Wind Energy Overview
In the last few years, wind energy has become a lower cost option for renewable energy technology. Wind
turbines are a common sight in Iowa, with a number of turbines and large wind farms scattered all across the
state.
This PDF includes information on both large and small electricity-generating wind systems:
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Small systems are typically those sized for residential, agricultural and small commercial use with an
electricity-generating capacity of less than 100 kilowatts.
Large systems, sized for industrial operations and utility companies, produce more than 100 kilowatts. A large
system or concentrated cluster of wind turbines is commonly referred to as a ‘wind farm’.
History of Wind Energy
Wind has played a long and important role in the history of human civilization. The first known use of wind as
an energy source dates back 5,000 years to Egypt, where boats used sails to travel from shore to shore. The
first true windmill, a machine with vanes attached to an axis to produce circular motion, may have been built as
early as 2000 B.C. in ancient Babylon. By the 10th century A.D., windmills with wind-catching surfaces as long
as 16 feet and as high as 30 feet were grinding grain in the Middle East.
The earliest written references to working wind machines in the Western world date from the 12th century.
These too were used for milling grain. It was not until a few hundred years later that windmills were modified to
pump water.
The familiar multi-vane “farm windmill” of the American frontier was invented in the United States during the
latter half of the l9th century. In 1889 there were 77 windmill factories in the U. S., and by the turn of the
century, windmills had become a major American export. Until the diesel engine came along, many
transcontinental rail routes in the U.S. depended on large multi-vane windmills to pump water for steam
locomotives.
Farm windmills are still being produced and used, though in reduced numbers, and show no sign of becoming
obsolete. They are best suited for pumping ground water in small quantities to livestock water tanks. Without
the water supplied by the multi-vane windmill, beef production over large areas of the West would not be
possible.
In the 1930s and 1940s, hundreds of thousands of electricity producing wind turbines were built in the U.S.
They had two or three thin blades which rotated at high speeds to drive electrical generators. These wind
turbines provided electricity to farms beyond the reach of power lines and were typically used to charge
storage batteries, operate radio receivers and power a light bulb or two. By the early 1950s, however, the
extension of the central power grid to nearly every American household, via the Rural Electrification
Administration, eliminated the market for these machines. Wind turbine development lay nearly dormant for the
next 20 years.
Following the OPEC Oil Embargo of 1973, climbing energy prices and questionable availability of conventional
fuels led to renewed interest in wind energy. Federal and state tax incentives and aggressive government
research programs triggered the development and use of many new wind turbine designs. Some experimental
models were very large. With a blade diameter of 300 feet, a single machine was able to supply enough
electricity for 700 homes. A wide variety of small-scale models also became available for home, farm and
remote uses.
In the 1970s there were nearly 50 domestic wind turbine manufacturers. Since then, the wind industry has
undergone massive consolidation, resulting in less than a dozen domestic manufacturers in 1997. This
consolidation followed the expiration of the tax incentives in the mid-1980s and the easing of the energy crisis,
both of which reduced market demand. A competitive marketplace to weed out inferior products further
contributed to consolidation.
Meanwhile, a new market for wind farms began in the early 1980s. This market evolved thanks in part to a new
Federal law, the Public Utility Regulatory Policies Act of 1978 (PURPA). This legislation requires utilities to buy
electricity from private, non-utility individuals and developers. Originally, California was home to most U.S. wind
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farm development because of the availability of wind and attractive buy-back rates. As the cost of the
technology has continued to decline, other areas of the country, namely the Great Plains, Pacific Northwest
and Northeast, are now beginning to see greater wind farm development.
Wind Technology
Wind Energy Systems
This section defines some of the terms used to describe wind energy systems.
Traditionally, wind turbine designs have been dictated by their function. Probably the most familiar wind turbine
is the “Dutch windmill,” a design developed centuries ago and used in Europe for grinding grain and pumping
water. In the United States, the fan-type “water-pumper” can still be seen throughout the countryside,
especially at remote watering wells in the western and plains states.
Today’s modern wind turbines generate electricity using slender aerodynamic blades and tall towers. Because
electricity is an indispensable part of life to most people, wind turbines are often interconnected with utility
lines, so that power is always available on demand. However, a wind turbine can also be a valuable part of an
energy supply system independent from the utility grid.
Wind and Wind Power
This section describes the characteristics of wind flow and explains how to measure and evaluate wind at a
prospective wind turbine site.
Like the weather in general, wind can be unpredictable. It varies from place to place and moment to moment.
Because it is invisible, it is not easily measured without special instruments. Wind velocity is affected by the
trees, buildings, hills and valleys around us. Wind is a diffuse energy source that cannot be contained or stored
for use elsewhere or at another time.
To be economically practical, a wind turbine should experience year-round average wind speeds of at least 12
mph. Sometimes people say, “The wind always blows at my place,” or “I live on the highest hill in the county.”
These observations, while they may be true, are too simplistic. A careful wind survey should be made before
buying a wind system. Wind turbine owners who buy without careful consideration may be disappointed in their
system’s performance.
Wind Data
An interactive wind map can be found on the Iowa Energy Center’s website.
This 6-year project is collecting wind data from across the state and developing maps of the average monthly
and annual wind speeds across Iowa using a GIS-based computer modeling technique. Detailed wind resource
data is available for 2,346 individual locations in the state. Iowa Wind Resource Assessment has been rated as
the most comprehensive public wind data collection project in the United States by the National Renewable
Energy Laboratory (NREL).
Wind Energy Systems
Main Components of a Wind Turbine
Rotor
The portion of the wind turbine that collects energy from the wind is called the rotor. The rotor usually consists
of two or more wooden, fiberglass or metal blades which rotate about an axis (horizontal or vertical) at a rate
determined by the wind speed and the shape of the blades. The blades are attached to the hub, which in turn
is attached to the main shaft.
Drag Design
Blade designs operate on either the principle of drag or lift. For the drag design, the wind literally pushes the
blades out of the way. Drag powered wind turbines are characterized by slower rotational speeds and high
torque capabilities. They are useful for the pumping, sawing or grinding work that Dutch, farm and similar
“work-horse” windmills perform. For example, a farm-type windmill must develop high torque at start-up in
order to pump, or lift, water from a deep well.
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Lift Design
The lift blade design employs the same principle that enables airplanes, kites and birds to fly. The blade is
essentially an airfoil, or wing. When air flows past the blade, a wind speed and pressure differential is created
between the upper and lower blade surfaces. The pressure at the lower surface is greater and thus acts to “lift”
the blade. When blades are attached to a central axis, like a wind turbine rotor, the lift is translated into
rotational motion. Lift-powered wind turbines have much higher rotational speeds than drag types and
therefore well suited for electricity generation.
Tip Speed Ratio
The tip-speed is the ratio of the rotational speed of the blade to the wind speed. The larger this ratio, the faster
the rotation of the wind turbine rotor at a given wind speed. Electricity generation requires high rotational
speeds. Lift-type wind turbines have maximum tip-speed ratios of around 10, while drag-type ratios are
approximately one. Given the high rotational speed requirements of electrical generators, it is clear that the lifttype wind turbine is most practical for this application.
The number of blades that make up a rotor and the total area they cover affect wind turbine performance. For a
lift-type rotor to function effectively, the wind must flow smoothly over the blades. To avoid turbulence, spacing
between blades should be great enough so that one blade will not encounter the disturbed, weaker air flow
caused by the blade which passed before it. It is because of this requirement that most wind turbines have only
two or three blades on their rotors.
Generators
The generator is what converts the turning motion of a wind turbine’s blades into electricity. Inside this
component, coils of wire are rotated in a magnetic field to produce electricity. Different generator designs
produce either alternating current (AC) or direct current (DC), and they are available in a large range of output
power ratings. The generator’s rating, or size, is dependent on the length of the wind turbine’s blades because
more energy is captured by longer blades.
It is important to select the right type of generator to match your intended use. Most home and office
appliances operate on 120 volt (or 240 volt), 60 cycle AC. Some appliances can operate on either AC or DC,
such as light bulbs and resistance heaters, and many others can be adapted to run on DC. Storage systems
using batteries store DC and usually are configured at voltages of between 12 volts and 120 volts.
Generators that produce AC are generally equipped with features to produce the correct voltage (120 or 240 V)
and constant frequency (60 cycles) of electricity, even when the wind speed is fluctuating.
DC generators are normally used in battery charging applications and for operating DC appliances and
machinery. They also can be used to produce AC electricity with the use of an inverter, which converts DC to
AC.
Transmission
The number of revolutions per minute (rpm) of a wind turbine rotor can range between 40 rpm and 400 rpm,
depending on the model and the wind speed. Generators typically require rpm’s of 1,200 to 1,800. As a result,
most wind turbines require a gear-box transmission to increase the rotation of the generator to the speeds
necessary for efficient electricity production. Some wind turbines do not use transmissions. Instead, they have
a direct link between the rotor and generator. These are known as direct drive systems. Without a
transmission, maintenance requirements are reduced.
