SPECIAL ISSUE PAPER 159
Marine current turbines: pioneering the development
of marine kinetic energy converters
P L Fraenkel
Marine Current Turbines Ltd, The Court, The Green, Stoke Gifford, Bristol BS34 8PD, UK
email: [email protected]
The manuscript was received on 21 March 2006 and was accepted after revision for publication on 17 November 2006.
DOI: 10.1243/09576509JPE307
Abstract: This paper gives the rationale and background to an already advanced research and
development (R&D) programme aimed at developing technology for the commercial exploitation of kinetic energy from marine currents. This is followed by a brief overview of the characteristics of the tidal stream resource, the technical principles by which it may be exploited, and
the key technical challenges that need to be overcome.
The paper includes a description of the pioneering ‘Seaflow’ Project involving the installation
and testing, since May 2003, of a prototype 300 kW tidal turbine 3 km off Lynmouth. The next
stage of Marine Current Turbines Ltd’s R&D programme is then described: this involves the
development of a 1 MW twin axial-flow rotor system, called ‘Seagen’ which is planned for installation early in 2007. The installation and testing of ‘Seagen’ will mark a landmark stage in the
R&D programme because it will form the basis for the commercial technology to follow. A
brief outline of future plans beyond ‘Seagen’ is also given.
Keywords: renewable energy, marine power, tidal power, energy converters
1
INTRODUCTION
Marine renewable energy resources, such as tidal
current kinetic energy conversion, are technically
difficult and potentially costly to develop, so until
recently there was no incentive to do so. However,
the growing realization that the unsustainable use
of fossil fuels is rapidly reaching the stage where
dramatic changes will be forced on us by factors
beyond our control has led governments to start to
seek ways to address this problem, such as encouraging the development of new renewable energy
technologies.
It is worth reminding ourselves of some of the key
developments which act as drivers for this recent
development. Atmospheric carbon dioxide concentrations are higher now than at any time in the last
500 000 years and have clearly departed from the
natural cycle. CO2 concentrations in the atmosphere
have risen as much in the last 150 years as in the previous 20 000 years [1]. This effect is commonly considered to be likely to cause significant global
warming which may in turn cause catastrophic
environmental problems [2].
JPE307 # IMechE 2007
The other, less talked-about key driver is that of
‘peak oil’. It is becoming the accepted wisdom that
in the near future for the first time a point will be
reached at which world oil production is no longer
capable of keeping up with growth of world oil
demand. Depletion of resources characterized by
the well-known Hubbert ‘peak oil’ curve will force
this to happen [3]. The result of demand exceeding
supply, as experienced briefly in 1973, will be a dramatic rise in energy costs.
Tidal stream (and ocean current) technology is one
of the most recent forms of renewable energy to be
developed. It has only been considered worthy of
official support by the UK government since 2001,
but today it is a key part of the Department of
Trade & Industry’s (DTI’s) R&D programme having
real potential to make a significant contribution to
the UK’s (and for that matter the rest of the
world’s) Kyoto targets.
The work so far completed by the author’s company, Marine Current Turbines Ltd. (MCT), has
successfully confirmed the (expected) technical
feasibility of collecting energy from tidal currents
and the work described in this article is intended to
Proc. IMechE Vol. 221 Part A: J. Power and Energy
160
P L Fraenkel
develop a commercial form of the technology that
is expected to demonstrate potential economic
feasibility.
Figure 1 illustrates Seaflow, the world’s first tidalcurrent-powered experimental turbine to function
in a truly offshore location. At the time of writing
this, 300 kW system has been operational for more
than three years and has exceeded its key design
goals, as will be discussed in detail later in this article.
2
2.1
TIDAL STREAM TECHNOLOGY: STRENGTHS
AND WEAKNESSES
The tidal stream resource
The tidal resource is driven by the relative motion of
the gravitational fields of the moon, the sun, and the
earth. The fluctuations in local gravity resulting from
these movements cause the rise and fall of sea level,
which in turn causes massive flows of water. Most of
these flows are far too slow to present enough energy
density for effective energy recovery, however in certain places where there are ‘pinch points’ due to flow
being constrained by land and seabed topography,
the currents can be accelerated to much higher
velocities. In such locations with peak velocities
exceeding about 2 –3 m/s, the energy density
becomes great enough to warrant deployment of
appropriate technology.
Because the kinetic energy in fluid flow is proportional to velocity cubed, the energy availability
Fig. 1
is highly sensitive to the velocity. Doubling the velocity results in eight times the energy density and
of course halving the velocity results in one eighth
of the energy. For example, the energy density in a
flow of seawater of 2 m/s is 4.1 kW/m2, but an
increase to 3 m/s represents an energy density of
13.9 kW/m2. Clearly it is crucial to find locations
with the highest possible flow velocities if energy is
to be extracted as cost-effectively as possible.
It should also be noted that the currents tend to
run in phase with the rise and fall of the tides, with
slack tide at high and low tide being the point at
which the currents reverse direction and come to a
brief halt, and the maximum velocity being when
the tide is at about the mean level. The ebb and
flood of the tides runs on an approximately 12.4 h
period diurnal cycle and superimposed on this is the
334 h spring-neap cycle, where the relative positions
of the sun and moon either reinforce their gravitational pull (springs) or are at right angles (neaps).
