SYNOPSIS
The
potential for extracting useful energy from waves has been studied for some
considerable time. This paper outlines
some of the history of wave energy, before proceeding to portray the revolution
in wave energy technology that has occurred in the last five years. The main devices currently under development
will be described, together with the effect of these developments on
electricity generating costs. The paper
closes by reviewing the environmental impacts of wave energy and estimating the
commercial potential of this resources.
INTRODUCTION
Waves
form a potentially large world-wide resource estimated at more than 2 TW1. There are several regions around the world
with high incident wave power levels (see Fig 1), which are particularly well
situated to exploiting this renewable energy source. To date, the attempts to design and deploy cost-efficient devices
have met with limited success (the main success being using wave energy to
power several hundred navigation buoys).
However, the last five years have seen a resurgence of interest in wave
energy throughout the world, with several companies currently developing and
deploying new devices that represent a significant improvement over older
concepts. This paper explains some of
the reasons for this development and the current activities that have resulted.
Fig 1 Global distribution of wave power levels in
kW/m of crest length
Author’s Biography
Tom Thorpe is a Principal Consultant in
renewable energy and environmental matters at AEA Technology, a position he has
held since 1989. Prior to that, he
worked for ten years in the offshore oil and gas field, first as a research
scientist on materials’ problems and then as a manager of the UK Department of
Energy’s research projects in this area.
He is a consultant to a range of industries as well as governments and
international bodies.
BACKGROUND
1999
saw the bicentenary of the first patent on a wave energy device by Girard (père et fils). However, work on wave energy began in earnest only in the early
1970s as a response to the oil crises.
There were several Government sponsored programmes throughout the world,
particularly in Japan, Norway and the UK.
These programmes advanced the technology considerably and their
achievements were impressive but they led to the installation of prototype
devices (including one in the UK).
The
failure of these programmes to deliver economic supplies of electricity left
the technology with a credibility problem that has been hard to overcome. Therefore, it is important to understand the
reasons for this failure.
Size of
Device
The
wave power resource around the UK is immense: approximately 120 GW or 2½ times
the total electricity demand.
Therefore, the early work on wave energy aimed at exploiting the maximum
amount of resource possible, which led to a target design of 2,000 MW for the
first wave energy schemes. Colossal
schemes were required to achieve this target, examples of which are given in
Table I. This entailed large
construction costs, prolonged construction times and significant technical
challenges. These factors led to high
generating costs and large capital costs for the first prototype, which made
all the technologies commercially unattractive.
Table I Example
of devices from the first UK wave energy programme
|
Name of device |
Weight of each unit |
Number of units in scheme |
Capital costs |
Generating costs2 |
|
NEL OWC |
22,500 t |
606 |
£ 6 billion |
19 p/kWh |
|
Bristol Cylinder |
20,000 t |
276 |
£ 5.6 billion |
19 p/kWh |
Technical
Maturity
In
the 1970’s, wave energy was a novel technology. It faced many technical challenges, required the synthesis of
many disciplines (e.g. oceanography, fluid mechanics, structural engineering,
etc.) and needed the development of completely new fields. At the time, the nearest technology that it
could draw on was coastal engineering (e.g. breakwaters and coastal defenses),
an area where cost was not of primary importance. In the author’s opinion, these early developments faced too many
challenges to solve within the overall heading of wave energy.
Make Up of
Research and Development
Most
of the early R&D programmes were totally government funded (either in
direct grants or through universities or research institutes) and many had a
large academic component. At the time,
these aspects were appropriate, because wave energy was an emerging technology,
which needed the research skills of academia and which could not provide
industry with the types of return that it required. However, industry-led development is generally required in order
to achieve commerciality.
CURRENT
DEVELOPMENTS IN WAVE ENERGY
Since
the mid-1990s, there has been a resurgence in wave energy, led mainly by small
engineering companies. As a result,
there are a number of projects being built (or about to be built) all over the
world. These companies have all learned
from the lessons taught by the early experiences in wave energy:
·
All
the devices are relatively small in size and output (the biggest is 2 MW). This reduces the prototype costs (making
funding easier) and the technological challenges. The technological challenges are further reduced for some devices
by installing them at the shoreline.
·
The
devices started to be constructed in the mid to late 1990’s. Consequently, they had the benefit of two
decades of experience in constructing and operating offshore oil and gas
platforms, with the corresponding scope for technology transfer.
·
Nearly
all of the devices described below are being built by industry (typically small
engineering companies), so the focus is on the economics of the technology from
the start.
The following describes a few of the more important projects.
