Plasma Actuation Effect on a MW class Wind Turbine

The first trial test for applying plasma actuation technology on a 1.75 MW field rotor was carried out. Specially developed plasma electrodes of 8 m in length were installed on the surface of the leading edge of each blade. An increase in turbine rotational speed has been identified for the plasma-on cases compared with the plasma-off cases for the same wind speed. Also, histogram of inlet wind speed showed a trend that inlet wind speed was shifted to higher speed region for the plasma-on case compared with the plasma-off case. The mechanism of plasma actuation on this behaviour was examined in qualitatively using CFD analysis of model turbine. Consequently, an averaged power increase of 4.9 % was achieved in the test period. Possibility of increase in wind turbine power even in a commercial scale large turbine has been proved by leading-edge flow separation control using the plasma actuation technology. INTRODUCTION Owing to fluctuated natural wind, wind turbine blade constantly suffered from flow separation, as a result wind turbine performance gets worsen. Active separation control of flow on the blade is one of the most effective techniques to reduce energy cost for wind turbines, since it has a potential both to increase energy captured and to extend blade life by reducing blade load. A non-thermal dielectric-barrier-discharged (NTDBD) plasma actuator is known as one of the active flow control device. The plasma actuator has a number of distinct advantages over other active flow control devices for flow control on the wind turbine blade [1]. Some of the advantages are as follows; 1. It can induce very thin jet flow (controls boundary layer flow efficiently), 2. It is fully electric and can provide fast response time, 3. It requires no moving parts (moving parts always brings mechanical troubles), and 4.It can laminate into the turbine blade surface (no additional drag force is generated even if the actuator is broken). Schematic view of the NTDBD plasma actuator is shown in Fig.1. The plasma actuator consists of thin electrodes separated by a dielectric insulator. One of the electrodes is exposed to the air and the other electrode is fully covered by a dielectric material. When a high enough A.C. voltage is supplied to the electrodes the air gets ionized at the corner edge of the exposed electrode and spreads out over a region of the covered electrode. This ionized flow serves as a body force on the ambient air and results in induced thin jet flow generates from the actuator. When the actuator was operated in unsteady mode (pulsed modulation mode), separated flow of the blade was controlled most effectively (e.g., [2-4]). Effectiveness of the plasma actuation operating in unsteady mode was also reported on a periodically oscillated air-foil [5]. The mechanisms of plasma actuation on separation flow control were studied in detail using specially developed high-order CFD scheme by JAXA’s group [6-9]. They reported that unsteady plasma actuation plays some role to strengthen the span-wise vortex in the blade boundary layer and this strengthened vortex brings effective separation flow control. The unsteady plasma actuation effect for rotating blade was studied both on a perpendicular axis wind turbine [10] and on a horizontal axis wind turbine [11]. Matsuda et al. carried out wind tunnel experiments using a 300 W rated small wind turbine having the plasma actuator located at the leading-edge of each of the turbine blade. By changing the inlet wind velocity, the turbine rotational speeds were measured for both the plasma-on case and the plasma-off case. Clearly the rise in turbine speed was observed for the plasma-on cases as compared to that for the plasma-off cases. The possibility of improving wind turbine performance using the plasma actuation was reported. Based on these attractive results, the world first field test with this plasma actuation technology was carried out using Mie University 30kW (Φ 10m) wind turbine [12]. Visualization test of rotor blade flow was also carried out using tuft method with wireless-LAN controlled monitoring camera fixed at the blade root. Under constant blade rotational speed of 20 rpm and fixed yaw angle, tuft behaviour was clearly visualized and it became clear that these separation flows were controlled and suppressed very effectively by this technique [13]. These test results infer that leading-edge flow separations were effectively controlled by the plasma actuation and torque augmentation in wind turbine was realized even under real wind conditions. Covered electrode Bare electrode Dielectric Barrie discharge