Here is a list of papers that are of special interest to those
involved with furling wind turbines:
Measurements are reported of velocity and pressure fields in the wake of a yawed rotor. Deflection of fluid by transverse pressure gradients is accompanied by vertical displacement of slow-moving wake fluid from the horizontal plane at hub height.
Restoring moment on yawed rigid rotors are caused largely by departures from radial independence. A method for calculating such moments is presented, and the results compared with published data.
The on-line approach to characterizing the load spectra and the
inflow turbulent scaling parameter produces results that are consistent
with other measurements. The on-line approximation of the turbulent
shear stress or friction velocity u* also is considered adequate. The
system can be used to characterize turbulence loads during turbine
deployment in a wide variety of environments. Using the WISPER
protocol, we found that a wide-range, variable-speed turbine will
accumulate a larger number of stress cycles in the low-cycle,
high-amplitude (LCHA) region when compared with a constant speed rotor
under similar inflow conditions.
This paper explores variable-speed operating strategies for wind
turbine applications. The objectives are to maximize energy production,
provide controlled start-up and reduce torque loading. This paper
focuses on optimizing the energy captured by operating at maximum
aerodynamic efficiency at any wind speed.
The control strategy we analyze uses rotor speed and generator power
as the feedback signals. In the normal operating region, rotor speed is
used to compute a target power that corresponds to optimum operation.
With power as the control objective, the power converter and generator
are controlled to track the target power at any rpm. Thus, the
torque-speed characteristic of the generator is shaped to optimize the
energy capture. The target power is continuously updated at any rpm. In
extreme areas of the operating envelope, during start-up, shutdown,
generator overload, or overspeed, different strategies driven by other
system considerations must be used.
The furling and soft-stall control strategies are compared using
steady state analysis and dynamic simulation analysis. The soft-stall
method is found to offer several advantages: increased energy
production at high wind speeds, energy production which tracks the
maximum power coefficient at low to medium wind speeds, reduced furling
noise, and reduced thrust.
The design and operational history of the Model 1000 is discussed in
detail.
This paper presents and discusses the results of detailed
strain gauge measurements taken from the surface of a composite blade
on a small horizontal-axis wind turbine. We believe that these are the
first such measurements to be taken on a small wind turbine. The
two-bladed upwind machine is located adjacent to Fort Scratchley, a
historic seaside landmark in Newcastle, and has a nominal rated output
of 5 kW at a wind and rotor speed of 10 m/s and 400 RPM respectively.
A series of field experiments were undertaken where the aeroelastic
response of the blade was measured with 26 surface mounted strain
gauges, along with the wind speed and direction, turbine speed and
direction and turbine power output. The results show a periodic
fluctuation in both the blade flapping and lead-lag directions with a
once pre revolution period.
For some years a group at the University of Newcastle,
Australia, has been researching aspects of small horizontal axis wind
turbines, and this paper presents results of field tests recently
performed on a 5kW prototype free yaw machine. This machine, which is
described in the paper, used two low solidity tapered and twisted
blades, and a control system which attempted to maintain a constant tip
speed ratio of 10, the theoretical optimum for the rotor. Measurements
included rotor speed, rotor power, turbine yaw, and wind speed and
direction. Testing was carried out using both standard power
performance tests, and what we termed instantaneous tests which used
high sampling rates to capture the machine's instantaneous power and
yawing. Results were collated using various binning procedures, with
the 'standard" results showing a rotor power coefficient of about 0.45
and that the tip speed ratio was maintained close to 10 over a wide
wind speed range. Instantaneous results showed some interesting
effects of yaw on machine performance, and these will be detailed and
discussed. As well, to provide for a more general categorization of
free yaw response, a 'yaw index' is introduced and applied to
instantaneous data with the turbine operating with different tail fins;
these results and the applicability of the index are also discussed.
Most small wind turbines are designed to autofurl in high
winds under aerodynamic and gravity forces in order to provide
overspeed protection. Although autofurling is a proven technology with
a long history, this totally passive mechanism is unfortunately not
well understood and can perhaps be improved through careful analysis
and redesign. Moreover, including an active yaw actuator such as a
linear motor to supplement the passive autofurling mechanism by
supplying a yaw moment can improve the turbine performance in high
winds and strengthen the protection function. This paper proposes a
mathematical model of small wind turbines and their autofurling
mechanism.
