DESIGN OF VERTICAL AXIS WIND TURBINE (H-SHAPED DARRIEUS)

The aim of this project was to design an H-shaped Darrieus vertical axis wind turbine (VAWT) and comment on the results.

After of the methodology used (double multiple stream tube), our attention was on the assumption and choices made during the implementation.

the results and their discussion are presented for the following three requests:

• Finding the optimal tip speed ratio by given a constant design value of undisturbed flow velocity.

• Finding the optimal tip speed ratios, according to variable values of undisturbed flow velocity (regulation).

• Commenting on results with fixed rotational speed and variable TSR (due to variable undisturbed flow velocity).

The profile for the three blades was the NACA 0021, for which a C.L and C.D distribution as a function of the angle of attack are given for the different values of the Reynolds number.

The main difference with the horizontal axis wind turbine (HAWT) is that in the VAWT the angle of attack changes every time during the revolution since the peripheral speed behaves in the same way. This makes the Re and so lift and drag varying with the blade motion. As consequences also the induction factor varies with azimuthal position

In this work the methodology chosen to model the VAWT is a combination of a double actuator disk is applied to each local stream tube. Thus, the turbine is modelled as a double multiple stream tube (DMST).

The algorithm of the Code in MATLAB was in the following scheme:

After The stream tube discretization, Interpolation to find CL and CD, Finite blade correction and by using Glauert correction we got these results:

As expected we realized the induction factor increases when the TSR increases and in particular the downstream values are higher than the upstream ones because the stream tubes in the downstream region take into account also the reduction of the velocity due to the upstream disks.

In these figures we can see lift (left) and drag (drag) coefficient with azimuthal angle variation for different values of TSR.

More results:

• normal force coefficient with azimuthal angle variation for different values of TSR (left)

• power coefficient (middle) and nominal power (right) with TSR variation

Regulation

The aim of this request was to simulate a regulation of the machine in order to guarantee in every wind speed condition the optimal rotational speed.

we got the increase in the performances was getting smaller as the undisturbed velocity increases. and drag coefficient with azimuthal angle variation for different values of undisturbed flow velocity.

Conclusion:

In this assessment the study was on a numerical tool based on Double Multiple Stream-Tube model, able to simulate the performance of a H-shaped VAWT, was performed. Beside the DSMT approach, the basic low-order correction (finite blade correction) was integrated to account for more complex aerodynamic phenomena, occurring during the wind-rotor interaction.

The main results are resumed here:

• The higher is the wind velocity, the higher are the performances but the optimal TSR remains 3. Thus, to keep the best efficiency, the machine must rotate at a velocity that is 2.4 times the wind one.

• Varying the wind velocity, a regulation is suggested in order to keep the best performances and to extract the highest power.

The inaccuracy of the analyses was due to the number of stream tubes used to discretize the diameter while the model presents some limits and critical issues. The modelling of the flow field was not comprehensive of vortexes generated by the blades, that are mutually affected, nor struts aerodynamic interaction with the flow.

PS:

• All The results and analysis extracted and done by MATLAB & Excel.

• Our project score was 30 out of 30, and it was top ranked project of course.

• More information are available upon request

• click HERE for downloading the Report.