Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine

Abstract

There is growing need for the green, reliable, and cost-effective power solution for the expanding wireless microelectronic devices. In many scenarios, these needs can be met through a small-scale wind energy portable turbine (SWEPT) that operates near ground level where wind speed is of the order of few meters per second. SWEPT is a three-bladed, 40 cm rotor diameter, direct-drive, horizontal-axis wind turbine that has very low cut-in wind speed of 1.7 m/s. It operates in a wide range of wind speeds between 1.7 m/s and 10 m/s and produces rated power output of 1 W at wind speed of 4.0 m/s. The wind turbine is capable of producing electrical power up to 9.8 W at wind speed of 10 m/s. The maximum efficiency of SWEPT was found to be around 21% which makes it one of the most efficient wind turbines reported at the small scale and low wind speed. These advancements open many new opportunities for embedding and utilizing wireless and portable devices.

Full text

Research Article
Ravi Anant Kishore, Anthony Marin, and Shashank Priya*
Efficient Direct-Drive Small-Scale Low-Speed
Wind Turbine
Abstract: There is growing need for the green, reliable,
and cost-effective power solution for the expanding wire-
less microelectronic devices. In many scenarios, these
needs can be met through a small-scale wind energy
portable turbine (SWEPT) that operates near ground
level where wind speed is of the order of few meters per
second. SWEPT is a three-bladed, 40 cm rotor diameter,
direct-drive, horizontal-axis wind turbine that has very
low cut-in wind speed of 1.7 m/s. It operates in a wide
range of wind speeds between 1.7 m/s and 10 m/s and
produces rated power output of 1 W at wind speed of 4.0
m/s. The wind turbine is capable of producing electrical
power up to 9.8 W at wind speed of 10 m/s. The max-
imum efficiency of SWEPT was found to be around 21%
which makes it one of the most efficient wind turbines
reported at the small scale and low wind speed. These
advancements open many new opportunities for embed-
ding and utilizing wireless and portable devices.
Keywords: small-scale wind turbine, wind tunnel, power
coefficient, wind energy, portable power
*Corresponding author: Shashank Priya, Department of Mechanical
Engineering, Center for Energy Harvesting Materials and
Systems (CEHMS), Bio-Inspired Materials and Devices
Laboratory (BMDL), Virginia Tech, 310 Durham Hall, Blacksburg,
VA 24061, USA, E-mail: spriya@vt.edu
Ravi Anant Kishore: E-mail: ravi86@vt.edu, Anthony Marin:
E-mail: marinvt@gmail.com, Department of Mechanical Engineering,
Center for Energy Harvesting Materials and Systems (CEHMS), Bio-
Inspired Materials and Devices Laboratory (BMDL), Virginia Tech,
310 Durham Hall, Blacksburg, VA 24061, USA
Introduction
Historical development of wind turbines
Wind energy is one of the most abundant renewable
energy resources and it has been targeted for centuries.
It is predicted that human beings have been using wind
energy in their daily work for about 4,000 years (http://
www.wwindea.org/technology/ch01/estructura-en.htm).
As early as 1700 BC, King Hammurabi of Babylon used
wind powered scoops to irrigate the plains of
Mesopotamia (Gasch and Twele 2012). Wind was also
used to grind grain and that is the reason why we still
speak of “windmills”, even though they are now hardly
used for grinding grains (http://www.wwindea.org/tech-
nology/ch01/estructura-en.htm). Figure 1 displays some
of the primitive windmills used by early human civiliza-
tions (Gasch and Twele 2012).
It can be noted that most of the world’s oldest wind-
mills had vertical axis of rotation. Initial designs (Figure 1
(a)–(c)) were very simple, especially from construction
point of view. Braided mats or sails were utilized to
generate drag and thus to rotate the windmills about
the central axis. The vertical-axis windmills had another
operational advantage that they were independent of
wind direction. Figure 1(d) and (e) shows some later
versions of vertical-axis mills developed in France and
Italy, respectively. In Figure 1(d), the millstone is
attached directly to the vertical drive shaft without any
intermediate gear or other mechanism to redirect the
rotational movement. Figure 1(e) shows one of the
advanced windmills created by Fausto Veranzio in Italy
(Gasch and Twele 2012). It can be seen that this windmill
is engineered with cup-shaped rotor blades which
improves the efficiency of the device. Veranzio also
developed a gearing mechanism for his windmills which
allows millstones to run at much higher speed, even
though the rotors were designed for low tip speed ratio
(Gasch and Twele 2012).
The horizontal-axis windmills are a relatively newer
invention than the vertical-axis windmills. Though the
first documentation of the horizontal-axis windmills
dates back to the twelfth century, the theoretical descrip-
tions regarding the driving power of horizontal-axis
devices, i.e. lift forces on the blades, was investigated
only during the beginning of the twentieth century
(Gasch and Twele 2012). One of the most popular early
horizontal-axis wind turbines (HAWTs) was the tower
doi 10.1515/ehs-2014-0004 Energy Harvesting and Systems 2014; 1(1-2): 27–43
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mills, shown in Figure 1(f), which existed in southern
Europe. The first written evidence of such windmills
dates back to the thirteenth century (Gasch and Twele
2012). There were some other types of horizontal-axis
windmills which existed in different parts of the world
(mainly in the Occident) during different periods of time:
Post windmill (1100s), Wipmolen Dutch (1400s), Dutch
smock mill (1500s), Paltrock mill (1600s), and Gallery
smock mill (1700s). Brief description about these wind-
mills can be found in Gasch and Twele (2012). However,
none of the historical HAWTs gained as much popularity
as the American farm windmill (sometimes also called
Western mill). These windmills were developed in the
mid-nineteenth century mainly to provide drinking
water to people and cattle in North America. Moreover,
they were used to assure the water supply for the steam
locomotives of the new railways expanding into the West
(Gasch and Twele 2012). Figure 2(a) shows an early adver-
tisement for the American windmills by U.S. Wind Engine
& Pumping Co., the company who developed this
(a) (b) (c)
(d) (e) (f)
Figure 1 Some of the world’s oldest windmills; (a) Ruins of a vertical-axis windmill in Afghanistan, approx. 700 AD (picture taken in 1977);
(b) Persian windmill; (c) Chinese windmill with flapping sails, approx. 1000 AD; (d) Vertical-axis windmills with flapping sails, France 1719 AD;
(e) Vertical-axis windmills with bodies driven by drag forces, Italy, approx. 1600 AD; and (f) Mediterranean tower mill with sails. All pictures
are adapted from Gasch and Twele (2012)
(a) (b)
Figure 2 American windmills (Western windmills): (a) an advertisement by U.S. Wind Engine & Pumping Co., developer of the American
windmills (http://www.ironmanwindmill.com/windmill-history.htm), and (b) a wind farm utilizing the American windmills for water pumping
(http://www.midamericawindmillmuseum.org/imgs/home/windmill_2.jpg)
28 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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windmill (http://www.ironmanwindmill.com/windmill-
history.htm). The most important component of this
windmill is the rotor, which is also called “rotor rosette”
because of its structural design. Its diameter varies
between 3 m and 5 m and has more than 20 metal sheet
blades. It also consists of a tail that allows the rotor to
turn automatically so that it always faces the incident
wind. It uses a crank shaft to drive a piston pump. The
American mills are still in existence, and several of them
are installed with a nearly unchanged design in
Australia, Argentina, and the USA (http://www.ironman-
windmill.com/windmill-history.htm). Figure 2(b) depicts
one of such wind farms utilizing the American mills for
water pumping (http://www.midamericawindmillmu-
seum.org/imgs/home/windmill_2.jpg).
