Videogrammetric characterisation of a flexible membrane wave energy converter

There have been many new ideas focused on harnessing ocean wave energy. But developing a concept from the original idea to a marketable product requires a company to pass through several stages of development. Initial stages include concept validation and design validation that involve scaled experimentation in accredited research facilities. One important objective of these stages is to evaluate the device’s ability to capture wave energy; an increasingly challenging, cross-disciplinary task as developers design unique conversion interfaces. As new technology goes, new measurement methods often need to be developed to monitor components of interest with minimum interference of the system. One of the most advancing, non-intrusive measurement methods is using numerical cameras to measure structure motions and free surfaces. This technique is known as videogrammetry.

Bombora Wave Power (Bombora) is developing a globally unique wave energy converter (WEC). The fully submerged air-filled compressible volume device utilises a flexible membrane as the conversion interface. This moving boundary couples with waves, driving airflow in the system that passes through an air turbine and generator. This paper presents details of the first application of underwater videogrammetry applied to the experimental investigation of the Bombora flexible membrane. Wave tank tests of the 1:15 scale model were conducted at the Australian Maritime College Model Test Basin located in Tasmania. The flexible membrane (conversion interface) consisted of 12 individual compartments of air, or cells, separated by diaphragms. Three cameras were set up in the basin to measure displacements of a carefully prepared membrane mounted to the test rig.

Physical experimental setup and overview of the membrane are presented, followed by explanation of videogrammetry and airflow measurement systems, and of post processing tools. Results based on data from both measurement methods include time series of cell volume flux during expansion/contraction under wave action, and time averaged volume of airflow in each cell per wave period. Membrane behaviour was assessed and discussed in terms of cell volume flux, with comparisons of the methods showing good correlation for the time series. Volume of airflow results show a 3% mean relative percentage difference between methods across the 12 cells. Sources of error in the videogrammetry methodology together with measurement uncertainty and precision are subsequently discussed. The main outcomes of this work are the demonstration of an adapted videogrammetry methodology useful for nonintrusive measurement of flexible membrane structures and empirically derived information for the physical and numerical development of the Bombora WEC.