A turbines-module adapted to the marine site for tidal farms layout optimization

Abstract

The ocean energy exploitation is arousing growing interest in the renewable energy sector. In theshort term, horizontal axis tidal turbines are the most promising technology due to the accumulatedknow-how in the field of wind energy. In order to maximize the performance of the devices in acluster, it is essential to optimize the layout. The marine environment offers different conditions thanatmospheric situations, in terms of confinement and turbulence intensity. Moreover, tidal currentsexhibit a highly predictable pattern in speed intensity and direction unlike the wind resource, whichhas a more random behaviour. Nonetheless, most of tidal sites are characterized by the inversionof flow where the two prevailing directions are not perfectly aligned and opposite, hence the anglebetween those directions should be a design variable. In this work we will consider as a case studythe site proposed in [1], where this angle is ±20°.For those sites with a flow inversion of almost 180°, the staggered configuration is preferable toavoid wakes interference as mentioned in [2]. Furthermore, many studies [3] had analysed positiveinteraction between neighbouring devices in a cluster, hence it is important to establish the optimalrelative position accounting for fluid dynamic positive effects, and not only negative aspects suchas wake interactions. For this reason, in this work we present a novel approach to determine thebest configuration of a cluster of few turbines, a ”module”, which will be the optimized ”buildingblock” for the whole farm. The procedure to be followed consist of two phases in which boththe characteristics of the site and those of the turbine are taken into consideration. To place thedevices in an optimal configuration, we first consider the change of flow direction during the tidalcycle for the site of interest, allowing only those configurations which avoid wake interference forboth prevailing flow directions; then, we assess the best layout by exploiting positive interactionsbetween devices in the cluster. The mutual fluid dynamic influence is analysed by means of a 3DBlade Element Momentum model of the turbine [4] implemented in the Open Source SHYFEMcode. A series of simulations is performed to outline the power production trend of the module, andconsequently find the optimal distancing between the machines. CFD simulations are also used toextract the module wake characteristics.