Is the tide turning for marine current turbines?

1 June 2001

Marine currents offer the possibility of generating power at lower costs than wind (or most other renewable energy resources) and of doing it on a grand scale. The key challenge is to demonstrate this and then to get the costs down to where the technology can form the basis of a self-sustaining clean-energy industry. Peter Fraenkel, Marine Current Turbines Ltd, London, UK

Marine current turbines function much like submarine windmills, being installed in the sea at places with high tidal current velocities, to take out energy from the huge volumes of flowing water. These flows have the major advantage of being an energy resource as accurately predictable as the tides that cause them, unlike wind, wave or solar energy, which are weather dependent. Energy to a timetable is inherently more valuable than randomly generated electricity. Tidal turbines also have minimal environmental impact.

Suitably fast currents can be found in places where movements of seawater are constrained by the topography of headlands and islands. This is familiar to most mariners; peak spring tide velocities of the order of 4 to 5 knots (2 to 2.5m/s) are needed, with depths of water between 20 and 35m, for economic exploitation.

Many sites have the energy and space to allow thousands of turbines to be deployed. Some locations, such as the Pentland Firth (between Scotland and Orkney) and the Alderney Race (between the Channel Islands and France), or the Big Russell (off Guernsey) have especially intense currents with potential for exploitation on a gigawatt scale. Other intense locations include the Severn Estuary (UK’s north Devon coast), the straits between Rathlin Island and Northern Ireland, the Straits of Messina between Italy and Sicily, and various channels between the Greek islands in the Aegean. Other large marine current resources can be found in regions such as south east Asia, Canada, the south east coast of South Africa, with many other places yet to be investigated.

New concept

It is unusual for an entirely new energy concept to be developed; even rarer if the technology to exploit it produces no pollution, delivers energy predictably and has the potential to generate thousands of megawatts of clean power from the sea at an attractively low cost. But marine current turbines promise just this.

Among the reasons marine current turbines look like being cost competitive are:

• The resource has four times the energy intensity of a good wind site and 20 times the energy intensity of sunshine in the Sahara, so marine current turbines need only a quarter the swept area of a wind turbine of the same power – and size equates with cost.

• Weight is not such a problem for a submerged machine, so the construction can be more like a ship than an aircraft – and ships are relatively low cost engineering products. Steel can be used rather than costly lightweight materials.

• The degree of over-engineering needed to cope with the “worst-case” operating conditions is much lower than for wind or wave powered devices, as conditions under water, even in the severest of weather, do not vary a great deal.

Although the relentless energy of marine currents has been obvious from the earliest days of seafaring, it is only now that the development of modern offshore engineering capabilities, coinciding with the need to find large new renewable energy resources, makes it technically and economically feasible to exploit it.

A frequently asked question is: “why, if this is such a good idea, has it not been done before?” As soon we start to evaluate the technical requirements for installing a tidal turbine in the sea, a number of seemingly daunting problems arise. For example, how do we hold a turbine rotor securely enough that it cannot be swept away, bearing in mind that the thrust on the rotor of a 1MW tidal turbine rotor running at full power is of the order of 100 tonnes force, which of course poses a significant structural or mooring problem? How, if you need to carry out work on the system, can you do this underwater with fast moving currents? Slack tide is a matter of minutes, and conditions at energetic locations are the underwater equivalent of a storm swept mountain top, so it is virtually impossible for divers or remotely operated vehicles to function effectively.

So, what makes the technology described here feasible? The key is a relatively recent technical breakthrough: the possibility of installing steel piles (large steel tubes) in holes in the seabed drilled from a jack-up barge. A jack-up barge can raise itself on legs like a table to provide a steady platform above the sea from which all the installation work can be completed.

Moreover, the patented turbine concept we are developing is mounted on a pile in such a way that it can be raised above the surface of the sea for maintenance or repair.

In other words, we have found a relatively low cost structure with the integrity to support a large turbine or turbines with solid reliability for many decades and the entire system can be installed, serviced and replaced without any need for underwater operations; everything is done from either a jack-up barge or surface work boats. Without this approach we do not think the exploitation of tidal currents would be a practical proposition.

Marine Current Turbines Ltd, based in the UK, has initiated a programme of tidal turbine development which has started with an initial four year R&D and demonstration phase leading to commercial manufacture. The plan is to conclude the initial R&D phase by around 2005, and to start commercial installations then. The goal is to complete almost 600 MW of installations by 2010. The long term potential is many times greater.