Towers
The tower on which a wind turbine is mounted is not just a support structure. It also raises the wind turbine so
that its blades safely clear the ground and so it can reach the stronger winds at higher elevations. Maximum
tower height is optional in most cases, except where zoning restrictions apply. The decision of what height
tower to use will be based on the cost of taller towers versus the value of the increase in energy production
resulting from their use. Studies have shown that the added cost of increasing tower height is often justified by
the added power generated from the stronger winds. Larger wind turbines are usually mounted on towers
ranging from 40 to 70 meters tall.
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Towers for small wind systems are generally “guyed” designs. This means that there are guy wires anchored to
the ground on three or four sides of the tower to hold it erect. These towers cost less than freestanding towers,
but require more land area. Some of these guyed towers are erected by tilting them up. This operation can be
quickly accomplished using only a winch, with the turbine already mounted to the tower top. This simplifies not
only installation, but maintenance as well. Towers can be constructed of a simple tube, a wooden pole or a
lattice of tubes, rods, and angle iron. Large wind turbines may be mounted on lattice towers, tube towers or
guyed tilt-up towers.
Installers can recommend the best type of tower for your wind turbine. It must be strong enough to support the
wind turbine and to sustain vibration, wind loading and the overall weather elements for the lifetime of the wind
turbine. Tower costs will vary widely as a function of design and height. Some wind turbines are sold complete
with tower. More frequently, however, towers are sold separately.
Basic Designs
Wind turbines are classified into two general types: horizontal axis and vertical axis. A horizontal axis machine
has its blades rotating on an axis parallel to the ground. A vertical axis machine has its blades rotating on an
axis perpendicular to the ground. There are a number of available designs for both and each type has certain
advantages and disadvantages. However, compared with the horizontal axis type, very few vertical axis
machines are available commercially.
Horizontal Axis
This is the most common wind turbine design. In addition to being parallel to the ground, the axis of blade
rotation is parallel to the wind flow. Some machines are designed to operate in an upwind mode, with the
blades upwind of the tower. In this case, a tail vane is usually used to keep the blades facing into the wind.
Other designs operate in a downwind mode so that the wind passes the tower before striking the blades.
Without a tail vane, the machine rotor naturally tracks the wind in a downwind mode. Some very large wind
turbines use a motor-driven mechanism that turns the machine in response to a wind direction sensor mounted
on the tower.
Vertical Axis Although vertical axis wind turbines have existed for centuries, they are not as common as their
horizontal counterparts. The main reason for this is that they do not take advantage of the higher wind speeds
at higher elevations above the ground as well as horizontal axis turbines. The basic vertical axis designs are
the Darrieus, which has curved blades, the Giromill, which has straight blades, and the Savonius, which uses
scoops to catch the wind.
A vertical axis machine need not be oriented with respect to wind direction. Because the shaft is vertical, the
transmission and generator can be mounted at ground level allowing easier servicing and a lighter weight,
lower cost tower. Although vertical axis wind turbines have these advantages, their designs are not as efficient
at collecting energy from the wind as are the horizontal machine designs.
Augmentors
Some experimental wind turbines have incorporated an added structural design feature, called an augmentor,
intended to increase the amount of wind passing through the blades. However, these augmentors do not
increase the energy capture of the machine enough to justify the added cost of employing them.
Operating Characteristics
All wind machines share certain operating characteristics, such as cut-in, rated and cut-out wind speeds.
Cut-in Speed
Cut-in speed is the minimum wind speed at which the wind turbine will generate usable power. This wind
speed is typically between 7 and 10 mph.
Rated Speed
The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power.
For example, a “10 kilowatt” wind turbine may not generate 10 kilowatts until wind speeds reach 25 mph.
Rated speed for most machines is in the range of 25 to 35 mph. At wind speeds between cut-in and rated, the
power output from a wind turbine increases as the wind increases. The output of most machines levels off
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above the rated speed. Most manufacturers provide graphs, called “power curves,” showing how their wind
turbine output varies with wind speed.
Cut-out Speed
At very high wind speeds, typically between 45 and 80 mph, most wind turbines cease power generation and
shut down. The wind speed at which shut down occurs is called the cut-out speed. Having a cut-out speed is a
safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways. In
some machines an automatic brake is activated by a wind speed sensor. Some machines twist or “pitch” the
blades to spill the wind. Still others use “spoilers,” drag flaps mounted on the blades or the hub which are
automatically activated by high rotor rpm’s, or mechanically activated by a spring loaded device which turns the
machine sideways to the wind stream. Normal wind turbine operation usually resumes when the wind drops
back to a safe level.
Betz Limit
It is the flow of air over the blades and through the rotor area that makes a wind turbine function. The wind
turbine extracts energy by slowing the wind down. The theoretical maximum amount of energy in the wind that
can be collected by a wind turbine’s rotor is approximately 59%. This value is known as the Betz limit. If the
blades were 100% efficient, a wind turbine would not work because the air, having given up all its energy,
would entirely stop. In practice, the collection efficiency of a rotor is not as high as 59%. A more typical
efficiency is 35% to 50%. A complete wind energy system, including rotor, transmission, generator, storage
and other devices, which all have less than perfect efficiencies, will (depending on the model) deliver between
10% and 40% of the original energy available in the wind.
Sizing Grid Connected Systems
The size, or generating capacity, of a wind turbine for a particular installation depends on the amount of power
needed and on the wind conditions at the site. It is unrealistic to assume that all your energy needs can be met
economically by wind energy alone. As a general rule, a wind system should be sized to supply 25% to 75% of
your energy requirements. Most residential applications require a machine capacity of between one and ten
kW. Farm use requires ten to 50 kW and commercial/small industrial uses typically require 20 kW or larger.
Because most buildings are connected to a utility line, many wind turbine owners have opted to interconnect
their systems. In effect, they use the utility as a backup system. Excess electricity from the turbine is
automatically fed to the utility and backup power is automatically supplied. While this does not constitute true
storage, it provides power on demand at any time, in any amount. The process to obtain approval for
interconnection from the utility company can be lengthy and complicated, and requires careful planning. The
possibilities for interconnection should be investigated early in the process of researching a wind system.
The variability of your energy consumption and the amount of money you are willing to spend on a wind
system should also guide your selection. For example, a user whose consumption is erratic or concentrated
during short periods of the day should size a wind turbine differently than a user with a fairly constant energy
demand. In the former case, wind turbine size should be a function of off-peak or average energy demand.
Off-Grid Systems
Off-grid power systems can result in higher cost energy, but the high cost of extending a power line to a remote
location often makes an independent energy system the most cost effective choice for remote homes and
equipment. If the average wind speeds at a location are greater than 12 mph, a wind turbine may provide the
least expensive form of energy.
Because wind is intermittent, it is often used in conjunction with batteries or with other energy sources, such as
a gas generator or solar electric panels, to make a hybrid system. Battery systems can supply the owner with
reserve power whenever energy demand exceeds that delivered by the wind turbine. This reserve power
comes in handy during calm spells, but in situations where the storage capacity is taxed beyond its limits, a
backup system, such as a portable gasoline or diesel generator, may be necessary. By combining two or more
sources of energy, the size of energy storage can be decreased.
Energy Storage
Electrical Storage
Batteries are the most common form of electrical storage. Where heat, rather than electricity, is the desired
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end product of a wind turbine application, hot water is the usual storage medium. However, the advantageous
economics of other heating systems make wind-powered heating a less attractive option.
Batteries can store and deliver only DC power. Unless an inverter is used to convert DC to AC, only DC
appliances can be operated from the stored power. The battery voltage must be the same as the voltage
needed to run the appliance. Standard battery voltage is six or 12 volts. For an appliance requiring 24 volts,
two 12-volt or four 6-volt batteries connected in series are required. For a 120-volt application, you will need a
series of ten 12-volt batteries.
The least costly batteries for wind applications are deep cycle, heavy-duty, industrial type lead-acid batteries,
such as those used in golf carts and forklifts designed for high reliability and long life. They can be fully
charged and discharged, while standard lead-acid batteries (e.g., automobile type) cannot. Gel-cell lead acid
batteries have improved the safety of the traditional liquid acid battery by containing the hydrogen that can be
produced during charging, and by preventing the liquid acid from spilling.
Battery conversion efficiency is approximately 60% to 80%. A battery’s capacity is rated in amp-hours, a
measure of its ability to deliver a certain amperage for a certain number of hours. For example, for a rating of
60 amp-hours, three amps can be delivered for 20 hours. Batteries should be routinely inspected for fluid level
and corrosion. The storage room should be well ventilated. If allowed to accumulate, the hydrogen gas
produced by some batteries can explode.
Substantial research on battery technology has taken place since 1990, when the Federal Clean Air Act
Amendment prompted new battery research for electric vehicles. Much of this research has focused on
developing batteries that can be rapidly charged and are lighter in weight. Many new types of batteries are
being developed. Nickel-cadmium batteries and nickel-iron batteries are two types that generally provide good
low-temperature performance and long life, but they are still more expensive than lead-acid batteries.