This paper describes the design, implementation, and
testing of a prototype version of a peak power tracking system for
small wind turbines in battery charging applications. The causes for
the poor performance of small wind turbines in battery charging
applications are explained, and previously proposed configurations to
increase the power output of the wind turbines are discussed. Through
computer modeling of the steady-state operation, the potential
performance gain of the proposed system in comparison with existing
systems is calculated. It is shown that one configuration consisting of
reactive compensation by capacitors and a dc/dc converter is able to
optimally load the wind turbine and thus obtain maximum energy capture
over the whole range of wind speeds. A proof of concept of the peak
power tracking system is provided by building and testing a prototype
version. The peak power tracking system is tested in combination with a
typical small wind turbine generator on a dynamometer. Steady-state
operating curves confirming the performance improvement predicted by
calculations are presented.
A new blade is designed for a small, variable-speed wind
turbine by relying on available theoretical design and analysis
methods. The performance predictions are compared to field test
measurements and are found to be optimistic. This feedback sheds light
on the interpretation of the theoretical results and is used to refine
the design method.
This paper describes some investigations of the dynamic yaw
behaviour of tail fins and wind vanes employing delta wing planforms.
Both single and twin wings were tested, the later were formed from two
single wings joined at their apices. Comparisons were made using
simple wind tunnel models with aspect ratios from 0.7 to 3.07, and
various arm lengths, wind speeds and planform areas. The twin wing was
found more highly damped than an equivalent single wing, with damping
increasing as the angle between the winds increased. Stall related
problems became evident as this angle approached or exceeded that for
static stall. A computational model based on unsteady potential
slender wing theory was developed to predict yawing motions of both
tail fin types and compared with experiments. The computational model
was found remarkably accurate for the single wind case even at high
initial angles of yaw, but generally inaccurate for the twin wing.
Possible reasons for this inaccuracy and the superior twin wing
response are given.
This paper summarizes experience gained to date in
attempting to model the yawing and furling behavior of small wind
turbines. This behavior is critical to passively controlling
overshoots in power and loads experienced by such turbines at high wind
speeds. The approach to modeling this behavior is described, and
predictions are compared to the limited reliable truck and field test
data that are available. These comparisons substantiate anticipated
uncertainties in the reliability of the modeling and help sharpen the
focus on areas needing improvement to increase the accuracy of the
predictions. These areas include especially the predictions of
aerodynamic loads on the rotor and the tail located aft in the wake of
the rotor. The need for more reliable test data to help guide this
effort is emphasized.
Small wind turbines are typically used for the remote or
rural areas of the world including: a village in Chile; a cabin
dweller in the U.S.; a farmer who wants to water his crop; or a utility
company that wants to use distributed generation to help defer building
new transmission lines and distribution facilities. Small wind
turbines can be used for powering communities, businesses, homes, and
miscellaneous equipment to support unattended operation. This paper
covers the U.S. Department of Energy/National Renewable energy
Laboratory Small Wind turbine project, its specifications, its
applications, the subcontractors and their small wind turbine concepts.
In this paper we discuss an on-line turbulent load
characterization system that has been designed to acquire loading
spectra from turbines of the same design operating in several different
environments and from different turbine designs operating in the same
environment. This system simultaneously measures the rainflow-counted
alternating and mean loading spectra and the hub-height turbulent mean
shearing stress and atmospheric stability associated with the turbulent
inflow. We discuss the theory behind the measurement configuration and
the results of proof-of-concept testing recently performed at the
National Wind Technology Center (NWTC) using a Bergey EXCEL-S 10-kW
wind turbine.
This report describes an experiment to measure the airflow
over a truck cab that can be used to conduct steady-state tests on an
8-kW wind turbine. The cab airflow measurements were made to document
the turbine inflow for analytical models. The airflow measurements
were made with an array of anemometers positioned to represent the
turbine rotor disk. The data showed that the influence of the truck
cab was primarily in the lower sector of the rotor disk. The influence
was negligible in the rest of the rotor disk.
The use of induction generators in wind power applications
has been common since the early development of the wind industry. Most
of these generators operate at fixed frequency and are connected
directly to the utility grid. Unfortunately, this mode of operation
limits the rotor speed to a specific rpm. Variable-speed operation is
preferred in order to facilitate maximum energy capture over a wide
range of wind speeds.
Many small wind turbines are designed to furl (turn) in
high winds to regulate power and provide overspeed protection. Furling
control results in poor energy capture at high wind speeds. This paper
proposes an alternative control strategy for small wind turbines- the
soft stall control method.
