12   Wave

If wave power offers hope to any country, then it must offer hope to the
United Kingdom and Ireland – flanked on the one side by the Atlantic
Ocean, and on the other by the North Sea.

First, let’s clarify where waves come from: sun makes wind and wind
makes waves

Most of the sunlight that hits our planet warms the oceans. The warmed
water warms the air above it, and produces water vapour. The warmed air
rises; as it rises it cools, and the water eventually re-condenses, forming
clouds and rain. At its highest point, the air is cooled down further by
the freezing blackness of space. The cold air sinks again. This great solar-
powered pump drives air round and round in great convection rolls. From
our point of view on the surface, these convection rolls produce the winds.
Wind is second-hand solar energy. As wind rushes across open water, it
generates waves. Waves are thus third-hand solar energy. (The waves that
crash on a beach are nothing to do with the tides.)

In open water, waves 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, and in the same direction. The wavelength of the waves
(the distance between crests) and the period (the time between crests) de-
pend on the speed of the wind. The longer the wind blows for, and the
greater the expanse of water over which the wind blows, the greater the
height of the waves stroked up by the wind. Thus since the prevailing
winds over the Atlantic go from west to east, the waves arriving on the At-
lantic coast of Europe are often especially big. (The waves on the east coast
of the British Isles are usually much smaller, so my estimates of potential
wave power will focus on the resource in the Atlantic Ocean.)

Waves have long memory and will keep going in the same direction for
days after the wind stopped blowing, until they bump into something. In
seas where the direction of the wind changes frequently, waves born on
different days form a superposed jumble, travelling in different directions.

If waves travelling in a particular direction encounter objects that ab-
sorb energy from the waves – for example, a row of islands with sandy
beaches – then the seas beyond the object are calmer. The objects cast a
shadow, and there’s less energy in the waves that get by. So, whereas sun-
light delivers a power per unit area, waves deliver a power per unit length
of coastline. You can’t have your cake and eat it. You can’t collect wave
energy two miles off-shore and one mile off-shore. Or rather, you can try,
but the two-mile facility will absorb energy that would have gone to the
one-mile facility, and it won’t be replaced. The fetch required for wind to
stroke up big waves is thousands of miles.

We can find an upper bound on the maximum conceivable power that
could be obtained from wave power by estimating the incoming power

Figure 12.1. A Pelamis wave energy collector is a sea snake made of four sections. It faces nose-on towards the incoming waves. The waves make the snake flex, and these motions are resisted by hydraulic generators. The peak power from one snake is 750 kW; in the best Atlantic location one snake would deliver 300 kW on average. Photo from Pelamis wave power www.pelamiswave.com.