Heat Storage
When heat is the desired end product, hot water is an alternate way to store energy. It is well suited to northern
climates where the heating season coincides with the windy season. There are two basic ways to produce heat
from a wind turbine. Electricity can be sent to resistance heaters immersed in water, or the wind turbine’s rotor
shaft can be mechanically coupled to a paddle or pump that agitates water, thereby heating it.
Resistance Heaters
The first method of heat storage involves electrical resistance heaters which can be DC or AC powered with
unregulated voltage and frequency levels. Thus, the buyer has considerable flexibility in choosing a machine
without the need for additional complex and expensive control or conditioning devices. The conversion
efficiency of a resistance heater is nearly 100%, and heat loss is minimized if the water storage tank is wellinsulated. Resistance heaters can also be used directly to heat air, as with baseboard electric home heaters.
Mechanical Heating
The second method of heating water is by mechanically agitating it, using either a pump or a paddle. The heat
is produced by the large frictional losses that are produced by agitation. This method of heating does not
require an electrical generator. Instead, the power from the rotating rotor shaft is used directly. Theoretical
conversion efficiencies are nearly 100%, but practical considerations can reduce this considerably. As yet, only
a few experimental models of this type of wind system have been tested.
Sizing of Off-Grid Systems
Sizing remote systems is substantially different than sizing a wind system for utility interconnection because
remote systems must be designed to supply the entire electrical demand. Before one can size the components
of a remote system, one must determine the load requirements of the site. This means quantifying the power
demand on a daily and seasonal basis. The goal is to compare the amount of energy needed at different times
of the day and year to when it is available on average from the wind. After taking into account the wind’s
intermittence, you can determine the size and type of energy storage or other energy sources needed to meet
your total demand.
Battery storage should be sized large enough to handle at least three windless days. Back-up generators are
often included in remote electrical systems as a supplemental backup. They help to power large, infrequently
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used loads and to preserve the life of the batteries by minimizing the number of times they are completely
discharged. Also, they run most efficiently at full load. For these reasons, generators are often sized larger
than the average expected load of the system so that they can also charge the batteries at the same time,
keeping run time and fuel consumption to a minimum. Many wind system dealers are familiar with remote
system designs and can assist you in selecting an optimum, cost-effective system.
Installation, Maintenance and System Life
Many dealers will sell and install a complete package, including wind turbine, tower and electrical work. Wind
turbines should be installed by qualified, experienced people. Local codes should be checked to determine if
contractors must be licensed. The local building inspector may be required to inspect the completed work. For
utility interconnected systems, a utility-designated inspector probably will make an inspection.
By their very nature, wind turbines are subject to extreme physical forces. The tips of a wind turbine rotor can
reach speeds of up to 300 mph. Hail, dirt and insects contacting the blades at these speeds can cause wear to
the blade edges. Bearings that support the rotor or other moving parts are also subject to wear. The lifetime of
these bearings depends on the wind conditions and level of maintenance care.
The manufacturer usually specifies what is required for the maintenance of a wind turbine. The entire wind
system, including the tower, storage devices and wiring should be inspected at least once a year. Routine
maintenance might include changing the transmission oil, greasing the bearings and visually inspecting the
condition of the blades, tower and electrical connections. Instead of doing the maintenance work yourself
(which may require climbing the tower), you can arrange a maintenance contract with the dealer.
According to manufacturers, the expected lifetime of wind turbines is 20 to 30 years. Although few
manufacturers have been around that long, this prediction is not unrealistic. Some electricity-generating wind
turbines built more than 50 years ago are still working today. Just like automobiles and washing machines,
wind turbines have to be properly and routinely maintained to maximize their life. Do not be surprised if minor
repair work is needed soon after initial operation. A reputable company will make such repairs quickly and at
no charge.
Wind and Wind Power
Like the weather in general, the wind can be unpredictable. It varies from place to place, and from moment to
moment. Because it is invisible it is not easily measured without special instruments. Wind velocity is affected
by the trees, buildings, hills and valleys around us. Wind is a diffuse energy source that cannot be easily
contained or stored for use elsewhere or at another time. It challenges us to harness it, but it first demands
considerable study.
To be economically practical, a wind turbine should experience year-round average wind speeds of at least 12
mph. Sometimes people say, “The wind always blows at my place,” or “I live on the highest hill in the county.”
These observations, while true, are too simplistic. A careful and precise wind survey should be made before
buying a wind system. Wind turbine owners who buy without careful consideration are frequently disappointed
in their system’s performance. At the going price for a complete wind turbine installation, it makes economic
sense to first thoroughly check out your wind resource.
Wind Speed and Power
Before venturing into the dynamics of wind flow, we should first differentiate between wind speed and wind
power.
Wind Speed
Wind speed is the rate at which air flows past a point above the earth’s surface. Wind speed can be quite
variable and is determined by a number of factors which will be discussed in this section.
Wind Power
Wind power is a measure of the energy available in the wind. It is a function of the cube (third power) of the
wind speed. If the wind speed is doubled, power in the wind increases by a factor of eight (23). This
relationship means that small differences in wind speed lead to large differences in power. For example,
assume that one person takes a speed measurement of 10 mph and another person, at the same time but at a
neighboring site, gets a reading of 12.6 mph. For this difference of 2.6 mph, there is a 100% difference in the
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available wind power (103 = 1000 vs. 12.63 = 2000)! This example points out that minor differences in wind
speed due either to site selection or measurement errors can have a major bearing on a decision to invest in a
wind turbine. For this reason, a thorough and accurate wind study is imperative before buying a wind turbine.
The amount of power available in the wind is determined by the equation w = 1/2 r A v3 where w is power, r is
air density, A is the rotor area, and v is the wind speed. This equation states that the power is equal to onehalf, times the air density, times the rotor area, times the cube of the wind speed. Air density varies according
to elevation, temperature and weather fronts. For the purposes of calculating wind power, the variations in
weather fronts are too small to significantly affect electric power output, so the formula for air density is: p =
(1.325 x P) / T where T is the temperature in Fahrenheit + 459.69 and P is the pressure in inches of Mercury
adjusted for elevation.
A standard value for air density can be used to reflect the typical average air temperature (59°F) adjusted to
sea level. The standard value may be used as an approximation for Iowa. The power equation can then be
simplified to the following, depending on whether you use English or metric units.
Simplified Power Equation
English units
Metric units
w = 0.0052 A v3
w = 0.625 A v3
where w is power in watts, and A is
the cross-sectional area in square
feet swept out by the wind turbine
blades, and v is the wind speed in
miles per hour.
where w is power in watts, and A is
the cross-sectional area in square
meters swept out by the wind turbine
blades, and v is the wind speed in
meters per second.
The metric version is shown because the metric system is frequently used by the wind energy industry. English
to metric conversion tables are posted here. For example, a wind speed of one meter per second is the same
as 2.24 miles per hour. One square meter is the same as 10.76 square feet.
Although this power equation shows an exponential increase in wind power as wind speed increases, in
practice, the actual power increase in a wind turbine is more linear than is predicted by the equation. This is
because a wind turbine is not perfectly efficient. The power curve of a wind turbine is actually the more
accurate way of predicting the turbine’s performance.
The area swept by most wind turbine blades is circular. The area can be calculated byusing the equation for
the area of a circle: Swept Rotor Area = A = (pi) r2 where r is the rotor radius (half the diameter, or the distance
from the hub to a blade tip). The equation states that the swept rotor area, A, is equal to (pi) or 3.14 times the
square of the rotor radius. Below is a sample power calculation.
Assume a wind speed of 15 mph, standard air density, and a rotor radius of 10 feet. The rotor area (A) is equal
to 3.14 times (10 ft.)2, or 314 square ft. Using the appropriate simplified power equation (English units), the
wind power is:
w
w
w
w
w
= 0.0052 A v3
= 0.0052 (314 sq. ft.) (15 mph)3
= 0.0052 (314) (3375)
= 5,511 watts
= 5.511 kilowatts
Wind power density (not to be confused with air density) is a term commonly used to describe the wind power
available per unit area swept by the blades, or w/A. Wind power density is normally expressed in metric units
which are watts per square meter (abbreviated W/m2). If using English units, square feet must first be
converted to square meters, and miles per hour to meters per second.
To determine the wind power density in metric units (W/m2) from the sample power calculation given above, it
is necessary to divide the calculated power (5,511 watts) by the rotor area (314 sq. ft.), which must first be
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converted to square meters. Dividing 314 sq. ft. by 10.76 equals 29.18 square meters. The wind power density
is:
A short-hand method for calculating wind power density in metric units (W/m2) when the wind speed is known
is:
Wind Power Density = 0.056 v3 where v is in miles per hour
Wind Power Density = 0.625 v3 where v is in meters per second
Using the previous example for a 15 mph wind speed: Wind Power Density = 0.056 (15 mph)3 = 189 W/m2
Because of the wind’s normal variability, and the effect of this variability on the cube of the wind speed, the
power equation should only be used for instantaneous or hourly wind speeds and not for long-term averages.
