TECHNICAL PAPER # 62
UNDERSTANDING WIND ENERGY
FOR WATER PUMPING
James F. Manwell
VOLUNTEERS IN TECHNICAL ASSISTANCE
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax:
Understanding Wind Energy for Pumping Water
1988, Volunteers in Technical Assistance
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Energy Research Laboratory, Department of Mechanical
Engineering, at the University of Massachusetts in Amherst.
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Cromack of "Understanding Wind Energy," another
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UNDERSTANDING WIND ENERGY FOR WATER PUMPING
There are many places in the world where wind energy is a
alternative power source for pumping water.
These include windy
areas with limited access to other forms of power.
In order to
determine whether wind power is appropriate for a particular
situation an assessment of its possibilities and the
should be undertaken.
The necessary steps include the following:
1. Identify the
users of the water.
2. Assess the
3. Find the
pumping height and overall power requirements.
4. Evaluate the
5. Estimate the
size of the wind machine(s) needed.
6. Compare the
wind machine output with the water
a seasonal basis.
7. Select a type
of wind machine and pump f rom the
possible suppliers of machines, spare
alternative sources for water.
10. Assess costs
of various systems and perform economic
find least cost alternative.
11. If wind energy
is chosen, arrange to obtain and install
and provide for maintenance.
II. DECISION MAKING
The following summarizes the key aspects of these steps.
1. Identify the
This step seems quite obvious, but should not be
paying attention to who will use the wind machine and its
it will be possible to develop a project that can have
to consider are whether they are villagers,
farmers, or ranchers; what their educational level is;
they have had experience with similar types of technology in
past; whether they have access to or experience with metal
shops. Who will be
paying for the projects? Who will be
the equipment; who will be responsible for keeping it
and who will be benefitting most?
Another important question
is how many pumps are planned.
A large project to supply
many pumps may well be different than one looking to supply
2. Assess the Water
There are four main types of uses for water pumps in areas
wind energy is likely to be used.
These are: 1) domestic
livestock watering, 3) irrigation, 4) drainage.
Domestic use will depend a great deal on the amenities
A typical villager may use from 15 - 30 liters per day (4-8
per day). When indoor plumbing is used, water consumption
may increase substantially.
For example, a flush toilet consumes
25 liters (6 1/2 gallons) with each use and a shower may
(60 gallons.) When
estimating water requirements, one must also
consider population growth.
For example, if the growth rate is 3
percent, water use would increase by nearly 60 percent at
of 15 years, a reasonable lifetime for a water pump.
Basic livestock requirements range from about 0.2 liters
quart) a day for chickens or rabbits to 135 liters (36
day for a milking cow.
A single cattle dip might use 7500 liters
(2000 gallons) a day.
Estimation of irrigation requirements is more complex and
on a variety of meteorological factors as well as the types
crops involved. The
amount of irrigation water needed is approximately
equal to the difference between that needed by the plants
and that provided by rainfall.
Various techniques may be used to
estimate evaporation rates, due for example to wind and sun.
These may then be related to plant requirements at different
stages during their growing cycle.
By way of example, in one
semi-arid region irrigation requirements varied from 35,000
(9,275 gallons) per day per hectare (2.47 acres) for fruits
and vegetables to 100,000 liters (26,500 gallons) per day
hectare for cotton.
Drainage requirements are very site dependent.
values might range from 10,000 to 50,000 liters (2,650 to
gallons) per hectare.
In order to make the estimate for the water demand, each
consumption is identified, and summed up to find the
will become apparent later.
It is desirable to do this on a
monthly basis so that the demand can be related to the wind
3. Find Pumping
Height and Total Power Requirement
If wells are already available their depth can be measured
If new wells are to be dug, depth must be estimated by
reference to other wells and knowledge of ground water
in the area. The
total elevation, or head, that the
pump must work against, however, is always greater than the
well depth. Other
contributors are the well draw down (the
lowering of the water table in the vicinity of the well
pumping is underway), the height above ground to which the
will be pumped (such as to a storage tank), and frictional
in the piping. In a
properly designed system the well depth and
height above ground of the outlet are the most important
of pumping head.
