Ocean Energy

Abstract

Oceans are the largest collector of solar energy on the earth’s surface. Considering oceans cover more than 70% of the earth’s
surface, the amount of energy stored by the oceans is enormous. The energy can be harvested from the ocean by taking advantage
of waves, tidal current, and the thermal gradients that exist within the body of water. The gravitational pull of the moon
primarily drives the tides, and the wind powers the ocean waves. In theory, these ocean-based renewable resources could meet
the world’s energy requirements many times over, but they are extremely difficult to harvest economically for large scale
production. In this chapter, various methods including three main techniques; wave power, tide power and ocean thermal energy
conversion, are discussed for harvesting energy from oceans.

Full text

Ocean Energy
Mårten Grabbe Urban Lundin and Mats Leijon
Department of Electricity and Lightning Research
Uppsala University
Sweden
Urban.Lundin@angstrom.uu.se
Mats.Leijon@angstrom.uu.se
1. INTRODUCTION ----------------------------------------------------------------------------------------------- 2
2. MARINE AND TIDAL CURRENT ENERGY ------------------------------------------------------------ 3
2.1 GENERAL ISSUES ON MARINE AND TIDAL CURRENT ENERGY-------------------------------------------4
2.1.1 Peculiarities of Marine and Tidal Currents--------------------------------------------------------- 4
2.1.1.1 Grid connection ----------------------------------------------------------------------------------------------6
2.1.2 Environmental aspects--------------------------------------------------------------------------------- 7
2.1.3 Economical aspects ------------------------------------------------------------------------------------ 8
2.2 DESCRIPTION OF MARINE CURRENT ENERGY CONVERTER TECHNOLOGIES --------------------------8
2.2.1 Marine Current Turbines------------------------------------------------------------------------------ 8
2.2.1.1 Turbine concepts ---------------------------------------------------------------------------------------------9
2.2.1.2 Support structure concepts-------------------------------------------------------------------------------- 10
2.2.1.3 Prototypes --------------------------------------------------------------------------------------------------- 10
2.2.2 Oscillating Hydrofoils ------------------------------------------------------------------------------- 13
2.2.3 Tidal barrages ---------------------------------------------------------------------------------------- 13
2.2.4 Future Marine Current Energy Converter Technologies. --------------------------------------- 14
2.3 PRESENT TIDAL CURRENT ENERGY MARKET------------------------------------------------------------ 15
2.4 FUTURE DEVELOPMENT ------------------------------------------------------------------------------------- 15
2.5 CONCLUSION -------------------------------------------------------------------------------------------------- 16
3. WAVE POWER [29, 30] ------------------------------------------------------------------------------------- 17
3.1 CONCLUSION AND SYNTHESIS------------------------------------------------------------------------------ 19
ACKNOWLEDGEMENTS ------------------------------------------------------------------------------------------------- 19
REFERENCES ------------------------------------------------------------------------------------------------------------- 20

2
1. Introduction
The oceans, covering more than 70 % of the Earth, have long been
appreciated as a vast renewable energy source. The energy is stored in
the oceans partly as thermal energy, partly as kinetic energy (waves and
currents) and also in chemical and biological products. Numerous
techniques for extracting energy from the sea have been suggested,
most of which can be included in one of the following categories:
• Marine and tidal current energy
• Wave energy
• Ocean thermal energy (OTEC)
• Energy from salinity gradients (osmosis)
• Cultivation of marine biomass
The kinetic energy present in marine and tidal currents can be converted
to electricity using relatively conventional turbine technology. Harnessing
the kinetic energy in waves presents a different set of technical
challenges and a wide variety of designs have been suggested. Ocean
thermal energy conversion is possible in locations with large temperature
differences, extracting energy using a heat engine. Salinity gradients can
be exploited for energy extraction through the osmotic process. The
cultivation of marine biomass can yield many useful products, including
renewable fuels for electricity generation.
Only a fraction of the global ocean energy resource can be found in sites
economically feasible to explore with available technology. However, this
fraction could still make a considerable contribution to the European
electricity supply and the marine renewable sector is currently the focus
of much industrial and academic research around the world.
Sites with attractive wave climate and intense tidal currents are abundant
in the vicinity of the European coastline. Thus, research and development
activities in the European marine renewable energy sector have mainly
concentrated on wave and tidal energy. Therefore wave and tidal
energy are reviewed in the following sections.

