solar powered water pumps

7/28/2019 Solar Powered Water Pumps

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SOLAR POW ERED WATER PUM PS: PROBLEMS, PITFALLS AND POTENTIAL

T. D. Short, M.A. MuellerUniversity of Durham, United Kingdom

ABSTRACT

For many years, solar (photovoltaic) powered waterpumping has been portrayed as being able torevolutionise water provision in rural and developingcommunities. Mass produced pumps and cheaper PVpanels have been promised, with the possibility ofbringing safe water to those people who currently lackthis basic human right. Although inroads have been

made to reaching such an ideal situation, the currentreality is somewhat different.This paper will considerthe challenges faced by electronic and electricalcomponents in a solar powered water pumping system.It will:

1. review how these problem have beenaddressed historically;

2 . investigate the ways in which the solutionshave failed

3. explore novel ways of utilising modemelectrical systems in order to allow fullexploitation of this potentially life-transforming technology.

INTRODUCTION

Very little has changed in recent yearsin the provisionof pumping energy through photovoltaics (PV). Whilstth e use of PV water pumps (P W s) has grown steadily

since their fust introduction in the late 1970s thetechnology has by no means reached itsfull potential.Moreover, present levels of penetration contraststrongly with the 1978 aim to installIO million unitsby the year 2000 [I]. By 1998 it was estimated that atotal of only 60,000 units had been sold since 1978[2].Whilst this goes some way to addressing the lack ofsafe water currently faced by up to1.2 billion peopleworld-wide [3], a recent study [4] suggests that nearlyhalf the pumps in some areas a re no longer functioning,only ten years after installation. So what is the

problem? Where are these system going wrong andfailing those who de pend on them?

PRESENT SYSTEMS : COMPONENTS &PROBLEMS

Solar water pumping systems are nominally verysimple. Figure 1 is a schematic of such a system,composed of a solar array, a motor,a pump and areservoir.The use of the reservoir removes the need fora battery, whilst the controllers presence is depende nton the type of motor-pump combination chosen andwhether or not Maximum Power Point Tracking isrequired. Each of the electrical components in thissystem has its own characteristics, and all are furtherdependent - hrough the motor - on the type of pumpemployed.

Figure 1 PV pu mp system schematic

Power Electroni cs. Machines and Drives, 16-1 8 April 2002,Conference Publication No.487.Q IEE 2002

c

c

L

Figure 2 PV and pump characteristics


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The photovoltaic array water collection, with the resultant health and socialimplications. The further potential for drasticconsequences ... for cattle and crops [ 9 ] onlyenhances the view that down time should be kept toaminimum and that reliability IS key. It seemsremarkable, therefore, that having mentioned the

failure of 17 out of 90 inverters in field testing, Hahn isable to conclude that PV P standard systems [i.e. usingAC motors driving centrifugal pumps] havedemonstrated their technical maturity and reliability.Kaunmuang [4] addsto this, noting that in a survey of48 9 PVPs, 220 units (45%) have failed. Most failuresare due to blockage of pumps and pipes and inverterfailures. The nature of places likelyto use PVPs doesnot ease the situation, with conditions likely to be

The PV panel has alolown DC current-voltage (I-V)characteristic such as that shown in Figure 2 , varyingwith the radiation incident on the panel and with aMaximum Power Point (MPP) curveas shown by thegrey line. If there is a direct connection between thearray and the motor then the operating point of thepanel is dependent on the characteristics of the motorand is unlikely to match the MPP. This canalso beseen in Figure 2 where the dashed lines show the I-Vdemand from typical DC permanent magnet motorsdriving centrifugal and reciprocating pumps.

The motor

Whilst the output from the PV array is DC, the motormay not be, depending on the systemas purchased.Several different possible motor types exist, each withtheir own problem. An immediate addition to expenseis that local electricians in developing countries are notalways trained to install DC systems, requiring eitherthe cost of specialist help [5], or of additional training.Should the motor be supplied with DC electricity, itcould be brushed (with inherent losses at the brushesand regular maintenance requirementsas brushes wearaway); brushless (requiringa complex controller [6]);separately excited (possibility of good MPP matching,but requiring a complex controller [7]); or switchedreluctance (also requiring a control circuit[SI) to namebut a fe w variations.

extremely difficult. Hammad[101 quotes temperatureas hot as 40C and dust blown froma constant desertwind, both of which are potential contributorstocontroller failure.

There are consequently two lessons to be leamt

regarding electronics and controllers:If any kind of controller is to be used, be itfor a D Cbrushless motor, an inverter for AC provision or aspecialised MPP Tracker (MPPT), it must beinherently extremely reliable, or easilyreparable/replaceable at local level. This placessevere limitations on such a controller;Given these limitations, it must be questionedwhether or not a controller should be used atall.Indeed, whilst electronic systems should haveincreased in dependability since Roger concludeddirect coupling petw een motor and am y ] is themost reliable pumping technique [9], the evidencepoints otherwise.