The power required to pump water is proportional to its mass
unit volume, or density (1000 kg/[m.sup.3]), the
acceleration of gravity
(g= 9.8 m/[s.sup.2], the total pumping head (m), and the
rate of water ([m.sup.3]/s).
Power is also inversely proportional to the
Note that 1 cubic meter equals 1000 liters.
Expressed as a formula,
Density x Gravity x Head x Flow rate
To pump 50
[m.sup.3] in one day (0. 000579 [m.sup.3]/s) up a total head of
15 m would require:
Power = (1000
kg/[m.sup.3) (9.8m/[s.sup.2]) (15m) (.000579[m.sup.3]/s) = 85 watts.
required would be more because of the less than
of the pump.
Sometimes needed pumped power is described in terms of daily
hydraulic requirement, which is often given in the units of
example, in the above example the hydraulic
is 750 [m.sup.3.]m /day.
4. Evaluate Wind
It is well known that the power in the wind varies with the
of the wind speed.
Thus if the wind speed doubles, the available
power increases by a factor of eight.
Hence it is very important
to have a good understanding of the wind speed patterns at a
given site in order to evaluate the possible use of a wind
there. It is
sometimes recommended that a site should have an
average wind speed at the height of a wind rotor of at least
m/s in order to have potential for water pumping.
That is a good
rule of thumb, but by no means the whole story.
First of all, one
seldom knows the wind speed at any height at a prospective
site, except by estimate and correlation.
Second, mean wind
speeds generally vary with the time of day and year and it
an enormous difference if the winds occur when water is
The best way to evaluate the wind at a prospective site is
monitor it for at least a year. Data should be summarized at
least monthly. This
is often impossible, but there should be some
monitoring done if a large wind project is envisioned. The
practical approach may be to obtain wind data from the
weather station (for reference) and try to correlate it with
at the proposed wind pump site.
If at all possible the station
should be visited to ascertain the placement of the
instrument anemometer) and its calibration.
Many times anemometers
are placed too near the ground or are obscured by vegetation
and so greatly underestimate the wind speed.
The correlation with
the proposed site is best done by placing an anemometer
a relatively short time (at least a few weeks) and comparing
resulting data with that taken simultaneously at the
site. A scaling
factor for the long-term data call be deduced and
used to predict wind speed at the desired location.
Of course, possible locations for wind machines are limited
the placement of the wells, but a few basic observations
be kept in mind. The entire rotor should be well above the
vegetation, which should be kept as low as possible for
a distance of at least ten times the rotor diameter in all
Wind speed increases with elevation above ground, usually
by 15-20 percent with every doubling of height (in the
range of most wind pumps).
Because of the cubic relationship
between wind speed and power, the effect on the latter is
5. Estimate Wind
A typical wind pump is shown in Figure 1.
Most wind pumps have a
relates actual water
flow at given pumping
heads to the wind
speed. This curve
reflects other important
as the wind speeds at
which the machine
starts and stops pumping
(low wind) and when
it begins to turn away
in high winds (furling).
Most commercial machines and those developed and tested more
recently have such curves and these should be used if
predicting wind machine output.
On the other hand, it should be
noted that some manufacturers provide incomplete or overly
estimates of what their machines can do.
should be examined carefully.
In addition to the characteristic curve of the wind machine,
must also know the pattern of the wind in order accurately
For example, suppose it is known how many
hours (frequency) the average wind speed was between 0-1
m/s, 2-3 m/s, etc., in a given month.
By referring to the characteristic
curve, one could determine how much water was pumped in
each of the groups of hours corresponding to those wind
ranges. The sum of
water from all groups would be the monthly
total. Usually such
detailed information on the wind is not
known. However, a
variety of statistical techniques are available
from which the frequencies can be predicted fairly
using only the long-term mean wind speed and, when
measure of its variability (standard deviation).
