Designing a Small Hydro Power Plant

Capable Of Producing 10 MW of

Electricity at Webuye along River

Nzoia

2015

Final Year Project

University of Nairobi

14/4/2015

I

Declaration

This work and material produced in this report is our original work and it has not been presented

or published elsewhere for academic purposes.

Chol Dhieu Gabriel

F18/34749/2010

Signed…………………………………………………………….

Paul Odhiambo

F18/29902/2009

Signed……………………………………………………………

John Odhach

F18/29942/2009

Signed………………………………………………………………

II

Dedication

We dedicate this project to our Almighty God who has been supporting us throughout the

duration of the project. We have also dedicated this work to our parent and sponsors.

III

Acknowledgement

We want to thank God almighty for His guidance and protection throughout this project.

We also want to appreciate our able supervisor Eng. Munyasi for his wise guidance. He has been

very supportive and essential in us achieving our objectives. We would also like to thank our

chairman, Prof.Ogola, for his support. He coordinated with the university’s transport department

to facilitate our travel to Webuye East County. We also want to thank Prof. Oduori for his

support and his introducing us to Eng. Sayi of KTDA power. We appreciate Eng. Sayi of KTDA

who provided us with a lot of information and reference material on the planning, design and

guidelines on small hydropower power plants. In addition to this, Eng. Sayi even went as far as

helping us locate hydrologist who came up with a practical flow duration curve. We would also

like to thank the Regional manager of WARMA and their entire staff for giving us volumes of

data on the River Nzoia. This data was very essential for the professional work that we

conducted. We would also like to give appreciation to Nzoia Water services Company

(NZOWASCO) for their invaluable advice regarding the River Nzoia. In addition to them, we

would also like to appreciate the Kenya Power staff at Webuye for providing us with the power

consumption data for the area and reliable future demand trends of the area. Also, we would like

to thanks the staff of the CDF offices in the constituency of Webuye for designating one of their

staff to guide us around the river. He was invaluable when we were conducting site evaluation.

He was also gracious enough to accompany us to Kakamega to the regional WRMA

headquarters. We would also like to thank Eng. Ndulu and Eng. Aduol for their constant support

especially in facilitating our transportation to Webuye. We would like to give our gratitude to

Mr. Mutai who drove us from Nairobi all the way to Webuye and also the many short trips

within Webuye safely. Last but not the least; we would like to thanks our parents, lecturers,

relatives, friends and sponsors for their tireless support throughout our studies at the university.

We owe our success in this project to all the above mentioned people.

IV

Abstract

The objective of this project is to come up with the design of small hydropower plant

capable of generating 10 MW at river Nzoia passing through Webuye East constituency. The

entire Bungoma County is suffering from power shortage. This is because the power from the

national grid is not sufficient to cater for the demand of the area. In addition to this, the

additional power from the hydropower plant will attract investors to the area. The hydropower

plant will have an immediate effect on the industries already set up. These industries have been

suffering from regular power outages and high cost of grid electricity.

To achieve a good design, the station location has to be selected as well as parameters

like head and discharge from the river has to be determined before any design analysis starts. We

carried out research by using questionnaires to get feedback from the local around the site

regarding the project. They were very friendly and they gave us a lot of information regarding

the River and the ownership of land and their thoughts on the project if at all it is going to be

implemented. Their responses and readiness to give up their ancestral land especially those living

around the river, motive us to proceed with the project.

In the data collection we used a hand held global positioning and ranging system (GPRS)

to determine the gross head by taking some points upstream and downstream and getting the

differences as our gross head. At the same time in analysis, we calculated the head losses due to

bends and friction in penstock which affect the final power output. The head losses were

subtracted from the gross head to get the net head that was used to calculate the power output

from generator or the power to be injected to the grid. On the other hand the flow rate was

obtained from a flow duration curve (FDC). Current flow data is not sufficient the design of

small hydropower plant. Thus, we had to visit Water Resource and management Authority

(WRMA) at their regional office in Kakamega to obtain the data pertaining river Nzoia and more

particularly the point around Nabuyole falls. Our skills as Mechanical engineering were out of

place in the flow data analysis hence we needed to consultant a Hydrologist who later came up

with FDC. From the analyzed data (plotted FDC) we selected our design discharge to be Q = 21

m3/s which was used for all our calculations. Therefore, the net head (Hn= 54.5 M) and design

discharge are the two most important parameters used for design analysis and selection of

hydropower components.

V

The next step was the design of civil structures e.g. weir, intake, settling basin, headrace,

head-tank (fore-bay), penstock, power house foundation and power house building. In addition to

this, the selection of electromechanical equipment i.e. turbine, generator, speed increaser, control

system was critical. The dimensions of the civil structures and electromechanical equipment

were calculated using empirical formulae and other formulae from reference materials. The

summary of dimensions and quantity of the hydropower components are tabulated under

summary section. The design analysis was done based on the economic viability and the site

configuration. The Francis turbine selected using the selection criteria discussed in chapter 2 of

electro mechanical equipment. The selection of this turbine was based on specific speed,

rotational speed, net head, power output and discharge from river. There are tables and charts

used for the selection of Francis turbine above other turbine types which is discussed in the

literature review. The generator type selected for this design was the synchronous type and the

rest of specifications of generator are in the conclusion part. The dimensions were calculated

using net head and design discharge discussed above.

The other important section was the cost of project. This is because it is the critical

determining factor in the planning of small hydropower plant. Hence, in the case of this project,

we got the cost estimate of civil structures and their work and the estimate cost

electromechanical equipment in the market. The cost also depend on other economic factors e.g.

the Dollar exchange rate affects the price of hydropower components thus make the cost of

project to go up or down. This is because most of the components are imported into the country

from specialized manufacturers. Therefore, the exact cost of this project can be determined

during the implementation though large deviations from the estimate are unlikely. The estimated

cost for this project is KSHS 212,524,000.00. This project can be funded by private

organizations, banks and the government Return on investment is very high since this plant is

most likely to generate revenue in the millions of shillings per day if the surplus electrical energy

is channeled into the national grid at the feed in tariff (FIT) rates. Though hydropower is a

renewable, green energy source it has some draw back which can be analyzed for sake of animal

life in the river and around the river. There is another environmental concern which is going to

affect people living around the plant i.e. the sound levels from the power house are at times very

high. This needs to be strictly controlled in order to reduce noise pollution to aquatics and people

living around the site during construction and operations

VI

Lastly, the project is capable of producing more than 10 MW of electrical energy.

However, pursuing this will be out of our small hydropower bracket. If this project is

implemented then the community and country at large can benefit. This project will create

employment for the locals and skilled Kenyans’. Therefore, during the implementation of the

project, we recommended that the other specialist in other fields to be involved. Therefore, in

conclusion, we recommended the implementation of this Project for it will greatly benefit the

Webuye residents’, industries and the country as a whole in meeting the desired electrical energy

output.

VII

Notation

Nomenclature

a, Difference between inlet and outlet radius of runner (m)

A, Area (m2)

B, Width of channel (m)

b, Height of shroud

C, Absolute velocity (m / s)

d, Diameter m

D, Diameter m

E, Specific hydraulic energy (J/kg)

F, Force N

g, Acceleration of gravity (m/s2)

H, depth of water in channel (m)

HN, Net head (m)

HG Gross Head (m)

I, Second area moment of inertia (m4)

K, Constant

k, conduction factor

L, Length m

m, Mass (kg)

N Number of measurements m

n, Rotational speed rpm

p, Pressure Pa

P, Power W

Q, Flow rate (m3

/s)

r Radius m

Re, Reynolds number

V, velocity (m / s)

U Peripheral velocity (m/s)

W Relative velocity (m/s)

VIII

Z Number of items m

WRMA: Water resource management authority

FDC: Flow duration curve

BEP: Best efficiency point

FIT: Feed in Tariff

IX

Table of Contents

DECLARATION ....................................................................................................................... I

DEDICATION .......................................................................................................................... II

ABSTRACT ............................................................................................................................. IV

CHAPTER 1.0 INTRODUCTION ............................................................................................ 1

1.1 Definition and classification of small hydropower ............................................................... 1

1.2 Objective of Project .............................................................................................................. 1

1.3 Reasons for the Project ......................................................................................................... 1

1.4 Advantages of a Small Hydro Power Plant: ......................................................................... 1

1.5 Disadvantages of a Small Hydro Power Plant ...................................................................... 2

1.6 Project Area .......................................................................................................................... 2

CHAPTER 2.0: LITERATURE REVIEW ................................................................................ 3

2.1 Introduction ........................................................................................................................... 3

2.2 Site Configurations ............................................................................................................... 3

2.2.1 Schemes can also be defined as:- ................................................................................... 3

2.2.1.1 Run-of-river schemes .............................................................................. 3

2.4 Fundamentals of hydraulic engineering ................................................................................ 4

2.4.1 Introduction .................................................................................................................... 4