3
2. Marine and Tidal Current Energy
Ocean energy, including wave and tidal current energy, has the potential
of playing a major role in the electricity market, providing reliable and
sustainable energy. Some of the most attractive features of tidal currents
include its highly predictable nature and the sizeable resource along the
European coastline [1]. It is technically possible to extract energy from
tidal currents with no pollution during operation and presumed low
environmental impact [2].
There are basically two ways of generating electricity from marine and
tidal currents: by extracting energy from free flowing water, or by building
a tidal barrage across an estuary or a bay in high tide areas. A tidal
barrage harnesses the energy in a similar way as run-of-river hydro power
plants and was the first ocean energy technology to be used in a large
scale project. The 240 MW tidal barrage La Rance was constructed during
the 1960s in France and it is still operational today. Since then, the focus
has been on capturing the energy in free flowing water, meaning much
less civil engineering work and less environmental impact at the site.
A great deal of attention was drawn to marine and tidal currents as a
possible source of energy during the oil crisis in the 1970s, but all in all the
abundant resources of tidal energy have remained untapped. However,
recent developments in power electronics, in the offshore industry and in
wind power technology have brought tidal energy much closer to an
introduction on the electricity market. At present, there are a number of
promising and more or less innovative concepts for Marine Current Energy
Converters (MCECs).
The tidal energy potential is substantial, but not all of the resource can be
extracted using the available technology. One might choose to call the
extension of present prototypes first generation devices, using
conventional engineering components and reaching a depth of 20 to 30
meters. Early devices might be rated in the range of 200 to 700 kWh [1],
but for reaching economical viability the effort to increase operational
lifetime and minimize maintenance is likely to be more important than an
increase in rated power.
Second generation devices are expected to follow within 10 years [3],
introducing specialized components, and more importantly, exploring
sites of depths below 40 meters where more of the tidal current potential
can be extracted.

4
The MCEC industry is still in its early days and no full-scale commercial
MCEC farm has yet been tested and proven. Hence, it is still too early to
foresee a winner in the tidal race. It is likely that the best option will
depend on site specific conditions and what follows can be considered
as a snapshot of ideas and designs that are evolving continually.
A selection of MCEC resembling wind turbines will be reviewed in chapter
1.3.1. The oscillating hydrofoil technique will be further discussed in
chapter 1.3.2. Finally, the concept of constructing tidal barrages will be
discussed in chapter 1.3.3.
2.1 General issues on Marine and Tidal Current energy
This chapter presents the basic characteristics and potential of marine
and tidal currents. The nature of the source also has implications for
environmental and economical aspects of extracting tidal current
energy.
2.1.1 Peculiarities of Marine and Tidal Currents
Marine and tidal currents are generally slow moving, but as water is
roughly 800 times the density of air a current of only 2 m/s would
correspond to the energy density at a wind speed of 18 m/s. Thus, as
many as 106 sites of interest for electricity generation have been identified
along the European coastline [1]. Strong currents are usually found in
straits and other shallow or narrow passages. Some of the locations with
extremely intense currents include the Pentland Firth, Alderney Race and
Severn Estuary in the UK. Particularly promising sites can also be found in
the Messina Strait (between Italy and Sicily) and between the Greek
islands in the Aegean Sea.
The tide is caused by the gravitational attraction between the moon and
the sun with the Earth’s oceans. Tidal currents change with the tide in a
highly predictable pattern, dominated by semidiurnal, diurnal and mixed
tidal streams. This predictable behaviour makes it easier to plan base
production power contributions to ensure a firm supply capacity. There
are also marine currents of more constant nature, albeit with seasonal
change, driven by the Coriolis force and variations in salinity and
temperature. A predictable energy source and possible long utilization
time are desirable features for achieving economical solutions for
renewable energy conversion.
The degree of utilization (the ratio of yearly produced energy to the rated
power of the device) is of importance when evaluating the economic
potential of renewable energy production [4]. This can be seen as the
investment cost is driven by the installed power, whereas the income is

5
dependent on the produced energy. The degree of utilization does not
only consider the technical aspect of device availability, but also includes
the natural variation of the energy source. For instance, cloudiness or
calm weather could affect the degree of utilization for solar photovoltaic
and wind turbines respectively. Marine currents offer a degree of
utilization in the order of 80%, which allows for comparatively high
investment costs per unit installed power while still achieving economic
viability. Even tidal currents show a relatively high degree of utilization, in
the range of 40 to 50%, compared to the 20 to 30% that is common for
land-based wind turbines [1].
Several resource assessments of the available tidal current potential have
been conducted, though it can be difficult to compare the estimates at
first sight. Different variables and factors in the assessments include
expected average efficiency, device availability, significant impact
factor (extractable, or available, percentage of the total resource),
spacing of MCEC arrays, lower limit of stream velocity for electricity
generation, ratio between first and second tide to name only a few. The
JOULE 1996 study [1] included all sites with a mean spring velocity above
1.5 m/s, and considered sites with lower velocities on a site by site basis.
Other reports [5, 6] concentrate on locations with mean flow speed
greater than 2 m/s.
The extractable potential in the UK has been reported to be 22 TWh per
year and 17 TWh per year for the rest of Europe [5]. Other reports give
estimates of 48 TWh per year for Europe [1] and as high as 58 TWh per year
in the UK alone [6] (note that these are estimates for tidal current potential
only). One aspect of the available potential is that it is not really
‘available’ with first generation MCECs operating at depths of 20 to 30 m.
Much of the extractable energy is to be found in sites of depths below 40
m [5].
In general, marine and tidal current energy, as well as many other
renewables, is characterized by high capital cost and low operational
costs. Some of the most advantageous characteristics of tidal currents
include its predictable nature, high degree of utilization, lack of pollution
during operation and little or no visual impact. However, it is a fluctuating
source and difficult conditions can be expected during installation and
maintenance.
Some parameters affecting design and operation of MCECs include:
• Fouling by unwanted marine growth, or bio-fouling, would eventually
increase the drag and lower the efficiency of the turbine. Antifouling

