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Journées Nationales sur la Récupération et le Stockage d’Energie (JNRSE) 2017, Lyon, May 9th-10th 2017
1
Small nomadic counter-rotating wind turbine : Design
and experimentation
Florian HUET*, Émile ROUX and Aurélien CARRÉ
Univ. Savoie Mont Blanc, SYMME, F-74000 Annecy, France
* Florian.Huet@univ-smb.fr
Abstract—Many studies show the efficiency of large-scale
wind turbines in wind energy harvesting for collective
applications. However, few examples relate to small one stage
wind turbines (OSWT) for mobile and individual applications.
This work details the modeling and the prototype realization of a
18 cm diameter wind turbine using counter-rotating propeller
technology (CRWT). This device has been tested in a wind tunnel
with wind velocities in the range of 3.5 to 8 m.s-1. First
experimental results with a wind of 4.2 m.s-1 (which is the
theoretical optimum wind velocity of the device) show that the
wind turbine can provide a power of 0.6 W currently dissipated
in a load resistance of 1.5 Ω. The power goes up to 2.8 W for a
wind of 8 m.s-1.
Keywords—Small-scale wind turbine, counter-rotating
propeller, wind energy, nomadic power.
I. INTRODUCTION
The green sources of energy are a major concern and
become even a necessity while we wish to deploy an activity in
nomadic or isolated natural environment. The energy demand
goes from some watts to recharge batteries of low capacity
(smartphone, GPS, lamp…) to some hundred watts to feed
isolated structures as a mountain refuge for example. The
technologies usually used for these applications exploit the
solar or wind energy. The power density supplied by the
outdoor solar energy can be very important, 15 000 µW.cm-2,
but it decreases strongly in 150 µW.cm-2 when weather
conditions are degraded [1]. On the other hand, the power
density supplied by a laminar airflow is stable is estimated at
380 µW.cm-3 for a wind velocity of 5 m.s-1. The ubiquity of the
natural wind in the relief or the sea regions favors the
exploitation of this energy source. This natural wind velocity
follows a Weibull distribution with average value is 3.94 m.s-1
and the maximal is about 5 m.s-1. The source of airflow can
also come from human motions and thus capture relative flows.
In these two cases, the harvesting energy depends strongly on
the velocity of the source and less on the apparent surface.
Many studies show the efficiency of large-scale wind turbines
in wind energy harvesting for collective applications. However,
few examples [2] relate to small wind turbines for mobile and
individual applications. This paper suggests a mechanical
concept of counter-rotating propeller to a small-scale wind
turbine (< 20 cm). The counter-rotating propeller solution is
well known in the aviation to increase the thrust of aircrafts for
the same initial mechanical power level. The efficiency of this
propeller is improved by 30 % compared with a conventional
propeller [3]. The integration of this structure type of large-
FIGURE 1. COUNTER-ROTATING WIND TURBINE CONCEPT AND PROTOTYPE
scale wind turbine was already realized to validate this theory
[4-7]. However, the use of this technology on small-scale
devices is a real innovation and allows envisaging an efficiency
improvement of the same factor.
II. COUNTER-ROTATING WIND TURBINE
A. Counter-rotating wind turbine concept
The CRWT concept exploits a general technology adapted
to propulsion propellers in aviation. In this paper, this structure
is composed of two different and optimized propeller stages
which rotate in the opposite direction (Figure 1a). The stages
directly engrave the two independent generators. The incoming
air flow is partially captured by the first propeller stage and is
converted into mechanical energy by the shaft rotation. The
outgoing airflow is deflected, creating a flow composed of an
axial and tangential velocity. The second propeller stage is
adapted to harvest this new form of wind field, but it turns in
the opposite direction. The optimization of the geometry of the
propeller blades maximizes the caption of wind kinetic energy.
The maximum power is theoretically restricted by the Betz
limit and it can be formulated from the following relation:
(1)
B. Propellers modelisation
The propellers geometries are optimized with Heliciel
software. The latter proposes a reverse design of the blades in
order to optimize the mechanical power according to a defined
wind field (axial and tangential velocities). The characteristics
of the upstream and downstream propeller stages are listed in
Table 1. After the first stage incidence angle optimization, the
airflow data output is injected in the second stage input data.
Journées Nationales sur la Récupération et le Stockage d’Energie (JNRSE) 2017, Lyon, May 9th-10th 2017
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TABLE 1. PROPELLERS CHARACTERISTICS
ROPELLER STAGE
PARAMETERS
UPSTREAM
DOWNSTREAM
BLADE NUMBER
6
6
PROFIL
NACA1412
NACA6412
BLADE FOOT / HEAD DIAMETER
40 mm / 180 mm
BLADE FOOT / HEAD WIDTH
17 mm / 7 mm
17 mm / 17 mm
THEORETICAL MAXIMUM POWER
0.8 W
2.2 W
THEORETICAL ROTATION VELOCITY
2800 rpm
6000 rpm
FIGURE 2. WIND TURBINE ELECTRICAL SCHEMATISATION
The axial velocity component is greatly reduced compared to
the source. The global geometrical parameters listed in the
Table 1 are determined using the design of experiments method
(DOE).
C. Electromecanical conversion
For electromechanical conversion, two independent
permanent magnet synchronous generators with an optimal
rotation velocity of 7200 rpm are used. The frequency of the
electrical signals informs directly about the rotation velocity of
the propellers. The generator is modeled (Figure 2) by three
coils in series with three resistors. To harvest the electrical
energy produced, a passive extraction circuit composed of a
diode bridge suitable for synchronous generators is envisaged.
But actually, the electrical voltages at the coil terminals are too
low to pass the threshold diode voltage. The power is measured
on an optimum load resistance of 1.5 Ω in parallel with each
coil. III. EXPERIMENTAL RESULTS
The tests are carried out in a wind tunnel with an air stream of
23×23 cm2. An OSWT and a CRWT (Figure 1b) prototype
were subjected to a wind source ranging from 3.5 m.s-1 to
8 m.s-1. The Figure 3 shows the produced electrical power by
the OSWT and the optimized CRWT and the maximum
recoverable power Pmax (Equation 1) as a function of the wind
velocity. The classical wind turbine starts at 4.6 m.s-1 and it
produce 0.01 W. The counter-rotating starts around 4.2 m.s-1
with small difference between the two stages. This device
produces 0.6 W which corresponds to the maximum efficiency
of 85 %. At 8 m.s-1, the OSWT produces 1.3 W and the
counter-rotating produces 2.8 W.
FIGURE 3. EXPERIMENTAL POWER RESULTS
IV. CONCLUSIONS AND PROSPECTS
A prototype of a counter-rotating wind turbine was realized
and tested. The latter is above all a means of observing and
measuring the effects of the airflow on the two stages
independently. The first experimental test show promising
results with an electrical power of 0.6 W for a wind velocity of
4.2 m.s-1 and a maximum power of 2.8 W at the maximum test
velocity. The optimum efficiency is 85 %. A semi-analytical
model integrating the optimized propellers theoretical data is
under development, in order to design a new small dimension
counter-rotating wind turbine. These first results also show the
need to integrate an intermediate transmission composed of a
mechanical amplifier and a mechanical differential to couple
both stages. This step will be included in the next prototype in
order to exploit optimally a single generator while dissociating
the propeller stages.
ACKNOWLEDGMENT
The authors would like to thank L. Bouderaux, A. Brenin,
T. Ringeisen and L. Roques, students of Polytech Annecy, for
their contributions in INoWind project and the MPH
department of Annecy IUT for the wind tunnel provision.
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