Current status of wind energy
The fourth edition of the Global Wind Energy Outlook
released on November 14, 2012 at Beijing by Greenpeace
International and the Global Wind Energy Council states
that wind power currently provides about 3.5% of global
electricity demand, and it is expected that the wind
energy share could reach up to 12% by 2020 (http://
www.gwec.net/publications/global-wind-energy-outlook/
global-wind-energy-outlook-2012/). Figure 3 shows the
global cumulative installed wind power capacity over
last 17 years (Global Wind Energy Council Report 2012).
At the end of year 2012, the world-wide total wind power
capacity was 282 GW, showing a growth of about 18.7%
(44 GW) over the preceding year. It is important to note
that although the year 2012 created a new record in total
installed wind power capacity, the wind market has
cooled down in relative terms. If we look at the annual
growth rate, it had continued to increase since the year
2004, peaking at 32.1% in year 2009. However, since then
the growth has decreased substantially. In 2012, the glo-
bal growth went down to 18.7%, which is the lowest rate
in the last two decades, according to a report by the
World Wind Energy Association (Gsänger and Pitteloud
2012).
Table 1 presents cumulative wind power capacity
from year 2008 to 2012 in top 10 countries and the same
variable worldwide (Global Wind Energy Council Report
2012; Gsänger and Pitteloud 2012). The data indicate that
even though the global wind power capacity exhibited
the low growth rate (18.7%) in year 2012, it increased over
133.5% during the last 5 years. It is also very interesting
to note that 73.7% of the total power capacity (282,430
MW) in 2012 was contributed by five countries, i.e. China,
USA, Germany, Spain, and India. China’s wind power
3,00,000
2,50,000
2,00,000
1,50,000
1,00,000
50,000
Wind power capacity (MW)
0
6,100 7,600 10,200 13,600 17,400 23,900 31,000
39,431 47,620
59,091
74,006
93,639
120,267
158,864
197,686
238,035
282,430
24.6%
34.2% 33.3%
27.9% 27.2%
37.4%
20.8%
24.1%
25.2%
26.5%
28.4%
32.1%
24.4%
20.4%
18.7%
29.7%
19971996 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 0%
Growth (%)
5%
10%
15%
20%
25%
30%
35%
40%
Figure 3 Total installed wind power capacity (MW) and world wind power market growth rate (%) 1996–2012. Information taken from
Global Wind Energy Council Report (2012)
Table 1 Cumulative wind power capacity outlook from 2008 to
2012 (Global Wind Energy Council Report 2012; Gsänger and
Pitteloud 2012)
Country 1) Total capacity (MW) Growth
rate
2012 (%)
2008 2009 2010 2011 2012
China 12,210 25,810 44,733 62,364 75,564 21.2
USA 25,237 35,159 40,180 46,919 60,007 27.9
Germany 23,897 25,777 27,215 29,075 31,332 7.8
Spain 16,689 19,149 20,676 21,673 22,796 5.2
India 9,587 11,807 13,065 15,880 18,421 16.0
UK 3,195 4,092 5,203 6,018 8,445 40.3
Italy 3,736 4,850 5,797 6,737 8,144 20.9
France 3,404 4,574 5,660 6,640 7,196 8.4
Canada 2,369 3,319 4,008 5,265 6,200 17.8
Portugal 2,862 3,357 3,702 4,093 4,525 10.6
Worldwide 120,986 159,837 197,040 238,035 282,482 18.7
R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine 29
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capacity continued to grow at the rate of over 21% in
2012. The United States has also gained the momentum
and displayed the annual growth rate of 27.9% in 2012,
which is the highest growth rate in last three consecutive
years. The installed wind power capacity in USA has
reached up to 60,007 MW by the end of year 2012. In
2012, the electricity produced from wind power in the
United States totaled to about 140 TW h, which is around
3.5% of net electricity generation by all the energy
sources (http://www.eia.gov/electricity/monthly/pdf/
epm.pdf). The U.S. Department of Energy envisions sup-
plying 20% of all U.S. electricity from wind power by
2030 (WIND AND WATER POWER PROGRAM 2011).
Classification of wind turbines
There are broadly three ways to classify the wind tur-
bines: (i) on the basis of the orientation of axis of rotation
(vertical or horizontal), (ii) on the basis of the component
of aerodynamic forces (lift or drag) that powers the wind
turbine, and (iii) on the basis of energy generating capa-
city (micro, small, medium, or large).
There are essentially two kinds of wind turbines, when
they are categorized on the basis of their orientation of the
axis of rotation: vertical-axis wind turbines (VAWTs) and
Horizontal-axis wind turbines (HAWTs). As the name sug-
gests, the rotor of VAWTs rotates perpendicular to the
ground while that of HAWTs spins parallel to the ground.
It was explained in the previous section that most of the
early wind turbines were vertical axis, because they were
relatively simple to construct (especially for the milling
purpose) and they also did not require any mechanism to
orient themselves in the direction of wind. In spite of these
attributes, none of the old designs of VAWTs survived for
long time. Currently, there are three most popular designs
of VAWTs: (a) Savonius VAWT, (b) curved-blade Darrieus
VAWT, and (c) straight-blade VAWT (Islam, Ting, and
Fartaj 2008). Figure 4(a)–(c) shows Savonius, curved-
blade Darrieus, and straight-blade Darrieus VAWT rotors,
respectively (http://www.elite.tugraz.at/Jungbauer/3.htm).
Savonius turbines are drag-type, because it utilizes drag
component of the aerodynamic force to rotate, while
Darrieus turbines are lift-type because it is the lift compo-
nent of the aerodynamic force that powers the Darrieus
rotor. In principle, Savonius rotors normally have two
cups or half drums attached to a central shaft in opposing
directions, as shown in Figure 4(a). The drum, which is
against the wind flow, catches the wind and creates a
moment along the axis. The aerodynamic torque by the
first drum rotates the rotor and brings the opposing
drum against the wind flow. The second drum now catches
the wind and causes the rotor to rotate even further and
thus completes a full rotation. This process continues until
there is sufficient wind to turn the axial shaft which is
normally connected to a pump or a generator (Islam,
Ting, and Fartaj 2008). Savonius turbines generally have
poor efficiency (less than 25%) and that is why they are not
so commercially successful but there are some advantages
such as simple construction with low cost, high static
and dynamic moment, wind acceptance from any
direction, low noise and angular velocity in operation,
and reduced wear on moving parts which justifies their
operation for low power applications (Akwa, Vielmo, and
Petry 2012).