6
paint can be used effectively, but may be toxic even in small
concentrations [7].
• Reliable sealing of the device is important, but this should not pose a
problem as sealing technology is well developed within other
industries. Cable junctions, entry and exit points are all potential leak
paths and some uncertainties remain where large seals with high
rubbing rate has to be used [7].
• Finding a suitable array configuration for a MCEC farm might not be as
straight forward as to use existing models from the wind power industry.
Wind turbines only use the bottom layers of the atmosphere and are
usually placed in open areas. Therefore the kinetic flux can be
expected to recover within a relatively short distance behind the
turbine [8]. MCECs, on the other hand, are likely to be placed in
channels or narrow passages where the flow will be constrained.
Hence it is not improbable that a tidal current farm might affect the
underlying flow to a greater extent than a wind farm would. The
amount of energy that can be extracted without any significant
impact on the underlying flow has been estimated at 10 to 20% [5, 9]
and slightly higher where the flow is less constrained.
• Cavitation occurs when the partial pressure locally falls below the
vapor pressure of water. This will be an important and limiting design
parameter for MCECs as it could lead to surface damage on the
turbine and a subsequent decrease in efficiency. To avoid cavitation,
a limited rotor tip-speed of around 7 m/s relative to the oncoming
water is recommended for first generation devices [1].
• Turbulence and vibration will also be important design issues. Different
load over the structure can give rise to vibration and eventually fatigue
in the material.
2.1.1.1 Grid connection
The integration of large amount of renewable energy will become an
increasingly important issue for the management and stability of electric
grids. While moving from centralized generation towards distributed (or
embedded) generation, the intermittency of most renewables is driving
additional system requirements and corresponding costs.
When discussing the effect of intermittency on grid stability, it is important
to recall the different time scales of natural variation for renewable
energy sources. Direct sunshine can show variation within seconds and
wind within minutes, whereas tidal currents have a predominant
semidiurnal cycle of 12.4 hours and marine currents only experience
seasonal change. Short term fluctuations in the total output power can be

7
reduced by geographically distributed production. Furthermore, the
requirements for backup capacity can be significantly reduced if the
electricity is generated by a well-balanced mix of renewables rather than
by one intermittent source only [10].
Preferred voltage for grid connection for wave and tidal energy devices
ranges from 10 kV to 380 kV [11]. Grid connection costs will depend on
distance to shore. In comparison the grid connection would be in the
order of 20% of the total investments for a 150 MW offshore wind farm
located 40 km from the nearest grid connection point [12].
2.1.2 Environmental aspects
Little is known about the environmental impact of large scale energy
extraction from marine and tidal currents. It is likely though that the
environmental impact will be low if appropriate care is shown during site
selection and deployment [2]. However, energy extraction will not be
without environmental detriment, and adequate research is required to
increase the understanding of the environmental footprint of MCECs and
mitigate any impacts [13].
There are no direct emissions of green house gases (GHG) or other
atmospheric pollutants during normal operation of a MCEC. Furthermore,
there are no indirect emissions from fuel transports. Most of the life-cycle
emissions, including a mere 12 g/kWh of CO2 (see table 1), are released
during manufacturing of the device and to a lesser extent during
construction and decommissioning [2].
Pollutant Tidal Current
(g/kWh)
Wave Energy
(g/kWh)
Wind energy
(g/kWh)
Average UK mix
(g/kWh) (1993)
CO2 12 14 - 22 12 654
SO2 0.08 0.12 – 0.19 0.09 7.8
NOx 0.03 0.05 – 0.08 0.03 2.2
Table 1: Life-cycle emissions from offshore renewables [2]
Very little intrusion on the public is to be expected during operation of
MCECs, but local shipping and fishing industry might be affected though.
Thus, care should be taken during site selection and choice of technology
to avoid conflict with other users. Potential environmental impact on the
benthic ecology, seabirds, pinnipeds and cetaceans is presumed to be
insignificant, although, further research is required before any conclusions
as to their magnitude can be drawn. Other factors to be regarded
include sediment dynamics and acoustic emissions. These factors,
together with recommended research areas, are thoroughly discussed in
[14].

