TECHNICAL PAPER # 62
 
                     UNDERSTANDING WIND ENERGY
                            FOR WATER PUMPING
 
                                By
                         James F. Manwell
 
                           Published By
                 VOLUNTEERS IN TECHNICAL ASSISTANCE
 
                               VITA
                  1600 Wilson Boulevard, Suite 500
                    Arlington, Virginia 22209 USA
              Tel:  703/276-1800 * Fax:   703/243-1865
                      Internet:   pr-in@vita.org
 
             Understanding Wind Energy for Pumping Water
                         ISBN:  0-86619-281-6
             [C] 1988, Volunteers in Technical Assistance
 
                               PREFACE
 
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
series.
 
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.
 
           UNDERSTANDING WIND ENERGY FOR WATER PUMPING
 
I.  OVERVIEW
 
 
There are many places in the world where wind energy is a good alternative for
pumping water. Specifically 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 from 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 for obtaining and installing the
    machines and for providing for their maintenance.
 
Decision Making Process
 
The following summarizes the key aspects of those suggested 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
 
Example:
 
    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] [multiplied by] m/day. For example, in the above
example the hydraulic requirement is 750 [m.sup.3] [multiplied by] 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 the 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 can 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 Machines Size
 
A typical wind pump is shown in Figure 1. Most wind pumps have a horizontal

40p05.gif (600x600)


axis (that is, the rotating shaft is parallel to the ground). Vertical axis machines,
such as the Savonius rotor, have usually been less successful in practice.
 
In order to estimate wind machine's size it is first necessary to have some idea
how it will perform in real winds. As previously mentioned, the power in wind
varies with the cube of the wind speed. It is also proportional to the density of
the air. Atmospheric density is 1.293 kg/[m.sup.3] at sea level at standard conditions but
is affected by temperature and pressure. The power that a wind machine produces,
in addition, depends on the swept area of its rotor and the aerodynamic characteristics
of its blades. Under ideal conditions the rotational speed of the rotor
varies in direct relation to the wind speed. In this case the efficiency of the
rotor remains constant and power varies as the cube of the wind speed (and
rotational speed).
 
With wind pumps, however, the situation is more complicated. The majority use
piston pumps, whose power requirements vary directly with the speed of the
pump. At high wind speeds the rotor can produce more power than the pump can
use. The rotor speeds up, causing its efficiency
to drop, so it produces less power. The
pump, coupled to the rotor, also moves more
rapidly so it absorbs more power. At a
certain point the power from the rotor equals
the power used by the pump, and the rotational
speed remains constant until the wind
speed changes.
 
The net effect of all this is that the whole
system behaves rather differently than an
ideal wind turbine. Its actual performance is
best described by a measured characteristic
curve (Figure 2), which relates actual water

40p06.gif (486x486)


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]
            where
    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])]
               or
    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
reliable.
 
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 Sources).
 
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 (5-10 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 performance.
 
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 of] (0.1) ([pi]) (DIAMR)[sup.3] (VMEAN)[sup.2] (GEAR)
                     --------------------------------------------------
                        (DENSW) (G) (HEIGHT) (TSR) (STROKE)
 
      where:
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
 
Example:
 
    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 of] (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, photovoltaics).
 
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)


 
It should be noted that the most economic option is strongly affected by the size
of the project. In general, wind energy is seldom competitive when mean winds
are less than 2.5 m/s, but it is the least cost alternative for a wide range of
conditions when the mean wind speed is greater than 4.0 m/s.
 
11. Install the Machines
 
Once wind energy has been selected, arrangements should be made for the
purchase or construction of the equipment. The site must be prepared and the
materials all brought there. A crew for assembly and erection must be secured,
and instructed. Someone must be in charge of overseeing the installation to
ensure that it is done properly and to check the machine out when it is up.
Regular maintenance must be arranged for.
 
With proper planning, organization, design, construction, and maintenance, the
wind machines may have a very useful and productive life.
 
Source:
 
James F. Manwell, VITA Volunteer, University of Massachusetts.
 
References:
 
Fraenkel, Peter. Water-Pumping Devices: A Handbook for Users and Choosers.
London: Intermediate Technology Publications, 1986.
 
Johnson, Garry. Wind Energy Systems. Englewood Cliffs, New Jersey: Prentice
Hall, Inc.
 
Lierop, W.E. and van Veldheizen, L.R. Wind Energy Development in Kenya, Main
Report, Vol. 1: Past and Present Wind Energy Activities, SWD 82-3/Vol. 1
Amersfoort, The Netherlands: Consultancy for Wind Energy in Developing Countries,
1982.
 
Lysen, E.H. Introduction to Wind Energy. SWD 82-1 Amersfoort, The Netherlands:
Consultancy for Wind Energy in Developing Countries, 1983.
 
Manwell, J.F. and Cromack, D.E. Understanding Wind Energy: An Overview.
Arlington, Virginia: Volunteers in Technical Assistance, 1984.
 
McKenzie, D.W. "Improved and New Water Pumping Windmills," Proceedings of
Winter Meeting, American Society of Agricultural Engineers, New Orleans,
December, 1984.
 
Vilsteren, A.V. Aspects of Irrigation with Windmills. Amersfoort, The Netherlands:
Consultancy for Wind Energy in Developing Countries, 1981.
 
Wegley, H.L., et al. A Siting Handbook for Small Wind Energy Conversion Systems.
Richland, Washington: Battelle Memorial Institute, 1978.
 
Wyatt, A.S. and Hodgkin, J., A Performance Model for Multiblade Water Pumping
Windmills. Arlington, Virginia: VITA, 1984.
 
Groups Involved with Wind Pumping in Developing Countries
 
Consultancy for Wind Energy in Developing Countries, P.O. Box 85, 3800 AB,
Amersfoort, The Netherlands
 
Intermediate Technology Development Group, Ltd., 9 King Street, Coven Garden,
London, WC2E 8HW, UK
 
IPAT, Technical University of Berlin, Sekr. TH2, Lentzallee 86, D-1000 Berlin 33,
West Germany
 
Renewable Energy Research Laboratory, Dept. of Mechanical Engineering, University
of Massachusetts, Amherst, Massachusetts 01003, USA
 
SKAT, Varnbuelstr. 14, CH-9000 St. Gallen, Switzerland
 
The Danish Center for Renewable Energy, Asgaard, Sdr. Ydby, DK-7760 Hurup
Thy, Denmark
 
Volunteers in Technical Assistance (VITA), 1815 N. Lynn Street, Suite 200,
Arlington, Virginia 22209-2079 USA
 
Manufacturers of Water Pumping Windmills
 
Aermotor, P.O. Box 1364, Conway, Arkansas 72032, USA
 
Dempster Industries, Inc., Beatrice, Nebraska 68310, USA
 
Heller Aller Company, Perry & Oakwood St., Napoleon, Ohio 43545, USA