Select Committee on Innovation, Universities, Science and Skills Written Evidence

Memorandum 10

Submission from Professor Stephen Salter

  1.0  Personal details: I am Emeritus Professor of Engineering Design at Edinburgh University. I have been working on renewable energy from sea waves since 1973 and more recently on applying power conversion ideas from wave to wind and tidal-streams. I have given previous evidence to Parliamentary Committees on renewable energy [1][2][3]. Very little has happened to change my views since those notes were written. The rate of atmospheric CO2 increase is still accelerating and most of its outcomes are at the top end of predictions. I fear that the rate of progress on renewables is too slow to prevent the triggering of at least six distinct climatic positive-feedback mechanisms and so my main present activity is aimed at the design of practical hardware to implement John Latham's proposal [4] for the direct reversal of global warming by increasing cloud reflectivity through the Twomey effect. Very small amounts of sea water injected as a micron spray into marine stratocumulus clouds can make them reflect more solar energy back out to space. Double present CO2 levels could occur with no temperature rise. Despite an enormous energy leverage and a wealth of literature confirming the background physics, official UK interest in the subject is strikingly similar to that in the early days of wave energy.

  Additions to my previous evidence are as follows:

  2.0  Tidal stream. Estimates for the tidal stream resource in the Pentland Firth have used equations taken from the wind industry. These are based only on the kinetic energy flux in an open flow field with just an adjustment for the higher fluid density. They may be inappropriate for long channels with rough beds and irregular walls because they ignore friction losses. We do not have accurate values for friction coefficients for the Pentland Firth but, if they are similar to those in the Menai Strait, then present peak bed dissipation would be over 50 GW. Any small reduction in velocity caused by turbine installations will release large amounts of energy. About one third of the present total friction loss could be extracted giving a possible resource of 10-20 GW, much higher than previous estimates.

  2.1  It may be possible to get a further increase by using speed- and pitch-control of turbines to change the phase of the power take-off relative to the tidal cycle. Data from the Proudman Laboratory show that there is a substantial phase lag (40-60 degrees) between the driving head of the Pentland Firth and the flow velocity through it. The channel has an apparent inertia greater than that of the mass of the water in it. This may be partly because of the need to accelerate through changes of cross section and partly because of the mass of water in the approaches. It would be better to have head and flow in phase with each other. Delaying generation will give the channel some virtual spring and so bring it closer to resonance. Many people, even trained engineers, find it difficult to understand phase. One way of looking at it is to argue that allowing more flow in the early part of the cycle and less in the later returning part will leave a "hole" in the water at the entrance and so make it look more attractive to flow in the next cycle. It is likely that smaller tidal-stream sites will have a resonance on the other side of the excitation period and so would benefit from a phase advance. This would make the combined outputs be steadier.

  2.2  I am advised by Professor David Pugh that more accurate estimates of bottom friction dissipation and flow impedance of the Pentland Firth, and other passages further north, will need the installation of a chain of (perhaps 20) acoustic Doppler velocity measurements linked to water depth readings. Sensors would be placed at points along the flow lines from the Atlantic to the North Sea and data recorded over the lunar cycle. The changes in depth measurement at each instrument will be used to calculate the mean surface slopes of the water.

  2.3  Although the Royal Navy spent much of the 19th century taking soundings of the world's oceans, the installation of a prototype tidal-stream device in the Orkneys was halted by a collision with an uncharted rock. This is a much more expensive way to improve chart accuracy than traverses with a side-scan sonar. However the latter is too expensive for small struggling tidal stream developers.

  2.4  Making use of the full resource will require new designs of turbine that can block a large fraction of the flow-window of the Pentland Firth which has a depth of about 70 metres over much of its area. Reference [5] describes a design.

  3.0  Synthetic fuel. As the full electrical output from the Pentland Firth would often exceed the peak Scottish demand, there will be a need for large inter-connectors to southern load centres or ways to convert irregular electrical supplies to produce natural gas substitutes and liquid fuels for transport. This can be done by electrolysis to produce hydrogen and oxygen followed by the use of both in a conversion something like the Fischer Tropsch process, developed in Germany in the 1920s. Peak production in 1944 was 6.5 million tonnes. Under the threat of oil sanctions the process was used in South Africa by SASOL. Historically the products have been somewhat more expensive than fuel from conventional sources but the gap would close if the carbon-neutral feedstock was municipal waste and there was a high land-fill tax. In the UK this has risen from £3-£24 per tonne and will be increased by £8 every year with further increase threatened by the EC. This seems a much more acceptable carbon-neutral source than any food stuff. Pilot plant is operating in Fife [6].

