A Tidal Energy Fence for Bristol, UK

Kepler energy tidal energy fence 2
Bristol Tidal Energy Fence (Pic: Kepler Energy)

Several months ago Kepler Energy announced it was bringing forward plans for a 30 megawatt (MW) tidal energy fence which will ultimately be located in the Bristol Channel. The plan is for it to be deployed in the stretch of water between Aberthaw and Minehead and it could be operational by 2021, subject to planning approval and financing. It is the first project Kepler has taken on since the company’s establishment in 2010. Altran, Gurit, Mojo Maritime, GE, Royal Haskoning and other companies have helped to develop the technology and explore possible deployment sites and, all being well, the first phase of the project will see a tidal fence of up to 1 kilometre in length, although future tidal fences could be much longer.

The technology for the project has been developed by Oxford University’s Department of Engineering Science for deployment in shallower tidal waters around the UK with a lower velocity (below ~2.5 m/s) and may also be deployed overseas. It consists of a stressed truss configuration Transverse Horizontal Axis Water Turbine (THAWT) and looks a bit like a water mill. Units can be operated either individually or as part of an array. Each unit has two rotors accompanied by a central generator, meaning that only four supporting bearings and three foundations supports are required. This reduces the weight below that of propeller type units. The turbine uses the very latest carbon composite material for the rotor blades, which have been configured so that the rotor (known as a triangulated stress truss) doesn’t need a supporting structure such as a central shaft. This helps to reduce parasitic drag (the drag created when an object moves through a fluid medium, such as water or indeed a gaseous atmosphere) which in turn increases efficiency and enables a simple construction design with reduced costs.

The design offers significant technological advantages over traditional axial turbines, for example theoretical analysis has shown that, because of the free water surface in tidal flows, tidal turbines are not subject to the Betz Power Limit, as wind turbines are. This means that the THAWT rotor design is much better suited to this optimisation than conventional propeller type turbines, and is therefore very effective at extracting the maximum amount of power from given stretches of tidal or river current. This enables the turbine to generate predictable, renewable electricity at very competitive costs in lower velocity and shallower waters where axial flow turbines cannot operate. The best feature of the truss design is its simplicity, which ensures the minimum amount of moving parts exposed to the tidal flow. The electrical equipment and controls will be housed in dry columns while the rotor installation will not require any high cost specialist vessels. The technology also ensures the device can use much longer ‘weather windows’ than axial turbines operating in harsh weather and wave locations.

Theoretical analysis and modelling, confirmed by testing, has shown outputs several times higher than those achievable by propeller type turbines placed in the same site. This advantage arises from the greater rectangular swept area of a THAWT rotor compared with the depth limited circular swept areas of multiple propeller type rotors and also the fact that greater powers can be extracted from tidal flows by optimising the blockage ratio (the swept area of turbine divided by flow area).

Kepler Energy tidal fence

The Tidal Fence can be constructed with a number of separate units (Pic: Kepler Energy)

The turbine is also scalable so it can be adapted to different marine sites. A typical turbine rotor would be 10 meters in diameter and 60 meters long. It would be deployed in a tidal flow with a mean depth of 20 metres and documented flume tests on a scale model show that the basic dimensions, when operating two units with a combined length of 120 metres, should generate more than 4.4 MW at a water velocity of 2 metres per second, and more than 5.2 MW at a water velocity of 2.5metres per second. The turbine should generate about about two to three times the power output of an array of conventional axial turbines, with even lower levelised costs. A 14 kilometre tidal fence would have a peak output in excess of 600 MW with lower levelised costs than offshore wind.

According to Peter Dixon, Kepler Energy’s chairman, the ability of the device to operate in lower velocity tidal water means there is greater scope for deployment in the UK and overseas.

“It means that we can achieve greater economies of scale as our projects are deployed” Mr Dixon explains. “We can happily co-exist with tidal lagoons, and the power peaks will occur at different stages of the tide, meaning that the combined output into the Grid will be more easily manageable. In addition, our levelised costs of production will be in the range £100 to £130 per MWh for utility scale production, so costs will be cheaper than lagoons and in time we will be cheaper than offshore wind generation.  Furthermore, investment risk is manageable since turbines are added incrementally to form the fence, with each one generating revenue as it is added.”

Kepler holds the exclusive global licence for the technology and is embarking on a funding round to take it through the development phase and the planning process. The project will also be subject to a rigorous environmental impact assessment during the planning process in order to ensure it presents no significant risk to marine wildlife. It has been designed to be environmentally benign with a rotor speed of around 11 RPM and a cordoned-off fence area to prevent accidental damage from passing marine traffic.

“Our Bristol Channel tidal fence has the potential to mobilise the carbon fibre industry in the UK as well as create new and skilled jobs in Wales and the West Country” Dixon adds. “For example, for a 400MW fence, we will need as much as 15,000 tonnes of carbon fibre, which will deliver a huge boost to the industry.  In summary, tidal fences offer a very practical and cost-effective source of utility scale renewable energy. Compared with locations in the deep, rough and windy waters of the Orkneys and Scotland, the Bristol Channel has the advantage of being relatively shallow, with lower tidal velocities and waves, and overall less stormy. The steel support columns we plan to use will be surface piercing and can be accessed using the same methods as for offshore wind turbines. They are designed to be watertight, so the electrical and control systems will be in a dry environment. The rotors themselves are simple, with no adjustment mechanisms, just like a water wheel, and Mojo Maritime have developed an efficient installation process for us which does not involve expensive vessels.”

The tidal energy fence also has many advantages over previous proposed marine energy projects, notably the Severn Barrage, while also being compatible with the Swansea Bay Tidal Lagoon. There were just too many environmental issues with the proposed barrage and advice provided to Kepler from its environmental specialists is that because the rotors will rotate slowly, environmental problems should be minimised, based upon experience of other tidal turbines. The tidal fence will also be built out unit by unit, so that risk can be managed through the construction process and so that cash is generated as each unit is installed and commissioned. In comparison, the Severn Barrage would have needed to be commissioned after a very lengthy construction process to find out if it worked correctly, and it wouldn’t have generated any revenue until it had been commissioned.

With regard to Swansea Bay and other lagoons, the tidal fence should be complementary because the two projects will not interfere with each other. They will also generate electricity at different stages of the tide, meaning that the net output of the two together is more valuable to the national grid as it will be smoother.

The outline plans for the project have been shared with various stakeholders including the Department of Energy & Climate Change (DECC), the Welsh Government, The Crown Estate and Bristol City Council, and a wider stakeholder consultation programme will be launched later in the year. It’s too early to say whether or not the newly elected Conservative government will favour the project, but the fact that it is a new British technology should carry some weight. Peter Dixon believes that at the very least the project will need the support of an extended Contracts for Difference (CfD) regime in order to promote its development, and other new technologies that promise low costs and low carbon output with the added benefit of global development potential.

About Robin Whitlock
I am an experienced freelance journalist with a wide and varied portfolio to my credit including web content, magazine articles, reporting, features, interviews, reviews and blogs. My special interests include environmental issues, particularly climate change, renewable energy, transport, green building and sustainable infrastructure. I have numerous secondary interests ranging from politics and current affairs to social justice, science, technology and innovation, historical topics and lifestyle subjects such as literature, psychology, contemporary spirituality and culture.

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