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Global renewable energy resources and use in 2050 231

6.7 Other possible renewable energy sources

A number of other possible RE sources are under consideration, many of which can be
grouped together as ocean energy sources. This list includes tidal energy, wave
energy, ocean current energy, and ocean thermal energy conversion (OTEC).
The global scope for tides as an energy source is very small, since the total tidal
energy resource base is only 75EJ, of which about 73EJ is dissipated at coastlines,
but in most cases the tidal range is too small for effective utilization. The only large
commercial plant for decades was on the Rance estuary in France, completed in 1966
and still supplying 0.5TWh or 0.002EJ of electricity annually. In 2011, a slightly
larger plant (0.55TWh) opened in South Korea, and several others are in the planning
stage. None have a power output of more than about 250MW [39]. The larger tidal
energy projects dam a bay and function in a manner similar to low head hydro plants,
but a less environmentally disruptive approach is to place turbines in tidal flows in
river estuaries or straits. One such installation in a tidal flow strait near Belfast has
a power output of 1.2MW [40].
The main potential ocean energy source is wave energy, which occurs because
some of the planet’s wind energy is transferred to the surface waters by shear forces.
Vast numbers of devices to capture wave energy have been invented, and several new
designs have undergone ocean trials. Lo ´pez et al. [41], in their review article, listed the
advantages and disadvantages of wave energy.
Its power density, at 2–3kWm , is an order of magnitude greater than solar energy, and
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almost an order of magnitude greater than wind energy.
Energy is available around 90% of the time, much greater than for wind or solar energy.
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Wave energy is well matched to demand, given that about 44% of the global population pres-
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ently live within 150km from a sea coast [40]. A related point is that wave energy is a poten-
tial RE source for nearly all countries with a sea coast, since it can be harvested on the open
seas or at the coast.
Drawbacks of wave energy mainly result from the very variable height, frequency, and
direction changes (for off-shore converters) of the waves, which all complicate design
of the conversion devices. In many offshore locations, the converters must be designed
to withstand the force of heavy waves, which increases both their cost and the diffi-
culty of maintenance. At present (2018), despite several sea trials with prototype
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devices, no wave energy is being generated, despite the $US 735 10 investment
over the 2004–14 period [40]. As an example of recent designs, the Pelamis Wave
Power device is segmented, and power is generated by the relative movement of
the segments. It was the first grid-connected wave energy converter, with different
versions deployed off both the Scottish and Portuguese coasts, but are now no longer
in operation.
The temperature difference between the tropics and the polar regions drives the
various ocean currents, such as the Gulf Stream. Some researchers have proposed tap-
ping into the kinetic energy of these currents as they pass through constrictions like the
Straits of Florida [42].