Inversin R. Allen Micro-hydropower Sourcebook

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less head than “hs” can be recovered, because the flow

in the draft tube is aerated, reducing somewhat the

weight of the column of water inside. There are also

losses caused by friction and turbulence within the draft

tube. It is not possible to recover the entire head repre-

sented by the distance from nozzle to tailwater level;

that would imply a submerged runner. Unlike reaction

turbines, a crossflow runner has to operate in air.

Pelton and Turgo runners can also operate with draft

tubes. However, these turbines are generally used at

high-head sites, and the distance between the turbine

and tailwater is therefore insignificant compared to the

head above the turbine.

One difficulty in using a draft tube with a runner that

operates in air is that air within the casing will gradu-

ally be entrained or dissolved in the water. The water

level in the draft tube will then rise, gradually submerg-

ing the runner and significantly reducing its efficiency.

Consequently, an air valve is incorporated in the turbine

casing (Fig.

6.8).

It is adjusted so that when the water

in the draft tube is higher than the permissible suction

head “hS” (set by the turbine’s need to operate in air),

the negative pressure in the casing pulls in additional

outside air, permitting the water column to drop

slightly.

Although a draft tube permits additional power to be

generated at a specific site, stuffing boxes must be

located where the shaft enters the casing and the casing

must be fairly airtight. Significant leaks in the casing

or draft tube will make the draft tube totally ineffec-

tive. Therefore, extra care and expense in the design

and fabrication of the turbine is required to gain extra

head, and therefore power, through the use

a draft

tube.

For micro-hydropower turbines, a draft tube used to

recover a major portion of the potential energy or head

between the turbine and tailwater is often cvlindrical in

shape-simply a length of pipe with its outlet end always

submerged below tailwater level. However, the water

descending a draft tube still has velocity and therefore

kinetic epergy, which represents a loss in head equiva-

lent to v‘/2g, where “v” is the water velocity in the

draft tube. If this is significant, as is more often the

case with reaction turbines, a portion of this head can

be recovered by gradually increasing the cross-sectional

area of the draft tube. This causes the velocity in the

draft tube to decrease as it descends; the decrease in

kinetic energy is converted to an increase in potential

energy manifested in the increase in negative pressure

or suction in the turbine casing.

A draft tube used with a small turbine to recover some

of this kinetic enerm is generally conical in shape.

Although a quickly diverging draft tube would seem

preferable to -reduce the velocity as much as possible

with the shortest possible draft tube, the maximum cone

angle is about 8’.

With greater angles, the flow

detaches from the walls, creating turbulence and an

accompanying loss of energy and head.

Basic relat.iolIships

“The Banki Water Turbine” (85) probably contains one of

the first descriptions in English of the design and opera-

tion of a crossflow turbine, along with test results of a

prototype. The initial portion of this publication is a

loose translation of the original paper by Donati Banki,

“Neue Wasserturbiie.”

For preliminq design purposes, equations and other

common relationships that can be useful in approximat-

ing overall runner dimensions and speed are as follows:

(6.8)

where

A = cross-sectional area of jet (m2)

s (jet thickness, m) x (runner length, m)

Q = flow (m3/s)

H = net head (m)

Making a first guess for a runner length and using the

definition for “A” above will permit the jet thickness to

be determined. The cross-sectional width

the

rec-

tangular nozzle of a crossflow turbine is approximately

equal to the jet thickness. The runner diameter is usu-

ally about 10 times the jet thickness. If the runner

dimensions seem unreasonable, another estimate for

runner length should be tried. Then, when the runner

diameter is found, its speed can be found as follows:

where

N = runner speed (rev/min)

D = runner diameter (m)

Fabrication

Because of the simplicity of its design and the fact that

the construction

the runner requires only cutting,

welding, and grinding (or machining, if available), the

crossflow turbine is the most popular turbine for local

fabrication. In addition, numerous publications describ-

ing the construction of crossflow turbines are available.

