Tidal stream turbines take a battering from turbulent flow and waves; they may even be hit by marine life and floating debris. Designers must be able to predict these extreme loads so that their turbines can generate electricity and withstand impacts over a 20- to 30-year operating life.
“Turbine designers must account for the stresses and strains of loads on the turbine blades and other components,” remarks Dr Tim Stallard, an academic in the Department of Mechanical, Aerospace and Civil Engineering. “Blade loads directly influence energy yield and lifespan. It is extremely difficult to predict the complex flow patterns caused by tidal currents and waves – and to model the loading due to these flows and impacts.”
Several turbine developers conduct offshore trials to evaluate prototype designs. “We are developing advanced computer models to simulate the conditions that occur at tidal stream sites,” says Dr Stallard. “The models will show how complex unsteady flows and impacts affect turbine loads.” Today these models are being evaluated against field trials and experiments – such models will be used to inform the next generation of turbine designs.
The University of Manchester is a world leader in CFD and turbulence modelling, techniques which it applies to marine environments for tidal turbine design. Two research projects - ‘Extreme Loading of Marine Energy Devices due to Waves, Current, Flotsam and Mammal Impact’ (X-MED) and ‘Reliable Data Acquisition Platform for Tidal’ (ReDAPT) – are developing computational fluid dynamics (CFD) methods. These models simulate the depth and time variation of unsteady tidal flows and their effects on turbine loads.
ReDAPT researchers are now evaluating the accuracy of these models. Predictions from CFD are being compared to load measurements obtained from a full-scale field trial of an 18m diameter turbine that is owned and operated by Alstom Ocean Energy. “Early findings are promising,” Dr Stallard reports.
Alongside the CFD work, a numerical methodology called Smooth Particle Hydrodynamics is being further developed to simulate what might happen if rigid and deformable debris and other floating objects hit turbine blades. Manchester scientists are collaborating with other institutions to conduct large-scale tank tests with a 3-bladed turbine to validate these methods.
CFD is also being used to model downstream flows in turbine arrays. The X-MED and Performance Assessment of Wave and Tidal systems (PerAWaT) projects look at how a turbine can disrupt the natural flow of water and alter downstream velocity and turbulence – which impact yield and loading of nearby turbines. Tank tests have also been conducted at Manchester to evaluate software that models energy yield from turbine farms - a crucial consideration for investors who depend on the revenue from energy yield for a return on their investment.
Project name: Extreme Loading of Marine Energy Devices due to Waves, Current, Flotsam and Mammal Impact (X-MED)
Lead investigator: Tim Stallard
Research group: Tidal stream turbines
Dates: February 2012 – January 2015
Dates: January 2009 – June 2014
Funding: EDF (as part of the ETI commissioned project ReDAPT)
Partners: Alstom Ocean Energy, EON, EDF R&D, DNV GL, University of Edinburgh and Plymouth Marine Labs
Dates: October 2009 – June 2013
Funding: Energy Technologies Institute (ETI)
Partners: DNV GL, EDF R&D, EON, University of Edinburgh, University of Oxford and Queens University Belfast
The models will show how complex flows and impacts affect turbine loads.