8
The long term benefits of MCEC, such as the avoidance of GHG emissions
and preservation of oil and coal, are assumed to far outweigh the
negative impacts. Good environmental impact assessment could also be
crucial for public support.
2.1.3 Economical aspects
More research and development is to be expected before MCECs will
find their way to full commercialization. A clear picture of the costs will not
be available until a full scale MCEC has been in operation for some time.
The economy of a farm would also be highly dependent on site specific
conditions, as can be seen in a case study [15] of two sites in British
Columbia, Canada. The cost of developing the resource at the two
different sites was predicted to 7.7 and 17.5 c /kWh respectively1.
Electricity cost of below 12 c /kWh have been set as an achievable goal
for first generation MCECs [1]2. Several more recent reports [3, 16, 17] on
potential future energy cost fall within the range of 3.8-9.5 c /kWh as
presented in the World Energy Assessment report [18]3.
Other electricity generation technologies, such as wind and nuclear, also
started off with what would now be regarded as an uncompetitive price.
The key issue here is that the physics behind marine and tidal currents is
very promising for energy conversion and grid connection. With a
comparatively high possible degree of utilization, the price of electricity
produced by MCECs can only be expected to decrease when the
technology is developed to better suit the nature of the energy source.
2.2 Description of Marine Current Energy Converter
technologies
The concepts of MCECs described in this chapter are a selection of some
of the more successful projects including a general review of the
technology used.
2.2.1 Marine Current Turbines
Generally, most applied research on MCECs so far has adopted the same
basic ideas as behind a wind power system: a turbine in order to convert
the linear movement of the current into a rotational movement, and a
1 Average 2002 exchange rate used, 1 ¼   &$' DQG   LQIODWLRQ UDWH SHU \HDU DGGHG 7KH
indicative cost analysis assumes 8 % discount rate combined with a 30 year facility life of the tidal current
farm and a capacity factor (mean power/rated power) of 20%. The calculations are based on cost analysis
of MCT technology as presented in [16].
2 Presented in [3] as 0,1 ¼N:KLQLQIODWLRQUDWHper year added.
3 Average 2001 exchange rate used, 1 ¼ 86'DQGLQIODWLRQUDWHSHU\HDUDGGHG

9
generator to produce electricity out of the rotational movement. What
follows will be a short survey of the different design issues for the main
parts of a marine current turbine.
2.2.1.1 Turbine concepts
2.2.1.1.1 Rotor types
There are basically two different rotor concepts, axial flow (propeller type)
rotors and cross flow rotors. Cross flow rotors are characterized by having
the axis of rotation perpendicular to the flow.
Regardless of the design, a turbine can only harness a fraction of the
available power in the free flowing water. The extractable power can be
expressed as
3
2
1vACP P
ρ
=
where • is the density of the water, A is the area swept by the turbine and
v is the mean velocity of the flowing water. The turbine power coefficient,
CP, is the percentage of power that the turbine can extract from the
current. The Betz limit of 59% is usually assumed to be the theoretical
upper limit for CP.
Characteristic for the cross flow rotor is that it does not need to be
oriented to the flow. This design also allows the gearbox and generator to
be located above or below the rotor to avoid interference with the flow
across the rotor. Axial flow rotors for MCECs resemble rotors commonly
seen for wind turbines, but are in general designed with shorter blades.
Furthermore, axial flow rotors need to face the oncoming current and thus
require a mechanism that allows the turbine to operate with the flow in
both directions. This can, for instance, be achieved by pitch control of the
rotor blades through 180o at turn of tide.
Due to low current velocities, the rotor of a marine current turbine will
experience low rotational speeds, typically in the range of 5 to 30 rpm [1].
Standard generators are in general designed for a higher rotational
Figure 1: Different types of cross flow rotors illustrated (from left
to right) by a Savonius rotor, a Darrieus type rotor and an H-
rotor.

10
speed. Thus, most marine current turbine prototypes have been equipped
with a gearbox between the rotor and the generator (see section 1.3.1.3).
Traditionally, a high speed generator together with a gearbox has also
been used in the wind industry, but today wind turbines with gearless
direct drive generators designed for a low rotational speed are
commercially available. In line with this development, MCEC concepts
with direct drive generators excluding the gearbox are suggested in
literature [19].
2.2.1.2 Support structure concepts
MCEC designs presented so far are either fixed to the seabed or moored
buoyant structures. The choice of suitable support structure will depend
on depth, unit size, seabed material and also economical considerations.
Figure 2: Different support structures for MCEC with axial flow rotor [17]
Specific site requirements imposed by the technology would be depths of
20 to 40 m for first generation MCEC, and between 40 to 80 m for second
generation devices [3]. Increased depth does not only mean higher
installation costs, it also leads to significantly higher forces on the structure
as a whole. The expected limit for any surface piercing structure is thought
to be around 30 to 50 m, depending on several factors such as flow
velocity and seabed material [1].
Experience shows the difficulties of deploying an MCEC in intense currents
and great depths. Larger jack up barges have been developed over the
years for installation of offshore wind farms. These barges, however, are
not necessarily developed for the extreme currents found at sites for
MCECs [20].
2.2.1.3 Prototypes
Several small scale prototypes have been tested over the years, but few
have reached an advanced state of development. A study in early 2005
includes a list of over 70 companies actively pursuing development in the