2.4.2.1 Head losses due to friction ...................................................................... 4

2.4.2.2 Loss of head due to turbulence ............................................................... 4

2.4.2.3 Trash rack (or screen) losses ................................................................... 4

2.4.2.4 Loss of head by sudden contraction or expansion .................................. 4

2.4.2.5 Loss of head in bends .............................................................................. 5

2.4.2.6 Loss of head through valves ................................................................... 5

2.4.3 Transient flow ................................................................................................................ 5

2.5 HYDRAULIC STRUCTURES (CIVIL STRUCTURES) ................................................... 5

2.5.1 Intake Weir..................................................................................................................... 6

2.5.2 Side intake ...................................................................................................................... 6

2.5.2.1 Location of Intake ................................................................................... 6

2.5.3 Settling Basin ................................................................................................................. 7

2.5.4 Headrace (channel) ........................................................................................................ 7

2.5.5 Head tank (fore-bay) ...................................................................................................... 7

2.5.5.1 Spillway at the head-tank ........................................................................ 7

2.5.6 Penstock ......................................................................................................................... 7

2.5.6.1 Penstock Material.................................................................................... 7

2.5.7 Tailrace .......................................................................................................................... 7

2.6 EVALUATING STREAMFLOW ........................................................................................ 7

2.6.1 Introduction .................................................................................................................... 7

2.6.2 Stream flow records ....................................................................................................... 8

2.6.3 Stream Flow Characteristics .......................................................................................... 8

2.6.4 Flow Duration Curves (FDC) ........................................................................................ 8

2.6.5 Evaluation of gross head ................................................................................................ 8

2.6.6 Estimation of net head ............................................................................... 8

2.6.7 SITE EVALUATION METHODOLOGIES ................................................................. 8

X

2.6.7.1 Introduction .......................................................................................... 8

2.8. ELECTROMECHANICAL EQUIPMENT ......................................................................... 9

2.8.1 Powerhouse .................................................................................................................... 9

2.8.2 Hydraulic turbines .......................................................................................................... 9

2.8.2.1 Types and configuration ......................................................................... 9

2.8.3 Specific speed .............................................................................................................. 10

2.8.4 Turbine selection criteria ............................................................................................. 10

2.8.4.1 Net head ................................................................................................ 11

2.8.4.2 Discharge .............................................................................................. 11

2.8.4.3 Cavitation .............................................................................................. 11

2.8.4.4 Rotational speed .................................................................................... 11

2.8.5 Runaway speed ............................................................................................................ 12

2.8.6 Turbine efficiency ........................................................................................................ 12

2.8.7 Speed increasers ........................................................................................................... 12

2.8.7 Generators .................................................................................................................... 12

2.8.7.1 Type of Generator ................................................................................. 12

2.8.7.2 Exciters ................................................................................................. 12

2.8.7.3 Speed Governors ................................................................................... 13

2.8.7.4 Switchgear equipment ........................................................................... 13

2.8.8 Automatic control ........................................................................................................ 13

2.8.8.1 Plant service transformer ...................................................................... 13

2.8.8.2 DC control power supply ...................................................................... 14

2.8.8.3 Headwater and tail-water recorders ...................................................... 14

2.9 ENVIRONMENTAL IMPACT AND ITS MITIGATION ................................................ 14

2.9.1 Burdens and impacts identification .............................................................................. 14

2.9.2 Impacts in the construction phase ................................................................................ 14

2.9.3 Water intakes, open canals, penstocks, tailraces .......................................................... 15

2.9.4 Impacts arising from the operation of the scheme ....................................................... 15

2.9.4.1 Sonic impacts ........................................................................................ 15

2.9.4.2 Landscape impact.................................................................................. 15

2.9. 5 Biological impacts ...................................................................................................... 16

2.9.5.1 In the reservoir ...................................................................................... 16

2.9.5.2 Trash-rack material ............................................................................... 16

2.9.6 Impacts from transmission lines .................................................................................. 16

2.9.6.1 Visual impact ........................................................................................ 16

2.9.6.2 Health impact ........................................................................................ 16

3.1 Design of electromechanical equipment ............................................................................. 17

3.1.1 Determination of power output .................................................................................... 17

3.2 Design of Francis turbines ............................................................................................ 18

3.2.1 Specific speed, NS ........................................................................................................ 19

3.2.2 Rotational speed, N ...................................................................................................... 19

3.2.3 Dimensions of Francis turbine ..................................................................................... 20

3.2.4 Cavitation design ......................................................................................................... 23

Knowing the specific speed, the required NPSH can be calculated as; .................................... 23

3.3 Design analysis of the draft tube ......................................................................................... 24

3.3.1 Inlet diameter Di ........................................................................................................... 24

XI

3.3.2 Exit diameter of draft tube, .......................................................................................... 25

3.3.3 Tail race water level, T ................................................................................................ 26

3.4 Generator Design ................................................................................................................ 26

3.4.1 Number of poles ........................................................................................................... 26

3.4.2 Speed of generator ....................................................................................................... 27

3.4.3 Exciter of generator ...................................................................................................... 27

3.4.4Generator type ............................................................................................................... 27

3.4.5Generator output ........................................................................................................... 28

3.5 Power transmission facility (speed increaser) ..................................................................... 28

3.6 Control facility of the turbine and generator. ...................................................................... 28

3.6.1 Speed governor ............................................................................................................ 28

3.7 DESIGN OF CIVIL STRUCTURES.................................................................................. 29

3.7.1 Penstock hydraulic calculations ................................................................................... 29

3.7.2 Determination of the penstock thickness, tp ................................................................. 29

3.7.3 Head loss in the penstock ............................................................................................. 31

3.7.3.1 Head loss due to entry and exit, hV ....................................................... 31

3.7.3.2 Head loss due to bend, hb ...................................................................... 31

3.7.3.3 Head loss due to friction, hf .................................................................. 31

3.7.4 Design of head race (open channel) ............................................................................. 32

3.7.5 Intake Weir (Dam) ....................................................................................................... 33

3.7.5.1 Weir height calculations ....................................................................... 33

3.7.6 Side intake .................................................................................................................... 33

3.7.7 Settling basin design .................................................................................................... 34

3.7.8 Head tank ..................................................................................................................... 34

3.7.8.1 Head tank capacity ................................................................................ 35

CHAPTER 4: PROJECT COST ESTIMATION .................................................................... 36

4.1 DIRECT COST ................................................................................................................... 36

4.1.1 Preliminaries (for civil structure work)........................................................................ 36

4.1.2 Intake weir ................................................................................................................... 36

4.1.3 Settling basin ................................................................................................................ 36

4.1.4 Fore-bay ....................................................................................................................... 36

4.1.5 Spillway ....................................................................................................................... 36

4.1.6 Penstock civil work ...................................................................................................... 36

4.1.7 Penstock pipes .............................................................................................................. 36

4.1.8 Canal ............................................................................................................................ 37

4.1.9 Power house ................................................................................................................. 37

4.1.10 Francis turbine ........................................................................................................... 37

4.1.11 Synchronous generator............................................................................................... 37

4.1.12 Transmission line ....................................................................................................... 37

4.1.13 Construction supervision ........................................................................................... 38

4.2 INDIRECT COST OF CONTRACTOR ............................................................................ 38

4.2.1 Engineering cost........................................................................................................... 38

4.2.2 Contingencies ............................................................................................................... 38

4.2.3 Administration ............................................................................................................. 38

CHAPTER 5.0: DISCUSSION, CONCLUSIONS AND RECOMMENDATIONS .............. 39

5.1 DISCUSSION ..................................................................................................................... 39

XII

5.3 CONCLUSION ................................................................................................................... 43

5.4 RECOMMENDATIONS .................................................................................................... 44

REFERENCES: ....................................................................................................................... 45

APPENDICES ......................................................................................................................... 47

Flow duration curve ......................................................................................................................

Moody Chart .................................................................................................................................

Satellite image of webuye .............................................................................................................

Photos of Nabuyole Falls ..............................................................................................................

List of tables

Table 2.10: Typical efficiencies of small turbines ............................................................ 18

Table 2.6: Range of heads ................................................................................................. 18

1

CHAPTER 1.0 INTRODUCTION

1.1 Definition and classification of small hydropower

Small hydro power (SHP) may be classified according to different criteria, such as head,

powerhouse layout and installed capacity of SHP. SHP stations are classified in terms of their

capacity. SHP capacity may vary at different times and in different countries but it has no strict

definition. Generally, SHP is the scheme with installed capacity up to 10 MW

1.2 Objective of Project

To design a small hydro power plant capable of producing 10 MW of electricity under a water

head of 57 m along River Nzoia in Bungoma County. This involves the selection of the most

efficient and economical turbines as well as an optimum selection of other plant components

(e.g. civil works, water resource potential and electromechanical equipment).Putting into

consideration the environmental impact and its mitigation and economic analysis

1.3 Reasons for the Project

Before picking on this project we were motivated by the following factors:

1. The desire to harness the existing hydro potential in remote areas of our country.

2. The need to provide a clean and cheap source of energy for the rural areas to supplement

the expensive fossil fuel sources of energy currently in use.

3. The need to hasten the pace of rural electrification programs by providing additional

electrical energy.

