Development of the next generation of controllers for wave energy devices

Project Description

This project is funded by a SFI Investigator Award and focusses on the development of nonlinear approaches to the modelling and control of wave energy devices. Oscillatory wave energy conversion (WEC) devices are required to operate in a wide range of sea states, presenting a variety of wave periods and amplitudes. WECs can be designed to resonate well at one particular frequency, but intelligent control systems must be employed to manipulate the power take-off (PTO) resistance so that the resonant frequency can effectively track the varying sea conditions. To date, linear WEC controllers have been employed to perform this adaptation. However, such controllers are based on linear hydrodynamic and PTO models, which obtain poor performance as oscillation amplitudes grow. In the wave energy case, the normal assumptions under which linearization is carried out (i.e. operation is maintained close to an equilibrium/linearization point) are violated, in part because of operation in high-energy seas and also because the objective of the controller is to exaggerate the motion of the device to resonance. This project aims to develop the next generation of wave energy controllers based on nonlinear hydrodynamic/PTO models which truly maximise wave energy capture. The maximisation of captured wave energy, through the application of intelligent control techniques, for a given device specification, is vital if wave energy is to become an economically viable form of renewable energy.  

Documents

Project Progress

75%

Project Timing

  • Start
    Oct 01 2014
  • End
    Dec 31 2020

10/01/2014 12/31/2020

79%

Control design for a hinge-barge wave energy converter

Project Description

The project is funded under the SFI MaREI Centre and is in partnership with the Wave Energy Corporation of America (WECCA).

This project will develop energy maximising control technology for hinge-barge wave energy converters. This will involve the development of suitable control-oriented hydrodynamic models, and the development of appropriate control, wave forecasting and excitation force estimation algorithms.

Documents

Project Progress

95%

Project Timing

  • Start
    Oct 01 2013
  • End
    Sep 30 2017

10/01/2013 09/30/2017

100%

Implementation of Latching Control in a Numerical Wave Tank

Project Description

The exploitation of wave energy resources represents an attractive opportunity to enlarge and diversify the renewable energy offer. Nevertheless, none of the existing technologies have achieved the complete economic viability to be competitive within the energy market. In order to increase the overall power production efficiency and face the sea state variability, the presence of the control is mandatory. A model of the system is required to tune the controller parameters and predict the resulting performances. Even though the relevance of nonlinearities is magnified by the presence of the controller, the device model employed is usually linear. An implementation of control in a fully nonlinear simulation model is desirable, but missing. Hence, this paper proposes to use the fully nonlinear Computational Fluid Dynamics (CFD) environment, implementing the latching control strategy in the open source software OpenFOAM.

Documents

  • Implementation of Latching Control in a Numerical Wave Tank Approved The exploitation of wave energy resources represents an attractive opportunity to enlarge and di- versify the renewable energy offer. Nevertheless, none of the existing technologies have achieved complete eco- nomic viability to be competitive within the energy market. In order to increase the overall power produc- tion efficiency and face the sea state variability, the presence of the control is mandatory. A model of the system is required to tune the controller parameters and predict the resulting performances. Even though the relevance of nonlinearities is magnified by the pres- ence of the controller, the device model employed is usually linear. An implementation of control in a fully nonlinear simulation model is desirable, but missing. Hence, this paper proposes to use the fully nonlinear Computational Fluid Dynamics (CFD) environment, implementing the latching control strategy in the open source software OpenFOAM. A case study has been analyzed to highlight the non- linear behavior of a device under latching control and to evaluate the differences between linear and nonlin- ear simulation models. The results show that the linear model overestimates the amplitude of motion, and, as a result, the extracted power. Moreover, the choice of the optimal control parameter is significantly affected by the nonlinear effect on the natural period of the device.

  • NWT Latching Control User Manual Approved This user manual is thought to be of assistance to whom is wishing to utilize the latching control algorithm proposed in (Giorgi and Ringwood, 2016). The controller is implemented in a Numerical Wave Tank (NWT), using the CFD open source software OpenFOAM. The application is obtained from the default mesh motion solver of OpenFOAM 2.3, called sixDoFRigidBodyMotion.

  • Motion solver of OpenFOAM 2.3 "sixDoFRigidBodyMotion" Approved ZIP File.