11
marine renewable sector [28]. Of these, three prototypes that have been
deployed at sea for some time will be detailed below.
Marine Current Turbines Ltd (MCT), Lynmouth, Devon, UK
The Seaflow has been the five-year project of a consortium of seven
organisations working with financial support from both the European
Commission and the UK Department of Trade and Industry (DTI). In the
summer of 2003, MCT in association with Seacore successfully installed the
300 kW Seaflow in the Bristol Channel near Lynmouth.
The turbine is mounted on a surface
piercing tubular steel monopile structure
fixed into a socket in the seabed. The
power train can be raised above sea level
for maintenance and the 11 m diameter
rotor can reverse the blades, pitching them
through 180o at turn of tide. The turbine has
been tested under remote operation, but
uses a dump load in lieu of a grid
connection [20].
The next step for MCT is to showcase the
commercial potential of tidal energy with
the twin-rotor system SeaGen rated at 1
MW. Installation in Northern Ireland’s
Strangford Lough is planned during 20064.
Hammerfest Strøm AS, Kvalsundet, Norway
Working with Statiol, ABB and Rolls-Royce, Hammerfest Strøm AS
connected a 300 kW tidal current turbine to the national power grid in
late 20035. Located in the Strait of Kvalsundet in northern Norway, the
turbine is deployed at a depth of about 50 m in an average current of 1.8
m/s.
Similar to the Seaflow, this tidal current turbine consists of an axial flow
rotor coupled to a gearbox to increase the low speed of the turbine shaft
to the desired operating speed of the generator. The three-bladed rotor
has a diameter of 20 m and is mounted on a completely submerged steel
structure with a gravity base foundation. An extension of the present
prototype with twice the output power is planned for commercial use.
4 http://www.marineturbines.com/home.htm - 2005-12-29
5 http://www.e-tidevannsenergi.com/index.htm - 2005-11-27
Figure 3: The Seaflow with the collar raised
above sea level (used with permission).

12
Figure 4: Installation in Kvalsundet. The nacelle and rotor is
lowered down to the already installed structure (used with
permission).
The ENERMAR project, the Messina strait, Italy
The ENERMAR system, owned by the Ponte di Archimede Company6, has
been installed in the Strait of Messina off the Sicilian coast since 2001. The
system is moored by four anchoring blocks where the water depth is 18-25
m and the expected current velocity is about 2 m/s.
The system consists of a buoyant support
platform and the patented Kobold
turbine. The platform, designed by the
Ponte di Archimede Company, houses
the gearbox, a 160 kW synchronous
generator and the necessary electrical
equipment. The Kobold turbine (cross flow
rotor, 6 m in diameter, equipped with
three blades with a span of 5 m) is placed
under the platform. The turbine has been
developed by the Department of
Aeronautical Engineering (DPA) at the
University of Naples with an automatic
pitch control mechanism and non-
symmetrical airfoil designed to be
cavitation free.
The Aircraft Design and Aeroflight Dynamics Group (ADAG) at DPA is
aiming to increase the global efficiency of the system using a direct drive
generator developed in cooperation with the Division of Electricity and
Lightning Research at Uppsala University, Sweden. The use of a gearbox
6 http://www.pontediarchimede.com/language_us/ - 2005-12-02
Figure 5: Artist’s impression of the
ENERMAR system (used with
permission).

13
and step-up transformer can be avoided with the new generator
designed for the low rotational speed determined by the turbine (5 rpm)
[19].
2.2.2 Oscillating Hydrofoils
Rather than utilizing the rotational movement of the more traditional
turbine concept to generate electricity, it is also possible to capture
energy using an oscillating hydrofoil.
Lift and drag forces causes the wing-
like hydroplane to move in the
oncoming current. The oscillating
motion can be controlled by
changing the angle of attack
relative to the stream.
The Engineering Business Ltd has
demonstrated the oscillating
hydrofoil technique proof of
concept with the 150 kW prototype,
the Stingray (Figure 6) [21]. As the
hydroplane and the supporting arm
oscillate, hydraulically powered
cylinders pump high-pressure oil
driving a hydraulic motor. The motor, in turn, drives an electric generator.
The Stingray has been tested during 2002 and 2003, achieving a mean
hydraulic power of 85,4 kW in an average current speed of 2 m/s over a
30 minute period. The demonstrator unit did not seem to have any
significant environmental impact and the projected future cost of
electricity by a commercial 100 MW farm is 0,1  /kWh [9]7. However, any
further development on Stingray has been put on hold due to economic
difficulties8.
2.2.3 Tidal barrages
Mankind has harnessed tidal energy in small scale water mills for centuries
[22]. In modern times, tidal barrages can be considered the first ocean
energy technology to reach maturity exemplified by the 240 MW La
Rance scheme operational in northern France since the late 1960s. The
early development of tidal barrages is probably due to its similarities with
conventional run-of-river hydropower plants with bulb turbines [23].
7 Exchange rate 1 GBP = 1,46 ¼
8 http://www.engb.com/ 2005-11-29
Figure 6: The Stingray (used with
permission).