The Darrieus-type VAWTs consists of two or more
blades which are attached to a vertical central shaft.
These blades can be curved (as shown in Figure 4(b)) or
they can be straight (as shown Figure 4(c)). Irrespective
of the curvature, the blades always have airfoil profile
that creates aerodynamic lift, when they are exposed to
the incident wind. This phenomenon creates moment
along the axis and causes the central shaft to rotate,
which ultimately runs the generator to produce electri-
city. The curved-blade Darrieus VAWTs have lower bend-
ing stress in the blades as compared to straight-blade
Darrieus VAWTs and therefore former is more commer-
cially successful (Islam, Ting, and Fartaj 2008). However,
on the small-scale power production, the straight-bladed
Darrieus VAWTs are more popular because of their sim-
ple blade design (Akwa, Vielmo, and Petry 2012). The
straight-bladed Darrieus VAWTs sometimes may have
variable pitch angle. It has been found that constant
pitch straight-bladed Darrieus VAWTs do not have self-
start ability (Kirke 1998). The variable pitch configuration
of the blades allows Darrieus VAWTs to overcome the
starting torque problem but it is overly complicated, mak-
ing them quite impractical for small-scale power genera-
tion (Islam, Ting, and Fartaj 2008).
At present, HAWTs are the most popular among all
windmill designs. This is primarily because HAWTs
(a) (b) (c)
Figure 4 VAWTs: (a) Savonius rotor, (b) curved-blade Darrieus
rotor, and (c) straight-blade Darrieus rotor (http://www.elite.tugraz.
at/Jungbauer/3.htm)
30 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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generally have much higher efficiency than VAWTs. The
maximum power coefficient of a modern HAWT has been
reported up to 45–50% while that of an efficient VAWT
normally lies below 40% (Eriksson, Bernhoff, and Leijon
2008) (power coefficient of a Savonius-type VAWT is even
lower, normally below 25% (Akwa, Vielmo, and Petry 2012)).
Figure 5 shows a HAWT commercialized by SIEMENS
(model: SWT-2.3-82 VS) (http://www.energy.siemens.com/
us/pool/hq/power-generation/wind-power/E50001-W310-
A123-X-4A00_WS_SWT-2.3-82%20VS_US.pdf).
As explained earlier, the rotor shaft of a HAWT is
positioned in horizontal direction, i.e. parallel to the
ground. The electric generator that is connected to the
turbine rotor via the primary and secondary shafts is
stored inside a nacelle box at the top of the tower.
HAWTs are lift-type wind turbines and are very sensitive
to changes in blade profile, design, and surface roughness.
Another limitation with HAWTs is that they cannot catch
the wind from all direction. They need a special mechan-
ism to turn the rotor so that it always faces the wind. This
was probably one of the main reasons why none of the
historical HAWTs were so successful. In fact, the American
windmill was the first HAWT which had a fully automati-
cally controlled yaw system. Yaw system of a HWAT is
basically a component which is responsible for the orien-
tation of the wind turbine rotor toward the wind. In small
size HAWTs, the yaw system comprises a simple roller
bearing connected between the tower and the nacelle. A
tail with a fin at the end is mounted on the back of nacelle
which produces corrective moment to turn the wind tur-
bine rotor into the wind. This type of yaw system is called
“passive yaw system”. The large-scale (MW) HAWTs how-
ever needs an active yaw mechanism. The active yaw
systems are normally equipped with a wind sensor that
senses the direction of wind and a servo motor that pro-
duces required torque to rotate the nacelle of the wind
turbine against the stationary tower.
A HAWT, in general, consists of a rotor, a gear box, a
generator, and a yaw system. The rotor of a HAWT includes
two to three blades connected together with a hub. The
hub is attached to a main shaft (sometimes also called
primary shaft or low-speed shaft), which passes through
bearings and connects to a gear-train. The gear train ampli-
fies the rotational speed and provides higher rpm to a
secondary shaft (sometimes also called high-speed shaft).
Thesecondaryshaftdrivesageneratorthatproduceselec-
tricity. The gear-box, the primary and secondary shafts,
and the generator are contained inside a nacelle box. The
nacelle box also contains a yaw system to orient the rotor
and a heat exchanger to cool down the generator. Figure 6
demonstrates all the major components of a large-scale
modern HAWT (Crossley and Schubel 2012).
Figure 5 Siemens HAWT (model: SWT-2.3-82 VS) (http://www.energy.siemens.com/us/pool/hq/power-generation/wind-power/E50001-
W310-A123-X-4A00_WS_SWT-2.3-82%20VS_US.pdf)
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Large-scale vs small-scale wind turbines
The definition of “small”- and “large”-scale windmill has
remained vague in the literature of wind energy. Small
wind turbine was initially defined on the basis of its
capability to produce electrical power sufficient enough
to cover individual household electricity demands (Zhang
2012). But the problem lies in the fact that the consump-
tion of electricity by a household itself is very debatable,
because it varies with time and place. For example, an
average American family needs a 10 kW turbine to cover
their full consumption, while a European household
requires a 4 kW turbine, which further reduces to a 1
kW turbine for an average Chinese household (Zhang
2012). Lacking any credible unanimous definition, the
range for the rated power capacity of small-scale wind
turbines (SSWTs) vary from few watts to few hundred
kilowatts. Kishore, Coudron, and Priya (2013) have
defined the nomenclature of HAWTs based on the size
of the wind turbine rotor as given below. We will use the
same nomenclature in this paper.
(i) Micro-scale wind turbine (µSWT): rotor diameter
≤10 cm,
(ii) Small-scale wind turbine (SSWT): 10 cm < rotor
diameter ≤100 cm,
(iii) Mid-scale wind turbine (MSWT): 1 m < rotor dia-
meter ≤5 m, and
(iv) Large-scalewindturbine(LSWT):rotor diameter> 5 m.
Need and applications of SSWTs
The advancements achieved in the field of power electro-
nics in the last few decades have vastly expanded the
1
2
Wind turbine
Components
1. Rotor: The rortor is made
up of blades affixed to a hub.
The blades are shaped like
airplane wings and use the
principle of it to turn wind energy
into mechanical energy. Blades can
be as long as 150 feet half the
length of football fileld.
2. Oitch Drive: Blades can be rotated to
reduce the amount of lift when wind speeds
become too great.
3. Nacelle: The rotor attaches to the nacelle,
which sits atop the tower and encloses the
various components.
