Innovative blade shape for micro wind turbines

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Innovative blade shape for micro wind turbines
Aur´
elien Carr´
e
Universit´
e Savoie Mont Blanc,
SYMME,
F-74000 Annecy, France
aurelien.carre@univ-smb.fr
´
Emile Roux
Universit´
e Savoie Mont Blanc,
SYMME,
F-74000 Annecy, France
emile.roux@univ-smb.fr
Laurent Tabourot
Universit´
e Savoie Mont Blanc,
SYMME,
F-74000 Annecy, France
laurent.tabourot@univ-smb.fr
Pierre Gasnier
Universit´
e Grenoble Alpes,
CEA-LETI, MINATEC,
F-38000 Grenoble, France
pierre.gasnier@cea.fr
Abstract—This study reports the design and fabrication of
innovative blades for a centimeter-scale propeller and the ex-
perimental testing of a wind harvester. A samara wing structure
is taken as a model to optimize the aerodynamics at low Reynolds
number. The performances of the 44 mm diameter horizontal-
axis micro wind turbine are tested in two dedicated wind tunnels,
with wind speeds from 1.2 m.s−1to 8 m.s−1. The output electrical
powers range from 50 µW to 80 mW, with a maximal overall
efficiency of 17.5%for 4 m.s−1. It appears that this bioinspired
prototype have better performances than almost all the previous
studies on small-scale wind harvesters in a large wind speeds
range, and gives hope to get even higher.
Index Terms—Energy harvesting, Micro wind turbine, Low
wind speeds, Biomimetics
I. INTRODUCTION
Wireless Sensor Nodes (WSN) are used for a large type
of measures (temperature, humidity, presence, vibrations or
even CO2 concentration) and are usually powered by batteries:
this requires a periodical maintenance with inevitable costs,
especially if the WSN is located in a difficult-to-access area. In
order to avoid it, some developments are now made in the field
of energy harvesting with Micro Electro-Mechanical Systems
(MEMS). The sources are varied: water or air flows, vibrations,
light... If we look at the power density of the energy source,
compared to batteries, air flows appear to be interesting even
at low wind speeds (down to 1 m.s−1) [1].
Airflow-driven energy harvesters, and more particularly
horizontal-axis wind turbines, exploit the power of the air,
which is expressed as:
Pair =1
2ρSU 3
0,(1)
with ρthe air density, Sthe rotor cross-section and U0the
air speed upstream of the system. The output electrical power
is then: Pelec =ηgene .CP.Pair, with CPthe power coefficient
of the turbine, which is limited to around 59%because of
the Betz limit, and ηgene the generator efficiency. The overall
efficiency of the harvester is thus: η=CP.ηgene.
For small systems (1 m diameter and less), the overall effi-
ciency can go from 30%for a 1.26 m rotor diameter [2], [3], to
20%for 40 cm [4]–[7], down to around 0.4%for the smallest
and less efficient system [8]. Electromagnetic generators are
often used for mechanical to electrical conversion. They are
more used than electrostatic technologies due to their better
power density [9].
In this paper, we will focus on centimeter-scale horizontal
wind turbines with a rotor diameter of around 7 cm and
less. Since the beginning of the 2000s, some great studies
have been made in this field. From 2 to 7.2 cm of diameter,
with a working wind speed between 2 to 11.8 m.s−1, the
output electrical powers for different harvesters in the literature
range from 0.5 to 227.4 mW [10]–[17]. The maximum overall
efficiencies ηfor each device and for our prototype are listed
in Table I.
TABLE I
OVE RVIE W OF TH E MA XIM UM E FFICI EN CY OF E XI STI NG HA RVE STE RS
Reference Rotor Maximal Maximal
diameter efficiency electrical power
[10] 7.2 cm 15%102.61 mW at 8 m.s−1
[11] 6.3 cm 8%9.95 mW at 4.7 m.s−1
[12] 4.5 cm 8%69.9 mW at 10 m.s−1
[13] 4.2 cm 9.5%130 mW at 11.8 m.s−1
[14] 3.5 cm 28%24.6 mW at 7 m.s−1
[15] 3.45 cm 12%4.5 mW at 4 m.s−1
[16] 2.6 cm 3.2%5.6 mW at 8.8 m.s−1
[17] 2 cm 3.9%4.32 mW at 10 m.s−1
This work 4.4 cm 17.5%80 mW at 8 m.s−1
It can be noticed that, except for the very interesting result
of Gasnier et al. in 2019 with an overall efficiency reaching
28%, the other harvesters highlight an issue in the process
of extreme miniaturization: only 15%of the wind power is
extracted in the best case. This is mainly due to the low
Reynolds number close to the blades in these low wind speed
conditions. Moreover, the lowest cut-in speed is 1.5 m.s−1,
which has to be improved to exploit even lower wind speeds.