14
The La Rance estuary, near Mont Saint-Michel, is blocked by a 750 m long
and 13 m high structure housing 24 bulb turbines each rated 10 MW [24].
The project as such can be considered a technical success after almost
40 years of operation, delivering enough energy to supply around 250000
households9. However, the construction of a dam or a barrage will
unavoidably have an impact on the physical environment and
subsequent changes in the estuarine ecosystem. In the case of La Rance,
a long period of time was required for a new equilibrium to establish after
the complete closure of the estuary during construction. Furthermore, with
the barrage in place, primary biological production in the La Rance basin
has been observed to be higher than in comparable estuaries [25].
Even if the environmental impact is neglected, there are only a handful of
sites suitable for the development of tidal barrage schemes. It requires an
estuary with a favorable reservoir complete with a short and shallow dam
closure, and is only considered economically feasible with a mean tidal
range of five meters or more [23]. One potential site is the Severn estuary
in the UK with a mean tidal range of 8,5 m [26]. In 1989 the Severn Tidal
Power Group proposed a scheme capable of generating 17 TWh
annually, a scheme that has recently gained renewed interest [27]. Tidal
barrages might find its place in favorable locations such as the Severn
estuary, but it requires large amounts of construction material and civil
engineering work.
2.2.4 Future Marine Current Energy Converter Technologies.
• So called “off-the-shelf” equipment and solutions will be
superseded by genuine new technology and focus will shift toward
optimization [1].
• The importance of high reliability to keep maintenance costs low
could make simple devices preferable even at some sacrifice in
efficiency [1].
• Using direct drive generators makes it possible to eliminate the
gearbox, effectively reducing the number of moving parts and thus
reducing both the initial cost of the system and maintenance [16].
• Better developed sealing and improved anti-fouling could diminish
the need for maintenance.
• Increased knowledge and recognition of site dependent and
technology dependent environmental impacts and improved
procedures for mitigating any impact.
9 http://www.electricite-de-france.com/html/en/decouvertes/voyage/usine/usine.html 2005-12-01

15
• A solution to the problem of deploying MCECs in energetic flows at
great depths, or where seabed typography does not allow for
conventional anchorage, could increase in the number of sites
possible to explore.
• From an environmental viewpoint, trying to limit the use of hydraulic
oil would be preferable.
2.3 Present Tidal Current Energy Market
At present there are many promising concepts striving to go beyond the
very costly prototype stages and reach full commercialization. Increased
financial support and investment is important for generating the
momentum needed to carry the MCEC industry forward.
While discussing financing and support issues, it can be fruitful to raise the
question of targeted or generic research. In some areas common to most
MCEC technologies, generic research could be called for. Areas of
interest include potential resource assessments, guidelines for testing and
certification of MCECs and environmental impact assessments [7]. But at
this stage of development, some of the questions can only be answered
when developers have had a chance to test their prototypes and verify
their ideas.
The most important aspect for unlocking the tidal current energy might be
to prove that the design lifetime, and hence economic viability, of first
generation MCEC technology can be reached. So far the most successful
prototypes have only been operational for a few years. In comparison,
tidal barrages can be considered a mature technology. The tidal barrage
at La Rance has proven its worth during almost 40 years in operation
without major breakdowns, producing electricity below Electricité de
France's average generation costs10.
2.4 Future development
Even though tidal currents are very predictable, they are still a fluctuating
energy source and thus have to be accompanied by different balancing
energy sources and/or storage techniques. Introducing large MCEC farms
into the electric grid makes it necessary to develop farm control
capabilities and strategies that have features equivalent to conventional
power plants. These features include capabilities to control the output
power of the total MCEC farm and robustness to faults in the grid.
The future influence of policies, subsidies and level of public support is not
yet clear, but could prove to have a strong impact on investment
10 http://www.electricite-de-france.com/html/en/decouvertes/voyage/usine/usine.html, 2005-10-20.

16
decisions in the MCEC industry. In Europe, the UK will most likely continue
to be a key market given the significant tidal energy potential in UK
waters. In time, a well developed European MCEC industry could also be
a leader in the global market for tidal current energy.
Today, the market for tidal current energy is only just beginning to develop
with initial MCEC farms expected within the next few years. The market is
likely to grow substantially up to year 2010 and beyond. However,
estimates of the installed power for the years 2020 and 2030 are scarce.
MCEC devices to have an impact on the electricity supply up to 2030 are
likely to be extensions and refinements of present concepts and
prototypes. Looking ahead to 2050, completely novel devices might be
developed as well as new technology and deployment vessels for deep
water sites.
2.5 Conclusion
There is a substantial potential for marine and tidal current energy in
European waters with an estimated extractable potential in the range of
39-58 TWh per annum [1, 5, 6]. Looking at the source of the energy, marine
and tidal currents do offer a high degree of utilization with no emission of
GHG during operation. However, further research and increased funding
is required to better adapt the MCEC technology to the nature of the
source.
Because MCECs can be kept small and modular, the technology can be
expected to see similar cost reductions in manufacturing as for instance
wind turbines. With favorable experience curves and economics of scale,
the potential future cost for tidal energy is in the range of 3.8-9.5 c /kWh
[18].
Working through all the details of such complex system as an MCEC is time
consuming and expensive. Thus, a long term commitment to R&D
financial support of a concept in its entirety is likely to suit the long
development times of MCEC technology better than complete funding of
a few stages in the project lifecycles. Once reaching the commercial
phase, market incentives similar to those seen for other renewables might
be needed further increase the installed capacity. In our efforts to
decrease GHG emissions, the potential of wave and tidal energy cannot
be neglected.
The market for tidal current energy is only just beginning to develop and is
expected to grow substantially up to year 2010 and beyond. However,
estimates of the installed power for the years 2020 and 2030 are scarce.
MCEC devices to have an impact on the electricity supply up to 2030 are