Shoreline
Devices
Being
constructed at the shoreline, these devices are the easiest to fabricate and
maintain but they capture much less energy than offshore devices. Most are oscillating water columns
(OWCs). These consists of a partially
submerged, hollow structure, which is open to the sea below the water line (Fig
2). This structure encloses a column of
air on top of a column of water. As
waves impinge upon the device they cause the water column to rise and fall,
which alternatively compresses and depressurises the air column. If this trapped air is allowed to flow to
and from the atmosphere via a turbine, energy can be extracted from the system
and used to generate electricity. This
is usually achieved using a Wells turbine, which has the property of rotating
in the same direction regardless of the direction that the air passes the
blades.
Fig
2 Operating principles of an oscillating water column (OWC)
A
number of OWC devices have been installed world-wide with several commercial
schemes currently being built:
·
The
European Pilot Plant3 on the island of Pico in
the Azores. This plant is already built
and operating. It will be used to
supply electricity to the local grid and as a test bed for various technologies
associated with OWCs.
·
The
Wavegen Limpet4, 5 is a 500 kW device on the island of Islay
(Scotland), which uses a novel construction method to reduce construction costs
(Fig 3). The device is constructed in a
hollow excavated from a cliff behind a rock bund; the bund is removed at the
end to allow ingress of sea water. This
device came on stream in November 2000 and is performing satisfactorily.
·
The
Energetech OWC6 is being built in Port Kembla, near Sydney,
Australia. This uses a novel, variable
pitch turbine (potentially with a higher conversion efficiency than the Wells)
and a parabolic wall behind the OWC to focus the wave energy on the OWC
collector, leading to potentially significant improvements in the economics of
OWCs (Fig 4).
Fig
3 Outline of the LIMPET (OWC)
Fig
4 Artist’s impression of the Energetech OWC
Offshore
Devices
This
class of device exploits the greater amount of wave energy available in deeper
water (> 40 m depth). They are
mainly floating devices held in place by different types of mooring. Because they face more powerful waves than
shoreline devices, they have greater technical challenges. In general, they are less developed than the
OWCs. There are several designs
currently being developed, each with its own pros and cons. Some of the promising ones are shown in Fig
5.
·
The
McCabe Wave Pump. Three narrow steel
pontoons are hinged together across their beam and point into the incoming
waves. The front and back pontoons move
in relation to the central pontoon, which is held relatively still by the
damper plate, by rotating about the hinges.
Energy is extracted from the rotation by hydraulic rams. This energy can be used to provide
electricity (~ 400 kW) or else to produce potable water by supplying
pressurised sea water to a reverse osmosis plant. A 40 m long prototype of this device was deployed off the coast
of Kilbaha, County Clare, Ireland and a commercial demonstration scheme is
currently being built5.
·
The
Pelamis.
The Pelamis device is composed of a number hollow, cylindrical sections linked
by hinged joints. These sections point
into the oncoming waves and move with respect to each other as the waves pass
down their length. Again, energy is
extracted by hydraulic rams at the joints, which drive electrical
generators. Here the emphasis is on
building the device with 100 % ‘off the shelf’ components. A device is being developed for deployment
in Scotland, which is rated at 375kW and is 130 m long and 3.5 m in diameter7.
· The Archimedes Wave Swing. This consists of a cylindrical, air filled chamber (the “Floater”), which can move vertically with respect to the cylindrical “Basement”, which is fixed to the sea bed. The air within the 10m – 20m diameter Floater ensures buoyancy. However, a wave passing over the top of the device, alternatively pressurises and depressurises the air within the Floater, changing this buoyancy. This causes the Floater to move up and down with respect to the Basement and it is this relative motion that is used to produce energy. This is the most powerful device currently under construction: a 2 MW Pilot scheme is being built for Portugal8.
There
are other promising offshore devices (e.g. Ocean Power Technology’s Wave Energy
Converter) which are undergoing deployment but less is known about them because
of commercial confidentiality.
CURRENT
STATUS OF WAVE ENERGY
As
a result of the above activities, the predicted costs of electricity from wave
energy has been steadily decreasing.
The assessment of the commercial prospects for wave energy is difficult,
because estimates of the cost of power from wave energy devices represent a
snapshot of the status and costs of these evolving designs at their current
stages of their development. The
electricity costs of a number of devices have been evaluated over the past 10
years using the a peer-reviewed methodology2, 5. A plot of the resulting costs against the
year in which the design of device was completed (Fig 6) shows a significant
reduction in generating costs. At best,
this is similar to improvement of generating costs for wind turbines in the UK9,
so that there are now several wave energy devices with predicted costs of about
5 p/kWh or less at 8% discount rate if the devices achieve their anticipated
performance.
The
McCabe Wave Pump
The
Pelamis
The Archimedes Wave Swing