4. Brake: A mechanical brake acts as a
back up to the braking effects of the blade
pitch drives or as a parking brake for
maintanance.
5. Low-speed Shaft: Attaches to the rotor.
7. High-speed Shaft:
Attaches to the generator.
YAW
8. Generator: Converts the
mechanical energy produced by the
rotor into electricity. Different designs
produce either direct current or
alternating current. The electricity may be
used by nearby appliances stored in
batteries or transferred to the power grid.
9. Heat exchanger: Keeps the genertor
cool.
10. Controller: A comouter system
runs self-diagnostic tests, starts and stops
the turbine, and makes adjustments as
wind speed vary. A remote operator can
run system checks and enter new
parameters via modem.
11. Anemometer: Meaasures wind
speed and passes it along to the
controller.
12. Wind Vane: Detects wind
direction and passes it along
to the controller, which adusts
the “yaw” or heading of the
rotor and nacelle.
13. Yaw Drive : Keeps the
rotor facing into the wind.
14. Tower : Because wind
speed increases with height,
taller towers allow turbines
to capture more energy.
6. Gear Box: The rotor turns the low-speed
shaft at speeds ranging from 20 revolutions
per minute (rpm) on large turbines to 400 rpm
on residential units. Transmission gears
increase the speed to the 1,200−1,800 rpm
required by most generators to produce
electricity. Some small-scale turbines use a
direct-drive system, eliminating the need for
a gear box.
3
4
5
6
7
8
9
10
11
12
13
14
Figure 6 Typical configuration of a modern large-scale HAWT (Crossley and Schubel 2012)
32 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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deployment of wireless electronics. Not only the electro-
nics have become smaller in size but also their power
requirements have reduced by orders of magnitude.
Currently, in majority of applications, lithium cell bat-
teries are used, which limits the operation time and pre-
sents maintenance challenges because these batteries
need to be regularly monitored and replaced. While the
need of the batteries is growing exponentially over time,
growth in battery capacity is proceeding along a flatten-
ing S-curve (Paradiso and Starner 2005). An alternative to
batteries might be energy harvesting devices that can trap
locally available environmental energy that mainly
includes solar, wind, tidal, magnetic field, and vibra-
tions. Out of these possibilities, the most reliable resource
in terms of both availability and power density is the
wind. However, the main concern is that the conventional
LSWTs have very high operating wind speed, normally
around 15 m/s, and thus they are installed several feet
above the ground level. One of the key requirements for a
wind turbine deployed for the purposes of charging the
wireless microelectronics is its reliability while operating
near the ground where wind is very weak due to bound-
ary layer effect and obstacles like trees, buildings, and so
forth. Unfortunately, most of the currently available
SSWTs that operate at low wind speeds have poor aero-
dynamic performance which makes them economically
unviable. The aerodynamic performance of a wind tur-
bine is primarily influenced by Reynolds number of the
airfoil used for the turbine blades. Reynolds number of
an airfoil is given by:
Re ¼ρcurel
μ½1
where ρand μare density and dynamic viscosity of the
flowing fluid, respectively, cdenotes the chord length of
the airfoil, and urel is the relative wind speed. The
Reynolds number of a wind turbine is proportional to
the chord length and the wind speed. For the SSWTs,
these two factors have very small value and therefore
they operate at much lower Reynolds number as com-
pared to the LSWTs. Figure 7 depicts the effect of
Reynolds number on the aerodynamic parameters (lift
and drag coefficients) for NACA 0012 airfoil (Kishore,
Coudron, and Priya 2013; Musial and Cromack 1988). It
is very interesting to note that the maximum lift coeffi-
cient decreases with decrease in Reynolds number while
the drag coefficient increases when Reynolds number is
reduced. This implies that the lift to drag ratio reduces
sharply with decrease in Reynolds number, which results
in poor performance of the SSWTs.
Literature review
There are very few papers available in literature that
study the performance characteristics of SSWTs. Vardar
and Alibas (2008) compared the rotation rates and power
coefficients of various 31 cm diameter wind turbine rotor
models. They were manufactured using four different
NACA profiles (NACA 0012, NACA 4412, NACA 4415, and
Lift
representation
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 5 10 15 20 25 30 35 40 45
Drag
representation
Eppler model for
high angle of
attack
Re = 2.4 × 106
Re = 1.3 × 106
Re = 6.6 × 105
Re = 3.3 × 105
Re = 1.7 × 105
Re = 8.0 × 104
Re = 4.0 × 104
Angle of attack, α (°)
Drag coefficient, Cd/lift coefficient, Ci
Re
increasing
Figure 7 Influence of Reynolds number on airfoil behavior (NACA 0012) (Kishore, Coudron, and Priya 2013; Musial and Cromack 1988)
R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine 33
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NACA 23012). Various geometric parameters such as
blade angle, twist angle, and blade number of these
turbine models were investigated at different wind
speeds. It was observed that the rotor with NACA 4412
profiles having 0° twist angle, 5° blade angle, and two
blades had the highest rotation rate while the one with
NACA 4415 profiles with 0° twist angle, 18° blade angle,
and four blades had the highest power coefficient. A
numerical study using blade element momentum theory
and lifting line based wake theory was conducted by
Duquette and Visser (2003) to assess the effects of blade
number and solidity on the performance of 2 m diameter
wind turbine. The study suggested that by increasing the
solidity from the conventional range of 5–7% to 15–25%
one can achieve better power coefficient values. Leung
et al. (2010) optimized different blade parameters of a
23.4 cm diameter wind turbine using Computational
Fluid Dynamics. The wind turbine used in this study
had fan-type, mono-thick (constant thickness along the
radius), and multiple numbers of blades instead of con-
ventional airfoil type tapered blades. This study sup-
ported the fact that the higher solidity gives better
performance to SSWTs, however it was also suggested
that the blades should not fully occupy the swept area
of the rotor to avoid blockage. It can be observed that all
of these studies primarily focus on designing and opti-
mizing the wind turbine rotor, more specifically the tur-
bine blades. They lack the overall comprehensiveness
addressing all the components together to develop a
complete SSWT. Probably, the first attempt to make a
complete SSWT was performed by Hirahara et al.
(2005). They developed a four-bladed 50 cm diameter
small wind turbine called µF500 using NACA 2404 airfoil.
This turbine exhibited very good performance with power
coefficient of 0.36 and overall efficiency of 0.25 in the
wind speed range of 8–12 m/s.
