Abstract:
Simplification of the aerodynamic control of large horizontal
axis wind turbines (HAWTs) has been identified as an important step
towards improved reliability and reduced cost. At present the
majority of large HMrrs use active control to regulate power and
loads. A simpler strategy is to use the inherent stalling of the
rotor blades in high winds to limit power and loads.
Unfortunately the performance of stall regulated HAWTs 1S poorly
understood; current performance models often fail to correctly
predict peak power levels. The benefits of passive control of power
and loads cannot be utilised because of this uncertainty.
This study examines the possible reasons for the poor performance
of current prediction techniques 1n high winds with the objective of
fonmulating a new model.
The available experimental evidence suggests that rotor stall is
caused by turbulent separation at the rear of the blade aerofoil,
growing in extent from the root in increasing wind. This 'picture'
of the stalling HAW! rotor forms the basis of the approach. The new
model consists of a prescribed vortex wake, first order panel method
(extended to represent the viscous region of trailing edge
separation) and three dimensional integral boundary layer directly
coupled in an iterative scheme.
A sensitivity study of rotor
indicates that the most important
performance to wake geometry
factor is the rate at which the
wake is convected downstream. However, it is found that stalled
power levels are insensitive to wake geometry; the study concludes
that the problem of poor prediction of high wind performance lies on
the rotor blades.
Before using the complete code to calculate the performance of a
rotor it 1S first tuned for the aerofoils used on the blade.
Aerofoil perfonmance characteristics measured in a wind tunnel are
synthesised by the model. Ideally these characteristics should
include measured pressure profiles below and above stall.
Validation of the complete code against detailed measurements
taken under controlled conditions on a three metre diameter machine
indicates significant differences in the perfonmance of aerofoil
sections on a wind turbine blade when compared to the same section
when tested in a wind tunnel. Derived lift coefficients show a
reduced lift curve slope and more gentle delayed stall.
Similar results are found when the code is applied to two Danish
stall regulated machines. These two machines although having very
similar geometries and using the same family of aerofoils do however
show differences in derived post stall drag. This is thought to be
due to the different thickness distributions of the two rotors.
The validation and applications of the new model show that it can
accurately predict the peak power level of stall regulated machines.