Passive vibration control of a floating hydrostatic transmission wind turbine and theoretical extensions

Short summary:
This proposal will develop novel passive damping technology to dampen the vibrations of the floating platform of a new type of wind turbine employing the hydrostatic transmission drivetrain, and develop a stability theory for coupled infinite-dimensional systems with nonlinear feedback.

Background:
In order to capture the highest quality wind resources, wind turbines are getting deployed further offshore with the floating wind turbine technology. However the floating wind turbines face more severe challenges from weather and wave conditions than their fixed-bottom counterparts. The motions of the floating platform not only cause large fluctuations in the rotor speed and generator power, but also cause considerable load variations on the tower base. Known vibration reduction methods are by torque control and by blade pitch control. But these methods are effective at the expense of interfering with the power generation and the latter will increase blade pitch actuator usage. Structural control, e.g., using tuned mass damper (TMD) or tuned liquid column damper (TLCD), might offer a good alternative solution. They are free from electrical faults, but a big disadvantage of TMD/TLCD type of dampers is that they have a large mass and/or a large amount of liquid, leading to substantial extra weight, this could be prevented if they made use of existing turbine components. However, practical considerations have shown that existing components cannot be used, so this is not a realistic solution for conventional wind turbines. A new type of wind turbine called hydrostatic transmission wind turbine (HSTWT), could provide the suitable mass and liquid component.

Control Application:
The proposed project will investigate how to make use of the hydraulic reservoir of the floating barge mounted HSTWT to dampen the vibrations of the floating platform, by acting as a novel damper. This will simply give the reservoir a dual function with very small extra costs. During the project we will use existing simulation model of the floating wind turbines developed by the National Renewable Energy Laboratory for detailed simulation analysis. This model will be modified to include the HST drivetrain, coupled dynamics of the barge-reservoir system, and pitch and torque controllers. The optimal damper design will be based on two simple models obtained from the above simulation model through system identification, and Particle Swarm Optimization algorithm.

Control theory:
The above tower (including the barge) - damper system is the interconnection of two passive systems. However this does not automatically lead to stability. We abstract this problem as stability theory and look even further: assuming the tower is flexible, which is described by partial differential equations, so that it is an infinite-dimensional system. When two systems influence each other in both directions, they are known as a coupled system. Coupled systems in engineering often consist of an infinite-dimension system interacting with a finite-dimensional system, i.e., a system that can be described by ordinary differential equations. Such coupled systems have been recently the topic of intense research in the linear case. The case of a nonlinear finite-dimensional system is open and challenging. Thus it will be very interesting to develop a stability theory for the interconnection of a passive linear infinite-dimensional system and a passive nonlinear finite-dimensional system. The equations of the coupled system can be rewritten as an abstract second order differential equation in a Hilbert space, with a nonlinear damping term. We aim to investigate the stability properties of such systems using monotone operator theory and Lyapunov functions.

This work requires an in-depth understanding of fluid mechanics, structural dynamics and control theory and engineering, all of which are well represented in the PI's multi-disciplinary background.

The key purpose of the proposed research is to produce new knowledge at the interface of the three disciplines (control theory, structural engineering and fluid dynamics) that will facilitate future technology developments in offshore wind turbines in a financially efficient manner.

Academic impact: The research will develop new structural control technology and new control theory for coupled infinite-dimensional systems with nonlinear feedback. It will create a synergistic approach across control theory with structural engineering and fluid dynamics in order to contribute to wind turbine technology, ultimately helping to address the big challenge of providing society with clean and renewable energy. It will therefore contribute to the UK economic, environmental and societal wellbeing.

Economic Impact: The proposed project investigates how to make use of the hydraulic reservoir of a new type of wind turbine named hydrostatic transmission wind turbine (HSTWT) to dampen the vibrations of the floating turbine platform, by acting as a novel damper system. The primary motivation of HSTWT is increased reliability, and reduced manufacturing, operation & maintenance costs for wind turbines by removing troublesome components and reducing tower top weight. Offshore wind turbines facing severe weather and wave conditions, that are difficult to maintain, would benefit from the use of HSTWT. HSTWT has a cheaper manufacture, operation and maintenance costs. This novel damper system will increase stability of the floating platform of HSTWT with negligible extra investment, which will improve HSTWT as a novel offshore application and speed up its widespread penetration, ultimately reducing the wind energy cost. The PI has existing collaborations with key companies in the energy sector (e.g. National Grid, FTI Consulting, XEMC Windpower). In addition, he co-lead an EU ITN consortium in the field of infinite-dimensional systems, named ConFlex (Control of flexible structure and fluid structure interactions). This 4 million Euros consortium has been formed recently, consisting of 14 universities and 11 industrial partners. The PI will use the fruitful interplay between the proposed EPSRC project and the work of this large EU-funded consortium to reach out to other industrial partners including Offshore Renewable Energy Catapult and National Renewable Energy Center CENER. Collaborating with these industrial partners will serve to monitor progress as well as disseminate the outcomes of the project, which will speed up the adoption of this new design. In particular, our industrial partner FTI Consulting will disseminate research results via their network of clients, by making them aware of any relevant results and build the necessary collaborative links.

Societal Impact: this project will contribute to the major societal challenge that is climate change. It will enhance the UK capabilities in renewables technology, contributing to maintain the UK's position as global leader in offshore wind farm installation. It will help achieve government's aim to cut greenhouse gas emissions by at least 80% below the level that it was in 1990. Beyond industrial links, the PI will continue his active engagement with the wider UK renewables energy community, including his ongoing participation in the Energy GRP (Global Research Priorities) at Warwick University, which will offer additional avenues for public exposure of the project outcomes.

People Impact: the project will provide skills development opportunity for both the PI and researcher involved. The PI will develop leadership skills as well as project and financial management. It will also allow him to continue to grow his profile in both control and wind energy communities. The researcher will be part of a multidisciplinary project and a strong research group lead by the PI, enabling him/her to develop new skills and contribute to an important challenge.