Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
John T. Woeste, Dean for Extension
D. S, Harrison, A. G. Smajstrla and F. S. Zazueta
1) 0 UMWEaT
OCT 18 1988
' I.F~AS. -Univ. of Flori<
Front cover photo: Windfall panel windrnill courtesy of Windfall Energy Corp., Buffalo,
NY; photo above: new improved windrnill courtesy of Parrish Windmill, Earth, TX.
About Windmills and Wind Generators
D. S. Harrison, A. G. Smalstria and F. S. Zazueta
D. S. Harrison is Visiting Professor (Professor Emeritus), A. G.
Smajstrla is Associate Professor and F. S. Zazueta is Visiting As-
sistant Professor, all of the Agricultural Engineering Department,
Institute of Food and Agricultural Sciences, University of Florida.
Wind is a free and inexhaustible source of energy. However, its
use as a power source depends upon the cost of equipment necessary
to harness it. For example, the many windmills supplying water for
farms and the "wind chargers," which were once more common
than power poles, are largely gone now victims of more reliable
power sources, such as the electric motor.
In Florida, the water pumped in one year by a typical windmill
(inside front cover) costing $2500 to $3500 could be produced by a
$1200, V4-hp electric motor, pump and well at a cost of less than
$60-80 in electricity. Therefore, windmills and wind generators are
really only practical where electricity is not available, or where gas-
powered generators cannot be attended, and wind is sufficient.
Recent increased fuel/energy costs and power shortages in Florida
have renewed interest in obtaining energy from the wind. Much
research in this area continues throughout the world. Large-scale,
commercial use of wind power has had limited success in some coun-
tries, and wind-driven generators of up to 1250 kilowatts (kw) (1678
hp) capacity have been constructed. The most successful and readi-
ly available types of units are the multibladed units used for pump-
ing water, and the propellor-type high speed unit and the panel mills
used to generate electricity. Some of the units have propellors that
are 6 to 16 feet in diameter, depending on the power needed. But
why aren't windmills and wind generators used more? What are
some of the problems?
Windmills are inefficient, with only about 35 percent efficiency
on the average. Theoretically, up to 60 percent of the wind's power
can be extracted. Extremely efficient propellors may obtain about
70 percent of this theoretical maximum. When losses due to gears,
pumps or generators and batteries are added, the overall efficiency
is reduced to 10 to 30 percent of the wind's total extractable power.
Florida is not windy enough for good windmill operation for power
generation, with the exception of some coastal areas or hills. Wind-
mills and generators develop little power or don't even start run-
ning until wind velocity is greater than 7 mph. Thus, they don't
generate enough energy to be useful in areas where the annual
average wind velocity is less than 8 mph, which is the case over most
of Florida (Table 1).
Table 1. Monthly Average Wind Power in Certain Florida (Military) Locationsa
Location speed (mph) Estimated wind power in month (hp/per 100 sq ft)
J F M A M J J A S O N D Avg.
Key West, NAS
West Palm Beach
Ft. Myers, Hed. Fid.
Tampa, McD. AFB
Avon Park, AAF
Orlando, Hernd. Apt.
Orlando, McCoy AFB
Cocoa Bch., Pat. AFB
Cape Kennedy, AFS
Daytona Bch., Apt.
Jax, C.F. NAS
Pan. City, TAFB
Valparaiso, Eg. AFB
Valparaiso, D. Fld.
Valparaiso, H. Fld.
Milton, W. Fld. NAAS
Pensacola, S.F. NAS
Pensacola, E. Fld.
Pensacola, Fo. SH. Fd.
