A Novel Vertical-Axis Wind Turbine for Distributed & Utility Deployment

A Novel Vertical-Axis Wind Turbine for Distributed & Utility Deployment J.-Y. Park*, S. Lee* +, T. Sabourin**, K. Park** * Dept. of Mechanical Engineering, Inha University, Korea + KR Wind Energy Research Institute, Korea ** KR Windpower, Inc., U.S.A. ABSTRACT KR Windpower, Inc. has developed a patented vertical-axis wind turbine line for both utility and distributed applications. Utility-scale turbines (1.5MW) are currently being installed in wind projects in Asia, while concomitant certification is ongoing. The KR wind technology has very high power-to-swept area efficiency and therefore works especially well around houses and buildings and in other applications where traditional three-blade horizontal axis turbines do not. Additionally, due to its low rotor speed (but with high torque, even at low wind speeds) and optimal rotor design, KR s patented turbine is vitually noiseless, making it especially attractive for installation in residential and recreational areas. Field testing of 1, 3, and 5kW turbines has resulted in cut-in speed of 2.5m/s and rated speeds of 9~12.5m/s. Thus, electricity is generated even at low wind speeds. This paper presents measured performances of a small-scale, vertical, wind turbine rated as 1 kw which has a tail consisted of a stabilizer and a rudder. Compared with the maximum power coefficient of 0.59 for a downsized model measured in a closed-type wind tunnel, the maximum turbine power coefficient averaged 0.5 at a tip-speed ratio of 0.5-0.6 in an open-type wind tunnel. The KR small-class turbines are based on IEC wind turbine class III, which specifies that the highest 10 minute-mean wind speed is 37.5m/s or less for the last 50 years. However, steady loadings by the extreme wind model of 52.5m/s were implemented, and the turbine was proven to be safe by its furling tail. A key element of KR Windpower s strategy is to provide a turbine product, including a wind/solar hybrid concept, which addresses power and aesthetic needs of developers and communities. Key Words: Wind Turbine, Vertical-axis, Distributed, Utility, Residential Nomenclature A : Projected area of turbine rotor C p : Power coefficient D : Rotor diameter (r=d/2) H : Turbine height ρ : Air density P t : Turbine output power P w : Wind power V : Wind speed λ : Tip Speed Ratio (TSR) ρ : Air density ω : Turbine rotor angular rotational speed

INTRODUCTION The trends in the wind power industry products can be summarized as energy cost saving, power reliability, grid support, and environment. The development of a reliable wind turbine should comply with community noise and aesthetic requirements as well as meet a strong need for high capacity (1). The Wind Turbine Generator Systems are classified as the horizontal axis wind turbine (HAWT) and the vertical axis wind turbine (VAWT) based upon whether the axis of rotation is parallel or perpendicular to the ground. The average electric power produced by the wind turbine strongly depends on the wind environment. In technical terminology, it is proportional to the efficiency of the rotor, air density, projected area of the turbine, and cube of wind speed. As an example, the average wind speed of 7m/s results in approximately 20% of the rated power at the wind speed of 12m/s. Thus, the ratio of the actual yearly output to the rated power of the turbine, which is called the capacity factor, should be increased to guarantee the economics of the turbine via increase in the rotor size or the turbine efficiency. The typical examples of VAWT are the Darrieus and the Savonius (Menet et al. (2), 2001; Menet (3), 2004; Blackwell et al. (4), 1978; Morcos et al. (5), 1996) VAWT's powered by the lift force and the drag force, respectively. These concepts were extensively examined in both United States and Canada in the 1970's and the 1980's. Despite their appealing features, those schemes have disappeared in the large-scale wind turbine market due to their low efficiencies compared with HAWT's. As was reported by Blackwell et al. (1978), the Savonius rotor of two blades achieved the power coefficient of 0.2 at the tip speed ratio of 0.8, the vertical devices have resulted in inherently low efficiencies and had structural problems in extreme winds. Nonetheless, the low rotational speed of the VAWT rotor implies that the machine will be quieter than the high-rotational speed HAWT s, thereby being potentially suitable for applications closer to population centers. The visual aesthetic is a critical issue in siting as their view shed is an important consideration. The high-speed propellers may offend some people in the community, while a slowly rotating machine may be considered as visual art (6). The KR jet-wheel-turbo turbine is composed of inlet-guide vanes, a side-guide vane, and a turbo-type rotor, and its functional assembly makes it unique from all the Savonius rotors or the ever-modified ones. To minimize inevitable negative torque generation for the conventional Savonius rotor and to accelerate the incoming jet into the rotor blade cascade, the round inlet-guide vanes are placed upstream of the rotor. While the air flows in the blade passage downstream after passing through the upstream blade for the Savonius rotor, the air passes over both sides of the blades and leaves the rotor through both side openings of the turbo-type rotor. The side-

