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I nternational Journal Of Engineeri ng And Computer Science I SSN:2319-7242

Volume 2 I ssue 2 Feb 2013 Page No. 416-432

EFFICIENCYIMPROVEMENT OF PELTON WHEEL ANDCROSS-

FLOW TURBINES IN MICRO-HYDRO POWER PLANTS: CASESTUDYLoice Gudukeya,Ignatio Madanhire

2

University of Zimbabwe, Department of Mechanical EngineeringP O Box MP169,Mount Pleasant, Harare, Zimbabwe

[email protected]

2University of Zimbabwe, Department of Mechanical EngineeringP O Box MP169,Mount Pleasant, Harare, Zimbabwe

[email protected]

Abstract:This research study investigated hydro power plant efficiency with view to improve on the power output while keeping the overallproject cost per kilowatt produced within acceptable range. It reviews the commonly used Pelton and Cross-flow turbines which areemployed in the region for such small plants. Turbine parameters such as surface texture, material used and fabrication processes are dealtwith the view to increase the efficiency by 20 to 25 percent for the micro hydro-power plants.

Keywords:hydro, power plant, turbine, efficiency, manufacture, micro

1.Introduction

Micro-hydro power plants are an attractive option forproviding electricity in off grid areas of the country [2]. Thesimple Pelton and Cross-flow turbines are predominantlyused for these projects as they are cheaper to construct forthis form of renewable energy. Current level of efficiency isestimated to be 60%, thus allowing for improvements on the

overall efficiency of whole micro-hydro system.At 60% turbine efficiency micro-hydro schemes seem to beunderutilising resources. Communities are benefitting byhaving their business centres, clinics and schools powered.However more electrical power can be attained withoutincreasing the resources but by only increasing the turbineefficiencies [4]. This can ensure that more households in thecatchment area get more than just lighting, but are able to useother devices such as refrigerators, stoves etc in their houses.

2. Justification

The improvement of turbine efficiencies will increase themicro-hydro scheme power output while keeping the project

within the monetary budget. Therefore more electricity isgenerated from the same resources head (H) and flowrate (Q)

The design of a micro hydro scheme should ensure betterenvironmental sustainability as the use of hydro energyenables communities to rely less on non-renewable sourcessuch as firewood, paraffin and coal thus ensuring that lessCO2 is emitted into the atmosphere leading to less globalwarming [3]. Renewable energy is also a cleaner and saferform of energy.

3. Overview of micro-hydro power plant

Hydro power is the harnessing of energy from falling water,

such as water falling through steep mountain rivers. Theenergy in flowing water is converted into useful mechanicalpower by means of a water wheel or a turbine [5]. Themechanical power from the turbine can be converted intoelectricity using an alternator or a generator.

Hydro power systems are classified as micro has the powergeneration capacity of less 100kW. They are relatively small

power sources that may be used to supply power to a smallgroup of users or communities, who are independent of thegeneral electricity supply grid [7].A micro-hydropower system (MHS) has the followingcomponents [8]:

A water turbine that converts the energy of flowingor falling water into mechanical energy. This drivesan alternator, which then generates electrical power

A control mechanism in the form of electronic loadcontroller to provide stable electrical power.

Electrical distribution linesTo develop an MHS the following features are needed asgiven in Figure 1 [9]:

an intake or weir to divert stream flow from thewater course;

a headrace, the canal or pipeline to carry the waterto the forebay from the intake;

a forebay tank and trash rack (gravel trap) to filterdebris and prevent it from being drawn into theturbine at the penstock pipe intake;

a penstock (pipe) to transport the water to thepowerhouse. This may be set up above the groundsurface or underground depending on thetopography of the site. At rocky sites, penstocks aresupported above ground on concrete blocks calledAnchors or Saddle (Pier) supports. The saddlesupports are provided along the straight length atregular intervals and anchors are provided athorizontal and vertical bends along the alignmentof the penstock. These are designed to carry thethickness and the diameter of the penstock

a powerhouse, being the building thataccommodates and protects the electro-mechanicalequipment, (turbine and generator), that convert the

power of the water into electricity;
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a tailrace through which the water is released backto the river or stream without causing erosion;

Figure 1:Components of a micro-hydro power system [8]

Figure 2:The inside of a power house[9]

3.1 Power generation

A hydro scheme requires water flow and a drop in height(head) to produce useful power. It is a power conversionsystem, absorbing power in the form of head and flow, anddelivering power in the form of electricity.To measure the potential power and energy on a site the firststep is to determine the flow rate and the head through whichthe water falls. These two parameters are defined thus:

Flow rate (Q) is the quantity of water flowing pasta point at a given time. Q is measured in cubicmetres per second (m3/s)

Head (H) is the vertical height from the level wherethe water enters the penstock at forebay tank to thelevel of turbine centerline. The typical unit ofmeasurement is metres (m).

Figure 3: Concept of the head[8]

The power available from a micro-hydro power system isdirectly related to the flow rate, the head and the force ofgravity as expressed by the power equation as below:

=

wherePgross is the theoretical power produced(kW)wateris the density of water (kg/m

3)

Q is the flow rate in the penstock pipe(m3/s)g is the acceleration due to gravity(9.81m/s

2)

hgross is the total vertical drop from intaketo turbine (m)

No power conversion system delivers 100% useful power as

some power is lost by the system itself in the form offriction, heating, noise etc. The efficiency of the systemneeds to be taken into account, since all the equipment usedto convert the power from one form to another do so at lessthan 100% efficiency. Figure 4 shows the power losses andefficiencies within a micro-hydro power system [7]:

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Figure 4: Power losses and efficiencies in a micro-hydro

system [7]

Typical system component efficiencies are shown in Table 1:

Table 1: Typical system component efficiencies [5]