Project Progress

31%

Project Timing

  • Start
    Sep 02 2015
  • End
    Sep 02 2020

09/02/2015 09/02/2020

80%

Techno-Economic Assessment & Optimisation of Wave Energy Convetrers (TEOWEC)

Project Description

The techno-economic assessment and optimisation project aims to improve the techniques used to assess and improve the economic performance of Wave Energy Converter technologies.

The motivation is to help developers discover the best device/wavefarm combination in the context of a resource/market combination. The research question to be addressed may be expressed as:

What combination of device concept, hull geometry, component equipment, device control and wavefarm parameters gives the best return on investment in the context of a particular location with a particular wave resource and market conditions?

The approach used is to apply optimisation techniques to an objective function formed from economic value measures (e.g. PI, IRR, NPV, CoE). The economic model used to calculate the objective function is based on a large scale (100+WM) wavefarm project. The value of the objective function is calculated using discounted cash flow techniques and both the device and wavefarm parameters are manipulated to improve its value.

Throughout these calculations there is a focus on capturing real world engineering considerations in device concept, hydrodynamic performance, sea-keeping, bearing arrangements, mooring, PTO, control, CAPEX & OPEX drivers, marine operations etc…

In addition there is an emphasis on implementing general purpose analysis techniques that may be applied to a wide range of wave energy technologies.

Expected outcomes of the project are:

  • Ability to provide high quality estimate of financial performance of a large wavefarm selected WEC designs.
  • Ability to undertake sensitivity analysis of the coupled techno-economic system; The sensitivity analysis facilitates the identification of the most promising avenues for achieving performance improvements and so helps with prioritisation of development efforts
  • Optimisation: Within limits of any given WEC concept, an optimised design for improved financial performance.
  • Assessment: Insightful assessments that inform extensions/modifications to a WEC concept with the objective of achieving step change improvements in economic performance.

The TEOWEC project has now been completed. The outputs of the TEOWEC project have been licensed to a new spin-out company Wave Venture Ltd.

Wave Venture provide specialist wave energy software and consultancy services based on the techno-economic analysis capabilities developed in the TEOWEC project.

Documents

Project Progress

100%

Project Timing

  • Start
    Jan 01 2010
  • End
    Nov 02 2014

01/01/2010 11/02/2014

100%

Wave forecasting for wave energy control applications

Project Description

This project examined both the forecasting requirements for wave energy device control systems and the comparative forecastability of ocean waves. The objective is to achieve optimal energy transfer from waves to wire and it is well known that the optimal control strategy requires a forecast of the wave excitation force some seconds (or,in some cases, tens of seconds) ahead. The sensitivity to forecast errors is also examined, as well as techniques which can reduce the sensitivity of the optimal control to errors in the forecast.

Documents

Project Progress

100%

Project Timing

  • Start
    Jan 01 2009
  • End
    Jan 01 2011

01/01/2009 01/01/2011

100%

Nonlinear parametric modelling and control for wave energy devices using numerical tank testing (NPM)