1.4 Advantages of a Small Hydro Power Plant:

i. Its suitability for decentralized development, fully using local materials and appropriate

technology with the participation of the local people.

ii. Its mature technology and small investment risk.

iii. Its low operating costs, easy maintenance and reliable power supply.

iv. Little environmental impact during construction, with some positive impact on the

environment.

v. The obvious social benefit to a developing local economy and improvements in the

material and spiritual life of local residents.

2

vi. Increasing revenue for the local government and income for local people.

vii. Creating more jobs and reducing the migration of rural people into cities.

viii. Developing tourism in rural areas.

1.5 Disadvantages of a Small Hydro Power Plant

i. Relatively high initial capital cost which might make it expensive for individual

institutions to afford.

ii. Must be sited where there is a water fall for example Nabuyole falls which in most cases

is accompanied by poor accessibility.

iii. Because its installation involves some site work, it is bound to interfere with the river

flow and ecosystem. This may lead to objection by the local people who might be

affected by such interferences.

1.6 Project Area

Webuye is an industrial town in Bungoma East District, Bungoma county in the Western

Province of Kenya. Located on the main road to Uganda, the town is home to the Pan African

Paper Mills, the largest paper factory in the region, as well as a number of heavy-chemical and

sugar manufacturers. Webuye municipality covers 69 square kilometers.

Villages near Webuye include Lugulu, Milo, Maraka and Misikhu. Webuye is home to

Broderick Falls of the river Nzoia.

Naming

In the pre-independence times, Webuye was known as Broderick Falls, after the first white man

to visit the nearby Nabuyole falls on River Nzoia. Itwaslater renamed after a cobbler,Nabuyole,

who used to repair shoes for railway workers.[1]

Railways

The town is located on the main railway from Mombasa to Uganda. The area around the town is

inhabited by both the Bukusu and the Tachoni.

Statistics

Webuye has a tropical climate, and the land around it is used mainly for subsistence agriculture.

The Latitude is 0.6166667°, Longitude 34.7666667°, average annual Temperature of 24°C /

75.2°F.Elevation = 1523m

3

CHAPTER 2.0: LITERATURE REVIEW

2.1 Introduction

This chapter is going to give the over view of small hydropower and its components. It is going

to provide review of site configuration, stream flow, site evaluation, civil structures and design,

electromechanical selection and environmental impact and mitigation. This chapter is very

important because it give technical background of the design used in this project.

2.2 Site Configurations

The objective of a hydropower scheme is to convert the potential energy of a mass of water,

flowing in a stream with a certain fall to the turbine, into electric energy at the lower end of the

scheme, where the powerhouse is located. The power output from the scheme is proportional to

the flow and to the head.

Schemes are generally classified according to the “Head”:-

High head: 100-m and above

Medium head: 30 - 100 m

Low head: 2 - 30 m

These ranges are not rigid but are merely means of categorizing sites.

2.2.1 Schemes can also be defined as:-

2.2.1.1 Run-of-river schemes

Run-of-river schemes are where the turbine generates electricity as and when the water is

available and provided by the river. When the river dries up and the flow falls below some

predetermined amount or the minimum technical flow for the turbine, generation ceases.

For this design, the scheme is run-of-scheme with weir built across the river to divert water to

intake, settling basin, channel, fore bay and finally to penstock. The river Nzoia has enough

water and it has water fall.

2.3 Planning a small hydropower scheme at river Nzoia

The most important parameters in planning small hydropower plant are design flow rate and the net head.

The two parameters determine the power to be produced and the success of the project. River Nzoia has

water fall which give us the head available for production of electricity. The river itself is gauged by

water resource and management authority. Hence, all the data pertaining to the river Nzoia were provided

4

by WRMA and from the data we got design flow rate. We used handheld global positioning system

(GPS) to get gross head (57m) and coordinates of location of intake and the power house. The other part

of planning was Environmental impact assessment and mitigation measures and Economic

evaluation of the project and financing potential. After having all those data in place, the

planning went ahead to select the plant components with their dimensions and other

specifications. The detailed of design are dealt with under chapter three.

2.4 Fundamentals of hydraulic engineering

2.4.1 Introduction

Hydraulic engineering is based on the principles of fluid mechanics, although many empirical

relationships are applied to achieve practical engineering solutions. Until now there does not

exist, and probably never will, a general methodology for the mathematical analysis of the

movement of fluids. Based on the large amount of accumulated experience, certainly there are

particular solutions to specific problems. Experience that goes back as far as 2500 years ago,

when a massive irrigation system, that is still operative, was built in Sichuan, China, and to the

Roman Empire’s builders of the aqueducts.

2.4.2.1 Head losses due to friction

The head loss due to friction was calculated in penstock. The loss was related to length of

penstock, friction factor and velocity as the main parameter. Losses in penstock reduced the

power output.

2.4.2.2 Loss of head due to turbulence

Water flowing through a pipe system, with entrances, bends, sudden contraction and

enlargements of pipes, racks, valves and other accessories experiences, in addition to the friction

loss, a loss due to the inner viscosity. This loss also depends of the velocity and is expressed by

an experimental coefficient K multiplying the kinetic energy v2/2g.

2.4.2.3 Trash rack (or screen) losses

A screen or grill is always required at the entrance of a pressure pipe. The flow of water through

the rack also gives rise to a head loss. Though usually small.

2.4.2.4 Loss of head by sudden contraction or expansion

When the pipe has a sudden contraction there is a loss of head due to the increase in velocity of

the water flow and to the turbulence.

5

2.4.2.5 Loss of head in bends

Pipe flow in a bend experiences an increase in pressure along the outer wall, and a decrease of

pressure along the inner wall. This pressure unbalance causes a secondary current.

The head loss produced in these circumstances depends on the radius of the bend and on the

diameter of the pipe. Such losses were calculated and subtracted from gross head.

2.4.2.6 Loss of head through valves

Valves or gates are used in small hydro scheme to isolate a component from the rest, so they are

either entirely closed or entirely open. Flow regulation is assigned to the distributor vanes or to

the needle valves of the turbine.

The loss of head produced by the water flowing through an open valve depends on the type and

manufacture of the valve. As for this project such losses were considered.

2.4.3 Transient flow

In steady flows, where discharge is assumed to remain constant with time, the operating pressure

at any point along a penstock is equivalent to the head of water above that point. If a sudden

change of flow occurs, for instance when the plant operator, or the governor system, open or

close the gates too rapidly, the sudden change in the water velocity can cause dangerous high and

low pressures. This pressure wave is known as water hammer and its effects can be dramatic: the

penstock can burst from overpressure or collapse if the pressures are reduced below ambient.

2.5 HYDRAULIC STRUCTURES (CIVIL STRUCTURES)

A hydropower development includes a number of structures, the design of which will depend on

the type of scheme, local conditions, access to construction material and also local building

traditions in the region. The following structures are common in a hydro scheme:

Diversion structure

Dam or Weir

Spillway

Energy dissipation arrangement

Fish pass

Residual flow arrangement.

Water conveyance system

6

Intake

Canal/channel

Tunnels

Penstock

Power house

Design aspects and common solutions for these structures are presented below:

2.5.1 Intake Weir

The diversion weir or intake weir is a barrier built across the river used to divert water through

an opening in the riverside (the ‘Intake’ opening) into a settling basin. For purpose of this project

wet masonry dam was adapted.

2.5.2 Side intake

The side intake is used to draw water from the river to the conveyance hence it is the type of

intake chosen for this design due to its simplicity. The full design is undertaken under design

analysis. Therefore, the height of the weir depends on the river slope.

2.5.2.1 Location of Intake

The location of the intake is selected considering the following conditions:

Extreme care must be taken in this selection for the development of small-scale hydropower as

the cost of the intake facilities significantly determines the development project economy.

(1) River Channel Alignment

For run-of-river types of hydropower plant, the appropriate section within the river channel to

construct the intake structure is where the channel is as straight as possible in order to ensure

steady and smooth flow of water to the intake and also to prevent scouring of the river banks

downstream of the intake site.

(2) Stability of Hillside Slope

The presence of a landslide or unsteady slope near an intake weir site causes concerns for

possible obstruction at the water intake by sediments from the landslide or erosion. Sufficient

consideration should, therefore, be given to the stability of nearby hillsides as part of the intake

location selection process.

7

2.5.3 Settling Basin

The settling basin is used to trap sand or suspended silt from the water before entering the

penstock. It may be built at the intake or at the fore-bay (head tank). For the purpose of this

design it is built at the intake

It must have a structure that is capable of settling and removing sediment with a minimum size

that could have an adverse effect on the turbine and also have a spillway to prevent inflow of

excess water into the headrace.

2.5.4 Headrace (channel)

Headrace is channel leading water to a fore-bay or turbine. The headrace follows the contour of

the hillside so as to preserve the elevation of the diverted water. Hence, rectangular type of

headrace was adapted with masonry type. The details design of the same is cover in chapter three

of this project.

2.5.5 Head tank (fore-bay)

Pond at the top of a penstock or pipeline; serves as final settling basin, provides submergence of

penstock inlet and accommodation of trash rack and overflow/spillway arrangement.

