Memorandum submitted by the International
The International Energy Agency is presently
launching a new Implementing Agreement on Ocean Energy focused
initially on Wave Energy and Marine Currents. Denmark, the European
Union, Portugal and the UK form the Interim Executive Committee.
Other countries have shown interest, namely Ireland, Japan, Norway,
Netherlands, USA, Mexico, India. The IA activities will be closely
co-ordinated with those of the European Thematic Wave Energy Network.
The Outline on Ocean Energy Technologies (Annex), prepared to
serve as a basis for the establishment of this new IA provides
a summary of the status of these technologies.
The initiative to start up this new IA with
activities on wave and tidal stream energy is the result of the
importance of these renewable energy resources, and the R&D
and demonstration activities, with the start of the commercial
exploitation of wave energy. Important achievements have been
made in Europe. Two 0.5 MW wave energy power plants of the Oscillating
Water Column (OWC) type have been built in the Azores, Portugal,
and in Scotland, UK, the latter being running commercially. In
addition a 2MW prototype of a Dutch offshore device is under construction,
its deployment off Portugal is planned for summer 2001. In Japan,
India and China prototypes of OWC and other device types have
been tested during the last decade. In China and Australia, OWCs
rated 100 kW and 600 kW respectively are under construction, the
latter being a commercial initiative. For tidal streams, two prototypes
rated 300 kW and 130 kW are planned to be deployed in 2001 in
the south-west of England (Bristol Channel) and in the Straight
of Messina, Italy. In USA, a few companies are being funded by
the Government to develop marine current devices.
AN OUTLINE OF OCEAN ENERGY CONVERSION
Several types of ocean energy sources with different
origins exist. The most developed conversion systems concern tidal
energy, which results from the gravitational fields of the moon
and the sun; thermal energy (Ocean Thermal Energy Conversion or
OTEC), resulting directly from solar radiation; marine currents,
caused by thermal differences in addition to tidal effects; and
ocean waves, generated by the action of the winds blowing over
the ocean surface. Harnessing the offshore wind energy can be
considered an ocean energy technology.
An outline of the resource availablity, the
state-of-the-art of the technology, existing or demonstration
plants, economics and environmental impacts for tidal, marine
currents and wave energy resource follows. This is intended to
serve as a basis to an Implementing Agreement on Ocean Energy
that is proposed herein.
Significant development of tidal energy technology
occurred during the past 30 years, particularly in France, Canada,
the USSR and China. A small number of commercial power plants
are in operation.
The tides are generated by the rotation of the
earth within the gravitational fields of the moon and the sun.
The relative motions of these bodies cause the surface of the
oceans to be raised and lowered periodically. Tidal energy is
predictable in both its timing and magnitude.
The locations where tidal power could be developed
economically are relatively few, because a mean tidal range of
five metres or more is needed for the cost of electricity to be
competitive with conventional power plants. Additional requirements
are a large reservoir, and a short and shallow dam closure.
A barrage constructed across an estuary is equipped
with a series of sluice gates and a bank of low head axial turbines.
The most common construction method involves the use of caissons.
Tidal energy technology can be considered largely mature with
a 240 MW commercial unit operating successfully since 1966 in
France at La Rance. Smaller units have been constructed in other
countries namely Canada (Bay of Fundy, 17.8MW, 30 GWh/yr, 1984),
USSR (Kislogubskaya near Murmansk, 0.4 MW, 1968), and China (Jiangxia,
3.2 MW, 11 GWh/yr, 1980) and others (CEC, 1992).
To widen the range of suitable sites a new approach
that uses an impoundment structure that sits nearshore on the
ocean floor has been patented by a US company. A contract with
Alaska authorities for the construction of a 2 MW tidal plant
of this type is being prepared.
Economics and Environmental Impacts
Tidal power incurs relatively high capital costs,
and construction times can be several years for larger projects.
The operation is intermittent with load factor 22-35 per cent.
Plant lifetime can be very long (120 years for the barrage structure
and 40 years for the equipment). The high capital costs and long
construction time have deterred the construction of large tidal
Tidal barrages can cause significant modifications
to the basin ecosystem. However they can also bring benefits like
flood protection, road crossing, marina and increased tourism.
Kinetic energy from the sea can be harnessed
using techniques similar in principle to those for extracting
energy from the wind, by using submarine converters similar to
"underwater windmills", but this option is still relatively
undeveloped. A number of studies have been completed on the energy
potential of marine currents but there have been few on the engineering
requirements for utilisation of this resource. Countries where
theoretical studies and experimental projects took place are UK,
Canada, Japan, Russia, Australia and China in addition to the
The start up of the exploitation of the marine
currents energy can make use of conventional engineering components
and systems but development is required to achieve reliability
and durability of the equipment at low operational and maintenance
costs. Further research will be necessary to develop more efficient
converters at lower electrical energy unit costs.
