UNIVERSITY OF NAIROBI

FACULTY OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING

DESIGN OF A GEOTHERMAL POWER PLANT

PROJECT INDEX: PRJ 129

BY

MIHANG’O PAUL GAKUNGU

F17/1809/2006

SUPERVISOR: DR. MANGOLI

EXAMINER: MR. OGABA

Project report submitted in partial fulfillment of the

requirement for the award of the degree

of

Bachelor of Science in ELECTRICAL AND ELECTRONIC ENGINEERING of theUniversity of Nairobi

Submitted on:

18TH MAY 2011

i

DEDICATION

To my parents Mr. and Mrs. Mihang’o for all the sacrifice they have made to see me go to

school, my Brothers, Sisters and friends for their continued support.

ii

ACKNOWLEDGEMENTS

Sincere thanks to my supervisor Dr. M. K. MANGOLI for his consistent guidance which

contributed greatly to the provision of knowledge as well as the completion of this project.

My special thanks to the entire teaching staff in the Department of Electrical and Information

Engineering in the University of Nairobi for giving me the foundation on which this work is

based.

I extend special thanks to my parents Mr. and Mrs. Mihang’o, for the sacrifices they ever made,

my brothers and sisters for their emotional and material support throughout my education.

Finally, my gratitude to all those who offered a word of encouragement towards completion ofthis work.

iii

DECLARATION AND CERTIFICATION

This BSc. work is my original work and has not been presented for a degree award in this or

any other university.

………………………………………..

Mihang’o Paul Gakungu

F17/1809/2006

This report has been submitted to the Department of Electrical and Information Engineering,

University of Nairobi with my approval as supervisor:

………………………………

Dr. M. K. MANGOLI

Date: ………………………

iv

TABLE OF CONTENTSDEDICATION................................................................................................................................. iACKNOWLEDGEMENTS............................................................................................................ iiDECLARATION AND CERTIFICATION .................................................................................. iiiTABLE OF CONTENTS............................................................................................................... ivLIST OF FIGURES ....................................................................................................................... viLIST OF TABLES........................................................................................................................ viiABSTRACT................................................................................................................................. viiiCHAPTER ONE ............................................................................................................................. 1INTRODUCTION .......................................................................................................................... 1

1.1 History of Geothermal Energy .............................................................................................. 1

1.2 Development of geothermal power ....................................................................................... 2

CHAPTER TWO ............................................................................................................................ 32.0 BACKGROUND AND LITERATURE REVIEW............................................................... 3

2.1 Geothermal energy – Working principle............................................................................... 3

CHAPTER THREE ........................................................................................................................ 63.0 METHODOLOGY................................................................................................................ 6

3.1 TYPES OF GEOTHERMAL PLANTS................................................................................ 6

3.1.1 Dry steam power plants .................................................................................................. 6

3.1.2 Flash steam power plants................................................................................................ 8

3.1.3 Binary cycle power plants ............................................................................................ 10

3.2 The flash vessel pressure effect........................................................................................... 12

3.2.1 The turbine inlet pressure effect ................................................................................... 14

3.2.2 The condenser pressure effect ...................................................................................... 14

3.2.3 The geothermal fluid enthalpy effect............................................................................ 16

3.3 Turbine Design.................................................................................................................... 17

3.3.1 Steam turbines control .................................................................................................. 18

3.3.2 Classification of steam turbines.................................................................................... 19

3.3.3 Compounding Effect..................................................................................................... 22

3.3.4 Operation and maintenance .......................................................................................... 22

3.3.5 Starting up the turbine .................................................................................................. 23

3.3.6 Running turbine ............................................................................................................ 25

3.3.7 Shutting Down.............................................................................................................. 27

3.3.8 Speed regulation ........................................................................................................... 28

3.3.8 Turbine protection ........................................................................................................ 29

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3.3.9 Thermodynamics of steam turbines.............................................................................. 30

3.4 Calculating turbine efficiency ............................................................................................. 31

3.4.1 Isentropic turbine efficiency......................................................................................... 31

3.5 The Generator...................................................................................................................... 33

3.5.1 Synchronous Generators............................................................................................... 34

3.6 Power Transformer.............................................................................................................. 36

CHAPTER FOUR......................................................................................................................... 37DISCUSSION............................................................................................................................... 37

4.1 Advantages of geothermal power........................................................................................ 37

4.2 Disadvantages of geothermal power ................................................................................... 39

4.3 Plant maintenance practices ................................................................................................ 40

CHAPTER FIVE .......................................................................................................................... 445.1 CONCLUSIONS................................................................................................................. 44

5.2 RECOMMENDATIONS .................................................................................................... 45

REFERENCES ............................................................................................................................. 46

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LIST OF FIGURESFigure 1Typical flow diagram for a geothermal power plant ......................................................... 4Figure 2 Schematic of a dry steam power plant.............................................................................. 7Figure 3 Schematic of a single flash steam power plant................................................................. 9Figure 4 Binary-Cycle Power Plants............................................................................................. 11Figure 5 Turbine Steam Rate versus condenser pressure (Pc)..................................................... 15Figure 6 Diagram of an impulse turbine ....................................................................................... 17Figure 7 photo of an impulse steam turbine.................................................................................. 20Figure 8 A diagram showing both Impulse and reaction turbines ................................................ 21Figure 9 A modern steam turbine generator installation .............................................................. 23Figure 10 steam cycle with superheat ........................................................................................... 33Figure 11 Generator stator windings............................................................................................. 35Figure 12 A simplified process flow diagram for a GPP............................................................. 40

vii

LIST OF TABLESTable 1 summary of main parts in a GPP ..................................................................................... 41Table 2 geothermal power growth since 2005 in various countries ............................................. 42Table 3 a summary showing various aspects in geothermal development ................................... 43

viii

ABSTRACTRecently Kenya has actively engaged to search of green Energy that is environment friendly,

reliable and affordable. This has led to the extensive search for potential areas to set up more

geothermal power plants especially in Naivasha where such plants have been set up. It’s believed

that there is over 7000MW geothermal potential and very little of this has been tapped; about

167MW at the Ol-karia 1 and 2 geothermal plants.