2.5.5.1 Spillway at the head-tank

Generally, the spillway will be installed at the head-tank in order to release excess water and

discharged it to the river safely when the turbine stopped it.

2.5.6 Penstock

A penstock is a close conduit or pressure pipe for supplying water under pressure to a turbine

from fore-bay or head-tank.

2.5.6.1 Penstock Material

The pipe materials chosen for this project is commercial steel for the penstock due to its strength

to withstand the harsh conditions.

2.5.7 Tailrace

It is the conveyance that return the water back in to the river after passing through the turbine.

2.6 EVALUATING STREAMFLOW

2.6.1 Introduction

8

All hydroelectric generation depends on falling water. This makes hydropower extremely site

dependent. Sufficient and dependable stream flow is required. Also the topographic conditions of

the site must allow for the gradual descent of the river in a river stretch be concentrated to one

point giving sufficient head for power generation.

2.6.2 Stream flow records

In Kenya, stream flow records can be obtained from Water Resource and Management Authority

(WRMA). WRMA has gauged river Nzoia and they take reading everyday throughout the years.

2.6.3 Stream Flow Characteristics

A program of stream gauging, at a particular site over a period of years, will provide a table of

discharges that can be organized into a usable form.

2.6.4 Flow Duration Curves (FDC)

Way of organizing discharge data is by plotting flow duration curve (FDC). An FDC shows for

a particular point on a river, the proportion of time during which the discharge there equals or

exceeds certain values. The flows over the years of river Nzoia were obtained from WRMA and

the FDC of those values was plotted. From FDC the design discharge was read from the curve.

The plotted curved is put at the appendix.

2.6.5 Evaluation of gross head

The gross head is the vertical distance that the water falls through in giving up its potential

energy. Hence, handheld Global positioning system was employed to get gross head of this

project at river Nzoia water fall.

2.6.6 Estimation of net head

Having established the gross head available, it is necessary to calculate the losses, from trash

racks, pipe friction, bends and valves. Hence, the losses for this project were calculated under

design analysis. The net head was obtained after subtracting those losses.

2.6.7 SITE EVALUATION METHODOLOGIES

2.6.7.1 Introduction

Adequate head and flow are necessary requirements for hydro generation. Consequently site

selection is conditioned by the existence of both requirements. The site evaluation was done to

get the exact place to install power house and where to construct things like intake weir, intake,

9

and channel, fore-bay settling basin, spillways, penstock, tailrace and powerhouse. The

topographic of the site was considered and geological area of the site dealt with to give easier for

construction.

2.8. ELECTROMECHANICAL EQUIPMENT

This chapter gives the main description of the electromechanical equipment, some preliminary

design rules and some selection criterion.

2.8.1 Powerhouse

The role of the powerhouse is to protect the electromechanical equipment that convert the

potential energy of water into electricity.

2.8.2 Hydraulic turbines

Hydraulic turbines transform the water potential energy to mechanical rotational energy.

Formulae are based on work undertaken by Siervo and Lugaresi11, Siervo and Leva12 13,

Lugaresi and Massa14 15, Austerre and Verdehan16, Giraud and Beslin17, Belhaj18, Gordon19

20, Schweiger and Gregori21 22 and others, which provide a series of formulae by analyzing the

characteristics of installed turbines. Based on the formulae given by the authors above Francis

turbine was chosen for this project. The details about Francis turbine are given below;

All the formulae of this chapter use SI units and refer to IEC standards (IEC 60193 and 60041).

2.8.2.1 Types and configuration

The potential energy in water is converted into mechanical energy in the turbine by:

The water pressure can apply a force on the face of the runner blades, which decreases as it

proceeds through the turbine. Turbines that operate in this way are called reaction turbines. The

turbine casing, with the runner fully immersed in water, must be strong enough to withstand the

operating pressure. Francis turbines belong to this category.

Francis turbines

Francis turbines are reaction turbines, with fixed runner blades and adjustable guide vanes, used

for medium heads. In this turbine water entry is radial but exits axially. Photograph 2.8 shows a

horizontal axis Francis turbine. They are usually used for head ranges from 25 to 350 m.

10

Photo 2.8: Horizontal axis Francis turbine

The draft tube of a reaction turbine aims to recover the kinetic energy still remaining in the water

leaving the runner. The kinetic energy is proportional to the square of the velocity. Hence, a draft

tube is required to reduce the turbine outlet velocity. An efficient draft tube would have a conical

section but the angle cannot be too large, otherwise flow separation will occur. The optimum

angle is 7º but to reduce the draft tube length, and therefore its cost, sometimes angles are

increased up to 15º.

2.8.3 Specific speed

The specific speed constitutes a reliable criterion for the selection of the turbine, without any

doubt more precise than the conventional enveloping curves, just mentioned.

2.8.4 Turbine selection criteria

The type, geometry and dimensions of the turbine will be fundamentally conditioned by the

following criteria:

Net head

Range of discharges through the turbine

11

Specific speed

Rotational speed

Cavitation problems

Cost

2.8.4.1 Net head

The gross head is well defined, as the vertical distance between the upstream water surface level

at the intake and the downstream water level for reaction turbines or the nozzle axis level for

impulse turbines.

The net head is the ratio of the specific hydraulic energy of machine by the acceleration due to

gravity. This definition is particularly important, as the remaining kinetic energy in low head

schemes cannot be neglected.

The first criterion to take into account in the turbine's selection is the net head.

2.8.4.2 Discharge

A single value of the flow has no significance. It is necessary to know the flow regime,

commonly represented by the Flow Duration Curve (FDC) as explain earlier under stream flow

evaluation.

2.8.4.3 Cavitation

When the hydrodynamic pressure in a liquid flow falls below the vapor pressure of the liquid,

there is a formation of the vapor phase. This phenomenon induces the formation of small

individual bubbles that are carried out of the low-pressure region by the flow and collapse in

regions of higher pressure. The formation of these bubbles and their subsequent collapse gives

rise to what is called cavitation. The cavitation calculation is dealt with under design analysis.

2.8.4.4 Rotational speed

The rotational speed of a turbine was calculated to be 354 rpm but that speed was very low for

generator to do direct coupling hence the speed increaser was used to step off the speed of

generator

12

2.8.5 Runaway speed

Each runner profile is characterized by a maximum runaway speed. This is the speed, which the

unit can theoretically attain in case of load rejection when the hydraulic power is at its

maximum. Depending on the type of turbine, it can attain 2 or 3 times the nominal speed. Table

2.9 shows this ratio for miscellaneous turbines.

It must be remembered that the cost of both generator and eventual speed increaser may be

increased when the runaway speed is higher, since they must be designed to withstand it.

2.8.6 Turbine efficiency

The efficiency characterizes not only the ability of a turbine to exploit a site in an optimal

manner but also its hydrodynamic behavior.

Average efficiency means that the hydraulic design is not optimum and that some important

problems may occur e.g. cavitation, vibration, etc. that can strongly reduce the yearly production

and damage the turbine. The turbine efficiency was chosen from the best practice of Francis

turbine. But only manufacturers can provide the most reliable efficiency for the turbine.

2.8.7 Speed increasers

Due to low rotational speed of Francis turbine for this design, the speed increaser was adapted to

increase the speed of generator to the required speed without directly coupling the two. Hence,

belt speed increaser was selected.

2.8.7 Generators

2.8.7.1 Type of Generator

Synchronous generator was selected due to its advantages compare to asynchronous.

Independent exciter of rotor is provided for each unit Applicable for both independent and

existing power network.

2.8.7.2 Exciters

In case of synchronous generator, an exciter is necessary for supplying field current to generator

and keeping the output voltage constant even if the load fluctuates.

13

Synchronous generators

The synchronous generator is started before connecting it to the mains by the turbine rotation. By

gradually accelerating the turbine, the generator must be synchronized with the mains, regulating

the voltage, frequency, phase angle and rotating sense. When all these values are controlled

correctly, the generator can be switched to the grid. In the case of an isolated or off grid

operation, the voltage controller maintains a predefined constant voltage, independent of the

load. In case of the mains supply, the controller maintains the predefined power factor.

2.8.7.3 Speed Governors

It is adopted to keep the turbine speed constant because the speed fluctuates, changes in load,

water head and flow. The change of generator rotational speed results in the fluctuation of

frequency. The governor consists of speed detector, controller and operation. Hence Dummy

load type was adapted.

2.8.7.4 Switchgear equipment

In many countries the electricity supply regulations place a statutory obligation on the electric

utilities to maintain the safety and quality of electricity supply within defined limits. Kenya is not

exception in this obligation. According to this project the plant is going to operate in such a way

that the safety is first priority during operation. Thus, switchgear must be installed to control the

generators and to interface them with the grid or with an isolated load. Also metering equipment

must be installed at the point of supply to record measurements according to the requirements of

the electric utility.

2.8.8 Automatic control

Small hydro schemes are normally unattended and operated through an automatic control

system. Because not all power plants are alike, it is almost impossible to determine the extent of

automation that should be included in a given system.

