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                        TECHNICAL PAPER # 11
                      UNDERSTANDING WIND ENERGY
              Dr. James F. Manwell & Dr. Duane E. Cromack
                            Illustrated By
                          Christopher Schmidt
                          Technical Reviewers
                             Theodore Alt
                          Christopher Turner
                          Christopher Weaver
                             Published By
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virgnia 22209 USA
               Tel:  703/276-1800   *  Fax:  703/243-1865
                       Understanding Wind Energy
                         ISBN:  0-86619-211-5
              [C] 1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Leslie Gottschalk
and Maria Giannuzzi as editors, Julie Berman handling typesetting
and layout, and Margaret Crouch as project manager.
The authors of this paper, Dr. James F. Manwell and Dr. Duane E.
Cromack, are professors with the Department of Mechanical Engineering
at the University of Massachusetts.   Dr. Manwell also has
background in solar energy, hydropower, thermodynamics, and electrical
and computer engineering.  Dr. Cromack has consulted for
the U.S. Government and private industries in wind energy.  Christopher
Schmidt is a professional illustrator in the fine arts,
technical, and medical areas, and attends the Pacific Northwest
College of Art.  He illustrated VITA's Renewable Energy Dictionary.
Theodore Alt, P.E., is a mechanical engineer who has been in
the energy field since 1942.   He has worked with the energy research
and development group of the Arizona Public Service Company
and the Government of Mexico's electric commission.   Christopher
Turner monitors and disseminates information about appropriate
technology, and has worked with wind energy in North
Carolina.  Christopher Weaver is an engineer with Energy and
Resource Consultants, Inc. in Colorado.   He has written two technical
papers for VITA on hydroelectric generation.
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
   By VITA Volunteers James F. Manwell and Duane E. Cromack
The sun is the original source of wind energy.   Sunlight warms the
sea, land, and mountains at different rates.   This creates inequalities
in the temperature of the earth's atmosphere.   These
thermal imbalances produce air in motion--or wind.   Wind machines
capture the energy of the wind and convert this energy into
mechanical motion or electricity.
The typical wind machine consists of a rotor or turbine, which
are usually mounted upon a tower.   The wind rotates the turbine or
rotor, which turns the shaft of an electrical generator or a
mechanical device.  If the wind system produces electricity, the
electrical power may be used immediately or stored in batteries
for later use.
The use of wind power is almost as old as recorded history.  The
Egyptians used sails to power their boats on the Nile River over
5,000 years ago.  The Chinese are thought to have been the first
to use windmills, and the Persians are known to have built windmills
in 200 B.C.  The Persian vertical shaft windmill, or "panemone,"
was used to power grain-grinding stones.   Medieval Europeans
used windmills for a wide range of activities, including
pumping water, sawing wood, grinding grain, and pressing oil--in
fact virtually any process that required mechanical energy.  The
traditional windmill was developed to its greatest extent by the
Dutch, who used windmills by the thousands (Figure 1).

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Early European windmills were of the "post mill" type (Figure 2).

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The entire machine was mounted on a post, and the mill itself was
built around the post.  The post, supported on the ground, served
as a pivot for turning the mill so that it could be faced into
the wind, or "yawed."   Subsequent mills were of the "cap design."
In this case only the top, or cap, of the mill, which held the
blades, was turned to face the wind.   Until the 1750s, millers
had to turn the machine by hand to face the wind.   After that
period, the invention of the fantail--a small windmill mounted at
right angles to the main blades--allowed the machines to be yawed
automatically (Figure 3).

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A new era for windmills began in the late 1800s in the United
States.  The settling of the semi-arid western United States
required the use of water, which had to be pumped out of the
ground.  The American multibladed farm windmill (Figure 4) was

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developed around that time to provide pumping power.   At one
time, hundreds of thousands of these machines were in use.  They
have been largely replaced today, but in many parts of the world
they are still used.
Near the beginning of the 20th century, the Danes first used wind
power to generate electricity (Figure 5).   The new wind generators

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found an active market in the American Great Plains, which already
had its wind-driven water pumpers in place.   The new machines
usually had an electrical output of less than 1,000 watts,
which was adequate to provide lighting and power for small appliances.
After the major U.S. rural electrification program had
begun in the 1930s, these wind machines could not compete with
cheap, reliable utility power and most of them were abandoned.
Nevertheless, some development in wind power continued into the
1950s, mostly on machines capable of much larger electrical
output.  The Danes, Russians, British, French, and Americans all
experimented with wind machines that could produce 100 kilowatts
(kW) or more.  By the early 1960s, however, interest in wind
power as a viable source of power production had waned, because
other energy sources appeared to make it obsolete.   During the
1970s many people realized that fossil fuels were not renewable
and were subject to interruption and that nuclear power was not
as reliable and inexpensive as some people had imagined. People
once again turned to wind power as an alternative to some of
those unexpected problems.