Objectives of the paper
In this paper, we demonstrate small-scale wind energy
portable turbine (SWEPT) targeted to operate near the
ground level with high efficiency. SWEPT is a HAWT of
rotor diameter about 40 cm, and it has been designed to
operate at wind speeds below 5 m/s. Kishore, Coudron,
and Priya (2013) and Kishore and Priya (2013) provide
detailed description about the design specifications, fab-
rication processes, experimental testing, and power char-
acteristics of two different generation prototypes of
SWEPT. Table 2 summarizes the significant similarities
and differences between the first and second generation
SWEPTs. It is very interesting to note that even though
both the prototypes are of about the same size, the per-
formance of the second generation prototype is much
superior to that of the previous one. The prime reason
behind this improvement is the difference between their
blade designs. There are mainly six blade parameters that
influence the aerodynamic performance of a wind tur-
bine: airfoil, number of blades, twist angle, chord length,
solidity, and tapering angle. The blades of the first pro-
totype were of conventional design like most of the
LSWTs, with linear twist and tapering from root to tip.
On the other hand, the second generation SWEPT has
blades specifically developed for the SSWTs at low wind
speed applications. Its blade profile contains NACA 0012
airfoil, which is suitable for applications at the low
Reynolds number. The blades were non-linearly twisted
by 37° with higher twist near the hub that creates higher
torque coefficient at low wind speed. The blades were not
tapered like those of a conventional wind turbine rather
they maintained constant chord length of 7.5 cm through-
out their span.
In spite of the fact that the second generation
SWEPT prototype had excellent efficiency of 21%, it is
not a direct-drive wind turbine. Like the first generation
prototype, it utilizes a gear train of gear ratio 80:10 to
amplify the angular speed so that its generator can run at
its rated rpm. The use of gear train however has several
Table 2 Comparison between the first and second generation
prototypes of SWEPT (Kishore, Coudron, and Priya 2013; Kishore and
Priya 2013)
First generation
prototype
Second generation
prototype
Axis of rotation Horizontal Horizontal
Rotor diameter 39.4 cm 40 cm
Number of blades 3 3
Tip radius 19.7 cm 20 cm
Hub radius 1.3 cm 3 cm
Twist angle 32° 37°
Blade type Tapered Constant chord
Chord length near root 5.7 cm 7.5 cm
Chord length near tip 2 cm 7.5 cm
Drive Gear train with
ratio 80:10
Gear train with ratio
80:10
Generator 24 V PM DC motor 24 V PM DC motor
Cut-in wind speed 2.7 m/s 3.0 m/s
Cut-out wind speed 5.0 m/s 5.5 m/s
Power coefficient 14% 32%
Optimal tip speed ratio 2.9 4.1
Maximum electrical power 0.83 W at 5 m/s 2.2 W at 5.5 m/s
Maximum overall
efficiency
9% 21%
34 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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limitations: (i) it increases the mechanical losses and thus
reduces the overall efficiency of the device, (ii) the static
friction between the gears increases the start-up wind
speed of the wind turbine, (iii) running gears produce
excess noise which is annoying, especially at high wind
speed, (iv) at high wind speed, when gears run at very
high rpm, they skid and thus limit the cut-out wind speed
of the wind turbine, and (v) the wear and tear in the gears
reduces the overall reliability and life span of the device.
These problems can be avoided, if the wind turbine is
direct drive. But, then the concern is the unavailability of
a small size generator with required attributes (high effi-
ciency, low rated angular speed of around 1,000 rpm, low
starting torque of the order of few mN m, and high
voltage to rpm ratio).
This paper also addresses the design and character-
ization of an axial flux generator developed to operate
SWEPT without any gear drive. Wind tunnel experiments
revealed that the axial flux generator allows the wind
turbine to start at wind speed as low as 1.7 m/s. The
wind turbine generates its rated electrical power of 1 W
at 4.0 m/s wind speed. In comparison to previous proto-
types that operate in a very narrow wind speed range of
about 2.5–5.0 m/s (Kishore, Coudron, and Priya 2013;
Kishore and Priya 2013), the current prototype operates
in a much wider operating wind speed range of 1.7–10 m/
s. The wind turbine was found to produce electrical
power up to 9.8 W at the maximum tested wind speed
of 10 m/s.
Generator design
There are several kinds of generators that are used for
wind turbines: direct or alternating current types, syn-
chronous or asynchronous, with or without permanent
magnets (PM), and self or external electrical field excited
machines. It has been suggested that the generators
equipped with PMs are more suitable for small size
wind turbines because of their higher efficiency com-
pared with other generators (Ani, Polinder, and
Ferreira). The PM electric generators are broadly divided
into two categories: radial flux and axial flux machines,
depending on the direction of magnetic flux in the air gap
between stator and rotor. The radial flux generators can
be further subdivided into inner rotor radial flux
machines and outer rotor radial flux machines, according
to the position of rotor with respect to stator. Similarly,
axial flux generators can be subcategorized into double
stator-single rotor machines and double rotor-single sta-
tor machines, based upon the number of rotors and
stators. Normally for SSWTs, an axial flux generator is
preferred because it occupies lesser space in the radial
direction than a radial flux generator of the same power
rating (Ani, Polinder, and Ferreira; Park et al. 2012;
Latoufis et al. 2012). A recent study by Marin et al.
(under review) revealed that double rotor-single stator
slotless axial flux generator design provides very promis-
ing results for μSWTs. The slotless axial flux generator
design has several other advantages over other types of
generators: (i) the topology of an axial flux machine leads
to a short radial length which makes it very compact
generator suited for small wind turbines; (ii) due to the
slotless air-gap winding, the values of mutual and leak-
age inductances are low; and (iii) the absence of the slots
provides noiseless operation with no cogging torque. In
this study, therefore, we focused on the design and fab-
rication of a double rotor-single stator slotless axial flux
generator.
As shown in Figure 8, a double rotor-single stator
axial flux generator consists of a stator containing a set of
coils and two rotors on either sides of the stator. Each of
the rotors contains a set of PMs. Holding the coils sta-
tionary has both functional and operational advantages.
It not only reduces the number of moving parts and thus
renders a commutator unnecessary but also decreases the
heating losses in the commutator and thus improves the
efficiency. The proposed generator design has a slotless
stator, and the coils are wound around the individual coil
holders, unlike a typical DC generator where core of the
machine consists of metallic slots and teeth around
which the coils are wound. This design eliminates cog-
ging torque and thus improves the start-up speed of the
wind turbine. Cogging torque is the friction caused by
the attraction between PMs and iron core present in a
typical DC motor/generator, which increases the start-up
torque and decreases the electrical efficiency at low wind
speed.