The first popular publication describing the desi@ and

construction of a crossflow turbine was VITA’s Low Cost

Development of Small Water Power Sites (56). In addi-

tion to providing summary information on developing a

hydropower site, it presents some details for construct-

ing a turbine. The vanes for this design are cut from

sections of steel pipe and are inserted in grooves cut

into the two circular end-plates. This design is also

summarized in the Energy Primer (8). Because of the

effort required to cut these grooves, many have omitted

this step and simply arc-welded the vanes to the end-

plates. Langhorne has documented his experience in

180 Turbines

constructing a crossflow turbine after the VITA design

and implementing a hydropower scheme (Fig. 6.9) in his

article, “Hand-Made Hydro Power” (70).

ln addition to reviewing one of several Nepalese efforts

to design turbines and implement small-hydropower

schemes, Small Water Turbines (97) presents detailed

machine drawings of all the components of the stan-

dardized designs manufactured by Butwal Engineering

Works. It also describes and illustrates jigs used to

facilitate fabrication. This publication is presently

being updated.

Fig.

6.9. This 6

kW scheme, using

a homemade

crossflow

turbine,

reduces demand placed by a private residence on

utility

power.

A complete set of detailed designs for a turbine with a

runner diameter of 200 mm (Fig. 6.10), manufactured by

the Nepalese company Balaju Yantra Shala, is found in

Crossflow Turbine, Type: BYS/T3 (4). Arter also con-

tributed to this design; details of his work and extensive

test results can be found in two theses in German,

Durchstrijmturbine (27) and Experimentelle Unter-

suchung einer DurchstrGmturbine des Typs Banki (34).

In addition to the previous publications, there has been

other university-based research on the crossflow tur-

bine. Design of Small Water Turbines for Farms and

Small Communities (43), prepared at the Massachusetts

Institute of Technology, quantitatively reviews the

design of a crossflow turbine. Although the objective of

this study was to develop a design that could be fabri-

cated in a simple workshop in a developing country, with

facilities to weld, drill, and cut steel parts, no prototype

was built or tested. Consequently, although this publi-

cation may give some insight into the design of a cross-

flow turbine, the preliminary design which it presents

lacks design improvements that would have been appar-

ent if prototypes had been built and field-tested.

Hothersall’s thesis, Micro-hydro Development for

P.N.G. (60), describes another effort to design and

fabricate a crossflow turbine. In “A Review of the

Cross-Flow Turbine,” Hothersall provides a technical

Fig. 6.10. In

a few weeks, SODERZA, a small NW deW-

opment

organization in Zaire with no previous

experience

in hydropower, buflt this

turbine, retying solely on

the pub-

lication Crossflow Turbine, Type: EYS/TI (4); consthtcted

civil

works (see Flg. 5.131); and in.stallc;“W. hammer mill

and alternator. A small-diameter runner and a stngle-

stage,

flat-bell speed increaser develowd the 3000-4000

rev/min

required to

operate the mill.

review of this turbine’s operation and performance (61).

Research with crossflow turbines is also being under-

taken at Colorado State University.

A major problem with locally fabricated crossflow run-

ners has been the eventual fracture of the blades

through metal fatigue where they are attached to the

end-plates. In “Blade Calculations for Water Turbines

of the Banki Type” (105), Verhaart addresses this prob-

lem. This report presents equations to determine the

maximum blade length for a given head or the maximum

head for a given turbine configuration to avoid blade

fracture. It also presents a computer program for per-

forming these calculations. Balaju Yantra Shala in

Nepal, wl ich manufactures crossflow turbines in quan-

tity, add* intermediate supporting disks to reduce the

effective length of each blade. Crossflow Turbine,

Type: T$YS/T3 (4) has a graph for determining the num-

ber of intermediate disks as a function of turbine width

and operating head. The author tested the theory

behind this graph by applying it to 80 operating turbines

and correctly predicting the 20 turbines that had exper-

ienced blade failure.

Turbines 18 1

Cost-reduction approaches

Among the lowest-cost crossflow turbines in use

today

are those in northern Pakistan (see VILLAGER-IMPLE-

MENTED WCRO-HYDROPOWER SCHEMES ZN PAKIS-

TAN, p. 248). In a number of these installations, the

cost of the casing has been eliminated; pillow blocks

mounted on timber or concrete support the turbine

bearings (see Fig. 10.55). The runner is simply covered

with wooden slats or a sheet of galvanized roofing to

prevent the spray from reaching the bearings, belts, and

generator. A draft tube cannot be used at such an

installation, but additional head is often gained by

locating the runner about 2 m below the floor of the

powerhouse, with belts to transmit the power upward

(see Fig. 10.43). Placing a turbine below floor level also

keeps leaking water off the powerhouse floor.