Studies to assess the marine currents resource
have been recently carried out in UK (DTI, 1993), European Union
(CEC, 1996b) and in far-eastern countries (CEC, 1998).
In Europe this resource is of special interest
for UK, Ireland, Greece, France and Italy. In this area 106 suitable
locations were identified and it was estimated that, using present
day technology, these sites could supply 48TWh/yr to the European
electrical grid network. In China it has been estimated that 7,000
MW of tidal current energy are available. Locations with high
potential have also been identified in the Philippines, Japan,
Australia, Northern Africa and South America. The predictability
of marine currents and the high load factor (20-60 per cent) are
important positive factors for its utilisation.
There remains much uncertainty with regard to
the detailed characteristics of this marine resource. The available
marine current data are limited and inconsistent. Even where velocities
were measured, these were single point measurements close to the
surface whereas the converters are to be deployed in deeper water
(at least 20m). The development of efficient 3D numerical flow
models in addition to long-term data collection will enable to
produce a comprehensive accurate assessment of this resource.
The technique that has been mostly considered
for the exploitation of marine currents is to use a turbine rotor,
set normal to the flow direction, that is mounted on the seabed
or suspended from a floating platform. The greatest technical
problems are likely to arise from the need for adequate operational
life and low maintenance costs from machinery operating in a hard
environment, although the offshore industry has solved similar
problems. First generation systems will be based on the use of
conventional engineering components and systems in order to achieve
reasonable reliability at low costs. A medium-sized turbine, of
10-15 m diameter and 200-700 kW rated power, deployed in as shallow
water as possible (ie 20-30 m water depth), is likely to be the
most economic overall solution for the first generation machines
(CEC, 1996b). Second generation systems can follow from this by
introducing specialised components, such as low speed multi-pole
electrical generators, hydraulic transmission systems, etc. Novel
concepts to be developed within R&D programmes will be the
third generation systems.
Design, construction and installation of turbines
in UK and Italy, that will supply electrical energy to the national
grids, are being funded by the European Commission (the turbines
will be commissioned in 2000 in UK (300 kW), and 2001 in the Strait
of Messina, Italy, 100 kW rated power). These projects are being
led by SME's. The construction of a 75 kW device is planned for
China with European collaboration.
Estimates of unit costs of electrical energy
vary between 0.05 and 0.15 ECU/kWh depending on the studies. The
assessment described in CEC (1996b) has estimated that costs of
less than 0.10 ECU/kWh would be achievable with first generation
machines in a good current regime with reasonable load factor.
The environmental impact of submerged marine
current turbines will be minimal; the main conflicts are expected
to be with shipping, navigation and fishing. Modification of sediment
transport as a consequence of current energy extraction may occur.
The conversion of ocean wave energy requires
new technology to an extent larger than for most of other ocean
energy sources. Considerable research on wave energy conversion
began only after 1973's oil crisis, and took place in UK, Japan,
Norway, Sweden and USA, a few years later in Denmark, Ireland
and Portugal, and, in the 1980s, in India and China. In the 1990s
activity in this subject started in Mexico, Australia and Netherlands.
Based on various energy-extracting methods a wide range of systems
was proposed. Only a few reached the demonstration stage.
Wave energy can be considered a concentrated
form of solar energy. Winds are generated by the differential
heating of the earth, and, as a result of their blowing over large
areas of water, part of their energy is converted into waves.
Just below the oceans surface wave energy flux, in time average,
is typically five times denser than the wind energy flux 20 metres
above the surface, and 10 to 30 times denser than the solar energy.
The best wave climates, with annual average power levels between
20 and 70 kW/m or higher, are found in the temperate zones (30
to 60 degrees latitude) where strong storms occur. However, attractive
wave climates are still found within ±30 degrees latitude
where regular trade winds blow, the lower power level being here
compensated by the smaller wave power variability.
The global wave power potential was estimated
to be 1 TW, which is the same order of magnitude of the world
consumption of electrical energy. Resource assessments have been
undertaken at national level and, more recently, at the European
level (CEC, 1996a). A global assessment based on visual observations
(Quayle and Changery, 1982) must also be referred. Based on the
European Wave Energy Atlas, the total annual deep water resource
along the Atlantic and Mediterranean coasts of Europe was estimated
to amount to about 320 GW. The highest annual wave power level
off the European coasts is 75 kW/m off Ireland and Scotland and
it decreases gradually to about 30 kW/m off northern Norway and
off the southern Atlantic Madeira and Canary archipelagos.