8.3 2.0 2.1 2.1 2.1
5.6 0.8 0.9 1.1 1.1
6.8 1.1 1.2 1.4 1.4
7.1 1.0 1.3 1.5 1.6
7.2 1.5 1.6 1.9 1.7
6.1 1.2 1.4 1.9 1.9
6.2 0.7 0.8 1.2 1.1
6.0 0.9 1.1 1.2 1.1
5.0 0.6 0.7 0.7 0.8
7.1 1.1 1.4 1.6 1.5
5.1 0.8 1.0 0.9 0.8
5.8 0.7 0.9 0.9 0.7
7.7 1.6 1.9 1.9 1.7
6.4 1.0 1.3 1.3 1.1
7.7 1.4 1.8 1.9 1.9
4.5 0.5 0.8 0.7 0.7
6.0 0.8 1.1 1.1 0.9
6.3 1.0 1.3 1.2 1.1
5.0 0.6 0.7 0.9 0.8
6.0 1.2 1.3 1.4 1.1
5.8 1.0 1.3 1.6 1.2
4.9 0.8 0.9 1.1 0.7
5.4 0.8 0.9 1.0 0.9
6.1 1.3 1.5 1.3 1.4
4.8 0.7 0.8 0.7 0.7
6.2 1.3 1.4 1.5 1.2
5.9 1.2 1.4 1.4 1.2
6.8 1.1 1.3 1.5 1.4
7.0 1.4 1.5 1.5 1.4
1.5 1.2 1.0 0.9 1.6 1.6 1.6
0.9 0.7 0.4 0.4 0.7 0.7 0.7
1.1 0.7 0.7 0.7 1.1 1.1 0.9
1.3 0.9 0.6 0.7 1.4 1.7 1.3
1.3 0.9 0.9 0.8 0.9 1.3 1.4
1.3 0.9 0.7 0.8 1.3 1.2 1.2
1.0 0.7 0.4 0.5 1.0 1.1 0.8
0.7 0.7 0.4 0.5 0.8 0.9 0.8
0.5 0.4 0.2 0.2 0.7 0.9 0.5
1.2 1.1 0.8 1.1 1.1 1.3 1.1
0.9 0.5 0.4 0.3 0.5 0.6 0.6
0.9 0.5 0.5 0.4 0.6 0.7 0.6
1.9 1.1 0.7 0.7 1.6 2.3 1.7
1.3 0.7 0.5 0.4 0.9 1.2 0.9
1.8 1.2 1.2 1.1 1.5 2.0 1.4
0.7 0.4 0.3 0.2 0.5 0.5 0.5
1.1 0.7 0.5 0.3 0.9 0.9 0.7
1.2 0.7 0.5 0.3 1.2 1.1 0.9
0.9 0.4 0.3 0.3 0.5 0.5 0.6
1.4 0.5 0.5 0.4 0.6 0.7 0.9
1.6 0.5 0.5 0.4 0.8 0.6 0.7
1.1 0.3 0.2 0.2 0.5 0.5 0.7
1.0 0.5 0.5 0.4 0.6 0.6 0.6
1.0 0.5 0.4 0.4 0.5 0.6 1.1
0.4 0.4 0.2 0.2 0.4 0.4 0.5
0.8 0.4 0.3 0.3 0.8 0.6 1.1
0.8 0.4 0.3 0.3 0.9 0.7 1.0
1.1 0.7 0.6 0.5 0.8 0.6 0.9
0.9 0.9 0.6 0.7 0.9 0.9 1.1
a From Park, J. 1981. The Wind Power Book.
Windmills and generators are expensive and do not generate
enough power to pay for such a large investment. To be competitive
with commercial power, wind-powered devices should cost in the
range of $4.50 to $8.00 per square foot of area swept by the pro-
pellor and wind speeds greater than 8 mph. Typical wind devices
marketed today cost $25 to $75 per square foot of swept area,
depending on accessories included. Because windmills are so expen-
sive, they have only found success in remote areas where other forms
of power are not available. But, perhaps, with increasing power
costs, new, less expensive designs may be developed and allow wind
power to become more widely used in the future.
Some companies in the U.S. are now offering panel windmills (front
cover) in the 10-100-kw range, with prices ranging from $20,000 to
$95,000. These should be on the market in late 1984 or early 1985.
Panel windmills produce power in low 6 to 10 mph wind velocities,
and produce power at any wind speed, even up to 100 mph so
states one manufacturer in its literature.
Many propellor windmills shut down at about 22-40 mph wind
speeds or they will self-destruct. Because of their ability to fune-
tion at both low and high wind speeds, some predict that within a
few years, only the panel windmills may be sold.