So far as the UK is concerned, various studies [4 –8]
have been carried out to determine locations suitable
for tidal stream energy generation, and although the
UK tidal stream data base is fairly limited at this
stage, there is probably no other country with more
detailed information available. Resource data
world-wide is sparse but work is in hand to try to
remedy this.
The most recent resource study by Black and
Veatch [8] gives an estimated UK extractable resource
of 22 TWh (electrical energy output per annum) for
tidal stream using a modified and probably more
Seaflow, the world’s first offshore tidal current turbine, rated at 300 kW and installed in
May 2003. The 11 m diameter rotor shown raised for maintenance on left and lowered
for power generation on right
Proc. IMechE Vol. 221 Part A: J. Power and Energy
JPE307 # IMechE 2007
Marine current turbines
accurate methodology. In short, depending on
assumptions and methodology, the estimates so far
completed suggest that something in the order of
5 – 10 per cent of the UK’s electricity supply (at present demand levels) could ultimately be met from
tidal stream projects.
At this stage, the lack of information means it can
only be guessed at the gross size of other tidal stream
resources. It seems reasonable to assume that there
is a world-wide extractable tidal current energy
resource at least of the order of 100 TWh and possible
significantly greater. However, even if the resource
proves to be limited to 100 TWh, this would require
an installed capacity exceeding 25 GW which is
clearly a multi-billion pound market. Moreover, the
UK in such a case would have the largest tidal
stream resource which suggests the potential for
developing a UK-based industry to address not just
a significant home market but also the potential
world market.
2.
3.
2.2
Environmental issues
The most important advantage of tidal or marine
current electricity generation systems is that
although they depend on a fluctuating resource
(with no energy available around slack tide), their
output is predictable, and therefore this technology
offers the possibility of dispatchable power, which
is inherently more valuable than the much more
randomly generated output of wind, wave, and
solar devices.
The ‘raison d’être’ for this new technology is that it
produces energy without pollution and thereby can
substitute for carbon-emitting power generation as
a means to mitigate atmospheric pollution. It is
believed that tidal turbine technology generally has
the potential to be used with minimal environmental
impact. Some of the key environmental issues which
are frequently raised are worth summarizing.
1. Effect on flow and sediment transfer. Tidal stream
energy exploitation is unlikely to have any significant effect on natural processes. This is because
analysis has indicated that extracting more than
10 – 15 per cent of the tidal stream energy from a
specific location is about the sensible limit [9].
The reason for this is that if excessive numbers
of turbines are installed, such that flow velocities
are reduced by more than a few per cent, the
excess turbines will be counterproductive by
effectively diminishing the performance of the
rest of the project. Therefore, the tidal stream
resource is unusual in being self-regulating as an
energy supply, because growing a project
beyond a sensible limit will reduce the overall
JPE307 # IMechE 2007
4.
5.
161
energy capture per turbine to such an extent
that it will reduce the overall return on investment. Therefore, any project can only have a limited effect on natural flow processes and will not
cause any localized effect outside the natural
spectrum of velocities.
Threat of impact on marine wildlife from turbine
rotors. The speed of underwater turbine rotors is
generally low compared with wind turbines or
with ship or boat propellers because of a need to
avoid cavitation (typical tip velocities will be
below 12 m/s). Also a tidal turbine rotor at a
good site absorbs about 4 kW/m2 of swept area
from the current, whereas typical ship propellers
release over 100 kW/m2 of swept area into the
water column; one is a gentle process and the
other is violent. Therefore, tidal turbines are
much less likely to be a threat to marine wildlife
than ship propellers. Underwater noise is also limited due to the low speed of operation and the
need to minimize cavitation.
Conflicts with other users of the sea. Tidal turbines
can only be applied in locations with unusually
high-current velocities, most often close to rocky
coasts, which tend to be hazardous for navigation
and hence are generally avoided by commercial
ship traffic. Arguably tidal turbines fitted with
navigation aids will provide a fixed reference for
mariners, which may be an aid rather than a hindrance to navigation. However, exclusion zones
for fishing may be needed around turbine farms,
which does have the benefit of protecting fish
stocks and the seabed in the area concerned.
Pollution. Tidal turbines if developed and applied
on a large scale can substitute for fossil fuel generation and thereby diminish atmospheric pollution. Lubricating oil or other potential
pollutants are present in small quantities, but
they are so well contained that they are most unlikely to escape. Only relatively small amounts of
anti-fouling paints (compared with ships) of the
most environmentally acceptable kind (copper,
glass, or PTFE based) need to be used. Decommissioning is relatively rapid and straightforward and
ought to leave conditions almost exactly as they
were before the project.
Energy return on energy invested (ERoEI). for a
tidal turbine looks like being better than for
most energy technologies. This has not yet been
investigated rigorously, but the ERoEI for wind
turbines has been found to be between 4 and 6
months (depending on the wind regime and the
technology) [10]. Since the weight of material
and the level of energy capture of the kind of
tidal turbines under development by MCT are
similar to those parameters for wind turbines,
the ERoEI seems likely to be of the same order.