guide vane also recovers wind energy by collecting the streamlines into the blade again, otherwise they would simply pass-by. The design details of KR VAWT can be found in the references (7). This paper presents the measured performance of a small-scale, vertical, wind turbine rated as 1kW which has a tail consisting of a stabilizer and a rudder. The aspect ratio H/D and the number of blades NZ can be adjusted to the site wind condition and the turbine price. To evaluate the performance change by both design parameters H/D and NZ, a downsized model of 580mm was tested in a closedtype wind tunnel (8). Based on the model performance data, the standard versions of 1, 3, and 5kW proto-turbines were made for intensive wind power testing on the site. In this paper, the 1kW, grid-tie turbine of 2.44m (D) 1.26 m (H) was tested for its electric power produced at specified wind conditions in an open-type wind tunnel. To eliminate the inevitable blockage effect by the size of turbine similarly compared with the test-section size of the wind tunnel, the flow deceleration effect of the incoming air to the turbine was analyzed through model testing and numerical simulation and implemented to the proto-type testing so as to keep the same similarity of incoming velocity distribution between the case of turbine sitting in an open space and the wind tunnel testing case. COMMUNITY-FRIENDLY JET-WHEEL-TURBO VAWT Basic Theory of Wind Turbine The output power P T from a turbine rotor and the wind kinetic energy per unit time P W are given by Equations (1) and (2). The rotor power coefficient C p is defined as the ratio between the rotor output power and the dynamic power of the air as shown in Equation (3). (1) (2) (3) The C p can generally be expressed as a function of the tip speed ratio, λ, defined as shown in Equation (4). The tip speed ratio λ at the maximum power coefficient is usually determined according to the turbine type. (4)

The rotor power coefficient is regarded as the energy transformation efficiency. The KR vertical-type wind turbine is designed to operate within 0.4 and 0.6 tip speed ratio to achieve the highest rotor efficiency ever developed. Principles of Jet-Wheel-Turbo Turbine The incoming wind flow-rate should be equal to the outgoing flow-rate to satisfy the mass conservation law if a control volume around a turbine is assumed. The outgoing wind-speed distribution and its direction strongly determine the turbine efficiency. As can be seen in Fig. 1, the upstream area (A 1 ) is smaller than the rotor area (A 2 ) of the same stream tube due to the rotation of the rotor, thereby resulting in unavailable wind energy upstream. The momentum transfer, which can be computed by the difference of the upstream and downstream wind speeds, may be maximized by reducing the wind speed (V 3 ) in the wake. Fig. 1 Stream tube passing the rotor plane The inlet- and side-guide vanes of the KR turbine are designed to make the streamlines as parallel to each other as possible (A 1 A 2 ), and the rotor wake is minimized by discharging the air through the upper and bottom openings of the rotor (7). KR 1kW VAWT Model Specifications The technical details of 1kW model are described in Table 1. The furling tail is attached to the guide-vane system which is passively forced to rotate by the stabilizer. Similarity of Incoming Flows to Turbine To determine the deceleration ratio of the velocity in front of the turbine to the free stream wind velocity, a numerical simulation was carried out for a turbulent flow

