Renewable energy from marine currents

This page describes the activity of the “Tidal Energy Team”.

We are fully convinced that the sustainable exploitation of waves, tides, thermal and salinity gradients in the Oceans can represent the “next big thing in energy,” as the experts from Ocean Energy Europe claim. With a nearly unlimited capacity potential, marine resources are expected to provide in the short term a significant contribution to the renewable energy mix. Particular interest is about the exploitation of the kinetic energy of marine (notably, tidal) currents. Instream turbines fixed to the seabed or supported by floating platforms represent the most mature technologies in the marine renewables sector, with a long list of pre-commercial projects. An installed capacity of 2.4 GW by 2030 increasing to 60-70 GW by 2050 is considered a realistic scenario. The achievement of these objectives implies technology advancements in many multidisciplinary sectors, to deliver improved efficiency, reliability, maintainability, environmental and social sustainability of the devices that are installed offshore.

The Institute of Marine Engineering contributes to the tidal energy sector with research activities that are intended to increase basic knowledge on the mechanisms of energy capture and conversion and to develop advance modelling tools as well as prototype testing techniques.

In this context, the “Tidal Energy Team” carries on research on hydrokinetic energy systems for all kinds of water streams like tidal, ocean, river currents, with primary focus on the following topics:

• Turbine hydrodynamics modelling
• Power conversion and control
• Turbine design, operation and energy production in tidal sites

The activities in these areas are integrated into two open laboratories, for the realization of the ULYSSES-1 physical model turbine and of the OCEANUS LINK digital platform, as described below.

Our activity on marine renewable energy is inspired to a multi-disciplinary approach and we are keen on sharing our knowledge and tools and collaborating with developers and researchers with complementary expertise.

The modelling of turbine performance and of device interaction with the surrounding flow is grounded on more than two-decades experience in the development of theoretical and computational models for the hydrodynamic analysis of marine propulsion systems. For tidal energy applications, these tools have been specialized to describe the fluid-dynamics mechanisms that have a major impact in the operation of instream tidal turbines. The result is a variable-fidelity approach, that allows users to select the most suitable analysis tool with respect to the objectives and constraints of the problem of interest. Specifically, three-levels of model complexity are available by in-house developed tools.

Simple and Fast: Boundary Integral Equation Model (BIEM) for inviscid flows:

BIEM is a well established methodology for the preliminary analysis and design of marine propellers. The our long-dated experience in this area has been transferred to hydrokinetic turbines. The Bee-M solver provides a fully-3D, unsteady flow methodology for inviscid flows around a single turbine in arbitrary (steady or unsteady) prescribed onset flow. Velocity shear in the water column or surface waves perturbations can be simulated. The loading distribution on the blade surfaces is evaluated and can be used as input for blade strenght analysis, cavitation risk assessment, noise emission characterizations. The methodology includes an original scheme to account for viscosity effects on blade loads. Realiable predictions of turbine performance over a full TSR range are obtained at costs that are comparable to those of less general Blade Element Models (BEM). See Salvatore et al. (2018) for details.

Full CFD: Navier-Stokes solver for viscous turbulent flows:

The general-purpose Navier-Stokes solver Chi-Navis is the result of about 3-decades of activity in our Institute to develop and continuously improve a CFD tool that can be applied to a wide range of marine engineering problems. This solver represents a powerful tool to perform physically-consistent simulations of energy capturing systems in the viscous, turbulent flow of a tidal site, by including the effects of the free-surface, bathimetry, surrounding obstacles. The user can choose among turbulence models, DES and LES schemes. The solver adopts a sophisticated overlapping grid methodology that reduces the grid generation burden. Numerical solutions are performed by OpenMP and MPI API parallelization. Applications include turbine flow and performance predictions in Dubbioso et al. (2019) and a novel approach to model tidal flow turbulence, in Gregori et al. (2022). Visit the Chi-Navis project for more details.

Affordable CFD: hybrid viscous/inviscid solver for engineering applications:

For many engineering applications, the cost and time necessary for a full CFD simulation is not affordable. In order to deliver accurate flow field solutions by minimizing the computational effort, a combined Navier-Stokes/BIEM methodology has been developed by integrating Chi-Navis and Bee-M into a single hybrid CFD tool. The viscous turbulent flow surrounding the turbines is solved by the Navier-Stokes model, whereas the 3D, unsteady perturbation induced by the turbines is predicted by BIEM and plugged into the Navier-Stokes equations as volume forces. The flow field solution is obtained through an iterative procedure. This way, turbine blades are not solid boundaries in the CFD domain and hence volume grids have a much simpler topology and smaller number of cells. The computational advantage of the hybrid CFD solver is apparent for applications to complex turbine layouts and arrays. See Salvatore et al. (2017) for details.