Proc. IMechE Vol. 221 Part A: J. Power and Energy
162
P L Fraenkel
Having summarized the advantages of tidal turbine
technology, the main ‘down-side’ compared with
wave or wind technologies is that the siting requirements for tidal turbines are specialized and relatively
rare. In practice, locations are needed with mean
spring peak tidal currents faster than about 4–
5 knots (2–2.5 m/s) or the energy density will be
inadequate to allow an economically viable project.
In short, the resource is limited to certain locations
with unusually strong currents, but even so it is a
non-trivial resource in terms of commercial potential.
3
TIDAL STREAM TECHNOLOGY: BACKGROUND
AND STATE OF THE ART
As long ago as 1976, the Intermediate Technology
Development Group (ITDG), the grand-parent company of MCT, was seeking to use renewable energy to
help people in remote areas of the world improve
their self-sufficiency. The author was working for
ITDG at that time and suggested that an ‘underwater
windmill’, driven by river currents could be used for
pumping irrigation water out of many fast-flowing
rivers which traverse otherwise arid regions [11].
Therefore, a river current turbine has been developed
for pumping irrigation water out of the Nile near
Juba in Southern Sudan. This had a 3 m-diameter
Darrieus-type of rotor driving a pump and was
mounted under a pontoon. It proved capable of
pumping 50 cubic metres of water per day through
a head of 7 m and ran for nearly two years during
the early 1980s. The Sudanese civil war unfortunately
put a stop to the project.
Falling oil prices through the 1980s prompted a
decline in official support in applying renewable
energy, but by the mid 1990s interest began to
develop in the large-scale use of renewables, largely
as a response to the perceived threat of global warming which was highlighted as the Kyoto process got
under way. To this end, by 1994, MCT’s’ parent company, IT Power, (which the author founded and also
worked for), in partnership with Scottish Nuclear (as
was), and The National Engineering Laboratory,
developed and demonstrated a 15 kW axial-flow
tidal turbine system, at the Corran Narrows on
Loch Linnhe [12]. This was intended as a proof-ofconcept project to lead to larger-scale developments.
This project was effectively the starting point for present activity; it proved the concept to be viable but it
also highlighted numerous technical challenges
including the difficulty of reliably mooring floating
tidal turbines. Unfortunately, the demise of Scottish
Nuclear as an independent entity brought that initial
work to an end.
In 1998, the ‘Seaflow Project’, to develop the
world’s first full-scale (300 kW) offshore tidal turbine
Proc. IMechE Vol. 221 Part A: J. Power and Energy
was initiated by IT Power. At that time, Seaflow
gained the support of the European Commission’s
Joule Programme and an industrial consortium was
formed to implement it, which included Marine Current Turbines that had been set up as a vehicle to
exploit the technology. The earlier experience with
the 15 kW proof-of-concept system had indicated
that the main difficulties of designing and developing
a viable water current turbine for use at sea relate to
the practical details of building, installing, and operating something large enough to survive offshore
conditions. Therefore, it was judged that testing
models, doing other land-based studies or even placing smallish devices into the sea, would not solve
the most challenging problems. Seaflow was, therefore, designed as the first ‘full-size’ tidal turbine, an
experimental system to test all the real problems of
developing viable offshore technology. Key issues
such as survivability, techniques for installation and
access, control, impact on the local environment,
etc. were all to be addressed. Seaflow will be discussed further in the context of MCT’s R&D programme later in this article.
By 2001, the DTI officially included ‘tidal stream’
as being eligible for support from the government’s
Renewable Energy R&D programme following completion of an independent consultant’s study on the
potential commercial viability of the technology
[13] which gave a positive evaluation to MCT’s
techno-economic model. Following this, the DTI
also agreed to cofinance the Seaflow Project.
Marine Current Turbines became an independent
entity from October 2000 and has developed a
business plan for the commercial development of
tidal current power generation systems. The ‘Seaflow
Project’ represented the first phase of this programme.
The tidal current ‘band-wagon’ really started rolling in the UK following the DTI’s declaration of support in 2001, and quite soon a number of new players
came on the scene. The government has spent in the
order of £10 million on tidal stream technology
development by 2005. Norway is the only other
country where significant spending has taken place
in this sector in support of a E11 million tidal turbine
project led by Hammerfest Strøm, but a number of
small projects have also been initiated in the USA
and Canada, where interest is growing. Therefore,
in conclusion, it can be seen that most tidal stream
technology projects are relatively recent, mostly
post-2000, compared with R&D on wave and wind
which goes back to the late 1970s.
4
MCT’S 300 KW ‘SEAFLOW’ PROJECT
The Seaflow turbine resembles in principle an underwater wind turbine, with a single 11 m-diameter
JPE307 # IMechE 2007
Marine current turbines
rotor, with full-span pitch control. It was installed in
a mean depth of seawater of 25 m approximately
1.1 km off the nearest landfall at the Foreland Point
lighthouse below Exmoor in North Devon in May
2003. It has been shown capable of exceeding its
300 kW rated power under favourable flow conditions (having delivered some 310 kW) and the
rotor efficiency also exceeded the design target of
37 per cent (typically achieving around 45 per cent –
see Fig. 2). The system is not grid-connected but
dumps its power into fan-cooled resistance heaters
capable of absorbing the maximum power.