An alternative is to use AC motors. Whilst the motoritself may be cheap, reliable and, in contrast to the DCmotor, simple, it requires an inverter which is

Discussion

potentially complex and hence expensive. In addition,according to Metwally [8] [als the inverter becomes Previously designed systems appear to have matchedcomp lex the reliability in service decrease s. This off-the-shelf comp onents, in order to standardise thestatement encompasses a considerable number of systems and reduce costs. However, it may not be theproblems encountered by PVPs in use in the field optimum solution for the application and the

which will now be considered. environment in which the system is being used. Drivesystems for PV powered pumps operating ina rural

The inverter a ndlor controller

Whilst it is clear that the optimum PVF design wouldbe a PV array directly connected to a DC motor, theproblem of matching the MPP curve of an arraystillremains. The added expense and complexity of somekind of controller is somewhat irrelevant in moredeveloped countries as spares and repairs are easy toaccess. This is not the case -in developing countries,however. If anything goes wrong with the pumpsystem, it may be a considerable time before anyone isavailable to inspect the pump. During this time, andany further time required to source replacement parts,water users must resort to more traditional methods of

community in the developing world have to be cheap,reliable and maintainable by an untrained user.Reductions in cost and gains in reliability can begained by minimising the complexity of powerelectronics and reducing tbe number of electro-mechanical stages in the drive system. Ideally thesystem shouldalso exhibit high performance.

Based on these constraints the drive system should bedc powered, brushless and placed in the boreholedirectly coupled to a reciprocating piston as suggestedby W hitfield eta1 [I I] . Moving beyond the assumptionof a rotational motor, however, the drive system couldhe based on the brushless permanent magnetorvariable reluctance principle, whilst the thrust forcecould be generated using either tangential forces fro maconventional linear motor topology or normal forcesfrom an electromagnet topology.


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Perris and Salameh [I21 proposed replacing a rotarymotor and gearbox with a linear dc motor drive systempositioned at ground level and c oupled to the pump viaa long connecting rod. Whilst this has moved towardsthe desired system, the use of a connecting rod is not

desirable in deep bore holes, say up to 100m. It is alsounclear whether the proposed m otor is brusbless.

RECIPROCATING INDUCED FLOW PUMP.

bead and the second term is the force dueto thesubmerged head. Apt. is the cross sectional area o f thepipe at the outlet valve andA,,i3,0n s the cross sectionalarea of the piston.

In order to investigate a suitable linear actuator a

typical specification of the pumping requirements isrequired. Boreholes are typically100 or 15 0 mm indiameter, with typical heads of 100 m. The sweptvolume at the piston typically vanes from 0.1 to 0.5U s , The frequency of operation can typically vary from10 to 20 Hz.

System DescriptionLinear Actuator.

Figure 3 shows the components in a reciprocatingpump. Motion of the piston is achieved using a linearactuator. During pumping the piston moves upwards,the outlet valve opens and the inlet valve closes.During the down stroke the outlet valve closes, and atthe same time the pressure in the chamber equalises tothe pressure represented by the submerged head. Theinlet valve then opens. Beneath the water chamber liesa sealed chamber containing the control system ifrequired

1I I

Figure 3 : Reciprocating InducedFlow Pump

The wo rst case static force acting on the piston is givenby equation 1. In order to open the outlet valve thethrust force from the actuator must be at least equal tothis force.

F = pgh,Apipe+ pghzApi,,, 1

The first term represents the force dueto the outlet

The function of the linear actuatoris to produce a forcewhich has to overcome the pressure of the submergedbead and the pressure of the outlet head behind theoutlet valve. The thrust force could be generated usingeither tangential forces from a conventional linearmotor topology or normal forces from anelectromagnet topology. In the latter case there wouldbe no physical contact between the actuator and thepiston.

A starting point for any de sign studyis to consider thestress capability of the options available. Forconventional linear motor topologies, a variablereluctance machine has a shear stress in the region of40 kN/mz, whereas machines based on the variablereluctance permanent magnet (VRPM) principleexhibit shear stresses in excess of 100 khVm'[13,14]. Ifan airgap of 1T could be achievedin the electromagnet

topology, a normal stress of400

kN/mz would beachieved. Clearly the latter option offers the bestchoice in terms of desired tlnust an d the ability to fitinside the physical envelope available in the pumpingchamber.