The nearest competitor, wind generation, has grown from almost nowhere ten years ago into a multi-billion dollar industry; marine current energy technology has the potential to equal or maybe even exceed wind energy in its future importance. This is because the scope for meeting future energy requirements solely from land-based resources will be constrained by conflicts over land-use; it is significant that the wind industry is also moving offshore.

Because of the higher energy intensity in the tidal stream compared with wind, the diameter of the rotors used in a tidal turbine is much less than for a wind turbine – around 18 m versus 55 m for a nominal 1 MW rotor.

Although the high density of water compared with air yields high energy densities at low velocities, it also yields correspondingly large forces on the rotor structure compared with a wind turbine. Hence, a submarine kinetic energy converter has loadings that are quite different from those of an equivalent wind turbine. Also, rotor speed has to be limited by the need to avoid significant cavitation as this has the effect of drastically reducing the lift/drag ratio and hence the efficiency of the rotor. With an axial flow rotor, cavitation develops at the rotor blade tips, in particular at the upper part of the swept path, where static pressure is least; in practice we believe blade tip velocity needs to be limited to around 12 to 15 m/s.

The technology

As already noted, in practice it is almost impossible to carry out extensive underwater operations using either divers or remotely operated vehicles in the kinds of locations of interest. Therefore Marine Current Turbines Ltd has developed a turbine concept based on using a monopile installed from a jack-up barge, which can drill the necessary hole and install the pile using its onboard crane. Seacore Ltd, one of the MCT’s commercial partners, are specialists in placing monopiles, and they have the capability to drill holes and place piles up to 4m in diameter even in hard rock such as granite. Seacore’s monopile installation technology was used in the recently completed Blyth offshore wind scheme in the UK.

The rotor of a tidal turbine runs at low rotational speeds (10 to 20 rpm) and generates correspondingly high levels of torque. Therefore the drive train poses some interesting technical challenges. The most likely solution in the short term will be to use a similar system to a wind turbine, namely a gearbox driving a generator. For most machines this will need at least a two-stage step-up, probably an epicyclic gearbox to keep the size and weight within reasonable limits. There are also possibilities for using hydraulic transmission or for developing special low speed direct driven alternators (a seemingly promising trend at present with wind turbines). The power train may be encased in an air filled nacelle using sealing arrangements for the drive shaft much like those used for ship propeller shafts, or alternatively it is possible to use dedicated sealed gearbox/generator units that can run immersed in water and therefore need no external casing. Submersible mechanical and electrical machinery is becoming relatively commonplace and large submersible pumps of similar power levels to the generators needed for a tidal turbine are standard commercial products.

The technology now under development by Marine Current Turbines Ltd consists of two 500 kW power trains mounted at both ends of a streamlined cross arm

The cross arm is mounted on the monopile in such a way that it can be raised to the surface by a lift mechanism.

Each variable pitch axial flow rotor drives a generator via a gearbox, much like a hydroelectric turbine or a wind turbine.

The system is connected to the shore by a marine cable lying on the seabed, which emerges from the base of the pile.

The submerged turbines will be grouped in arrays under the sea, at places with high currents, in much the same way that wind turbines in a wind farm are set out in rows to catch the wind. The main difference is that marine current turbines of a given power rating are smaller, can be packed closer together and involve negligible land use or other environmental impact. As a result there are savings in costs both for the underwater cable connections and for the installation work that is required.

Becuse tidal turbine technology is modular, small batches of machines can be installed with only a short period between investment in the technology and the time when revenue starts to flow. This contrasts with large hydro electric schemes, tidal barrages or other projects involving major civil engineering, where the lead time between investment and gaining a return can be many years.

The cost overheads involved in starting to develop a new project are high (connection to the grid, planning, mobilisation, etc), so turbines will generally be installed in batches – probably of around 20-30 MW to get reasonable economies of scale.

Many of the potential sites so far investigated are large enough to accommodate hundreds of turbines, so there is likely to be scope for numerous upgrades of the installed capacity at a given location and these will allow considerable economies of scale to drive down generating costs.