17
likely to be extensions and refinements of present concepts and
prototypes. Looking ahead to 2050, completely novel devices might be
developed as well as new technology and deployment vessels for deep
water sites.
3. Wave power [29, 30]
Wave energy is a concentrated form of wind
energy issued from the sun. The 20 first meters of
water of Atlantic Ocean are sweep over by a
swell energy of about 2,5kW/m2. Most part of
the European coasts is concerned.
From the economical point of view, costs have
strongly decreased. The investment is about
1000-3000  /kW, depending on local conditions,
for an annual operation of about 4000 hours.
Current electricity costs range between 50 and
100  /MWh.
Ocean waves can produce electricity using an
Oscillating Water Column (OWC). This system
consists in a large column open in the sea at the
bottom and a turbine at the top. When a wave
hits the OWC, the water inside the column rises, and the air inside is
compressed and moves upwards. When the wave falls the air is sucked
back down. A two-way turbine spins when the air is forced upwards by a
wave, and continues to spin in the same direction when the wave drops
and the air is drawn back down again. The air turbine at the top of the
column is connected to an alternator generating electricity.
The first successful OWC device was produced in Japan to power light for
navigation. Most OWC's are experimental. An OWC used in Norway is one
of the most advanced wave-to-energy generators in the world,
developing an electric power of 500 kW. This had a 19.6 meter steel
chimney pipe that went 7 meters into the ocean. This device, along with
most others, suffered from the unpredictability of the ocean waves and
damage to the plant may occur when gale or storm are raging.
Other devices, such as “Salter Ducks”, use the bobbing motion of waves.
When they were tested, rough seas damaged them.
The Tapered Channel Station, or TAPCHAN, directs waves into an ever-
narrowing channel in which the water is forced to climb 10 to 15 meters
into a dam. Water from this dam then flows back down to the sea through
a hydro turbine to produce electricity.
Figure 1 Average wave po
wer on
European coasts (kW/meter of
coast)

18
Trials have been conducted on various types of wave electricity-
generation equipment since the early 1970’s, but Oscillating Water
Column (OWC) systems have been the most successful.
OWC’s have been built and tested in Japan, Norway, India, China,
Scotland and Portugal. Portugal and Scotland use this technology to
produce small amounts of electricity. The Islay Oscillating Water Column,
In the west coast of Scotland, has been running for 10 years and produces
a power of 75 kW.
These units are designed to generate up to 2 MW, for short periods, but
have not been operated during extended periods.
Wave electricity generation is a source of clean, renewable energy and
does not produce any greenhouse gases. The unpredictable
characteristics of the sea waves remain a problem with wave electricity
generation.
Facilities need to be built to withstand the effects of waves under
exceptional circumstances. Indeed, waves can exert forces 10 times
stronger than normal waves.
Electricity generating facilities using waves may cause changes to coast
lines and local ecosystems.

19
3.1 Conclusion and synthesis
There are several possibilities to exploit energies from the sea. The four
technologies discussed here have a large potential of development
although they are still not economically competitive. Anyway,
environmental impacts should be systematically studied.
Energy Offshore wind
power Wave power Tidal currents Tidal power
Development in
UE
612 MW in 2004
2000MW in
development
Availability Intermittent Intermittent Intermittent Intermittent
Predictability In progress Good Excellent Excellent
Predicted costs 50-100 ¼0:K
since 2015
50-70¼0:K
since 2015
50-70¼0:K
since 2015
Acknowledgements
Christian Ngô and his co-workers at ECRIN are hereby gratefully
acknowledged for their contribution on wave energy.