Designs for crossflow turbines usually include a nozzle

made of sheet metal bent to a prescribed shape and

positioned to minimize clearance between the nozzle

and runner. hi addition, a short section of pipe is com-

monly used to make a smooth transition from a cylindri-

cal penstock to a rectangular nozzle. This is another

piece which has to be fabricated and which slightly

increases cost. Nozzles of simpler designs include one

made of flat metal sheets (see Fig. 10.55), one made by

forming a rectangular opening in one end of an oil drum

(see Fig. 10.561, and one made by using rectangular steel

channel (Fig. 6.11). With these simple designs, proper

transition pieces are sometimes omitted, and there is

often considerable clearance between nozzle and run-

ner: the crossflow runner functions entirely as an

impulse unit.

Fabrication has been simplified, but

sometimes at the expense of slightly increased losses in

head.

With these simple nozzle designs, no valves are used to

regulate the flow. The frequency of the ac voltage

generated is left to fluctuate somewhat. Flow into the

penstock is sometimes adjusted, with the penstock flow-

ing partially full during periods of part-load. If a gener-

ator is driven by a turbine using such simple nozzle

designs with no flow control, use of a load controller

can still permit close frequency control (see Load con-

troller, p. 204).

Francis turbines

The Francis turbine is generally a medium-head reaction

turbine, although it has been used under heads of

l-500 m. Its runner is composed of buckets formed of

complex curves and is more difficult to fabricate than

other turbines. In addition, a Francis turbine *usually

includes a scroll casing that distributes water around

the entire perimeter of the runner and several series of

vanes to guide and regulate the water into the runner,

If a turbine for a specific site is to be built in a local

workshop, most site developers prefer to use a turbine

that can be made more easily. Even commercial Fran-

cis turbines are rarely used at the lower end of the

“micro” range. For these reasons, the operation and

182 Turbines

Ffg. 6.11. This runner, fabricated In a rwul workshop, is

covered

with

wooden

box

during

opemtion

Runner and

nozzle are independently

mounted

a concrete founda-

tion

fabrication of the Francis turbine will not be covered

here.

Propeller turbines

A propeller turbine is a reaction turbine commonly used

at low-head sites. It resembles a marine propeller that

intercepts all the water descending the penstock. With

large conventional units, the propeller turbine has a

configuration similar to that of a Francis turbine. It is

a vertical-axis machine surrounded by a scroll case.

The rlow enters the case radially inward before being

deflected axially and passing through the runner.

Because flow is parallel to the shaft, this turbine is also

referred to as an axial-flow turbine. The blades of the

propeller can be either fixed or adjustable. In the latter

case, it is referred to as a Kaplan turbine. In addition, a

Kaplan turbine has variable wicket gates, reflating

flow to the runner, which are mechanically coupled to

the movable blades. Because this design is more com-

plex than others, this configuration is not often used

with micro-hydropower schemes.

There are several other configurations for this type of

turbine, all of which permit the flow to enter or leave

with less significant changes in direction. Micro-hydro-

power turbines are commercially available in all three

basic configurations described below:

The runner of a tubular turbine (Fig. 6.12a) is placed

in a bend

kink in the water passageway so that the

shaft can go through the wall, either upstream

downstream of the runner, to an externally mounted

generator.

(a)

Fig. 6.12. Three configuratIons usfng a propeller or

Kaplan

turbine: (a) tubular, 0

buI4

and (cc) rtght-angle dtive.

To eliminate bends in the passageway, the bulb tur-

bine (Fig. 6.12b) includes the generator in a bulb,

which is an extension of the propeller hub, either

upstream or downstream of the

runner,

and this

eliminates the need for the shaft to go through the

walls of the p=Jsageway. This turbine also reduces

the excavation requirements at very low-h.Tad sites.