System Component Efficiency

Canal 95%

Penstock 90%

Turbine 60 -80%

Generator 85%

Step-up and down transformers 96%

Transmission 90%

To determine the actual power output from the MHS thetheoretical power must be multiplied by the systemefficiency factor, total:

=

where Pactual is the actual power produced (kW)

water is the density of water (kg/m3)

Q is the flow in the penstock pipe (m3/s)

g is the acceleration due to gravity (9.81m/s2)

hgross is the total vertical drop from intake to

turbine (m)

= x x x x x

It is common to assume an overall system efficiency of 50 60%. However, in practice actual power output may be aslow as 30% of gross input power for very small installationsand as high as 70% for larger schemes [3]. Therefore, duringthe detailed design stage, it is important to recalculate the

power output based on the actual design and manufacturersdata for the proposed equipment.The turbine efficiency is typically the lowest of thecomponent efficiencies [7]. Hence it has the highest chancesof being improved and once this has been done, the overallsystem may be improved.

3.2 The turbine

The turbines convert energy in the form of falling water intorotating shaft power. They basically consist of the followingcomponents [2]:

intake shaft - a tube that connects to the piping orpenstock which brings the water into the turbine

water nozzle - a nozzle which shoots a jet of water(Impulse type of turbines only)

runner - a wheel which catches the water as itflows in causing the wheel to turn (spin)

generator shaft - a shaft that connects the runner tothe generator

generator- a unit that creates the electricity exit valve - a tube or shute that returns the water to

the stream from where it came

powerhouse - a small shed or enclosure to protectthe water turbine and generator from the elementsSelection of the turbine depends on the site characteristics,the main factors being the head available, the flow rate andthe power required as well as the speed at which the turbineis desired to run the generator [6]. All turbines have a power-speed characteristic and an efficiency-speed characteristic.For a particular head they will tend to run most efficiently ata particular speed, and require a particular flow rate.Water turbines are often classified as being eitherReactionorImpulseturbines, depending on whether the runner isfully immersed in water or whether it operates in air.

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3.2.1 Reaction turbine

In a reaction turbine, the runners are fully immersed in waterand are enclosed in a pressure casing. The runner blades areangled so that pressure differences across them create liftforces, like those on aircraft wings, and the lift forces causethe runner to rotate [5]. The casing is scrolled to distributewater around the entire perimeter of the runner hence therunner and the casing are carefully engineered so that the

clearance between them is minimized. Examples of reactionturbines are the Francis and Kaplan type of turbines.

In a Francis turbine shown in Figure 5, water enters aroundthe periphery of the runner, passes through the guide vanesand runner blades before exiting axially from the centre ofthe runner. The water imparts most of its pressure energy tothe runner and leaves the turbine via a draught tube. AFrancis turbine is most effective on medium to high head ofwater.

Figure 5: Francis turbine [7]

A Kaplan turbine as shown in Figure 6 is a reaction turbinethat is a propeller type which consists of a propeller fitted

inside a continuation of the penstock tube, its shaft taken outwhere the tube changes direction. Usually three to six bladesare used, three in the cases of very low head units. Waterflow is regulated by the use of swiveling gates upstream ofthe propeller. Kaplan turbines are more sophisticatedversions of propeller turbines and are used in large-scalehydro sites. Swiveling of the propeller blades simultaneouslywith wicket gate adjustment has the effect of maintaininghigh efficiency under part-flow conditions. Wicket gates arecarefully profiled to induce tangential velocity of whirl inthe water.

Figure 6: Kaplan turbine

3.2.2 Impulse turbine [4]

In an impulse turbine, the runner operates in air and is turnedby one or more jets of water which make contact with the

blade. The water remains at atmospheric pressure before andafter making contact with the runner blades. In this case,

pressure energy is converted into the kinetic energy of a highspeed jet of water in the form of a nozzle. Then it is

converted to rotation energy in contact with the runnerblades by deflection of the water and change of momentum.The nozzle is aligned so that it provides maximum force onthe blades. An impulse turbine needs a casing only to controlsplashing and to protect against accidents.

3.2. 3 Advantages of impulse over reaction turbine

Impulse turbines are cheaper than Reaction turbines because

no specialist pressure casing and no carefully engineeredclearances are needed. These fabrication constraints makeimpulse turbines more attractive for use in micro-hydrosystems. Other advantages of using impulse turbines overreaction turbines in micro-hydro systems are that impulseturbines are more tolerant to sand and other particles in thewater. They also allow better access to working parts and areeasier to fabricate and maintain [3].They are also less subject to cavitations. They have flatterefficiency curves if a flow control device is built in (e.gnozzle area change, spear valve, change of number of jets,guide vanes, partitioning of flow).The major disadvantage of the Impulse turbines is that they

are mostly unsuitable for low head-to- power ratios.However, the Cross-flow and multi-jet Pelton are impulseturbines that are suitable for medium head-power ratios. Animpulse turbine can also operate on a low head, if the powertransmitted is also low and slow speed is acceptable.It is apparent then that for micro-hydro projects, Impulseturbines are more suitable than Reaction turbines. Table 4shows groups of Impulse turbines and the head levels they

best function under.

Table 2: Types of impulse turbines and optimum operatingheads

Turbine

Runner

Head (Pressure)

High Medium Low

Impulse Pelton

Turgo

Multi-jet Pelton

Cross-flow

Turgo

Multi-jet Pelton

Cross-flow

In this study the Pelton wheel and the Cross-flow turbine areconsidered in detail

4. Pelton wheel turbine [6]

A Pelton turbine consists of a series of cups mounted aroundthe periphery of a circular disc. The shaft of the Peltonturbine is mounted either horizontal or vertical. The turbineis not immersed in water but operates in air with the wheeldriven by jets of high pressure water which hit the cups. Thekinetic energy of the water jets is transferred to the turbine asthe water jet is deflected back. The kinetic energy of a jet ofwater is converted into angular rotation of the cups as the jet

strikes. The high-velocity jet of water emerging from anozzle impinges on the cups and sets the wheel into motion.The speed of rotation is determined by the flow rate and thevelocity of water and these are controlled by the spear valve.

