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 the
short term, horizontal axis tidal turbines are the most promising technology due to the accumulated
know-how in the field of wind energy. In order to maximize the performance of the devices in a
cluster, it is essential to optimize the layout. The marine environment offers different conditions than
atmospheric situations, in terms of confinement and turbulence intensity. Moreover, tidal currents
exhibit a highly predictable pattern in speed intensity and direction unlike the wind resource, which
has a more random behaviour. Nonetheless, most of tidal sites are characterized by the inversion
of flow where the two prevailing directions are not perfectly aligned and opposite, hence the angle
between those directions should be a design variable. In this work we will consider as a case study
the site proposed in [1], where this angle is ±20°.
For those sites with a flow inversion of almost 180°, the staggered configuration is preferable to
avoid wakes interference as mentioned in [2]. Furthermore, many studies [3] had analysed positive
interaction between neighbouring devices in a cluster, hence it is important to establish the optimal
relative position accounting for fluid dynamic positive effects, and not only negative aspects such
as wake interactions. For this reason, in this work we present a novel approach to determine the
best configuration of a cluster of few turbines, a ”module”, which will be the optimized ”building
block” for the whole farm. The procedure to be followed consist of two phases in which both
the characteristics of the site and those of the turbine are taken into consideration. To place the
devices in an optimal configuration, we first consider the change of flow direction during the tidal
cycle for the site of interest, allowing only those configurations which avoid wake interference for
both prevailing flow directions; then, we assess the best layout by exploiting positive interactions
between devices in the cluster. The mutual fluid dynamic influence is analysed by means of a 3D
Blade Element Momentum model of the turbine [4] implemented in the Open Source SHYFEM
code. A series of simulations is performed to outline the power production trend of the module, and
consequently find the optimal distancing between the machines. CFD simulations are also used to
extract the module wake characteristics.