Since the mid-1970s a number of countries have begun major programs
to develop modern wind systems.   Some of the programs have
focused on large-scale power generation, others on medium-scale
systems for commercial use, and still others on improved "intermediate
technology" devices, most suitable to Third world applications.
Wind power provides for two basic types of needs:   (1) For remote
applications, where an electricity grid (supply) is not available
or the need is for mechanical power such as water pumping, wind
can serve the function quite well, provided an adequate wind
source is available.  (2) In other areas, where electricity grids
are available, wind power can serve as an alternative to conventional
forms of power generation.  It can help to decrease the
amount of purchased fuel and replace some of the conventional
generating capacity.
Where surface water is scarce and there is adequate wind, wind
machines are a reliable and economical way to pump water from
deep or shallow wells for isolated ranches, villages, and farms.
Wind power can provide water for irrigation, drinking supplies,
livestock, and other uses.  Wind power can also be harnessed to
provide power for grinding grain and sawmill operations.
For sites not connected to an electric grid, wind machines can
generate electricity for pumping water, grinding grain, heating
homes, running appliances, and lighting.   In those areas where
utility service is already available, wind power can contribute
to the operation of lights, electric stoves, air conditioners,
and other appliances.  In some applications, wind power may also
provide heat for warming homes and water.
Wind is air in motion.  As such, it possesses energy.  A windmill
operates by slowing down the wind and capturing some of its
energy in the process.  Consider an area A([m.sup.2]) perpendicular to
the wind direction.  If the wind, with density p (kg/[m.sup.3]), flows
through it with a velocity V(m/s), the power (watts) in the wind
is given by:
                       P = 1/2p[AV.sup.3]
This equation summarizes the following key facts:
     (1)   The power varies directly as does the density.  It
          should also be noted that the density decreases with
          increasing temperature and decreasing atmospheric
          pressure (e.g., caused by increasing altitude).  At sea
          level and 15 [degrees] C, p = 1.225 kg/[m.sup.3].  Under other conditions,
          the density is given by p = .464 P(mm Hg)/
          (T([degrees] C) + 273).
     (2)   For a horizontal axis windmill of radius R, the power
          is proportional to A = [pi] [R.sup.2].
     (3)   The power varies with the cube of the wind speed.
          This means that the power increases by a factor of
          eight when the wind speed doubles.
A windmill cannot extract all the power in the wind.   Theoretically,
a wind machine rotor can extract at most 59.3 percent of
the power. Other factors contribute to even greater decreases in
efficiency.  Typical rotor efficiencies, called power coefficients,
or Cp, range from 20 to 40 percent.
Most wind machines operate through the use of sails, blades, or
buckets connected to a central shaft.   The extracted wind energy
causes the shaft to rotate.  This rotating shaft can be used to
drive a pump, power a generator or compressor, or do other work.
Two aerodynamic principles come into play in wind-machine operation:
lift and drag.  The wind can rotate the rotor of a wind
machine by pushing against it (drag) or by lifting the blades
(aerodynamic lift).  Wind drag is the force you feel when you turn
the palm of your hand into a strong wind.   Drag is the primary
motive force in some slow-speed machines such as the Savonius
rotor (Figure 6).

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A common example of aerodynamic lift is the force that acts on
the wings of an airplane.  Airplane wings have a special shape
called an airfoil (Figure 7).   The airfoil produces a low pressure

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area above the wing and a high pressure area beneath it as the
airplane flies.  The difference in pressure between the top and
bottom of the wing actually lifts the plane and keeps it in the
Lift force is used on most wind machines today, whether they are
the relatively slow, multibladed water pumpers, or the high-speed
two- or three-bladed electric generators.
The blades of most present day wind generators are, in effect,
airfoils.  When the wind hits these blades the pressure difference
lifts the blade and allows it to move with great speed and
efficiency.  Any drag force on the blades decreases power production.
The relationship of the blade speed (measured at the tip)
to the wind speed is the tip speed ratio.   If the blades are moving
five times faster than the wind, the tip speed ratio is 5:1.
Tip speed ratios are typically in the range of one to six.  Drag
machines always have a tip speed ratio of less than one.
The higher the design tip speed ratio, the lower is the required
ratio of total blade area to swept area (called solidity).  For
electric power generation, the trend is toward higher tip speed
ratios, both because high rotational speeds are required at the
generator and because fewer blades are needed so relative costs
are less.  In addition, higher power coefficients are obtainable
at the higher tip speed ratios.
