Abstract
This paper analyses two different configurations of horizontal axis Tidal Stream Turbines (TSTs) using a Blade Element Momentum Theory (BEMT) model. Initially, a ‘conventional’ three bladed and bare turbine is assessed, comparing against experimental measurements and existing literature. Excellent agreement is seen, increasing confidence in both the implementation of the theory and the applicability of the method. The focus of the paper lies on the analysis of a ducted and open centre turbine. An analytical adjustment to the BEMT model is applied, using empirical expressions detailed in the literature which are devised from Computational Fluid Dynamics (CFD) studies. This is modified to a symmetrical duct profile, calibrating certain geometrical parameters against blade resolved CFD studies of a bi-directional device. The results are validated with a coupled CFD blade element model (RANS BEM), where both models align very closely (within 2%) for most tip speed ratios (TSRs), including the peak power condition. Over predictions are seen at higher TSRs of up to 25% in power and 13% in thrust at TSR = 5, due to model limitations in replicating fully the complex flow interactions around the hub and the open centre. The presented approach benefits from significantly lower computational requirements, several orders of magnitude lower than reported in the RANS-BEM case, allowing practicable engineering assessments of turbine performance and reliability.
Highlights
15 Figure [1] Andritz Hydro Hammerfest 1.5MW rated Tidal Stream Turbines (TSTs) with installation into the Pentland Firth, Scotland as part of the MeyGen Phase 1A deployment The second is a high solidity, ducted and open-centre turbine design
The tip/hub loss describes the reduction in hydrodynamic efficiency along the blade, becoming more influential towards the tip and hub as per its definition. The magnitude of this efficiency decreases with tip speed ratios (TSRs), 424 where the tip losses are clearly more significant in all cases considered
577 An analytical model which aims to characterise the effects of flow through a duct as a function of the inlet efficiency, diffuser efficiency and base pressure is considered. Empirical expressions for these parameters are formulated in the literature, based on Computational Fluid Dynamics (CFD) studies of various different unidirectional ducts, as functions of numerical coefficients and duct geometry
Summary
15 Figure [1] Andritz Hydro Hammerfest 1.5MW rated TST (left, image credit: Atlantis) with installation into the Pentland Firth, Scotland (right, image credit: MeyGen) as part of the MeyGen Phase 1A deployment The second is a high solidity, ducted and open-centre turbine design. High fidelity models are commonly used in design refinement, or to perform detailed assessments of turbine components under specific operating conditions. These can be used to determine wake formation to measure the impact of the turbines on the tidal flow, as well as to describe the interactions of multiple turbines in an array. The availability of such models for ducted, high solidity and open centre turbines is limited At present, these types of devices are analysed using blade resolved CFD, which has a high computational requirement and is not practical for multiple calculation applications. Less computationally intensive alternatives have been applied (Fleming et al 2011; Turnock et al 2011; Belloni et al 2016) based on a coupled Reynolds Averaged Navier Stokes with blade element momentum (RANS-BEM), where case studies report good comparison with fully blade resolved studies, at a fraction of the processing time (McIntosh et al 2012)
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