Kepler Energy reveals plans for tidal energy scheme in Bristol Channel

Kepler Energy reveals plans fo...
Kepler Energy's Transverse Horizontal Axis Water Turbine (THAWT) uses a stressed truss configuration
Kepler Energy's Transverse Horizontal Axis Water Turbine (THAWT) uses a stressed truss configuration
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Kepler Energy's Transverse Horizontal Axis Water Turbine (THAWT) uses a stressed truss configuration
Kepler Energy's Transverse Horizontal Axis Water Turbine (THAWT) uses a stressed truss configuration
THAWT compared to a horizontal axis turbine
THAWT compared to a horizontal axis turbine
THAWT undergoing tank tests
THAWT undergoing tank tests
Rendering of THAWT being installed in the Bristol Channel
Rendering of THAWT being installed in the Bristol Channel
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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.

THAWT undergoing tank tests
THAWT undergoing tank tests

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

Source: Kepler Energy

Truss 1m 2

View gallery - 4 images
Bruce H. Anderson
What we have here is a vertical axis turbine turned on its side. But since water is more dense than air, some of the challenges with a terrestrial installation (most have not fared well for wind power) may be mitigated. There will be questions regarding its impact on sea life. The unit may foul quickly. The test unit really needs to be put in place in the actual environment to get some power, enviromental, and fouling data. And one might ask if this would work in a river, where the flow would be more constant than tides.
It is still intermittent, so every milliwatt of power must be basked up by an equivalent hot spinning thermal generator, which will be running in a highly inefficient mode.
So just another subsidy generator, in other words...
If the numbers are to be believed, this is one very efficient water turbine. The Kepler website says .... "Documented flume tests on a scale model have shown that the basic 10m diameter, 120 m long unit should generate more than 4.4 MW at a water velocity of 2m/sec."
The power available from intercepting a rectangular block of sea water traveling at 2m/s measuring 120m x 10m, is 4.86 MW, and Kepler reckon they can extract 4.4MW of that. Hmmm ... that's a conversion efficiency of 91% which is outrageously excellent. Could someone close to this project confirm this please ?
I understand how the Betz limit doesn't apply in this situation.
Wave energy is great as it is driven ultimately by the sun. It is renewable.
Tidal energy is sucking energy out of earths kinetic energy of rotation. It is noT renewable. The earth will slow down and the days will get longer. I dont think this is such a great idea.
Their most-optimistic & cheapest *cost* price is already more expensive than retail supply. Almost certainly, their estimate will be out by a factor of 10x... and that's not even starting on longevity, maintenance, storage, or the questionable efficiency to start with...