Windmills for Pumping Water
The windmill almost everyone remembers is the multibladed type
that has 15 to 40 galvanized steel fan blades and is used to pump
water for farm or livestock use. The fan drives a mechanism which
converts rotary motion to an up-and-down motion used to drive a
piston pump. The pump cylinder is placed below the water level in
the well, and a pump rod descending from the windmill drives it.
The many blades of the propellor develop a high starting torque,
which is needed to overcome the high starting load of a piston pump.
Larger mills can lift water 400 to 600 feet from a deep well to a
tank. Maximum pumping capacities are shown in Figure 1 and range
from 100 to over 2000 gallons per hour depending on the windmill
diameter and height of lift or head. These ratings, however, assume
wind velocities of 15-18 mph. With lower velocities pumping rates
decrease rapidly as shown in Figure 2.
Figure 1. Typical maximum pumping rates of windmills. (Parsons, 1974)
One commercial company now manufactures 8- and 10-foot wind-
mills for pumping water using a 2-inch direct and adjustable stroke
piston pump, for 5 and 15 mph winds which delivers 105-225 and
140-300 gallons per hour (gph), respectively, for $1118-1880. Pump-
ing lifts are limited to 100-370 feet.
In areas of Florida where windmill use may be practical, the max-
imum pumping rate will be achieved only about 15 to 20 percent
of the year. Moderate winds of 8 to 12 mph occur only a small percen-
tage of the time in most areas and a windmill will operate at about
half its rated output at these speeds. At a 10 mph wind speed, a
10-foot-diameter windmill pumping against a 100-foot head will
pump about 4500 gallons per day, or 1.63 million gallons of water
per year. Spring and summer are generally more windy than fall and
winter in Florida, and more water will be pumped during these
periods, most of the time.
Wind speed has an important effect on pumping output. The power
available from the wind is proportional to the cube of the wind speed,
s3. The output increases rapidly then, with increasing wind speed.
When the wind speed doubles, the power increases eight times. Con-
versely, output drops off just as rapidly when wind speed decreases.
Figure 2. Typical windmill output versus wind velocity. (Parsons, 1974)
In other words, if a windmill is rated for a 15 mph wind, the cube
law says that at 5 mph (typical in Florida) it will produce 1/27 (about
4%) of its rated amount.
Windmill output is also reduced by a safety feature in windmill
design. The propellor is mounted off-center from the tower axis so
it will "furl" or swing out of the main wind direction when wind
speeds climb above a critical wind velocity (normally 22-25 mph).
While this furling feature protects the windmill mechanism, it also
limits the energy obtainable to that of wind speeds of less than the
critical velocity, no matter how fast the wind blows (Figure 2).
Since the wind doesn't always blow, water must be stored for use
during calm periods. The simplest storage is a stock watering trough
or tank placed beside the mill. The water from this tank can then
be used for limited irrigation or stock watering. If pressure is need-
ed, a water tank can be located on a tower near the windmill.
Small wind generators cannot produce enough energy to operate
the usual household load of refrigerator/freezer, heating appliances,
0. 30 0
5 0 100
WIND VELOCITY, MILE PER HOUR
windspeed (mph) factor
television sets, and domestic water systems. Their main use is in
remote areas unserved by power lines, and for such uses as unat-
tended beacon lights and communications equipment, isolated camps
with minimal lighting and low-demand electrical equipment, and for
The maximum power output or rated wattage of most wind
generators is developed at wind speeds of 23 to 30 mph. Below this
speed the output drops as shown in Figure 3 (in proportion to the
cube of the air velocity). For example, a generator with a 16-foot-
diameter propellor develops only about 150 watts (0.02 hp) in an
8 mph wind. The potential monthly kilowatt-hour (kwh) energy out-
put will be limited in most areas of Florida because of lack of wind.
A typical household uses a minimum of 400 to 600 kwh of electrici-
ty each month. Any dependable, continuous use wind system, then,
must include storage batteries.
The potential monthly kwh output may be estimated for any area,
when the average wind speed is known. Multiply the maximum rated
kw output of the wind generator by the factors listed below:
Example: a wind generator rated at 2 kw and located in an area with
an average wind speed of 10 mph would generate 100 kwh per month
(2 x 50).