A key patented feature of MCT’s technology is that
the turbine rotor and power-train are mounted on a
steel tubular pile set in a hole drilled in the seabed
and tall enough always to project above the surface
of the sea (Fig. 1). The entire rotor and power
system can be physically raised, using hydraulic
rams, so as to slide up the pile to a position above
the surface to facilitate maintenance or repairs
from a small boat. We believe this is a vital requirement as the use of divers or any other form of underwater intervention is virtually impossible in locations
with such strong currents.
Seaflow is the first sea-powered renewable energy
system world-wide to have been installed in exposed
open-sea conditions, and it has survived through
three winters with regular Force 8 conditions with
no significant technical failures. As with all new
163
technologies, there have been teething troubles, but
nothing that could not be repaired by a crew of two
gaining access from a small boat. This has demonstrated the feasibility of developing technology to
survive in unforgiving conditions exposed to the
incoming Atlantic storms. Indeed after the departure
of Seacore’s jack-up barge ‘Deep Diver’ on 31 May
2003, nothing larger than a small workboat has
been needed to service the system and keep it working, confirming the potential for servicing this technology at low cost. ‘Offshore projects’ and ‘low cost’
are phrases that rarely appear in the same sentence.
The axial-flow rotor uses electrically driven servos
built into the hub to permit full-span pitch control
with provision for turning the blades 1808 to achieve
reverse pitch so as to allow efficient operation with
both the ebb and flood tides. In practice, the single
rotor Seaflow system is only generally operated unidirectionally with the ebb tide, which arrives so that
the rotor is upstream of the pile, because operation
on the flood tide involves running the rotor through
the pile wake. The pitch control system can also be
used to limit the power as a means for achieving a
specific power rating, although Seaflow is designed
to handle the maximum power ever available at the
location selected which is marginally over 300 kW.
The rotor design was developed using a computer
model modified from a wind turbine rotor design
model and based on blade element theory. It uses an
Fig. 2 Typical power versus current speed at hub height for two test runs, one at neaps (light grey
points) and one at springs (dark grey points), showing underlying calculated curves for
three values of CP (based on sampling rate of 2 Hz)
JPE307 # IMechE 2007
Proc. IMechE Vol. 221 Part A: J. Power and Energy
164
P L Fraenkel
uncambered NACA 44 series foil and the design efficiency was predicted at 37 per cent (i.e. Cp –power
coefficient) by the design model in ‘free-stream’
conditions. Water current turbines have a significantly
different load spectrum compared with wind turbines,
in that centrifugal forces and gravity forces which are
significant for wind turbines are relatively unimportant for water current turbines; in this case, the dominant loads are caused by lift forces which tend to
cause ‘flap-wise’ bending of the rotor blades (i.e. in
the axial direction). Because of the relatively high
mass flow and low velocities, these forces are significantly higher for tidal turbines than for wind turbines.
For example, a 1 MW wind turbine with typically a
60 m-diameter rotor has an axial thrust on the rotor
at rated velocity of around 400 kN but a 1 MW tidal
turbine with a rotor of about 20 m diameter would
experience a thrust of 1000 kN or more at rated
power. The implication of this is that the rotor
blades see extremely high bending forces which are
also dynamic (i.e. fatigue is a major issue) due to turbulence, velocity shear, vortex shedding, and the
effect of any passing waves. Therefore, fatigue is a
dominant design driver. A key part of the Seaflow
test programme was to establish the true magnitudes
and frequencies of the various key load cases, in particular the fatigue inducing variances.
An example showing a scattergram of spot readings of current velocity versus shaft power is given
in Fig. 2. It can be seen that there is a lot of scatter largely because at that time the control system was not
well optimized so rotor blade settings sometimes
happened to be such that a high efficiency was
achieved and sometimes they were some way from
being correct. However, even with more effective
control, there is still significant scatter due to the
fluctuations resulting from turbulence and other
effects. Wind turbine rotor performance measurements tend to display scatter in a similar manner.
What is encouraging is that under the most favourable conditions, Cp values exceeding 40 per cent are
consistently achieved. The enhancement in
performance over what was predicted by modelling
is largely due to blockage effects (i.e. it is not in a
free-stream).
The design philosophy for Seaflow was necessarily
cautious because any serious component failure
could have killed the project, so structural integrity
was paramount. As a result, the rotor, which had
originally been planned as a steel fabrication, was
finally made from composite materials, using a
carbon fibre reinforced mainspar with glass fibre
reinforced ribs and external skinning to achieve an
adequate fatigue life from a much lighter rotor. The
rotor blade detail design and manufacture was by
Aviation Enterprises Limited, a company with cutting edge capability in this field.
Proc. IMechE Vol. 221 Part A: J. Power and Energy
An issue unique to axial-flow tidal turbines is the
static pressure variation experienced by a rotor
blade as it rotates through the water column. This
is enough to induce large forces in the rotor blade
skins akin to ‘breathing’ as the blades move from
the top of the swept circle to the bottom. With
Seaflow, this problem was dealt with by flooding
the rotor blades to achieve internal and external
pressure equalization.