2.8.8.1 Plant service transformer

Electrical consumption including lighting and station mechanical auxiliaries may require from 1

to 3 percent of the plant capacity; the higher percentage applies to micro hydro (less than 500

kW). The service transformer must be designed to take these intermittent loads into account. If

14

possible, two alternative supplies, with automatic changeover, should be used to ensure service

in an unattended plant.

2.8.8.2 DC control power supply

It is generally recommended that remotely controlled plants are equipped with an emergency 24

V DC back-up power supply from a battery in order to allow plant control for shutdown after a

grid failure and communication with the system at any time. The ampere-hour capacity must be

such that, on loss of charging current, full control is ensured for as long as it may be required to

take corrective action.

2.8.8.3 Headwater and tail-water recorders

In a hydro plant, provisions should be made to record both the headwater and tail-water. The

simplest way is to fix, securely in the stream, a board marked with meters and centimeters in the

style of a leveling staff, however someone must physically observe and record the

measurements. In powerhouses provided with automatic control the best solution is to use

transducers connected to the computer via the data acquisition equipment.

2.9 ENVIRONMENTAL IMPACT AND ITS MITIGATION

2.9.1 Burdens and impacts identification

Impacts of hydropower schemes are location and technology specific. A high mountain diversion

scheme situated in a highly sensitive area is more likely to generate an impact than an integral

low-head scheme in a valley.

2.9.2 Impacts in the construction phase

Schemes of the diversion type, multipurpose reservoir, inserted on an irrigation canal or built

into a water supply system produce very different impacts from one another, from both a

quantitative and qualitative viewpoint. Even the location of the powerhouse will be at the base

and shall not alter the ecological system.

15

2.9.3 Water intakes, open canals, penstocks, tailraces

The impacts produced by the construction of these structures are; noise affecting the life of

animals, danger of erosion due to the loss of vegetation through excavation work, turbidity of the

water and downstream sediment deposition, etc. To mitigate such impacts it is strongly

recommended that the excavation work should be undertaken in the low water season and the

disturbed ground restored faster. In view of its protective role against riverside erosion it is wise

to restore and reinforce the riverbank vegetation that may have been damaged during

construction of the hydraulic structures. The ground should be repopulated with indigenous

species, best adapted to the local conditions. Vehicle emissions, excavation dust, the high noise

level and other minor burdens contribute to damaging the environment when the scheme is

located in sensitive areas. To mitigate the above impacts the traffic operation must be carefully

planned to eliminate unnecessary movements and to keep all traffic to a minimum.

2.9.4 Impacts arising from the operation of the scheme

2.9.4.1 Sonic impacts

The allowable level of noise depends on the local population or isolated houses near to the

powerhouse. The noise comes mainly from the turbines and, when used, from the speed

increasers. Nowadays noise inside the powerhouse can be reduced, if necessary, to levels in the

order of 70 dB, almost imperceptible when outside.

2.9.4.2 Landscape impact

Each of the components that comprise a hydro scheme - powerhouse, weir, spillway, penstock,

intake, tailrace, and substation and transmission lines - has potential to create a change in the

visual impact of the site by introducing contrasting forms, lines, color or textures. The design,

location, and appearance of any one feature may well determine the level of public acceptance

for the entire scheme. Most of these components, even the largest, may be screened from view

using landscaping and vegetation. The powerhouse, with the intake, the penstock, and tailrace

and transmission lines must be skillfully inserted into the landscape.

16

2.9. 5 Biological impacts

2.9.5.1 In the reservoir

In integral low head schemes, peaking can result in unsatisfactory conditions for fish

downstream because the flow decreases when the generation is reduced. The lower flow can

result in stranding newly deposited fish eggs in spawning areas. The eggs apparently can survive

periods of de-watering greater than those occurring in normal peaking operation but small fish

can be stranded particularly is the level fall is rapid.

2.9.5.2 Trash-rack material

Almost all small hydropower schemes have a trash rack cleaning machine, which removes

material from water in order to avoid it entering plant waterways and damaging

electromechanical equipment or reducing hydraulic performance.

2.9.6 Impacts from transmission lines

2.9.6.1 Visual impact

Above ground transmission lines and transmission line corridors can have a negative impact on

the landscape. These impacts can be mitigated by adapting the line to the landscape, or in

extreme cases burying it.

2.9.6.2 Health impact

In addition to the visual intrusion, some people may dislike walking under transmission lines

because of the perceived risks of health effects from electromagnetic fields.

17

CHAPTER 3.0: DESIGN ANALYSIS

In this chapter we are going to carry out analysis on the small hydropower components. The

design analysis is based on net head and design flow obtained earlier on in previous chapter. The

analysis is done using some empirical formulae from reference materials cited under references.

Hence, the net head = 54.5 m and design discharge = 21m3/s

3.1 Design of electromechanical equipment

3.1.1 Determination of power output

P = g * Q * H * o * ρW

Where

P = power developed

g = gravitational acceleration

Q = design flow rate

H = head

o = overall efficiency

ρW= density of water

In our case;

Turbine efficiency, t= 0.94 (Francis turbine)

Transmission efficiency, m= 0.98 (Belt type)

Generator efficiency, g = 0.97 (synchronous generator)

Hence

o= t* m * g

o= 0.94 * 0.97 * 0.98

o= 0.89

Thus, power developed by generator is given by;

P = g * Q * H * o * ρW

P = 9.81 * 21 * 54.5 * 0.89 * 1000

P = 9,992,515 W

P = 9.992MW

P ≈ 10 MW

18

3.2 Design of Francis turbines

After the analysis, the Francis turbine was selected using net head, flow rate, rotational speed,

specific speed, power output and cost. The selection was arrived at using charts and tables as

shown below.

Turbine type Best efficiency

Kaplan single regulated 0.91

Kaplan double regulated 0.93

Francis 0.94

Pelton n nozzles 0.90

Pelton 1 nozzle 0.89

Turgo 0.85

Table 2.10: Typical efficiencies of small turbines

Turbine type Head range in metres

Kaplan and Propeller 2 < Hn

< 40

Francis 25 < Hn

< 350

Pelton 50 < Hn

< 1'300

Crossflow 5 < Hn

< 200

Turgo 50 < Hn

< 250

Table 2.6: Range of heads

19

These calculations are based on Lugaresi and Massa equations.

3.2.1 Specific speed, NS

NS = 1.924 /

Since Hn= 50m

= 1.924 / (54.5)0.512

= 0.2484

For Francis turbine the range of specific speeds is:

0.0η ≤ NS ≥ 0.33

Hence the specific speed is within the range thus acceptable.

3.2.2 Rotational speed, N

N =NS* E3/4 / √Q

But

Hn= 54.5 m

E = g * Hn= 9.81 * 54.5 = 534.6

Q = 21m3/s

20

Hence

N = 0.2484 * (534.6) 0.75 / √21

N = 6.026 t/s

Where t/s is turn per second. But N is always given in RPM.

Therefore in RPM is given below.

N = 6.026 RPS * 60 seconds / minute

N = 361.56 RPM

3.2.3 Dimensions of Francis turbine

Outlet diameter D3 is given by;

D3 = 84.5 * (0.31 + (2.488 * NS)) * √Hn/ (60 * N)

Inlet diameter D1 is given by;

D1= (0.4 + 0.095/ Ns) * D3

The inlet diameter D2 is given by;

D2 = D3/ (0.96 + 0.3781 * Ns)

For NS<0.164; D1 = D2

Using the above equations the diameters of the runner of Francis turbine are:

D3 = 84.5 * (0.31 + (2.488 * 0.2484)) * √η4.η/ (θ0 * θ.02θ)

D3 = 1.601 m

21

D1 = (0.4 + 0.095/0.2484) * 1.601

D1 = 1.253 m

D2 = 1.601 / (0.96 + 0.3781 * 0.2484)

D2 = 1.519 m

Hence, all these dimensions of diameters are given in figure of Francis turbine runner under

literature review above.

