Technological Advance in Generating Electricity from Tides

The Open University publication on renewable energy, Renew reports in its Jan/Feb 2007 edition (newsletter 165) on the following developments in the technology for developing electricity from tides and tidal currents.

Marine Current Turbines (MCT), the foremost British company in this field, has installed a commercial scale trial 1.2 MW tidal current turbine in the narrows of Strangford Lough, N. Ireland in September 2006. This trial is aimed at proving the economics and technical efficiency of a tidal turbine of this size.

In concept, tidal current turbines bear a similarity to wind turbines in that their blades are driven by the tidal currents beneath the sea’s surface (rather than the wind above the surface) and the captured energy is transformed into electricity via a turbine. The advantage that tidal current turbines have over wind turbines lies in the constancy of tides and the movement of offshore tidal currents.

MCT aims to install arrays of such turbines offshore. Joe Verdi from MCT informed the House of Commons Welsh Affairs Select Committee in March 2006 “In roll-out we see an array, subject to consents being given, in about 2008. We intend to have something like a 10 or 13MW site, presumably off Anglesey. The resource around north Anglesey – again it is a potential resource, subject to planning – is of the order of around 180MW, so we see that rolling out starting off with 30 and then 50 and then going to 100MW within five to eight years. Wales is blessed with marine current and potentially we would feel there is about 8%; in fact, there is 1GW potentially available.”

Joe Verdi stated that investment is a problem “We have to be able to demonstrate to any investor that there is a firm revenue stream. It is very much a catch-22 situation: while our prototype or first commercial demonstrator costs money, a lot of it is getting their confidence to say they will order [however] with say, for 20 machines, your purchasing power increases tremendously. You get economies of scale in installation as well. Your grid connection costs, whether you put one machine in or whether you put 20 machines in, is about the same. That supports the project finance. Once you get the first array in, in terms of MW I firmly believe the utilities will then sit up and see that it is a viable technology and they will be prepared to take more risk than they are taking now.”

A rival industrial consortium, Swanturbines Ltd, is developing a novel tidal current device. The consortium is led by Swansea University and has 8 partners, including CB&I, John Brown and Corus.

The Swanturbine concept is similar in some way to that of MCT, but it attempts to reduce the complexity of the system by eliminating the gearbox and other potentially unreliable components, thus reducing maintenance costs which in an offshore location can be significant It is also mounted on the seabed by a patented construction method (unspecified at present) rather than via a pile driven into the seabed, and the tower of the installation can be extended like a telescope to rise out of the water to give access to the machinery for maintenance.

James Orme from Swanturbine says “By developing a novel method of installation and removal we will reduce the cost of setting up a tidal farm dramatically. We are also not limited by depth and so we will be able to extract 96% of the UK resource with these devices. That’s significant potential.”

Swanturbine have done testing on the River Towy, and are currently looking for a site to install a demonstration device in 2007. James Orme has told the House of Commons Welsh Affairs Select Committee that the Swanturbine device is similar to MCT’s tidal turbine “The main difference is the method we use to support the turbine underwater. . . . Studies undertaken by our industrial partner CB&I John Brown have indicated that the installation and maintenance costs are the primary drivers in the viability of the system over the lifetime. This cost is therefore reduced by a clever method of installation.” The other difference is that “it does not use a gear box, so we have increased efficiency in the drive train. It means that we have a clever generator, which runs at low speed rather than the high-speed type which you might find elsewhere. It is similar in a sense to the Enercon wind turbines, which have the largest market share in Germany. We believe that has a number of advantages; reduced maintenance requirements and more reliability if you get rid of the gear box; and there is less noise underwater, which should prove less intrusive to marine life.”

In commenting to the House of Commons Welsh Affairs Select Committee, Dr Ian Masters from the School of Engineering at the University of Wales in Swansea stated “I started my evidence saying that we should invest £500m in this technology. If we went back thirty years to 1976 . . . and if we had the foresight to invest in the UK wind industry then, we would now have a viable wind industry. We currently import all our turbines from Denmark, Germany and Spain . . . . £500m is not very much when you consider that 3.2 billion people live within 100km of the sea . . . the DTI has made a start; there is the £50m wave tidal demonstration scheme, and support through the technology programme, but it is not of that order of magnitude.”