Arc shaped magnets
Tear-drop shaped coils
Figure 8 A CAD model of the “double rotor-single stator slotless
axial flux generator”without fixtures
R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine 35
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Once the configuration of the generator was selected,
the next step was to determine and optimize the various
design parameters which are needed to achieve high
efficiency under the given size constraints. Some of the
main parameters which affect the performance of an axial
flux generator are number of poles, air gap between
stator and rotor, shape and size of magnets, and shape
of the coils. The number of pole pairs is given by the
following equation (Latoufis et al. 2012):
p¼120 fnom
nnom
½2
Where p denotes number of pairs of magnetic poles,
fnom is the nominal armature EMF frequency, and nnom
represents the nominal angular speed of the rotor in rev/
min (rpm). The rpm of rotor essentially depends on the
wind speed; it increases as wind speed is increased. We
selected the optimal angular speed of the wind turbine at
its rated wind speed of 4.0 m/s, which is around 900
rpm, as the nominal rpm to calculate the number of pole
pairs. Considering the applications of the SSWT, a DC
voltage output is desired. Therefore, the AC output of
the generator needs to be rectified, and thus no particular
value of EMF frequency is mandatory. Higher number of
magnet poles provide higher power-to-weight ratio of an
electric generator, but it also increases the leakage of
magnetic flux (Chalmers and Spooner 1999). We took
the optimal number of magnetic poles equal to 8, which
results in the EMF frequency of around 60 Hz at the rated
wind speed of 4.0 m/s.
The procedure to obtain the optimized values of other
three parameters, air gap between stator and rotor, shape
and size of magnets, and shape of the coils is described
below. To determine the optimum air gap, the electromag-
netic induction model was utilized that has been described
by Marin et al. (under review). The transformation factor
ΦðtÞwhich governs the rotational energy to electrical
energy conversion is determined through the relationship:
Ut
ðÞ¼ð~
v~
B

d
~
lffi
_
θrΦðtÞ½3
where ~
vis the tangential coil velocity, ~
Bis the magnetic
flux density cutting the coil, lis the conductor length, Ue
is the EMF or instantaneous voltage generated, and _
θis
the rotor angular velocity. By assuming that the coil
velocity is orthogonal to magnetic field vectors, the line
integral in eq. [3] reduces to eq. [4]:
UðtÞ¼
_
θrð
Lcoil
0
Bt;r;θ;zðÞ
rr;θ;zðÞ
rdl cos ’ðr;θ;zÞðÞ½4
where ’is the angle between ~
v~
Band the differential
conductor length d
~
l,ðr;θ;zÞare coordinates within the
coil volume, rðr;θ;zÞis the distance from the center of
rotation to the coordinate ðr;θ;zÞ, and ris the radius at
edge of rotor. By discretizing the coil volume, eq. [4] is
reduced to eq. [5] as:
ΦðtÞffiXBðt;r;θ;zÞΔLcoil
rðr;θ;zÞ
r

dl cos ’ðr;θ;zÞðÞ
½5
ΔLcoil ¼Lcoil
#of volumes ½6
Using the above formulation, a transformation factor (Φ)
was calculated. In order to calculate voltage and power
using the transformation factor, Kirchoff’s voltage law
was applied in eqs [7] and [8].
0¼ΦðtÞr_
θReþRL
ðÞ
iðtÞ½7
iðtÞ¼ ΦðtÞr
ReþRL
_
θ½8
ULðtÞ¼iðtÞRL¼ΦðtÞr
ReþRL
_
θRL½9
PLðtÞ¼iðtÞ2RL¼ΦðtÞ2r2
ReþRL
ðÞ
2_
θ2RL½10
With the assumption that ΦðtÞhas sinusoidal variation
the average power can be calculated as:
Pavg ¼Φ2
maxr2_
θ2
2ReþRL
ðÞ
2RL½11
We examined two different shapes of magnet: (i) rectan-
gular and (ii) arc-shaped. It was numerically found that
the arc-shaped magnets produce about 60% higher
power than rectangular magnets of the same thickness
and size. Considering the size of the generator (about the
same diameter as the hub of the wind turbine), arc-
shaped magnets of dimension 26.4 mm (or) 10.5 mm
(ir) 6.35 mm (t)40° were selected, where, “or”and
“ir”denote outer radius and inner radius, respectively,
“t”denotes the thickness, and 40° is the angle made by
inner and outer arc at the center. Figure 9 shows the
contour for magnetic field strength at the mid-plane
between two rotors. Dark red and dark blue indicate the
highest field strength in two polarities. It can be noted
that the magnetic field is in tear-drop shape, therefore, to
36 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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match the profile of magnetic field, coils were also
designed to have a tear-drop shape rather than the com-
mon circular shape. Lastly, the numerical simulations
predicted that the maximum power output by the gen-
erator occurs when the air-gap between the rotors is
about 8 mm (Figure 10). This distance was increased to
10 mm during fabrication due to the manufacturing con-
straints using human hands.
Table 3 summarizes the major design specifications of the
axial flux generator developed for SWEPT. Figure 11(a)
depicts the fabricated stator and rotor parts, while
Figure 11(b) demonstrates a fabricated prototype of the
axial flux generator set along with other main compo-
nents assembled inside the nacelle box.
Prototype design
Figure 12 shows the current generation prototype of
SWEPT. It can be noted that SWEPT is three-bladed
HAWT. It employs NACA 0012 airfoil template for the
blade design. Each of the blades has constant chord
length of 7.5 cm throughout the span. The hub and tip
diameters of the wind turbine rotor are 3 cm and 40 cm,
respectively. The blades are non-linearly twisted by 37
from hub to tip, and the solidity of the rotor is about
30%. As shown in Figure 11(b), SWEPT’s nacelle box
consists of an axial shaft, two bearings, a generator set,
and a tail post. The nacelle is mounted over a tower using
a thrust bearing and a radial bearing. The central shaft is
3/16 inch (4.76 mm) in diameter. The bearings used are
high-load steel ball bearings, which are oil lubricated and
unshielded from both the sides to minimize the frictional
losses. The hub was connected with the central shaft
using press-fit, while the blades are attached with the
hub using dove-tail joints. The nacelle box also contains
a tail with a fin at the end, which produces corrective
moment to orient the wind turbine rotor into the direction
of wind. The blades, hub, and nacelle box were con-
structed using a 3-D printer (Objet Inc., USA). The printer
material used was VeroWhite plastic.
Experimental set-up
Figure 13 depicts the schematic of experimental set-up
used in this study to characterize the wind turbine. All
the experiments were conducted in Subsonic Open Jet
Wind Tunnel facility available in Aerospace & Ocean
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1567
Air
g
a
p
between ma
g
net sets (mm)
Power output (W)
8910
Figure 10 Effect of air gap on power output of the generator
Figure 9 Contours showing the magnetic field strength at the
mid-plane between two rotors
Table 3 Major design specifications of the axial flux generator
developed for SWEPT
Number of pole pairs 8
Number of the magnets 16
Shape of magnets Arc-shaped
Dimension of magnets 26.4 mm(or) 10.5 mm(ir)
6.35 mm(t)40°
Shape of coils Tear drop
Number of coils 8
Number of turns per coil 150
Coil wire material AWG 30 copper
Stator thickness 5 mm
Air gap between rotors 10 mm
Outer radius 3 cm
Inner radius 5 mm
R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine 37
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Engineering department at Virginia Tech. The details
about the wind tunnel and the quality of flow are given
in Kishore, Coudron, and Priya (2013) and Kishore and
Priya (2013). We used non-contact type optical digital
Tachometer “DT-209X”(SHIMPO Instruments, USA) to
measure the angular speed of the wind turbine.
