Feature

Going radical to get power from the sea


George Marsh

George Marsh looks at some of the latest technologies and tidal power concepts being investigated by companies and research organisations around the world.

The power locked up in the tide-wave that surges round the earth every day, powered by earth-moon orbital interaction, is massive. This ‘tide-wave’ is unrelated to a so-called tidal wave or tsunami that results from a seismic disturbance – rather we're talking energy-generation power.

Existing technologies can unlock modest (relatively) amounts of this tidal power for human use. Tidal turbines capture kinetic energy due to flow while potential energy is converted when heads of water created by holding the tide back behind barrages etc are allowed to fall through turbines. These means are constrained in scale by their need for favourable locations, the global supply of which is limited, and by the fact that each technology targets just one of the two types of energy available, kinetic or potential.

In recent times, however, two Dutch coastal engineers have proposed a system that would capture both energies together and could wrest very large amounts of power from the sea. Because the system patented by Kees Hulsbergen and Rob Steijn exploits the dynamics of wave power – the tide-wave in this case – it is called Dynamic Tidal Power (DTP). Currently it exists only as a concept but a study being undertaken by Dutch and Chinese interests is aimed at establishing the feasibility of implementing such a project.

DTP requires locations where strong tides run along the shore line, ie parallel to it, and that are in depths shallow enough to permit construction. At such a location, a long breakwater or dam is built out from the shore at right angles, typically with a cross piece at the far end so that the construction resembles a ‘T’. The role of this structure is to obstruct the tidal flow so that water piles up on its upstream side, making it higher than the level on the downstream side.

The ‘T’ section on the seaward end limits ‘leakage’ of trapped water round the end. Conventional low-head turbines set into the breakwater convert both the potential energy due to the head of water and the kinetic energy due to the tidal flow, into electricity. The turbines are bi-directional so that each time the oscillating tide reverses its direction, generation can continue.

The name Dynamic Tidal Power is appropriate because the system exploits the dynamics of a local tidal system, modifying the wave phase and amplitude relationships to maximise the diff erence in water levels on both sides of the barrier and hence the power that can be obtained. A large tidal range (variation in water level between low and high water) is not needed because the system creates its own head of water behind the barrier. Typically this can be a couple of metres or more. The system also captures extra energy that is present in the tide- wave due to the fact that it is normally accelerating or decelerating.

Studies suggest that a single project could generate enough electricity to power thousands of homes or even complete towns. According to one estimate, a DTP dam with 8GW installed capacity and a capacity factor of 30% could deliver some 21T Wh annually, enough to meet the needs of three million Europeans. Tidal power is utterly predictable and its ‘nulls’ can be countered by governing the rate of head depletion so that flow is maintained during tide reversals, and more fully by combining the outputs of two projects located a suitable distance apart.

A sting in the tail, though, is the scale required to make the system viable. To influence the dynamics of the tide-wave the dam/breakwater length needs to be significant compared to the wavelength of the tide itself. In practice this means anything from, say, 30 km to 100 km long. Because power generation capacity increases roughly as the square of dam length increase, any attempt to build a pilot evaluation scheme at reduced-scale would be of little value. Even a dam length of 1 km would have minimal eff ect.

Yet a full-scale system requires a massive structure that is built from the seabed up and then maintained and operated for the life of the scheme. Moreover, finding places where navigational, ecological, fishing and other constraints would not rule out a scheme might be difficult. A high level of project risk seems inevitable initially.

Overall, DTP is a real teaser because the costs and difficulties are high but the benefits could be higher still. There can be few definite answers until a first system is built. Hopefully, the present three-year feasibility study being conducted by the Dutch POWER (Partners Offering a Water Energy Revolution) consortium in collaboration with the Chinese can pave the way for this to happen.

Cross flow

Other radical concepts departing from the prevailing bottom-mounted open rotor, horizontal axis ‘propeller in the sea’ model include vertical axis cross-flow devices. These can be either straight-bladed, as with the Darrieus type system known to the wind energy community, or helically bladed. In both cases, turbines rotate always in the same direction, irrespective of the direction of tidal flow.

