In order to make use of ocean renewable energy, a combination system of a bi-directional impulse turbine and a bi-directional flow collector for tidal current energy conversion is investigated in this paper. It is the advantage that this turbine system does not need an operation of orientation change according to the reversal of regular tidal orientation when fixed on the seabed. The experimental investigations by using both a circulating water tank and a towing tank showed that the turbine power output could be increased by adopting the flow collector proposed in this study. Then the flow collector with fixed spiral vane named spiral flow collector was investigated by both a circulating water tank test and CFD analysis. The experimental result of the spiral flow collector showed that the performance improvement was found on the increase of axial velocity in the turbine which contributed to the increase of the turbine power output. The results of CFD analysis showed that 180 deg of the skew angle of the fixed spiral vane was suitable in view of the angular moment at the turbine inlet in this case.

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In ocean-based renewable energies, such as wave motion, ocean current, tidal current and ocean thermal energy, many studies have been conducted on the wave energy conversion system and they have been tested in many countries. In this study, a tidal current energy is focused. Collecting a tidal flow is very important because it is generally known that a power available from a stream of water is proportional to the cube of the free stream velocity of the current. In this research, a bi-directional turbine system is investigated for a tidal flow that changes the direction periodically. Numerical investigations are conducted on a bi-directional flow collector with guide vane to collect the flow, in which the effects of vane skew angle and blade number are investigated. Calculated results showed that the collector with eight guide vanes was the highest angular moment at turbine inlet. The axial velocity at turbine inlet was larger for smaller value of the vane skew angle, whereas the tangential velocity at the turbine inlet was larger for larger value of the vane skew angle. As the results of both the profiles of axial and tangential velocity, the angular moment took the local maximum value at around the angle of 180 deg. in this study.