University of Bath

Active control for multi-degree-of-freedom wave energy converters

This project aims to reduce the cost and accelerate the commercialisation of wave energy by developing active control systems for wave energy converters.

The aim is to develop active control systems for a wide range of wave energy converters (WECs) in order to balance energy capture against component loading in order to minimise the cost of energy. Wave energy still faces many technological challenges on the path to commercialisation. At the heart of much of the mistrust associated with the various concepts is the cost of energy relative to other renewable sources. Nevertheless the potential resource is recognised as highly significant and great efforts are being input to pursue the goal of affordable energy production.

Regardless of the WEC architecture, it must be controlled to maximise efficiency across the wide range of operating conditions it will experience. The control system can also be used to limit load transmission to aid survivability. Many control strategy studies have been limited to idealised single degree-of-freedom WECs which are not realistic. Additionally, many control strategies are very complex, difficult to implement in real time and rely on the future prediction of wave forces which is difficult to achieve. This project is developing a control strategy which is intuitive to implement and tune, and is robust to modelling errors and other uncertainty. It is also applicable to a wide range of WECs with multiple degrees-of-freedom. The WaveSub WEC under development by Marine Power Systems Ltd has been used as a target device and has been the subject of experimental studies conducted to validate numerical models previously.

Science

A nonlinear time-domain simulation model of the WaveSub WEC has been produced and an optimally tuned passively damped configuration has been used as a performance benchmark. The passive and actively controlled systems were tested with a range of realistic sea states and good performance is observed for the actively controlled system. Mean power increases of between 13% and 86% are seen compared to the optimal passive system. Experimental testing will follow in due course to verify the findings. Some outputs from this project can be found in the following papers:

Hillis, A.J., Whitlam, C., Brask, A., Chapman, J. and Plummer, A.R. Power capture gains for the WaveSub submerged WEC using active control, in Proceedings of the 13th European Wave and Tidal Energy Conference, 1st – 6th of September 2019, Napoli, Italy.

Hillis, A.J., Plummer, A.R., Zeng, X., Chapman, J. Simulation of a power electronic conversion system with short-term energy storage for actively controlled wave energy converters, in Proceedings of the 6th Offshore Energy and Storage Summit, 10th – 12th July 2019, Brest, France.

Impact

The developed control strategy shows promise to provide a substantial increase in power capture for a wide range of WEC concepts for a minimal additional device cost. The control system is key to unlocking the generation potential of any WEC and is the bridge between all the subsystems in the device. As such it is an enabling tool to drive the holistic design of a complete WEC system to reduce the cost of energy and accelerate the commercialisation of wave energy.