This table gives only a rough estimate. Actual output also depends
on such factors as wind gustiness, design characteristics of the
generator, nearby obstructions, etc.
The basic components of a wind generator system consist of the
161 propellor dia
loo 1000 /6'dia.
5 10 15 20 25 30
WIND VELOCITY, MILE PER HOUR
Figure 3. Typical wind generator output versus wind velocity. (Parsons, 1974)
(1) A 2-to-4 blade propellor that directly drives or is geared to a
(2) A tower that is either free-standing or that mounts on an ex-
isting building to reach more dependable, higher speed winds above
trees and other obstructions. The unit usually is mounted 30 to 40
feet above the ground.
(3) A brake or furling device to stop the unit for repairs.
(4) Wet cell batteries to store energy for use during calm periods.
(5) A control panel, including a device to prevent reverse battery
current from driving the wiridmill during calm periods, and a voltage
(6) A speed-controlling device to prevent damage in a high wind.
Some generators have a variable pitch propellor that is direct-
connected to a centrifugal governor to prevent overspeeding. Some
use governor flaps. Some have rotors offset from the main support
axis to turn them out of the wind at high speed.
(7) A standby generator.
A 12-V DC system permits use of conventional automotive lights
and accessories that are readily available at competitive prices. This
low voltage has little shock hazard. However, bare wires or loose
terminals do present hazards. Proper fusing is critical.
A DC system can also be adapted for partial or limited use of AC
equipment. Two-hundred-watt inverters are available from auto
supply stores to convert 12-V DC to 115-V AC to operate standard
electric razors, radios or other small appliances.
Electrical storage capacity can be provided by one or more 12-V,
heavy duty, lead-acid storage batteries (Table 2). Series or parallel
connections can be used when two or more batteries are needed to
obtain the voltage and watt-hour storage desired.
Table 2. Determining Storage Battery Requirements
(a) Determine daily watt-
hour needs by multiplying
wattage of each lamp or
device by hours of ex-
60-watt kitchen light
used 4 hr/day
60 x 4 = 240
60 x 2 x 1 = 120
30 x 4 = 120
100 x 3 = 300
60-watt bath and bedroom
lamps (2) used 1 hr/day
30-watt portable TV
used 4 hr/day
100-watt reading lamp
Total daily watt-hours
(b) Divide watt-hours by 12
(volts) to get ampere-hour
(c) Divide calculated AH need
by the rated AH of battery
to get daily battery
(d) Finally, multiply daily
battery capacity re-
quirements by probable
number. Calm days are
common in Florida.
(Disregard this factor if
generator is used in
780/12 = 65 AH
65/205 = 0.317
3 x 0.317 = 0.951
Thus, 1 battery of
selected size (205 AH)
Estimated 3 calm days
Small wind generators, up to a maximum rated capacity of 1000
watts (1.34 hp), cost about $4 to $5 per maximum rated watt. Larger
generators may vary from $1.00 to $2.50 per rated watt output. A
unit rated to deliver a maximum of 2000 watts (2.68 hp) may cost
about $5000, while one with a 6000-watt (8.0 hp) rating may cost
about $7000. These costs are for generator only and do not include
the tower, batteries, control panel or 115-V inverter. These items
increase the cost by about $1.50 per rated watt. Approximate costs
for additional equipment are listed in Table 3.
Table 3. Approximate Costs of Some Wind Generator Accessories
Item Approximate cost
Steel tower, per foot 30
Auto, 12-volt, heavy duty 110 AH 50
Truck, 12-volt, heavy duty, 205 AH 150
Battery service kit and tester 25
12-volt DC to 115-volt AC, 200-watt 75
12-volt DC to 115-volt AC, 1000-watt 500
Generator; heavy duty, gas, 115-volt AC, 800
Estimation of Potential Power
Power produced by wind (theoretical) is a function of the cube
of the windspeed. It is very important to have accurate windspeed
data when making predictions of wind turbine performance.
Historically, windspeed data have been collected by the National
Weather Service at airports and agricultural research stations.
