Unsteady hydrodynamic effects on the dynamic performance of low speed vertical axis current turbine

Abstract

Malaysia’s rivers and ocean energy can be the best resource for green marine renewable energy. The generation of electricity by the burning of fossil fuels are expensive and produce undesirable greenhouse gases. Malaysia’s sea has average speed of 1 m/s, which is twice less than the minimum speed that can operate the conventional turbines. Low-Speed Vertical Axis Turbine (LS-VACT) as a drag device represents a promising technology to exploit marine currents. It can be applied to harness current energy in rivers, coastal area and ocean due to their relative simplicity with reduced installation and maintenance costs. The purpose of this research is to investigate performance of the turbine and the influence of added mass, damping and arm-length to its performance at low current velocities. To achieve that, numerical simulation was conducted using MATLAB program by utilizing the hydrodynamic coefficients and derivatives of the hydrodynamic forces and moments acting on the turbine buckets. The simulation program was validated through the experiments of the LS-VACT. This developed simulation program can be used as a fast and useful tool to achieve design improvements for this turbine at several speeds and various loads. This computer programming can match and integrate the full-scale turbine to a suitable generator with different powers and loads efficiently. The simulation results showed that the performance of LS-VACT agreed within 10% with the experiment results and having the same trend at various flow speeds. A parametric study was performed to analyse the effects of added mass and arm-length at several current speeds. LS-VACT has the highest power coefficient of 0.196 at 0.32 m/s. Also, the peak power (8.6W) and the maximum torque (19.4N.m) values were recorded at a flow velocity of 0.64 m/s. At low water flow speed of 0.17 m/s and 0.32 m/s, the added mass has a significant influence on the LS-VACT performance. At this condition, the inertia forces were dominant at low Keulegan-Carpenter number (K-C) of 3 to 9. The torque and the power magnitudes of the turbine decreased about 18 % and 52.7% respectively. At K-C number above 10, the boundary layer separated with formation of vortex shedding occur. The drag forces were found to be dominant in this situation. At the current speed of 0.32 m/s and arm-length of 0.27 m, the maximum torque of 10.11 N.m and corresponding power of 1.75 W was achieved. However, further increase of the arm-length results in decreasing torque and power. The dynamic performance of LS-VACT was carried out and it can facilitate improvements in its design at low current speed.