Getting the costs down

As with all completely novel technologies, the costs will not be competitive to start with, but with technical improvement and economies of scale from mass-production and improved methods of installation, there is considerable scope for cost reduction.

The first generation of tidal turbine systems will have costs comparable or possibly lower than wind turbines less than ten years ago, and today wind turbines are considered to be economically competitive with any other method of power generation. Wind turbine generating costs have fallen in real terms by a factor of four since the first commercial wind farms were installed in California in the early 1980s.

But marine current technology has the major advantage that it only needs to fall by a factor of around two to reach cost levels where windmills are now. This can be achieved within a few years given the growth in demand for clean methods of power generation that can be expected in the coming decade.

In the meantime there are numerous niche markets for this technology where power can be supplied to small island communities which are at present using costly diesel generation. Tidal current turbines can be readily integrated with a diesel generation system as the predictability of the currents permits the diesels to be shut down completely at periods of high flow.

Today, most governments, including that of the UK, are making major efforts to encourage the take-up of new methods for power generation to minimise carbon dioxide and other atmospheric pollution. To this end, new subsidies, grants and other methods for encouraging the development of clean power can be expected. It is likely that as utilities the world over are pushed into seeking greater percentages of their generating capacity from clean renewable resources, so technologies such as this will become increasingly attractive.

The route to commercialisation

Marine Current Turbines Ltd was originally founded by IT Power Ltd (ITP), a technical consultancy company. Since ITP is primarily a consultancy company, MCT was formed recently as a more appropriate vehicle to develop commercial technology. MCT has formed a consortium of companies with a common interest in developing tidal stream technology, including IT Power, Seacore, Bendalls Engineering and Corus UK Ltd (formerly British Steel). There is also an international dimension through part funding of the first phase of the R&D project by the European Commission (contracted to IT Power) and the involvement of Flender, manufacturer of wind turbine gearboxes and their associated company Loher which builds marine electrical generators and seabed mounted pumping equipment for the offshore oil and gas industry.

The concept of using tidal currents as an energy resource has not been seriously taken up until relatively recently. Only limited resources have been available so far to permit experimentation and research (the European Commission has been the largest donor by far, but even the EC has only funded about half a dozen projects in this field). As a result, most of the work so far has been either theoretical or else small scale experimentation. “Full-size” pilot projects are now needed to take the technology forward, as the main uncertainties relate to implementation, operation, cost and reliability.

The concept MCT is developing has reached the stage where it is ready for a “full-scale” pilot project, and it is planned to install a 300 kW single rotor demonstrator and test bed off the coast of Devon, UK, in 2002. This will be 30 times more powerful than its largest predecessor, which was developed by the some members of the same team in 1995.

The EC has part funded this work and a design has been developed and costed, a site has been identified and surveyed and permissions have been requested from all the relevant authorities.

  The industrial partners have pledged to contribute a significant financial component and an application has been made to the UK DTI for the necessary top-up finance to carry this phase of a planned R&D programme to a successful conclusion.

Given that the proposed project just outlined is successful, a second phase will follow a year later to develop and install a twin rotor commercial prototype, of about 700 kW.

A third phase is also planned to be the first tidal turbine “farm”, consisting of at least four twin rotor turbines capable of delivering of the order of 3 to 5 MW between them and it is planned for 2004-05.

This third phase will be partially self financing from revenue from the sale of electricity. The development of commercial tidal turbine projects will follow immediately thereafter, around 2005-06.

Marine Current Turbines Ltd’s business plan envisages the possibility of installing of the order of 300 MW worth of turbines by 2010, so the technology could make a significant contribution to the UK government’s target for 10 per cent renewable energy generation by that year.

MCT has completed detailed technical and economic analyses and believes the concept it has under development has the potential to generate electricity within five years for less than 4p/kWh, providing reasonably large projects (minimum size 20 to 30 MW) are carried out (to share the fixed overheads between sufficient turbines). We believe that in the longer term, thanks to the high energy intensity of tidal currents, generating costs of less than 3p/kWh can be achieved.

MCT’s analysis has been scrutinised by Binnie, Black & Veatch in a review funded by the UK DTI, and BB&V have confirmed that these unit costs seem feasible with the technology as envisaged (see panel, p38).

Therefore Marine Current Turbines has one of the few large scale renewable energy concepts with the potential to compete directly with fossil fuel on a generating cost basis within the next ten years.

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