Offshore wind design

Shallow water tidal energy – going with the flow

To date, free-flow tidal power generation has in the main been confined to stormy exposed seaways with fast flowing currents and harsh environmental challenges for installation, operation and maintenance. LIC Energy UK’s work with Kepler Energy is helping to bring a new, low-cost and low investment risk, shallow water technology to the maritime renewable energy mix.

( Text content written by Twenty6 for Danny Bonnett, LIC Energy )

Mention tidal energy schemes and most people instinctively think of ‘robust wind turbines adapted to work on the seafloor’ in deep, aggressive, fast-flowing channels such as the turbulent Pentland Firth between Scotland’s north coast and Orkney.

This is not illogical. Because power output is proportional to the cube of velocity, the massive tidal flows found around Britain’s rugged coast are commercially attractive, despite harsh operating environments.

Now, however, there is an equally appealing alternative. Kepler Energy’s development of a second generation of extremely efficient and low-cost tidal turbines is opening up major opportunities in more benign and accessible local waters with calmer conditions and longer weather windows.

With state-of-the-art novel technology from Oxford University’s Department of Engineering Science, using composite materials, Kepler’s highly-innovative ‘tidal fence’ offers a cost-efficient turbine manufacture, deployment and maintenance system designed specifically for shallow water use close to major population centres where taking power ashore does not involve expensive interconnectors.

Kepler’s long-term vision includes the eventually construction of many kilometres of ‘fence’ harvesting sustainable power from the Moon’s gravitational field along coasts and estuaries in the Indonesian islands, Korea, Japan, China and UK. Crucially, the system works efficiently throughout the cycle of rising and falling tides.

However, subject to planning permission and finance, an early pilot project should see a 2.2MW rotor installed and operating by 2020/21 in the Bristol Channel, most probably near Aberthaw and Minehead. The aim is to combine extreme reliability with ease of installation and maintenance, plus minimal or no ecological impact.

LIC Energy UK (LIC) was quick to recognise the synergies of working closely with Kepler whose aims are totally consistent with LIC’s own innovation, simplicity and safety principles honed through extensive offshore wind experience.

A departure from traditional turbine designs

By concentrating on efficient power generation in near-shore waters, the ‘fence’ system has abandoned the familiar ‘propeller’ blade arrangement of axial flow turbines normally associated with tidal energy.

Instead, the much lighter Kepler Transverse Horizontal Axis Water Turbine (THAWT) has the ability to span across the tidal flow, allowing the economics of scale to reduce the unit cost of extracting energy from the tides.

The hydrofoils are conventional, even though it involves no yaw or complex pitch-changing mechanism and electrical equipment is contained in a dry column.

The real challenge has been to make a structure light enough to be handled easily, reliable enough to ensure a long, low-maintenance lifespan, but also strong enough to withstand large forces in generating economic quantities of power.

To achieve this, the THAWT solution uses the hydrofoils to form a three-dimensional truss to span across the flow utilising modern carbon composite material to provide both the strength and stiffness required.

As an example of how innovative design has been carefully matched to the practicalities of operation, the density of the carbon composite hydrofoil blade system is important. Because it is very close to the density of water, lifting and installation of the rotor are particularly easy

Typically standing some 10m in diameter – and therefore high – the 60m long THAWT units can be freestanding, or installed as end-to-end ‘fence’ arrays extending over many kilometres

Understanding new physical parameters

The key to THAWT design is in the understanding that the physical environment around wind turbines and sub-sea or tidal turbines is completely different – not least because water is denser than air and water provides a more accessible free surface, from which potential energy can be extracted in combination with the natural kinetic energy of the tidal flow itself.

Since its foundation in 2010, Kepler’s approach has been to understand this physical environment intimately, and its consequences for energy extraction, and then apply this new knowledge to turbine design with expert partners like LIC Energy UK.

The rectangular profile that THAWT units present head-on to the tidal flow fill a large proportion of the flow area, whatever the state of the tide. Energy capture per unit increases as more of the flow cross-section is covered, an enhancement coming from the ‘blockage effect’, provides for the ability to extract potential energy. This manifests itself in increased velocity through the THAWT device, above that of the tidal flow being presented to the device. These turbines also generate on both rising and falling tides.

Greater energy conversion efficiency

One of the low risk investment attractions of Kepler Energy’s concept is that manufacturing, installation and maintenance costs are intentionally low. Because modules can be installed one-by-one and the wired up immediately for power generation, they create an early incremental income.

This means that by optimising blockage effects on a circa 14km long fence, 600MW could be generated at peak. This is utility-scale quantities of power!

LIC Energy UK’s involvement

LIC was attracted by the advanced virtues of Kepler’s concept and keen to apply its team’s skill set to the civil engineering challenge. Kepler’s vision for civil works is to use standard plant, with an absolute minimum of sub-sea work. An onshore production line will produce most components; offshore work must be a smooth and swift assembly operation. Electrical components will be kept dry to give a very long operating lifetime, but also straightforward to maintain. These objectives are totally consistent with LIC’s core principles.

LIC believes that a highly detailed understanding of client needs is the secret of good delivery. Kepler agreed and attended workshops at LIC’s Bristol offices to help develop and share the brief, while also expanding upon the economic drivers.

LIC then considered two superstructure concepts – one steel, the other concrete. The Bristol team developed the concrete solution to a greater level, preferring the natural weight of such a structure because it reduces buoyancy issues. LIC’s experience with large-scale concrete gravity bases has also taught it the value of good quality concrete as a long-lasting structural material in the marine environment.

The concrete structure’s installation will be carried out in sections (four per column) to keep lift weights within the limits of ‘standard plant’ prescribed by Kepler. The LIC team also engineered a system for applying pre-compression to sections, over the full column height, to provide the narrow crack widths needed for high durability. This will be achieved without the traditional pre-tensioning cables that can be the bane of older pre-stressed concrete structures, especially when exposed to salt.

Further, an innovative concept to preload the lower blocks into the seabed, in a similar way to the preloading of jack-up barge legs, will ensure long-term reliable bearing of the column skirt on/in the ground. This will minimise the length of bored or driven piles, reducing their required moment capacity. The pile installation methodology will be drilled or driven, or a combination of the two (known as DDD). The chosen pile driving methodology is very geology specific, and likely to impact on the exact installation methodology to best balance costs, schedules and structural performance.

While working with Kepler, LIC addressed and resolved a number of challenges in the continuous search for cost-efficiency and simplicity. The application of an open mind, a first principles approach, plus diverse experience, allowed LIC to bring a host of original ideas to the table.

Tidal energy in a wider context

The Kepler project is being developed in close liaison with stakeholders that include the Department of Energy and Climate Change, the Welsh Government, the Crown Estate and Bristol City Council.

The Bristol Channel area could also see the potential development of tidal lagoon technology whereby incoming tides are trapped and released slowly through other forms of power-generating turbine. Kepler sees no conflict of interest and points out that the two systems could be complementary in supporting the regional electricity grid because peak power generation occurs at different states of the tide.

Given the focus on innovation and efficiency, the Kepler Energy team also anticipates that its technology will eventually be cheaper than offshore wind energy.

As a commercial knock-on, it is estimated that installing a 400MW fence could also stimulate the creation of an industrial sector capable of creating some 15,000 tonnes of carbon fibre that the THAWT project will need.