The Pentland Firth

The Pentland Firth. Image source: The Crown Estate.

The funnel of water between the north-east tip of Scotland and the Orkney Islands contains some of the most powerful tidal energy in the world. The exploitable resource at the time of the fastest running tides may be as much 8 gigawatts. One source I have talked to suggests the figure could even be as high as 20 gigawatts. As I write this note, the National Grid’s website gives an estimate of the electricity use at this moment – about 55 gigawatts at peak-time on a winter evening. Be in no doubt, the Pentland Firth is the single most important source of renewable energy in the UK. The power concentrated in this narrow stretch of water could comfortably provide London’s electricity need. And it is entirely predictable to within a few percent every minute of every day. The tidal power will peak twice every 24 hours in a cycle that the Grid will be able to plan for decades in advance.

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The Crown Estate is aware of the potential. As the UK body responsible for licensing the use of the seabed, it is an enthusiastic proponent of exploiting the energy contained in the Pentland Firth. Today (17 November 2008) it has opened a bidding round for developers interested in putting tidal turbines in the Firth. Licences are expected to be awarded by next summer. It has relatively modest ambitions to get 0.7 gigawatts of power installed by 2020.

We shouldn’t underestimate the difficulties involved in getting electricity from this fast-flowing water. The speed and turbulence of the tides are almost unparalleled at any inshore site in the world. Like wind, the power of a flow of water is proportional to the cube of the speed. Water moving at 10mph has eight times the power of water at 5mph.

The designs proposed by the brave or foolhardy companies seeking to exploit the power are usually a mixture of a simple turbine with minimal moving parts and a huge steel structure, full of hundreds of tonnes of ballast to hold the turbine in place. It is only the 30 years of experience of hydrocarbon extraction from the North Sea that gives the developers any chance of installing turbines that are robust enough to last more than a few days. Thus far, the major contenders for the licences have only experimented with their turbines in waters with a small fraction of the power of the Firth so we will probably see multiple failures before we get decent amounts of power from the tides.

The first operators will be able to use the reasonable quality grid connection to the Dounreay experimental fast breeder reactor site on the coast nearby. But major development will require a new power infrastructure. Last year the Crown Estate proposed a new high voltage direct current cable down the east coast of England, coming in to London and then on to Holland. This would have the huge advantage of providing the UK with better connections to the northern European grid, helpful for balancing power supply and customer needs.

In the next few days we expect the government to announce an expansion in public spending on hospitals and schools. Is it too late to suggest that it would be more productive to promise more active support for tidal power and a commitment to back the new cable off the east coast?



(The value of exploiting the energy in the Pentland Firth is one of the case studies in chapter 3 of my new book, Ten Technologies to Save the Planet, in which I discuss the major opportunities for extracting energy from waves, tides and currents around the world.)

Ten Technologies to Save the Planet was listed as one of the Financial Times Science Books of the Year 2008.