Page 259 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

248 Fundamentals of Ocean Renewable Energy

first row. Of course, flume experiments such as this are unidirectional, and so
bidirectional (tidal) flows will require further consideration, for example, the
‘first’ row during the flood phase of the tidal cycle will become the ‘last’ row
during the ebb.
Apart from potentially making use of regions of accelerated flow to strate-
gically site devices (as discussed above), it is clearly advantageous, from
both economic and practical perspectives, to minimize lateral and longitudinal
extents of tidal energy arrays. Many leased tidal energy sites are situated in
relatively narrow straits, where the principal flow dimension is much greater
than the lateral channel dimension. Therefore, due to physical and navigational
constraints, a more compact lateral array configuration (and hence spacing)
is desirable. Excessive longitudinal spacing will significantly increase subsea
cable lengths (e.g. the range of 5D–15D longitudinal spacing to avoid wake
effects equates to 100–300 m for a 20 m diameter turbine), but such potentially
increased cabling costs would depend on the design of the subsea connections
and cable route between the devices and the substation (e.g. [12]).
Blockage
The theoretical upper limit to power extraction by a wind turbine is constrained
by the Betz limit (C p = 0.59), where C p is the rotor power coefficient (see
Section 3.13.1). However, in wind energy conversion, the cross-sectional area
(CSA) of the turbine is very small in comparison to the CSA of the wind
resource. By contrast, when tidal turbines are placed in a tidal channel, the
2
turbine blockage ratio increases, resulting in a theoretical C p that can consider-
ably exceed the Betz limit for array scales/configurations that are characterized
by high blockage ratios [13]. However, the situation is complicated by the fact
that by increasing the blockage ratio of a channel, there will be a corresponding
reduction in the free-stream flow due to the increased drag resulting from tidal
energy conversion. In other words, placing too many turbines in a tidal channel
will merely block the flow.
To maximize turbine efficiency (i.e. the power available per turbine), tidal
arrays should occupy the largest possible fraction of a channel’s CSA [14].
However, practical constraints, such as navigation and the passage of marine
life, will require gaps between the turbines in an array. In such a scenario, energy
is dissipated in the wakes, and so the turbines are less effective than if they were
3
deployed in a fence that spans the entire channel [15].
An interesting extension of the work on simple tidal channels (e.g. [15]) is
application to split channels [16]. Split channels are fairly common amongst
tidal energy sites—for example, the Bay of Fundy, Puget Sound, and the

2. The ratio between the total swept area of the array to channel CSA.
3. The assumption of a complete fence is equivalent to assuming that the channel CSA is occupied
by a single turbine [15]—a slightly unusual concept, but one that can be useful for analysis.