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.
REFERENCES
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.
6. http://www.globalenergyinc.com/920209.html
Further papers on relevant matters can be downloaded
from http://www.see.ed.ac.uk/~shs
July 2007
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