The tide turns for marine current turbines14 December 2000
Marine currents offer the possibility of generating power at low costs and on a grand scale. Peter Fraenkel reports on the technology needed to get the best from this predictable energy resource
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 predictable as the tides that cause them, unlike wind or wave energy. 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 sea water 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/sec) 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 Bristol Channel (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.
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 to a predictable timetable 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.
The reasons marine current turbines look like being cost competitive include:
• The resource offers typically four times the energy intensity of a good wind site and 30 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 critical for a submerged machine, compared with wind turbines, so the construction can be from low cost steel - more like a ship than an aircraft.
• 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 as you 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 you 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 100t force? This 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 lasts 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 marine current turbines 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.
UK-based Marine Current Turbines (MCT) has developed a patented turbine concept which is 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. In other words, the company claims to have found a relatively low cost structure with the integrity to support a large turbine or turbines reliably for many decades. 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.
Seacore, one of the the company's commercial partners, is a specialist 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 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 20rpm) 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 rotor driving a generator through a gearbox. 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. 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 MCT consists of two 500kW power trains mounted at each end of a streamlined cross arm. The arm is mounted on the monopile. The variable pitch axial flow rotors each drive a generator via a gearbox, much like a hydroelectric turbine or a wind turbine. The turbine 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.
Because 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 quite high (connection to the grid, planning, mobilisation, etc), so turbines will generally be installed in batches. 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 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 will 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
MCT was originally founded by IT Power, a technical consultancy company. Since IT Power 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 (formerly British Steel). This consortium has been awarded grant support worth £962,000 (approximately US$1.4M) by the UK's Department of Trade and Industry (DTI) to ensure that the first phase of the research and development programme can go ahead. 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 ISET - which is part of the University of Kassel in Germany - and of Jahnel-Kestermann Getreib, manufacturer of wind turbine gearboxes. The German partners are also being given some financial support by the government of Germany.
The concept of using tidal currents as an energy resource has not been seriously taken up until relatively recently (the 1990s). 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 to date 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 300kW demonstrator and test bed off the coast of Devon, UK, in 2002. A design has been developed and costed, a site off Lynmouth has been identified and surveyed and permissions are being requested from all the relevant authorities.
A second phase of the R&D programme will follow a year later to develop and install a twin rotor commercial prototype of around 700 to 800kw. This will be installed by 2003-04. A third phase is also planned to be the first tidal turbine farm, consisting of at least four twin rotor turbines capable of delivering in 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. MCT's business plan envisages the possibility of installing 300MW worth of turbines by 2010, so the technology could make a significant contribution to the UK government's target for 10% renewable energy generation by that year.