A high tip speed ratio is not always desirable, however.  Power is
the product of torque ("twisting force") and rotational speed.
Thus, low-speed machines have relatively high torque compared
with high-speed machines.  In particular, fast machines have very
poor starting torque characteristics.
For many mechanical applications, such as water pumping, high
torque is of primary importance.   Thus, machines used for those
purposes tend to be slower, higher-solidity machines.   Although
these machines do require a relatively greater blade area, because
of their lower speed the blade shapes can be simpler.   For
example, slower machines can use sails or curved flat plates
effectively, whereas faster machines need more streamlined blade
shapes to minimize the adverse effects of drag.
An important consideration in any wind machine design is structural
integrity.  The forces that give rise to the torque and
hence power also have components parallel to the wind direction.
These forces contribute to the bending of the blades and a thrust
that tends to push the machines over.   The thrust force is given
               [F.sub.T] = [C.sub.T]1/2p[AV.sup.2]
Under ideal conditions, [C.sub.T] = 8/9.   The machine and tower are
usually designed to withstand at least four times the force that
would be produced when the machine is operated at its greatest
output.  The thrust force is distributed equally over the blades,
and for blade design purposes can be assumed to act at two thirds
of the way out on the blade from the hub.
The essential characteristic of the wind is its variability.  The
power output of a wind machine will vary accordingly.   Average
wind speeds vary from place to place.   They also vary with the
time of day and with the seasons.   The average wind speed normally
increases with height above the ground.   For example, each time
the height above ground is doubled (e.g., from 10 m to 20 m),
the wind speed increases by at least 10 percent, which increases
the available power by 30 percent.
The most important measure of a site's potential for wind power
is the annual average wind speed.   For example, sites with mean
wind speeds less than 3 m/s are seldom good sites.   Those with
averages above 3 to 4 m/s may be feasible, depending on the
application and the cost of other forms of energy.   Sites with
averages in the range of 6.5 to 8 m/s or higher are excellent
candidates for wind power development.   At any prospective site,
however, it is important to consider the seasonal and diurnal
(time of day) wind speed variations and ensure that they are
compatible with the load.
Nearby weather stations can provide data on wind speed.   In flat
terrain, readings from the three or four closest stations will
provide a rough estimate of average wind speed.   In mountainous
areas the wind speed is more site-specific and requires more
detailed analysis.
To determine the wind resourse at a proposed site, the following
information should be obtained:   monthly mean wind speed; frequency
distribution of wind speed (the percent of time the wind speed
blows at a given strength); and daily variation of wind speed.
The monthly mean wind speed will indicate if power will be available
when most needed.  It will also help determine the kind of
turbine that is needed.  The frequency distribution of wind speed
and direction will provide an estimate of potential power and
help to identify the best location for a wind system.   The daily
variation of wind speed will tell the likelihood that power will
be available at those times during the day when it is most
If these data are available, an anemometer, or wind sensor,
should be used to obtain readings on or near the proposed site.
The hand-held type is the least expensive and is usually available
in outdoor and aircraft supply stores.   Although it does not
average the wind speed, it will give a rough idea of the wind
resource.  A cup anemometer can be set up and left alone to
measure wind speed (See Figure 8).

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Wind characteristics are best analyzed by taking hourly wind
speed data at a site for at least 12 months.   When that is not
possible, data may be taken for a shorter period, and then compared
with data from another, nearby site, such as an airport,
for which long-term data are available.   When complete data are
available these are often summarized in velocity and power
duration curves, which can then be used in estimating energy
production for various wind machine designs.   If only summary
data are available, such as mean wind speeds, a variety of statistical
techniques have been developed that make it easier to
determine the amount of wind resources available.
Often, no data are available for a particular site.   In this
case, the shapes of bushes and trees can give an indication of
the wind resource at a given site.   Bushes will generally be
shorter in locations with strong winds.   Trees will have off-center
crowns and trunks, and branches will be swept leeward.
Other environmental indicators of strong winds may include sand
scours and crescent-shaped sand dunes.   These indicators will be
particularly prevalent if the wind direction is relatively constant.
The operation of a wind machine as well as its power output
depends on the wind speed.  There are four important wind speed
ranges to consider.  In the first range, when the wind is less
than the cut-in speed, no power is produced.   The wind machine
may rotate at these low speeds, but it would not be performing
useful work.  In the second range, between the cut-in speed and
the rated wind speed, useful power will be produced.   The amount
of power will depend on the wind speed.   In a machine optimally
matched to wind speed variations, the power output will vary
directly as the available power in the wind, i.e., as the cube of
the wind speed.  For most machines, however, the relation is
usually less than cubic.  In the third range, where the wind is
above the rated speed, but less than the cut-out wind speed,
power output is usually constant, at rated power.   Partially
furling the blades (pitching them out of the wind) or moving the
rotor out of the wind prevents more power from being produced.
