High-efficiency twin-rotor direct-drive marine current turbine: Design, tests, and performance evaluation

Existing marine current turbines (MCTs) suffer from low energy utilization in low-velocity currents because of high starting velocity (>0.7 m/s) and low efficiency in slow currents (<30 %). Despite the prevalence of low-velocity currents and the growing energy demand in regions with such currents, the technology for harnessing these resources has progressed slowly. Thus, we propose a direct-drive parallel-axis twin-rotor MCT (TRMCT) to more effectively harness slow currents. Herein, we describe the structural components of the TRMCT and construct a MATLAB simulation model to compare the performance of the developed TRMCT and conventional MCTs. Finally, we evaluate a 2-kW TRMCT prototype in a series of tests: the onshore test, the towing test, and the sea test. The simulation and test results indicate that the TRMCT shows a minimum starting velocity of 0.2 m/s and a maximum energy conversion efficiency of up to 35 % under slow current conditions, outperforming the existing low-velocity MCTs in both metrics. Notably, the highest power coefficient of the TRMCT rotor reaches 0.44, and the efficiency of its electromechanical conversion system is up to 80 %. In the simulations, the TRMCT delivers respectively 20.4 % and 106 % higher power generation than two isolated same-size MCTs and a single large-rotor MCT with equivalent swept area and the same rated power. In the prototype tests using the same rotor, the TRMCT exhibits 9 % higher efficiency than the conventional single-rotor MCTs (SRMCT). The results of the sea test confirm the feasibility and stability of the TRMCT prototype under real sea conditions.