Publications

Survey of Energy Resources 2007

Harnessing the Energy in the Tides

There are two fundamentally different approaches to the exploitation of tidal energy. The first is to exploit the cyclic rise and fall of the sea level through entrainment and the second is to harness local tidal currents in a manner somewhat analogous to wind power.

Tidal Barrage Methods
There are many places in the world in which local geography results in particularly large tidal ranges. Sites of particular interest include the Bay of Fundy in Canada, which has a mean tidal range of 10 m, the Severn Estuary between England and Wales, with a mean tidal range of 8 m and Northern France with a mean range of 7 m. A tidal-barrage power plant has indeed been operating at La Rance in Brittany since 1966 (Banal and Bichon, 1981). This plant, which is capable of generating 240 MW, incorporates a road crossing of the estuary. It has recently undergone a major ten-year refurbishment programme.

Other operational barrage sites are at Annapolis Royal in Nova Scotia (18 MW), the Bay of Kislaya, near Murmansk (400 kW) and at Jangxia Creek in the East China Sea (500 kW) (Boyle, 1996). Schemes have been proposed for the Bay of Fundy and for the Severn Estuary but have never been built.

Principles of Operation.
Essentially the approach is always the same. An estuary or bay with a large natural tidal range is identified and then artificially enclosed with a barrier. This would typically also provide a road or rail crossing of the gap in order to maximise the economic benefit.

The electrical energy is produced by allowing water to flow from one side of the barrage, through low-head turbines, to generate electricity.

There are a variety of suggested modes of operation. These can be broken down initially into single-basin schemes and multiple-basin schemes. The simplest of these are the single-basin schemes.

Single-Basin Tidal Barrage Schemes.
These schemes, as the name implies, require a single barrage across the estuary, as shown in Fig. 13-2. There are, however, three different methods of generating electricity with a single basin. All of the options involve a combination of sluices which, when open, can allow water to flow relatively freely through the barrage, and gated turbines, the gates of which can be opened to allow water to flow through the turbines to generate electricity.

Ebb Generation Mode.
During the flood tide, incoming water is allowed to flow freely through sluices in the barrage. At high tide, the sluices are closed and water retained behind the barrage. When the water outside the barrage has fallen sufficiently to establish a substantial head between the basin and the open water, the basin water is allowed to flow out though low-head turbines and to generate electricity.

The system can be considered as a series of phases. Fig. 13-3  shows the periods of generation associated with stages in the tidal cycle. Typically the water will only be allowed to flow through the turbines once the head is approximately half the tidal range. This method will generate electricity for, at most, 40% of the tidal range.

Flood Generation Mode.
The sluices and turbine gates are kept closed during the flood tide to allow the water level to build up outside the barrage. As with ebb generation, once a sufficient head has been established the turbine gates are opened and water can flow into the basin, generating electricity as shown in Fig. 13-4 .

This approach is generally viewed as less favourable than the ebb method, as keeping a tidal basin at low tide for extended periods could have detrimental effects on the environment and on shipping. In addition, the energy produced would be less, as the surface area of a basin would be larger at high tide than at low tide, which would result in rapid reductions in the head during the early stages in the generating cycle.

Two-Way Generation.
It is possible, in principle, to generate electricity in both ebb and flood. Unfortunately, computer models do not indicate that there would be a major increase in the energy production. In addition, there would be additional expenses associated in having a requirement for either two-way turbines or a double set to handle the two-way flow. Advantages include, however, a reduced period with no generation and the peak power would be lower, allowing a reduction in the cost of the generators.

Double-Basin Systems.
All single-basin systems suffer from the disadvantage that they only deliver energy during part of the tidal cycle and cannot adjust their delivery period to match the requirements of consumers. Double-basin systems, as shown schematically in Fig. 13-5, have been proposed to allow an element of storage and to give time control over power output levels.