Table 1 1kW KR VAWT Specifications Nominal Power (1 kw) Type Vertical axis type Blade number 7 Rotor Diameter Height Rotor hub material Blade structure Swept area Rotational speed range Rotor speed control 2.44 m 1.26 m SS400 2-Dimensional, arc-type blade 3.07 12.0 ~ 48.9 rpm I.G.V. & S.G.V. position control Blade # of blades 7 Material PVDF Membrane Bearing Operating Conditions Main thrust bearing Main radial bearing Guide vane bearing Cut-in wind speed Rated wind speed Furling wind speed Survival speed Taper Roller Self-adjusting Taper Roller 3.0 m/s 12.5 m/s 17 m/s 50 m/s Gear type/overall gear ratio Power Transmission Lubrication Type Rated power Girth Gear/6.35:1 No lubrication PMG 1.8 kw Generator Rotational speed range Rated voltage 0 ~ 310 rpm 500 Vdc Protection level IP 54 I.G.V. + S.G.V. Control mechanism Passive control by stabilizer and rudder Control Extreme wind control Furling tail & spring back around the turbine. Figure 2(a) shows the observation locations in front of the turbines and velocity vectors at one cross-section of the turbine. The average wind speed is observed to decrease to 0.7 times of free-stream one in front of the turbine according to Fig. 2(b), in which x/d represents the normalized distance from the center of the turbine. Measurements with the downsized model of 580mm in

(a) (b) Fig. 2 Simulated velocity field; (a) velocity vectors at one cross-section around turbine, (b) deceleration ratio at observation locations in front of turbine Fig. 3 Measured velocity distributions upstream and in front of turbine for downsized model of 580mm the wind tunnel test resulted in 0.87 times free-stream one in front of the turbine, which is higher than the simulated one. The discharging velocity distribution from the wind tunnel is set to have the similarity as close as possible to the measured one for the downsized model. The free stream velocity is computed by multiplying 1/0.87 to the test section exit velocity to account for this deceleration effect. Stabilizer and Rudder Design The KR turbine experiences non-symmetric torques generated by the inlet- and side-guide vanes. Unless an extra control force for the guide vane is applied for the alignment with the wind direction, the guide-vanes will have an offset angle to the wind direction, resulting in a significant drop of efficiency. The KR small-class turbine was designed to have the tail consisting of the stabilizer and a rudder. As can be seen in Fig. 4, the net torque from the guide-vane and the stabilizer is found to have zero-crossing angles at 20 o and 180 o by a numerical simulation. To make 20 o offset to move to 0 o meaning the alignment with the wind, the rudder of an

NACA0012 airfoil is attached to the stabilizer with an incidence angle of 70 o. Under extreme wind conditions (9), e.g. 25m/s, the tail consisting of the stabilizer and the rudder is furled via a hinge against an elastic spring so that the guide-vane system rotates to a yaw angle of 110 o, thereby the air-braking acts on the rotor as can be shown in Fig. 5.. Fig. 4 Torques for guide-vane and stabilizer at various angles of incoming wind (V =7m/s) Fig. 5 Yaw angle of guide-vane and rotational speed of rotor by change of setting angle between wind and stabilizer with rudder (V =7m/s) EXPERIMENTAL RESULTS Wind Tunnel Specification The performance of the proto turbine was tested in an open-type wind tunnel shown in Fig. 6. The wind tunnel consists of a vane-axial fan, a diffusing part, a reducer and a test section wrapped by silencing materials. The test section has a

cross-section of 2.3m 2.1m and affords a change in the wind speed from 0 to 16.7m/s continuously within low turbulence intensity levels. The wind speed in the wind tunnel is automatically computed by using a digital micro-manometer with density corrections by the measurement of room temperature and humidity. Performance Prediction and Measurement The PMG-type generator is connected to a girth gear via a pinion gear to measure the grid-tie performance by applying variable loads to the system. The rotational speed of the rotor and the electric power were simultaneously measured and stored in a data logger via RS-232 communication. The performance of the KR 1kW proto-type turbine was predicted at the design stage by considering the effects of the blade number and the aspect ratio (H/D) on the power coefficient (C p ). Increasing the number of blades results in an asymptotic increase in the power coefficient, and the aspect ratio H/D near 0.5 gives the maxima of the power coefficient for the case of NZ=12, as shown in Fig. 8. Fig. 6 Schematic view of open-type wind tunnel and test turbine Fig. 7 Apparatus setup for performance measurement While the KR 1kW proto-type turbine which has 7 blades and the aspect ratio of 0.52 is supposed to have a maximum power coefficient of 0.52, the measured performance in the wind tunnel test results in 0.47 as a maximum as shown in Fig. 9(a). This discrepancy is considered to come from the one-sided, compliant blade