The Reynolds number for a blade with a chord c, under a
wind speed U0with νthe cinematic viscosity of the air is:
Re =U0.c
ν(2)
In the case of centimeter-scale harvesters, cand U0are
very small, giving Re around 103−104, which characterizes
a laminar (or low turbulent) flow. The lift-to-drag ratio of
the blades is then way beyond the ones seen on large scale;
consequently, the wind turbine will be less efficient than a
121978-1-6654-8445-9/22/$31.00 ©2022 IEEE WPW 2022

larger one [2]. The main goal is thus to find a new blade design
at small dimensions with better aerodynamical properties, in
order to optimize the power coefficient of the propeller and
thus the overall efficiency of the harvester. As an example,
Kunz et al. mentioned in 2003 that it is possible to optimize
the lift-to-drag ratio, even at small scale, in particular with the
use of thin and cambered blades [18].
A radically different method is to use biomimicry: in a
numerical study by Holden et al. published in 2015, the
mechanical efficiency (or power coefficient CP) of a maple
seed is evaluated during its rotational movement. The results
show a value of 59%, very close to the maximum being the
Betz limit at 59.3%[19]. Therefore, in the present study, it is
chosen to follow this path and to develop it.
II. BIOMIMETIC CONCEPTION OF THE PROPELLER
A. Cinematics of a maple seed
The natural specimens with movement that come closest
to micro turbine rotors are maple seeds, also called samaras.
Their length is comprised between 1 and 18 cm [20], and they
are composed of a dry fruit and a wing.
During their fall from a tree, the combination of the two
parts creates an autorotation that permits the seed to land far
from its initial point. This rotation is measured to be between
80 and 150 rad.s−1, or between 760 and 1430 rev.min−1, and
the fall speed is around 1 m.s−1[20], [21]. Moreover, the
Reynolds number of samaras blades during their rotation is
found to be around 103−104[22], [23]. All those parameters
are interestingly close to what can be observed for the working
conditions of centimeter-scale wind harvesters at low wind
speeds [14].
Thanks to those results, maple seeds are taken as models for
a new design of turbine blades at small dimensions. The aim
is thus to copy their shape, with the best precision possible.
B. Reverse engineering and fabrication
The first step towards reverse engineering is to scan a
samara to get its geometry. If possible, a 3D scan permits to
have the entire volume. Here, it was only possible to measure
one face of a samara. The seed was stuck to a plane and the
position of around 3 000 000 points were measured. The result
of the scan is shown on Fig. 1.
Fig. 1. Scan of the samara with colors representing the height of the points.
One can notice that the seed is globally curved: this is
why the center is blue and the extremities are red or white.
Moreover, the hole in the middle of the seed is intentional. The
most interesting part is the undulation that can be seen on the
trailing edge of the wing: the colors are between orange and
white, so the points on this area are between 1.3 and 1.9 mm in
height. This shows an important curvature in the wing structure
relatively to the thickness, which gives the samara its particular
aerodynamic properties [19].
Thus, this undulation is reproduced in the design of the
blades for the turbine rotor, as well as the outer contour of the
samara and the thickness of the different parts. Fig. 2 proposes
a view of the reconstructed model from below.
Fig. 2. View of the wing undulation in the CAD reconstruction.
For the fabrication of the propeller, it is chosen to use stere-
olithography (SLA) which is a type of additive manufacturing.
Machining and molding processes were discarded because of
the complexity of implementation for this type of parts. The
SLA technique is chosen mostly thanks to its precision of
fabrication, which in our case with a Formlabs Form 2 printer
is about 100 µm. However, the tiny veins on the wing that can
be seen on the scan on Fig. 1 are too small to be replicated: the
wing is then flattened, keeping only the undulation. Moreover,
the manufacturing precision is not sufficient to have the real
thickness of the samara: this one is measured at 0.1 mm. A
few parts printed with this thickness show badly manufactured
or deteriorated areas and encourage to thicken the wing a bit;
the final CAD model is then designed with 0.4 mm for the
wing, to keep a thin structure and to ensure a good fabrication.
C. Complete harvester
In order to make a propeller based on the samara design, the
previous CAD model is cut to remove the most massive part,
corresponding to the seed. The wing is then linked to an 8 mm
diameter hub and is duplicated to make a variable number of
blades. Two angles can also be changed: the pitch angle (noted
αp) and the coning angle (noted β). The pitch angle is inducted
by the blade rotation along its own axis; it impacts its angle
of attack. The coning angle is the one between the rotation
plane of the propeller and the blade axis; it is observed on
samaras during their autorotation. Depending on the coning
angle value, the propeller diameter is about 44 mm.
For the mechanical to electrical conversion, a micro perma-
nent magnet generator is used. It is made of a samarium-cobalt
(SmCo) cylindrical magnet, which presents a good remanent
induction, and a copper wire wrapped around it. The magnet
is fixed to a rotated shaft, this one being supported by two
micro ceramic bearings to minimize the friction. A coreless
generator is chosen because of its low starting torque that
is better for low wind speeds. The outer dimensions of the
generator are 14×11×25 mm3. The propeller with eight blades
and the electromagnetic generator are represented on Fig. 3.