Above the cut-out speed, the machine is totally shut down and
remains so until the wind speed decreases back to the normal
operating range.  The operating characteristics are usually summarized
in a power versus wind speed curve.
Wind energy systems include the following major components:
rotor, hub assembly, main shaft, main frame, transmission, yaw
mechanism, overspeed protection, electric generator, nacelle,
power conditioning equipment, and tower.
High-speed wind machine rotors usually have blades with a cross
section like that of an airplane wing (airfoil).   The blades are
usually made of wood (solid or laminated), fiber glass, or metal.
Slower machines usually use flat or curved metal plates or sails
mounted on a spar (See Figures 9, 10, and 11).

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Hub Assembly and Main Shaft
The blades are attached by a hub assembly to a main shaft.  The
main shaft rotates in bearings supported in the main frame.  If
the blades are designed to rotate (pitch control), the hub can be
fairly intricate.  With fixed pitch, attachment is relatively
Main Frame with Support Bearings
The main frame of the wind machine serves as the point of attachment
for various components, such as the main shaft, transmission,
generator, and nacelle.  It usually contains a yaw bearing
assembly as well.
Transmission Mechanism
A transmission assembly (gear box, chain drive, or the like) is
required to properly match the rotational speed to the desired
speed of a water pump, electric generator, or air compressor
because the rotational speed of the wind wheel (rotor) does not
match that of the pump or generator to which it is to be connected.
Yaw Mechanism
Horizontal axis machines must be oriented to face the wind by a
process called yawing.  Upwind machines (those with blades upwind
of the tower) usually incorporate a tail vane, small yaw rotors
(fantails), or a servo mechanism to ensure that the machine
always faces upwind.  Downwind machines (blades downwind of the
tower) often have the blades tilted slightly downwind (coned) so
that they also act as a tail; this angle ensures proper orientation.
Vertical axis machines accept wind from any direction;
thus, they do not need a yaw control (See Figure 12).

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Overspeed Protection
All wind machines must be protected from high winds.   A number of
different methods are used.  In some machines, the blades can be
turned around their long axis (pitch control) and aligned so that
they do not produce any lift, hence no power.   Blades with fixed
pitch often use brakes to slow the machine.   The brakes are
either aerodynamic (e.g., tip brakes) or mechanical (e.g., disc
brakes on the main shaft).  Other machines use various mechanical
means to turn the rotor out of the wind.
Electric Generator
The electric generator is attached to the main support frame and
coupled to the high-speed end of the transmission shaft.  Alternating
current generators often run at 1,800 rpm in the United
States or 1,500 rpm in much of the world to maintain system frequencies
of 60 Hz and 50 Hz, respectively.
The most popular types are:
     1.    For small independent wind systems, direct current (DC)
          generator alternators with built-in rectifier
          diodes are often used to change AC to DC.
     2.    For larger independent systems, or those that may be
          run in conjunction with a small diesel electric grid,
          synchronous generators are common.  These machines produce
          alternating current (AC) and must be able to be
          regulated precisely, to ensure proper frequency control
          and matching.
     3.    Wind machines connected to a utility grid may have
          induction generators.  These induction machines produce
          AC current, but are electrically much simpler to connect
          to a grid than a synchronous generator.  They
          normally require a utility connection to maintain the
          proper frequency and cannot operate independently without
          special equipment.
Electric Power Conditioning Equipment
The need for electrical equipment in addition to the generator
will depend primarily on the type of generator.   For small DC
systems, at least a voltage regulator is needed.   Battery storage
is often used to provide energy in times of low winds.   Sometimes,
an inverter (to convert DC to AC) is used if some of the
load requires alternating current.   For grid-connected systems, a
control panel is needed that will typically include circuit
breakers, voltage relays, and reverse power relays.   Synchronous
machines require special synchronizing equipment and frequency
The nacelle is the housing that protects the main frame and the
components attached to it.  This enclosure is particularly important
for wind electric systems, but is often left out in water
A tower or other support structure is needed to get the wind
machine up into the air, away from the slower and more turbulent
winds near the ground.  A wind machine should be at least 10 m
higher than any obstructions in the surroundings, such as trees.
Towers are typically of truss design or of poles supported by guy
wires.  Guy wires are cables attached to the tower and anchored in
the ground so that the tower will not move or shake from the
force of the wind.  Towers must be designed to resist the full
thrust produced by an operating windmill or a stationary wind
machine in a storm.  Special concern must be given to the possibility
of destructive vibrations caused by a mismatch of wind
machine and tower (See Figure 13).

