Text and charts from The Wind Power Book by Jack Park (copyright © 1981) are reprinted by permission of the author and Cheshire Books.
Harnessing the wind is a fantastically appealing idea.
Anyone who's stepped out into a howling autumn blow knows
instinctively that there's tremendous power in the
movement of air, but the would-be windplant owner must
decide whether his or her area has enough wind year round
to justify the expense of such a system.
Windplant designer and builder Jack Park—owner of Helion
Inc., president of the American Wind Energy Association,
and author—has addressed that subject in a new text: The Wind Power Book. In the opinion of
MOTHER EARTH NEWS' technical staff, it is the clearest, most
understandable explanation yet of the factors that anyone
must consider before either purchasing or constructing a
wind energy system.
In fact, we were sufficiently impressed by the
book to secure the right to reprint a portion of
the volume's introduction ...which we hope will help to
give everyone who reads it a sound understanding of the
whys and wherefores of wind power.
The Uses of Wind Power
Today, the wind can be harnessed to provide some or all of
the power for many useful tasks such as pumping water,
generating electricity, and heating a house or barn. Let's
examine a few of these more closely.
Pumping water is a primary use of wind power. Daniel
Halliday and others began manufacturing multi-bladed
windmills for this purpose in the mid-nineteenth century.
Halliday's work coincided with advancements in the iron
water-pump industry. Soon the combination of wind machines
and iron water-pumps made it possible to pump deep wells
and provide the water for steam locomotives chugging across
the North American plains. The demand for wind-powered
deep-well pumps created a booming wind power industry at the
turn of the century. Sears sold those machines for about
$15, or $25 for a tower.
The wind has also been harnessed to provide mechanical
power for grain grinding, sawmill operation, and even
driving a washing machine. While I don't envision next
year's Kenmore washing machine to come complete with a
tower, blades, and drive shaft in lieu of an electric cord
and plug, mechanical power from a wind machine can prove
Electricity can power just about anything, and its
generation from wind power seems to grab the lion's share
of attention. You can pump water, run washing machines,
grind grain, heat houses, and read books if you have
electric power. As soon as electric generators from old
cars became available, farmers started building "light
plants," or homemade wind generators. Such early mechanics
magazines as Popular Science and Modern Mechanix
demonstrated how to convert water-pumping windmills into
wind chargers by using junked generators, bicycle chains,
and the family wind machine.
Many Midwest farmers already had gasoline or kerosene
generators to charge their batteries, and the addition of
wind power helped reduce fuel costs and wear-and-tear on
generators. Out of all of this backyard activity grew the
pre-REA windcharger industry. Some half-million wind
systems once existed in the United States alone, but it's
not clear from historical records whether this number
includes the water pumpers along with the wind chargers.
Farmers used wind-generated electricity to power a radio,
one or two lights for reading, eventually an electric
refrigerator or a wringer washing machine, and not much
else. Electric irons for pressing clothes, electric
shavers, and other gadgets built to run on direct current
appeared, but most of these proved unrealistic uses for
wind-generated electric power. In fact, they may have
contributed to the demise of wind electricity when rural
electrification began. Electric appliances performed much
better on an REA line, which wasn't subject to dead
batteries. "Let's go over to the Joneses, Pa. They got one
of them new powerlines. Maybell says her refrigerator don't
defrost no more!"
Rural electrification put numerous wind chargers out of
business. In the Midwest you can drive for miles on an
empty dirt road, following a long electric powerline to
only one, or perhaps two, homes at the end of the road.
Leave one road and follow the next. It's the same story.
REA lines were installed and wind generators came down.
Sears catalogs touted all the marvelous gadgets one could
buy and plug into the newly installed powerline.
Electric stoves, hot curlers, electric air conditioners,
two or more televisions ...these aren't very realistic
loads to place on a wind-charged battery. However, wind
power can contribute to the operation of these
devices, especially if grid power is already doing part of
the job. With such cogeneration (wind power used
together with grid power), the more wind power available,
the less grid power needed.
In another application, wind power can provide heat for
warming our households, dairy-barn hot water, or just about
anything else for which heat is used as long as the heat is
not needed in a carefully controlled amount. This wind
heating concept is called the wind furnace, and
it's one of our most useful applications of wind power.
Wind furnaces can use wind-generated electricity to produce
the heat, or they can convert mechanical power into heat
Wind machine design must begin with a realistic assessment
of energy needs and available wind resources. When
confronted by inexperienced people observing my wind
machine, I'm asked most often, "Will it power my house?"
Taking this question to its most outrageous extreme, I'm
often tempted to reply, "Just how fast would you like your
house to go?" But usually I just ask, "How much power do
you need at your house?" Blank stares, mumbled confusion,
sometimes ignorant silence follow. Then, "Well, will it
power the average house?"
