Aerodynamic Optimization and Open Field Testing of ...

Aerodynamic Optimization and Open Field Testing of a 1 kW Vertical Axis Wind Turbine

PO. ID 101

Gabriele Bedon1, Marco Raciti Castelli1, Uwe Schmidt Paulsen2, Luca Vita2, Ernesto Benini1 1University of Padua, Department of Industrial Engineering – 2DTU Wind Energy, Department of Wind Energy

Abstract

Optimization Results

The design of a Darrieus wind turbine rotor is a complex process due to the nonstationarity of the flow field inside the machine and the dynamic stall effects at rotor blades. The combination of a genetic algorithm with a Blade Element – Momentum (BEM) code can be successfully adopted to improve the aerodynamic design of existing wind turbines.

The optimization Pareto front is reported in Figure 5. The individual represented by the red cross is chosen for the following considerations. The chosen configuration presents the chord distribution shown in Figure 6.

In this work, starting from the Venco Twister 1000-T vertical-axis wind turbine (VAWT), considered as a baseline configuration, the WOMBAT (Weatherly Optimization Method for Blades of Air Turbines) algorithm is adopted to provide the optimal chord distribution for a SCSc rotor characterized by the same swept area. The optimization is performed considering two objectives: the enhancement of the power coefficient for a wind speed of 7 m/s - the wind speed that maximizes the power production probability at the test site - and the maximization of the power coefficient for a wind speed of 12 m/s - the nominal wind speed of the baseline turbine.

Fig. 5: Pareto front from WOMBAT. Fig. 6: Chord distribution for the chosen individual. The optimal chord distribution is not uniform:

An optimized three-bladed rotor is designed according to the geometrical constraints imposed by the original turbine configuration and its aerodynamic performance is investigated at the DTU Wind Energy test ground.

• The minimum value of the chord at the rotor top and bottom leads to a lower aerodynamic resistance of the blade sections where the power production is very low.

Case Study and Methodology

• An increase of the chord length is registered towards the centre of the rotor, in order to maximize the aerodynamic load where the radius is increasing, thus enhancing the power production.

The Venco Twister 1000-T, tested at DTU Wind Energy, is considered as a case study in order to proceed subsequently with the experimental activity on the optimized prototype. Venco Twister 1000-T is a 2-meter high turbine characterized by three linear twisted blades placed at 1 meter from the rotational axis. A picture of the turbine is shown in Figure 1. The test conducted at the DTU Wind Energy test site highlighted a maximum power coefficient for the total conversion of around 0.20, as shown in Figure 2.

•The successive chord reduction represents a compromise between the increase of the aerodynamic load and the decrease of the interference factor, that would reduce the flow velocity across the rotor section. The rotor power coefficient and the power production for different rotational speeds are shown in Figures 7 and 8 as a function of the free-stream wind speed.

Fig. 7: Rotor Power Coefficient.

Fig. 8: Rotor Power Production.

Rotor Prototype Fig. 1: Venco Twister 1000-T.

Fig. 2: Twister 1000-T experimental CP curve.

In the optimized configuration, a fixed thickness to chord ratio is maintained along the blade span, whereas the chord distribution is free to be optimized. The NACA 0015 profile is chosen, since it provides one of the highest performance in combination with a variable chord distribution. The electrical generator needs to be maintained in the central position, due to the fact that the bearings are not designed to be stressed with a bending moment. The so-called ”cut SCS” (SCSc) variant is therefore adopted as optimization geometry, shown in red in Figure 3.

The rotor prototype is obtained by assembling several parts, designed in order to fit the original generator geometry and preserve the blade optimized shape. Rotor blades are manufactured adopting the rapid prototyping technique. They are made of ”Proto-plus”, a material with a density of 0.59 g/cm3 and a tensile strength of 40 MPa. Every blade is subdivided into three portions, in order to make their manufacturing and final assembly easy. As can be seen from Figure 9, rotor blades are assembled with the metal reinforcement bars; epoxy resin is then sucked up from the bottom to the top, in order to fill the holes between the steel and the blade parts, providing additional consistence. The prototype, installed on the 11-meter high tower at Risø - DTU Wind Energy, test site is shown in Figure 10.

Fig. 3: SCSc vs SCS geometry.

Fig. 4: WOMBAT Algorithm.

The rotor is mounted on a permanent magnet generator equipped with eddy current brakes. The optimization procedure will therefore consider the range between 0 and 270 rpm to find the best performance. A total of 50 individuals are evaluated and evolved for 100 generations, using the WOMBAT algorithm, represented in Figure 4. The genes are considered as control point coordinates for a spline curve that describe the trend of the chord. The fitness of the profile is calculated considering: • the individual power coefficient at a wind speed equal to 7 m/s, considered as a design wind speed. In fact, for this wind speed, the energy production probability for the Risø test site is maximized. • the power production at 12 m/s, considered as the maximum production. This value is the same declared by Venco for the original Twister 1000-T turbine configuration.

Fig. 9: Connection of blade sections.

Fig. 10: Installation at DTU Wind Energy test site.

Conclusions The aerodynamic design of a commercial vertical axis wind turbine is improved by adopting an optimization algorithm combined with a Blade Element - Momentum simulation code. Two target wind speeds are selected, in order to provide an optimized configuration suitable for the same working condition as the baseline configuration. The optimized rotor configuration results to be characterized by a variable chord distribution, obtained in order to maximize the energy conversion at two design wind speeds. A complete simulation campaign is conducted, showing that rotor performances are highly increased with respect to the baseline configuration. However, an experimental test is needed in order to confirm the predictions and to estimate the aerodynamic and electrical losses, not included in the simulation model. For this reason, a prototype obtained with the rapid prototyping technique is realized and installed on an 11-meter tower at the test site of DTU Wind Energy.

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