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Abstract.OpenFoam is used to compare computational fluid dynamics (CFD) with blade element momentum theory (BEM) for a variable speed -variable pitch HAWT (Horizontal Axis Wind Turbine). The wind turbine is first designed using the BEM to determine the blade chord, twist and operating conditions. The wind turbine blade has an outer diameter of 14 m, uses a NACA 63-415 profile for the entire blade and root to tip twist distribution of 15 deg ( Figure 3). The RPM varies from 20-75 for freestream velocities varying between 3-10.5 m/s (variable speed) and a constant RPM of 78.78 for velocities ranging between 11-25 m/s (variable pitch). OpenFOAM is used to investigate the wind turbine performance at several operating points including cut-in wind speed (3 m/s), rated wind speed (10.5 m/s) and in the variable pitch zone. Simulation results show that in the variable-speed operating range, both CFD and BEM compare reasonably well. This agreement can be attributed to the fact that the complex three-dimensional flow around the turbine blades can be split into two radial segments. For radii less than the mid-span, the flow is three-dimensional, whereas for radii greater than the mid-span, the flow is approximately two-dimensional. Since the majority of the power is produced from sections beyond the mid-span, the agreement between CFD and BEM is reasonable. For the variable-pitch operating range the CFD results and BEM deviate considerably. In this case the majority of the power is produced from the inner sections in which the flow is three-dimensional and can no longer be predicted by the BEM. The results show that differences in pitch angles up to 10 deg can result to regulate the power for high wind speeds in the variable-pitch operation zone.

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In this paper a variable speed variable pitch horizontal axis wind turbine (HAWT) is designed using the blade element momentum (BEM) model and experimentally tested in an open section wind tunnel. The BEM offers a fast design approach since it is essentially a twodimensional strip theory model and is used for all preliminary designs and for real time wind turbine control. Two wind turbines were designed that have the same rated power (0.45 kW). In the first design the blade is composed of a constant thickness blade (t/c = 15%). In the second design the blade is composed of a variable thickness blade varying from t/c = 25% at the hub to t/c=21% at the tip. The model results show that thick blades are less sensitive to errors in the pitch angles as the turbine switches from variable speed to variable pitch control. The thick blades were also easier to manufacture using CNC machines. Three sets of experimental results were obtained for pitch angles of -5 o , 0 o and 10 o . The experimental measurements compare favorably for the 0 o and 10 o which correspond to cases in which the power coefficient is large (order of 10%). The experimental measurements for the -5 o does not compare well with the model. This is attributed to two issues. First the blades operate in the stall regime and hence the BEM model is not valid, and second the power coefficient is relatively small (order of 1%). Overall the BEM theory compares well with experiment, with a correlation coefficient, R 2 , of 0.9. Nomenclature A = swept area a = local axial induction factor a ' = local swirl or angular induction factor B = number of blades C = chord of local airfoil C p = coefficient of performance C l = lift coefficient C d = drag coefficient R = blade length t/c = thickness to chord ratio U rel = local relative speed V = wind speed  = local angle of attack  = air density  = rotational speed of wind turbine  = local flow angle  T = local twist angle