To illustrate why, consider two places where the average speed is 15 mph. At the first location the wind always
blows at 15 mph, giving a power density of 189 W/m2. At the second site the speed fluctuates. The wind is at
10 mph half the time (power density = 56 W/m2), and at 20 mph the other half (power density = 448 W/m2).
The mean power density here is (56 W/m2 x 1/2) + (448 W/m2 x 1/2) or 252 W/m2. At both locations the mean
speed is exactly 15 mph, but there is 33% more power at the site with varying speeds.
In the real world, the wind varies constantly. Actual wind power density at most sites can range from 1.7 times
to 3 times greater than that calculated from only the mean wind speed. For a typical 15 mph site in Iowa, the
actual average wind power density is about 400 W/m2.
What Causes Wind
As long as there is sunlight, there will be wind. The wind is a by-product of solar energy. Approximately 2% of
the sun’s energy reaching the earth is converted into wind energy. The surface of the earth heats and cools
unevenly, creating atmospheric pressure zones that make air flow from high- to low-pressure areas.
Trade winds on a tropical island are fairly dependable, providing a nearly constant wind flow throughout the
day and night. Unfortunately, we have no trade winds in our part of the world, and weather systems move
through every few days. With alternating stormy and fair weather, wind speeds can range from gale force to
total calm within a 24-hour period. An Iowa wind turbine owner must cope with these large variations. Daily and
seasonal changes are important considerations for applications where electricity use is time-dependent.
Seasonal winds in Iowa are strongest in winter and early spring and weakest in summer. Daily winds generally
are strongest during the afternoon and lightest during the early morning. To make the most efficient use of the
energy supplied by the wind turbine (assuming little is to be fed into the utility grid), users should adjust their
energy consumption to match the availability of the wind. Weather forecasts are valuable in planning for high
and low wind periods.
Wind direction is also variable, although the strongest winds generally prevail out of the southwest and
northwest. Knowledge of the prevailing wind direction is important for siting the wind turbine in the least
obstructed setting possible.
Geography and Wind
If all of Iowa consisted of flat and smooth land, there would be little wind variation from place to place. But with
the addition of hills, valleys, river bluffs and lakes, a complex and highly variable wind regime is created. Trees
and buildings add to the complexity of the wind on a smaller scale. Each geographical feature influences wind
flow in certain ways, as detailed below. The effect of trees and buildings is discussed later in this booklet.
Hills, plateaus and bluffs provide high ground on which to raise a wind turbine into a region of higher wind
speeds. Valleys, which are lower and sheltered, generally have lower wind speeds. However, all valleys are
not necessarily poor wind sites. When oriented parallel to the wind flow, valleys may channel and improve the
wind resource. A constriction to the valley may further enhance wind flow by funneling the air through a smaller
area. This is often the case in narrow mountain passes or gaps that face the wind.
Valleys often experience calm conditions at night even when adjacent hilltops are windy. Cool, heavy air drains
from the hillsides and collects in the valleys. The resulting layer of cool air is removed from the general wind
flow above it to produce the calm conditions in the lowlands. Because of this, a wind turbine located on a hill
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9
may produce power all night, while one located at a lower elevation stands idle. This phenomenon is more
likely to occur on high terrain features that reach at least several hundred feet above the surrounding land.
High terrain features can accelerate the flow of wind. An approaching air mass is often squeezed into a thinner
layer so it speeds up as it crosses the summit. Over a ridge, maximum acceleration occurs when the wind
blows perpendicular to the ridge line. Isolated hills and mountains may accelerate the wind less than ridges
because more of the air tends to flow around the sides. The downward, or “lee,” side of high terrain features
should be avoided because of the presence of high wind turbulence.
Land areas adjacent to large bodies of water may be good wind sites for two reasons. First, a water surface is
much smoother than a land surface, so airflowing over water encounters little friction. The best shoreline site is
one where the prevailing wind direction is “on-shore.” Second, when regional winds are light, as on a sunny
summer day, local winds known as sea or lake breezes can develop because the land and water surfaces heat
up at different rates. Because land heats more quickly than water, the warm rising air over the land is replaced
by the cooler air from over the water. This produces an on-shore breeze of typically 8 to 12 mph or more. At
night the breeze stops or reverses direction, as the land cools more quickly.
Surface Roughness
The surface over which the wind blows affects its speed. Rough surfaces, such as areas with trees and
buildings, will produce more friction and turbulence than smooth surfaces such as lakes or open cropland. The
greater friction means the wind speed near the ground is reduced.
The approximate increase of speed with height for different surfaces can be calculated from the following
equation:
v2 = v1 x (h2/h1)n where v1 is the known (reference) wind speed at height h1 above ground, v2 is the speed at
a second height h2, and n is the exponent determining the wind change. Values for n are listed in the following
table for different types of wind cover. If the wind comes across a fallow crop field, you do not have to reach as
high for greater wind speeds as you would in a forest or suburb.
Ground Cover
n
smooth surface ocean, sand
.10
low grass or fallow ground
.16
high grass or low row crops
.18
tall row crops or low woods
.20
high woods with many trees suburbs, small towns
.30
Here is an example of how this method is used. Suppose you are interested in buying a wind turbine and have
taken measurements for a year with a wind speed instrument on a 30 ft. tower in an area of low woods. The
average speed is 10 mph. You want to estimate the speed at the planned 100 ft. height of the wind turbine.
In your calculation v is equal to 10 mph, h1 is 30 ft and h2 is 100 ft. Since your surroundings consist of low
woods, the correct value for n is .20. Plugging these values into the formula, the average wind speed at 100
feet is:
v2 = 10 mph x (100 ft/30ft).20
v2 = 10 x (3.33).20
v2 = 10 x 1.27
v2 = 12.7 mph
Again, this method only provides a rough estimate of wind speeds, not a precise value. it is most useful when
using average and not instantaneous wind speeds. In addition, this formula should only be used for relatively
flat terrain because hills and mountains often have unpredictable influences on wind characteristics.
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Lastly, within dense vegetation, such as a forest or an orchard, a new effective ground level is established at
approximately the height where the branches of adjacent trees touch. Below this level there is little wind. in a
dense cornfield, this height would be the average corn height. In a forest, it would be the average height ofthe
tree canopy, and so on. When using the wind speed equation all heights should be expressed above the
effective ground level.
Trees and Buildings
Trees and buildings are the most common obstacles to wind in the vicinity of a potential wind turbine site. They
act to disturb the air both upwind and downwind of the obstruction by reducing wind speed and increasing
turbulence.
If it is impossible to avoid all obstacles entirely you may want to use these siting rules of thumb:
1. Site the wind turbine upwind at a distance of more than two times the height of the obstruction.
2. Site the wind turbine downwind a minimum distance of 10 times, and preferably 20 times, the height of the
obstruction.
3. Site the wind turbine hub at least twice the height of the obstruction above ground, if the wind turbine is
immediately downwind of the obstruction.
The upwind and downwind directions can be defined as being aligned with the prevailing wind direction.
Buildings that are wider than they are tall have an influence on air flow at greater distances downwind than do
taller buildings. For example, at a distance 20 times the building height downwind, only very wide buildings
produce more than a 10% decrease in the available wind power (those in which width divided by height = 3 or
more). On the other hand, tall, narrow buildings will create power reductions of less than 10% at distances as
close as 5 building heights downwind. Most residential structures, such as houses, barns and garages, are as
wide or wider than they are tall so the previously mentioned rules of thumb should be followed.
Vegetative Indicators of Wind
Trees growing in an area of high winds are often permanently deformed. Severe deformation, such as when
the tree trunk is bent away from the prevailing wind direction, occurs at wind speeds of 15 to 18 mph. However,
“brushing” or “flagging” can be seen in a tree exposed to average speeds as low as 8 to 10 mph which prevail
from one dominant direction. An examination of the vegetation in an area can be a rough indicator of the wind
strength there.
We see brushing commonly in deciduous trees, like maple, oak and elm, where the branches and twigs bend
downward like the fur of a pelt that has been brushed in one direction.
Flagging is common in coniferous trees, like pine and spruce. It is indicated by branches that stream downwind
and by short or missing upwind branches.
The absence of deformation does not necessarily imply that the wind resource is weak. Some tree species are
more sensitive to the wind than others. Trees within a continuous forest, for example, are too sheltered, and
strong winds may blow from more than one major direction. Therefore, use tree deformation only as a rough
guide and not as a primary tool in selecting a wind turbine site.
Iowa Winds
Iowa is fortunate to lie at the eastern edge of the Great Plains where winds blow strongly and steadily,
particularly in the winter and spring. The relative flatness of the terrain means that most areas of the state are
well exposed to the wind. In addition, most of the state consists of cropland with few trees to reduce wind
speeds near the ground. Iowa is, consequently, an especially attractive state for wind power development.