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The water must have just enough speed to move out frombetween the cups and fall away under gravity from thewheel.

Figure 7: Pelton turbine [6]

Figure 8: Pelton buckets

4.1 Characteristics of Pelton wheel cups/buckets [7]

The central part of each bucket has a splitter ridge. This isthe part that the water jet hits first. This splitter ridge should

be sharp and smooth, so that it cleanly splits the jet into two

halves. The force on the bucket comes from it catching thewater and taking out as much of the waters momentum as itcan. The more the waters momentum absorbed by each

bucket, the better the turbine efficiency. Getting a goodtorque from this force requires the force to act at as large aradius as possible. To achieve this, the position of the end

point of the splitter ridge is important. The optimum positionwill have most of the water hitting the bucket when it isnearly at right angles to the jet. The angle of the splitter ridgeis set so that it is approximately at right angles to the jetwhen the full cross-section of the jet is hitting it. The rest ofthe internal shape is designed to allow water to flow freelyround, and out at the edges. The water should emerge at a

favourable an angle to give maximum moment from themomentum change. This is achieved by having the cupsurfaces smoothly curved, with a reasonably large radius.The jet should always strike the surface between the splitterridge first and never at the edges of the bucket. Thesurface between the splitter ridge and the outside end of the

bucket has to be shaped so that it directs all the water fromthe ridge into bucket cups.Ideally the bucket should take the water around a completeU, removing all the energy from the jet, and leaving thewater stationary in air and then have it fall under its ownweight into the tailrace. In reality this is not achievable

because the next bucket would hit the water left hanging in

mid-air. The edges of the bucket are consequently angledoutwards; just enough to make sure that the water clears thenext bucket.

Figure 9: Water jet being deflected back at an angle of 1650.

For optimum efficiency the speed of the jet, vj, should beabout twice the speed of the bucket, vb. Because the runneris moving away from it, the speed at which the jet enters the

bucket is given by . The jet is split by the ridge and

flows round the two cups to emerge at the sides of thebucket. The inside of the bucket should be as smooth aspossible and evenly curved so that the water does not loosemuch speed as it goes round. Lack of smoothness reducesefficiency. If there were no losses, the water jet would leave

at a velocity equal to . If the jet speed, vj, is twice the

bucket speed, vb, then the water, relative to the bucket, leavesat a speed of: = 2 =

So if the bucket is moving at a velocity of vb and the water iscoming out of the bucket at a speed of vb, in almost theopposite direction, this would mean that the speed of thewater coming out relative to the housing is nearly zero; itsenergy having been completely removed by the bucket [7].

The rotational speed of the buckets hence the runner is

determined by the jet velocity as illustrated in Figure 10.

Figure 10: Water jet hitting the bucket

The net head acting on the Pelton wheel

H = Hg - hfwhere Hg = gross head

hf is the head loss due to friction,which is given by the DarcyWeisbach Equation

=42

2

where f = Darcy friction factor (dimensionless)

L = length of the penstock (m)V = volumetric flow rate (m/s)

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g = acceleration due to gravity (m/s2)D* = diameter of penstock (m)

then

velocity of the jet at inlet = (2gH)

in figure 10 this is = V

v = v1= DN / 60

where N = speed of the wheel in rpmD = diameter of the wheel

the velocity triangle at inlet will be a straight line where

Vr= Vv = 0 and = 0

from the velocity triangle at outletVr1 = Vr

and V1 = Vr1cos v1

the force exerted by the jet of water in the direction ofmotion is:

F = aV [V + V1]

where a is the area of the jet given as:a = d2 / 4

d = diameter of the jet

the angle is an acute angle, therefore positive sign is takenwork done by the jet on the runner per second

= F x v = aV[V + V1] x v (Nm/s)

power given to the runner by the jet

= aV[V + V1] x v (W)

work done / s per unit weight of water striking/s

= aV[V + V1] x vweight of water striking/s

= aV[V + V1] x v

aV x g

= [V + V1] x vg

the energy supplied to the jet at inlet is in the form of kineticenergy and is equal to mV

2/ 2

and kinetic energy of jet per second

= aV x V2

2Therefore the kinetic energy of the water jet is transferred tothe buckets and hence the runner. So the runner rotationalspeed depends on the jet velocity.

4.3 Pelton wheel turbine efficiency [7]

The parameters of the runner that affect efficiency are:diameter of runner, number of jets, number of buckets of arunner and method used to attach buckets to a runner.A small runner is cheaper to make than a larger one. It takesless material and the housing and associated components can

be made smaller. To use smaller runners requires dividingthe flow of water among a large number of jets. Multiple-jet

Pelton turbines can have up to six jets before interferenceeffects between jets lead to too much inefficiency. Multiple-

jet machines have the following advantages over the single-jet machines:

higher rotational speed smaller runner and case some flow control is possible without extra

equipment such as the spear valve

less chances of blockage in most designs due toreduced surge pressures

In order to optimize efficiency, the number of buckets on theturbine runner is generally between 18 and 22. For the Peltonwheel turbine to function as expected, the buckets must beattached securely onto the runner. This is to avoid bucketsflying off the runner during operation or buckets movingfrom their original positions, hence compromising efficiency.There are a number of ways of mounting buckets onto therunner hub. The whole runner and buckets may be cast asone piece or buckets may be cast separately and are then

bolted, clamped or welded onto the runner [4]. Casting ofbuckets and mounting them to the runner is the processwidely used in the manufacturing of turbines. It is cheap,reliable and can produce complex shapes easily.