Apparently many people would like to install a $1,000 wind
machine and "switch off" good old Edison. This is a
fantasy. It might be reasonable to use the wind to power
your house if old Edison is a $30,000 powerline away from
your new country home, but most end uses for wind power
will be somewhat less extravagant.
A successful wind power system begins with a good
understanding of the intended application. For example,
should you decide that water pumping is the planned use,
you must determine how high the water must be lifted and
how fast the water must flow to suit your needs. The force
of the wind flowing through the blades of a windmill acts
on a water pump to lift water. The weight of the water
being lifted and the speed at which the water flows
determine the power that must be delivered to the pump
system. A deeper well means a heavier load of water. Speeding up the flow means more water to be lifted per
second. They both mean more power required to do the job,
or a larger load.
This concept of load is crucial to the understanding of
wind power. Imagine that instead of using a windmill you
are tugging on a rope to lift a bucket of water from the
well. This lifting creates a load on your body. Your
metabolic processes must convert stored chemical energy to
mechanical energy. The rate at which your body
expends this mechanical energy is the power you
are producing. The weight (in pounds) of the bucket and the
rate (in feet per second) at which you are lifting it
combine to define the power (in foot-pounds per second)
produced. How long you continue to produce that power
determines the total amount of mechanical energy you have
The kind of application you have in mind pretty clearly
defines the load that you'll place on your windpower
system. Knowing something about that load will allow you to
plan an energy budget. "What the #$°/a&,"
you ask, "is an energy budget?" Let's explain that with an
When you collect your paycheck, you have a fixed amount of
money to spend. You probably have a budget that allocates
portions of your money to each of the several bills you
need to pay. With any luck, something is left over for
savings, a few beers, or whatever else you fancy. Energy
should be managed the same way, and if you live with wind
power for very long, you'll soon set up an energy budget.
Setting up an energy budget involves estimating,
calculating, or actually measuring the energy you need for
the specific tasks you have in mind. If you plan to run
some electric lights, you must estimate how many, how long,
and at what wattage. If you plan to run a radio for three
hours each evening, you'll have to add that amount of
electrical energy to your budget. If you want to pump
water, you should start with estimates of how much water
you need per day and calculate how much energy is required
to pump that much water from your well into the storage
Wind furnaces require that you calculate the amount of heat
needed. In some cases, you need to calculate only the heat
required to replace the heat lost from your house when it's
windy. Such a system works only when it's needed. Your
energy budget will now be in heat units, probably British
Thermal Units (BTU) ...which can easily be converted to
horsepower-hours (HPH) or to kilowatt-hours (KWH): energy
units more familiar to wind machine designers.
Your utility bills and the equipment you already own will
help define your energy needs. For example, average
electrical energy consumption in U.S. residences is around
750 KWH per month, or about one kilowatt-hour per hour. Or,
more specifically, most residential well pumps are rated at
one to three horsepower. You can easily determine how long
your pump runs and arrive at the total energy required per
day, per week, or per month. In short, you really need to
get a handle on your energy needs before you can proceed to
the design of a wind power system.
You will also need to determine your energy paycheck. There
are two possible approaches:  Go to the site where you
intend to install the wind machine and analyze the wind
resource, or  go searching for the best wind site you
can find. The former approach is more direct. You own some
property, there is only one clear spot, and that's where
the tower will be planted ... along with your hopes for a
successful project. The latter approach offers more avenues
for refinements and better chances for success. In any
event, the larger the paycheck, the less strain on your
energy budget ...and the smaller and less efficient a
windmill needs to be.
In either approach you need to measure, estimate, or
predict how much wind you can expect at your chosen site.
I've talked with folks who claimed to be wind witchers,
possessing the ability to use a wet index finger and
predict the wind speed with great accuracy. I've talked with
people who installed wind generators back in the days
before rural electrification. Most of these machines were
installed in areas now known to be quite windy, with
average wind speeds of 14 to 16 miles per hour (MPH). When
asked, these folks almost always guessed that the wind
averaged 30 MPH or more. Those old wind chargers were
installed haphazardly. "Heck, anywhere you stick one, it
will work just fine."
Once you have established an energy budget, you have
effectively established a standard for the performance of
your wind power system. That puts you in a different league
from the pre-REA folks. Your system will be good if it
meets the energy budget. Theirs was good because it was all
they had. Your site analysis should be careful and
conservative. If it is, the wind system you plan will
probably serve its purpose. If not, you'll have to be happy
with what you get, just like the folks before the REA. As
you gain familiarity with your system, you might learn how
to save a little power for later. Or maybe you'll build a
larger wind machine because you like wind power so much.