A cross between the bulb and tubular turbine that

permits straight-through flow and an externally

mounted generator for easier access is a right-angle

drive unit (Fig. 6.12~). Because only the right-angle

drive is located within the bulb, the size of both the

bulb and the passageway is reduced.

The runner of a propeller turbine usually has from three

blades (for low-head units) to six. The water under

pressure enters the t-urbine through a series of angled

guide vanes designed to induce a tangential

whirl

component of velocity. On passing through the runner,

water pressure is reduced as energy is imparted to the

runner. The water then passes on to the tailrace

through a draft tube, which recovers some of the

remaining head. This head arises from both the pressure

energy caused by the difference in elevation between

the turbine and the tailwater level and from the velo-

city energy remaining in the exiting flow (see discussion

of draft tubes in Croaaflow

turbines,

p. 178).

With its high specific speed, the propeller turbine per-

mits reasonable speeds in spite

low operating head.

Runner

diameter is one factor affecting runner speed.

With large runners, some form of speed increase is

required to attain speeds of commonly used generators.

However, the speed of small-capacity, small-diameter

propeller runners associated with some micro-hydro-

power units may permit direct coupling to four- or six-

pole generators.

In large hydropower plants, flow through propeller tur-

bines is regulated by varying the pitch of the runner

blades and wicket gates while the turbine is in opera-

tion.* A hollow turbine shaft accommodates a regulat-

ing rod whose movement, controlled by a governor, is

transmitted to the runner blades through a suitable link

mechanism. This involves a level of sophistication and

cost usually unacceptable for developers of micro-

hydropower sites. To reduce the complexity of the

flow-regulating mechanism, some commercial units vary

only the wicket gates.

Micro-hydropower plants using propeller turbines gener-

ally do not have tightly closing wicket gates to regulate

flow. To start, stop, and dewater the runner, butterfly

valves are often used at the inlet to the turbine. They

are not usually used to regulate flow, because they

can

introduce significant losses. A butterfly valve included

in the draft tube (Fig. 6.13) can be used to start and

stop a runner and also to regulate flow if good part-load

efficiencies are not necessary.

* A wicket gate is composed of angularly adjustable

blades, usually just upstream of the runner, which

control the flow through the turbine.

Turbines 183

keep in mind the rapid closure possible with a butterfly

valve (see &Itterfly p. 159) and its impact on

surge pressures in the penstock (see discussion of surge

pressures in Sl

m p. 133).

As with all reaction turbines, propeller runners face a

problem with cavitation. As water passes over the run-

ner, the pressure may decrease to the point that water

vaporizes, forming bubbles. When these bubbles are

carried downstream to area5 of greater pressure, they

collapse violently, producing very large instantaneous

pressures that fatigue and erode any nearby surface.

Cavitation leads to pitting of the runner, which can

eventually eat away entire portions of the blades,

thereby reducing turbine efficiency. It is accompanied

by vibration and a noise similar to that of gravel in a

rotating

vessel.

Fig.

6.13. Butterfly

valves have been lncorpomted in

these

draft tubes to start and stop 2O-inch

propeller runners

installed

in an open-flume configuration

To reduce equipment cost further, several approaches

may be taken:

e If adequate flow is always available and the plant is

connected to the grid or a load controller is

used

with an isolated plant, no flow regulation is neces-

sary and a fixed-blade propeller can be used.

o If flow adjustment is not required often--perhaps

several times a year, possibiy to accommodate-

reduced streamflow during the dry season--a

runner

with manually adjustable blades can bc used. A

manually adjustable propeller runner consists of a

central hub to which the blades are bolted in a man-

ner that allows the blade angle to be adjusted. The

adjustment can be made through a “window” in the

turbine throat after the turbine has been shut down

and dewatered.

Although proper design of the runner can decrease the

likelihood of cavitation, the principal factor affecting

its occurrence is the turbine setting-the distance of the

turbine above the tailwater. If it is set too hinh above

the tailwater, the reduced (suction) pressure ii the tur-

bine will cause even a well-designed

runner

to cavi-

tate. The tendency for cavitation also increases when a

runner operates at high speeds. Therefore, for a given

head, a small and consequently lower-cost, high-speed

runner must be set lower than a large, low-speed runner’.