4.4 Materials for Pelton wheel turbine bucketsThe chief material requirements are high strength, abrasionresistance, casting suitability and ability to withstandextended use in water. Materials in common use areAluminium, Copper Alloys, Grey Cast Iron, Stainless Steeland Plastic.

4.5 Flow control for Pelton wheel turbine efficiencyIn order to maximise the efficiency of the turbine, it isimportant to be able to control the flow rate. The need tocontrol this flow arises due to site conditions that includechange in water flow rate with changing seasons. There arevarious options that are used to control the flow of the water

jet, and these include the varying of number of jets, the spearvalve and the replacement of nozzles [5].

Varying number of jets:If a multi-jet has shut off valvesfitted on each of its jets, it can be run at different flow rates

by simply altering the number of jets playing on the runner.Figure 11 shows a four-jet turbine. If jets are of equaldiameter then the flow will simply vary in proportion to thenumber of active jets. It may be economic to use unequaldiameter jets, and divide the year into more seasonal flows.

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Figure 11: A four-jet Pelton turbine [5]

It is possible to utilise the nozzle replacement approach in amulti-jet turbine to provide greater variation in flow. Thiswill reduce the cost of adding further jets, but will requireincreased input from an operator of the plant.

Spear Valve:The spear valve is a streamlined spear headarranged to move within the nozzle allowing variation ineffective orifice cross-sectional area without introducingenergy loses. Hence the water flow rate is varied with aconstant j et veloci ty. Large turbines may include morethan one spear valve around the periphery of the rotor. Thespear is moved by a handle or automatically by a mechanicalspeed governor as shown in Figure 12.

Figure 12: Spear valve and Jet Deflector plate on a single jetturbine

Figure 13 shows the effect of the spear valve on the waterjet. When the spear valve is moved in the forward directionmore the water jet diameter is smaller, and vice versa is true.

Figure 13: Effect of spear valve position to the water jetdiameter

Spear valves are very effective where continuous flowregulation is considered necessary and their use in such casesmay be cost effective. In addition they can stop a foreign

body which has fallen into the penstock from blocking thenozzle.However in some cases, multi-jet Pelton wheel turbines arechosen over single-jet with a spear valve because the spearvalve is expensive. It can also present the danger of

instantaneously blocking the nozzle should it becomedetached while the turbine is operational.

Replacement of nozzles:One nozzle is removed and replacedwith a more appropriate one to suit the seasonal change.Thus wet season flow would require a larger nozzle size thandry season flow. It is possible to divide the yearly flowvariation into two, three or more parts and provide a suitable

nozzle for each flow. This is a low-cost method ofcontrolling flow. It is important to be certain that sufficientoperator skill and input is available and the plant has to bestopped when nozzles are changed. The water jet may bediverted by a jet deflector plate, by breaking the jet and byshut-off valves.

Jet deflector plateThe water jet may be deflected away from the buckets of therunner when the plate is rotated into the jets path as shownin Figure 12. This is very useful in stopping the turbinewithout shutting off the flow in the penstock hence avoidingthe dangers of pressure surges. The process of stopping theturbine is then completed by diverting the flow at the forebay

tank. Breaking jet

The turbine may be stopped by moving the spear valve in theforward direction until the water striking the runner isreduced to zero or by use of the Jet Deflector Plate. Howeverwhen the water has been completely stopped from reachingthe runner, due to inertia the runner goes on revolving forsome time. To stop the runner in a short time, a small nozzlemaybe provided which directs the jet of water on the back ofthe buckets. This jet is called the Breaking Jet and is shownin Figure 14.

Figure 14: The breaking jet

Shut-off ValvesA gate valve or butterfly valve may be placed in the turbinemanifold. Pelton wheels are driven by long penstocks inwhich surge pressure effects, due to valve closure, can be

very dangerous and lead to damage caused by bursting of thepenstock. If these valves are fitted, they must always beclosed slowly, particularly during the last phase just beforeshut off.

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4.6 Friction

Turbine efficiency can be considered separately as hydraulicefficiency, mechanical efficiency, volumetric efficiency andoverall efficiency [7].

i. Hydraulic efficiencyHydraulic efficiency is defined at the ratio of powergiven by water to the runner of a turbine to the

power supplied by the water at the inlet of theturbine. The power at the inlet of the turbine is thehighest and this power goes on decreasing as thewater flows over the buckets of the turbine due tohydraulic losses as the buckets are not as smooth.Hence the power delivered to the runner of theturbine will be less than the power available at theinlet of the turbine. From Figure 10, mathematicallythe hydraulic efficiency of a turbine is:

h = power delivered to runnerpower supplied at inlet

= work done per secondK.E. of jet per second

=aV[V + V1] x v(aV) x (V)

2/ 2

= 2 [V + V1] x v

(V)2

ii. Mechanical efficiencyThe power delivered by water to the runner of aturbine is transmitted to the shaft of the turbine. Dueto mechanical losses, the power available at theshaft of the turbine is less than the power deliveredto the runner of a turbine. The ratio of the poweravailable at the shaft of the turbine to the powerdelivered to the runner is defined as mechanicalefficiency. Hence, it is written as:

m = power at the shaft of the turbine

power delivered by water to the runner

iii. Volumetric efficiencyThe volume of the water striking the runner of aturbine is slightly less than the volume of watersupplied to the turbine. Some of the volume of thewater is discharged to the tail race without strikingthe runner of the turbine. Thus the ratio of thevolume of the water actually striking the runner tothe volume of water supplied to the turbine isdefined as volumetric efficiency. It is written as:

v = volume of water actually striking the runnervolume of water supplied to the turbine

iv. Overall efficiencyOverall efficiency is the ratio of power available atthe shaft of the turbine to the power supplied by thewater at the inlet of the turbine. It is written as:

o = volume available at the shaft of the turbinepower supplied at the inlet of the turbine

= m x h

Friction has the greatest impact on hydraulic efficiency andin turn hydraulic efficiency is one of the factors affecting theoverall efficiency. This makes friction an important factor inoverall turbine efficiency. For example in the analysis offlow friction of Pelton turbine hydraulics it is indicated thatthe flow friction in the Pelton buckets has a substantialimpact on the system efficiency.

