Back to Home Page of CD3WD Project or Back to list of CD3WD Publications

Home - English - French - German - Italian - Portuguese - Spanish
                        TECHNICAL PAPER # 62
                     UNDERSTANDING WIND ENERGY
                            FOR WATER PUMPING
                         James F. Manwell
                           Published By
                  1600 Wilson Boulevard, Suite 500
                    Arlington, Virginia 22209 USA
              Tel:  703/276-1800 * Fax:   703/243-1865
             Understanding Wind Energy for Pumping Water
                         ISBN:  0-86619-281-6
             [C] 1988, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance (VITA) to provide an introduction to specific
state-of-the-art technologies of interest to people in developing
countries.  The papers are intended to be used as guidelines to
help people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar
organization for further information and technical assistance if
they find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteers technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Margaret Crouch as
editor and project manager and Suzanne Brooks handling typesetting,
layout, and graphics.
The author of this paper, VITA Volunteer James F. Manwell, heads
the Renewable.  Energy Research Laboratory, Department of Mechanical
Engineering, at the University of Massachusetts in Amherst.
Dr. Manwell is also the co-author with his colleague Dr. Duane E.
Cromack of "Understanding Wind Energy," another paper in this
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
There are many places in the world where wind energy is a good
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 alternatives
should be undertaken.  The necessary steps include the following:
1.     Identify the users of the water.
2.     Assess the water requirement.
3.     Find the pumping height and overall power requirements.
4.     Evaluate the wind resources.
5.     Estimate the size of the wind machine(s) needed.
6.     Compare the wind machine output with the water
       requirement on a seasonal basis.
7.     Select a type of wind machine and pump f rom the
       available options.
8.     Identify possible suppliers of machines, spare
       parts, repair, etc.
9.     Identify alternative sources for water.
10.    Assess costs of various systems and perform economic
       analysis to find least cost alternative.
11.    If wind energy is chosen, arrange to obtain and install
       the machines and provide for maintenance.
The following summarizes the key aspects of these steps.
1.  Identify the Users
This step seems quite obvious, but should not be ignored.  By
paying attention to who will use the wind machine and its water
it will be possible to develop a project that can have continuing
success.  Questions to consider are whether they are villagers,
farmers, or ranchers; what their educational level is; whether
they have had experience with similar types of technology in the
past; whether they have access to or experience with metal working
shops.  Who will be paying for the projects?  Who will be owning
the equipment; who will be responsible for keeping it running;
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 a
single site.
2.  Assess the Water Requirements
There are four main types of uses for water pumps in areas where
wind energy is likely to be used.   These are:  1) domestic use, 2)
livestock watering, 3) irrigation, 4) drainage.
Domestic use will depend a great deal on the amenities available.
A typical villager may use from 15 - 30 liters per day (4-8 gallons
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 take 230
(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 the end
of 15 years, a reasonable lifetime for a water pump.
Basic livestock requirements range from about 0.2 liters (0.2
quart) a day for chickens or rabbits to 135 liters (36 gallons) a
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 depends
on a variety of meteorological factors as well as the types of
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 liters
(9,275 gallons) per day per hectare (2.47 acres) for fruits
and vegetables to 100,000 liters (26,500 gallons) per day per
hectare for cotton.
Drainage requirements are very site dependent.   Typical daily
values might range from 10,000 to 50,000 liters (2,650 to 13,250
gallons) per hectare.
In order to make the estimate for the water demand, each user's
consumption is identified, and summed up to find the total.  As
will become apparent later.  It is desirable to do this on a
monthly basis so that the demand can be related to the wind resource.
3.  Find Pumping Height and Total Power Requirement
If wells are already available their depth can be measured directly.
If new wells are to be dug, depth must be estimated by
reference to other wells and knowledge of ground water characteristics
in the area.  The total elevation, or head, that the
pump must work against, however, is always greater than the static
well depth.  Other contributors are the well draw down (the
lowering of the water table in the vicinity of the well while
pumping is underway), the height above ground to which the water
will be pumped (such as to a storage tank), and frictional losses
in the piping.  In a properly designed system the well depth and
height above ground of the outlet are the most important determinants
of pumping head.