In the paper, results are presented from a Dutch research
project, carried out by the Netherlands Energy Research Foundation,
ECN. The project aimed at the development of an improved engineering
model for the prediction of the unbalance in the inflow of a wind
turbine rotor at yawed conditions. The yaw model was derived from
inflow measurements which are performed in the rotorplane of a 1.2 m
diameter wind turbine, placed in the open jet wind tunnel of the Delft
University of Technology, DUT in Holland. The model has been validated
with load measurements in yaw which were made on several model wind
turbines placed in windtunnels and on a full scale wind turbine. In
many validation cases, the new yaw model resulted in a considerable
improvement of the prediction of the loads at yawed conditions.
This paper details the 7 year developmental history of a
series of exceptionally rugged, proven SWECS designed for remote
battery charging applications. the most notable feature of this
equipment is the use of side turning rotor overspeed control
techniques, which has created a very simple and highly reliable system
of low mechanical complexity.
This paper deals with the development of an International
Standard for small wind turbines within IEC and with the experimental
validation of design rules given in the standard within a research
project supported by CEC DG12 ( Joule II Programme). Furthermore it
describes recent activities of ECN to develop a certification procedure
for small wind turbines.
Detailed velocity measurements have been performed in the
open jet wind tunnel in the near wake of the rotor model. The wake
velocities have been measured with hot-wire equipment, providing a very
fast and accurate measuring method. For yaw angles 10º, 20º, and 30º
of the rotor, radial traverses are performed in 12 azimuthal direction
at an axial distance of x/R = 0.05. At every position of the
traverses, velocity samples are taken every 0.5º azimuth angle of the
rotor. In previous experiments, it has been shown that the velocity
signal is well determined, The passage of blades, vortex sheets and
the influence of the tip vortex can be recognized clearly. As results
will be presented: azimuthal distribution of the velocity at stationary
probe positions, velocity distribution over the rotor plane for several
yaw angles and the effect of yaw on the induction factor.
After testing three trailing edge flap configurations which
didn't significantly improve the water pumping performance of a wind
turbine, additional research was performed to understand why the
trailing edge flap configurations were unsuccessful. A camera was
mounted at the root of one of the blades with tufts taped to the
outboard half of the blade suction surface. Pictures were taken with a
remote control device for wind speeds up to 12 m/s. No separation was
seen in any of the photos, so vortex generators were not tried. A
theoretical analysis of the wind turbine was then begun, and the
theoretical power curve came very close to the actual power curve. The
theoretical analysis showed that a decrease of four degrees in the
pitch angle would result in not only a substantial decrease in the
furling wind speed, but also result in an increase in the wind turbine
power coefficient from 0.28 to 0.40. Whether these improvements are
obtainable will be verified through field testing.
Wind tunnel tests have been conducted to measure the wake
and surface flow associated with a 50.4 cm diameter untapered,
untwisted rotor. A disk model of the same diameter was also tested.
Torque and thrust were measured with a unique generator-mounted
balance. Wake surveys were conducted using a scanning total pressure
probe and special hot film anemometer. Blade surface patterns were
observed using small tufts and a strobe light. The untwisted rotor
produced thrust coefficients higher than 1, and wake regions of
velocity reversal at high tip speed ratios. For high tip speed ratios
the rotor wake flow is quite similar to the solid disk. Blade surface
flow patterns reveal the blade stall progression associated with angle
of attack changes as the tip speed ratio varies.
This paper presents the results from a measurement
programme on a 1000 watt commercial wind turbine performed at CRES'
Test Station. The aim of the measurements was to evaluate the IEC
draft standards for design loads on small wind turbines and was
co-funded by the European Union within Joule II Programme and by the
GSRT. Within the same project, measurements on two other machines were
also performed. The measurement campaign includes the operational
characteristics as well as the fatigue and ultimate loads on some of
the critical components of the machine. The comparison with the IEC
standards is the subject of another work which includes the other two
wind turbines. Here, the collected data is analyzed and the effect of
turbulence on the dynamic loading of the machine is examined. Finally,
some relevant conclusions are drawn.
This page was last updated on 05-May-2005.
Questions about the NWTC Furling web pages? Contact Marshall Buhl.
The National Wind Technology Center is operated by the National Renewable Energy Laboratory (NREL), a national laboratory of the U.S. Department of Energy.