The direct influence is determined by the friction, whichdirectly retards the bucket motion and thus determines the

power output. The indirect way is determined by the changesof relative flow and the pressure distribution in the bucket.The flow friction theorem points out that the total reductionof the system efficiency is equal to the total efficiencyreductions by two different ways. In the total efficiencyreduction, the effect from the indirect way dominates. Inaddition, frictions in the bucket, whether at the bucketentrance or at the exit, always cause a reduction in systemefficiency. The efficiency drop resulting from the flowfrictions represents the greatest part in the total loss in the

system efficiency of a Pelton turbine.

4.7 Water jet and Pelton wheel buckets

Of great importance is the interaction of the water jet withthe Pelton buckets. The jet discharged from the nozzle of aPelton turbine is a key item of hydropower systems and its

precise shape and position are highly relevant to theoptimum design of the turbine buckets to match theincoming flow.In the first configuration, the interaction between the runnerand an axial-symmetric jet characterized by a given velocity

jet profile were investigated, whereas in the secondconfiguration the runner was coupled with the needle nozzle

and the final part of the penstock and the interaction betweenthe jet and the bucket were analyzed. The results confirmedthat the turbine efficiency is affected by the water jetinteraction with the turbine was shown.

5 Cross-flow turbinesThe turbines comprise of a drum shaped runner consisting oftwo parallel discs connected together near their rims by aseries of curved blades. It has its runner shaft horizontal tothe ground in all cases [8].

Figure 15: Cross-flow turbine

The Cross-flow turbine consists of a cylindrical water wheelor runner with a horizontal shaft, composed of 18 up to 37

blades, arranged radially and tangentially. The blade's edges

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are sharpened to reduce resistance to the flow of water. Ablade is made in a part-circular cross-section. The ends of theblades are welded to disks to form a cage, instead of the bars,the turbine has trough-shaped blades.In operation, the water jet of rectangular cross-section passestwice through the rotor blades, arranged at the periphery ofthe cylindrical runner. The water strikes the blading firstfrom the periphery towards the centre, imparting most of its

kinetic energy, and then crosses the runner from insideoutwards striking the blades on exit and imparting a smalleramount of energy on leaving the turbine as shown in Figure16.

Figure 16: Flow of water across the turbine

Going through the runner twice providesadditional efficiency. When the water leaves the runner, italso helps clean the runner of small debris. The Cross-flowturbine is a low-speed machine that is well suited forlocations with a low head but high flow.

The effective head driving the Cross-flow runner can beincreased by the inducement of a partial vacuum inside thecasing. This is done by fitting a draught tube below therunner which remains full of tail water at all times. Anydecrease in the level induces a greater vacuum which islimited by the use of an air bleed valve in the casing. Carefuldesign of the valve and casing is necessary to avoidconditions where water might back up and submerge therunner. This has the additional advantage of reducing spraydamage on the bearings since the internal vacuum causes airto be sucked in through the bearing seals, so impeding themovement of water out of the seals.

The efficiency of Cross-flow turbine depends on thesophistication of its design. For example a feature such asvacuum enhancement is expensive because it requires the useof air seals around the runner shaft as it passes through thecasing, and airtight casing. Sophisticated machines attainefficiencies of 85%. Simpler ones range typically between60% and 75%.

Two major attractions of the Cross-flow turbine are that it isa design suitable for a wide range of heads and power ratingsand it lends itself easily to simple fabrication techniques.This is a feature that is of interest in developing countries,e.g. the runner blades can be fabricated by cutting a pipelengthwise in strips.

The turbine housing may be entirely made of mild steelplates. Mild steel is tougher than grey cast iron and is goodon impact and frost resistance and is rigid enough to

withstand high operational stress and enable smoothoperation. The design and hydraulic layout result inminimized vibrations and noise level. Special care should betaken in the layout of the main bearings for the runner andthe guide vane. The casing is designed in such a way thatdifferent options for the bearing system are available to copewith the specific site requirements, flywheel or belt drive. Asealing system is included in the side covers. The guide vane

unit can easily be taken out through a side cover forinspection, cleaning or replacement. To obtain theguaranteed efficiency, the cylindrical runner is fabricatedwith high precision.

5.1 Flow control in Cross-flow turbine

Flow control is done by a guide vane. The shaft of the guidevane is parallel to the rotor shaft, it fits neatly inside thenozzle to keep leaks at the side in the closed condition withinlimits. It guides water to the runner and controls the amountof water entering the runner. The device is operated by ascrew and nut which is connected to a hand wheel. It canalso be coupled to automatic operation, to the hydrauliccylinder of a speed governor. In addition, rubber gaskets areused to seal up the turbine housing.

The correct design speed of a turbine depends on the powerrating of turbine, the site head and the type of turbine [9].Small turbines designed for micro-hydro applications may attimes have techniques for altering the flow rate of water. Onlarger machines a method of altering flow is different. Cross-flow has guide vanes which alter the water flow rate.

Different turbine types respond differently to changed flowat constant head. Typical efficiency characteristics of threeturbines are given in Figure 17. The curves shown assume

pressure turbines which have facilities for varying water flowrate at constant head, for example, spear valves or multiple

jets on Pelton turbines, partition devices of flow controlvanes on Cross-flow turbines.

Figure 17: Part-flow efficiency of various turbines

6

Micro-hydro plants overviewThe turbines for this study are for micro-hydro power plantsat sites located in Malawi, Mozambique and Zimbabwe asshown in Table 3.