Ratio of width to diameter, (B/D)

n = B1 / D1; n = B2 / D2

Where the value of n varies from 0.1 to 0.45

Flow ratio, Kf

The flow ratio is the ration of the velocity of flow at the inlet to the theoretical jet velocity. Thus,

Flow ratio, Kf= Vf1 / √ (2gH)

The value of Kf varies from 0.15 to 0.30

Speed ratio Ku

The speed ratio is the ratio of peripheral speed at the inlet to the theoretical velocity. Thus,

Speed ratio, Ku = u / (√ (2gH))

The value of Ku ranges from 0.6 to 0.9

Using above equations yield:

22

Width B1

B1 = 0.45 * 1.601 m

B1 = 0.7205 m

Flow velocity, Vf1

Vf1 = Kf√ (2gH)

Vf1 = 0.3 * √ (2 * 9.81 * η4.η)

Vf1 = 9.81 m/s

Rim velocity (tangential), u1

u1 = π * D1 * N / 60

u1 = π * 1.θ01 * 3θ1.ηθ / 60

u1 = 30.31 m/s

Velocity of whirl at inlet Vw1

L= Vw1 * u1 / (g * h)

Vw1 = L * (g * h) / u1

Since the best efficiency of the Francis turbine is L= 0.94

Therefore

Vw1 = 0.94 * 9.81 * 54.5 / 30.31

Vw1 = 16.58 m/s

Guide vane angle (α) and the runner vane angle ( )

Tan α = Vf1 / Vw1 = 9.81 / 16.58

Tan-1(9.81 / 16.58)

α = 30.θ°

But

Tan = Vf1 / (Vw1 – u1)

So,

Tan = 9.81 / (16.58 – 30.31)

Tan-1(9.81 / (16.58 – 30.31))

= -35.5°

Width at outlet

n =B2 /D2 where n = 0.45

23

So

B2 = 0.45 * D2

B2 = 0.45 * 1.519

B2 = 0.6836 m

Hence u2 is given by;

u2 = π * D2 * N/60

u2 = π * 1.η19* 3θ1.ηθ / 60

u2 = 28.76 m/s

Velocity of whirl at outlet (Vw2)

Vw2 = g * H * h/ u2

Vw2 = 9.81 * 54.5 * 0.94 / 28.76

Vw2 = 17.47 m/s

Guide vane angle (ϕ2) and the runner vane angle at the outlet (β2) = 90°

Tan ϕ = Vf2 / u2

Tan-1 (9.81 /28.76)

ϕ = 18.8°

3.2.4 Cavitation design

If the water pressure in the runner is lower than the vapor pressure, cavitation may occur. The

impact of gas cavities collapsing close to the wall surface causes cavitation erosion. In order to

avoid the water pressure to drop below the vapor pressure, the turbine can be submerged. The

required level of submergence, expressed as Net Positive Suction Head (NPSH) depends on the

main dimensions and the speed number of the runner. The specific speed is a non-dimensional

expression for rotational speed at a given head at best efficiency point.

From previous calculation:

NS =0.2484

Knowing the specific speed, the required NPSH can be calculated as;

24

Where the parameters a and b are empirical constants, and, according to Brekke, dependent on

the speed number.

NS<0.55 gives a=1.12 and b=0.055

NS>0.55 gives a=1.12 and b=0.1· NS

Cm2 = Vf2 = flow velocity at outlet = 9.81 m/s

U2 = Vw2 = whirl velocity at outlet =17.47 m/s

NPSHrequired = 1.12 * (9.81)2 / (2 * 9.81) + 0.055 * (17.47)

2 / (2 * 9.81)

NPSHrequired = 6.35

NPSH has to fulfill the following requirement to avoid cavitation

NPSHrequired < hatm − hva − Hs

hva from the steam table at a temperature of 24°c = 0.03625 bar

1 atm = 1.01325 bar

hva= 0.03625 bar / 1.01325 bar * 1 atm

hva= 0.03578 atm = 0.3685 mWc

6.35 < (10.3 - 0.3685)

6.35 < 9.931 thus no cavitation occurs.

Where

hatm= atmospherically pressure, 1 atm = 10.3 mWc

hva= vapor pressure

Hs= submerging of the turbine. A negative value of Hs implies that the turbine is set below tail

Water level.

From the above calculation, the turbine is not subject to cavitation even without being

submerged. Thus submerging the turbine below the tailrace water level is not necessary to avoid

cavitation.

3.3 Design analysis of the draft tube

Conical Draft tube was selected for this design due to its advantages over other type.

Flare angle of 6°

3.3.1 Inlet diameter Di

Di = 1.601 m

25

Vertical height of draft tube

y = 2.75 * Di

y = 2.75 * 1.601 m

y = 4.403 m

3.3.2 Exit diameter of draft tube,

Consider the triangle below.

Tan 6° = x / y but y = 4.403 m

Tan 6° = x / 4.403

x = 4.403 * tan 6°

x = 0.4628 m

Total increment = 0.4628 m * 2

= 0.9256 m

D0 = 0.9256 m + 1.601 m

D0 = 2.5266 m

26

3.3.3 Tail race water level, T

T = 0.8 * Di

T = 0.8 * 1.601 m

T = 1.2808 m

3.4 Generator Design

3.4.1 Number of poles

Np= 120 * f / N

Where

Np= number of poles

F = frequency of supply i.e. 50 Hz in Kenya

N = rotational speed (RPM)

Np= 120 * 50 / 361.56

Np= 16.59 ≈ 17 poles

Table of Standard Rotational Speed of Generator

Referring to the original turbine speed and the rated generator speed, either direct coupling or

indirect coupling with power transmission facility (gear or belt) is selected so that the suitable

ratio of speed between turbine and generator can be matched. The total cost of turbine,

transmitter and generator shall also be taken into consideration. For small hydropower plant, 4 –

8 poles are selected to save the cost. Hence, 17 poles are not economically. Therefore, the speed

27

increaser is used to raise the speed of turbine to the standard speed of generator without directly

coupling the two.

Since the speed of the turbine was calculated as 362 RPM, it is seen that this speed is low and

hence needs to be increased. The ideal speed can be achieved by increasing the rotational speed

of turbine by a factor of four.

3.4.2 Speed of generator

Ideal speed = 4.1 * 362

= 1484.2 RPM ≈ 148η RPM

Hence

Np= 120 * f / N

Np= 120 *50 / 1485

Np= 4.04 poles ≈ 4 poles

For small hydropower plants, 4 – 8 pole generators are selected to reduce the cost of the

generator. The size and cost of high speed generators is less in comparison to low speed

generators. Hence, 4 poles fall within acceptable limit and results to a cheaper generator.

The type of coupling to be used is the flexible coupling of belt drive to increase the speed of

turbine to acceptable speed of generator.

3.4.3 Exciter of generator

In the case of a synchronous generator, an exciter is necessary to supply the field current to the

generator and keep the output voltage constant even when the load fluctuates.

Type of exciters

i. Brush type

ii. Brushless type

For small hydropower plants the brushless type of exciter is recommended due to its low

maintenance costs. The best efficiency of this type of generator is 97%.

3.4.4Generator type

A synchronous generator with three phases is selected because it is economical and most

reliable.

28

3.4.5Generator output

The output of the generator is shown in KVA and calculated as follows;

Pg (KVA) = 9.81 * H * Q * o* ρ / Pf

Where

Pg = required power output

H = net head

Q = design discharge (m3/s)

o=overall efficiency i.e. turbine efficiency, t*transmission efficiency, m * generator

efficiency, g

ρ = density of water

Pf= power factor = 0.8

Hence

Pg (KVA) = 9.81 * 54.5 * 21 * 1000* 0.89/ 0.8

Pg (KVA) = 12,490 KVA

Pg (KVA) ≈ 12,η00 KVA

3.5 Power transmission facility (speed increaser)

The speed increaser is always used to reduce the set-up cost especially when the turbine speed is

very low. Hence, the speed of the turbine is stepped up by a factor to a certain convenient value.

For this design a factor of 4.1 is adopted to increase the rotation speed. This saves on cost since

low speed generators are big and expensive.

In addition to this, in the case of small- hydropower plants, V- belts or flat belts coupling are

usually adopted to reduce overall costs since gear type transmissions are very expensive. The

efficiency of the belt type transmitter for this design is 98%.

3.6 Control facility of the turbine and generator.

3.6.1 Speed governor

The speed governor is adopted to keep the rotation speed of the turbine constant. The change in

the speed of rotation of the turbine is due to changes in load, water head and water flow rate.

29

For this design, a dummy load type governor is recommended since it is cheap. The capacity of

the dummy load is calculated as follows;

Pd= Pg * Pf * S.F

Where

Pd= capacity of the dummy load

Pg = rated output of the generator

Pf = rated power factor of the generator

S.F = safety factor according to the cooling method being employed (1.2 – 1.4)

Pd(KW) = 12.5 MVA * 0.8 * 1.4

Pd= 14 MW

3.7 DESIGN OF CIVIL STRUCTURES

3.7.1 Penstock hydraulic calculations

In our case;

Q = 21 m3/s

Net Head = 54.5 m

Penstock flow velocity = 4.5 m/s

This is from common practice that flow velocity in small hydropower plant penstocks’ range

from 2 m/s to 5 m/s.

Find internal diameter.

