Abstract
In the year 2000, 15GW of wind power was installed throughout the world, producing 100PJ
of energy annually. This contributes to the total electricity demand by only 0.2%. Both the
installed power and the generated energy are increasing by 30% per year world-wide. If the
airflow over wind turbine blades could be controlled
... read more
fully, the generation efficiency and thus
the energy production would increase by 9%.
Power Control
To avoid damage to wind turbines, they are cut out above 10 Beaufort (25 m/s) on the wind
speed scale. A turbine could be designed in such a way that it converts as much power as
possible in all wind speeds, but then it would have to be to heavy. The high costs of such a
design would not be compensated by the extra production in high winds, since such winds are
rare. Therefore turbines usually reach maximum power at a much lower wind speed: the rated
wind speed, which occurs at about 6 Beaufort (12.5 m/s). Above this rated speed, the power
intake is kept constant by a control mechanism. Two different mechanisms are commonly
used. Active pitch control, where the blades pitch to vane if the turbine maximum is exceeded
or, passive stall control, where the power control is an implicit property of the rotor.
Stall Control
The flow over airfoils is called attached when it flows over the surface from the leading
edge to the trailing edge. However, when the angle of attack of the flow exceeds a certain
critical angle, the flow does not reach the trailing edge, but leaves the surface at the separation
line. Beyond this line the flow direction is reversed, i.e. it flows from the trailing edge
backward to the separation line. A blade section extracts much less energy from the flow
when it separates. This property is used for stall control.
Stall controlled rotors always operate at a constant rotation speed. The angle of attack of the
flow incident to the blades is determined by the blade speed and the wind speed. Since the
latter is variable, it determines the angle of attack. The art of designing stall rotors is to make
the separated area on the blades extend in such a way, that the extracted power remains
precisely constant, independent of the wind speed, while the power in the wind at cut-out
exceeds the maximum power of the turbine by a factor of 8. Since the stall behaviour is
influenced by many parameters, this demand cannot be easily met. However, if it can be met,
the advantage of stall control is its passive operation, which is reliable and cheap. Problem Definition
In practical application, stall control is not very accurate and many stall-controlled turbines do
not meet their specifications. Deviations of the design-power in the order of tens of percent
are regular. In the nineties, the aerodynamic research on these deviations focussed on: profile
aerodynamics, computational fluid dynamics, rotational effects on separation and pressure
measurements on test turbines. However, this did not adequately solve the actual problems
with stall turbines.
In this thesis, we therefore formulated the following as the essential question:
Does the separated blade area really extend with the wind speed, as we predict?
To find the answer a measurement technique was required, which 1) was applicable on large
commercial wind turbines, 2) could follow the dynamic changes of the stall pattern, 3) was
not influenced by the centrifugal force and 4) did not disturb the flow. Such a technique was
not available, therefore we decided to develop it.
Stall Flag Method
For this method, a few hundred indicators are fixed to the rotor blades in a special pattern.
These indicators, called stall flags are patented by the Netherlands Energy Research
Foundation (ECN). They have a retro-reflective area which, depending on the flow direction,
is or is not covered. A powerful light source in the field up to 500m behind the turbine
illuminates the swept rotor area. The uncovered reflectors reflect the light to the source, where
a digital video camera records the dynamic stall patterns. The images are analysed by image
processing software that we developed. The program extracts the stall pattern, the blade
azimuth angles and the rotor speed from the stall flags. It also measures the yaw error and the
wind speed from the optical signals of other sensors, which are recorded simultaneously. We
subsequently characterise the statistical stall behaviour from the sequences of thousands of
analysed images. For example, the delay in the stall angle by vortex generators can be
measured within 1° of accuracy from the stall flag signals.
Properties of the Stall Flag
The new indicators are compared to the classic tufts. Stall flags are pressure driven while tufts
are driven by frictional drag, which means that they have more drag. The self-excited motion
of tufts, due to the Kelvin-Helmholtz instability, complicates the interpretation and gives
more drag. We designed stall flags in such a way that this instability is avoided. An
experiment with a 65cm diameter propeller confirms the independence of stall flags from the
centrifugal force and that stall flags respond quickly to changes in the flow.
