Abstract
Tidal Stream Devices (TSDs) are relatively new renewable energy converters. To date only a few prototypes, primarily horizontal-axis turbine designs, are operational; therefore, little reliability data has accumulated. Pressure to develop reliable sources of renewable electric power is encouraging investors to consider the technology for development. There are a variety of engineering solutions under consideration, including floating tethered, submerged tethered, ducted sea-bed bottom-mounted and sea-bed pile-mounted turbines, but in the absence of in-service reliability data it is difficult to critically evaluate comparative technologies. Developing reliability models for TSDs could reduce long-term risks and costs for investors and developers, encouraging more feasible and economically viable options.
This research develops robust reliability models for comparison, defining TSD reliability block diagrams (RBD) in a rigorous way, using surrogate reliability data from similarly-rated wind turbines (WTs) and other relevant marine and electrical industries.
The purpose of the research is not to derive individual TSD failure rates but to provide a means of comparison of the relative reliabilities of various devices. Analysis of TSD sub-assemblies from the major types of TSDs used today is performed to identify criticality, to improve controllability and maintainability. The models show that TSDs can be expected to have lower reliability than WTs of comparable size and that failure rates increase with complexity. The models also demonstrate that controls and drive train sub-assemblies, such as the gearbox, generator and converter, are critical to device reliability.
The proposed developed models provide clear identification of required changes to the proposed TSD system designs, to raise availability, including duplication of critical systems, use of components developed for harsh environments and migration of equipment onshore, wherever practicable.