Parts of Iowa fall into a major area of the United States which is very suitable for wind power generation. Of
Iowa’s 144,950 km 2 of total land area, 39.1%, or 56,700 km 2 is potentially available windy land. According to
studies prior to actual measurements, the state’s wind electric potential was estimated at 62,900 Megawatts, at
a hub height of 50m and assuming 25% efficiency and 25% energy loss. This estimate assumes the availability
of land and transmission line access.
The prevailing winds in Iowa come out of the southeast and northwest, but significant winds also come from
the south, southwest, west and north directions. It is the northwest winds which provide the greatest wind
This guide was created by the Iowa Energy Center.
11
power potential. East and northeast winds (because they occur less frequently) provide the lowest wind power
levels.
There are average variations in windspeeds daily and annually which are valuable to consider when trying to
assess the load capacity of wind power. Ultimately, wind is an intermittent power source since there is no
definite way to predict whether there will be wind when there is a corresponding high power demand. However,
as part of a mix of power sources, wind can be quite reliable and inexpensive. In Iowa, average hourly yearround wind speed varies between 11.5 mph at low wind sites, to 17.5 mph at high wind sites. Keep in mind that
these are average wind speeds. Actual wind speeds at any given time may be much higher or lower. There is a
slight rise in average wind speeds between 1 pm and 6 pm daily. Afternoon heating of the earth with respect to
the air tends to increase air currents, and therefore wind. There is usually a temporary drop in wind speeds at
sunrise and sunset when the temperature difference between the air and the earth is lower.
On an annual basis, July and August are the lowest wind months. November through March are the highest
wind months. The wind is more powerful during the winter because of colder temperatures. When the
temperature is low, air density is high. At high air density, the same wind speed delivers more air, and thus
more power. In the winter time, much of Iowa has Class 4 winds, meaning that this time of year is very good for
wind power generation.
In general, wind speeds in Iowa increase from southeast to northwest. But there are high wind areas in central
and north central Iowa that rival the more famous Buffalo Ridge sites in northwest Iowa. Exposed cropland and
elevated sites in each region will have the highest winds in that region. Lower and more sheltered locations will
tend to have less wind power. This is especially true in the autumn and winter when vertical air mixing is
restricted, causing wind speeds to be less than at more elevated locations. Estimated wind speeds for each of
2,000 towns and cities and wind maps of Iowa can be found on the Wind Index page.
Usually there are layers of faster moving air at higher elevations above the ground. Wind towers are made as
tall as possible to take advantage of these higher wind speeds. The change in wind speeds at different heights
is called wind shear. Iowa has a higher than average wind shear, especially in locations that are hilly or
forested. The wake effects of hills, trees and buildings can cause turbulence which lowers wind speeds up to
50 meters above the ground. Wind shears are less on hot and windy days where there is more vertical mixing
of the air. It is often reversed in the early morning hours before dawn. But, in general, the taller the tower, the
greater the wind power.
The daily and seasonal variations in wind speed have a great effect on planning for a distributed generation
system. For example, if you were interconnecting a wind turbine to a manufacturing company which uses more
energy in the morning or during the summer, and shuts down at night, you might only be able to use directly
25% of the electricity the turbine generates. If you were interconnecting the turbine to a school which uses little
power while classes are out of session in the summer, your ability to use the available power would be higher.
Farmers often need a lot of electricity to run grain dryers in the fall when the winds are high.
When wind farms are connected to the grid, all of the low cost wind power can be utilized. Now that electricity
can be bought and sold on the spot market, the intermittency of wind is less of a factor in determining the
suitability of investing in wind power. As the market for green energy emerges, wind power will be increasingly
sought after. Part of the cost of wind energy is the “wheeling charge” to move the power from its source to an
urban center. Wind power produced in Iowa creates value for midwest urban centers.
Measuring Wind
On-site wind measurements should be taken prior to deciding to purchase a wind turbine. The data collected
will determine the wind resource and help with wind turbine selection and economic value. You should
consider several questions about taking measurements:
•
•
•
•
•
•
Who should take them?
Exactly what should be measured?
What instruments should be used, and at what cost?
Where should the measurements be taken?
For how long should they be taken?
How will the data be analyzed?
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Before answering any of these questions, you need to back up and examine how the collected data will be
used. This depends on the intended purpose of the wind turbine. If a wind turbine is being considered for
annual electric power production, then the wind data will be used to estimate that production. In this case,
mean wind speeds and wind speed distributions will be important. If, however, a wind turbine is intended to
furnish power during periods of the peak demand, a more detailed evaluation will be needed. At a minimum,
the variation of speed with time-of-day and season should be estimated, and possibly the duration of windless
periods for applications where battery storage is to be used.
Who should take the measurements?
There are two options. You can take the measurements yourself or you can hire a specialist. The second
option costs more, but a professional is better equipped to do the job right and can be a valuable resource for
other wind energy information.
What should be measured?
Obviously, wind speed must be measured. Wind direction measurement is optional, but is recommended if you
can afford it, or if the site is influenced by obstructions or irregular terrain. How frequently measurements are
taken will depend on the intended use of the contemplated wind turbine. Cost considerations once again play a
role. As a general rule, the more frequently data is recorded, the more expensive the recording equipment is
and the more data there is to analyze. Hourly recording will suit the needs of virtually any application. Both
short-term information, such as the day-night (diurnal) speed variations and the duration of calm spells, and
long-term information, such as monthly and annual averages, can be provided in this way. Daily recording is
usually sufficient to characterize average wind conditions where hourly detail is unnecessary.
What instruments should be used?
Anemometers measure wind speed. Most are designed with cups mounted on short arms connected to a
rotating vertical shaft. Some use a small propeller. The anemometer rotates in the wind and generates a signal
proportional to wind speed. In most cases the signal is electrical, although some anemometers produce
mechanical signals. Wires lead from the anemometer to an indicator (display) or recorder that is made for
indoor or outdoor use.
Indicators give current information on a dial or digital display or with blinking lights. They present only visual
wind values and do not have any storage capability. If data are to be collected, they require a person to
monitor the system and manually record the data into a logbook. Indicators are impractical for most wind
feasibility studies and in general are not recommended.
Recorders store past information using counters (mechanical or electronic), paper strip charts, magnetic tapes
or electronic data chips or cards. Electronic recorders and other storage devices can be easily interfaced with a
personal computer for subsequent data analysis. The simplest recorder is a counter that accumulates “run-ofthe-wind”. It totals the miles of wind that blow past the anemometer, just as a car odometer records
accumulated road mileage. If 240 miles of wind passage are recorded in a 24-hour period, the average speed
for the period is 10 mph. This recorder, known as a wind-run recorder odometer or accumulator, can be read at
any time interval-hourly, daily or monthly-to suit the needs of the intended wind turbine application. A logbook
is required to enter the recorded data. Electronic recorders offer the greatest power and flexibility among
recorder types. They can provide a variety of statistics such as sequential hourly wind speed averages and
wind speed frequency distributions. More sophisticated recorders may be used to record wind speed by
direction and/or time of day, as well as compute statistical parameters such as standard deviations and
maximum/minimum values.
The equipment for complete wind speed measurement costs from several hundred dollars for the simple
indicators and recorders to several thousand dollars for the higher quality, sophisticated recorders. PCcompatible software is also available from some equipment suppliers to automatically analyze your data. Some
systems can be rented. If you hire a specialist to do the wind study, he or she will provide the necessary
equipment, but you should inquire about the type of equipment the specialist is using, and the amount and
quality of data to be gathered.
This guide was created by the Iowa Energy Center.
13
Wind direction is sensed with a wind vane, which also sends a signal to an indicator or recorder. Costs are
comparable to those incurred in measuring wind speed. Some wind equipment is designed to measure speed
and direction.
The anemometer should give readings no less accurate than 5% of the actual wind speed. Wind vanes should
be within 10 degrees of the true wind direction. Inexpensive hand held wind sensors should not be used for
wind turbine studies. Generally, they are inaccurate and must be held too close to the ground or one’s body.
Where should measurements be taken?
Measurements should be taken at the intended wind turbine location and at the anticipated hub height, the
distance above the ground at which the turbine rotor hub will be located. If the instruments cannot be placed
there, they should be placed near the intended location in a similar environment. Measurements should be
taken in an open area and not above the roof of a building where the wind flow is likely to be disturbed. Wind
instrument towers, much less substantial and costly than towers, can be purchased, rented or provided by a
wind specialist.
For how long should measurements be taken?
The longer the monitoring period, the more confident you can be of the results. Ideally, one full year, and
preferably several years, of wind measurement is recommended. This will provide data for all the seasons, and
on interannual changes, should the period be more than one year. Because of cost or scheduling problems,
some people may not wish to make lengthy measurements. Shorter-term measurements can still be of value,
but they also will have greater limitations.
Tips for Short Term Monitoring
1. The measurement period should be no less than three months, and preferably six to 12 months.
Measurements should be targeted to spring or fall when winds can be measured from both southerly and
northerly wind directions.