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Fig. 3. The complete harvester: a bioinspired propeller and a micro generator.
III. EXP ER IM EN TAL RE SU LTS
A. Testing materials
In order to characterize the harvester on a large band of
wind speeds, two different wind tunnels are used: a first one
for ”low speeds”, from 0.5 to 2.3 m.s−1, and a second one for
”high speeds” between 3 and 8 m.s−1. A test at 2.3 m.s−1has
been made with both to confirm that the results are equivalent.
Moreover, to guarantee a good flow quality, the harvester rotor
area is verified to be less than 2%of the two tunnels areas.
In order to measure the electrical power produced, the
generator is connected to a variable resistive load, which value
Rload is set between 5 Ωand 90 kΩduring the tests. An
oscilloscope is connected in parallel to the load to measure
the voltage: its own impedance (1 MΩ) is taken into account,
giving an equivalent resistive load Req. The output power of
the harvester Pelec is therefore the power dissipated by Joule
effect in the load: Pelec =U2
RM S /Req, with URM S the RMS
voltage.
The output power produced by the harvester for a various
types of samara-based propellers is measured, in order to
choose the best combination of the three variable parameters:
pitch angle, coning angle and number of blades Nb. This leads
to an optimal propeller with the following values: αp= 30◦,
β= 5◦and Nb= 8. The results presented in the following
section are those for this propeller.
B. Power and efficiency results
The harvester is tested for air speeds between 1.2 and
8 m.s−1. Its cut-in speed is around 2 m.s−1and when the wind
goes down it stops at 1 m.s−1. It produces between 50 µW at
1.2 m.s−1and more than 80 mW at 8 m.s−1. The evolution of
electrical output power is illustrated in Fig. 4. It is interesting
to note that it follows an exponential trend: there is no concave
part that would reflect a fall in efficiency. This phenomenon
can be seen in the study by Gasnier et al. for the strongest
wind speed of 7 m.s−1[14]. In other words, the harvester
keeps here a good efficiency on a large range of wind speeds.
For each wind speed U0and according to the value of the
resistive load, the rotational frequency of the rotor fchanges.
This permits to determine the tip-speed ratio λ, defined as the
ratio between the speed of the blades tip Vtip and the wind
speed:
12345678
Wind speed U0(m.s−1)
0
10
20
30
40
50
60
70
80
Electrical power produced (mW)
Fig. 4. Maximum output power for each wind speed tested.
λ=Vtip
U0
=R.ω
U0
=R.2π.f
U0
(3)
with Rthe propeller radius and ωits rotational speed. The
overall efficiency of the harvester ηis also calculated: η=
Pelec/Pair . Its evolution for all the wind speeds tested is given
in Fig. 5.
0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00
Tip-speed ratio λ
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
Overall eciency η(%)
1.2 m.s−1
1.4 m.s−1
1.7 m.s−1
2 m.s−1
2.3 m.s−1
3 m.s−1
4 m.s−1
5 m.s−1
8 m.s−1
Fig. 5. Overall efficiency as a function of the tip-speed ratio for various wind
speeds.
The maximum value of ηranges from 2.5%for a 1.2 m.s−1
wind to 17.5%for 4 m.s−1. It increases when the wind speed
gets stronger, reaches its maximum at 4 m.s−1and stays
around 17%beyond. The optimal tip-speed ratio is between
0.5 and 1.5. The overall efficiency is also plotted as a function
of the resistive load on Fig. 6.
101102103104
Equivalent resistive load Req (Ω)
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
Overall eciency η(%)
1.2 m.s−1
1.4 m.s−1
1.7 m.s−1
2 m.s−1
2.3 m.s−1
3 m.s−1
4 m.s−1
5 m.s−1
8 m.s−1
Fig. 6. Overall efficiency as a function of the resistive load for various wind
speeds.
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It shows that the optimal value of Req depends on the
air speed: it goes down with the wind increasing. Globally,
it is situated between 300 and 3000 Ω, with a bandwidth
maintaining 90%of the efficiency of about 200 to 300 Ω.
IV. DISCUSSION AND CONCLUSION
In terms of maximal overall efficiency, our bioinspired
harvester outperforms the ones of the literature with diameters
of 7 cm and less [10], [11], [13], [15]–[17], except for
the one from Gasnier et al. which demonstrates very high
performances [14]. In order to make a deeper comparison,
the power density of the harvester is calculated for each wind
speed as the ratio between Pelec and the rotor area (Fig. 7).
It can be noted that also here, the performances are better
than the ones of almost all the existing systems. One interest-
ing point is that the bioinspired harvester covers a large range
of wind speeds, from very low to moderate, and that it keeps
its performances all the way.
Nevertheless, the efficiency and the power density are the
worst at low wind speeds: the work must continue on this
point, one way of improvement being to analyze deeper
samaras behavior and structure. Another idea to get better
performances is to change the material of the generator magnet
to NdFeB, which has a higher remanent induction than SmCo
and could have a better efficiency, in particular at low air
speeds.
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