  4.0  Wave Energy. Waves from offshore deep water sites around the UK offer a larger ultimate resource than tidal streams, with a different pattern of variability but quite long reliable forecasting, certainly long enough for grid controllers and the electricity market. The technology is recovering from the damage caused during the eighties by the UKAEA [1] but progress is still slow. The problems are that some over-confident newcomers are not using existing information and are not doing enough small-scale testing of tank models to identify the worst loading conditions. Pressure from non-technical investors to cut corners and get quick results is very hard for inventors and engineers to resist if their incomes depend on doing as they are told. All developers claim to be front-runners in the field with leading-edge and patented, but simple and proven, technology. Some of the statements made in fund-raising advertisements do not bear close examination.

  4.1  The success of complete generation systems may be at risk if failure occurs in a single, perhaps very cheap, small component. We need to test large batches of small parts and sub-assemblies in parallel on some form of test raft in the correct chemical and biological environment. Failures would then be useful pointers to design improvement instead of financial disasters for investors. When the small component does fail, attempts are made to conceal news of the disaster so the mistake is repeated by competitors. What we need is a system of reporting and widespread circulation of every detail of accidents and near accidents as was made compulsory since the early days of the aircraft industry and was operated on a voluntary basis in the early days of the wind industry, where it led to enormous improvements in reliability.

  4.2  Some ideas, design approaches and technology from the offshore industry can be usefully transferred to wave energy but methods for moving and installing offshore structures are not in this category. The costs of installation vessels can vary by more than an order of magnitude depending on the needs of the oil industry. There is a need for independent installation methods perhaps involving propulsion modules that can easily be attached and removed from wave plant.

  5.0  Sea bed attachments. There is also a need for sea bed attachments that can easily be connected or disconnected without the need for heavy lifting gear, and also for robotic vehicles to prepare the sea bed side of the connection. The design of these has a considerable overlap with underwater vehicles that could survey the sea bed off Dounreay for the sources of radioactive particles and recover them safely. So far 1,200 particles, each typically the size of a grain of sand and a lethal alpha-emitter have been found, with numbers rising as detection equipment improves. It is not known how many have been blown inland.

  6.0  Test facilities. Several types of wave energy device are potentially vulnerable to currents and most marine-current devices would be vulnerable to waves. Finding ways to reduce this vulnerability will greatly increase the size of the resource by extending the number of sites. Waves and currents interact with one another in an extremely complicated way especially if they approach from opposite directions. It is important to test renewable energy plant (and other structures) in any combination of directions of waves and currents. Such a facility would be too expensive for any single developer but preventing a single accident could save the cost many times over. Work at Edinburgh University on a model of a test tank has shown that any complex pattern of currents can be produced by a single vertical-axis variable pitch-rotor placed in the "cellar" of a circular tank. The previous Edinburgh wide tank with a long straight line of wave makers has had to be demolished but it has been partially rebuilt with wave-makers around a 90 degree arc. We can therefore be confident that the two halves of the technology can be combined.


1.  Lords Select Committee on the European Communities 1987-88. Alternative Energy Sources pp 178-206.

2.  Commons Energy Committee 1991-92 Volume III pp 62-77.

3.  Commons Science and Technology Committee Report on Wave and Tidal Energy, April 2001.

4.  Bower K et al. Computational assessment of a Proposed Technique for Global Warming Mitigation via Albedo Enhancement of Marine Stratocumulus Clouds. Atmospheric Research vol 82 pp 328 336 2006.

5.  Salter S H, Taylor J R M T. Vertical-Axis Tidal Current Generators and the Pentland Firth. Proc.I.Mech.E vol 221 Part A. Journal of Power and Energy Special Issue pp 181-295 April 2007.


  Further papers on relevant matters can be downloaded from

July 2007

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