2. Because only part of the year will be measured, the data for the rest of the year must be estimated. This can
be done by comparing the readings at the wind turbine site with those taken at the same time at a nearby
weather station. The difference between these short-term measurements is assumed to represent the
difference between the actual average wind speeds at the two locations. Where the long-term average at a
weather station is known, the average at the site can be estimated.
3. The accuracy of the wind instruments should be checked at the end of your measurement program.
Anemometers may lose accuracy with prolonged operation.
How to Analyze Wind Data
Many do-it-yourselfers rely on monthly and annual wind speed averages to evaluate a site’s potential. The data
analysis required in these cases is fairly straightforward and is usually explained in wind instrument manuals.
The interpretation of more detailed or lengthy data, such as daily or hourly readings over a year, might require
the expertise of a specialist. If you choose to do all of the data analysis yourself, a local meteorologist or wind
turbine dealer may be willing to check your results at no charge.
Wind Energy Issues
Legal Issues (links to new page)
This section discusses utility interconnection, zoning, ordinances, building codes, land use and liability and
insurance. Check with your utility provider for more information on specific issues.
Important legal issues may determine the feasibility of installing or operating a wind energy system. The type
and size of the machine chosen, the tower height and regulations in the city, town or neighborhood where the
wind turbine will be located could legally prevent installation or operation, and may influence your decision
whether to buy.
Social and Environmental Issues (links to new page)
This section discusses turbine aesthetics and visibility, noise, television interference and effects on wildlife.
Consider social and environmental issues when installing or operating a wind energy system.
This guide was created by the Iowa Energy Center.
14
Economic Issues (links to new page)
This section addresses economic questions and charts a course toward a well-informed investment decision.
The energy supply market is very competitive, led by utilities and fuel companies that meet nearly all our
energy demands. Alternative energy sources like wind power provide new options and certain advantages, but
to be truly competitive with conventional energy sources, they also must be economical.
Throughout their lives, people make informed decisions on major purchases and financial investments. In the
case of home and car purchases, the decision is usually which one to buy, rather than whether to buy,
because these items are considered necessities. Many sources of professional advice are available to assist
the buyer in making a sound decision for these common types of investments. For wind turbines, where the
choice to buy is voluntary, good advice can be hard to find.
How does the average homeowner, farmer or business owner choose between wind energy and the energy
source he or she now uses? A wind energy system requires a large initial capital outlay, but over its lifetime it
will provide years of energy with no fuel cost.
How much do wind turbines cost and will they eventually pay for themselves? Will utility rates change in the
future and by how much? This information will prepare prospective buyers to determine the potential financial
gain available from the wind system chosen to fit a particular energy profile.
Making a Decision (links to new page)
This section reviews nine points to consider when making a wind energy decision.
Should you invest in a wind turbine? The choice is ultimately yours and yours alone. A well-informed decision
will require much thought and personal research.
Plan to review other literature and contact a number of wind turbine dealers or consultants to answer questions
specific to your site and application.
Plan to visit a few installed wind machines near you. This is a good way to become familiar with local wind
turbine dealers and their performance with clients.
Legal Issues
Utility Interconnections
The majority of wind turbines being installed today are connected to electric utilities. This provides back-up
power during periods of calm and allows the owner to sell surplus electricity to the power company. These
connections are permitted under a Federal law called PURPA. Electric utilities around the State have adopted
standards to ensure that the power transferred into their system is of suitable quality, and to ensure the safety
of the connection between their network and the wind turbines. They have also adopted “buy-back” rates for
the surplus electricity. In Iowa, these standards and rates are subject to review by the Iowa Utilities Board. You
should be aware, however, that the interconnection process can be complicated and time consuming.
Connection standards vary from utility to utility, so it is essential that you contact your local utility office for the
standards that apply to you. Power companies in general want assurances that an interconnected wind system
will not endanger utility personnel, distribution equipment or customers. Furthermore, utilities also are
concerned about the quality of power transferred to their power grid. The utility may require the customer to
purchase one or more control and protection devices. This will add to the total cost of a wind system. The wind
turbine owner also may have to carry special liability insurance in case the wind turbine causes a problem or
damage to the utility system.
Early in your own wind energy investigations, you should find out how your utility deals with wind turbine
interconnections. The company should inform you about the costs and conditions involving interconnection,
including its buy-back rate for surplus energy. The buy-back rate varies with utility and often from year to year.
This information also will help you decide whether to opt for a stand-alone system.
Zoning Ordinances, Building Codes and Land Use
Most cities and towns have ordinances to ensure that structures and activities are safe, proper and compatible
with existing or planned development. Most municipalities either use existing ordinances regarding structure
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15
heights or require that an exemption from an existing zoning ordinance (a variance) be obtained from the
zoning board. Some towns, particularly those in rural areas, have few or no codes to restrict the use of a wind
turbine. Most restrictions occur in populated areas where height, safety or aesthetics are issues. There are
federal regulations about the height of structures near the approach path to runways at local airports. If you are
within 10 miles of an airport, you need to check these regulations with the airport.
Municipal building codes often require that a building permit be obtained prior to any installation, and that the
building inspector approve the completed installation. The local building inspector or town clerk can provide
information on ordinances and codes. The landowner may want to obtain a title search of the deed to
determine if any prior agreements exist that would prevent a wind turbine from being installed on the property.
Liability and Insurance
The wind turbine owner faces a liability if his or her property poses any threat to the general public. Although
chances are remote, a wind turbine could throw a blade, or the tower could collapse onto neighboring property.
In addition, because of its visibility, a wind turbine may attract unwanted visitors.
Many homeowner insurance policies can be extended to insure against a liability brought about by damage or
injury caused by a wind turbine. The wind turbine itself can be protected by insurance coverage against
damage as a result of fire, lightning, ice or theft. Because insurance coverage varies from company to
company, check with your agent for specifics.
To minimize the likelihood of damage or injury (and possibly the cost for insurance), you should consider
reserving a set-back distance of at least one tower height from property lines and structures, and building a
safety fence or anti-climb device around the wind turbine tower. Keeping your wind equipment in top shape
should prevent most problems.
Social & Environmental Issues
Aesthetics and Visibility
The visibility of a particular wind system will depend on many factors, including tower height, proximity to
neighbors and roadways, local terrain, and tree coverage. Some people may object to a wind turbine being in
their field of view, and this could be an issue when applying for a zoning permit. Therefore it may be worthwhile
to investigate your neighbors’ concerns before investing in a wind turbine. In most areas of the state, wind
turbines are an uncommon sight, so it is natural to expect some reservations about their introduction.
Objections are more likely to occur in populated and tourist areas.
Noise Factors
The most characteristic sounds of a wind turbine are the “swish, swish, swish” of its turning blades and the
whirring of the generator. Improved designs have made wind turbines much quieter over the last decade.
Within several hundred feet of a machine, these sounds may be distinguishable from the background noise of
local traffic or the wind blowing through the trees, but they usually are not disruptive or objectionable. The
impact on any particular neighbor will depend on how close they live, whether they are upwind or downwind,
and the level of other noise sources such as traffic.
Television Interference
Small wind turbines, such as those sized for residential and farm use, have not been found to create
interference with television signals. In fact, small wind systems are commonly used today to power remote
telecommunication stations for both military and commercial uses. Most wind turbines use blades made of
wood, fiberglass or composite materials that don’t cause reception problems. Many years ago, a few wind
turbines equipped with long, metallic blades did cause some localized problems, but they are no longer
commonly used.
Effects on Wildlife
In general, wind turbines are not a hazard to wildlife. Cattle are willing to graze next to them, and sufficient
clearance between the blades and ground prevents injury to cattle, deer and other animals. There have been
reports of bird kills in a few areas of the world where large numbers of wind turbines are concentrated.
However there is little evidence that single installations or small clusters of wind turbines kill birds in significant
numbers. By nature, local birds avoid wind turbines by flying around them, and migrating birds tend to fly well
This guide was created by the Iowa Energy Center.
16
above wind turbine height. Some concern may be warranted in areas where birds concentrate, such as in
wildlife refuges or shoreline feeding and nesting areas.
Making a Decision
Should you invest in a wind turbine? The choice is ultimately yours and yours alone. A well informed decision
will require much thought and personal research.
You should plan to review other literature and contact a number of wind turbine dealers or consultants to
answer questions specific to your site and application. You also should plan to visit a few installed wind
machines near you. This is a good way to become familiar with local wind turbine dealers and their
performance with clients.
Following is a review of the major points involved in a wind energy decision, condensed into a series of nine
steps.
1. Evaluating Energy Requirements
The easiest way to determine your present energy usage is to consult all your fuel bills for the previous year.
Distinguish between your use of electricity, oil, natural gas and any other fuel that might be reduced or
replaced by a wind system. Note how your consumption varies by the month or season so that your energy
use pattern can be compared to the availability of winds in your area. Although the fuel bills will not indicate
how your energy usage varies with time of day, you should determine this in at least a rough sense to compare
with wind availability. When is it the greatest? When is it the least?