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Wind power has two major uses today:   mechanical power and electric
power production.  By far, the most important use of mechanical
power is in water pumping, although wind power is sometimes
used directly for aeration of ponds or other mechanical loads.
Within the electric power production category, there are two main
applications:  (1) power for remote applications, and (2) utility-connected
machines.  Wind electric generating machines (WEGM)
or wind electric conversion systems (WECS) used in remote applications,
separate and distant from any utility grid, are typically
connected to storage batteries.   When supplemented by another
electric generator such as fossil fuel or hydro, the WEGM or WECS
is termed a hybrid system.  Large machines (100-2,500 [kw.sub.E]) are
being developed to be operated by the utility companies, much the
same an they would operate any other power plant.   An application
that is becoming more common in industrial countries is the
development of wind farms.  This involves private groups who form
consortia to purchase wind machines, and sell power to utilities
as small power producers.
Small machines (1.5-50 [kw.sub.E]) are being used by individuals, farmers,
and small businesses in remote locations to augment their
power supply and decrease the power purchased from electric
A minor and frequently inefficient use of wind power is in heating
applications.  This is carried out either through electrical
generation, the power from which is dissipated in resistors, or
mechanically by using a water brake or churn.
The equipment, materials, and resources needed to construct and
operate a wind system depend largely on the type of system being
planned.  Wind systems are divided into three categories:  (1)
simple technology, (2) intermediate technology, and (3) complex
The simple technology systems include those that can be built
easily using locally available components.   They are typically
small machines with low power output, operating at low rotational
speeds for water pumping.  Savonius rotors, made of recycled
drums and erected on wooden truss towers, fall into this category,
as do sailwing machines patterned after traditional designs.
Although such machines can be built using locally available
wood and cloth materials, most of them could be improved
substantially by incorporating a few imported, manufactured components,
especially bearings.
The intermediate-technology wind machines are more sophisticated
than those in the first category.   These WECS include deep well
water pumpers of modern design plus small wind electric machines.
They are made primarily of steel, which should be available in
the form of sheet stock, rods, bars, and structural forms (angle
iron).  The blades themselves are likely to be made of curved
steel plates (slow-speed machines) or carved wood, either solid
or laminated (high-speed machines).   Most of the components can
be made at a local machine shop or blacksmith shop.   In addition
to conventional hand tools, such equipment as drill presses,
sheet metal cutters, lathes, milling machines, arc welders, and
gas torches should be locally available.   Specialty components,
such as bearings, gears, chains, sprockets, and electrical equipment
(when applicable) might need to be purchased elsewhere.
The high-technology, complex WECS represent the third category of
machines.  This category includes the high-speed wind electric
systems of high power output (200-2,500 [kw.sub.E]).   These machines
require special equipment, as well as materials more exotic than
steel or wood.  Many of the components, such as gearboxes, generators,
control system electronics, and electrical switchgear,
are likely to be produced by separate suppliers.   The blades are
likely to be made of fiber glass, constructed either in the
manner of fiber glass boats or with a filament winding technique
such as is used in the helicopter industry.   The nacelle also is
likely to be of fiber glass.   Special materials and equipment
might also be used in building such items as brakes, pitch control
systems, yaw controls, or electrical slip rings.   The main
frame could be built at a standard machine shop.   The tower must
be designed specifically for the machine; it probably has to be
constructed by a firm familiar with support structures.
Construction of simple-technology WEC machines requires a journeyman
skill level.  Builders should be familiar with basic hand
tools, and be able to read construction plans.   For example, a
literate farmer, capable of making, maintaining, and using simple
implements such as plows or animal-operated irrigation pumps,
should be able, with some instruction, to construct and operate a
simple wind machine.
To build intermediate-technology machines requires a higher skill
level.  The designs could certainly be produced elsewhere, but a
good understanding of the principles behind the design is desirable.
Builders must have the skills of a competent machinist or
blacksmith, and must be able to operate the simple tools described
earlier.  They also must have some special skills in
order to handle certain aspects of the construction, such as
making blades or hooking up the electrical equipment.   A person
familiar with rigging should supervise the installation of the
machine.  The design of the machine should be such that normal
operation and repair could be carried out by the owner.
The production of high-technology machines requires the highest
skill level.  An engineer familiar with the design should oversee
the construction and testing of at least the first few machines.
Persons, with a variety of skills, such as welders, machinists,
electricians,  sheet metal workers, and fiber glass workers are
required.  Much of the work also requires precision, and familiarity
with the latest building techniques and materials.   The
various subcontractors should have their own work force to ensure
the proper design and construction of the individual components.