Using io lower-cost runner thtlrefore implies a turbine

more accessible to flood waters and possibly

exca-

vation to accommodate the necessary draft tube.

a If flow varies significantly, an installation can use

multip!e fixed-blade propeller turbines, usually of

different size, with units switched on or off depend-

ing on Lhe flow available (see Alternative hub&e

configurcations, p. 42).

The efficiency of well-designed propeller turbines

attains 10%; with adjustable blades and wicket gat.es, a

high efficiency can be maintained down to 30% of

design flow. However, efficiency quickly drops when a

fixed-Siade propeller turbine is operated at part-load.

Runaway speeds of a propeller turbine are a higher per-

centage of nominal operating speed than those asso-

ciated with impulse turbines. Runaway speeds are

commonly 2.5-3 times design operating speed. Flew

through the turbine must be stopped or diverted to pro-

tect the turbine and generator from overspeed during

loss of load. With small turbines, a butterfly valve

closed either by water pressure or a counter-balance

weight can be used. During operation, the valve is held

open by a solenoid energized directly from the genera-

tor output or through a sensing relay. Any design should

Proper turbine setting to avoid cavitation can be deter-

mined through the use of a parameter called Thoma

sigma (for a description of this parameter, s+:e TUR I

BINE PERFORMANCE UNDER NEW SlTE COHD’-

TIONS, p. 191). Either model or full-size trstirig deter-

mines the value of this parameter for a specific: runner.

For preliminary planning purposes, experience: curves’ -

curves based on a compilation of experience to date--

can be used.

When the propeller turbir,c is used at a very-low-head

site, small reductions in not head can have a significant

impact on power output. Consequently, unlike the set-

ting of impulse and most Francis turbines, the setting

and design of the powerho JSC and tailrace for a scheme

using a propeller turbine are intricately tied to the

physical configuration and the ravitatiou characteristics

of this turbine and the design of the draft tube to be

used.

In addition, even with propeller turbines used at low-

head micro-hydropower installations, large volumes

water Fre used. This requires a penstock of more

generous proportions. Because a large penstock is con-

siderably more rigid than small-diameter plastip pipe

commonly used for high-head installations, alignment of

the penstock with the turbine becomes more critical.

Fabrication

Although propeller turbines are appropriate at low-head

sites because of their relatively high speed, they are not

184 Turbines

widely used with micro-hydropower schemes, largely

because of their more complex design. For low-head

sites, crossflow turbines are frequently selected. Extra

gearing is then required, but this is seen a small price to

pay for a turbine that is otherwise seen as easy to

design and build.

Proper design of a propeller turbine is essential to

*obtain a runner that is efficient and free of cavitation.

Numerous parameters--number of blades and their pro-

file, width, and pitch, runner hub diameter and overall

diameter--must be determined and the guide vanes and

draft tube correctly designed. Simplifications are pos-

sible, but they require a good understanding of the

theory of propeller turbine design.

Two popular approaches to local fabrication of propeller

turbines are to form runner blades from steel plate or to

modify a marine propeller. Blades are also cast, either

separately from or integrally with the hub, but this

requires additional facilities and skills. The turbine cas-

ing, guide-vane assembly, butterfly valve, and draft

tube can be fabricated of steel sheet.

Few references present designs for propeller turbines.

In Design of Small Water Turbines for Farms and Small

Communities (43). Durali oresents a comnlete set of

working drawings’for a pripeller turbine designed to

generate 5 kW under a head of 10 m. Whereas he never

actually fabricated the turbine, Ho did this later for a

Master’s thesis (58). For this effort, the major compo-

nents of the turbine-the blades, hub, and housing-were

made of fi lerglass-reinforced plastic. The thesis con-

tains details on the technique used in its manufacture.

Cost-reduction approaches

Several approaches to reducing cost and sophistication

of propeller turbines have already been mentioned.

These include the use of fixed-blade runners (Fig. 6.14)

and a tubular configuration (Fig. 6.15).