See Lysen, 1983,
and Wyatt and Hodgkin, 1984.
Many times there is little information known about a
machine or it is just desired to know very approximately
size machine would be appropriate.
Under these conditions the
following simplified formula can be used:
Power = Area x 0.1
Power = useful
power delivered in pumping the water, watts
= swept area of rotor (3.14 x Radius
Vmean = mean wind
By rearranging the above equation, an approximate diameter
wind rotor can be found.
Returning to the earlier example, to
pump 50 [m.sup.3]/day, 15 m would require an average of 85
the mean wind speed was 4 m/s.
Then the diameter (twice the
radius) would be:
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Diameter = 2
[Power/(3.14) x 0.1 x [Vmean.sup.3])]
Diameter = 2 x
[85/(3.14 x 0.1 x [4.sup.3])] = 4.1 m
6. Compare Seasonal
Water Production to Requirement
This procedure is usually done on a monthly basis.
It consists of
comparing the amount of water that could be pumped with that
actually needed. In
this way it can be told if the machine is
large enough and conversely if some of the time there will
excess water. This
information is needed to perform a realistic
The results may suggest a change in the size
of machines to be used.
Comparison of water supply and requirement will also aid in
the necessary storage size.
In general storage should
be equal to about one or two days of usage.
7. Select Type of
Wind Machine and Pump
There is a variety of types of wind machines that could be
The most common use relatively slow speed rotors with
many blades, coupled to a reciprocating piston pump.
Rotor speed is described in terms of the tip speed ratio,
is the ratio between the actual speed of the blade tips and
free wind speed.
Traditional wind pumps operate with highest
efficiency when the tip speed ratio is about 1.0.
Some of the
more recently developed machines, with less blade area
to their swept area, perform best at higher tip speed ratios
(such as 2.0).
A primary consideration in selecting a machine is its
Generally speaking, wind pumps for domestic use or
livestock supply are designed for unattended operation.
should be quite reliable and may have a relatively high
Machines for irrigation are used seasonally and may be
to be manually operated.
Hence they can be more simply
constructed and less expensive.
For most wind pump applications, there are four possible
sources of equipment.
These are: 1) Commercially
of the sort developed for the American West in the late
1800s; 2) Refurbished machines of the first types that have
abandoned; 3) Intermediate technology machines, developed
the last 20 years for production and use in developing
and 4) Low technology machines, built of local materials.
The traditional, American "fan mill," is a well
with very high reliability.
It incorporates a step down
transmission, so that pumping rate is a quarter to a third
rotational speed of the rotor.
This design is particularly suitable
for relatively deep wells (greater than 30m--100').
problem with these machines is their high weight and cost
to their pumping capacity.
Production of these machines in
developing countries is often difficult because of the need
Refurbushing abandoned traditional pumps may have more
than might at first appear likely.
In many windy parts of the
world a substantial number of these machines were installed
in this century, but were later abandoned when other forms
power became available.
Often these machines can be made operational
for much less cost than purchasing a new one.
cases parts from newer machines are interchangeable with the
older ones. By
coupling refurbishing with a training program, a
maintenance and repair infrastructure can be created at the
time that machines are being restored.
Development of this infrastructure
will facilitate the successful introduction of newer
machines in the future.
For heads of less than 30m, the intermediate technology
may be most appropriate.
Some of the groups working on such designs
are listed at the end of this entry.
These machines typically
use a higher speed rotor and have no gear box.
On the other
hand they may need an air chamber to compensate for adverse
acceleration effects due to the rapidly moving piston.
are made of steel, and require no casting and minimal
Their design is such that they can be readily made in
shops in developing countries.
Many of these wind pumps have
undergone substantial analysis and field testing and can be
Low technology machines are intended to be built with
available materials and simple tools.
Their fabrication and maintenance,
on the other hand, are very labor intensive.
In a number
of cases projects using these designs have been less
than had been hoped.