Although the energy in the wind is free, the wind system that
extracts the work is not.  System-installed cost is often associated
with the rated output, e.g., dollars per kilowatt or
dollars per horsepower.  To evaluate the economics of a system
accurately, one must consider at what wind speed the machine is
rated or how much total energy should be produced in a given wind
regime.  Despite this caveat, the costs of wind machines usually
fall within specific ranges.   For example, water pumpers usually
cost from $4,000 to $8,000 per horsepower (hp) for units less
than one hp.  In sizes of 5 to 15 hp, they usually cost between
$1,000 and $2,000/hp.  Simple designs that can be built locally
and that produce mechanical shaft power can cost in the range of
$1,000 to $1,500/hp, but they also could involve higher labor,
maintenance, and operational requirements.
Complete wind electric systems typically cost from $1,500 to
$3,500/kW for machines in the range of 5 kW and from $1,000 to
$2,500/kW for machines in the range of 30 kW.
Evaluating the economics of a wind system requires a knowledge of
the system's useful energy output and its value, as well as the
cost of the machine.  Complete analyses usually consider other
factors as well, such as maintenance costs, loan interest rates,
and discount rates.  One useful indicator of economic viability
is the payback period, which can be calculated easily.   The payback
period, in years, is determined simply by dividing the system
cost by the annual value of energy produced.   The payback period,
then, is the number of years it takes to pay back the
original cost.  The following example illustrates a simple economic
     Wind Machine:  Rated power = 10 kW at 10 m/s
     Cost = $1,500/kW or $15,000 installed
     Wind Resource:  Annual average wind speed = 6.5 m/s
     Annual Productivity of Machine = 35,000 kilowatt hours (kWh)
          (assuming a typical wind regime)
     Value of Power = $.15/kWh
     Payback Period = Cost/value of annual productivity
          = 15,000/(.15) (35,000) = 6.67 years.
As discussed earlier in this paper, wind machine rotors have
power coefficients in the range of .2 to .35 for slow machines
and .35 to .45 for fast machines.   In addition, transmissions,
generators, and pumps all have efficiencies associated with them.
Transmissions can have efficiencies in the range of 90 to 97
percent, depending on the type.   Generators can have efficiencies
as high as 95 percent, but small generators often have lower
efficiencies.  In addition, the efficiency can drop off substantially,
when the generator is operated at less than 25 to 50
percent of its rated output.   The overall efficiency of the
gearing and pump of a water-pumping windmill can be about 60
percent.  When all the losses are considered, the overall maximum
efficiency of a high-speed machine can be in the range of 25 to
38 percent.  For slow machines, overall efficiencies can be in the
range of 12 to 21 percent.  It is important to note that efficiencies
can fall off substantially at wind speeds other than those
corresponding to the maximum; due to the inherent mismatch between
piston pumps and windmills, the overall efficiencies of
water pumpers drop off sharply at higher wind speeds.   The ultimate
performance of the machine, as a function of wind speed,
including all the inefficiencies, is summarized in the power
curve described earlier in this paper.
Windmills are rotary machines that require maintenance at regular
intervals to keep them operating smoothly.   Close attention to
proper design and construction will ensure that the machines have
a long service life with minimum repair.   Normal maintenance
includes lubrication of moving parts, and regular inspection of
all the equipment for signs of fatigue, wear, or damage.  The
brushes used in direct-current electrical generators must be
checked periodically, and replaced when necessary.   All electrical
connections should be fastened tightly to make sure that the
vibrations do not loosen connections when the WECS is operating.
All electrical connections must be clean and free of dirt to
ensure that electric operations are done without arcing of connection
The metal towers must be painted as needed to minimize rusting.
Some machines have manual reset after shutdown due to such causes
as vibration or overspeed.  Since the main body of the wind
machine is high above the ground, access to it must be provided
for any repairs or maintenance.   Access can be as simple as a
tall ladder for low machines.   Other machines can be lowered
readily to the ground.  Still others are equipped with a built-in
ladder to reach a work platform at the top of the tower.
The energy storage requirements for wind systems vary, depending
on the type of wind machine and how it is used.   Water-pumping
windmills can use ponds or elevated tanks to store water and to
help match the wind requirements with the water requirements.
