Aerodynamic Characteristics of Horizontal Axis Wind Turbine with Archimedes Spiral Blade

To investigate the aerodynamic characteristics of an Archimedes spiral wind turbine for urban-usage, both experimental and numerical studies were carried out. The Archimedes spiral blade was designed to produce wind power using drag and lift forces on the blade together. Instantaneous velocity fields were measured by two-dimensional PIV method in the near field of the blade. Mean velocity profiles were compared to those predicted by the steady state and unsteady state CFD simulation. It was found that the interaction between the wake flow at the rotor downstream and the induced velocity due to the tip vortices were strongly affected by the wind speed and resulting rotational speed of the blade. PIV measurements revealed the presence of dominant vertical structures at downstream the hub and near the blade tip. Unsteady CFD simulation results agreed well with those of PIV experiments than the steady state analysis. The power coefficient (Cp) obtained by CFD simulation demonstrated that the new type of wind turbine produced about 0.25, relatively high value compared to other types of urban-usage wind turbine. INTRODUCTION Among all renewable resources, wind energy has been proven to be a relatively matured technology and has tremendous potential in commercialization and mass production. Today the major application of wind power is electricity generation from large grid-connected wind farms. With the expansion of power grid and the reduction of electricity scarce areas, small-scale wind turbine has now been applied in fields such as city road lighting, mobile communication base stations, offshore aquaculture and sea water desalination in several countries [1]. In the small wind turbine field, two types of wind turbine will be classified as HAWT (Horizontal axis wind turbine) and VAWT (Vertical axis wind turbine). Archimedes spiral wind turbine, as new concept structure which using the Archimedes spiral principles [2], is one of the HAWT, but different from traditional HAWT that uses the lift force to take power from wind energy, the Archimedes spiral small wind turbine is mainly depended on the drag force. A schematic diagram of 0.5 kW class Archimedes spiral wind turbine is shown in Fig. 1. The Archimedes spiral wind turbine can be fully stripped of its kinetic energy by reversing the wind. This special structure decides the special aerodynamic characteristics in the small scale wind turbine. Particularly, in the circumstance such as around buildings, the advantage of the Archimedes spiral structure will more obviously according to the facts that there is always wind direction change and wind speeds are low[3]. In the previous researches related on the Archimedes spiral wind turbine, Theoretical and experimental researches have been carried out. Timmer and Toet set one model in the DNW wind tunnel at TU Delft with 0.14m radius and 3mm wall thickness to investigate the aerodynamic characteristics. In their report, the maximum power coefficient was measured at the optimum tip speed ratio. In 2009, the Archimedes spiral wind turbine model was improved by TU Delft. As a result, the highest efficiency measured was 12% [4]. Recently, Lu et al. [5] developed a design method of the Archimedes spiral wind turbine blade and performed a numerical simulation using ANSYS CFX v12.1. In the present study, an experimental study is conducted to find out the aerodynamic characteristics and evolution of the tip vortex structures in the near wake of the Archimedes wind turbine model installed in an open type wind tunnel. The