The main basin would behave essentially like an ebb generation single-basin system. A proportion of the electricity generated during the ebb phase would be used to pump water to and from the second basin to ensure that there would always be a generation capability.

It is anticipated that multiple-basin systems are unlikely to become popular, as the efficiency of low-head turbines is likely to be too low to enable effective economic storage of energy. The overall efficiency of such low-head storage, in terms of energy out and energy in, is unlikely to exceed 30%. It is more likely that conventional pumped-storage systems will be utilised. The overall efficiency of these systems can exceed 70% which is, especially considering that this is a proven technology, likely to prove more financially attractive.

Possible Sites for Future Tidal Barrage Developments.
Worldwide there is a considerable number of sites technically suitable for development, although whether the resource can be developed economically is yet to be conclusively determined (Boyle, 1996). These include, and this is not a definitive list (Fig. 13-6):

Tidal lagoons
Tidal barrage systems are likely to cause substantial environmental change; ebb generation results in estuarial tidal flats being covered longer than in a natural estuary - this might not be acceptable; a barrage, even with locks, will cause obstruction to shipping and other maritime activity. Artificial lagoons (www.tidalelectric.com) have been proposed as alternatives to estuarial barrages. Electricity would be generated using sluices and gated turbines in the same manner as 'conventional' barrage schemes.

The principal advantage of a tidal lagoon is that the coastline, including the intertidal zone, would be largely unaffected. Careful design of the lagoon could also ensure that shipping routes would be unaffected. A much longer barrage would, however, be required for the same surface area of entrainment. Some preliminary studies do suggest, however, that in suitable locations the costs might be competitive with other sources of renewable energy. However, there has not yet been any in-depth peer-reviewed assessment of the tidal lagoon concept, so estimates of economics, energy potential and environmental impact should be treated with caution.

The Severn Estuary, which lies between England and Wales, and the mouth of the Yalu River, China have both been suggested as potential locations for lagoon-style development.

Tidal Current Technology
Principles and History. Presently the development of tidal barrage schemes has been limited. This has been partly a result of the very large capital costs of such systems associated with the long construction times and fear of environmental impact.

Many engineers and developers now favour the use of technology which will utilise the kinetic energy in flowing tidal currents. The most thoroughly documented early attempt to prove the practicality of tidal current power was conducted in the early 1990s in the waters of Loch Linnhe in the West Highlands of Scotland (www.itpower.co.uk/researchdevelopment.htm ). This scheme used a turbine held mid-water by cables, which stretched from a sea-bed anchor to a floating barge.

The mid to late 1990s was primarily a time of planning and development as far as tidal current power was concerned, and it was not until the beginning of the 21st century that further systems became ready to test. In 2000 a large vertical-axis floating device (the Enermar project [www.pontediarchimede.com]) was tested in the Strait of Messina between Sicily and the Italian mainland. Marine Current Turbines Ltd (www.marineturbines.com) of Bristol, England, has been demonstrating a large pillar-mounted prototype system called Seaflow in the Bristol Channel, which lies between England and Wales. Fig. 13-7  shows the Seaflow system with its nacelle raised into the 'maintenance position'. It is intended that the same company will install a further large prototype system, SeaGen, in Strangford Narrows in Northern Ireland, probably in late-summer 2007 (Fig. 13-8 ). Although conceptually similar to Seaflow, it would be equipped with two rotors and have a rated capacity of 1.2MW.

In Norway, the Hammerfest Strøm system (www.tidevannsenergi.com) demonstrated that pillar-mounted horizontal-axis systems can operate in a fjord environment. In the USA the first of an array of tidal turbines were installed in December 2006 in New York's East River (www.verdantpower.com ). Once fully operational this should be the world's first installed array of tidal devices.

In 2007, The European Marine Energy Centre (EMEC) (www.emec.org.uk), which was established in 2004 to allow the testing of full-scale marine energy technology in a robust and transparent manner, became fully equipped for the testing of tidal, as well as wave energy, technology. The tidal test berths are located off the south-western tip of the island of Eday, in an area known as the Fall of Warness.

