We British live on an island - or strictly a set of closely spaced islands - which colours our judgement about many things, for good and for bad. Nonetheless, in regard to using the surrounding seas to generate electricity, if this can be done, we could not be better placed in the world. The term "sea-power" covers various technologies, but can be roughly sub-categorised into tidal-stream turbines and surface-wave "rockers". Undersea rockers have been devised too but none of this technology has as yet been implemented on a serious scale. There are related hydro-schemes too: for example, it has been proposed that a barrage (dam) could be placed in the Severn Tidal Estuary - and in the Bristol Channel - (between England and Wales) whose geography constrains relatively large displacements in sea-levels from high to low tide and hence considerable amounts of energy could be extracted there, allowing that various kinds of water-borne traffic need to pass through the estuary too.
Such ideas have been around for some years now, but it looks that the U.K. is poised to begin the extraction of this potentially large source of power. A novel device described as looking like an "upside-down windmill", left from Belfast yesterday, and is expected to launch a revolution in sea-power, ultimately providing for one fifth of Britain's electricity demand. The device is due to be installed near the mouth of Strangford Laugh, in Northern Ireland, and appears to usher-in an alternative but truly renewable technology to the nuclear "new build" for which Gordon Brown and his French counterpart, Nicolas Sarkozy, have agreed the joint construction of a new generation of nuclear-reactors for "home-use" and as saleable technology for the rest of the world. We may well witness both kinds of energy production as part of the energy-mix that is expected to provide energy in the U.K. by 2050.
The new tidal device is named SeaGen and has a capacity of 1.2 MW, and though four times as powerful as its prototype, SeaFlow, is still small in its output, compared to a typical coal, gas or nuclear power station which is closer to 1,000 MW (1 GW), but it does represent the first commercial scale power system ever that is "fuelled" by the renewable forces of the sea-currents. Hydropower schemes use dams and are always criticised for their likely detrimental impact on local ecology, because they impede the natural flow of water, its fauna and its nutrients and associated flora. The proposed scheme in Northern Ireland has a radically different design from barrages and hence avoids these problems. In effect, SeaGen simply sits in the water and uses the ocean currents to turn its turbines, in contrast to a dam/barrage, which is a massive and permanent structure that takes years to build and is there for good. The SeaGen device will be installed at the mouth of the Strangford Lough, in the "narrows" with a width of just 500 metres, where the currents move at above 7 knots (nautical miles per hour), but it will be closely monitored to see whether the spinning turbine blades cause any injury to swimming mammals such as seals. However, it is thought that such creatures are too quick to suffer harm in this way.
A feasibility study of the Severn Barrage has been instigated by Mr Brown, which is expected to last until the year 2010, and if the full construction is made the project will cost £14 billion (costs always escalate on building projects, and so it might be considerably more expensive that that, especially as the full project is thought impossible to complete before 2020). It is said that the Severn Barrage could provide 5% of Britain's electricity.
In contrast it is thought that SeaGen could be constructed relatively easily and the turbines installed in the most suitable locations in a far more flexible way and at much smaller capital cost. Exploiting ocean currents is reckoned to hold the potential of supplying 5% of the nation's electricity (coincidentally the same as the Severn Barrage). The rest of that hoped-for "fifth" (20%) would presumably come from other kinds of sea-power, e.g. wave-power "rockers" etc.
A wave-energy power station off Cornwall is expected to begin feeding its output into the national grid, and the Severn Barrage has already begun a two-year feasibility study. It is thought hat around the coast of Scotland, with its strong currents (and rough seas, it must be said, especially the North Sea, among the roughest in the world and mechanically fretting to whatever is built there) could provide a number of energy-rich locations from which to extract sea-power based electricity. Britain has around half of Europe's tidal-stream potential and about 10 - 15% of that identified in the world as a whole, making it uniquely placed.
The capacity factor of a hydro-turbine is thought to be around 40%, and so each SeaGen would produce 1.2 MW x 0.4 = 0.5 MW. The average amount of electricity drawn in the UK amounts to about 40 GW, or 40,000 MW. Hence if the technology is to produce 5% of that, we need 0.05 x 40,000/0.5 = 4,000 of them. I wonder how quickly this amount of engineering can be fabricated and installed? Let's say, 1 a week; that's about 50 a year, and so it would take 80 years to put the lot in place. To make a serious impact we will need them (and that's just to produce 5% of our electricity, 95% coming from other sources) in say 25 years and so they need to be installed at a rate of 160 a year or 3 a week. It could be done, I'm sure, but what about the rest of it? If nuclear can be maintained to provide around 20%, we then have 25% and if they do build the Severn Barrage (another 5%), we have 30% from nuclear plus "water"-power and the rest from coal and gas. If other ways to extract the grand total of 20% of our electricity from sea-power is managed, on that same time-scale, we have matched about 45% of our present demand for electricity. My point is that it is going to take a long time before we are weaned-off fossil fuels and nuclear power, and even then, if all the engineering can be done (this is big-scale stuff) we might still only have 25% of the amount of electricity we enjoy currently (from combined sea-power plus the Barrage).
I try to remain optimistic about the survival of humans, but to my sight of the horizon, there still beckons a collection of localised societies that not only cannot travel very much, in consequence of depleting and hugely expensive oil supplies, but which have to get by with maybe a quarter of current electricity supply. It is also worth noting that electricity is not the same as energy, but about one fifth of the total. If 40% of our total energy is used in the form of transportation fuel and about 20% as electricity (both of which will be curbed significantly within this putative 25 years), that still leaves another 40% for heating buildings etc. to be found. There are many conclusions that can be drawn even from such approximate numbers as these.
 "The rise of British sea power." http://www.independent.co.uk/environment/green-living/