Anemometer used in the experiments was PASPORT,
Model PS-2174 (PASCO, USA). “OhmSOURCE Model OS-
260”by IET Labs, Inc. was used as the resistance box to
vary the loading conditions of the wind turbine. The
multi-meter used to measure the output voltage of the
wind turbine generator was purchased from RadioShack.
It can be seen from Figure 13 that the resistance box and
the multi-meter are connected in parallel with the gen-
erator of the wind turbine. The load resistance can be
varied using the resistance box at any given wind speed,
and the corresponding output voltage can be recorded.
Since the voltage output of this generator was alternat-
ing, we measured the root mean square (rms) value to
calculate the electrical power. We recorded continuous
data for voltage and angular velocity at the given loading
conditions and wind speeds, and then the arithmetic
mean was calculated as the representative value for the
respective variables. The experiments were carried out at
various wind speeds between 1.5 m/s and 10 m/s.
Results and discussion
Generator characteristics
To investigate the performance of a generator in labora-
tory, normally it is coupled with a dynamometer motor
and the wind turbine is emulated by varying the speed of
the dynamometer motor. In this study, however, we
attached the generator directly to the wind turbine rotor
and its characteristics were studied using wind tunnel
experiments. Figure 14 depicts the output voltage profiles
of the generator at various loading conditions and at
constant wind speed of 3.2 m/s. It can be noted that the
waveforms are practically sinusoidal. Increase in the load
Bearing 2
Tail post
Bearing 1
Thrust
bearinng
Mounting
rod
Radial
bearing
(b)(a)
Rotor disc Rotor with arc
shaped magnets
Stator disc
Output
terminals
Tear-drop shape coil
Coil holder
Stator with tear-drop
shaped coils
Stator Rotor 1
Rotor 2 Hub
Shaft
Figure 11 Axial flux generator design for SWEPT: (a) fabricated stator and rotor parts showing the shapes of the magnets and coils and
(b) axial flux generator inside the nacelle box
Figure 12 An experimental prototype of “SWEPT”
38 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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resistance increases both the amplitude and the fre-
quency of the voltage signal.
The internal impedance of the generator was mea-
sured by varying the load resistance and the wind speed
in such a manner that the constant rotational speed of
the wind turbine rotor is maintained. Figure 15 shows the
variation of output rms voltage vs rms current at three
different angular speeds. The slope of the curves gives
the internal impedance of the generator at different fre-
quencies (rpm). It should be noted that the value
obtained is internal impedance, not internal resistance.
The coil resistance of the generator was measured to be
19.6 Ωusing a multi-meter. Looking at the slopes of the
curves in Figure 10, it can be determined that the internal
impedance is around 20.4Ωat 304 rpm (20.3 Hz), and it
increases slowly with increase in the angular speed of the
Generator
Optical
tachometer
Resistance
box
Volt-meter
Data logger
CPU
Wind
Monitor
Wind turbine
Figure 13 Schematic diagram of the experimental set-up
10
8
6
4
2
0
–2
–4
–6
–8
–10 Time (sec)
0 0.02 0.04
20 Ω50 Ω70 Ω
0.06 0.08 0.1
Voltage (V)
Figure 14 Output voltage profiles at various loading conditions
(wind speed ¼3.2 m/s)
R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine 39
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rotor. The angular speed of the rotor is related to EMF
frequency by following equation:
f¼np
120 ½12
where ndenotes rpm of the rotor and prepresents num-
ber of pole pairs. Increase in angular speed increases the
EMF frequency which ultimately increases the reactance
of the stator coils. However, it can be calculated that the
reactance of the generator is very small in comparison to
the resistance of the stator coils and therefore the gen-
erator’s performance will be primarily influenced by
internal resistance.
Figure 16 demonstrates the variation of root mean
square (rms) voltage across the load as a function of
load current under various loading conditions and the
wind speeds. Voltage across the load is given by V¼IR,
where Vis the voltage, Idenotes the current, and Ris the
load resistance. It can be noted from Figure 16 that at a
fixed value of load resistance, voltage increases almost
linearly with increase in current. Also, as expected, slope
of the lines increases with increase in load resistance.
Increase in wind speed increases both the current and the
voltage across the load, when load resistance is fixed.
However, at the fixed value of wind speed, voltage and
current has inverse relationship. Increase in load resis-
tance increases the voltage but decreases the current.
Figure 17 shows the relationship between voltage and
the angular speed at different load resistance and wind
speeds. Theoretically, voltage by the generator is directly
proportional to the angular speed of the shaft. As shown
in Figure 17, for a fixed wind speed, voltage increases
almost linearly with rpm. For a given value of wind
speed, increase in load resistance causes increase in
both, voltage by the generator and the angular speed of
the rotor. Also, when external load is constant, increase
in wind speed increases voltage as well as rpm. Figure 18
depicts the alternating power produced by the generator
as a function of the angular speed of the shaft at various
loading conditions and wind speeds. In can be noted that
at a fixed load resistance, power by the generator
increases with increase in shaft rpm. Also, for a given
load, increase in wind speed increases the rotor speed as
well as the power output because of the increase in
aerodynamic torque on the rotor blades. For a fixed
18
16
14
12
10
8
6
4
2
0
0 0.2 0.4 0.6
rms current (A)
rms voltage (V)
y = –20.36x + 4.21
y = –22.74x + 9.37
y = –23.25x + 16.90
1,155 rpm
650 rpm
305 rpm
0.8
Figure 15 Variation of generator voltage with current at different
constant rpm conditions
24
20
16
12
2.0 m/s
3.0 m/s
4.0 m/s
5.0 m/s
6.0 m/s
60Ω
80Ω
100Ω
120Ω
140Ω
160Ω180Ω
200Ω
8
4
00 0.05 0.1
rms current (A)
rms voltage (V)
0.15 0.2 0.25
Figure 16 Voltage vs current at various loading conditions and
wind speeds
24
20
16
12
8
4
00 500 1,000
An
g
ular s
p
eed (r
p
m)
rms voltage (V)
1,500
20Ω
2.0 m/s
3.0 m/s
4.0 m/s
5.0 m/s
6.0 m/s
7.0 m/s
40Ω
60Ω
80Ω
100Ω
120Ω
200Ω
2,000
Figure 17 Voltage vs angular speed at various loading conditions
and wind speeds
5
4
3
2
15.0 m/s
4.0 m/s
3.0 m/s
2.0 m/s
6.0 m/s
7.0 m/s
20Ω
40Ω60Ω
80Ω
100Ω
120Ω
160Ω
200Ω
00 500 1,000 1,500
An
g
ular s
p
eed (r
p
m)
ac power (W)
2,000
Figure 18 Power vs angular speed at various loading conditions
and wind speeds
40 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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value of wind speed, the angular speed of the rotor
increases with increase in load resistance. However,
there exists an optimal load where power is highest,
when wind speed is fixed.