The facility offers five tidal test berths at depths ranging from 25 m to 50 m in an area 2 km across and approximately 3.5 km in length. Each berth has a dedicated cable connecting back to the local grid. The first tidal device (www.openhydro.com) was installed at the end of 2006. This is operated by the OpenHydro Group and is a novel annular-turbine system held by twin vertical pillars. The system can be seen in its maintenance position in Fig. 13-9.

The physics of the conversion of energy from tidal currents is superficially very similar, in principle, to the conversion of kinetic energy in the wind. Many of the proposed devices have therefore an inevitable, though superficial, resemblance to wind turbines. There is, however, no total agreement on the form and geometry of the conversion technology itself. Wind-power systems are almost entirely horizontal-axis rotating turbines. In these systems the axis of rotation is parallel to the direction of the current flow. Many developers favour this geometry for tidal conversion. Vertical-axis systems, in which the axis of rotation is perpendicular to the direction of current flow, have not been rejected. It is of interest to note that Enermar used a novel Kobold vertical-axis turbine.

 The environmental drag forces on any tidal-current energy-conversion system are very large, when compared with wind turbines of the same capacity. This poses additional challenges to the designer. Designs exist for devices which are rigidly attached to the seabed or are suspended from floating barges, such as the early Loch Linnhe device. It is generally accepted that fixed systems will be most applicable to shallow-water sites and moored systems for deep water. There may be exceptions to this, however.

Energy Available in Tidal Currents.
The superficial similarity between the kinetic energy flux in tidal currents and energy available from the wind encouraged the design of technology with more than a passing resemblance to wind turbines. Early assessments of the available energy also, rather unfortunately, encouraged the consideration of resource availability in terms of the kinetic energy flux alone, without taking due account of the nature of the free surface between the sea water and the atmosphere, the frictional interactions between the flowing water and the flow boundaries, or the complex turbulent nature of the flow.

It is very tempting to consider only the kinetic energy flux in moving water when assessing available energy. This can be very easily calculated for water passing through a cross section by using equation 1

where:
  is the density of water (kgm-3)
U is the component of flow velocity perpendicular to the section area (ms-1), which is normally a function of position within the cross section.
A is the cross section area (m2)

Further analysis rapidly reveals that, although the value of the kinetic energy flux can suggest the presence of extractable energy, the actual potential of a site to deliver energy is a more complex relationship involving understanding of the nature of the total flow environment.

Fig. 13-10  shows the expected kinetic energy flux in a simple, static head driven channel (Bryden, Grinsted and Melville, 2005) of length 4 km, width 500 m, an inlet depth of 40 m and an outlet depth of 39 m. The kinetic energy flux increases along the channel. The figure clearly shows a head drop immediately downstream of the inlet, resulting from acceleration of the flow from stationary in the inlet ocean, resulting in a sharp head drop.

It is interesting to speculate on the influence of extracting energy at the mid-point of the channel. Fig. 13-11  shows, for the same channel, the influence of extracting energy equivalent to 25% of the kinetic flux in the undisturbed channel, with the output expressed in terms of channel depth and kinetic flux. The kinetic energy flux in the channel is actually higher downstream from the energy extraction than upstream. If the available energy is only considered in terms of kinetic flux, this would be a bewildering result in which energy appears to be coming from nothing and contradicting the conservation of energy. This is not, of course, the case.  

At least part of the mystery can be solved by comparing Figs. 13-10 and 13-11. The kinetic flux in the exploited scenario is substantially less than that in the unexploited case.

It can be demonstrated (Bryden, Couch, Owen, Melville) that many of the properties of the simple channel model can be expressed in terms of a simple parameter given in equation 2, which appears to govern at least some of a channel's response to energy extraction.

where:
f is the ratio of energy extraction to the actual kinetic flux in a channel
L is the channel length (m)
g is the acceleration due to gravity (ms-2)
n is the Manning Roughness Coefficient
R is the hydraulic radius (m)

Fig. 13-12  shows the result of a sensitivity study into the influence of changes in the channel length, width, depth and roughness, expressed in terms of the parameter B.

