Integration of oscillating water column wave energy converters within multi-use maritime structures

Ocean energy presents arguably one of the most rich renewable energy solutions currently under exploration, and consists of a variety of potential resources including tidal barrages, salinity gradients and ocean thermal energy. However two sources, tidal currents and ocean waves, are considered by many as the most promising and have subsequently observed the greatest development in recent decades. Ocean waves offer a predictable, dense and virtually untapped energy resource with potential to significantly contribute towards the rising global energy demands. A number of prototype failures and subsequent lack of long term commercial deployments has consequently impacted the development of Wave Energy Converter (WEC) technologies, such that the technologies are considered immature and currently economically uncompetitive with renewable energy counterparts such as wind and solar. To combat the economic argument, a number of solutions have been devised to reduce the high costs currently associated with ocean wave energy, one of which is integration within maritime structures to create synergistic multi-purpose platforms. While concepts have been formulated for the integration of various WEC technologies, the Oscillating Water Column (OWC) WEC is favoured as a predominant devices for implementation due to its rigid design, capability for incorporation within solid edifices, and relative ease of maintenance due to all moving parts above water. The OWC device's operational principle, in its most elementary form, utilises incident wave interaction to oscillate a trapped column of water inside the chamber, subsequently operating in a 'piston-type' motion to force air in and out of a turbine. The economic benefits of OWC device integration encompass both the capital and operating expenditure, from costs shared during the construction, through to the reduction in maintenance and grid connection costs, ultimately making the concept more competitive within the renewable energy sector. With some full scale demonstration cases and commercial devices currently operational, the vast majority of maritime structure integrated WECs target bottom-mounted breakwaters, which are typically depth limited due to the economic constraints associated with deep water construction. This type of integration restricts the operational range of the concept to onshore/nearshore regions; however, with the expansion of many blue economy industries into offshore regions, opportunities arise for exploration of wave energy conversion to serve in deeper waters. In order to migrate from the nearshore integrated concepts, integration within floating offshore structures, such as breakwaters and offshore platforms, must be explored for viability from both economic and operational perspectives. Understanding the hydrodynamic performance of OWC devices integrated within maritime structures, both _fixed and floating, is the focus of this research project. Initial stages of the research project considered a detailed model scale experimental investigation regarding integrated OWC device performance, which was conducted to explore two specific parameters respective impact on OWC device energy absorption; firstly, the cross-sectional geometry, and secondly, breakwater integration. An isolated OWC device of rectangular cross-section was compared to a previously researched device with a circular cross-section of equivalent area to explore the impact on energy absorption, where negligible difference in performance was observed between the geometrically varying devices. Following this realisation, both respective devices were incorporated within a model scale, gravity-based breakwater to compare the extraction efficiency of the devices between both isolated and integrated configurations. The results obtained indicated that the energy absorption capacities of the OWC devices are significantly improved through breakwater integration, with the rectangular OWC device recommended due to its orthogonal construction allowing for less complex incorporation. This research provided a foundation for the performance of OWC device integrated maritime structures, and enhanced the potential for OWC device integration within floating offshore structures. Development of the project generated a second comprehensive model scale investigation designed to establish a proof-of-concept for a floating breakwater integrated with multiple OWC devices. A generic ˜ìvÑ-type, soft-moored breakwater was integrated with a modular number of OWC devices and subjected to both regular and irregular sea states to analyse how variations to device configuration, breakwater width, pneumatic damping, wave height and motion constraints impact two overarching parameters; the energy absorption of the integrated OWC devices, and the performance of the floating breakwater. The investigation yielded substantial insights regarding the beneficial impact OWC device integration can have on the motion characteristics of the floating breakwater, while minor reduction was simultaneously observed for wave transmission and reflection. The investigation also highlighted the importance of device spacing with respect to OWC device performance, where insufficient spacing was found to have a detrimental impact on energy absorption where destructive device-device interference was observed. Through specific configuration of the aforementioned design parameters, the WEC/breakwater concept was able to obtain total device conversion efficiencies of up to approximately 80% at resonance in regular waves, and observed equivalent performance in irregular waves. This project reveals that maritime structure integration of OWC WECs provides significant benefits to the hydrodynamic performance of the integrated devices, which in association with the previously established economic benefits, further strengthens the viability of the concept, and provides foundation for future development.