0

50

100

0 0.5 1

Efficiency(%

)

Flow (as a fraction of maximum flow)

Part-flow efficiency of

different turbines

Pelton &

Turgo

Cross-flow

Francis
http://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Efficient_energy_usehttp://en.wikipedia.org/wiki/Efficient_energy_usehttp://en.wikipedia.org/wiki/Welding

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Table 3: Micro-hydro power plant by site

SITE NAME GROSS

HEAD

(m)

DESIGNED

FLOW

RATE

(m3/s)

TURB

INE

TYPE

DESIGNED

EFFICIEN

CY (%)

A BONDO,

MALAWI

51 0.325 Pelton 60%

B DAZI, NYANGA 138 0.035 Pelton 60%

C NYAMWANGA,

HONDE

VALLEY

26 0.277 Cross-

flow

60%

D CHITUNGA,

MOZAMBIQUE

35 0.180 Cross-

flow

60%

SITE A: Bondo

The Bondo MHS has a gross head of 51m and a designflowrate of 0.325m

3/s. A special characteristic at this site is

the high flowrate. This is due to the vast amount of waterthat flow in Bondo River. Figures 18 to 20 show the weir,

part of the canal at Bondo project and the Pelton turbine in

use at this site.

Figure 18: Weir at Bondo MHS

Figure 19: Part of the canal at Bondo MHS

Figure 20: The Pelton wheel turbine at Bondo MHS

The Bondo project benefits two schools in the area; aprimary and a secondary schools and teachers houses, oneclinic including the waiting mothers home and staff housesand one business centre. The business centre has somegrocery shops, a battery charging centre, hair salon and an

electric grinding mill. There are seven villages in the Bondocatchment area with a total of 3084 households. However,evidence shows that if more power could be produced furtherdevelopments would be possible.

SITE B: Dazi micro-hydro project

At the site the head is 138m, the design flowrate is0.035m3/s. A pelton turbine is in used. A specialcharacteristic of this site is the very high head thatcompensates for the low flows. The project benefits a schoolin Figure 21, a clinic and lighting for about 1000 people innine villages. More power would allow the 1000 people to

use electrical gadgets such as refrigerators and stoves.

Figure 21: A classroom lit by power from the Dazi MHS

SITE C: Nyamwanga project

The Nyamwanga MHS is located 27km North East of HaunaTownship in Chief Zindis area in Mutasa District . Thegross head is 26m, the flowrate is 0.277m3/s. A Cross-flowturbine is in use at this project as shown in Figure 22. The

project benefits one school, and one shopping centre withfifteen shops. The plan for the shopping centre is shown inFigure 23:

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Figure 22: The Cross-flow turbine for Nyamwanga project

Figure 23: Plan for Nyamwanga business centre that isbenefitting from the project.

SITE D: Chitunga project

The rainfall pattern is shown in Figure 24.

Figure 24: Average rainfall graph for Chitunga

At the site, the gross head is 35.12m, the designed flowrateis 0.180m3/s. The project benefits one school, one clinic, anenergy centre, local authority offices, four grinding mills and

366 households. A cross-flow turbine is in place at thisproject site.

7. Research design process

Visits were done to the workshops where turbines arefabricated were. There the study of the factorymanufacturing of turbines was done by following through the

fabrication process. Visits were also made to the project sitesto physically test turbine efficiencies.

7.1 Physical testing of turbine efficiency

Physical testing of the turbine rotational speed was doneusing a tachometer. A tachometeris an instrument thatmeasures the rotational speed of a shaft or disk. Tachometersmay either be manual or digital. A digital one is shown inFigure 25 [8].

Figure 25: Digital Tachometer

The digital gadget displays the RPM on the screen. Therotation of the turbine was measured using a digitaltachometer model by placing the tip of the tachometer on a

point on the centre of the runner at site C. The rotation of theturbine was then used to calculate the efficiency of theturbine as follows:

= ( )( + )

and =

and =2

60

where P = turbine power output (W) = density of water (kg/m3)u = bucket rotational velocity (m/s)vj = jet velocityQ = flowrate (m3/s)R = runner radius (m)

N = runner speed (rpm)k = constant of friction

Also efficiency could be assessed by checking theperformance of the turbine. A load (e.g. an industrial weldingmachine) is carried to site. A typical example is one which

operates at 25amps and about 35Amps when striking an arc.The community is then disconnected and the load isconnected. The operating current is measured and multiplied

by the system voltage. This gives the power output which is

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then compared to the designed power output. From this, onecan tell if the turbine is still performing as designed.It is important to make certain the correct number of bucketson a runner. This guarantees that the outgoing bucket doesnot interfere with water jet now hitting the next bucket. Toensure the correct number of buckets on a runner, asimulation program called Autodesk Inventor was used tosimulate the turbine rotation. This is a 3D mechanical solid

modelling design software for creating 3D digital prototypesused in the design, visualisation and simulation of products.

8. Results analysis

8.1 Design and manufacture of turbines

The friction within the Pelton buckets, the interaction of thewater jet with the buckets, the accuracy in the cutting ofCross-flow blades and the method of welding of the blades tothe runner all have major roles in the efficiency of a turbine.

8.1.2 Pelton turbine manufacture

The head and flowrates were collected from each of the four

study sites. This data was used to calculate the gross poweroutput hence the size of the runner, number of buckets,number and size of water jets. The efficiency of the turbine isdesigned at 60%. Two of the four sites that were analysedhave Pelton wheel turbine working on them. Data from thesites is given in Table 4.

Table 4: The two MHS sites that have the Pelton wheelturbine installed on themSITE GROSS

HEAD

(m)

FLOW

RATE

(m3/s)

CAPACITY

(kW)

TURBINE

TYPE

EFFICIENCY

(%)

A 51 0.325 88 Pelton 60%

B 138 0.035 20 Pelton 60%

The capacity for each site is the actual power output .

