With its large tidal range, Britain's Bristol Channel has a huge potential for generating tidal electric power. The problem is that, until now, schemes for tapping that power have required building dams and barrages so gigantic they would have given even the most wild-eyed Victorian engineer pause. As a more economical alternative, Kepler Energy has announced plans for a 30 MW tidal energy fence to be built in the Channel. With an estimated cost of £143 million (US$223 million), the underwater fence would be built in the water somewhere along the line between Aberthaw and Minehead and could be operational by 2021.

According to some estimates, if the tides that flow in and out of the Bristol Channel could be properly harnessed, they could supply up to five percent of Britain's energy needs. It's a challenge that has attracted engineers for over a century, but while on paper there's a tremendous amount of energy waiting to be tapped, a viable plan of how to actually go about it has proven elusive.

The main stumbling block is the nature of tidal power technology. Most conventional systems rely on horizontal axis turbines. This is like an underwater wind turbine and, like its terrestrial counterparts, it has severe limitations as to where it can be placed and under what conditions it can generate a practical amount of energy without ripping itself apart. Larger outputs require larger blades, which is a tricky enough problem with wind turbines, but water turbines tend to cavitate, which reduces efficiency and damages the turbine, so there's an upper limit to how big the machine can get. Because of these various factors, horizontal axis turbines are confined to deep waters over 30 m (98 ft) deep and tidal flows of over 2.5 m/s (8.2 ft/s).

The top alternative is Dynamic Tidal Power (DTP). This requires large barrages thrusting many miles out into the sea to channel the tide toward the turbines. Theoretically, such an installation could produce 10 to 15 GW of energy with a 40 to 50 km (25 to 31 mi) wall, but it's a massive investment and it has to be built first before the engineers can determine how well it works, so there's a high level of uncertainty.

For its project, Kepler Energy is using its Transverse Horizontal Axis Water Turbine (THAWT) technology. Developed by Oxford University’s Department of Engineering Science, it's designed for deployment in shallower, lower velocity tidal waters. It's based on a stressed truss configuration with carbon composite hydrofoil blades. According to Kepler, this simple design with a minimum of moving parts allows the dynamos and other electrical equipment to be installed in dry columns. In this, the generating units consist of two sets of blades sitting on three columns with a single generator between them.

Kepler regards the Bristol channel as ideal for the installation and plans for the turbines to be set up as a subsurface tidal fence 1 km (0.6 mi) long, while future fences could be over 10 km (6.2 mi) long. The idea is that the fence causes the current to back up against it as it tries to flow past. This phenomenon, known as blockage, produces a head of water that increases the efficiency of the turbines. Blockage increase with the length of tidal fence, so the longer the fence, the greater the output by each turbine.

According to Kepler, the new system does not require specialist vessels to overlook them, and can operate over a wider range of sea conditions than the alternatives. The Kepler blades cover a much greater area than horizontal axis turbines with much less depth needed. They also claim that the simplicity of the design makes it much less expensive than more conventional designs.

Kepler Energy is currently seeking funding for the project as it moves into the development and planning phases, which includes a rigorous environmental assessment. It has presented its plans to the Department of Energy & Climate Change, the Welsh Government, The Crown Estate, and Bristol City Council, and plans to start a stakeholder consultation program

“As our tidal technology can operate in lower velocity tidal waters, there is greater scope for its deployment in the UK and overseas." says Peter Dixon, Chairman of Kepler Energy. "It means that we can achieve greater economies of scale as our projects are deployed. 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 levelized costs of production will be in the range £100 to £130 (US$156 to US$202) 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.”

The video below shows a test of the Kepler Energy technology

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