From an engineering standpoint, the available wind power is
proportional to the cube of the wind speed. Put another way,
if the wind speed doubles, you can get eight times the wind
power from it unless the tower collapses. If you are off
by a factor of two on your wind speed estimate, you'll be
off by a factor of eight on the power you think is coming
to you. Remember those farmers who guessed their average
wind speed to be 30 MPH, although it's been measured at
around 15 MPH. A factor of two.
Folks who have lived in an area for a long time can usually
tell you what seasons are windy and the direction of the
wind during those seasons. But they're not very good at
estimating wind speeds. You'll have to measure the wind speed
yourself, or use some accurate methods to estimate it.
Once you have established your energy budget and collected
adequate wind resource data, you can begin the task of
system design. Whether you intend to design and build the
windmill yourself, assemble one from a kit, or buy one off
the shelf, these are necessary preliminaries. Too
frequently, designers disregard energy budget and site
analysis. Instead, they make arm-waving assumptions on the
use of the wind machine and where it might be installed,
and use other criteria like cost as the design goal. Some
of the best wind machines on today's market have been
designed this way, but it's very important to recognize
that most manufacturers of mass-produced wind machines have
left the tasks of energy budget and site analysis up to the
buyer. The information provided in later chapters of The
Wind Power Book will help you make these
analyses before you select a factory-built wind
machine suited to your needs or set about designing and
building your own system.
A thorough energy budget should tell you something about
the time of day, week, or month when you need certain
amounts of energy and power. For example, if everybody in
your house rises at the same time each morning, your
electric lights, hot curlers, coffeepot, television set,
toaster, stove, water heater, and room heaters probably
become active all at the same time. An enormous surge in
electricity use occurs, the same problem Edison has.
Their generators idle along all night doing virtually
nothing until everybody wakes up at once. If you had a wind
electric system, it would be really nice if the wind at
your site were strongest when the loads on the system were
greatest. Chances are, though, that your consumption
doesn't coincide with the wind. So, you can either try to
synchronize your loads with the wind or store
wind-generated energy until you need it.
The methods of energy storage are legion, but only a few
are practical. If wind is being used to pump water, your
energy storage might be a familiar old redwood water tank.
Electrical storage has traditionally taken the form of
batteries, which is still the most reasonable means of storage in
many installations. Cogeneration allows you to send
wind-generated electricity out to utility lines (running
the meter backward) when you don't need it all. In effect,
old Edison becomes the energy storage for the wind system.
There are a number of exotic ways one might choose to store
excess wind energy. You might dynamite an enormous mine
under your house and pump it up with air from a
wind-powered compressor. This compressed air can then power
a small generator, sized for your loads, as well as provide
aeration for the tropical fish tank. If you own enough
land, you can bulldoze a large lake and pump water up to it
with wind power. A small hydroelectric turbine will produce
electricity as you need it. In fact, you might sink two
telephone poles out in the yard and use a wind-powered
motor to power a hoist, lifting a '56 Oldsmobile up to 100
feet. As it descends, the motor that lifted it becomes a
generator. Such a mechanism could provide you with 500
watts of electricity for about 15 minutes ... maybe enough
to burn the toast! There are as many possibilities for
energy storage as there are crackpot inventors around, and
some of these possibilities are just as crazy.
Some systems provide energy storage as an inherent part of
the design. The wind furnace, where wind power is being
used exclusively for heat production, is a good example.
Heat storage is energy storage: You may store energy by
heating water, rocks, or a large building with the excess
heat. But probably you'll be heating with wind power when
you need heat the most. Wind chill can draw heat from a
house much more quickly than heat loss occurs under no-wind
conditions. So little, if any, storage would be necessary.
But for most applications, some energy storage is
Wind system design is a process of balancing energy needs
against wind energy availability. Besides picking a good
site and buying or building the right wind machine, you
have to select a suitable storage system, plan all wiring
or plumbing, build a tower, support it with guy wires, and
get building permits and neighbor approval.
The Wind Power Book is organized to help lead you through the design process. Whether you intend
to design and build the entire system or just to assemble
it from factory-built parts, a systematic approach will
help you achieve a wind power system worthy of your efforts.
Energy and Power
A clear distinction must be made between energy and power, two different but closely related quantities. Briefly,
power is the rate at which energy is extracted, harnessed,
converted, or consumed. It equals the amount of energy per
unit time, or
Power = --------
An equivalent relation between these entities is
Energy = Power X Time
The amount of energy extracted or consumed is therefore
proportional to the elapsed time. For example, a typical
light bulb draws 100 watts of electrical power. One watt (1
W) is the basic unit of power in the metric system, Leave
the light bulb on for two hours, and it will consume 200
watt-hours (100 watts times 2 hours equals 200 watt- hours,
or 200 WH). Leave it on for ten hours, and it consumes
1,000 watt-hours, or one kilowatt-hour (1 KWH), the more
familiar metric unit.