A= Q / V

A = 21/ 4.5 = 4.667 m2

But area of a circle

Or A = πD2/4.2

Making D the subject

D = 2* √ (A / π)

D = 2* √ (4.θθ7 / π) = 2.437 m

Thus D ≈ 2.4 m

3.7.2 Determination of the penstock thickness, tp

tp= P * r / σ

30

P =Ph + Ps

Where

P = total pressure

Ph = pressure due to water hammer

Ps = static water pressure

σ = stress

Ph= ρw * Cp* V

For water under ordinary conditions, Cp= 1120

So;

Ph= 1000 * 1120 * 4.5

Ph= 5.04 MPa

Static pressure, Ps

Ps = ρw* g * H

Ps = 1000 * 9.81 * 57

Ps = 0.5572 MPa

Factor of safety, n = 4

σyp= 957MPa

But

P =Ph + Ps

P = 5.6 + 0.5572

P = 6.157 MPa

σallowable = σyp / n

= 957 * 106 / 4

σallowable = 239.25 MPa

Hence

= P * r / σallowable

= 6.157 * 106

* (2.4 / 2) / (239.25 * 106)

31

tp= 0.03088 m = 30.88 mm ≈ 31 mm

3.7.3 Head loss in the penstock

3.7.3.1 Head loss due to entry and exit, hV

hV= K * V2 / (2 * g)

But K = 0.2

hV= 0.2 * (4.5)2 / (2 * 9.81)

hV= 0.2064 m

But two valves lie at the entry and exit;

hVT= 2 * 0.2064 m

hVT≈ 0.42 m

3.7.3.2 Head loss due to bend, hb

hb= C * V2 / (2 * g)

For a deflection angle of 45° C = 0.09

hb= 0.09 * 20.25 / (2 * 9.81)

hb= 0.093m

3.7.3.3 Head loss due to friction, hf

hf= f * (LP / DP) * (V2

/ (2 * g))

N/B: For the purpose of accuracy we used the program from the website

www.lmnoeng.com/moody.php

This enabled us to get the value of ‘f’ from the Moody’s chart more accurately.

f = 0.009

hf= f * (LP / DP) * (V2

/ (2 * g))

hf= 0.009 * (432.9 / 2.4) * (20.25 / (2 * 9.81))

hf ≈ 1.θ7θ m

Total head losses in the, hT

hT = 1.676 m + 0.093 m + 0.42 m + 0.311 m

hT = 2.5 m

Hence, the gross head from the site was 57 m and the net head is found by subtracting the head

losses. The head losses were calculated from above.

Net head is 57 – 2.5 = 54.5 m.

32

From the design rule;

hL ≤ 0.0η Hgross

2.5 ≤ 0.0η * η7 m

2.5 ≤ 2.8η m

From the above rule the head loss comply with it hence the design is safe.

3.7.4 Design of head race (open channel)

Q = A * R2/3

* / n

Where

Q = design discharge of head race = 21 m3/s

A = area of the cross section = b * h

b = width of the channel

h = depth of the channel

R = A / P

P = wetted perimeter = b + 2h

SL= longitudinal slope of the head race ≈ 1/1η00

n = coefficient of roughness = 0.015

The most economical channel shape is rectangular.

For rectangular channel section;

The rectangular channel cross section is most economical when;

h = b / 2 and R = h / 2

Hence

A = b * h

But

h = b / 2 or b = 2 * h

Thus

A = 2h2

Therefore

Q = A * R2/3

* / n

21 = 2h2

* (h/2)2/3

* (1/1500)0.5

/0.015

h = 2.1314 m

33

Hence

b = 2 * h

b = 2 * 2.1314 m

b = 4.2628 m

The length of the channel will be measured at the site to get accurate one.

3.7.5 Intake Weir (Dam)

3.7.5.1 Weir height calculations

Under normal conditions, the weir height should be planned to exceed the calculated value by

the following method to ensure the smooth removal of sediment from the upstream of the weir

and the settling basin.

3.7.6 Side intake

Weir height, D1, determined in relation to the bed elevation of the scour gate of the intake weir.

D1 = d1 + hi

Where;

d1 = height of the bed of the scour gate to the bed of the inlet (usually 0.5 – 1.0 m)

hi= water depth of the inlet ( usually determined to make the inflow velocity approximately (0.5

– 1.0 m/s)

Qd= A * V

But

Qd= 21 m3/s

V = 1 m/s

A = 21m3/s / 1 m/s = 21 m

2

A = b * hi = 21 m2

We choose b = 5 m

Where

b = width of the side intake

hi= height of the side intake

hi= 21 / 5

hi= 4.2 m

34

3.7.7 Settling basin design

The settling section’s function is to settle sediments / grain size of (0.η – 1 mm). The minimum

length (L) is calculated by the following formulae based on the relation between the settling

speed, υ, flow velocity, V, and the water depth, hs. The length of the settling basin, Ls, is usually

determined so as to incorporate a margin to double the calculated by the formulae below;

L ≥ (V / υ) * hs

Ls = 2 * L

Where

L = minimum length of the settling basin (m)

Ls = length of the settling basin

hs= water depth of the settling basin (m)

υ = marginal settling speed for sediments to be settled (m/s). it is usually around0.1 m/s for a

target grain size of 0.5 to 1.0 mm.

V = mean flow velocity in the settling basin (m/s). It is usually around 0.3 m/s but up to 0.6 m/s

is tolerated in the case where the width of the settling basin is restricted.

V = Qd / (b * hs)

Where;

Qd= design discharge (m3/s)

b = width of the settling basin (m)

but

Qd= 21 m3/s ; V = 0.6 m/s

b * hs= 21 / 0.6 = 35 m2

We choose b = 7m and hs= 5 m.

L ≥ (V / υ) * hs

L ≥ (0.θ / 0.1) * 5

Ls= 30 m * 2 = 60 m

3.7.8 Head tank

Function of the head tank;

Control the difference of discharge in the penstock and the head race because of the load

fluctuations.

Finally remove litter (sand, drift wood, etc.) in the flowing water.

35

3.7.8.1 Head tank capacity

Definition of the head tank capacity

The head tank capacity is defined as the water depth from hc to ho in the head tank of length L as

shown in the diagram.

Vsc= As *dsc= B * L *dsc

Where;

Vsc=head tank capacity

dsc=water depth from uniform flow depth of a head race when using maximum discharge (ho) to

critical depth from top of a dike for sand trap in a head tank (hc).

B = width of the head tank

L = length of the head tank

Determine the head tank capacity;

In the case where only the load is controlled

Vsc= 20 * Qd

Vsc= 20 * 21

Vsc= 420 m3/s

The head tank capacity should be secured only to absorb the pulsation from the head race that is

about 10 times to 20 times the design discharge (Qd).

Vsc= B * L *dsc= 420 m3/s

B = 2 * 4.3 = 8.6 m

L * dsc= 420 / 8.6 = 48.7 m2

dsc= ho / 2

Where;

ho= height of the head race = 2.1 m

Hence

dsc= 2.1 / 2 = 1.05 m

Therefore

L = 48.837 m2

/ 1.05 m = 46.5 m

L = 46.5 m

The dimensions of the head tank were chosen as a matter of convenience.

36

CHAPTER 4: PROJECT COST ESTIMATION

This chapter is dealing mainly on the estimate cost of the project if the implementation is going

to take place.

4.1 DIRECT COST

4.1.1 Preliminaries (for civil structure work)

From cost analysis of projects of similar size, the preliminaries for civil works cost was

approximated to be KSHS 4,000,000.00

4.1.2 Intake weir

From cost analysis of projects of similar size, the intake weir construction cost was approximated

to be KSHS 5,500,000.00

4.1.3 Settling basin

From cost analysis of projects of similar size, the settling basin cost of length 60 metres was

approximated to be KSHS 3,500,000.00

4.1.4 Fore-bay

From cost analysis of projects of similar size, the fore-bay cost of length 46.5 metres was

approximated to be KSHS 2,200,000.00

4.1.5 Spillway

Length of spill way = 15 M

Cost per metre = KSHS 100,000.00

Total cost = 15 * 100,000.00 = 1,500,000.00

Kshs 1,500,000.00

4.1.6 Penstock civil work

From cost analysis of projects of similar size, the penstock civil works cost was approximated to

be KSHS 700,000.00

4.1.7 Penstock pipes

Internal Diameter (Di) = 2.4 M

Length of penstock (LP) = 432.9 M

Thickness (tp) = 31 mm = 0.031 M

37

Density of steel = 8000 kg / M3

Price of steel = Kshs 15 per kg

Volume of steel = Π * Lp * (((Di + 2 * tp)/2)2 – (Di / 2)

2) = Π * (((2.4 + 0.031 * 2)/2)2 – (2.4 / 2)

2)

* 432.9

Volume of steel = 102.5 M3

Mass = density * volume

= 8000 * 102.5

= 819,925 kg

Cost = (price / kg) * mass

=15 * 819,925

=KSHS 12,298,875.00

4.1.8 Canal

From cost analysis of projects of similar size, the low slope canal of length 503.5 M cost was

approximated to be KSHS 2,000,000.00

4.1.9 Power house

From cost analysis of projects of similar size, the power house construction cost was

approximated to be KSHS 4,000,000.00

Total civil works

The total cost of the civil works was the sum of the above mentioned nine components. The total

cost was thus KSHS 35,698,750.00

4.1.10 Francis turbine

This price was found directly from one of the turbine manufacturers called Saimpro hydraulics.