surface, which is made of membrane. The grid-tie overall electric power coefficient is summarized for various wind speeds with corrections described previously in Fig. 9(b), where the overall transmission efficiency by gear, generator and inverter losses is estimated as 63%. Figure 10 shows the hybrid concept of wind and solar energy, which addresses both power and aesthetic needs of developers and communities. (a) (b) Fig. 8 Maximum power coefficients measured in closed-type wind tunnel; (a) (C p ) max vs. NZ at H/D=0.8, (b) (C p ) max vs. H/D at NZ=12 (a) (b) Fig. 9 Power coefficients vs. tip speed ratio at various wind speeds of 1kW model; (a) turbine power coefficient, (b) grid-tie system power coefficients

Fig. 10 Drawings of a hybrid system of wind and solar energy CONCLUSIONS In an effort to adjust the aspect ratio H/D and the number of blades (NZ) of KR VAWT s to satisfy the requirement of the capacity factor at a given site, the 1kW, gridtie turbine of 2.44m (D) 1.26 m (H) was tested for its electric power at specified wind conditions via an open-type wind tunnel. While the KR 1kW proto-type turbine of 7 blades and the aspect ratio H/D of 0.52 is considered to have a maximum power coefficient of 0.52, the measured performance in the wind tunnel test results in 0.47 as a maximum. To eliminate the inevitable blockage effect by the size of the turbine similarly compared with the test-section size of the wind tunnel, the flow deceleration effect of the incoming air to the turbine was implemented to the proto-type testing so as to keep the same similarity as the incoming velocity distribution between the case of turbine sitting in the open space and the wind tunnel testing case. All KR small-class turbines are designed to meet the structural safety requirements for IEC wind turbine class III. Under extreme wind conditions, e.g. 25m/s, the tail consisting of the stabilizer and the rudder is furled via the hinge against the elastic spring to protect the rotor from over-speeding. A key element of KR Windpower s strategy is to provide a turbine product, including a wind/solar hybrid concept, which addresses power and aesthetic needs of developers and communities. REFERENCES (1) "US Wind Power Markets and Strategies" Emerging energy research, 2007 (2) Menet, J.-L., Valdes, L.-C. and Menart, B. A., 2001, A Comparative Calculation of the Wind Turbine Capabilities on the Basis of the L-σ Criterion, Ren. Energy, Vol. 22, pp. 491-506 (3) Menet, J.-L., 2004, A Double-Step Savonius Rotor for Local Production of Electricity: A Design Study, Renewable Energy, Vol 29, pp. 1843-1862.

(4) Blackwell, B.B., Sheldahl, R.E. and Feltz, L.V., 1978., Wind-tunnel Performance Data for Two and Three-Bucket Savonius Rotors, Sandia Laboratories Energy Report, SAND76-0131, also Journal of Energy, Vol 2, pp. 160-164. (5) Morcos, V. H., and abdel-hafez, O.M.E., 1996, Testing of an Arrow-head Verticalaxis Wind Turbine Model, Renewable Energy, Vol. 7, Issue 3, pp. 223-231. (6) Bell B. "A Technical Opinion on A Novel Vertical-Axis Wind Turbine Concept" Garrad-Hassan Report, 2007 (7) Lee, S., Nam, S.-K., Power Generation System having Vertical Wind Turbine of Jet-Wheel Type for Wind Power, PCT/KR2007/002902 (8) Kim, B.-K., et al., An Experimental Study on the Performance of the Vertical-Axis Wind Turbine, Journal of Fluid Machinery, Vol. 10, No. 5, pp. 17-24, 2007 (9) "Guideline for the Certification of Wind Turbines" Germanischer Lloyd, 2003