Variable reluctance electromagnet actua tor

Figure 4 shows the proposed electromagnet actuator inmore detail. The actuator is supplying the force todisplace the water and hence provide pumping power.The thrust force is provided by the force of attractionbetween the iron barand the C-core and is calculated

from Maxwell Stress:

2

where B is the airgap flux density and po is thepermeability of air.


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-

m m*d,am/er

:e 4 Variable reluctance electromagnet ach

The a irgap flux density is calculated from a reluctancenetwork of the device, show n infigure 5 . It is assumedin this simple network that all the flux flows in thelimbs of the C-core and o nly crosses the airgaps under

the poles; no account of fringing is considered.Saturation is considered very simply by choosing theiron permeab ility to limit theflux density in the iron to2.5 T.

n o r ,

r,. p o k L

0. iq aS. ,mn *r

Figure 5 : Reluctance network of the electromagnetactuator

The actuator is providing the pumping actionso thatthe iron bar has to move through a displacement equalto the pump stroke. The airgap operating range istherefore equal to the stroke, which affects theresulting force produced.

Electrically the a ctuator can be modelled as a resistorand inductance in series with the solar m a y providingthe voltage supply. The solar array consists of anumber of solar panels connected in series to providethe necessary voltage and a-number of these stringsconnected in parallelto provide the required current.At a certain solar irradiationa constant current isgenerated, but the current rise is limited by the coilinductance. Since the airgap changes the coilinductancealso changes. In order to achieve optimum

performance the coil cnnent should rise to the sourcecurrent as early as possible in the pumping period. Byconsidering the inductance at the maximum airgapposition a voltage can be chosen to ensure that thedesired current is achieved in an ap proporiate time.

This brief overview highlights that the design of thesystem is highly dependant on the individualcomponents in the system, which is illustratedfurtherin a basic design study.

DESIGN STUDY

The actuator cannot fill the whole spa% of theborehole, a path is required for the water around theperime ter of the actuator. Propose d dimen sions fora10 0 nnn borehole are given in Table 1. Thesedimensions have been used throughout the designstudy.

Table 1 : Typical dimensions (inmm )

Because of the physical limitations it is anticipated thatthe iron will he saturated, andthe slot dimensions arechosen to ensure a reasonable temperature rise.

The airgap is dependent on the displacement of thepump piston. For exam ple at 10Hz operation thepiston stroke is 6.4 mm in a l O O m m borehole and 2.4nnn in a 150 mm borehole for a swept volume of0.5U s in each case. With the actuator providing thepumping action the iron bar moves through adisplacement equal to the stroke.

Figure 6 hows how the force produced by the actuatorvaries from an airgap of 8mm to Im m for eachborehole. In each case the number ofrums is set to500 and the maximum coil current is equal to 13A.Saturation of the magnetic circuit limits the forc eavailable.


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120w

8000

- l 0 ~ O ~ 10w

0 2 4 6 8 10airgap (mm)

I. . . . 100 m m -1sOmm I

U0 4000

. - - . . _____.0000

Figure 6 : Force vs airgap

At the maximum airgap the force produced has toovercome the pressure of the submerged depth and thepressure of the outlet head behind the outlet valve. Forthe 1 0 0 " borehole a force of 77.1N is required per

metre of submerged depth. The force hehind the outletvalve depends upon the diameter of the pipe at thatdepth. Knowing the stroke and the force at this airgapa working head can be calculated for different pipediameters.

Figures 7 & 8 show the relationship hetween head andpipe diameter for different swept volumes atfrequencies of 10Hz and 20 Hz respectively.

As the swept volume increases the stroke increases fora fixed frequency. The thrust force from the actuatordecreases therefore and only by decreasing the pipediameter can the pump operate at high heads. Thereisa penalty to be paid in terms of flow rate at the outletpipe. At 20 Hz the stroke is smaller for the equivalentswept volume compared to10 Hz, leading to a greaterthrust force from the a ctuator and better performance athigher heads.

10 0

80

I O

- 6 0 -0 .1 5 11sE . . .2 1s030

_ 50

0 20 4 0 60 80

d iamete r ( m m l

Figure 7 : Head vs pipe diameter at different sweptvolumes at f=lOHz

80

lo

60

I .

. .4030

20

10

I

. . .3 11sml- 0.35 1s0 50 100

diameter ( m m )

Figure 8 : Head vs pipe diameter at different sweptvolumes at f=20Hz

The current required to achieve the starting force islimited by the inductance of the coil when the airgap isat a maximu m. A simple analysis of the magnetic

circuit shows that at an airgap of 8" the coilinductance is equal to 40mH for a 100 mm borehole.However, as the piston moves the airgap decreases andso the inductance increases. In addition the voltagewill decrease as the current increases because of thesolar panel I-V characteristic (figure 2) . A fulldynamic model is required to investigate the currentwaveform during the pumping action, which is beyondthe scope of this paper.

