrate that a device can operate successfully in the shallow water tidal resources found in estuaries, lochs and tidal narrows. The aim is to develop a 1MW full-scale commercial device which can be deployed in linear arrays with installed capacity of at least 50MW.

Staying put

Tidal stream devices can be further categorised by their placement mechanism. Devices need to be positioned in one place to capture the tidal stream and to avoid presenting a hazard to shipping. They can be fixed in position or weighted to sit on the sea floor. Floating devices may be tethered to the seabed or other mooring structure.

Whichever technique is used needs to be able to cope with the thrust reaction caused by taking energy out of the system. This can be in the order of 100t per MW. The technique chosen will be largely determined by water depth as pilings, the main method currently in use to fix tidal stream devices in position, can only easily and cheaply be installed in depths of up to 40m. In order to exploit the richer tidal energy sources found in deeper waters, developers will need to focus on engineering suitable device support structures.

SeaGen is secured to the seabed by four pin-pilings drilled to a depth of 9m and is a fixed permanent structure until it is decommissioned. The blades and power units can be raised above the surface for maintenance and repair. OpenHydro's demonstration device can also be raised and is fixed to the seabed between a twin monopile structure.

The fixed foundations must be large enough to prevent the structure breaking loose and also to cope with the high structural loads. By contrast, Lunar Energy's RTT will use technology from the offshore oil and gas sector with three ballast filled legs on which the turbine, duct and housing are positioned.

At >2,500t the structure is load-bearing and self-supporting, and can be positioned in water up to 60m deep. Heavy-lifting vessels, as used in the oil and gas sector, are used to install the device.

Lunar Energy claims that using a gravity-based foundation gives the RTT an advantage over fixed installations in that it can be deployed quickly with minimal preparation.

However, to exploit deeper-water sites some sort of floating structure may be required. These include buoyant submersible structures floating at the required depth and attached to a mooring system. Alternatively, a floating surface structure could be attached to the seabed via anchors, such as in Statkraft's MORILD demonstration project in Norway.

Stability while in operation is one of the engineering challenges posed by floating units, as is overcoming the difficulties inherent in coupling electricity cables to floating structures.

MCT engineers have been working on the development of deeper water technology, based on their experience with Seaflow and SeaGen. MCT has patented a completely submerged deep water multi-rotor system which can be brought to the surface for maintenance, repairs or replacement.

An alternative concept, currently under development by CRE+E, is the Sea Snail. This free-positioned marine environment support structure uses hydrofoils in a configuration similar to the familiar upturned aerofoil used on racing cars.

Hydrofoils are mounted on a frame so as to induce a down force from the stream flow and counteract the overturning moment of the tidal current.

Mathematical modelling followed by trials of a small scale device has demonstrated the technical viability of the concept. CRE+E is now seeking commercial collaboration for a mark II demonstration device.

In addition to supporting tidal stream generators, the Sea Snail has a variety of potential applications including seabed surveillance prior to tidal stream device installation.

The current state-of-play can be broadly likened to the early days of the wind power sector when numerous turbine designs were investigated before horizontal axis turbines became the standard.

There are a variety of tidal stream technologies at various stages of development; only one has reached the commercial stage while others are at the prototype/demonstration phase. Entec concludes: "The success of the tidal stream industry is reliant on the technology developers proving their concepts technically, and then moving into a manufacture and project development phase to steadily increase the number installed and benefit from 'learning by doing' and economies of scale."

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