In the English system, energy is measured in foot-pounds,
British Thermal Units, and a host of other units that don't
concern us here. One foot-pound (1 ft.-lb.) is the amount
of mechanical energy needed to raise one pound one foot
high. One British Thermal Unit (1 BTU) is the
amount of thermal energy needed to heat one pound of water
1°F. Power is most often measured in horsepower and in
BTU per hour. One horsepower (1 HP) is the power required
to raise a 550-pound weight one foot in one second:
1 horsepower = 550 -----------
Note that the units of power are expressed in units of
energy per time, as one would expect.
Conversions between metric and English units require that
you know a few conversion factors. For example, one
horsepower equals 746 watts, and one kilowatt-hour is
equal to 3,413 BTU. Thus,
100 W = --- HP = 0.134 HP
10,000 BTU = ------ KWH = 2.93 KWH
Rarely is it a simple task to estimate the wind energy
available at a particular site; the wind speed is
constantly changing. During one minute, 300 watts of power
may be generated by a windmill, or 300 watt-minutes of
energy (which equals 5 WH). During the next minute the wind
may die, and you get absolutely no energy or power from the
machine. The power output is constantly changing with the
wind speed, and the accumulated wind energy is increasing
with time. The wind energy extracted by the machine is the
summation or total of all the minute-by-minute (or whatever
other time interval you care to use) energy contributions.
For example, if there are 30 minutes during a particular
hour when the windmill is generating 5 WH and another 30
minutes when there is no energy generated, then the machine
generates 150 WH (5 X 30 = 150) of wind energy. If there
are 24 such hours a day, then 3,600 WH or 3.6 KWH are
generated that day.
Wind Energy and Wind Power
Energy and power are derived from the wind by making use of
the force it exerts on solid objects, pushing them along.
Buildings designed to stand still against this force
extract very little energy from the wind. But windmill
blades are designed to move in response to this force, and
wind machines can extract a substantial portion of the
energy and power available.
The wind energy available in a unit volume (one cubic foot
or one cubic meter) of air depends only upon the air
density p (Greek "rho") and the instantaneous wind speed V.
This "kinetic energy" of the air in motion is given by the
-------------- = 1/2pV2
To find the kinetic energy in a particular volume of air,
you just multiply by that volume. The volume of air that
passes through an imaginary surface—say the disk
swept out by a horizontal-axis windmill oriented at right
angles to the wind direction—is equal to
Volume = AVt
where t is the elapsed time (in seconds) and A is the area
(in square feet or square meters) of the surface in
question. Thus, the wind energy that flows through the
surface during time t is just
Available Energy = 1/2pV3At
Wind power is the amount of energy which flows through the
surface per unit time, and is calculated by dividing the
wind energy by the elapsed time t. Thus, the wind power available under the same conditions as above that
is given by the formula
Available Power = 1/2pV3A
Both energy and power are proportional to the cube of the
If all the available wind power working against a windmill
rotor could be harvested by the moving blades, this formula
could be used directly to calculate the power extracted.
But getting such an output would require that you stop the
wind dead in its tracks and extract every last erg of its
kinetic energy. This is an impossible task. Some non-zero
windspeed must occur downstream of the blades to carry away
the incoming air, which would otherwise pile up. Under
ideal conditions, the maximum power that can be extracted
from the wind is only 59.3% of the power available,
Maximum power = -----pV3A
In practice, a wind machine extracts substantially less
power than this maximum. For example, the windmill rotor
itself may capture only 70% of the maximum power. Bearings
will lose another few percent to friction ...generators,
gears, and other rotating machinery can lose half of
whatever power remains. Pushrods, wires, batteries, and
monitoring devices will lose still more. The overall
"system" efficiency of the entire wind machine is the
fraction of the wind power available that is actually
delivered to a load or to a storage device:
Efficiency = ---------------
Thus, the power extracted by a particular wind machine with
system efficiency E is given by the formula
Extracted Power = 1/2pV3AE
The final output of a wind machine is greatly reduced from
the power that is really available in the wind. In
practice, values of E commonly range from 0.10 to 0.50,
although higher and lower values are possible.
One more factor is needed before the formula above can be
used in your calculations: a conversion factor, K, that
makes the answer come out in the appropriate units, whether
metric or English.
The final formula combines everything so far presented:
This is a very important formula, perhaps the most
important in The Wind Power Book.
EDITOR'S NOTE: Another book that should definitely be a part of any
prospective wind power user's library is a new treatise by
Donald Marier (editor of Alternative Sources of Energy
magazine) called Wind Power for the Homeowner: A Guide to
Selecting, Siting, and Installing an Electricity-Generating
Wind Power System.