Another approach to reducing overall cost is to elimi-

nate the powerhouse entirely. A bulb turbine, with the

generator located with the turbine within the water pas-

sageway, e!iminates the need for a powerhouse. Even if

an externally mounted generator is used--for example,

with a tubular or right-angle drive unit-only the gener-

ator itself needs to be covered, weatherproofed, and

located above flood stage. A small structure can

enclose and protect the associated controls and switch-

gear.

Waterwheels

Chronologically, waterwheels represent an initial step in

the evolution of machines to harness the energy of fall-

ing water (Fig. 6.16). They are large, bulky, relatively

slow-turning machines that are better suited to gener-

ate mechanical power to drive pumps or small grain

mills and sawmills than to drive high-speed electric

generators. However, with appropriate gearing, high

Fig. 6.14. A fixed-blade 20-inch propeller

runner,

with

guide-vane assembly and turbine thraat, fabricated by a

small U.S.

company.

The

lower bearing

that is

visible is

waterlubricated.

speed can be obtained (Fig. 6.17). With the need to

generate large quantities of electric power, water-

wheels have been replaced by modern, smaller, high-

speed turbines.

The Energy Primer (8) briefly describes the basic types

of waterwheels. A detailed description of sizing of an

overshot waterwheel is found in “A Design Manual for

Water Wheels” (92); however, this publication does not

cover its actual construction. For those interested in

using a waterwheel to drive a piston pump for pumping

water, it does provide information on pump selection,

methods of driving pumps, and a simple pump design.

Pumpe a.s turbines

One method for reducing the cost of a hydropower

installation that is gaining popularity is to use pumps in

reverse as turbines (Fig. 6.18). Pumps, of generally

small capacity, have been used for years in industrial

applications to recover energy otherwise wasted in

Turbines 18 5

Fig. 6.15. A completed B-inch, fired-blade propeller W-

blne, with draft tube,

which

generates l-10 kW

under a

head

of 2-8

Fig. 6. !6. ‘This overshot waterwheel

WQS one of

many

that

were wk!ely used in the United States

to provide

mechani-

cal power for a variety of industries.

industrial applications. On the large scale, pump/tur-

bines have been used around the world with pumped-

storage hydropower schemes.

Pumps provide several advantages:

o Because they are I-lass-produced by numerous manu-

facturers, they cost less than turbines.

e Because they are standard and available off the

shelf, delivery times are minimized.

Fig.

6.17.

waod-lined canal (see Fig. 5.79) decends

about

5 m, passing underground Oeft photograph, foreground) to

stage of gean’ng and a flat tMi in the mill house adds a

strike an undershot waterwheel Crfght

photograph

and

left

second

stage to

mise

the qpeed

to the 40QO

rev/min

needed

to opemte the hammer mill.

photograph,

center).

Spur gears ore used for the first

186 Turbines

prepared by a pump manufacturer that n arkets pumps

as turbines. In addition, the Micro-hydropower Hand-

book (761 summarizes a simpie technique for converting

the-required turbine performance characteristics, based

on site cof-ditions, to the necessary pump characteris-

tics. The method gives only a rough approximation. If

the use ~1” a pump as a turbine is being considered

seriously, the manufacturer should be contacted.

Actual tests are recommended to verify a pump’s per-

f;rmance as a turbine, especially for larger, more costly

units.

Fig.

6.18. An axial-ftow pump manufacturered

in-country

used

as Q turbine.

There is no question that pumps can be used as turbines.

Rather, the concern is with their efficiency, cavitation

characteristics, and operating rulge.

One major difference between turbines and pumps used

as turbines is thai flow through the latter is determined

by the head undci which they operate; there is no effi-

cient way of controlling flow through a pump. On the

other hand, hydraulic turbines have efficient flow con-

trols, but this is one reason they cost more.

lJsi?!g pumps where constant head and flow conditions

arc available presents no difficulties. This is the reason

they are used in industrial applications where process

energy has to be dissipated. This is also the case at

hydropower sites where adequate water is available to

onerate a unit at design flow whenever it is in use.

Under these circumstances, a constant flow passes

through the turbine. A constant power is generated that

can either be fed into a grid or, in an isolated applica-

tion, be controlled by of a load controller. Pumps as

turbines are less frequently used if a hydopower plant

has to harness a range of flows. However, by using at

least two pumps at a site, preferably of different capa-

cities, it is possible to harness a significant portion of

the energy available from varying streamflows (see

Alternative turbine configurations, p. 42).