The House of Commons Welsh Affairs Select Committee also heard about tidal lagoons from Peter Ullman of Tidal Electric. Peter Ullman explained “Tidal lagoons are a form of low-head hydro-electricity. The way it functions is that the low-head hydro-electric turbine is installed in an impoundment structure. It has come to be called a lagoon because it is a much more descriptive term, but it is a structure built out of rock, sand, gravel, in a conventional marine construction fashion. It sits a mile or so offshore, and is self-contained . . . . What you create with the impoundment structure is a difference in water level, and then the difference in water level is harvested – the energy is harvested by allowing the water from the high pool to go into the low one. Then what you have is a full situation where the tide drops away to do the opposite; so it is a two-way generation using conventional low-head hydro-electric.”

Peter Ullman noted that “In Swansea Bay we are proposing a project that will be 60 megawatts, and it is 5 square kilometres, which is a significant part of the area of the Bay. It is by no means the whole area of the Bay because the Bay itself is many times that, but whenever you do that you change things . . . . It is expected that this will enhance the biodiversity of the area. The inner shore tidal zone and the near-shore tidal zone is famously barren; not that there is nothing that lives there, but there are precious few creatures there that take up the habitat. Because of the size and the natural structure of this, it will create a wildlife habitat. The flipside of that advantage is that because it is big it is going to change things in the Bay in terms of currents, sediment transport, and the general flow of traffic in Swansea Bay. That is why we have Associated British Ports as part of our team, to assess the sediment and transport issues to make sure that we are not causing difficulties.”

On the effect of silt build-up within the lagoon Peter Ullman commented “Siltation occurs in inverse proportion to the depth so if you have a very shallow area you will get more siltation than if you have a deeper area; so we have designed the inside of the structure to be deep, that is a metre or more of depth at all times, which would reduce the siltation. Given that the general calculus is that it is unlikely to be a great amount in either direction, it still leaves you with the possibility that you are wrong, and some unusual event happens and silt does get entrained on the inside.” Under those circumstances “you change the velocity of the flow in and out of the structure. When you increase the velocity in a particular direction you will create a scouring; so if you have had too much siltation you aim the velocity at the build-up and you scour it, and then you rotate this wall such that you self-scour the entire structure. That is the likely solution. In some sort of disaster scenario you can also dredge.” To deal with the opposite problem, scouring out too much and undermining the soils, Tidal Electric plan to install a screen which would retain the silt.

Peter Ullman also commented on increasing the efficiency and generating capacity of the lagoon by means of its design. “At the moment the design for Swansea Bay is a single pool. . . . . the project would need to be a multiple pool – whether three, four or ten pools remains to be seen for a further design stage. The point of single, double or triple, is that because the tides happen twice a day the optimal times for generating are high tide and low tide, and the transitional periods in between are less optimal. If you have a single pool you can generate about 48% of the time. By dividing the pool into three you can pulse generate from each of the pools and increase the amount of time during the day. In Swansea Bay, for example, by going from a single pool to a triple pool you increase the time of day from 48% to 81%. This allows you to hit tea-time and times at which the grid is needing power a lot more often than not. The cost would be somewhat higher for the impoundment structure because you have to build these internal walls, but lower for the equipment because you do not need as much equipment. At the end of the day there is some improvement in the cost of the output, that is the unit cost of power, by going to a multi pool.”

On the potential for other sites for lagoons, Peter Ullman noted in addition to south Wales and the north-west Wales coastline, he has “been asked to look at the Liverpool docks area, which is a potential site; all of the Severn estuary; and the Thames estuary.”

Asked about the Severn Barrage, Peter Ullman argued that “There are four basic problems with the barrage: it blocks navigation; it impedes fish migration; it changes the head pond in the area back a bit; and it changes the tidal regime downstream. Put those altogether and it creates an economic problem. When you bundle all that together it has been unrealistic to proceed with this. This is before you tackle the environmental issues involved.”

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