1.6.1 Non-solar renewables
There are also two other sources of renewable energy that do not depend on solar radiation:
Tidal energy is often confused with wave energy, but its origins are quite different, as this short video clip shows.
Ocean tides are caused by the gravitational pull of the Moon (plus a small contribution from the Sun’s gravity) on the world’s oceans, causing a regular rise and fall in water levels as the Earth rotates. The power of the resulting tides can be harnessed by building a low dam or ‘barrage’, behind which the rising waters are captured and then allowed to flow back through electricity-generating turbines. The strong underwater currents found in some locations are mainly tidal in origin. Various devices for exploiting this energy source, such as Marine Current Turbines’ 'Seagen” prototype in Strangford Narrows, Northern Ireland (see Figure 12) are at the demonstration stage.
Geothermal energy is heat from deep within the Earth, which in some locations heats rocks near the surface. These can heat water in underground aquifers (water bearing rocks), producing hot water that can be used for heating purposes, or in some cases to produce steam for electricity generation.
Watch the video ‘Geothermal Energy’ to learn more:
In this short course we have decided to concentrate on the solar direct and solar indirect renewables, omitting further detailed coverage of tidal and geothermal energy.