The gearbox follows typical wind turbine practice
in that it uses a planetary firststage and has spur
intermediate and high-speed stages. The gear ratio
is 57 : 1 so that a rated input speed of 17.4 r/min produces a nominal 1000 r/min output. However, the
power-train runs immersed and has water-tight casings, which gives excellent passive cooling and has
no need for supplementary heat exchangers. The
generator is flange mounted on the gearbox with a
spring-loaded hydraulically released brake in a
sealed housing between gearbox and generator. The
input shaft to the gearbox has face seals and the gearbox is pressurized with compressed air fed from the
above water housing to approximately the seawater
static pressure at operating depth. The gearbox
manufacturer, Jahnell –Kestermann GmbH (from
Bochum, Germany) is well known for both wind
turbine and marine gearboxes and this unique
submersible gearbox uses know-how gained from
manufacturing gearboxes for wind turbines and for
submerged dredger bucket drives. The gearbox
output and generator are offset from the main
input shaft allowing cables for the rotor pitch mechanism servos and instrumentation to be run through
the centre of the main shaft to a multiple electrical
slip-ring unit mounted at the back of the gearbox;
this allows interconnection from the above-sealevel control system and low-voltage power supplies
to the rotating components.
The generator is an induction machine designed for
use as a subsea motor for seabed pumping equipment
used by the oil and gas industry. Because the system
runs at variable speed, typically nominal þ or 225
per cent, it has electronic power-conditioning equipment and is controlled by a frequency converter.
Interconnection of subsea power and instrumentation components to the above-sea control equipment
was implemented where possible using submarine
cables, in many cases with underwater mateable
connections.
Because Seaflow is over 3.3 km from the nearest
practical grid connection and is only intended as a
relatively short lived experimental test bed, it was
decided at an early stage not to grid connect it, but
to provide a dump load capable of absorbing full
rated power, since the high cost of such a long grid
connection seemed to be unjustifiable for a component that would not add significant value to the
JPE307 # IMechE 2007
Marine current turbines
project. The dump load and backup power supplies
needed in lieu of the grid were estimated to be
much less costly than a lengthy marine cable. This
did add significantly to the design work-load, as the
dump load and auxiliary power supplies had to be
developed, sourced, and tested. In the event, fancooled air heaters situated in the housing above the
water were used to absorb power. A 15 kVA diesel
generating set with a bank of batteries provides the
necessary power for maintenance functions (e.g.
powering a small on board crane, power tools, etc.),
navigation lights, and not least the parasitic loads
to start the tidal turbine. Parasitic loads include the
electronics, control PC, fan for the dump load,
hydraulic pump to release the brake, etc. A small
solar photovoltaic panel and separate storage battery
provide backup for the navigation light as it was not
always possible to run either the tidal turbine or the
diesel generating set largely because the exposed
location precluded visits by the engineering team
during winter months. Safe access, gained by jumping from a rigid inflatable boat (RIB) onto the
access ladder is only possible with wave heights of
,50 cm, which are rare in winter.
The structure is supported on a tubular steel
monopile, 2.1 m in outside diameter and 52 m long,
welded from 6 m ‘cans’. The wall thickness varies
to suit the load distribution. The pile penetrates
18 m into the seabed and stands in a mean depth
of water of about 24 m.
A ‘collar’ fabricated mainly from steel rectangular
sections surrounds the pile and is free to slide up
and down vertically (Fig. 1). The collar carries the
power-train and rotor and can be raised by pulling
it up using a tubular strut that can be ratcheted up
or down by a pair of hydraulic rams and removable
locking pins that engage with a perforated rack running the length of the strut. This hydraulic lifting
mechanism is similar to that used for raising and
lowering the legs of a jack-up barge of the kind
used by MCT’s partner Seacore for installation.
A 6 m 3 m 3 m box-like housing consisting of
a frame with external cladding is bolted on top of
the pile and carries the dump load, hydraulic lifting
mechanism, the control PC and the electronic
frequency converter and transformer, diesel generating set, batteries, emergency equipment with
just enough room for two operators to monitor the
system and to maintain, repair, and when necessary
make modifications. Significant emergency equipment is needed, notably navigation lights, a fog
horn with fog sensor, life-raft, and fire extinguishing
system. There is also a small hydraulic folding crane
mounted on a strong point on top of the housing
with a man-basket to permit virtually all maintenance functions to be completed in the absence of a
servicing vessel.
JPE307 # IMechE 2007
165
The engineering team was pleased that the system
has been reliable enough that all necessary repairs
and maintenance functions have successfully been
carried out using no more than the on-board equipment for over three years. Visits have always been
carried out from a small RIB with twin outboards,
which is about the least possible cost for intervention
for any off-shore project; an essential requirement if
low-cost power is eventually to be produced.
Installation of Seaflow was carried out from a jackup barge equipped with rock drilling equipment. The
procedure, which was implemented in May 2003,
was to position the jack-up on site during slack tide
in the neap period and drop its legs so that it could
be jacked out of the water. Once standing on its
legs with an adequate air gap beneath its hull, the
jack-up forms a stable working platform, although
there was a lot of analysis needed in this case of
determining the limits of its operating envelope
from the point of view of adverse combinations of
current, water depth, waves, and wind.
A rotary rock drill is driven from an extension on the
back of the jack-up and a steel sleeve is allowed to
follow the drill into the hole to prevent debris being
swept back in by the currents. Seawater is used to
lubricate the drill and transport the spoil so no alien
materials were introduced into the water column.
