Hello to everybody,
First of all, as I wasn’t sure if this question should go with CONTROL topics or it is more related with AERODYNAMICS I have decided to post it here.
I’m designing a VS-FP (passive stall,10 kW rated) WT blade for my Master Degree Final Project to be placed in a low wind town surroundings (with Weibull parameters k=1.79 and c=5.5). My doubts are the followings:
- As far as I know, in variable speed turbines we make the rotational speed increase proportionally with wind velocity until a certain point so that we work in the optimum TSR for the turbine. I have seen in some books that the Cp remains constant and Cp=Cpmax within this range of velocities as it can be seen in the attached figure.
My concern is that when I design the blade with BEM corrected method using Matlab I optimize the twist angle of each section of the blade to reach maximum glide ratio (Cl/Cd). I choose a design velocity (the most probable in the emplacement) and this gives a different chordal Re number in each section, so the angle of attack in which the maximum Cl/Cd ratio are evaluated for the precise Re number that occurs in each spanwise point.
Mantaining a constant TSR within a range of velocities will make the angles of attack to remain constant and equal to those of the design, but with different wind velocities (from the velocity used in the design), wouldn`t we have our blade sections working in different reynolds regimes, and therefore nearly optimally (but not exactly)?
- My second question is directly related to the control system issue. How can I set the limit of the gap where my WT will work in variable speed? I mean, it will start working variable speed with the Cut-in velocity, but how can I establish a criteria to decide at which wind velocity it will switch to constant velocity. Could it be a tip noise matter? Which are the practical drawbacks of being working in VS through a wide range of velocities?
Thank you all in advance for your precious time.