Page 55 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Chapter 3

Tidal Energy

Tidal energy is presently one of the more favoured forms of marine renewable
energy because, due to its origins in (astronomical) tide generating forces, it
is predictable. This is in contrast to other more stochastic renewable energy
sources such as wind and wave and, in part, solar. However, regardless of its
predictability, tidal energy shares a key feature with the majority of renewable
energy sources—it is intermittent, from diurnal (once per day) and semidiurnal
(twice daily), to fortnightly (spring-neap) timescales. In this chapter, we explain
the origin of the tides, and how tides evolve as they propagate over shelf
sea regions. It covers methods of analysing and predicting the tides, and how
tides can be used to generate electricity through arrays of tidal stream devices,
and tidal range schemes (lagoons). The primary objective of this chapter is to
equip those working within or researching marine renewable energy with an
understanding of the fundamentals of tidal energy from both oceanographic and
engineering perspectives.

3.1 TIDE GENERATING FORCES
Newton’s Law of Universal Gravitation explains how every object in the uni-
verse attracts every other object with a force that is proportional to the product
of their masses, and inversely proportional to the square of their distance apart
m 1 m 2
F = G (3.1)
r 2
where F is the force between two masses m 1 and m 2 , G is the gravitational
2
3
constant (6.674 × 10 −11 m /kg per s ), and r is the distance between the
centres of the masses (Fig. 3.1).
In the case of the Earth-Moon system, the Earth and Moon orbit each other
about their common centre of gravity, and so the gravitational attraction is
1
balanced by the outward directed centrifugal force (Fig. 3.2). Because the mass
of the Earth is two orders of magnitude greater than the mass of the Moon,

1. The outward directed centrifugal force is an apparent force, whilst centripetal force, the
component of force acting on a body in curvilinear motion that is directed towards the centre
of the axis of rotation, is an actual force.
Fundamentals of Ocean Renewable Energy. https://doi.org/10.1016/B978-0-12-810448-4.00003-3
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