The power required to pump water is proportional to its mass per
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 volume flow
rate of water ([m.sup.3]/s).   Power is also inversely proportional to the
pump efficiency.  Note that 1 cubic meter equals 1000 liters.
Expressed as a formula,
           Power = 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.
  Actual power required would be more because of the less than
  perfect efficiency of the pump.
Sometimes needed pumped power is described in terms of daily
hydraulic requirement, which is often given in the units of [m.sup.3],
m/day.  For   example, in the above example the hydraulic requirement
is 750 [m.sup.3.]m /day.
4.  Evaluate Wind Resource
It is well known that the power in the wind varies with the cube
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 pump
there.  It is sometimes recommended that a site should have an
average wind speed at the height of a wind rotor of at least 2.5
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 windmill
site, except by estimate and correlation.   Second, mean wind
speeds generally vary with the time of day and year and it makes
an enormous difference if the winds occur when water is needed.
The best way to evaluate the wind at a prospective site is to
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 most
practical approach may be to obtain wind data from the nearest
weather station (for reference) and try to correlate it with that
at the proposed wind pump site.   If at all possible the station
should be visited to ascertain the placement of the measuring
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 there for
a relatively short time (at least a few weeks) and comparing
resulting data with that taken simultaneously at the reference
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 by
the placement of the wells, but a few basic observations should
be kept in mind. The entire rotor should be well above the surrounding
vegetation, which should be kept as low as possible for
a distance of at least ten times the rotor diameter in all directions.
Wind speed increases with elevation above ground, usually
by 15-20 percent with every doubling of height (in the height
range of most wind pumps).  Because of the cubic relationship
between wind speed and power, the effect on the latter is even
more dramatic.
5.  Estimate Wind Machine Size
A typical wind pump is shown in Figure 1.   Most wind pumps have a

40p05.gif (600x600)

relates actual water
flow at given pumping
heads to the wind
speed.  This curve also
reflects other important
information such
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 possible in
predicting wind machine output.   On the other hand, it should be
noted that some manufacturers provide incomplete or overly optimistic
estimates of what their machines can do.   Sales literature
should be examined carefully.
In addition to the characteristic curve of the wind machine, one
must also know the pattern of the wind in order accurately to
estimate productivity.  For example, suppose it is known how many
hours (frequency) the average wind speed was between 0-1 m/s, 1-2
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 speed
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 accurately,
using only the long-term mean wind speed and, when available, a
measure of its variability (standard deviation).   See Lysen, 1983,
and Wyatt and Hodgkin, 1984.
Many times there is little information known about a possible
machine or it is just desired to know very approximately what
size machine would be appropriate.   Under these conditions the
following simplified formula can be used:
  Power = Area x 0.1 x [(Vmean).sup.3]
  Power = useful power delivered in pumping the water, watts
  Area   = swept area of rotor (3.14 x Radius squared), [m.sup.2]
  Vmean = mean wind speed, m/s
By rearranging the above equation, an approximate diameter of the
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 watts.  Suppose
the mean wind speed was 4 m/s.   Then the diameter (twice the
radius) would be:
  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 be
excess water.  This information is needed to perform a realistic
economic analysis.  The results may suggest a change in the size
of machines to be used.
Comparison of water supply and requirement will also aid in determining
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 considered.
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, which
is the ratio between the actual speed of the blade tips and the
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 relative
to their swept area, perform best at higher tip speed ratios
(such as 2.0).
A primary consideration in selecting a machine is its intended
application.  Generally speaking, wind pumps for domestic use or
livestock supply are designed for unattended operation.   They
should be quite reliable and may have a relatively high cost.
Machines for irrigation are used seasonally and may be designed
to be manually operated.  Hence they can be more simply
constructed and less expensive.