Typically, a storage volume of at least three days' demand is
desirable.  However, the desired storage volume will depend on
the wind characteristics (duration per day and velocity) at the
Stand-alone wind electric systems require storage (usually in the
form of batteries) because wind energy varies hour by hour over a
wide range of velocities.  The total storage requirement for
these systems is typically three to five days, depending on the
wind conditions and the load requirements.   Wind electric systems
connected to large utility grids usually do not need storage if
the electric utility purchases excess power.   If the utility does
not purchase the power, some storage is advisable.   Wind machines
coupled to a small isolated grid, such as an isolated grid powered
by diesel generators, may require storage--in terms of a few
hours--to smooth the system output and suppress electrical transients
(sudden changes of load, voltage, or current).   Wind heating
systems use thermal storage, usually water.   The storage is
usually sized for two or three days of the maximum heating requirement.
Some wind electric systems use only a portion of
their output for normal AC loads.   The remaining output is used
for heating, and augments the thermal storage.
Depending on load requirements, climatic conditions, degree of
development of the area, and proximity to power lines, there are
a number of alternatives to wind power.   In any comparison, the
identified wind resource must be adequate for wind power to be
For electric power load requirements, the usual alternative is
utility electric service.  Whether or not to use a wind system
depends on the relative cost.   Reliability will be higher with
the utility.  Smaller grids that use diesel generators are also
reliable, but the power is expensive.   Wind power may be highly
competitive here.
In mountains or hilly terrain with ample rainfall, hydroelectric
power is an alternative to wind power.   Habitation tends to be
clustered more in valleys (where the rivers are) rather than at
mountain peaks, thus transmitting hydroelectric power should be
easier than wind power.  Hydropower is more controllable than
wind power, and a pond is much cheaper than batteries.   Otherwise,
system costs for hydropower and wind systems are roughly comparable,
except where major civil work (e.g., a dam) is required.
For remote areas in regions with good solar energy potential,
photovoltaic (PV) cells are an alternative to wind power.  At
present, PV cells are much more expensive than wind systems; so,
if the region has a good wind source, PV cells will probably not
be economically competitive.   Where the wind resource varies
greatly over the year, a hybrid system comprising both solar
cells and wind power could prove advantageous.
For water pumping, the main alternatives to wind power are animal
power, gasoline or diesel pumps, photovoltaic cells, and utility
electric power.  Animal power, the oldest of the alternatives, is
slow and may involve an inefficient use of resources.   Fossil
fuel pumps are convenient, but their operating costs are very
high.  Photovoltaic cells, as mentioned before, are very expensive.
On the other hand, a complete water-pump system using a PV
panel coupled with a submersible electrically driven pump is easy
to install, compared with a wind system.   It would have many
fewer moving parts and could prove more reliable in the long run.
Utility power is only an option in regions where a grid is already
in existence.  Even in those areas, the cost of bringing a
separate power line to the site of the water may render this
option more expensive than others.
For heating applications, there are also a number of alternatives
available:  fossil fuels, wood, and solar energy.  Fossil fuels
(e.g., oil, natural gas) burned in a furnace are very convenient
sources of heat, and the technology of furnaces is well developed
and relatively simple.  The disadvantage of these fuels is their
high cost and inaccessibility.   Coal is another fossil fuel that
has been commonly used for heating, but it can produce substantial
amounts of pollutants, especially when burned in a small
Wood is a very competitive source of heat in many areas of the
world.  It is much cleaner than coal and often readily available.
In other areas, however, wood usage has outstripped the regenerative
capability of the forests; thus, obtaining wood for fuel may
be difficult.
Direct use of sunlight for heating is another alternative.  The
technology for use of solar energy is developing rapidly.  Active
solar systems, using collectors separated from the load, are used
for space heating, domestic hot water, process applications, crop
drying, etc.  Passive solar systems, where the collectors are
incorporated into the load, are excellent choices for many applications,
such as heating residential buildings.   The disadvantage
of solar energy is that at the time when it is most needed for
heating--in the middle of winter--solar radiation is scarcest.
The wind resource, however, is strongest in the winter in many
locations; for that reason, the use of wind power may be more
cost effective than the use of direct solar energy.   In addition,
obtaining high temperatures with wind power, using electric resistance
heaters, is simpler than obtaining it through the conversion
of sunlight.
One of the main advantages of wind power and other forms of
solar-derived energy is that all involve clean renewable sources
of energy.  All are relatively safe, and the "fuel" is not subject
to arbitrary interruption.  Because wind power provides power
in the form of a rotating shaft, the power is of the highest
grade--it can be used to perform work as well as to provide heat.
On the other hand, there are also land use questions and environmental
issues that must be considered with wind power development.
The wind is a relatively diffuse source of energy.   Wind
machine rotors must sweep a large area, and many machines must be
made available to supply an amount of energy comparable to that
supplied by fossil fuels.  The competing options in the choice of
technology, as well as use of the prospective site, must be
examined carefully.