2. Evaluating Energy-Efficiency Measures and Other Energy Alternatives
Before considering a wind turbine, you should have already applied sound energy-efficiency measures to your
home, farm or business. Doing this will provide a double benefit, because you may not need as large and
expensive a wind turbine as you might have thought. You may even be happy enough with your fuel savings to
dismiss any further thoughts of buying a wind turbine.
On the other hand, you already may have taken several steps toward energy-efficiency, and the results of your
economic evaluation of a wind turbine may prompt you to consider alternative ways of cutting fuel costs.
Potentially viable options include solar energy systems, wood stoves and ground source heat pumps.
Remaining with your present energy supplier could even be the best buy. Whatever your final decision, it
should be based on sound economic principles and be compatible with your energy needs and lifestyle.
3. Evaluating Legal, Social and Environmental Issues
A survey of these issues is crucial to your decision-making, because certain issues can alter or even end your
plans for a wind turbine.
Contact the local zoning board, town clerk, or building inspector to identify applicable zoning ordinances and
building permit requirements. Liability coverage and insurance needs should be discussed with an insurance
agent. To avoid unforeseen public objections to the sight of a wind turbine in the neighborhood, discuss your
intentions with neighbors. Obtain a title search to determine if prior agreements or easements exist which
would prevent a wind turbine from being installed on your property.
4. Evaluating Wind Resources
Taking wind measurements for at least three months, and preferably a year or longer, is the best way to
determine your wind conditions. The necessary instruments can be purchased, rented or provided as part of a
site evaluation study performed by a hired consultant or wind turbine dealer. A wind turbine site should
experience wind speeds averaging no less than 12 mph, for economic viability. This average wind speed is
required for most applications.
Before investing in measurements, however, decide if your site has the potential for having sufficient winds.
Consult the wind index and also evaluate your site’s elevation relative to the surrounding terrain and its access
to the prevailing winds. Look at vegetative indicators. Remember that the power in the wind is a function of the
cube of the speed. A 10% error in a wind speed estimate can mean a 33% error in a wind power calculation.
This guide was created by the Iowa Energy Center.
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5. Determining Wind System Application
If you have already decided what a wind turbine will provide (electricity, for example), then you should next
determine an appropriate machine size, or generating capacity. As a rule of thumb, a wind turbine should be
sized to supply anywhere between 25 and 75% of your electrical demand. How much you are willing to spend
on a system also will affect size selection.
If you have not yet decided what fuel source to displace, the previous steps should give you a clue. Try to
match fuel consumption to wind availability. Seasonal winds are strongest in winter and early spring, and daily
winds usually peak during the afternoon. If you are considering storing the electricity, as with batteries, then the
daily variation in winds is less important.
Most people choose electricity as the wind turbine’s product. Interconnection of the wind system with the utility
grid eliminates the need for separate storage and provides the convenience of unlimited back-up power from
an existing energy source. If you are only looking for alternative energy sources for hot water or space heating,
there are other alternatives that should be considered for their practicality and cost-effectiveness. Active and
passive solar systems, and wood stoves or furnaces might be better suited for such heating needs.
6. Shopping for a Wind System
Once you have decided how wind energy will work for you, you should examine the wind turbine products and
accessories available on the market. If you haven’t already, this is the time to contact one or more dealers to
discuss your particular interests and get preliminary cost estimates. Choosing a good wind dealer is probably
just as important as selecting a good wind system, because most dealers service what they sell. Dealers
should provide you with references. Don’t hesitate to demand excellence. You should be as particular about
service and maintenance of your wind machine as you are for your automobile.
Become familiar with manufacturer’s product literature, and read what you can about specific wind turbines in
popular magazines, newspaper articles and elsewhere. Talk with local wind turbine owners, who can be your
most reliable information source. Ask the owners about how they chose their wind systems and whether their
expectations were realized. Find out what problems they have had with their machines’ performance, utility
cooperation and so on. Ask how responsive the dealer has been to service calls since the installation.
As you narrow your choices to just a few machines, compare wind turbine warranties and note differences
between those provided by the manufacturer and those offered by the dealer. Inquire whether the dealer’s
warranty is transferable or assumable by the manufacturer should the dealer go out of business. Compare the
maintenance requirements of machines. Higher maintenance means higher annual costs. You should compare
the terms and prices of service contracts offered by different dealers.
7. Determine the Requirements for Utility Interconnection
If you plan to interconnect the wind system with the utility’s power lines, you must first contact the local utility
office. The utility in turn should provide you with a written description of its costs and conditions involved with
interconnection. Determine the need for safety and power conditioning devices, additional monthly service and
demand charges, buy-back rates and electrical inspection of the installation. Be prepared to submit a detailed
schematic diagram (including electrical plans) of the planned wind system.
8. Evaluating Wind System Economics
This is the time to evaluate the financial consequences of your pending decision. The initial costs and annual
expenses of wind turbine ownership must be weighed against the benefits of long-term fuel cost savings. Wind
Energy Economics discusses the variety of costs and savings pertinent to you. Most of them can be
incorporated into the simple pay-back method of economic analysis to determine the pay-back period of your
investment and the cost per kilowatt-hour of wind turbine-generated electricity.
The Iowa Energy Center’s Alternate Energy Revolving Loan Program may be able to help offset start-up costs.
9. Refine System Design
After having gone through the previous seven steps, you may want to make some final adjustments to your
initial wind turbine design concept. If they will significantly affect system costs or performance, then you should
retrace the appropriate steps.
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Conclusions
You are now prepared to make a sound decision. The key to satisfaction with a wind system starts with
understanding what any given machine can and cannot do. If you take the time and effort to plan an installation
that will cost and perform as expected, a wind system should give you years of reliable service.
Cost
The cost of a wind system has two components: initial installation costs and operating expenses. The initial
installation cost includes the purchase price of the complete system (including tower, wiring, utility
interconnection or battery storage equipment, power conditioning unit, etc.) plus delivery and installation
charges, professional fees and sales tax.
The total installation cost can be expressed as a function of the wind system’s rated electrical capacity. A gridconnected residential-scale system (1-10 kW) generally costs between $5,000 and $8,000 per installed
kilowatt. That’s $50,000-$80,000 for a 10 kW system. A medium-scale, commercial system (10-100 kW) is
more cost-effective, costing between $4,000 and $6,000 per kilowatt. Large-scale systems of greater than l00
kW cost in the range of $1,500 to $4,000 per kilowatt, with the lowest costs achieved when multiple units are
installed at one location. In general, cost rates decrease as machine capacity increases. The curve’s width
reflects the range of costs available. For exact figures applicable to you, contact a manufacturer or dealer.
Remote systems with operating battery storage typically cost more, averaging between $,4,000 and $5,000 per
kilowatt. Individual batteries cost from $150 to $300 for a heavy-duty, 12 volt, 220 amp-hour, deep-cycle type.
Larger capacity batteries, those with higher amp-hour ratings, cost more. A 110-volt, 220 amp-hour battery
storage system, which includes a charge controller, costs at least $3,000.
The other cost component, operating expenses, is incurred over the lifetime of the wind system. Operating
costs include maintenance and service, insurance and any applicable taxes. A rule of thumb estimate for
annual operating expenses is 2% to 3% of the initial system cost. Another estimate is based on the system’s
energy production and is equivalent one to two cents per kWh of output.
Several methods can be used to determine the financial benefit of a wind system investment. We will consider
two important ones, estimating the payback period, and comparing the cost per kilowatt-hour from a wind
turbine to purchased electricity. These methods are more appropriate for grid-connected systems than for
remote, off-grid systems.
Payback Period*
A common and simple way to evaluate the economic merit of an investment is to calculate its payback period,
or break-even time. The payback period is the number of years of energy-cost savings it takes to recover an
investment’s initial cost. To determine the payback, the investor first estimates the wind turbine’s total initial
cost, annual energy-cost savings, and annual operating costs. Dividing total initial cost by the difference
between annual energy-cost savings and annual operating costs gives the payback period:
Total Initial Cost/(Annual Energy Cost Savings – Annual Operating Costs) = Payback time, in years
The initial cost is inclusive of all expenses to evaluate, buy, install and start-up a wind system. For illustrative
purposes, consider the total initial cost of a 5 kW residential system and a 50 kW commercial system:
Residential 5 kW system = $15,000
Commercial 50 kW system = $100,000
Annual electric savings is the retail value of electricity from the wind system that you would have otherwise
bought from the utility company. It is determined by multiplying the retail cost of electricity given on your
electric bill by the number of kilowatt-hours the wind turbine is supposed to produce in a typical year. A
manufacturer or dealer can provide an estimate of the wind system’s annual output as a function of your
location’s average wind speed. Assume the cost of electricity to be 6 cents per kWh and the annual output
from the residential and commercial systems at a 14 mph site to be 10,000 kWh and 100,000 kWh,
respectively. The annual energy-cost savings from both systems would be:
Residential $0.06/kWh x 10,000 kWh = $600
Commercial $0.06/kWh x 100,000 kWh = $6,000
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Annual operating costs are estimated by multiplying the wind system’s energy output by a typical operations
and maintenance cost, such as 1 cent per kWh. For the two wind system examples, the annual operating costs
are:
Residential $0.01/kWh x 10,000 kWh = $100
Commercial $0.01/kWh x 100,000 kWh = $1,000
Now that all components of the payback equation are defined, the payback period can be calculated.