For most wind pump applications, there are four possible types or
sources of equipment.  These are:  1) Commercially available machines
of the sort developed for the American West in the late
1800s; 2) Refurbished machines of the first types that have been
abandoned; 3) Intermediate technology machines, developed over
the last 20 years for production and use in developing countries;
and 4) Low technology machines, built of local materials.
The traditional, American "fan mill," is a well developed technology
with very high reliability.  It incorporates a step down
transmission, so that pumping rate is a quarter to a third of the
rotational speed of the rotor.   This design is particularly suitable
for relatively deep wells (greater than 30m--100').   The main
problem with these machines is their high weight and cost relative
to their pumping capacity.  Production of these machines in
developing countries is often difficult because of the need for
casting gears.
Refurbushing abandoned traditional pumps may have more potential
than might at first appear likely.   In many windy parts of the
world a substantial number of these machines were installed early
in this century, but were later abandoned when other forms of
power became available.  Often these machines can be made operational
for much less cost than purchasing a new one.   In many
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 same
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 machines
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.   The machines
are made of steel, and require no casting and minimal welding.
Their design is such that they can be readily made in machine
shops in developing countries.   Many of these wind pumps have
undergone substantial analysis and field testing and can be considered
Low technology machines are intended to be built with locally
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 successful
than had been hoped.  If such a design is desired, it should first
be verified that machines of that type have actually been built
and operated successfully.  For a sobering appraisal of some of
the problems encountered in building wind machines locally, see
Wind Energy Development in Kenya (see References).
Although most wind machines use piston pumps, other types include
mono pumps (rotating), centrifugal pumps (rotating at high
speed), oscillating vanes, compressed air pumps, and electric
pumps driven by a wind electric generator.   Diaphragm pumps are
sometimes used for low head irrigation (5-106 m or 16-32').  No
matter what type of rotor is used, the pump must be sized appropriately.
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 head
increases the volume pumped will have to decrease accordingly.
The piston travel, or stroke, is generally constant (with some
exceptions) for a given windmill.   Hence, piston area should be
decreased in proportion to the pumping head to maintain optimum
Selecting the correct piston pump for a particular application
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:  1)
the rotor size (diameter); 2) the design tip speed ratio; 3) the
gear ratio; and 4) the stroke length.   The first two have been
discussed earlier.  The gear ratio reflects the fact that most
wind pumps are geared down by a factor of 3 to 4.   Stroke length
increases with rotor size.  The choice is affected by structural
considerations.  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 the
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.  This 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] [(VMEAN).sup.2] (GEAR)
                   (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 would be:
             DP = [square root] (0.1) (3.14) [(4.1).sup.3] [(4.0).sup.2] (3.5) = .166m
                                -------------------------- -------------  ----               
                                (1000)   (9.8)  (15)           (1.0)       (0.3)
8.  Identify Suppliers of Machinery
Once a type of machine has been selected, suppliers of the equipment
or the designs should be contacted for information about
availability of equipment and spare parts in the region in question,
references, cost, etc.  If the machine is to be built locally,
sources of material, such as sheet steel, angle iron, bearings,
etc. will have to be identified.   Possible machine shops
should be visited and their work on similar kinds of fabrication
should be examined.
9.  Identify Alternative Power Sources for Water Pumping
There are usually a number of alternatives in any given
situation.  What 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 pumps),
internal combustion engines gasoline, diesel, or biogas), external
combustion engines (steam, Stirling cycle), hydropower (hydraulic
rams, norias), and solar power (thermodynamic cycles,
10.  Evaluate Economics
For all the realistic options the likely costs should be assessed
and a life cycle economic analysis performed.   The costs include
the first cost (purchase or manufacturing price), shipping, installation,
operation (including fuel where applicable), maintenance,
spare parts, etc.  For each system being evaluated the
total useful delivered water must also be determined (as described
in Step 6).  The life cycle analysis takes account of costs
and benefits that accrue over the life of the project and puts
them on a comparable basis.  The result is frequently expressed in
an average cost per cubic meter of water (Figure 3).

40p11.gif (600x600)