Tidal devices exploit kinetic
energy in the tidal flow of water caused by the interaction of the
gravitational fields of the earth, moon and sun. Tidal currents are
magnified by topographical features, such as headlands, inlets and
straits, or sea bed channels. Many of the devices that use these
currents are broadly similar to submerged wind turbines. Sea water
is around 800 times denser than air but average flow speeds for
viable projects are of the order of one fifth of corresponding wind
speeds, giving a power density of around 5-10 times. This means
that for a tidal device of the same rating as a wind turbine, the
rotor will be smaller. Individual tidal devices are unlikely to
reach the same rated power as offshore wind turbines; however, as
to have sufficient space to deploy such large devices requires
development of deeper water sites that generally have less
significant tidal flows. EMEC has identified four main types of
tidal energy convertors that are outline below.
Tidal energy device concepts
(graphics courtesy of EMEC).
The flow of water turns the rotor
by generating lift due to the flow around the blades. This
rotational movement is used to generate electricity. The
device can be housed within a duct to accelerate the flow through
the rotor, thus increasing energy capture.
This device extracts energy in a
similar way to a horizontal axis turbine, but the axis of rotation
of the rotor is vertical and perpendicular to the flow of
water. Some vertical axis turbine concepts work by the
principle of drag rather than lift.
A hydrofoil is attached to an
arm. The flow of water around the hydrofoil causes it to
oscillate by generating lift. This oscillating movement is
used to generate electricity.
A duct has the effect of
accelerating the flow through the narrowest section. This
flow causes a pressure differential that drives an air
turbine. The rotational movement of the turbine is used to