Page 263 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

252 Fundamentals of Ocean Renewable Energy

to minimize such destructive interferences through optimization of various
parameters such as array geometry and device spacing.

Smoothing Power Fluctuations
Wave energy comes in pulses, and so is unsuitable for direct conversion and
transmission to the electricity grid [22]. The addition of devices to an array
reduces the variance in power (Fig. 9.9A); however, at the expense of reduced
average power per WEC due to the park effect (Fig. 9.9B). In sea trials with
WECs, Rahm et al. [22] found that there was an 80% reduction in the standard
deviation of electrical power output from three WECs, compared with the
standard deviation from a single WEC. Based on model simulations of a larger
number of WECs [21], normalized variance of power Var (where variance
2
P
2
Var = σ and σ is the standard deviation) reduced from ∼0.9 to ∼0.4 for 4
to 64 WECs, respectively (Fig. 9.9A).
Geometry of WEC Layout
It is possible to further smooth power fluctuations through careful consideration
of the geometric WEC layout [22,23]. It has been shown that ‘global’ array
layouts can increase power by 5% or reduce power by 30% [24]. Göteman et al.
[25] simulated four global geometries, each with the same number of WECs
(Fig. 9.10). Device spacings for the different configurations varied within the
range 20–55 m, other than for the ‘random’ layout, where the minimal device
spacing was set to 6 m. Because the incident waves propagate in the x-direction,
clearly those devices at the rear of each array (increasing x coordinate) absorbed
less power. However, it was not always the first row that absorbed the most
power; for the rectangular configuration, it was the third row that absorbed most
power, demonstrating a positive interference effect from the other WECs in the
array. All four configurations resulted in similar energy absorption (Fig. 9.11A);
however, power variance differed significantly between the four cases: ‘rectan-
gular’ and ‘random’ layouts led to the highest and lowest variance, respectively
(Fig. 9.11B). Engström et al. [26] investigated WEC array layouts that were
only slightly randomized, that is, to represent uncertainty in mooring positions
and temporal drift, again finding a reduction of variance in these more realistic
‘randomized’ layouts. This could be explained by considering regular WEC
layouts that are aligned with the dominant wave direction (e.g. Fig. 9.10B).
In this case, the electricity produced by all of the WECs distributed along the
wave crest will generally be in phase for each WEC row. Staggering the devices
slightly in the direction of wave propagation would introduce more phase
diversity into the array, leading to more stable (less variable) aggregated power
output. Minimizing the lateral dimension of the WEC array, and maximizing
the longitudinal dimension, will have a similar effect, provided the longitudinal
spacing is considered, in conjunction with the expected wavelengths. For
example, a WEC array of 4 × 5 has lower variance if there are five rows along
the direction of wave propagation, as compared to four [21].