Many times, windspeed data are not directly applicable for wind-
power use because of the averaging interval or the height of the
measurement. Wind data averaged over periods in excess of 30
minutes produce errors of about 20 percent when predicting wind-
power. When new data are collected for windpower assessments,
they should be averaged over time intervals less than 10 minutes,
preferably at 1-minute intervals, in order to reduce the error to less
5 M 250 O
I, 300 MM 400
300 150 300
20 0 C-~~300 IOO
200 M 300 "' ;:
300 100 20010
NOITE: TO CONVERT WATT~ PER 012 TO 200
H.P. PER 100 FT DIVIDE BY 80.27.
Figure 4. Mean Annual Wind Power (w/m2) estimated at a 50m40
height. (Elliott, 1977).
than 2 percent. Longer averaging intervals (Table 1 and Figure 4)
can be used if original data are cubed as sampled and the cube root
of the average cube is used.
Wind data collected at the centerline height of the proposed wind
turbines are also important. Predictive equations for adjusting wind-
speeds with height are empirical and often do not correctly repre-
sent the terrain being studied. The 1/7 power law used for corree-
ting windspeed for height has been shown to be in error by over 200
percent in a terrain of rolling hills and by 10 percent in a flat, treeless
terrain. If the centerline height is uncertain at least two
measurements, one below and one above, should be used to
characterize the wind site. This is especially true where terrain is
complex due to trees, buildings or mountains.
Table 1 and Figure 4 give average monthly and annual theoretical
power for various locations. In Figure 4, wind power is given in watts
per m2 Of windmill fan area, and in Table 1 hp per 100 square feet
of windmill fan area. Note that averages in Table 1 are considerably
less than in the generalized map in Figure 4.
Winds in Florida are generally not of sufficient duration and
magnitude for dependable applications for pumping large quantities
of water or powering wind generators. Panel mills could change this
because they operate at lower wind speeds; however, the potential
power in the wind in Florida is still very low. Wind power data show
Florida as the poorest choice of all the states for wind energy, utiliz-
ing regular windmills (Table 1 and Figure 4). Table 1 gives more ac-
curate long term data on wind speed.
Anonymous. 1984. Windfall Panel Windmill. Special Bulletin, Windfall
Energy Corp., Buffalo, NY.
Clark, R. N. 1980. Data requirements for wind energy. ASAE Paper 80-4517,
Am. Soc. Agr. Engrs., St. Joseph, MI.
Clark, R. N. and A. D. Schneider. 1980. Irrigation pumping with wind energy.
Trans. ASAE 23(4): 850-853.
Elliott, D. L. 1977. Synthesis of National Wind Energy Assessments. Report
No. BNWL/WIND-5. Battelle Labs., Richland, WA.
Park, Jack. 1981. The Wind Power Book. Cheshire Books, Palo Alto, CA.
Parsons, R. A. and W. C. Fairbank. 1974. Windmills and wind generators.
Ext. Leaflet, Coop. Ext. Sve., Univ. of Calif., Davis, CA.
Perry, T. O. 1899. Experiments with windmills. U.S. Geological Survey; U.S.
Gov't. Printing Office, Washington, DC.
Soderholm, L. H. and J. F. Andrew. 1974. Wind-electric power. ASAE Paper
74-3503, Am. Soc. Agr. Engrs., St. Joseph, MI.
This publication was promulgated at an annual cost of $570.80, or 34
cents a copy, to inform Florida citizens and agriculturists and county Ex-
tension personnel of the potential, requirements and design for wind
energy in Florida. 10-1.7M-85
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORI-
DA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, K. R.
Tefertltler, director, In cooperation with the United States Department il FA~
of Agriculture, publishes this Information to further the purpose of the
May 8 and June 30, 1914 Acts of Congress; and is authorized to pro-
vide research, educational Information and other services only to indi-
viduals and Institutions that function without regard to race, color, sex or national ori-
gin. Single copies of Extension publications (excluding 4-H and Youth publications) are
available free to Florida residents from County Extension Offices. Information on bulke
rates or copies for out-of-state purchasers is available from c. M. Hinton, Publications
Distribution Center, IFAS Buliding 664, University of Florida, Gainesville, Florida
32611. Before publicizing this publication, editors should contact this address to deter-