Buckets are sand cast individually and bolted to a runner.Figures 25 to 30 show this process. The pattern is used to

produce the mould, the hollow section in the sand box wherethe molten metal is poured in to produce the required Pelton

buckets.

Figure 26: Pelton bucket that has been sand cast.

Figure 26 is showing a single bucket that has been sand cast.The feeder and the riser are still attached to the bucket. Thefeeder is the entrance through which the molten metal is

poured into the mould and the riser is the way excess metalgoes through.

Figure 27: Pelton bucket that has been machined

In Figure 27 the bucket has been machined to remove the

feeder and riser. It has 2 holes drilled through it for bolting tothe runner. It is important to have the holes expertly drilledotherwise the buckets will not sit on the runner perfectly andthe turbine is misaligned. This reduces the efficiency of theturbine as some energy is lost to heavy vibrations and noisesince the water jet will not be hitting on the correct spotanymore.

Figure 28: Alignment of buckets on the runner

Figure 28 shows how individual buckets are aligned on therunner. They sit at an angle to each other. This allows for thefull force of the jet to be applied on each bucket one afteranother without hindrance from the previous bucket. Thisensures maximum efficiency of the turbine.

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Figure 33 shows closely the blades welded into the runner.The curving of the blades guides the flowing water as itflows from one end of the turbine to the other.

Figure 34: The turbine casing.The turbine casing is shown in Figure 34. The cutting torchis used to cut the different parts which are then weldedtogether.

8.2 Effect of roughness due to manufacturing

The processes used to manufacture turbines are sand casting,oxyacetylene welding and flame cutting. Differentmanufacturers of turbines provided quotations that includedthe turbine efficiencies and methods used to manufacture theturbines.Each manufacturing process has a different roughness. This

combined with material roughness determines the friction ofa turbine and it will then translate to turbine efficiency, inparticular hydraulic efficiency. The hydraulic efficiencyaffects overall turbine efficiency directly [6].

Table 6 compares different manufacturing processes againstthe overall turbine efficiency associated with each process.

Table 6: The turbine efficiencies associated with roughnessof different manufacturing processes

ManufacturingProcess

TurbineEfficiency Roughness

Pressure die casting A 85.0 1.6

Investment casting B 85.0 3.2

Milling C 85.0 6.3

Laser D 80.0 6.3

Electron beam E 80.0 6.3

Oxyacetylene welding F 60.0 25.0

Sand casting G 60.0 25.0

Flame cutting H 60.0 50.0

Figure 35 shows a graph that was drawn to relate this

roughness to the turbine efficiencies

Figure 35: Turbine efficiency & roughness for differentmanufacturing processes

The graph indicates that as the manufacturing processroughness increases the overall efficiency of the turbine

produced decreases. This emphasizes the need to usemanufacturing processes with minimal roughness.

This diagnosis of roughness affecting the overall efficiencyof turbines tallies with the findings that the flow friction in

the Pelton buckets has a substantial impact on the systemefficiency of the turbine. The efficiency drop resulting fromthe flow frictions represents the greatest part in the total lossin the system efficiency of a Pelton turbine.

Mechanical efficiency is the other determining factor of theoverall efficiency. One way to improve this efficiency incasting procedures is to cast segments comprising of anumber of buckets or whole runners, comprising the runnerand all the buckets.

8.3 Effect of the water jet movement

Of great importance in the mounting of the Pelton turbine isits interaction with the water jet. To maximize efficiency the

jet must hit precisely the centre of each Pelton bucket. Thatway the jet gives the maximum drive to each bucket as notedearlier in the design of the buckets that the rotation of theturbine depends on the water jet velocity. This fact matcheswith the experimental studies of the jet of a Pelton turbine.Itwas concluded that the jetdischarged from the nozzle of aPelton turbine is a key item of hydropower systems and its

precise shape and position are highly relevant to theoptimum design of the turbine buckets to match theincoming flow.

8.4 Turbine material

The type of material used to fabricate a turbine affects itsefficiency. Good materials are hard and must be able to resisterosion and corrosion. This ensures that the turbines willfunction at their best efficiency for the expected life span onthe micro hydro power plant. Stainless steel is mainly usedfor Pelton and Cross-flow turbines and mild steel is used forcasings.

This study revealed that other materials that maybe used forturbines are Aluminium, Cast Iron, Copper based alloys suchas Brass and Bronze, and Sheet steel. The properties of thesematerials and their prices per kg are given in Table 7.

0.0

20.0

40.0

60.0

80.0

100.0

A B C D E F G HManufacturing process

Turbine efficiency & roughness for different manufacting processes

Rough

ness

Turbi

ne Eff

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Table 7: Properties of different materials and their prices perkg

MATERIAL MATERIAL ROUGHNESS(x100-2)

BRINELLHARDNESS

(x100)

PRICE/KG

(US$/kg)

A Aluminium 0.2 1.2 1.32

B Grey castiron

25 3.02 0.83

C Brass 1 3.6 5.54

D Bronze 1 3.6 5.54

E Stainlesssteel

3 4.15 1.47

F Sheet Steel 15 4.5 0.29

Figures 36 and 37 show the relationships between roughnessand price per kg of material and hardness and price per kg ofmaterial respectively.

Figure 36: Material roughness compared to material price

Figure 37: Material hardness compared to material priceThe two graphs shows why Stainless steel is the mostcommonly used material in the manufacturing of turbines forMHS. It has a high hardness, a very low roughness and price

is close to the lowest. Sheet Steel maybe the cheapest and itshardness is slightly higher than that of stainless steel but itsroughness is much higher than Stainless Steel. Such aroughness means that it has a much higher friction thanStainless Steel and this is not good for the hydraulicefficiency of the turbine and hence the overall efficiency.Brass and Bronze are too expensive for MHS. This is

because they are alloys of Copper which is a very expensive

metal. Its price is about $8 per kg.

