Levelised Cost of Energy for High Tip-Speed Ratio Tidal Turbines Operating in Less Energetic Flows

Abstract

The development of the tidal stream industry has been steady over the years. However, this growth is concentrated in areas of the world where the current magnitudes are high, mostly in the northern parts of the UK. Other sites characterized as less energetic sites remain to be untapped due to the high cost of energy of the technology compounded by the low energy yield in these sites. However, there is a benefit to be gained if the technology is developed to accommodate less energetic sites with flow velocities of less than 2 m/s, such as the waters of Southeast Asia, Southern UK, as well as Mexico’s continuous Yucatan current, and Taiwan’s Kuroshio. Research and development of tidal stream technologies that may be able to harness the energy in these currents have been continuous, but development is yet to start due to technology mismatch. Designing to increase the rated tip-speed ratio of the turbine blades with the notion that faster-rotating blades reduce the overall cost of the generator and its subsequent components, leading to greater savings compared to the usual downsizing, is an option to further the development in these regions. This design approach has been shown to be technically feasible, but the economic feasibility of such a design remains to be seen. The economic benefit is then evaluated using a cost model starting from generator cost modeling and then computing the cost of other components, including auxiliaries, cables, foundations, as well as commissioning and financing. A one-at-a-time sensitivity analysis is then used to evaluate the resulting LCOE when considering array size, turbine diameter, and rated speed. It has been found that there is a region of substantial cost reduction at about 50 RPM regardless of the tip-speed ratio, where the LCOE of turbine operating in less energetic sites at this range are better compared to the LCOE of conventional turbines designed for operating at the same less energetic site. It was also found that 5 m diameter turbines operate at a reasonable LCOE at array sizes of greater than 5 turbines, although 10 m diameter turbines have the most attractive LCOE. Nonetheless, both configurations lead to a better LCOE compared to diesel and the conventional tidal turbine counterparts, which could help in the transition to greener energy technologies, as well as green rural electrification, where diesel remains to be the main source of electricity.