If such a design is desired, it should first
be verified that machines of that type have actually been
and operated successfully.
For a sobering appraisal of some of
the problems encountered in building wind machines locally,
Wind Energy Development in Kenya (see References).
Although most wind machines use piston pumps, other types
mono pumps (rotating), centrifugal pumps (rotating at high
speed), oscillating vanes, compressed air pumps, and
pumps driven by a wind electric generator.
Diaphragm pumps are
sometimes used for low head irrigation (5-106 m or
matter what type of rotor is used, the pump must be sized
A large pump will pump more water at high wind speeds
than will a small one.
On the other hand, it will not pump at all
at lower wind speeds.
Since the power required in pumping the
water is proportional to the head and the flow rate, as the
increases the volume pumped will have to decrease
The piston travel, or stroke, is generally constant (with
exceptions) for a given windmill.
Hence, piston area should be
decreased in proportion to the pumping head to maintain
Selecting the correct piston pump for a particular
involves consideration of two types of factors:
1) the characteristics
of the rotor and the rest of the machine, and 2) the
site conditions. The
important machine characteristics are:
the rotor size (diameter); 2) the design tip speed ratio; 3)
gear ratio; and 4) the stroke length.
The first two have been
The gear ratio reflects the fact that most
wind pumps are geared down by a factor of 3 to 4.
increases with rotor size.
The choice is affected by structural
Typical values for a machine geared down 3.5:1
range from 10 cm (4") for a rotor diameter of 1.8 m
(6') to 40 cm
(15") for a diameter of 5 m (16').
Note that it is the size of the
crank driven by the rotor (via the gearing) that determines
stroke of the pump.
The key site conditions are:
1) mean wind speed and 2) well
depth. These site
factors can be combined with the machine parameters
to find the pump diameter with the use of the following
equation assumes that the pump is selected so that
the machine performs best at the mean wind speed.
DP = [square root] (0.1) (Pi) [(DIAMR).sup.3]
(DENSW) (G) (HEIGHT) (TSR) (STROKE)
DP = Diameter of piston, m
Pi = 3.1416
DIAMR = Diameter of the rotor, m
VMEAN = Mean wind speed, m/s
GEAR = Gear down ratio
DENSW = Density of water, 1000 kg/[m.sup.3]
G = Acceleration of gravity, 9.8 m/[s.sup.2]
HEIGHT = Total pumping head, m
TSR = Design tip speed ratio
STROKE = Piston stroke length, m
Suppose the wind
machine of the previous examples has a gear
down ratio of
3.5:1, a design tip speed ratio of 1.0 and a
stroke of 30
cm. Then the diameter of the piston
[square root] (0.1) (3.14) [(4.1).sup.3] [(4.0).sup.2] (3.5) = .166m
Suppliers of Machinery
Once a type of machine has been selected, suppliers of the
or the designs should be contacted for information about
availability of equipment and spare parts in the region in
references, cost, etc.
If the machine is to be built locally,
sources of material, such as sheet steel, angle iron,
etc. will have to be identified.
Possible machine shops
should be visited and their work on similar kinds of
should be examined.
9. Identify Alternative
Power Sources for Water Pumping
There are usually a number of alternatives in any given
might be a good option depends on the specific
conditions. Some of
the possibilities include pumps using human
power (hand pumps), animal power (Persian wheels, chain
internal combustion engines gasoline, diesel, or biogas),
combustion engines (steam, Stirling cycle), hydropower
rams, norias), and solar power (thermodynamic cycles,
10. Evaluate Economics
For all the realistic options the likely costs should be
and a life cycle economic analysis performed.
The costs include
the first cost (purchase or manufacturing price), shipping,
operation (including fuel where applicable), maintenance,
spare parts, etc.
For each system being evaluated the
total useful delivered water must also be determined (as
in Step 6). The life
cycle analysis takes account of costs
and benefits that accrue over the life of the project and
them on a comparable basis.
The result is frequently expressed in
an average cost per cubic meter of water (Figure 3).