Kinetic energy extraction of a tidal stream turbine and its sensitivity to structural stiffness attenuation

Highlights

Blade deflection will alter the power output.

Blades could be designed to reach their optimum setting at a given deflection.

Overall performance sensitivity to deformation is low but should not be neglected.

Abstract

The hydrodynamic forces imparted on a tidal turbine rotor, whilst causing it to rotate and hence generate power, will also cause the blades to deform. This deformation will affect the turbine’s performance if not included in the early design phase and could lead to a decrease in power output and a reduction in operational life. Conversely, designing blades to allow them to deform slightly may reduce localised stress and therefore prolong the life of the blades and allow the blades to deform in to their optimum operational state. The aim of this paper is to better understand the kinetic energy extraction by varying the material modulus of a turbine blade. Shaft torque/power, blade tip displacement, and axial thrust results are presented for 2, 3 and 4 bladed rotor configurations at peak power extraction. For the rotor design studied the FSI model data show that there is a low sensitivity to blade deformation for the 2, 3 and 4 bladed rotors. However, the results reveal that the 3 bladed rotor displayed maximum hydrodynamic performance as a rigid structure which then decreased as the blade deformed. The 2 and 4 bladed rotor configurations elucidated a slight increase in hydrodynamic performance with deflection.

Keywords

  • CFD;
  • FEA;
  • Tidal stream turbine;
  • FSI;
  • TST

1. Introduction

The EU has targeted renewable Energy to provide 20% of the total energy mix by 2020 [1] in comparison the UK target is 15% of the UK energy demands from renewable sources by 2020 [2]. From statistics published by the Department of Energy and Climate Change (DECC) since 2008, wind energy has been the largest contributing renewable source to the energy mix. In 2012 a total of 8.8% of the UKs electricity demand came from renewable resources, more than half (5.48%) of which came from the wind sector [3]. The annual growth of the wind energy sector is significant, yet remains dependent on unpredictable wind conditions and hence a steady base load. In order to meet and sustain the targets set by 2020 and beyond, the UK and EU at large must continue to address the imbalance in the renewable energy mix.

To add to this renewable mix the potential for sustainable production through wave and tidal energy conversion has resulted in large investment from industry and governments. For example, the Carbon Trusts Marine Energy Accelerator program has identified a practical and economical resource of tidal current and wave energy resource of 70 TWh/yr around the UK coast, which would contribute to 20% of UK’s total industrial, commerce and domestic electricity demands based on 2012 usage [4].

The long term predictability of tides is the main advantage of tidal power over wind and solar, since it allows any phase change in power productions between wind and or other tidal stream and wave sites to be balanced. The two leading techniques in energy conversion for tidal range power generation are impoundment schemes such as a barrage or tidal lagoon. These techniques yield reward for large scale solutions, as documented in the La Rance Tidal Power Plant run by EDF Energy which produces 0.54 TWh/year. In the short term damage to the local ecosystem occurred however this was mainly during the construction stages [5]. Large capital investments and resulting ecological damage and aesthetic disturbance creates acceptance of such schemes difficult, although the tidal lagoon based in Swansea Bay, South Wales, UK has submitted an application for development which is still awaiting consent [6]. An alternative to tidal range power generation is via the use of a Tidal Stream Turbines (TSTs). These devices are optimised for their location, to maximise kinetic energy extraction from free flowing water. Being submerged a TST is by its nature significantly less intrusive than impoundment schemes and minimises impacts on the marine and costal environment. In spite of some TST designs involving support structures that penetrate the water’s surface this technique is still considerably less aesthetically intrusive than other large scale renewable energy technologies.

The introduction of TSTs into the UK energy mix can only be a positive step since the UK has some of the strongest currents in the world. Areas around Orkney Islands, Pentland Firth, Anglesey and Pembrokeshire, have been studied as viable sites for installing tidal energy devices [7]. Ideal conditions for tidal stream turbines are; a free stream velocity of 2–3 m/s and a depth of 20–30 m, at least for early stage implementation with deeper water designs (>40 m) introduced as the industry matures. Tidal Energy Limited is currently developing their DeltaStream device for 12 month test in the Ramsey Sound, Pembrokeshire. Marine Current Turbines (MCT) developed and installed the first commercially operating tidal stream turbine. The SeaGen S is a 1.2 MW capacity gull-wing horizontal axis TST installed in Strangford Lough, Northern Ireland since 2008. The 16 m diameter twin rotors positioned at either end of the supporting gull wing are pitch controlled. From environmental monitoring reports actively pitch controlling the blades limits the maximum rotational speed of the rotors to 14RPM [8]. The hydrodynamic forces required to produce such motion will also cause considerable loading on the blades which will result in blade deformation. The magnitude of the deformation will be dependent on the external and internal structure of the blades and the materials used.

2. Performance of 2, 3 and 4 bladed rotor

The aim of this work was to investigate, by the means of coupled 2 way FSI modelling, the extent of blade deformation and the associated changes in hydrodynamic performance characteristics. Note is made of the kinetic energy extraction sensitivity by varying the material modulus and hence the stiffness of the rotor structure. Although not discussed in this paper the axial loading and shaft torque data generated will be used as part of the development of a conditioned based monitoring system [9]. The data generated in this study and a further 2-way transient model will be subsequently validated using a lab scale rotor.

Generally when using Computational Fluid Dynamics (CFD) the hydrodynamics of a turbine blade are studied while in its undeformed state and therefore is considered as being rigid. While this approach might serve well at the prototype phase, it is unlikely to suit the operation of a large scale device. Blade deflection will depend on the internal structure, the materials used and the profile shape, for example long slender blades. Using a 3D coupled boundary element method and finite element method Yin [10] showed that when scaled to 20 m a rotor with long slender blades led to excessive blade bending and high localised stresses when run outside its optimal operational conditions. Therefore, when scaling a device to economically viable dimensions, numerical modelling can play an important role in informing engineers on the magnitudes of deflections and potential for stress raisers. . There are however a number of issues facing the development of turbine blades via numerical simulation, such as the number of development iterations and the time it takes to run the numerical simulations when using fully coupled 2 way FSI simulations. Numerical run-time, to some extent, is a product of processing speed and the accuracy required to capture the physics of the real system, the latter being dependent on the numerical approach.

There are a number of theories that can be used to model the hydrodynamic performance of TSTs with a good degree of accuracy. The most prominent of these is the Blade Element Momentum Theory (BEMT) which is a combination of the momentum and blade element theories, [11]. Resulting from their work on experimental verification of numerical predictions using a 0.8 m diameter rotor, Bahaj [12] and Batten [13] demonstrated that power and thrust measurements showed good correlation with those obtained using the BEMT. Barltrop [14] also used the BEMT to investigate the effects of waves on marine current turbines and their influence on blade root bending moments. Other approaches include the Vortex method which has the ability to include 3D flows. Malki et al. [15] have developed a coupled BEM–CFD model which is based on momentum source terms from a BEM model being fed into a RANS model. The authors state that where the incoming flow is non-uniform, as is likely in most practical cases, this method can give more realistic predictions than the classical BEM method.

Be the first to comment

Leave a Reply

Your email address will not be published.


*