Once the socket has reached the required depth
(which is a function of the foundation design
requirements which are driven by issues such as
the pile natural frequency after installation), the
sleeve is left protruding about 1 m above the seabed
to prevent any debris entering the hole. The pile
was sealed at both ends and floated to site, being
positioned by tugs; the jack-up crane then picked
up one end of the pile and in a delicate operation
timed for slack tide, lifted it vertically and presented
it into the sleeved socket. The pile was then held vertically by the jack-up’s pile hydraulic handling
system and slowly sunk into the socket by filling it
with water. Once in position concrete grout was
pumped through grout pipes provided in the pile
so that it would emerge from the base of the pile
and well up into the annular space between the
pile and the sleeved socket. After the grout had set
the water in the pile could safely by pumped out.
The jack-up then backed off and repositioned itself
about 10 m from the pile so that its crane could be
used to assemble the collar, rotor, and power-train
and then the pile top housing onto the pile. All
these items were brought to site on the jack-up’s
deck, having been partially assembled and pretested
on shore.
Seaflow was operated for the first time on 31 May
2003, the day the jack-up departed the site. It has
been under test since then and will probably be
decommissioned during the summer of 2007 by
Proc. IMechE Vol. 221 Part A: J. Power and Energy
166
P L Fraenkel
which time it has been expected to have gained all
the useful data that can be gleaned from it.
MCT owns the project and is responsible for the
design, but various other participants included Seacore Limited (a leading offshore engineering company), IT Power (a renewable energy consultancy
and former parent company), Bendalls Engineering
(a steel fabricator from Carlisle), Corus UK (part of
the Anglo-Dutch steel company – formerly British
Steel) and also German partners in the form of
ISET (a Renewable Energy R&D company attached
to Kassel University), and Jahnell – Kestermann (a
major manufacturer of gearboxes). The total project
cost was approximately £3.5 million of which 60
per cent was subsidized by the UK government, the
EC, and the German government and 40 per cent
came from MCT and the partners.
A wealth of useful data has flowed from the Seaflow
test programme to inform the development of
commercial technology to follow. It has also been successful in confirming that various key conceptual
ideas actually work effectively in practice, including
the fundamental concept, the axial-flow rotor, the
marinized (submerged) power-train, the use of a
surface-breaking monopile and structure, together
with low-cost intervention for maintenance from
small boats. Most importantly, experience has shown
the system to be harmless to the local environment, or
at least no obviously harmful effects have been
observed so far, and environmental checks are
ongoing.
MCT has had to cope with a significant number of
‘bugs’, as is to be expected with any new technology
of this complexity, but fortunately none of these
have been ‘show stoppers’ and as a result, the system
has functioned better after two years use than it did
initially and can be operated automatically or by
remote control using an internet connection. Most
importantly, the team has learnt a number of requirements for future technology which should make commissioning and early stage reliability much easier and
less costly for the planned future project phases.
5
MCT’S 1 MW ‘SEAGEN’ PROJECT
The next stage in MCT’s R&D programme is to
develop and build the prototype for the commercial
technology to follow a system, which has been
named as Seagen. While Seaflow proved technical
feasibility, Seagen is needed to prove the economic
and commercial feasibility.
The Seagen system has its rotors mounted at the
outer ends of a pair of streamlined wing-like arms
projecting either side of the supporting pile (Fig. 3).
Each rotor is 16 m in diameter and drives a 600 kW
power-train consisting of a gearbox and generator.
Proc. IMechE Vol. 221 Part A: J. Power and Energy
Fig. 3
Artist’s impression of Seagen 1 MW tidal turbine
Therefore, the total rated power per installed unit is
up to 1200 kW(e) (depending on siting conditions).
The reasons for the twin rotor configuration are primarily that this permits bidirectional operation with
the rotors clear of the pile wake when the rotors are
downstream of the pile; 1808 rotor blade pitch control
allows efficient operation when the current reverses.
Also, two rotors clearly deliver twice as much energy
as one would, but at less than twice the cost, so
enhanced cost-effectiveness is another reason.
Essentially, Seagen produces three times the power
of Seaflow at around twice the cost, giving a significant improvement in cost-effectiveness. Seagen will
be installed in 2007 and will be grid connected.
Seagen is a £10 million project, and it involves some
of the same partners as Seaflow. It is also supported
by new shareholders of MCT and strategic partners,
EDF Energy (the UK subsidiary of one of the largest
utilities in the world–Electricité de France), by Guernsey Electricity (the Channel Island utility which happens to have strong currents around its coast), and
by BankInvest (a Danish specialist investment bank
focusing on innovative and clean-energy technologies). The UK government, through the DTI, is
again supporting MCT’s R&D, having committed to
provide a grant worth £4.3 million.
At the time of final editing (October 2006), manufacture of Seagen is virtually complete and dry-testing of systems has started. It is planned that Seagen
will be installed as early in 2007 as possible and it
will be immediately followed by work to develop an
array of similar systems to be installed in an open
JPE307 # IMechE 2007
Marine current turbines
sea location, where economies of scale will yield a
further improvement in cost-effectiveness.
The goal of MCT’s business plan is to have a technology that can be deployed in commercial power
projects by 2007 to 2008 and which will rapidly
become cost-competitive with offshore wind projects.
