Modelling and Control of Tubular Linear Generators for Wave-Power Applications

Change log
Ridge, Alexander Nicholas 

Linear generators are an attractive choice for use in wave-power applications. They have the ability to be coupled directly to the motion of a point-absorber or float, eliminating the need for potentially complex mechanical/hydraulic intermediate stages which are often required for designs based on rotary generators. This decreases system complexity / maintenance requirements and increases system reliability, key factors for economic operation of wave energy converters out at sea.

Linear generators are a relatively new technology, as is their application to the field of wave-power energy extraction. The literature on the modelling and control of linear generators thereby naturally lags behind that of rotary machines. This dissertation makes contributions to four key areas in applying a linear generator to the challenge of wave power energy extraction – thrust ripple reduction, thermal modelling, dynamic testing / maximum power extraction and sensorless control. These areas were selected for their importance in the optimal use of a linear generator in a wave-power scenario and for which there was little existing published knowledge. Experimental validation was used throughout this work to ensure a directly-applicable nature for its results. The work in this thesis generally starts at the point of an existing linear machine design and looks towards the optimal application of that machine to energy extraction in a wave-power scenario. As the design and application of machines are however inherently linked, some of this work will naturally feed back into the design process.

The work on thrust ripple reduction presents a successful methodology to allow precise control of the force from the machine in an open-loop manner, regardless of the machine’s position. The thermal modelling work mainly focuses on the construction of thermal models for a linear machine when stationary. This was considered a first and important step in thermal modelling of a linear machine, giving a ‘worst case’ thermal rating. Detailed thermal models that are able to predict the temperatures of individual machine components with minimal experimental calibration are presented and validated. The realistic dynamic testing of linear generators for a wave-power scenario is an important area to develop due to the risky, harsh and expensive-to-access nature of testing in the ocean. A dynamic test rig that is capable of providing a linear generator with a mechanical environment equivalent to what it would experience being attached to a float on the sea was successfully designed, built and tested. This test rig has no fixed motion profile, but instead solves a hydrodynamic model in real-time to determine the forces that should be applied to the linear generator. Initial investigations into maximum power extraction algorithms using this test rig confirm its usefulness and versatility as a piece of equipment for future research. Last but not least the potential for sensorless control is investigated. If achieved, this can increase device reliability by eliminating externally-mounted sensors which is attractive for harsh marine environments. A sensorless control technique is developed which can output a number of different estimations of machine position at a full range of machine speeds, including stationary. This control technique shows great promise and requires no additional electrical components besides those which would already exist for general machine control.

McMahon, Richard
Wave-power applications, Linear generators
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
George and Lillian Schiff Studentship