Residential payback period:
$15,000/($600 – $100) = $15,000/$500 = 30 years
Commercial payback period:
$100,000/($6,000 – $1,000) = $100,000/$5,000 – 20 years
Other Economic Factors
The simple payback method does not account for all the actual costs and savings associated with a wind
turbine investment over its operating lifetime. Additional costs and savings might include the following:
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increases in energy costs relative to general inflation
interest paid on borrowed money
insurance
utility buy-back
state and federal tax benefits
wind turbine resale value
To an extent, these items can offset one another, depending on your particular circumstance. To determine the
impact of one or more of the above factors on your investment, it is necessary to perform a life-cycle cost
analysis. This comprehensive method calculates the wind system value by considering all the costs and
savings on a yearly basis throughout a wind turbine’s lifetime, and discounting them back to a present value.
For most applications, the payback year estimated by this method will be fairly close to that estimated by the
“simple” method.
Energy-Cost Increases
Although a wind turbine’s energy savings may average out to be relatively constant from year to year, the
value of that savings may actually increase annually. During the last few decades, energy costs have risen
more rapidly than the general inflation rate. For instance, if electric utility rates increase at 6% per year versus
a 3.5% general inflation rate, the real increase in energy cost is 2.5%. As long as this trend continues, the cost
savings from a reduced energy load will rise each year. This shortens the payback period on your investment.
The extent of future energy cost increases will depend upon many factors, including fuel availability,
government policies, international activities and consumer demand.
Cost of Borrowing Money
It is likely that part of the initial purchase price of a wind system will be paid in cash and the remainder in
borrowed money. Lending institutions base their interest charges upon a number of factors, including the
amount of risk involved in a particular loan and current economic conditions.
You should contact several lending institutions to determine the interest rates and term (number of years to
repay a loan) available to you for a particular wind machine. Usually a fixed monthly payment is arranged,
consisting of two parts: repayment of a portion of the principal amount borrowed, and interest charges on the
remaining principal. The principal repayments are part of the initial costs of the wind turbine and the interest
charges are an annual expense that reduces the energy-cost savings.
Energy Center’s Alternate Energy Revolving Loan Program
The Energy Center offers a revolving loan program called the Alternate Energy Revolving Loan Program
(AERLP). The program is designed to encourage the development of alternative energy production facilities
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within Iowa. The AERLP was created by the Iowa state legislature in 1996 as an amendment to the 1990 Iowa
Energy Efficiency Act and it is funded by Iowa’s investor owned utilities.
Insurance Expenses
For the same reasons you protect a car and home investment with insurance, a wind system should be
similarly covered. Insurance coverage should apply to the system itself as well as to any potential machinerelated accidents. Policies and premiums for insurance vary by insurance company. Coverage may be
included under an existing homeowner’s or general liability policy at little or no added cost, or a separate policy
may be required.
Utility Buy-Back Rates
In the previous payback examples, it was assumed that all the electricity produced by the wind machine was
used by the owner. However, there may be occasions when more energy is generated than is needed at a
particular time. An on-site storage system would save the power for use at a later time or at a different location.
The use of the local utility company electric lines for sales of surplus electricity, as well as a back-up supply, is
another option in Iowa.
Utilities are required to buy electricity that is fed into their power lines. In Iowa the buy–back rate for this
electricity varies by utility, but in most cases it is less than five cents per kWh. Iowa has a buy-back rate that
may be negotiable, and people have negotiated a wide range of rates. In any case, this rate is considerably
less than the retail rate at which you buy electricity. This means that a wind turbine’s output has far more value
when used directly by the owner. Therefore, a wind turbine’s payback period is affected by the buy-back rates
if excess electricity is sold to the utility. Assume that 1,000 kWh from the residential system example cannot be
used when produced and is supplied to the utility at 3 cents per kWh. The wind system owner would receive a
$30 credit on his or her electric bill. If the owner later purchases 1,000 kWh at the retail rate of 10 cents per
kWh, it costs $100. The owner, therefore, has lost $70 in an even energy exchange.
When buy-back rates are lower than the retail price of a kWh of purchased electricity, a wind machine should
be sized to minimize the amount of energy sold to the utility. In the rare instances where buy-back rates are
equal to or higher than the consumer’s purchase price, this constraint does not apply.
It is important for commercial customers to realize that their rate structure may change dramatically if their
utility usage decreases.
Resale Value
The resale value of a wind turbine refers to the potential salvage at the end of its useful life. Future resale
value will depend on many factors, including the extent of changes in the design of new systems and upon the
attractiveness of wind machines relative to other alternative energy technologies.
The salvage value of the wind energy system increases the ultimate rate of return on the investment. A
moderate estimate for salvage value might be a selling price after 20 years that is worth 10% of its cost at
today’s dollar value. For example, if we assume a 5% annual inflation rate, today’s $15,000 machine might sell
for about $4,000 in 20 years. This salvage value is equal to $1,500 at today’s prices.
Calculating the Cost Per kWh*
The information gathered to find the payback period easily enables us to compute the cost per kWh of windgenerated electricity. The calculation for cost per kWh is a two-step process.
Step 1
Annual Cost = (Initial Cost/Expected Life) + Annual Operating Costs
This formula defines the annual cost over the wind system’s lifetime. Wind turbine manufacturers estimate a
useful life of between 20 and 30 years for their product. In this example, let’s assume a 25 year estimate of
useful life. The annual cost is equal to the total initial cost (defined previously) divided by the expected life of 25
years, plus the annual operating costs for maintenance (defined previously).
Step 2
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Cost Per kWh = Annual Cost/Annual Energy Output
This formula takes the annual cost calculated in Step 1 and divides it by the projected annual energy output
(defined previously). Using our previous examples of residential and commercial machines, the cost per kWh
calculation is:
Residential:
($15,000/25 years) + $100/year = $600 + $100 = $700/year
Commercial:
Annual cost = ($100,000/25 years) + $1,000/year = $4,000 + $1,000 = $5,000/year
Cost per kWh = ($5,000/year)/100,000 kWh/year = $0.05 per kWD
These costs can be compared with your utility company rates. These examples indicate that both wind system
applications should produce energy over their life at a lower cost per kWh than purchases from the utility.
Considerations for Remote Systems
The economics of remote wind systems are different than grid-connected systems, for several reasons. One is
that a decision has to be made to either install an independent energy system or to pay to have the power line
extended to your location. As a rule of thumb, it is probably cost-effective to install a wind system to meet the
energy needs of a small, remote load (e.g., residence, water pump) if the power line is more than a half-mile
away and the site has an average hub-height wind speed of at least 12 mph. However, for an energy intensive
application, such as a business or factory, a line extension may be justified.
Another reason for the difference between remote and grid-connected system economics is that there are
many types of independent energy and storage technologies available to choose from, with wind energy being
just one. They can be used exclusively or combined into hybrid systems.
A third reason is that the selection and sizing of your system components is critical to energy supply
dependability. An intermittent power application like water pumping has different design criteria than a home or
communication repeater station where electric needs are more continuous. These issues are very user- and
site-specific, thus making the discussion of remote system economics non-generic.
The cost to extend an electric utility line is a function of many factors:
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The particular utility company
Distance from the utility
The terrain and ground cover along the line extension route
The need to obtain rights of way from affected land owners along this route.
Zoning regulations that control the use of overhead versus underground lines
The line capacity.
Similar to a grid-connected system, a remote system has initial and lifetime operating costs. Initial costs will
include equipment components such as batteries, controls and an inverter to supply AC loads. In addition to
servicing, maintenance and insurance, annual operating costs will include battery replacement every few
years, depending on the battery type and the number of discharges.
The cost effectiveness of a wind system relative to a solar electric or propane generator cannot be determined
solely by comparing the initial and annual operating costs. This is because these systems rely on different fuels
that are available at different times. For example, a solar system without a battery can’t work at night. The
availability of energy output from these systems must be compared to the time periods when energy is needed,
both on a daily and seasonal basis.
Battery systems can store excess energy for use when your energy system is not operating. However, the size
and cost of the battery system depends on the degree of match or mismatch between when energy is
produced and when it is used. It is economically beneficial to minimize battery size by properly matching the
wind resource availability with load requirement. Therefore a careful analysis of your energy needs is essential
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to design an optimal remote energy system. Remote system dealers and installers can assist you in
developing an appropriate system design.
* All examples used in this booklet should not be used as real-world cost comparisons. In order to get a true
idea of your costs, you will need to input numbers specific to your situation.
This guide was created by the Iowa Energy Center.
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