Optimisation of tidal stream turbines array configurations

Tidal energy is on the verge of commercial viability and full scale prototypes are meeting the challenges of the marine environment. The primary focus of the sector has concerned Horizontal Axis Tidal Turbines (HATTs); comprising a turbine supported by a tubular stanchion operating on a bi-directional, or yaw system. The direction of tidal flow, however, varies over the ebb or flood phases of a tidal cycle. This pa-per utilises tidal velocity data measured in Ramsey Sound (Pembrokeshire, Wales, UK), a macrotidal strait and proposed HATT installation site and combines with Computational Fluid Dynamics (CFD) to assess the impact of misalignment between a HATT and its surrounding free stream velocity. The majority of the veloci-ties within the northern area of Ramsey Sound tend to fall within a misalignment of ±20° for velocities greater than the economic viable threshold of 2 ms-1. However, bathymetry and coastline configuration influence both flow magnitude and direction. At the outer margins for the Sound, the velocities are acted upon by various promontories, reefs and shelving areas, which deflect and retard the flow, resulting in a flow direction greater than 20°, particularly towards the outer edges of the Sound. Utilising field data for numerical simulations will help inform the industry and increase investor confidence in this technology, whilst avoiding costly scaled ex-perimentation. It was found that an axial flow misalignment of ±10° results in approximately a 7% reduction in peak power, 3% in peak torque and 5% in peak thrust. The axial wake recovery length was shorter for the ±10° cases, recovering to 90% by 7D downstream, as opposed to 10D downstream for the aligned turbine.

Many coastal areas around the UK are configured such that tidal currents are accelerated by headlands and islands making them attractive tidal energy sites. Efforts to reduce energy dependency on fossil fuels have prompted examination of these macrotidal and nearshore sites for potential tidal-stream resource development. A number of prospective tidal energy sites have been identified around the UK, however, little is known about the specific, and often complicated, hydrodynamics in many of these systems. Understanding the ways in which coastal features affect tidal velocities at potential tidal stream turbine (TST) development sites will help understand the resource and identify potential constraints on tidal energy output and the key physical parameters to consider when identifying suitable sites for tidal stream turbines.
This paper presents and examines tidal velocity data measured in Ramsey Sound (Pembrokeshire, Wales, UK), a macrotidal strait that accommodates circulation from the Irish Sea with peak spring tide velocities exceeding 3 m s-1. Although this coastal area is soon to become the first tidal-stream turbine (TST) demonstration site in Wales, little known about the intricacies of the local tidal dynamics.
Ramsey Sound exhibits a marked flood-dominated asymmetry. Furthermore, local bathymetric features affect flow fields that are spatially heterogeneous in three dimensions, patterns that depth-averaged velocity data (measured and modelled) tend to mask. By examining physical characteristics including flow velocities across the blade sweep (15 m diameter) of a tidal-stream turbine (TST), estimated power output, water depth, and bed slope, it is shown that spatial and temporal variability in the flow field renders much of Ramsey Sound unsuitable for tidal power extraction. Although first-order appraisals may make energetic tidal straits attractive sites for development, realizing their resource potential depends on flow conditions that are fundamentally local. Tidal-energy developers must therefore rely on detailed characterisation of tidal energy sites prior to device field trials and installation.

A procedure for the optimisation of hydrokinetic turbine array layout through surrogate modelling is introduced. The method comprises design of experiments, computational fluid dynamics simulations, surrogate model construction, and constrained optimisation. Design of experiments are used to build polynomial and Radial Basis Function surrogates as functions of two design parameters: inter-turbine longitudinal and lateral spacing, with a view to approximating the capacity factor of turbine arrays with inline and staggered layouts, each of which having a fixed number of turbines. For this purpose, two scenarios have been used as case studies, considering uniform and non-uniform free-stream flows. The major advantage of this method in comparison to those reported in the literature is its capability to analyse different design parameter combinations that satisfy optimality criteria in reasonable computational time, while taking into account complex floweturbine interactions and different turbine types.

This paper presents a study for estimating the size of a tidal turbine array for the Faro-Olhão Inlet (Potugal) using a surrogate optimization approach. The method compromises problem formulation, hydro-morphodynamic modelling, surrogate construction and validation, and constraint optimization. A total of 26 surrogates were built using linear RBFs as a function of two design variables: number of rows in the array and Tidal Energy Converters (TECs) per row. Surrogates describe array performance and environmental effects associated with hydrodynamic and morphological aspects of the multi inlet lagoon. After validation, surrogate models were used to formulate a constraint optimization model. Results evidence that the largest array size that satisfies performance and environmental constraints is made of 3 rows and 10 TECs per row.