Project Description

Offshore floating wave energy converters (WECs) are a new and rapidly growing technology, driven by the need to reduce CO2 emissions and provide renewable energy sources. The design process of such a device involves the use of computational tools and several stages of physical tank testing of scale models. Physical tank tests are generally expensive and can only provide insight into the physics taking complex scaling effects into account. Full scale models cannot be used, as WECs are generally large in dimension and thereby too large for controlled tank tests. Different computational tools exist, which are either linear or non-linear. Linear models, such as boundary element models, are used to describe the system of WEC-mooring-PTO-waves by means of a linear, predominantly frequency-based, descriptions and are traditionally used in control design. The associated assumptions of inviscous fluid, irrotational flow, small waves and small body motion, however, are a major limitation of this modelling approach, since WECs are designed to operate over wide wave amplitude ranges, experience large motions with waves breaking at the body and generating viscous drag and vortex shedding.
With consideration of the full range of effects, the physics can be described using the Navier-Stokes equations. This branch of computational model falls into the category of computational fluid dynamics (CFD). It is very computationally expensive and not suitable for use in the design process of WECs with regards to maximising the efficiency (through design iterations and development of control algorithms) in real sea states, where long-time simulations are required.
This project aims to combine the strengths both types of modelling concepts. In particular, the mathematical description of the technical system including the main device, its auxiliaries such as the mooring and power-take-off (PTO), and also the waves, that excite the device, is a necessity for the design of controllers but also when simulating the interaction of WECs in an array. Instead of using linear coefficients in this model, we propose to generate non-linear parametric models using system identification techniques, based on WEC responses obtained from numerical tank tests performed using CFD. These will take into account wave run-up in front of the structure, time varying degrees of submergence, which results in changes of buoyancy and restoring forces. It will also model viscous effects, such as vortex shedding and drag, and turbulence.
The project is divided into three main sections. Within the first stage, an experimental setup will be developed, that can be used in a numerical wave tank in CFD. Such an experiment would need to provide the data from which radiation effects due to the moving body, and diffraction effects due to waves exciting the WEC, can be identified. Possible solutions may include decay tests, where the body is released from unstable positions and the response is measured. For the fixed body, i.e. to get diffraction results, waves of varying frequency may be generated and the surface elevations, depending on the incoming wave, can be processed.
The next part of the project involves system identification. The CFD tests can be standardised and routines need to be developed that identify the necessary coefficients from the CFD results. The Centre of Ocean Energy Research at NUI Maynooth has considerable experience, over more than 20 years, in the development and application of system identification methods for a wide range of industrial applications. A crucial step in this stage involves the determination of appropriate model parameterisations which capture the essential nonlinear dynamics of families of WEC types. The parametric description can then be used in the third stage of the project to design a true nonlinear control algorithm to optimise energy conversion. The benefit of such controllers is that they respect the true nonlinear dynamics of WECs and are likely to give realistic energy maximisation in any irregular, non-linear sea.
Therefore, the project will provide an important step in extending the range of tractable computational techniques to the design and control of WECs, shortening the leap to expensive tank testing and providing realistic mathematical models upon which to base energy-maximising control designs.
The final outcome of this project will be a modular software suite, which can be licensed to end users. Furthermore, a company will be set up, that can provide the proposed technology as a service. The company can charge for the CFD model, system identification routine, development of the motion model and controller design separately. Also, as the CFD model is already available, a final long-time full-scale simulation with the controller in place can be offered. This would be similar to the full scale device being deployed in controlled conditions.

Documents

Project Progress

100%

Project Timing

  • Start
    Feb 01 2012
  • End
    Jan 31 2014

02/01/2012 01/31/2014

100%

BE Student projects

Project Description

  • Wavemaker design for a micro-scale wave tank
  • Micro-scale design of a hinge-barge wave energy converter
  • Machine-vision based motion capture system for a wave tank

Documents

Project Progress

100%

Project Timing

  • Start
    Oct 01 2010
  • End
    May 20 2011

10/01/2010 05/20/2011

100%

Control of an Array of Wave Energy Converters

Project Description

Summary

The goal of the project is to determine the need and potential of overall wave farm control with respect to design configuration. The project aims to both develop a numerical model for modelling an array of wave energy devices and algorithms dedicated to the control design of the whole system.
The wave energy group has been involved in wave energy for 6 years working both on the control and hydrodynamic side for individual wave energy devices. A previous project (IF/2002/309), successfully completed by the wave energy group at NUI Maynooth, developed a suite of control algorithms for a wave energy application producing potable water using a reverse-osmosis process resulting in a successful Irish patent application. A current project (EI/CTFD/IT/325-WAVELEC) concerns the development of a suite of control algorithms to optimize the performance of a typical wave energy converter for electricity production. During the course of the WAVELEC project, a mathematical model of an entire wave energy device including mechanical system dynamics, hydrodynamics and hydraulic power take-off mechanism has been developed. This led to a numerical model simulating the motion of a wave energy device in random seas and which was subsequently licensed to the wave energy company WAVEBOB Ltd.

The present project will use the knowledge gained from these different experiences and apply it to an array of devices. Arrays present a number of significant challenges which are not currently solved. For the hydrodynamic side, it will be important to understand how the different oscillators interact with each other and how their relative positions and distances affect the performance of the system. Indeed, the interaction of devices in an array is an important and difficult problem in hydrodynamics: it can lead to substantial increase or reduction in absorbed energy per device, depending on device geometry and control, and also on array geometry and its relation to incident wave length and direction.

For a control perspective, the approach used for single devices will be used with additional constraints such as interactions between neighbouring devices and practical issues related to the communication between the devices. It is anticipated that the developed algorithms and/or numerical model will be patented and licensed to interested wave energy device developers.

Documents

Project Progress

100%

Project Timing

  • Start
    Sep 01 2010
  • End
    May 31 2014

09/01/2010 05/31/2014

100%