As part of the design study it is sufficient to determinethe minimum voltage required for the coil current torise to its required value before pumping starts whenthe airgap is equal to the stroke. In order to start thepumping cycle the current must rise to the required

value in the shortest time possible, so as not tointroduce too long a delay in the pumping cycle.Setting the rise time equal to the time constant shouldnot present a significant delay. The coil resistance is1.2 R giving a time constant of0.033 seconds. ThePVvoltage required is therefore at least 24V.

Once the coil current rises to the maximum availablefrom the PV panel some control may be required tomaintain this current taking into account the risinginductance and operating point on the I-Vcharacteristic.

Design Summary

Based on the fmdings in the design study performanceparameters have been generated using the simplemodel for a low and high head. The results are shownin Table 2. The results imply that the pump system canoperate over a wide rangeof beads, only requiring anincrease in.supply voltage as the head increases.


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pumping - successes and problems". in

3.

4.

Table 2 : Performance at low and high heads in aIOOmmborehole .

5.

Control System

For ease of maintenance the control system should bevery simple. Its main function is to de-energise theactuator once the stroke has been achieved. A tripswitch is the simplest methodof achieving this control.

However, the energy stored in the actuator coil has tobe removed as soon as possible, which could beachieved by dum ping the energy into a resistor. Thepiston will then fall under the pressure of thesubmerged head. When the piston has reached itsoriginal position the control system should switch thesupply back across the coil provided the current hasreached zero. Some form of current sensing will herequired to ensure that the coil current has fallen tozero before re-energisation in the next pump cyc le.

CONCLUSION

The results of the design study show that the linearelectromagnet actuator has the potential to' meet thespecification of an ideal pumping system. It is fullysubmerged; it is a directly coupled systemmechanically and electrically and it is very simple inconstruction. The proposed actuator design fora 100mm borehole can provide enough force to pump at arange of heads and swept volumes, provided the pipediameter and pumping frequency are. chosenappropriately. Further workis required to investigatein more detail the dynamic performanceof the actuatorand the control system.

In conclusion a solution has been proposed, whichshows potential in termsof reliability, cost and e ase of

maintenance.

REFERENCES

1. Barlow, R., McNelis, B., and Derrick, A.,"Solar Pumping: An Introduction and Updateon the Technology, Performance, Costs andEconomics." World Bank Technical PaperNumber 168. 1993, London: IntermediateTechnology Publications Ltd.Derrick, A., "20 yearsof PV powered water.

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Report, ISBN 924 156202 1,200 0.Kaunmuang, P., et al., "Assessment ofphotovoltaic pumping systems in Thailand-one decade experience." Solar EnergyMaterials and Solar Cells, 2001. 67(1-4): p p ~529-534.Cawood, W., Maphephetheni: "Renewahles atwork in a South African rural developmentprogram" , in Renewable Energy World.2001. pp. 86-99.Langridge, D.. Lawrence, W., and Wichert,-W., "Development of a Photovoltaic PumpingSvstem using a Brushless DC Motor andHelical Rotor Pump." Solar Energy, 1996.56(2): pp. 1 51-160.Akbaba. M.. Oamber. I.. and Kamal. "A.., . ,I .Matching of Sepa rately Excited DC M otors toPhotovoltaic Generators for Maximum PowerOutput." Solar Energy, 1998. 63(6):pp. 375-385.Metwally, H.M.B. and Anis, W.R.,"Performance Analysis of PV PumpingSystems using Switched Reluctance MotorDrives." Solar Energy, 1996.56(No. 2): pp.161.168.Roger, J.A., "Water and Photovoltaics in

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-developing-countries." Solar Cells, 1982.6(3): pp. 295-308.Hammad, M.A., "Characteristics of solarwater pumping in Jordan." Energy, 1999.24(2): pp. 85-92.Whitfield, G.R., Bentle y, R.W., and M ogotsi,B. "Improving the cost-effectiveness of smallsolar photovoltaic water pumping systems. inRenewable" Energy Sources '91. 1991:Prentice, pp. 5 1-56.Penis, C. and Salameh, Z., "Photovoltaic-powered Piston-type Water Pump Controlledby a L inear Motor." Progress in Photovoltaics:Research and Applications, 1995.3: pp. 265-271.Weh, H., Hoffman, H., and Landrath,J. "NewPermanent Magnet Excited SynchronousMachine with High Efficiency at Lower

Speeds." in Intemational Conference onElectrical Machines. 1988. Pisa, Italy,Septembe r 1988, pp. 35-40.Suooner. E. "Hieh Toraue Machines". in

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