Design of a novel floating offshore wind turbine.

Abstract

Over the past decade, the renewable energy sector nationally and globally has experienced a large growth due to factors such as increases in government subsidies and cost reductions. One of the fastestgrowing sectors within the renewable energy space is wind energy, however, the growth comprises mainly onshore wind farm developments but is limited by land availability. Typical wind turbine farms are large special deployments that accommodate large amounts of land. Offshore wind resources are normally higher density than that land-based resources due to fewer obstructions to the wind. The resource available offshore along the eastern and western coastline of South Africa has significant potential which may be tapped with offshore wind turbines. Findings from the investigation found four potential sites along the coastline of South Africa which are viable for offshore wind turbine exploration. The results of the study found that these sites can be connected to onshore bulk substations and located in zones that are outside the areas of shipping traffic to not affect economic trade. These locations allow suitable access to coastal areas and ports which can reduce strain on the national electrical grid as well as reduce transmission losses from inland power stations. The sites which were investigated have a mean wind speed of 9.5 m/s with power densities between 500 W/m2 and 1000 W/m2. However, the ocean seafloor depth between 1km and 3km deep poses a challenge, new floating wind turbine concepts and wind farm configurations technologies indicate that these may be overcome to harness the large energy potential. The study indicates that the offshore potential sites are suitable for offshore wind turbine development and grid integration. The sites which are closer to the shoreline (between 10km and 50km from the shore) have short-medium term deployment potential. The previously designed, vertical axis, ocean hydrokinetic turbine was optimized for wind conditions by evaluating a larger set of aerodynamic profiles. It was found that the symmetric profiles exhibited stable characteristics when under loading in the upwind and downwind side of the turbine resulting in a smoother transfer of torque to the rotor shaft. The results showed a smooth torque profile on the rotor with minimal main rotor vibration. Even though the aspect ratio of 1.5 has a larger operating range for large tip speed ratio (TSR) range, the aspect ratio of 1 has a higher coefficient of power range resulting in higher turbine power output. The need for a suitable control volume for small scale testing led to the design and construction of a suitable wind tunnel for the small scale wind turbine testing. The wind tunnel was designed to accommodate a small scale vertical turbine at a velocity of 9 m/s to 9.3 m/s based on the mean wind speed of the potential sites. The turbulence intensity was examined and found to be minimum across the testing section. A small scale wind turbine was fabricated based on the vertical axis helical blade which was designed at the optimum twist angle for an aspect ratio of 1, due to the nature of power and performance at selected TSR’s. The study had proven that the results from the test are in good comparison to that of the simulation. For utility scale turbines with larger power outputs (5MW to 10MW) the rotor speed would be between 10 and 15 RPM for maximum power output. The helical turbine, in comparison to reference HAWT and VAWT turbines within the market, show an improvement to the power curve efficiency. The helical blade profile shows positive results on the transmission of mechanical power from the blade to the rotor torque in comparison to other VAWT systems within the market. This allows for reduced impact on mechanical and electrical components of the turbine. When deployed in a standardised wind farm layout, the helical turbine outperforms the reference HAWT and VAWT turbines with respect to capacity factor and annual energy production. The investigation found that there are suitable offshore wind farm locations along the coastline of South Africa which can provide suitable clean energy as well as diversify the country’s energy mix as per the national development plan and long-term energy targets. The study concluded that the vertical axis turbine design can provide much needed power which may be fed into coastal regions at bulk point of supply or to offshore oil & gas rigs and possibly coupled with hydrogen technology in the future.