Environmental Engineering Reference
In-Depth Information
Table 2.2
Technically achievable wave power resource
UK potential (GW)
World potential (GW)
Shoreline
0.03
1 - 50
Near shore
0.3 - 0.7
10 - 500
Offshore
7 - 10
200 - 5000
one-third of the waves and the
zero up-crossing period T
e
is defi ned as the average time
counted over ten crossings or more between upward movements of the surface through the
mean level. It can be shown that the average power in one metre of wave crest is proportional
to
T
e
and to the square of
H
S
.
The world wave resource is not yet fully analysed but there is no doubt that it is sizeable.
Table 2.2 gives an estimate of the scale of the technically achievable resource.
2.7.2 The Technology
The conversion of wave power into electricity requires a device that intercepts the waves and
converts a proportion of their energy fi rst into mechanical and then into electrical form. The
conversion of wave energy into mechanical energy demands a central stable structure incor-
porating an active element which moves relative to it under the forces exerted by the waves,
and can react against the central structure to produce forces and displacements that generate
mechanical power.
Wave energy research and development has been taking place for over 25 years now, and
signifi cant progress has been made towards the development of viable technologies able to
exploit the large energy potential of the world ocean wave climates. Ocean waves are often
powerful, but with extremely low frequencies, of about 0.1 Hz (equivalent to 6 rpm or to
periods of around 10 s), and the success in generating electricity demands that this frequency
is raised to 500 - 1500 rpm.
The technology for wave energy conversion is still at an early stage compared to wind and
photovoltaics and a variety of different approaches are currently under development. Because
of the nature of the resource and for effi cient conversion, the swept volume of the device
must be of the order of several tens of cubic metres per metre of device width. The devices
are therefore physically large and have to be designed to withstand without damage extreme
waves that may occur very rarely. The transition of a concept from a model tested in a labo-
ratory wave tank to a working prototype requires considerable expenditure and it is only very
recently that serious funding from governments and industry has been forthcoming.
It is not the intention here to describe in detail the various concepts and associated hard-
ware. The devices can be classifi ed in a variety of ways depending on their intended location
or their geometry and orientation. At least twelve different devices are being developed
worldwide and it is still too early to guess which of them will be capable of providing energy
competitively and reliably. As an example of the ingenuity of recent developments, Figure
2.21 shows an artist's impression of a Pelamis array. The structure of the Pelamis comprises
four semi-submerged cylinders linked by hinged joints. The relative motion between the
