Global warming is one of the serious issues in the world mainly due to the burning of fossil fuels and emission of carbon dioxide to the atmosphere. The importance of alternative energy is steadily rising in the 21st century.
Ocean energy conversion can be classified into two categories; Wave energy and tidal energy. However, the efficiency of most wave energy devices is subject to weather and seasonal variations. The main advantage of tidal sources is that it is constant throughout the year and makes energy production predictable and attractive for investment. Tidal current turbines convert the kinetic energy within currents to produce power. The horizontal-axis tidal current turbine is one of the machines used to harness tidal current energy, which appears to be the most technologically and economically viable one at this stage.
Currently, single rotor horizontal axis tidal turbines are used for tidal current power production. By the Betz theorem, single rotor turbines can obtain a maximum power coefficient of 59.3% whereas a dual rotor can obtain a maximum of 64%.
In this thesis, a study was done to investigate the performance of a counter rotating marine current turbine. In order to obtain an optimal turbine design for application, the basic data and performance analysis was conducted on the turbine using CFD, experiments and PIV measurements. From this study, the following were concluded:

(1) The performance of a single rotor marine current turbine with the selected blade design was analyzed using CFD and model experiments. From the CFD results, the flow pattern on the pressure side of the blade was seen to affect the performance. From the CFD results, at the design flow speed of 1m/s, a power output of 43.18W and a power coefficient of 0.441 was obtained. In experiments, at a flow speed of 1.05m/s, a power output of 48.7W and power coefficient of 0.431 was obtained. From these results, it was seen that BEMT is a useful tool for marine current turbine blade design.

(2) CFD analysis and experiments were done to study the effect of the blade distance between the front and rear rotors of the counter rotating turbine. At a blade distance of 150mm and flow speed of 1.2m/s, the highest power coefficient value of 0.47 was obtained. In the comparison between CFD and experiments, the highest power coefficient was 0.457 at a flow speed of 1.16m/s in experiments while the highest power coefficient obtained in CFD was 0.457 at a flow speed of 1.2m/s. The results of the CFD and experiments were shown to be in good agreement.

(3) The front and rear rotor blade angles were varied and CFD and model experiments were done on counter rotating turbine. When the front blade was set at 5° and the rear blade was set at 0° and the flow speed was at 1.4m/s, the CFD results showed a power output of 123.5W. From the experimental results, at a flow speed of 1.37m/s, the power output was 123.41W and the power coefficient for both results was 0.461.

(4) The counter rotating turbine was then placed into a duct that was designed to increase the flow velocity to the turbine. However, it was seen that the turbine obstructed the internal flow of the duct. The thickness of the rear ends of the duct was then modified and analyzed in CFD. When the thickness of the duct rear edges were increased, a low pressure region would form near the wake and improve the turbine performance in the duct. Despite this, more research into the optimization of the turbine and duct is needed.

(5) A magnetic coupling method was then applied onto the experimental model of the turbine. The mechanical loss was measured and compared to the original oil seal coupling method. The average mechanical loss for the oil seal method was 0.344Nm and the magnetic coupling was 0.286Nm, which was an approximately 13% decrease in mechanical loss. Also, the oil seal type model obtained a maximum power coefficient value of 0.411 at a flow speed of 1.13m/s and the magnetic coupled model obtained a maximum power coefficient of 0.435 at a flow speed of 1.39m/s.

(6) From the PIV measurements, the velocity vectors and kinetic energy in the flow was shown. In the single rotor turbine PIV measurements, the flow of central vortex and the tip vortices was observed. As the central and tip vortices dissipated the further away from the turbine before the flow velocity recovered. In the case of the counter rotating turbine, the flow in the wake was slower than the flow of the recirculating water tank and a large central vortex was not observed near the central region.