Turbine cost = KSHS 94,000,000.00. The exchange rate used was 1pound = kshs 147

4.1.11 Synchronous generator

The price was found from Siemens alternator manufacturer. Hydropower generator unit cost =

kshs 23,000,000.00

4.1.12 Transmission line

From cost analysis of projects of similar size, the transmission line of 1.5 kilometres was

approximated to be KSHS 18,500,000.00

38

4.1.13 Construction supervision

From cost analysis of projects of similar size, the construction supervision cost was

approximated to be KSHS 3,000,000.00

Total direct cost

The total direct cost of the above components was found to be Kshs 174,200,000.00

4.2 INDIRECT COST OF CONTRACTOR

4.2.1 Engineering cost

Approximately 5% of direct cost = 5% * Kshs 174,200,000.00

= KSHS 8,710,000.00

4.2.2 Contingencies

Approximately 10% of direct cost = 10% * Kshs 174,200,000.00

= KSHS 17,420,000.00

4.2.3 Administration

Approximately 7% of direct cost = 7% * Kshs 174,200,000.00

= KSHS 12,194,000.00

Total of indirect cost

The total indirect cost of the above elements was found to be Kshs 38,324,000.00

Total cost of project

The total cost of the project is the sum of direct and indirect costs. This produces a sum total of

KSHS 212,524,000.00

39

CHAPTER 5.0: Discussion, Conclusions and Recommendations

5.1 DISCUSSION

The object of this project was to carry out design of small hydropower at River Nzoia at

Webuye East constituency in Bungoma county western Kenya. River Nzoia has water fall near

the station of Nzoia Water Company the only water plant in Bungoma supplying clean water to

many areas in Bungoma County and beyond. The river itself has enough water to be used to

generate enough electricity for use in the area and the surplus can be injected in to the national

grid for sale. The area is very fertile as the local people around the river grow a lot of crops for

subsistence and for commercial purpose. Besides, the area has many industries working there

and it host pan Africa paper mill which was supplying books in the region. Webuye has market

and commercial centers and lodges and hotel around. The major challenge facing the local

population around that area is lack of enough power and hence, that is the purpose of this project

to address the challenge and provide the sustainable and most reliable power. Hydropower is the

most clean renewable source of energy compare to other source of power.

Being the current problem to be tackle, we therefore went to the site to assess the

potential site for installation of hydropower plant. We took the measurement of head at

difference point upstream where to locate the intake and downstream where to put powerhouse

which will house electromechanical equipment. Our aim was to get the location that will give us

the maximum head and to be economical too in term of construction cost. However, we got the

gross head as the difference of head upstream above sea level and downstream above sea level.

The measurement of the head and the coordinates of the location were found by using GPRS.

Also the river was having gauging station done by water resource management Authority which

takes reading on daily basis to yearly. Therefore, we got the data pertaining river Nzoia and we

have to take them for analysis in form of flow duration curve (FDC). This is a curve generated

by plotting the flow of the river recorded daily for duration of many years. After we got the FDC

we read the design flow rate from the curve and it became our design flow rate of the river. As

we know when the river level goes down to predetermine level, the flow of river will drop and

the design flow become less than what was meant to drive turbine hence, the turbine will not

work and the plant will be shut down with immediate effect. Thus, the choice of the flow rate of

40

the river plays a very critical role in the plan of the small hydropower to eliminate the shutdown

of the plant.

After getting flow rate and the net head (which was found after subtracting all the losses

along the components) then the actual plan went ahead and the power output was calculated

safely. The design was divided in to two major parts i.e. the design of civil structures (intake

weir, intake, settling basin, channel, head-tank /fore-bay, spillway, penstock, powerhouse

foundation and power house building) and electromechanical equipment (turbines and its

accessories, generator and its accessories, control system and transmission line). The

specifications of each after design are listed under summary section of chapter 5. The selection

of dimensions and other specifications was not accidental it was after analysis and due to

economical consideration. The turbine chosen was Francis turbine and again its specifications

are list in summary section. The turbine was chosen among the other variety of turbines using

certain criterion e.g. specific speed, rotation speed, net head, flow rate, power output and above

all the cost. After using all those criteria with the help of chart and table in the section of

electromechanical above the Francis turbine met almost all the criteria for selection and hence

for this project it is the recommended turbine. At the same time the speed of turbine was 362

RPM which was very low for generator; therefore we have to choose belt drive, speed increase to

top up the speed to the speed of generator in the market. To achieve that we had to choose a

factor of 4.1 to step up the speed of turbine to1485 RPM which is good enough for generator

speed. Again the cost of generator goes hand in hand with the high speed of generator. And at the

same time the number of poles for small hydropower of generator, are supposed to be within a

range of 4-8 poles. The number of poles of generator was calculated using the step up speed and

frequency of 50 Hz for case of Kenya and we got the number of poles that fall within that range

hence the design was safe. The generator type was synchronous type of three phase, the

specifications of generator are given above.

However, the site is economical viable for the generation of electricity of more than what

is given in this project if other points on the site where to be chosen for installation of power

house and the intake point. The cost of the project was estimated under the chapter of cost

estimate. But for contractor to get most reliable cost it will be in order to get the real cost at the

site especially the civil structures location. The project if implemented will generate enough

revenue to the local around the area and local authority that can be enough for their development

41

for things like more schools, roads, agricultures, etc. It can always generate revenue to the

central government. It can provide electricity for local around the plant and beyond can enjoy the

clean power for their daily up keep. It can attract investor who can invest in the area and provide

employment and cheap commodities to civil population around the area.

5.2 SUMMARY OF DESIGN

The following is the summary of the design analysis

no component Dimensions/ specifications. Number

1 Penstock( commercial steel) Internal diameter 2000 mm

Thickness 30.8 mm

Length 432.9 m

1

2 weir Height = 5.2m 1

3 Side intake Velocity = 1m/s

Width, b = 5m

Height, h = 4.2m

1

4 Settling basin Velocity, v = 0.6m/s

Width= 7m

Height = 5m

Length = 60 m

1

5 Channel:

Rectangular.

Masonry concrete.

Length, L= 503.2m

Width, B = 4.2628 m

Depth, H = 2.1314 m

1

42

6 Head tank Head-tank capacity, Vs= 420 m3/s

Depth. Dsc= 1.05m

Length, L = 46.5m

Width, B= 8.6m

1

7 Turbine (Francis turbine) Shaft power= 9.6 MW

Rotational speed = 354 RPM

Specific speed = 0.2596

Runner outer diameter D3 = 1.6122 m

Runner Inner diameter D1 = 1.2 m

Design flow = 21m3/s

Guide vane inlet angle α = 31°

Guide vane outlet angle ϕ = 22°

Runner inlet width B1 = 0.7 m

Runner outlet width B2 = 0.55557 m

Turbine efficiency, t = 94%

1

8 Draft tube Flare angle = 6°

Inlet diameter, Di = 1.6122 m

Outlet diameter, D0 = 2.54417 m

Height = 4.43355 m

Tail race water level, T = 1.28976 m

1

9 Generator (synchronous) Phase = 3

Pole = 4

1

43

Rotational speed = 1400 RPM

Rated power = 11.5 MVA

Power output = 9.2 MW

Frequency = 50Hz

Generator efficiency, g = 97%

Brushless type exciter

Power factor Pf = 0.8

10 Speed increaser Belt type

Speed increaser efficiency = 98%

Ratio = 4.1

1

11 controllers Dummy load type

Dummy load capacity, Pd= 12.88 KW

1

Site configuration

Design discharge, Qd = 21 m3/s

Gross head, Hg = 57 m

Net head, Hn = 54.5 m

Total head loss Hl = 2.5 m

5.3 CONCLUSION

The objective of this project was to design for construction a small hydropower plant capable of

producing 10 MW along river Nzoia at Webuye. The site is capable of producing far more than

10 MW in our design criteria. The power consumption of Webuye (4.7 MW) is less than the

estimated output hence there is surplus power (5.3 MW) that can be injected in to the National

grid to generate income for the local authority in the area. The revenue can be utilized for the

44

benefit of the local community. The project when completed will be capable of generating a lot

of revenue. The summary of the design components of hydropower with their specifications are

listed in the table under the summary section of this chapter.

Also the scheme for this design will be run-of-river scheme as discuss above under literature

review because it is cheaper to install and the nature of the river is capable of providing water.

All the specifications are provided above including the cost estimate of project. Thus, the

objective of project was achieved.

5.4 RECOMMENDATIONS

The following are the recommendation we suggested:

1- The river is capable of producing more than10 MW hence we recommend the changing

of focus from small hydropower a medium hydropower plant.

2- This project needed more time for a more detailed report to be created. Hence, anything

left out in this project is due to time constraints.

3- This project is site specific hence we would encourage its fast implementation.

4- The local government or any other agency should look for funding to implement the

project since it is economically feasible. It will also improve the living standards of the

local people.

5- The further detailed design needed is on civil structures preferably by civil engineers and

surveyors as these structures are beyond the scope of this project.

6- There is need creating a power distribution network beyond the powerhouse and this need

further research on this project.

45

REFERENCES:

1. R. K. Rajput (2011), Fluid mechanics and Hydraulic Machines4th

edition, 2010,2011pp

867-946, pp 1052-1171

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46

14. G. Schmausser& G. Hartl, “Rubber seals for steel hydraulic gates”, Water Power & Dam

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Power & Dam Construction, November 1987

47

APPENDICES

Photos of Nabuyole Falls

48