상호영향을 고려한 조류발전용 터빈의 배치에 따른 수치해석적 성능 연구

Abstract

There are plenty of renewable energies in the ocean. Among the renewable energies, tidal energy is the reliable and unlimited resource since the tides are caused by gravitational force exerted by the moon and the sun. In order to utilize tidal energy, many companies have been developed various turbine systems which generally consisted of horizontal-axis turbine(HAT) and vertical-axis turbine(VAT) and developed systems have been performed field test for evaluation of turbine efficiency and grid connection.

Tidal farm have multi-arrayed turbine systems for utilizing tidal stream energy. It is also required that each unit has to be deployed in order to avoid hydrodynamic interference between turbines and environmental effects. For horizontal-axis turbine(HAT) farm, the European Marine Energy Centre(EMEC) proposed guidelines that turbines must be spaced far apart, but there are no regulation and suggestion for vertical-axis turbine(VAT). Moreover performances of adjacent VATs are able to be increased or decreased, so that it is important to find suitable VATs arrangement.

The present paper deals with the numerical study on performance vertical-axis turbine system as arrangement considering interaction using CFD. All numerical study was performed using FLUENT which was based on Reynolds averaged Navier-Stokes(RANS) equations and contents of this study were consist of four parts.

For the study of first, hydrodynamic aspects on three-dimensional effects were investigated for VAT. Performance of VAT can be evaluated to convenient 2-D calculation for simple geometry but there are some discrepancy between experiment and 2-D results. In this respect, it is important to investigate the differences of flow characteristics between 2-D and 3-D. Numerical approach was made to reveal the differences of flow physics between 2-D estimation and rigorous 3-D simulation. It was shown that the 3-D effects were dominant mainly due to the variation of tip vortices around the tip region of rotor blade, causing the loss of lift for steadily translating hydrofoil and the reduction of torque for rotating turbine blade. Due to the 3-D effect, efficiency of 3-D turbine had discrepancy about 16% than 2-D efficiency on TSR=3.

For the study of second, numerical calculations were performed to investigate interactions between adjacent two turbines in terms of rotational direction, distance between turbines, diameter. For VATs, it has advantage that each turbine rotates counter-clockwise and clockwise direction, its power coefficient was higher about 9.2% than two times of single turbines' and such improvement caused by increasing velocity between turbines. Performance change were almost disappear that each turbine were spaced 15times of turbine diameter apart. On the other hand, for adjacent HATs, it doesn't have advantage and efficiency was decreased just about 4% than two times of single HAT on high TSR.

For the study of third, non-uniform inflow characteristics and turbine performance on the flow condition were investigated. Thermal and nuclear power plants on shore commonly use the sea water for cooling facility. Discharged cooling water has the high kinematic energy potential due to amount of water flux. Discharged channel can be assumed to a kind of confined water and flow characteristics around its inlet was similar with jet flow due to flux about 50ton per second. In this respect, numerical analyses were made to investigate for turbine performance on the non-uniform inflow condition in terms of turbine diameter to inlet size, axial distance, single and dual inlet. Basically, Performance of VAT operated on the non-uniform condition was decreased 15% compared with uniform condition. For the single inlet with HAT and VAT, the mean power coefficient appeared to be gradually decreased with increasing distance, and maximum power was obtained when the turbine diameter was same with the inlet diameter. For the dual inlet with VATs, better performance was obtained, compared with single inlet, due to turbine interaction when the turbine rotated clockwise-counterclockwise.

For the study of the last, performance of hextuple VATs according to various arrangements was investigated. It is reasonable that plenty of turbine systems are able to deploy in the ocean, but numerical calculations were performed to investigate turbine performance for only six VATs, the arrangement was included single-type and canard-types. Canard-types were consisted of Dual, Triple, Hextuple.

Numerical calculations were performed to compare with efficiency and power for single and Canard-types. For the single-type, performance was almost same with single turbines'. For the Dual, Triple, Hextuple, performances were increased about 13, 16, 18 percent each compared with single turbines'. Velocity deficits of far wake were compared for each types, velocity recovery of canard-types was slower than single-type. It means that longitudinal distance of canard-types for maximum power was required more than single-type.

To find out the best arrangement in terms of total power and power per unit, two-rows non-staggered and staggered arrangements were considered with respect to various scenarios for three sites. Among the canard-types, Hextuple was generally the best arrangement in aspect of total power and power per unit for scenarios, but dual and triple were mostly useful for flexible arrangements. Finally, hexagonal staggered dual and triple arrangements utilizing VATs were proposed.