Notes and further reading

page no.

73Waves are generated whenever the wind speed is greater than about 0.5 m/s.
The wave crests move at about the speed of the wind that creates them
. The
simplest theory of wave-production (Faber, 1995, p. 337) suggests that (for
small waves) the wave crests move at about half the speed of the wind that
creates them. It’s found empirically however that, the longer the wind blows
for, the longer the wavelength of the dominant waves present, and the greater
their velocity. The characteristic speed of fully-developed seas is almost ex-
actly equal to the wind-speed 20 metres above the sea surface (Mollison,

The waves on the east coast of the British Isles are usually much smaller.
Whereas the wave power at Lewis (Atlantic) is 42 kW/m, the powers at the
east-coast sites are: Peterhead: 4 kW/m; Scarborough: 8 kW/m; Cromer:
5 kW/m. Source: Sinden (2005). Sinden says: “The North Sea Region expe-
riences a very low energy wave environment.”

74Atlantic wave power is 40 kW per metre of exposed coastline.
(Chapter F explains how we can estimate this power using a few facts about
waves.) This number has a firm basis in the literature on Atlantic wave
power (Mollison et al., 1976; Mollison, 1986, 1991). From Mollison (1986), for
example: “the large scale resource of the NE Atlantic, from Iceland to North
Portugal, has a net resource of 40–50 MW/km, of which 20–30 MW/km is
potentially economically extractable.” At any point in the open ocean, three
powers per unit length can be distinguished: the total power passing through
that point in all directions (63 kW/m on average at the Isles of Scilly and
67 kW/m off Uist); the net power intercepted by a directional collecting de-
vice oriented in the optimal direction (47 kW/m and 45 kW/m respectively);
and the power per unit coastline, which takes into account the misalignment
between the optimal orientation of a directional collector and the coastline
(for example in Portugal the optimal orientation faces northwest and the
coastline faces west).

Practical systems won’t manage to extract all the power, and some of the
power will inevitably be lost during conversion from mechanical energy to
. The UK’s first grid-connected wave machine, the Limpet on Islay,
provides a striking example of these losses. When it was designed its con-
version efficiency from wave power to grid power was estimated to be 48%,
and the average power output was predicted to be 200 kW. However losses
in the capture system, flywheels and electrical components mean the actual
average output is 21 kW – just 5%E of the predicted output (Wavegen, 2002).

Photo by Terry Cavner.