Basic relationships

Performance characteristics of a pimp operating in the

pumping mode and generation mode differ; the pumps

cannot be assumed to have the same characteristics in

both modes. Numerous articles describe methods for

estimating the performance of a pump serving as a tur-

bine. A technical description is found in the article

“Centrifugal Pumps Used as Hydraulic Turbines” (66). A

more qualitative comparison of performance character-

istics is found in “Centrifugal Pumps as Hydraulic Tur-

bines for the Small Hydropower Market” (lOl), a paper

MisceUaxwous

In addition to the major turbine types described pre-

viously, other designs are being developed. Among

these are the in-stream or water-current turbine, the

Schneider engine, the Segner turbine, and marine

thrusters.

In-stream or watercurrent turbine

Several organizations around the world are currently

involved in research and development related to the in-

stream or water-current turbine. For example, experi-

mental work is being undertaken by Nova Energy Sys-

tems on the St. Lawrence River and by the Intermediate

Technology Development Group (ITDG) on the Nile

(Fig. 6.19). The objective of these turbines is to harness

the energy of water flowing down a gradual gradient

without the need for civil works such as dams or pcn-

stocks. They extract some of the kinetic energy of a

large river rather than harnessing the potential energy

of a smaller stream. The operation of some of the in-

stream turbines tested is analogous to that of a Darrius

wind turbine. This type

turbine is composed of verti-

cal airfoils that are submerged in a stream and rotate

about a central vertical shaft which extends above the

water level to provide a power take-off (Fig. 6.20). The

turbine can be operated below a barge or floating pon-

toon moored to the shore. ITDG has also successfully

tested turbines with rotors similar to a conventional

horizontal-axis windmill but arranged on an inclined axis

so that the power take-off is above water level. ITDG’s

work has concentrated on the development of water-

pumping turbines with shaft power up to 1 kW, but elec-

tricity generation has also been demonstrated. Further

details csn be found in Water Current Turbines: An

Appraisal Handbook (23).

Harnessing the power in large flowing rivers is an

attractive idea, but its appeal may be somewhat miti-

gated with the realization that the power density in a

flowing stream is low. The power density in a river

flowing at 1.0 m/s is 500 W/m2 of cross-sectional area.

As with a wind turbine, a theoretical maximum of only

about 60% of this energy can be recovered. Further-

more, machines designed for low-cost manufacture in

developin countries typically are 50% efficient, so that

150 W/m

of intercepted area would be available at the

shaft. The power available from water current is pro;

portional to the cube of its velocity, so that 600 W/m”

would be available from a stream flowing at 2.0 m/s;

Turbines

187

Fig. 6.19. A

prototype

ITDG in-stream turbine being

tested on the

Nile

in the

Sudan. The

turbine is

immersed

m the si

!ream moving at 1.3 m/s and provides 1 kW

shaft

power to

pump

water. The two mooring

cables are

joined

to u single cable to the

bank

of the river.

howevex, such velociti

howevex, such velocities are not often found in a river

in which such a turbine could be used.

hour ba

so if used to generate electricity, it is available on a 24-

hour basis and could irrigate a significant area if used

for DUU

Although the output from a turbine intercepting an area

for pumping. Even with a streamflow velocity of 1 m/s,

of evlan several square meters is modest, and even more of evlan several square meters is modest, and even more

water-pumping water-current Lurbines are economically

compet

competitive with diesel pumps where fuel is expensive.

centrifugal pump ,,;

two-stage belt

speed increaser

mooring cable

)

mooring cable

(adjustable length)

turbine rotor

Fig. 6.20. Details of the in-stream turbine shown in Fig. 6.19.

floating

II \

direction

._ -__~--.

of curent

delivery pipe

188 Turbines

At a test site on the Nile, a prototype “low-cost” ttir-

bine had a payback period of less than two dry seasons

when used for irrigating a 0.4 ha riverside plot for vege-

table cultivation.