It is also planned to initiate demonstration projects in North American waters in parallel to the
first UK array (which will enable economies of scale
in procurement of the turbines at the same time as
those for the planned array) and MCT is actively
seeking US and/or Canadian strategic partners to
head such a program. The first North American project may then be ‘rolled out’ into a larger project soon
after, once its efficacy is demonstrated.
5.1
Seagen: technical details
Seagen has inherited most of the successful features
from Seaflow, but it also differs in quite a number of
respects. It still has full-span pitch control with
carbon/glass fibre composite rotor blades. The
power-train is again submersible, with a pair of planetary gearboxes driving induction generators,
although in this case a UK design from Orbital2 manufactured by Wikov in the Czech Republic. The main
support structure is again a rolled-steel monopile,
although this time 3 m in diameter.
A major difference however is the so-called ‘crossarm’ structure, such as a pair of wings, to carry the
power trains either side of the pile, far enough apart
for the rotors not to cut into the pile wake. The
cross-arm wings have some dihedral primarily to
help raise the power trains higher out of the water
for a given collar lift. The dihedral also ensures that
the rotor blades cut the cross-arm wakes in a scissorlike manner so that only part of a rotor blade is in
the wake at any moment. The cross-arm wing section
is elliptical and designed to minimize the wake
thickness; some CFD analysis was carried out for
MCT by QinetiQ at Haslar to optimize the cross-arm
geometry.
The rotors and power trains are held by threepoint mountings under the side wings and designed
so that when raised above the water, a flat-top barge
may be positioned underneath and the power-train
and rotor can be lowered as a complete unit onto
the barge before being replaced by reversing this
process.
Many aspects of this technology have been
patented or are pending patents, and Seagen is a
registered design. The mechanism for locking the
cross-arm in place when the system is operational
is the subject of the most recent patent application.
This is because the extreme load cases considered
in the design process have to allow for dynamic
asymmetrical loads which can be generated by a
JPE307 # IMechE 2007
167
combination of turbulence, lack of uniformity in
the flow, velocity shear, and passing waves and
these act through the rotor axes giving rise to huge
yawing moments, where the cross-arm and collar
interface with the pile. Finding a robust mechanism
to handle these loads proved to be a significant challenge, but it is believed that an excellent solution to
this problem is obtained.
As with Seaflow, the power system is variable
speed, variable voltage, and variable frequency with
a control system able to vary both the frequency converter’s parameters and the rotor blade pitch angles
in order to optimize performance. The control strategy is to achieve a successful start-up by initiating
rotation without generation and then under the correct conditions starting the generation process at a
current velocity of about 0.7 m/s. The system then
seeks to maximize the power until the current
speed reaches a level where rated power is achieved,
which will typically be at about 2 –2.3 m/s depending
on the local site conditions. For higher currentvelocities, the pitch-control mechanism sheds power
by reducing the angle of attack of the blades to maintain as close to rated power as possible. When the
tidal stream velocity starts to reduce, the control
system maintains rated power as long as the velocity
is high enough and thereafter, as the speed ramps
down, it maintains maximum power for the conditions, until the velocity falls below 0.7 m/s, at
which time the system cuts out and the rotor is
parked with its blades ‘feathered’. When the tide
turns (slack tide) and the flow direction reverses,
the control system pitches the rotor blades 1808
ready for operation in the reverse direction once
the velocity again exceeds the cut-in speed.
Other design features of interest are that the collar
and cross-arm, carrying the pair of rotors and their
power trains, can be lifted by a pair of hydraulically
activated vertical struts driven by rams, situated in
the above water housing. This housing also provides
a control centre where the control PCs are located
and where ancillary equipment such as a hydraulic
power pack, a small air compressor, safety equipment, and a dehumidifier are located. Transformers,
to deliver power into a marine cable at 11 kV, and
power-conditioning equipment are located in the
top of the pile on three levels and the interior
water-cooled pile surface is used to provide cooling,
with a fan to circulate the internal air and the aforementioned dehumidifier to take out condensation.
The interior of the pile and housing are sealed to
minimize ingress of moisture and sea salt.
It is planned to carry through an extremely
thorough testing programme where possible prior
to installation to ensure bought-in items are to
specification (this was not always the case with Seaflow). The turbine will then be commissioned and
Proc. IMechE Vol. 221 Part A: J. Power and Energy
168
P L Fraenkel
Fig. 4
Artist’s impression of arrayed Seagen turbines showing one raised for maintenance
tested firstly to obtain performance data and then to
try various control strategies, before seeking to
obtain operational reliability of the highest possible
order.
A continuous environmental monitoring programme will be run in parallel with the main programme, to confirm that the system is not causing
significant adverse environmental impacts, and that
if any such impacts are detected, steps can rapidly
be taken to identify any such problem and develop
effective mitigation measures. A large number of
potential environmental issues are being investigated, including possible interactions with marine
mammals and fish, effects on benthos (seabed life),
underwater noise, size of wake, etc. It is believed
that this technology has the potential to generate
electricity with minimal adverse environmental
impact, which these days is an important selling
point, and therefore it is important to establish
that this is a correct assumption and also to identify
any adverse effects and find methods for mitigating
them.
