News

German researcher's successful visit for wave energy research


12 June 2008

Elisabeth Baden has just spent the last 6 months completing her Masters degree thesis as a visiting researcher from Germany. Her research topic is realted to wave energy. As she was preparing to return home, she shared some thoughts on her experiences in Sydney.

Why did you come to The University of Sydney?
"After doing my undergraduate in Mechanical Engineering at theUniversity of Duisburg-Essen in Germany, I decided to do my Masters degree in Coastal Geosciences and Engineering at the University of Kiel, Germany. Especially because coastal-related issues are best dealt with on a global scale, the Faculty strongly supports the idea of students going abroad for the duration of the thesis project. Fortunately, I had been in contact with Dr Tim Finnigan, who kindly agreed to supervise my work. Coming to Australia as a visiting researcher has been a terrific experience and I am very glad to have had the opportunity to carry out experimental research in such an exciting field of study."

What research have you been doing?
"The World's Oceans carry an enormous energy potential, challenging Engineers to find solutions to extract power for commercial electricity generation. The conversion of wave energy into electricity is a technology still in its infancy but ongoing research and existing prototypes yield promising results to make it a commercially viable energy source. Conditions at sea tend to be rather inhospitable, especially in those areas favourable for wave energy conversion. However, once deployed, the redemption times appear to be rather short and maintenance costs at a moderate extent.

There are various types of wave energy converters, which rely on different principles. The device dealt with in my research is a vertical bottom-pivoted cylinder, which pitches in response to a passing wave. Experiments are being carried out in the wave flume of Sydney University's fluids laboratory in order to investigate the cylinder's ability to absorb energy from waves in idealised conditions as well as real ocean conditions, simulated by the wave paddle. "

Elisabeth's thesis is entitled A comparison of the performance response of a bottom-pivoted wave energy device in regular and mixed-sea conditions.

Abstract

This study aims to assess the ability of a vertical, bottom-pivoted, cylindrical point absorber to extract power while pitching in response to regular and irregular waves. Investigations were carried out on a fundamental level and, although representative damping was applied, no consideration was given to the practical aspects of power- take-off mechanisms.

The proposed wave energy converter was tested in regular waves; however, real ocean conditions were simulated as the device was subjected to a mixed-sea state while its performance was studied. Experiments were conducted in a two-dimensional wave flume set to intermediate water depth. Since it was desired to match the mean energy content of the selected set of regular waves with the energy content in a particular sea state, a Bretschneider spectrum was chosen to be produced by the wave paddle. This allowed specifying the size of the area under the spectral curve as a function of the significant wave height. The cylinder's motion was constrained by a rotary dashpot, which was capable of providing adjustable damping rates, of which three were applied in this study. The relationship between the power output and the applied damping rate was studied, as well as the correlation between the applied frequency in the set of regular waves and the natural frequency of the device. Furthermore, the capture width as a ratio between the power extracted and the incident wave power was determined. For convenience of comparison with other wave energy converters it was stated with respect to the cylinder's size.

In regular wave conditions, it was found that the cylinder responds with periodic motion while generating considerable amounts of power, its magnitude increasing with increasing wave amplitude. Moreover, the cylinder performed best when subjected to wave frequencies coinciding with its resonant frequency across the low and intermediate damping range. In the case of highest damping, the power output appeared to increase with increasing wave frequency, which gave reason to believe that the damping rate altered the resonant case. The point absorber effect was validated as the capture width exceeded a value of 1 in the case of high and intermediate damping. When exposed to an irregular sea state, the device yielded similar amounts of power, compared to the mean output in regular wave conditions, with the power output being proportional to the applied damping rate.

In general, the analysis of the device's performance yields promising results with respect to converting wave motion into electricity. However, optimization of the device's shape and dimensions are investigated in ongoing research at the University of Sydney as these features are expected to further enhance the device's efficiency.