To increase the power density through the turbine, Nova

Energy Systems has experimented with incorporating

surfaces upsteam of the turbine to increase the effec-

tive velocity of the flow through the rotor. This

increases the cost and complexity of the turbine and is

unlikely to be worthwhile for small. machines using only

a small fraction of the available streamflow

Schneider engine

The Sch,ieider engine is another type of turbine, prima-

rily for lower-head application. This turbine incorpo-

rates a series of straight lifting foils mounted between

two continuous chains, similar to a continuous belt of

Venetian blinds running over a pair of parallel axles.

The large number of moving parts is a concern with this

design.

The U.S. Department of Energy (DOE) has partially

funded the development of a hydropower plant at a

canal drop in the Turlock Irrigation District in Califor-

nia to evaluate the performance of this type of turbine.

After a short time of continuous running, the 140 kW

unit jammed. A prebminary examination revealed the

failure of water seals and a main roller bearing and

cracks in the blade assembly and moving blades. These

failures occurred before the viability of the basic con-

cept was proven (95).

Segner

turbine

A few years ago, in looking for an approach to generat-

ing large: quantities of power than possible by using

traditional mills, yet at a lower cost than using its

crossflow turbines, Balaju Yantra Shala, a Nepali engi-

neering firm and mechanical workshop, resurrected the

Segner turbine, first developed in the mid-1700s (79).

The turbine is comprised of a vertical pipe with nozzles

mounted at the end of radial arms at the bottom. Watet

is fed by a flume into the upper end of the pipe and

released tangentially through the nozzles, as is the case

with a pcpular design for lawn sprinklers. The pipe,

generally 1 to 5 m long, essentially serves as a penstock

hut is free to rotate and is supported by a thrust bearing

at its base. Lower-than-design flows can be accommo-

dated simply by reducing or stopping flow through any

of the nozzles. This type of turbine can handle up to

300 f/s to generate power in the 2-10 kW range. Power

is tapped off the upper end of this rotating pipe section.

Probably the principal difficulty with the Segner turbine

is that it requires a site with a vertical drop.

Marine thrusters

Marine thrusters are propellers used for the propulsion

of ships or the maneuvering of oil rig supports. They

are also being used

for

low-head applications and have

the advantage of being commercial “off-the-shelf”

equipment, which can reduce plant costs. The DOE

funded the installation and testing of a prototype

240 kW unit in the Modesto (California) Irrigation Dis-

trict. The maximum turbine efficiency was about 60%,

possibly caused by mismatching of the turbine with the

site conditions, and some cavitation was also

exper-

ienced (95).

TURBINE SPECIFICATIONS

In requesting price quotations from suppliers of hydrau-

lic turbines and packaged units (Fig. 6.21), it is impor-

tant that as much of the information listed below as

possible accompany each inquiry. Not knowing all this

information may imply that the details of the project

have not been worked out in sufficient detail and that

further work is necessary before firm bids can be solic-

ited. Because a supplier of equipment can spend consid-

erable time preparing a tender, it is best to specify a

single configuration or to minimize the number of alter-

native configurations and extras. An advantage of doing

so is that it is much easier to compare tenders from the

various suppliers.

It is also advisable to avoid unnecessarily overspecifying

the equipment for a site, because this msy prevent the

supplier from proposing equipment that may actually be

better suited to the needs of the developer.

The following is a checklist to help ensure that a site

developer submits the necessary specifications to sup-

pliers of equipment so that they have adequate informa-

tion to prepare tenders:

Customer’s name, address, and telephone number or

cable address.

An indication of whethek ,n’y an estimate is desired

or whether a full detailel: prllposal or bid is required.

If possible, a map or sketch showing the layout of

the proposed site; drawings and photographs of exist-

ing structures and foundations are particularly

important and useful.

For quotes on hydraulic turbines, the following informa-

tion also should be submitted to suppliers:

Average net head available. For low-head sites, if

the head vari.ticeably with the flow, also

include the minimum and maximum heads. Penstock

profile and specifications (see below) must also be

described if only the gross head is provided.

Flow available for power generation. If virtually the

--7

entire stream is to be used, include a flow-duration

curve based on as many years of data as possible (see

Data proceasing, p. 29) or, if unavailable, provide

some indication of flow variation during a typical

year. Is there provision for storage? If so, how

much?

Turbines 189