Abstract
The contribution of wave energy to the renewable energy supply is rising. To extract a considerable amount of wave power, Wave Energy Converters (WECs) are arranged in several rows or in a 'farm'. WECs in a farm are interacting (e.g. the presence of other WECs influence the operational behaviour of a single WEC) and the overall power absorption is affected. In this paper wake effects in the lee of a single WEC and multiple WECs of the overtopping type, where the water volume of overtopped waves is first captured in a basin above mean sea level and then drains back to the sea through hydro turbines, are studied using the time-dependent mild-slope equation model MILDwave. The wake behind a single WEC is investigated for long-crested and short-crested incident waves. The wake becomes wider for larger wave peak periods. An increasing directional spreading results in a faster wave regeneration and a shorter wake behind the WEC. The wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, with varying lateral and longitudinal spacing. The wave power redistribution in and behind each farm lay-out is studied in detail using MILDwave. In general, the staggered grid results in the highest overall wave power absorption.
Highlights
Several wave energy converters (WECs) have been intensively studied and developed during the last decade and currently small farms of WECs are getting installed
The power absorption of each individual WEC in a farm is affected by the wakes of its neighbouring WECs
In the latter study reflection and transmission characteristics are coupled through the degree of porosity of the structure, which makes the adaptation of the absorption characteristics of a WEC in a farm to the incident wave climate impossible
Summary
Several wave energy converters (WECs) have been intensively studied and developed during the last decade and currently small farms of WECs are getting installed. In this paper the wake behind a single WEC and multiple WECs based on the overtopping principle, where waves overtop in a basin above mean sea level and where the stored water drains back to the sea through hydro turbines (Cruz 2008), is studied in a time-dependent mild-slope equation model MILDwave, developed at Ghent University (Troch 1998) In this phase-resolving model each combination of reflection and transmission characteristics, and absorption characteristics, can be modelled for all individual WECs in a farm according to the methodology presented in Beels et al (2010). The shape of the absorption function S(y) through the WEC (when the direction of wave propagation = y-direction) is changed (Beels et al 2010, 2010b) This way the degree of absorption (and transmission) of the WEC, given in the power matrix of the WEC, can be tuned for a fixed amount of reflection on the WEC as specified by the developer. Waves with a peak period of 5.2 s and a significant wave height of 1 m have the highest frequency of occurrence in the southern part of the North Sea and are considered in this study as the basic case
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