In straight-blade machines the blades can be fixed or oscillating, the latter being potentially more efficient. An example of the oscillating blade type is the Kobold turbine, pioneered in Italy and trialed in the Strait of Messina since 2001.

Kobold is a vertical axis turbine having three or more vertical hydrofoils/blades which vary their pitch cyclically so as to achieve maximum lift when moving against the current (upstream) and maximum reaction to the current when moving downstream – hence optimum torque. This device, deployed from a tethered floating platform, can off er a high swept area compared with ‘propeller style’ machines since its horizontal spread can be increased by locating its blades at greater radius from the centre spindle.

Exemplifying the helical type system is the Gorlov helical turbine (GHT), pioneered by Russian born American professor Alexander Gorlov. This can be used as a vertical axis device or in any other orientation. The GHT utilises helical blades/foils to drive rotation, so producing smoother torque than straight-blade rotors. Other claimed advantages include self-starting, fish-friendliness, minimal turbulence and ability to work in low flow.

A seabed-mounted GHT array was last year placed in the ocean off Maine, USA, by the Ocean Renewable Power Company and other examples have been deployed by the Korean Ocean Research and Development Institute.

Unfortunately, another company that has investigated the use of a vertical axis helical device, in this case within a duct, has given up on the concept. The duct was intended to accelerate the flow of water past Neptune Renewable Energy's Proteus turbine so as to increase electrical output, but test results were disappointing. This illustrates the substantial technical risk that haunts tidal enterprise at present as developers seek solutions that are both hydrodynamically and economically viable.

Meanwhile, Australian inventors Aaron Davidson and Craig Hill are keeping faith with the shrouded turbine concept developed by their company Tidal Energy Pty Ltd. A vertical-axis foil-based impeller is contained within a hydrodynamic shroud that comprises a series of guide foils. Tests have supported the inventors' contention that this configuration can be three times more efficient than a comparable open rotor turbine since the Betz limit that applies to the latter does not constrain the shrouded device. Bottommounted via a tripod system, the unit is self-aligning with the tidal flow.

An approach that avoids high costs associated with seabed-mounted turbines is to use tethered floating turbines instead, a solution championed by Oceanflow Energy with its Evopod system. Each Evopod comprises one or more horizontal-axis turbines mounted via struts to a submerged nacelle that keeps the turbines operating in the smoother flow below surface wave action. Each turbine assembly is so tethered that it aligns itself automatically to the direction of tidal flow. Turbines can be recovered individually to sheltered waters for repair or servicing.

Subsidiary Oceanflow Development has secured a seven-year lease from the UK's Crown Estate for an area of seabed off South Kintyre, Scotland which, from this year, will enable it to test a quarter-scale sub-50kW gridconnected unit in an open-sea environment. According to Graeme Mackie, Oceanflow's managing director, “Floating tethered turbines will be cheaper to install and operate than seabedmounted devices and these tests will help demonstrate the potential of the technology to exploit Scotland's extensive tidal stream resource, which is mainly in exposed deeper water sites.”

Non-rotating

Finally, one further approach is to capture energy using an oscillating horizontal blade or hydrofoil, rather than a rotating turbine. A blade attached via a moving arm to a seabed- mounted frame is pitched relative to the oncoming current so that it is driven up (or down) to the maximum extent allowed by its pivoted mounting arm. At that point, the pitch is reversed so that the blade is driven down (or up) again.

This oscillating motion is maintained at a speed determined by the strength of the current. The blade drives the arm, whose angular movement is converted hydraulically to rotational drive for a generator that provides electricity. Key advantages of the technology are its suitability for shallow water and the fact that more energy can be harvested by making the blade wider, without requiring any increase in water depth.

A pioneer has been the Stingray, a 150 kW prototype of which was developed and trialled a decade ago by Engineering Business Ltd. Another UK company, Pulse Tidal Ltd, placed a conceptually similar system in the River Humber estuary in 2009. Chief executive Bob Smith reports that Pulse Tidal intends to deploy a fullscale 1.2 MW demonstration Pulse-Stream device in 2014, following a recent agreement with Crown Estate for the lease of an area of seabed off Lynmouth, Devon.

George Marsh Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.

 

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