We developed an optical model of the method to find an optimum set-up. With the present
system, we can take measurements on turbines of all actual diameters. The stall flag responds
to separated flow with an optical signal. The contrast of this signal exceeds that of tuft-signals
by a factor of at least 1000. To detect the stall flag signal we need a factor of 25 fewer pixels
of the CCD chip than is necessary for tufts. Stall flags applied on fast moving objects may
show light tracks due to motion blur, which in fact yields even more information. In the case
of tuft visualisations, even a slight motion blur is fatal. Principal Results
In dealing with the fundamental theory of wind turbines, we found a new aspect of the
conversion efficiency of a wind turbine, which also concerns the stall behaviour. Another new
aspect concerns the effects of rotation on stall. By using the stall flag method, we were able to
clear up two practical problems that seriously threatened the performance of stall turbines.
These topics will be described briefly.
1. Inherent Heat Generation
The classic result for an actuator disk representing a wind turbine is that the power extracted
equals the kinetic power transferred. This is a consequence of disregarding the flow around
the disk. When this flow is included, we need to introduce a heat generation term in the
energy balance. This has the practical consequence that an actuator disk at the Lanchester-Betz
limit transfers 50% more kinetic energy than it extracts. This surplus is dissipated in
heat.
Using this new argument, together with a classic argument on induction, we see no reason to
introduce the concept of edge-forces on the tips of the rotor blades (Van Kuik, 1991). We
rather recommend following the ideas of Lanchester (1915) on the edge of the actuator disk
and on the wind speed at the disc. We analyse the concept induction, and show that correcting
for the aspect ratio, for induced drag and application of Blade Element Momentum Theory all
have the same significance for a wind turbine. Such corrections are sometimes made twice
(Viterna & Corrigan, 1981).
2. Rotational Effects on Flow Separation
In designing wind turbine rotors, one uses the aerodynamic characteristics measured in the
wind tunnel on fixed aerodynamic profiles. These characteristics are corrected for the effects
of rotation and subsequently used for wind turbine rotors. Such a correction was developed by
Snel (1990-1999). This correction is based on boundary layer theory, the validity of which we
question in regard to separated flow.
We estimated the effects of rotation on flow separation by arguing that the separation layer is
thick so the velocity gradients are small and viscosity can be neglected. We add the argument
that the chord-wise speed and its derivative normal to the wall is zero at the separation line,
which causes the terms with the chord-wise speed or accelerations to disappear. The
conclusion is that the chord-wise pressure gradient balances the Coriolis force. By doing so
we obtain a simple set of equations that can be solved analytically. Subsequently, our model
predicts that the convective term with the radial velocity (vrvr/r) is dominant in the equation
for the r-direction, precisely the term that was neglected in Snels analysis.
3. Multiple Power Levels
Several large commercial wind turbines demonstrate drops in maximum power levels up to
45%, under apparently equal conditions. Earlier studies attempting to explain this effect by
technical malfunctioning, aerodynamic instabilities and blade contamination effects estimated
with computational fluid dynamics, have not yet yielded a plausible result.
We formulated many hypotheses, three of which were useful. By taking stall flag
measurements and making two other crucial experiments, we could confirm one of those three
hypotheses: the insect hypothesis. Insects only fly in low wind, impacting upon the blades at
specific locations. In these conditions, the insectual remains are located at positions where
roughness has little influence on the profile performance, so that the power is not affected. In
high winds however, the flow around the blades has changed. As a result, the positions at which the insects have impacted at low winds are very sensitive to contamination. So the
contamination level changes at low wind when insects fly and this level determines the power
in high winds when insects do not fly. As a consequence we get discrete power levels in high
winds.
The other two hypotheses, which did not cause the multiple power levels for the case we
studied, gave rise to two new insights. First, we expect the power to depend on the wind
direction at sites where the shape of the terrain concentrates the wind. In this case the power
level of all turbine types, including pitch regulated ones, will be affected. Second, we infer
heuristically that the stalled area on wind turbine blades will adapt continuously to wind
variations. Therefore, the occurrence of strong bi-stable stall-hysteresis, which most blade
sections demonstrate in the wind tunnel, is lost. This has been confirmed by taking special
stall flag measurements.
4. Deviation of Specifications
The maximum power of stall controlled wind turbines often shows large systematic deviations
from the design. We took stall flag measurements on a rotor, the maximum power of which
was 30% too high, so that the turbine had to be cut out far below the designed cut-out wind
speed. We immediately observed the blade areas with deviating stall behaviour. Some areas
that should have stalled did not and caused the excessive power. We adapted those areas by
shifting the vortex generators. In this way we obtained a power curve that met the design
much more closely and we realised a production increase of 8%.
show less