5.2
Commercial tidal stream technology:
the future
In the face of the developing Global Warming and
Peak Oil crises, there is an urgent need to prove
and bring on stream new clean-energy technologies, such as tidal turbines. It can be anticipated
that security of supply will increasingly preoccupy
the governments of industrialized countries all of
which need to find huge new clean-energy supplies. The technology under development by MCT
Proc. IMechE Vol. 221 Part A: J. Power and Energy
has the potential to contribute to solving this problem and it has been hoped and expected it to be
commercially viable well within the next 5 years
(Fig. 4).
It is hoped that commercial feasibility will be effectively demonstrated through the Seagen project. The
key to arriving at this result is to gain the operational
experience needed to develop the reliability of the
systems. The prototype will inevitably be overengineered as a means to minimize risks of failure,
so there will be a pressing need to value-engineer
the system in order to get costs down and to ensure
the resulting product can reliably deliver electricity
from the seas for several decades with minimal
environmental impact.
MCT plans to commission several hundred megawatts of turbines by 2012. The potential thereafter
runs to many gigawatts of capacity even for this
first-generation technology which is limited to
water depths in the range of 20 m to about 50 m.
However, the company has already made patent
applications and started research into more radical
second-generation systems that are expected to be
functional in much deeper currents, up to 100 m or
more (300 ft) with significantly larger rotors,
higher rated power levels, and correspondingly
greater economies of scale.
In conclusion, MCT believes it is well on track to
delivering commercial tidal stream technology with
the potential to supply electricity on a large-scale,
at low cost and without pollution. It is believed that
the concepts under development by MCT will
become one of the primary techniques for extracting
energy from the seas.
JPE307 # IMechE 2007
Marine current turbines
REFERENCES
1 Etheridge, D. M., Steele, L. P., Langenfelds, R. L., and
Francey, R. J. Historical CO2 records from the Law
Dome DE08, DE08-2, and DSS ice cores. Division of
Atmospheric Research, CSIRO, Aspendale, Victoria,
Australia and J.-M. Barnola Laboratoire de Glaciologie
et Géophysique de l’Environnement, Saint Martin
d’Hères-Cedex, France V.I. Morgan Antarctic CRC and
Australian Antarctic Division, Hobart, Tasmania,
Australia, 1998, available from http://cdiac.esd.ornl.gov/
trends/co2/lawdome.html and http://www.gci.org.uk/
contconv/cchist.html
2 Climate impact of quadrupling atmospheric CO2: an
overview of GFDL climate model results, Geophysical
Fluid Dynamics Laboratory at NOAA (National Oceanic
and Atmospheric Administration of the US Department
of Commerce), 2004, available from http://www.
gfdl.noaa.gov/tk/climate_dynamics/climate_impact_
webpage.html
3 King Hubbert, M. Nuclear energy and the fossil fuels,
1956 (American Petroleum Institute, Texas), available
from
http://www.hubbertpeak.com/hubbert/1956/
1956.pdf
4 Tidal stream energy review. Report no. ETSU T/05/
00155/REP, UK DTI, Prepared by Engineering and
Power Development Consultants Ltd, Binnie & Partners, Sir Robt. McAlpine & Sons Ltd and IT Power Ltd
for the ETSU, UK Deptartment of Energy, Crown Copyright 1993.
5 Marine currents energy extraction: resource assessment.
Final report, EU-JOULE contract JOU2-CT93-0355
JPE307 # IMechE 2007
6
7
8
9
10
11
12
13
169
(Tecnomare, ENEL, IT Power, Ponte di Archimede,
University of Patras) 1995.
Feasibility study of tidal current power generation for
coastal waters: Orkney and Shetland. Final report, EU
contract XVII/4 1040/92-41, ICIT, March 1995.
The potential for the use of marine current energy in
Northern Ireland, Peter L Fraenkel – Marine Current
Turbines Ltd, Adrian Bell –Kirk McClure Morton Ltd,
Prof. Trevor Whittaker– Queens University Belfast &
Les Lugg– Seacore Ltd, published by the Department
of Enterprise, Trade and Investment, Belfast, NI, 2003,
available
from
http://www.detini.gov.uk/cgi-bin/
moreutil?utilid¼41&site¼5&util¼2&fold¼&parent¼
Black and Veatch Consulting Ltd. Tidal stream energy
resource assessment. Technical report, Carbon Trust,
London, September 2004.
Bryden, I. G. Extracting energy from tidal flows,
Hydraulic Aspects of Renewable Energy, 19th March
2004, Glasgow.
The energy balance of modern windturbines. Windpower Note No. 16, December 1997, available from http://
www.windpower.org/media(444,1033)/The_energy_
balance_of_modern_wind_turbines%2C_1997.pdf
Fraenkel, P. Water pumping devices, 1986 (Intermediate Technology Publications, London), revised 1997.
Fraenkel, P. L., Macnaughton, D., Paish, O., and
Hunter, R. Tidal stream turbine development. In Proceeding of Conference on Clean Energy 2000, Institute
of Electrical Engineers, London, 17 – 19 November 1993.
Commercial prospects for tidal stream power. Binnie,
Black and Veatch, and IT Power Ltd, Redhill. DTI/
ETSU, Harwell, April 2001.
Proc. IMechE Vol. 221 Part A: J. Power and Energy