Aluminium has a price close to Stainless Steel and the lowestroughness, but is susceptible to damage if debris such asstones and sand find their way to the turbine. This is becauseit has the lowest hardness. The roughness of Grey Cast Ironis too high. This compromises the turbine efficiency despitehaving a low price and a fairly high hardness.

8.5 Effect of increasing turbine efficiency on MHS power

out put

The resultant change in actual power output due to increase

in turbine efficiency of the four MHS that were studied isshown in the Table 8.

Table 8: Differences in power output due to increase inturbine efficiency

SITE TURBINE TYPE

CURRENTTUBINE

EFICIENCY

CURRENTPOWEROUTPUT

(kW)

NEWTURBINE

EFFICIENCY

POSSIBLEPOWEROUTPUT

(kW)

Bondo (A) PELTON 0.60 88.00 0.85 124.67

Dazi (B) PELTON 0.60 20.00 0.85 28.33

Nyamwanga(C)

CROSSFLOW

0.60 34.00 0.80 45.33

Chitunga (D) CROSSFLOW

0.60 30.00 0.80 40.00

Figure 38: Difference in MHS power output for the foursites analysed

In all the four cases there is a remarkable increase in poweroutput coming from the same resources. Therefore using the

materials and manufacturing processes that give the highestpossible yield from each site and resources is most ideal.

0

5

10

15

20

25

30

A B C D E F

Materialroughness

Material

Material roughness compared to price of material

ROUGHNES

S (x100^-2)

PRICE/KG

0

1

2

3

4

5

6

A B C D E F

Materialhardness

Material

Material Hardness compared to material price

BRINELL

HARDNE

SS

(x100)

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

A B C D

POWER

OUTPUT

SITE

DIFERENCE IN POWER OUTPUTS

CURRENT

POWER

OUTPUT

(kW)POSSIBLE

POWEROUTPUT

(kW)

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8.6 Performance measurements

After the turbine has been set up and the micro hydro is juststarting to run, it is of utmost importance to measure theefficiency of the turbine in order to verify if it is meeting theexpected efficiency. This is done using a tachometer. This isdone at the initiation of a project and after it has run for sixmonths to check if all is performing as expected. During the

period of study the tachometer was used to check the

performance of the turbine at Site C as shown in Table 9.

Table 9: Tachometer readings at Site CTACHOMETER

READINGS

1 2 3 4 5

495 rpm 515 rpm 511 rpm 492 prm 498 rpm

The average of these numbers was used to calculate turbinepower output.The designed power output for this site is 34kW and turbineefficiency is designed at 60%. The calculated power outputfrom these readings was 33.3kW. The two power outputs arecomparable to each other, therefore it was concluded that the

turbine was still operating at 60%.The need for performance measurement is invaluable. Whendone on regular intervals problems are identified andrectified before the MHS needs to be shut down.

9.Financial analysisThere is a limit on the project cost per kilowatt for eachMHS. It entails that total project cost of each site depends onits capacity. A project is considered financially viable if the

budget does not exceed $6000 per kW. Table 10 shows thecosts that were involved in the selected projects.

Table 10: Costs involved in the selected four projectsSITE TURBINE

TYPE

TURBINE

EFFICIENC

Y

MHS

CAPA

CITY

TURBINE

COST (US$)

TOTAL

PROJEC

T

COST(U

S$)

PROJ

ECT

COST

PER

kW

A Pelton 60% 88kW 5000.00 509

444.00

5

789.14

B Pelton 60% 20kW 5000.00 81 775.20 4

088.76

C Cross-

flow

60% 30kW 7000.00 174

719.00

5

823.97

D Cross-

flow

60% 34kW 7000.00 158

340.00

4

166.84

Using quotations from suppliers new MHS capacity werecalculated, total project cost and project cost per kW asshown in Table 11.

Table 11: Costs involved with better efficiency turbines

SITE TURB

INE

TYPE

TURBIN

E

EFFICIE

NCY

NEW

CAPACITY

TURBINE

COST (US$)

TOTAL

PROJECT

COST(US$)

PROJEC

T COST

PER kW

A Pelton 85% 125kW 6500.00 510 944.00 4 087.55

B Pelton 85% 28kW 6500.00 83 275.20 2 974.12

C Cross-

flow

80% 40kW 8200.00 175 919.00 4 397.98

D Cross-

flow

80% 45kW 8200.00 159 540.00 3 545.33

New turbines have a better efficiency; therefore the capacityof a MHS is improved. These turbines are more expensivethan the current turbines in use and this translates to highertotal project costs. However, because a turbine with betterefficiency also improves the MHS capacity the projects aremade even more viable because in the end the project cost

per kW is reduced. The change in project cost per kW for

each site is shown in Table 12.

Table 12: The difference in project costs per kw

SITE CURRENT PROJECT

COST PER kW

EXPECTED PROJECT

COST PER kW

A 5 789.14 4 087.55

B 4 088.76 2 974.12

C 5 823.97 4 397.98

D 4 166.84 3 545.33

This reduction is shown graphically in Figure 39.

Figure 39: The reduction in project cost per kW on each site

The graph is showing that even though turbines of higherefficiencies are more expensive the overall project cost perkW actually decreases.

I. 10.RESEARCH RECOMMENDATIONSIt is highly recommended to consider investing in processesthat produce turbines with higher efficiencies such as:

Pressure die casting of turbines. They may becast as whole turbines or as segments made upof a number of buckets.

Investment casting of whole turbines. CNC milling of Pelton buckets. Cutting of Pelton runner and Cross-flow runner

and blades using CNC laser, CNC plasma.

Electron beam and laser welding of blades torunners.

0

1000

2000

3000

4000

5000

6000

7000

A B C D

$

/kW

SITE

REDUCTION IN PROJECT COST PER kW

CURRENT

PROJECT

COST PER

kWPOSSIBLE

PROJECT

COST PER

kW

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