This implies that the parameter, B, at least in terms of the simple channel model, appears to offer the prospect of a simple assessment of channel sensitivity to energy extraction.

Further analysis (Bryden and Couch, 2007) of the nature of energy extraction from simple channels can show that it is possible to extract more energy than the total kinetic flux. Equation 3 shows the maximum energy extraction, expressed as a function of channel parameters.

This takes the form of the kinetic flux multiplied by a term influenced by channel parameters. It is obvious from this equation that knowledge of the undisturbed kinetic energy flux is necessary for the determination of the potential for energy extraction, but that it is also necessary to know additional facts about the geography of the site.The simple channel models used to generate equations 1 to 3 are recognised as abstractions and that real tidal environmental flows are far more complex than such simple approaches can fully describe. Even models of more complex channel (Garrett and Cummins, 2005) might not be sufficient, as many energetic tidal regions are multiply connected with inter island channels in a complex geography.

Progress is, however, now well advanced in the understanding of complex flows in three dimensions and it is now possible (Couch and Bryden) to assess the impact of energy extraction, even in multiply connected flow domains typical of real high energy tidal zones. Even issues associated with environmental disruption from energy extraction, such as localised flow distortion shown in Fig. 13-13  and disruption close to the sea bed as shown in Fig. 13-14, are being addressed and, with this increased knowledge, uncertainties about the resource potential for tidal currents and environmental constraints are potentially quantifiable.

Development Options for Tidal Currents. The environment that tidal devices will operate in is very different from that experienced by wind turbines, and there are some rather difficult problems associated with installation, survivability and maintenance which need to be solved before true commercial exploitation can be achieved. Proposed development options often involve the use of dedicated installation and maintenance vessels, which suggests that tidal currents might only be economically developed in large sites, where major developments can be installed, justifying the use of an expensive infrastructure.

Small sites could perhaps be developed, however, using technology which can be installed and maintained using less expensive techniques. The Sea Snail, which has been developed by the Robert Gordon University, which can be installed using a small sea-going tug, could be an option. This sea-bed located device, which is shown in Fig. 13-15, is held to the sea bed using variable-position hydrofoils which generate substantial down force, thus reducing the need to use substantial ballast.

Many industrial, commercial and public bodies have suggested that there is a high degree of synergy between the development of a tidal-current generation industry and the offshore oil and gas industry. This offers the intriguing prospect of a new renewable industry developing in partnership with the petroleum industry and could, perhaps, result in accelerated development, as a result of the availability of expertise and technology, which would otherwise have to be developed from scratch.

Unlike the wind, tides are essentially predictable, as they derive from astronomic processes. Wind-power systems are dependent upon random atmospheric processes. This results in it being difficult to integrate large wind-power developments into strategic electricity distribution networks. The predictability of the tides will make this integration much easier.

Although prototype tidal-current devices are now available and have mostly proved successful in their operation, there are still issues requiring resolution before the resource can be fully exploited. With the exception of the New York East River development, knowledge of the performance of devices in arrays is somewhat limited, although theoretical models are at last becoming available. It is also becoming obvious that turbulence levels in high-energy tidal flows can be considerable. Turbulent amplitudes exceeding 30% of the time-averaged flows have been measured and this will prove challenging to systems designers. Similarly there is an ongoing need for enhanced understanding of the behaviour of tidal-current devices in the presence of incident waves. These gaps in understanding should not, however, prevent ongoing deployment of pre-commercial, or even early-stage commercial technology, provided that technology developers are aware of the design constraints that knowledge gaps impose and recognise that they themselves are part of the research process. This will ultimately allow efficient technology development and hence allow cost-effective exploitation of the tidal-current resource.