The variation in shaft speed vs load resistance at
various wind speeds is shown in Figure 19. Increase in
wind speed increases the aerodynamic torque on the
wind turbine rotor and therefore, at a fixed load resis-
tance, shaft speed increases with increase in wind speed.
However, at a fixed wind speed, rpm first increases with
increase in load resistance, but it starts to saturate when
load is too large. This can be explained if we understand
the interaction between the stator and the rotors of the
generator when the generator is at different loading con-
ditions. When the connection is open circuit, there is no
current in the stator coil and therefore no induced mag-
netic field. But, when a finite amount of resistive load is
applied, it delivers current which creates a magnetic field
around the stator coils that reacts with the magnetic field
of the rotors. The strength of the reacting magnetic force
between stator and rotors depends on the current which
is ultimately determined by the magnitude of the load
resistance. Higher load resistance results in lower current
and thus lowers magnetic reaction, which causes
increase in shaft rpm.
Wind turbine performance
The direct-drive operation of the current generation
SWEPT (with axial flux slotless generator) has two
immediate advantages over the previous generation
gear-drive prototypes: (i) the cut-in wind speed decreased
to 1.7 m/s in comparison to 2.7 m/s of first generation
prototype and 3.0 m/s of second generation prototype
and (ii) the range of operating wind speed increased to
1.7–10 m/s in comparison to 2.7–5.0 m/s of first genera-
tion prototype and 3.0–5.5 m/s of second generation
prototype. Figure 20 shows the electrical power output
of the wind turbine as a function of the load resistance at
different wind speeds between 2.0 m/s and 7.0 m/s. At a
fixed wind speed, the electrical power first increases with
increase in load resistance, it reaches to a maximum
value at an optimal load, and it decreases thereafter
with further increase in the load resistance. Also, it can
be seen that the electrical power increases with increase
in wind speed because theoretically, power output of a
wind turbine is proportional to cube of the wind speed.
The optimal load decreases from 120 Ωat 2.0 m/s to 40 Ω
at 7.0 m/s. The peak electrical power produced by the
wind turbine is about 1 W at 4.0 m/s of wind speed,
which increases to around 4 W at the wind speed of
7.0 m/s.
To investigate the performance of the wind turbine at
higher wind speeds, we continued the wind tunnel
experiment till 10 m/s. However, above 7.0 m/s of wind
speed, the wind turbine was allowed to run only at its
optimal load in order to obviate its operation at very high
rpm. Figure 21 shows the variation of peak electrical
power of the wind turbine as a function of wind speed.
The experiment revealed that the wind turbine is capable
of producing electrical power up to 9.8 W at the wind
speed of 10 m/s. The net efficiency of the wind turbine is
1,800
1,600
1,400
1,200
1,000
800
600
400
200
00 50 100 150
Load resistance (Ω)
Angular speed (rpm)
200 250
2.0 m/s
3.0 m/s
4.0 m/s
5.0 m/s
6.0 m/s
7.0 m/s
Figure 19 Angular speed vs load resistance at various wind speeds
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
00 50 100 150
2.0 m/s
4.0 m/s
6.0 m/s
3.0 m/s
5.0 m/s
7.0 m/s
200
Load resistance (Ω)
ac power (W)
Figure 20 Power output vs load resistance at different wind speeds
12
y = −0.0005x3 + 0.139x2 − 0.3958x + 0.3658
R2 = 0.997
10
8
6
4
2
00246810
Wind s
p
eed (m/s)
Peak ac power (W)
Figure 21 Peak electrical power vs wind speed
R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine 41
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shown in Figure 22. The wind turbine has maximum
efficiency of about 21% at the rated wind speed of 4.0
m/s. It is also interesting to note that the peak efficiency
of the wind turbine lies between 19% and 21% at lower
wind speeds (2.0–5.0 m/s), however it decreases with
further increase in the wind speed. This happens because
the blades of the wind turbine have been specifically
designed for low wind speed operations (rated wind
speed of 4.0 m/s), which causes the wind turbine to
stall when wind speed is increased above 5.0 m/s.
Lastly, Figure 23 demonstrates the peak efficiency of
SWEPT vs that of a conventional large-scale megawatt
wind turbine. LSWTs typically have overall efficiency of
about 30–35%. However, there exists a cut-in wind speed
of about 4.0–5.0 m/s where LSWTs do not operate. It is
very interesting to note that in spite of the fact that SWEPT
is a very small wind turbine and it operates within the cut-
in wind speed region of the convention wind turbines, it
still has about 60–70% efficiency in comparison to LSWTs.
Conclusions
In summary, this study presented the design, develop-
ment, and experimental testing of an ultra-low start-up
speed, highly efficient, direct-drive SWEPT. An axial flux
generator was developed which is suitable for the small
size wind turbines at low wind speed applications. Wind
tunnel experiments were conducted to characterize the
performance of the generator and the wind turbine. The
major findings of the study can be summarized as
follows:
(a) The cut-in speed of the wind turbine was found to be
1.7 m/s.
(b) The wind turbine operates in a very wide operating
range of wind speeds, from few meters per second to
10 m/s.
(c) It produces electrical power of 1 W at its rated wind
speed of 4.0 m/s.
(d) The peak power by the wind turbine increases to 4 W
at 7.0 m/s, which further increases to 9.8 W at 10 m/s.
(e) The peak efficiency of the wind turbine was found to
be around 21%. However, it decreases with increase
in wind speed above 5.0 m/s.
Acknowledgments: The authors gratefully acknowledge
the financial support provided by NIST program and NSF
I/UCRC: Center for Energy Harvesting Materials and
Systems (CEHMS). We also thank senior design students:
Jonas Alles, Robert Langhans, and Will Robbins for their
contribution during design and development of the wind
turbine.
30%
25%
20%
15%
10%
5%
0% 0 50 100
Load resistance (Ω)
Efficiency (%)
150 200
2.0 m/s 3.0 m/s
6.0 m/s
4.0 m/s
7.0 m/s5.0 m/s
Figure 22 Net efficiency of the wind turbine vs load resistance at
various wind speeds
35%
Cut-in (LSWT)
SWEPT
Large-scale wind
turbine (LSWT)
30%
25%
20%
15%
10%
5%
0%
12
Efficiency (%)
34567
Wind s
p
eed (m/s)
8 9 10 11 12 13 14 15
Figure 23 Power efficiency curve of SWEPT vs a conventional LSWT
42 R. A. Kishore et al.: Efficient Direct-Drive Small-Scale Low-Speed Wind Turbine
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