Nearshore and mainly at the shoreline the wave
power level is in general smaller than offshore because of wave
breaking in shallow waters. Other phenomena as refraction, and
difraction in indented coastlines, can cause significant resource
variations within lengths of 1 km or much less, especially at
The conversion of the available resource could
supply the whole or a substantial part of the electrical energy
demand in several countries as eg Ireland and Portugal, in Europe,
whereas in remote areas the conversion of a small fraction of
the available resource could meet the whole electrical energy
The large number of different concepts under
investigation at present in various parts of the world suggests
that the best technology has not yet been indentified. Prototypes
of just a few types have been tested in the sea, the best known
of which are the oscillating water column (OWC) and the convergent
The OWC device comprises a partly submerged
concrete or steel structure, open below the water surface, inside
which air is trapped above the water free surface. The oscillation
of the internal free surface produced by the incident waves makes
the air to flow through a turbine that drives an electric generator.
The self-rectifying axial-flow Wells turbine had been used in
almost all the prototypes. Eight prototypes have been tested since
1985 in Norway, Japan, UK, India, China and Portugal, the rated
power ranging between 20 and 500 kW. Most of the devices are located
on the shoreline where maintenance and installation are easier
and moorings and underwater electricity transmission are avoided.
In two cases (India and Japan) bottom-fixed nearshore devices
have been located just outside or incorporated in a protecting
breakwater (a way of building the plant at marginal costs). A
floating OWC device was constructed in Japan with the double purpose
of producing electrical energy and providing shelter.
The Tapchan comprises a gradually narrowing
channel with wall heights above mean water level. As the waves
propagate down the channel, the wave height is amplified until
the wave crests spill over the walls to a reservior which provides
a stable water supply to a conventional low head turbine. A demonstration
Tapchan device with rated output of 350kW operated from 1985 to
the mid-1990s in Norway. This is considered a relatively mature
A shoreline or nearshore advice ("Pendulor"),
based on a pendulum or oscillating flap acted upon directly by
the waves, has been under development for a number of years in
Japan and more recently in China (this included sea tests of models).
The construction in China of a prototype rated 30kW is being planned.
Although some floating devices have shown good
prospects to be competitive with conventional electricity production,
no offshore device has yet been demonstrated at full scale. The
proposed floating devices are intended for electricity production
and, in some cases, for fresh water supply. These devices, that
will exploit the more powerful wave regimes available in deep
water (typically more than 40 m water depth) require flexible
moorings and electrical transmission cables. Reduced models of
some have been temporarily deployed in the sea. The contracts
for construction of two floating systems are underway in Scotland
and Australia. The development and construction of a wave power
vessel has been contracted with the Scottish authorities. A prototype
of a floating wave pump to supply fresh water or electrical energy
has recently undergone sea tests in Ireland.
It is difficult to realistically estimate the
unit costs of electrical energy produced from the waves since
the few existing schemes have been prototypes with the additional
costs incurred by such a stage of development. However the estimated
costs have shown a steady decrease with time, despite the little
financial support received in recent years. It should be noted
that the cost of energy produced is a function of local wave climate
and, in the case shoreline devices, is site specific.
It appears that several devices have already
the potential to provide cheaper electrical energy for small islands
and remote coastal communities that depend on expensive Diesel
Wave power generation is generally considered
environmentally benign. For shoreline power plants, the main negative
impacts are visual intrusion and noise from air turbines. Nearshore
and offshore plants may constitute obstacles to coastal marine
traffic and, when deployed in large numbers, may promote modifications
to coastal dynamics. Other impacts, namely on the ecosystems,
on fishing and on recreation and tourism may occur. Most of these
burdens can be minimised and, in some cases, eliminated.
A detailed environmental cause-and-effect study
of any intended deployment should be required. A strategy for
the assessment and quantification of environmental impacts needs
to be developed.
Commission of the European Communities, DGXVII
(1992), The Potential for Tidal Energy Community within the
European Union, prepared by ETSU and CCE.
Commisson of the European Communities, DGXII
(1996a), Atlas of Wave Energy Resource in Europe, Final
Report. JOULE Contact No JOU2-CT93-0312.
Commission of the European Communities, DGXII
(1996b), Wave Energy Project Results: The Exploitation of
Tidal Marine Currents, Report EUR16683EN.
Commission of the European Communities, DGXVII
(1998), Promotion of New Energy Sources in the Zhejiang Province,
China, Final Report. Program SYNERGY Contract No 4.1041/D/97-09.
Department of Trade and Industry, UK (1993),
Tidal Stream Energy Review, Report No ETSU T/05/00155/REP.
Quayle, R G and M J Changery (1982), "Estimates
of coastal deepwater wave energy potential for the world",
Proc. Oceans '81 Conf, Boston, Vol 1, p 903-907
15 March 2001