In order to complete the hydrodynamics modelling suite, the geometry manipulation tool Autogrid, specifically designed for marine turbines, has been developed. Surface grids used by BIEM and volume grids for both hybrid N-S/BIEM and full N-S solvers, are built by a fully automated process for surface discretization and volume meshing. The user controls the mesh quality through a simple set of input parameters. This way, the typical barrier to reliable CFD simulations represented by the grid generation task is circumvented, and quality grids can be built by a fast and controlled procedure.

The hydrodynamic design of turbine blades matching developer’s requirements is obtained through recursive calculations in which an initial guess shape is iteratively improved. An original design algorithm has been developed in which three modules are combined:

• Blade geometry manipulation, by the Autogrid solver
• Turbine hydrodynamics performance prediction, by the Bee-M solver
• Turbine performance analysis against objectives and constraints and update of blade geometry parameters

In the present formulation, design parameters control the radial distributions of blade pitch and chord. The design objective is to maximize the power output with a constraint on the thrust that is generated at high TSR, to avoid excessive loading on turbine components. The results are parametrized for a user’s defined range of design TSR values. As output the 3D model of the turbine is printed in various formats, including STL for rapid prototyping.

For a new-design or an existing turbine, the evaluation of the Annual Energy Production (AEP) for a given tidal site and turbine installation conditions can be obtained by the Sitesim module. Site resource data are characterized through the velocity probability density over a relevant time period, site depth, vertical velocity profile. By assuming a Maximum Power Point Tracking (MPPT) control in the variable speed range and a user defined condition in the overspeed range, turbine operational profiles for both mechanical and electrical components are determined. See Sarichloo et al. (2022) for details.

The full characterization of a tidal energy system implies that the study of energy capture (hydrodynamics) is combined with the study of the conversion of mechanical energy into electricity. This requires the modellization of the Power Take-Off (PTO) system as a single system integrating key components as the main shaft, the gearbox (if any), the bearings, the generator and the power electronic convertor. In most cases, mechanical/electrical power conversion of tidal turbines follows well proven technologies derived by the mature wind-energy sector. However, the peculiarities of the tidal energy resource impose ad-hoc technology solutions. A significant difference between wind turbines and tidal turbines is that the devices have to be free of maintenance service for long periods (about 5 year), because access to the turbine is extremely expensive at sea. This lead to design a simple, robust, and well proven Power Take-Off (PTO) system which is asked to have good efficiency, high reliability, low cost and long service life with affordable maintenance requirements. See Rafiei et al. (2020) for more information about state-of-art PTO configurations.

Our activity deals with the development of computational models in Matlab/Simulink to simulate the PTO dynamic response and propose solutions for improved efficiency, reliability, low cost and long service life with affordable maintenance requirements.

Our Institute has a consolidated experience in the integration of computational modelling with experimental studies that are carried out in our world-class testing facilities.

In the tidal energy sector, we have been partner of the Tidal Turbine Round Robin Test initiative in the framework of the EU-FP7 Marinet and H2020 Marinet-2 projects. A first-of-its-kind model-scale test campaign which has delivered new knowledge on the hydrodynamics of tidal turbines and a unique open dataset of experimental data for the validation of CFD models.

In parallel to this collaborative effort, our team is working at the realization of fully-equipped model scale turbines for research activity. The first realization is a 3-bladed horizontal axis, direct-drive turbine with a 3 kW Permanent Magnet Synchronous Generator (PMSG). The rotor hub is designed to host blades with variable size corresponding to rotor diameters between 650 and 750 mm. The nacelle is designed to allow for both ground based or floating installation. The model turbine represents a physical lab aimed at increasing knowledge on the operation of tidal turbines. Model test results will be also processed to deliver open-access experimental datasets for the validation of CFD models.

The hydrodynamic analysis and design models, the geometry manipulation tools, the AEP model, have been structured as elements that interact within a comprehensive computational platform. The step forward is the exploitation of this software as an online platform with open access by registered users through a standard web browser.

Oceanuslink is the name given to the online platform. It will provide:

• Computational tools for the study of marine currents energy systems
• Documentation to describe the methodologies underlying the computational tools
• Communication tools to stimulate interaction among users

Django (Python) is used for the development of the user interfaces, databases of user projects, multi-user access functionalities, input data repositories, as well as communication protocols between the solvers. Communication among users will be possible via forums and internal chatting. The ambition is to deliver Oceanus link as the first open access platform for a community of users exchanging data, experience, ideas in the marine currents energy sector.

• Francesco Salvatore,
• Danilo Calcagni,

• Sepehr M. Rafiei,
• Zohreh Sarichloo,
• Pedram Ghorbanpour,
• Matteo Gregori,