In deciding whether to use wind power in a region, a number of
questions must be addressed:
     1.   Is there a sufficient wind resource available?
     2.   Can reliable, maintainable machines be built or obtained
         at a resonable cost?
     3.   Is the infrastructure in place to ensure that the
         machine can be operated over its economic lifetime?
         Will parts and the people to service it be available?
     4.   Is wind power a better choice than the other alternatives
         available?  Should the system chosen incorporate
         other technologies as well?
     5.   Will wind power meet with public acceptance?  Is there
         anything about the society in the region where it is to
         be introduced that might cause it to reject the use of
         wind power?  If so, how can the concerns of the society
         be met and still allow the technology to be introduced?
     6.   Are the economics such that the wind system is truly
         desirable?  Will the system be built largely with local
         materials and resources and thus help the local
         economy, or will it involve only imported machinery
         that may be as much of an economic drain as would the
         Purchase of oil?
All of the above questions must be answered before the development
of a wind system can begin.  Given the right situation, the
wind is undoubtedly an excellent source of producing power for
today's world.
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American Wind Energy Association
1609 Connecticut Avenue, N.W.
Washington, D.C. 20009 USA
Mid-American Solar Energy Complex
8140 26th Ave. So.
Bloomington, Minnesota 55420 USA
NASA-Lewis Research Center
Large Systems Technical Information
21000 Brook Park Road
Cleveland, Ohio 44135 USA
Northeast Solar Energy Center
470 Atlantic Ave.
Boston, Massachusetts 02110 USA
Pacific Northwest Laboratories
Wind Characteristics and Siting Information
Battelle Boulevard, P.O. Box 999
Richland, Washington 99352 USA
Rockwell International Energy Systems Group
Small Systems Technical Information
P.O. Box 464
Golden, Colorado 80401 USA
Sandia Laboratories
Vertical Axis Wind Turbine
Information Division 5712
Albuquerque, New Mexico 87185 USA
Southern Solar Energy Center
61 Perimeter Park
Atlanta, Georgia 30341 USA
U.S. Department of Agriculture
Agricultural Systems Information
Agricultural Research Services
Beltsville, Maryland 20705 USA
Western Solar Utilization Network
921 S.W. Washington, Suite 160
Portland, Oregon 97205 USA
Aeolian Energy Inc.
R.D. 4
Ligonier, Pennsylvania 15658 USA
P.O. Box 576
South Dartmouth, Massachusetts USA
Air Track Marketing, Inc.
Three Bridges Road
Box 108C
Federalsburg, Maryland 21632 USA
American Energy Savers, Inc.
912 St. Paul Rd.
Box 1421
Grand Island, Nebraska 68801 USA
P.O. Box 291
127 West Main St.
Millbury, Massachusetts 01527 USA
Bergey Windpower Co., Inc.
2001 Priestley Ave.
Norman, Oklahoma 73069 USA
Bertoia Studio Ltd.
644 Main St.
Bally, Pennsylvania 19503 USA
Carter Wind Systems, Inc.
Rt. 1, Box 405-A
Burkburnett, Texas 76354 USA
Enertech Wind Systems
P.O. Box 420
Norwich, Vermont 05055 USA
Future Energy R&D Corp.
Carretera Estatal No. 113
Zona Industrial
Quebradillas, Puerto Rico 00742
Hummingbird Wind Power Corp.
12306 Rip Van Winkle
Houston, Texas 77024 USA
Home Energy Systems
C/O J&G Energy
Ohio & Missouri Streets
Kanopolis, Kansas 67454 USA
Jacobs Energy Research, Inc.
Rt. 1, Box 171-D
Audubon, Minnesota 56511 USA
Jacobs Wind Electric Company
2720 Fernbrook Lane
Minneapolis, Minnesota 55441 USA
KW Control Systems, Inc.
RD 4, S. Plank Rd.
Middletown, New York 10940 USA
North Wind Power Company
P.O. Box 556
Moretown, Vermont 05660 USA
Oakridge Windpower Inc.
P.O. Box 634
Battlelake, Minnesota 56515 USA
P.M. Wind Power Inc.
P.O. Box 89
Mentor, ohio 44060 USA
Sencenbaugh Wind Electric
P.O. Box 11174
Palo Alto, California 94306 USA
Whirlwind Power Company
207 E. Superior St.
Duluth, Minnesota 55802 USA
WINCO, Division of Dyna Technology, Inc.
7850 Metro Parkway
Minneapolis, Minnesota 55420 USA
Windrive Marketing International
P.O. Box 32007
Kansas City, Minnesota 64111 USA
16341 Eight Mile Rd.
Stanwood, Missouri 49346 USA