20
References
[1] Non Nuclear Energy – JOULE II, Wave energy project results, The Exploitation of
Tidal Marine Currents, EU JOULE contract JOU2-CT94-0355, Technomare SpA,
IT Power Ltd, 1996.
[2] Environmental Impact, WP 3.3 Final Report, 2003, European Thematic Network on
Wave Energy, NNE5-1999-00438
[3] The Potential for the use of Marine Current Energy in Northern Ireland, Department
of Trade and Industry; Department of Enterprise, Trade and Investment; and
Northern Ireland Electricity, 30 June 2003 (available online,
http://www.detini.gov.uk/cgi-bin/moreutil?utilid=41&site=5&util=2&fold=&,
2005-08-29).
[4] M Leijon, H Bernhoff, M Berg, O Ågren, 2003, Economical considerations of
renewable electric energy production – especially development of wave energy,
Renewable Energy 28, p 1201-1209.
[5] Black & Veatch, 2004, Tidal Stream Resource Report, Peer Review Issue 1, Report
Submission 107799/D/2100/05/1.
[6] Department of Trade and Industry, UK, 1993, Tidal Stream Energy Review, ETSU-
T/05/00155/REP.
[7] The International Energy Agency, 2003, Status and Research and Development
Priorities – Wave and Marine Current Energy, DTI report number FES-R-132.
[8] S J Couch, I G Bryden, 2004, The impact of energy extraction on tidal flow
development, Presented at 3rd International Conference on Marine Renewable
Energy, Blyth 2004 (available online, http://www.supergen-
marine.org.uk/documents/papers/Public/Resource%20Estimation/Marec_2004_Cou
ch_Bryden.pdf, 2005-10-22).
[9] Department of Trade and Industry, 2005, Stingray Tidal Stream Energy Device –
Phase 3, T/06/00230/00/REP, The Engineering Business Ltd.
[10] The International Energy Agency, 2005, Variability of Wind Power and other
Renewables: Management Options and Strategies (available online,
http://www.iea.org/Textbase/publications/free_new_Desc.asp?PUBS_ID=1572)
[11] GreenNet WP1, 2003, Dynamics and basic interactions of RES-E with the grid,
switchable loads and storage, Energy Economics Group (EEG), Institute of Power
Systems and Energy Economics,Vienna University of Technology (available
online, http://www.greennet.at/)
[12] GreenNet WP2, 2004, Cost and Technical Constraints of RES-E Grid Integration,
Energy Economics Group (EEG), Institute of Power Systems and Energy
Economics,Vienna University of Technology (available online,
http://www.greennet.at/)
[13] S. L. Dacre, I. G. Bryden, C. R. Bullen, 2002, Environmental impacts and
constraints of tidal current energy: The Pentland Firth feasibility study, Proceedings
from MAREC 2002 two day international conference on Marine Renewable
Energy, Newcastle, UK, September 2002.
[14] Department of Trade and Industry, 2002, A scoping study for an environmental
impact field programme in tidal current energy, Robert Gordon University, ETSU

21
T/4/00213/REP (available online at
www.dti.gov.uk/renewables/publications/pdfs/t0400213.pdf, 2005-09-23).
[15] BC Hydro, 2002, Green Energy Study for Brittish Columbia, Phase 2: Mainland,
Triton Consultants Ltd.
[16] Department of Trade and Industry, 2001, The commercial prospects for tidal stream
power, Binnie Black & Veatch, ETSU - T/06/00209/REP (available online at
http://www.dti.gov.uk/renewables/publications/pdfs/t0600209.pdf, 2005-09-15).
[17] Scottish Executive, 2001, Scotland’s Renewable Resource 2001 – Volume II:
Context, Garrad Hassan and Partners Ltd, Document 2850/GR/03, Issue B
(available online,
http://www.scotland.gov.uk/library5/environment/SRS2001Vol2.pdf).
[18] United Nations Development Programme, 2004, World Energy Assessment
Overview: 2004 Update, ISBN: 92-1-126167-8.
[19] E Segergren , K Nilsson , D P Coiro and M Leijon, 2004, Design of a Very Low
Speed PM Generator for the Patented KOBOLD Tidal Current Turbine,
EnergyOcean 2004, West Palm Beach, Florida, 28-29 June 2004.
[20] European Commission, 2005, SEAFLOW - World’s first pilot project for the
exploitation of marine currents at a commercial scale, EUR 21616.
[21] Department of Trade and Industry, UK, 2003, Stingray Tidal Stream Energy Device
– Phase 2, T/06/00218/00/REP, The Engineering Business Ltd.
[22] United Nations Development Programme, 2000, World Energy Assessment (WEA),
ISBN: 92-1-126126-0.
[23] M T Pontes, A Falcão, 2001, OCEAN ENERGIES: RESOURCES AND
UTILISATION, World Energy Council 18th Congress, Buenos Aires, October
2001.
[24] H Andre, 1978, Ten Years of Experience at the “La Rance” Tidal Power Plant,
Ocean Management, 4 (1978) 165—178.
[25] J P Frau, 1993, Tidal Energy : Promising Projects La Rance, a successful industrial-
scale experiment, IEEE Transactions on Energy Conversion, Vol. 8, No. 3,
September 1993.
[26] M Sathiamoorthy, S D Probert, 1994, The Integrated Severn Barrage Complex:
Harnessing Tidal, Wave and Wind Power, Elsevier Science Limited Applied
Energy 49, p 17-46.
[27] Department of Trade and Industry, UK, 2002, The Severn Barrage – Definition
study for a new appraisal of the project, Sir Robert McAlpine Limited, ETSU -
T/09/00212/00/REP.
[28] Scottish Enterprise, 2005, Marine Renewable (Wave and Tidal) Opportunity
Review (available online, http://www.scottish-enterprise.com/sedotcom_home.htm)
[29] “Sea energy” Working group (ECRIN)
[30] J.Ruer, “Innovation et énergie”, journée ECRIN-OPECST, octobre 2004, CD-ROM