Page 306 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Other Aspects of Ocean Renewable Energy Chapter | 10 291

than the rotor, possibly even the full water column. Drag on the support structure
serves to reduce the flow without generating any power [36]. Whereas a depth-
averaged representation of the support structure may be sufficient in a 3D model,
the rotor itself may require a 3D parameterization in the model (e.g. [40]).
Although it is possible to tune modelling methodologies for tidal energy
extraction, datasets are only available for scaled laboratory experiments of
turbines at present. Once large-scale arrays are commissioned, and data are
made publicly available, it will be possible to validate these techniques against
appropriate field data.

Wave Models
In contrast to simulating the tidal energy resource (e.g. [31]), the corresponding
IEC technical specification for wave resource assessment (IEC 62600-101
TS) [41] does not provide specific guidance on when to consider WEC
device/array feedbacks in models that simulate the wave energy resource.
Rather, the academic community tend to focus on how wave energy extraction
could influence the nearshore wave climate from an environmental (impact)
perspective. However, IEC 62600-101 TS does state that when it is considered
appropriate to include the effects of the WEC array on wave propagation in the
numerical model, any modifications made to the numerical model to account for
the effects of a WEC array should be documented and justified.
In a fairly preliminary study, Neill and Iglesias [42] made use of a 1D
cross-shore wave model (UNIBEST-TC) to simulate the nearshore impacts of
wave energy extraction, with a focus on subtidal bars. They used wave buoy
observations coincident with the model boundary to create a joint probability
distribution of wave period and wave height, then simply calculated the ‘deep
water’ wave power, and reduced this power by an appropriate amount (e.g. 10%)
to represent the presence of a WEC array, simulated by a proportional reduction
in wave height and period at the model boundary. Although 1D cross-shore wave
models can be insightful, it is more common to make use of 2D (spectral) wave
models for resource assessment, such as the SWAN model that was described in
Section 8.4.
SWAN has an OBSTACLE command that can be used to input character-
istics of a (line of) subgrid obstacles through which waves are transmitted/re-
flected [43]. By application of this OBSTACLE command, several studies have
attempted to account for the presence of WEC arrays in the wave model (e.g.
[44]). The transmission coefficient (K t ) is defined as the ratio of H st (wave height
in the lee of the wave array) to H si (incident wave height)

H st
K t = (10.8)
H si
and acts as an energy sink in the wave action balance equation (8.39), extracting
a fraction of the incident wave energy to represent a wave energy converter.
An example K t measured under wave tank conditions for the WaveCat device