Following the ever rising demand for energy in Kenya it is of essence to have an intensive study

done on the way to utilize this untapped energy to benefit the citizens by availing clean,

affordable power. This will be in line with the Vision 2030 and Millennium Development Goals

that will see the country having more manufacturing and processing industries and hence the

overall national development.

This report therefore is a study on efficient and effective tapping of geothermal energy to

generate power that is environment friendly and affordable. There is need utilize this naturally

occurring energy to provide power for development to be realized since energy is major factor in

any country’s economic growth.

The setting up of Geothermal Development Company has seen Kenya explore more geothermal

power and even production drill wells that are already promising to provide vast amount of

energy. It is therefore necessary to come up with ideas that will harvest the geothermal power

effectively and this can be done by being knowledgeable on the best designs to implement

geothermal power generation

1

CHAPTER ONE

INTRODUCTION

1.1 History of Geothermal EnergyThe word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal

energy is heat from within the earth. Geothermal energy is generated in the earth’s core, almost

4,000 miles beneath the earth’s surface. The double-layered core is made up of very hot magma

(melted rock) surrounding a solid iron center. Very high temperatures are continuously produced

inside the earth by the slow decay of radioactive particles. This process is natural in all rocks.

Surrounding the outer core is the mantle, which is about 1,800 miles thick and made of magma

and rock. The outermost layer of the earth, the land that forms the continents and ocean floors, is

called the crust. The crust is 3–5 miles thick under the oceans and 15–35 miles thick on the

continents. The crust is not a solid piece, like the shell of an egg, but is broken into pieces called

plates. Magma comes close to the earth’s surface near the edges of these plates. This is where

volcanoes occur. The lava that erupts from volcanoes is partly magma. Deep underground, the

rocks and water absorb the heat from this magma. When wells are drilled and the super heated,

underground water ejected to the surface the realized geothermal energy is used to produce

electricity.

Geothermal energy is a renewable energy source because the water is replenished by rainfall as

well as the water is pumped back through the injection wells and the heat is continuously

produced deep within the earth.

First, geothermal power station was built more than a hundred years ago, at Landarello, Italy in

1904, and second one was built in Wairekei in New Zealand.

In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric

power plant in the United States at The Geysers in California. The original turbine lasted more

than 30 years and produced 11 MW net power.

.

2

1.2 Development of geothermal power

The binary cycle power plant was first demonstrated in 1967 in Russia and later introduced to the

USA in 1981. This technology allows the use of much lower temperature resources than were

previously recoverable. In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-

line, producing electricity from a record low fluid temperature of 57°C (135°F).

Geothermal electric plants have until recently been built exclusively where high temperature

geothermal resources are available near the surface. The development of binary cycle power

plants and improvements in drilling and extraction technology may enable enhanced geothermal

systems over a much greater geographical range. Demonstration projects are operational in

Landau-Pfalz, Germany, and Soultz-sous-Forêts, France, while an earlier effort in Basel,

Switzerland was shut down after it triggered earthquakes. Other demonstration projects are under

construction in Australia, the United Kingdom, and the United States of America.

This report will consider the various geothermal power plant designs of the already existing plant

and discuss ways that such plants efficiencies can be enhanced for optimal performance with

minimal environmental impact. It also focuses on the internal plant design by discussing the

various parts of a geothermal power plant, their operation and maintenance practices, efficiency

as well as merits and demerits of such plants both economically and environmentally.

It brings together all designs and tries to elaborate on each to have see how such different

designs can be implemented in Kenya’s geothermal potential fields.

3

CHAPTER TWO

2.0 BACKGROUND AND LITERATURE REVIEW

2.1 Geothermal energy – Working principle

Geothermal is defined as the heat energy that originates from the hot rocks deep beneath the

surface of the earth. The shallow-lying magma, beneath the surface of the earth, heat up the

deeply circulating ground water to form hot water and steam. Geothermal energy reaches the

surface in the form of hot water, steam or its mixture mostly at high pressures when a borehole is

drilled into the geothermal reservoir. Geothermal resources are classified into low temperature (<

90°C), medium temperature (90-150°C) and high temperature (>150°C). The uses of geothermal

resources depend on their temperature; high temperature resources are mainly used for electricity

production or as combined heat and power (electricity) production, medium temperature

resources are used for electricity production in binary units and for direct uses while low

temperature resources are mainly for direct uses (heating).

Geothermal power plants transform the heat energy in the geothermal fluids into a form of

energy suitable for human uses, mainly electricity and useable heat. In typical single flash

Geothermal power plant, the naturally heated steam and water is brought to the surface by a well

drilled into the geothermal reservoir. The super heated mixture of water and vapor is flashed

under low pressure and a better part of the liquid flashes into vapor.

The steam realized is used to drive a steam turbine which in turn drives a generator that produces

electricity. In a double flash geothermal power plant, the hot water is further flashed to produce

low pressure steam which is used to drive a low pressure steam turbine. In combined heat and

power geothermal power plant, the separated water is further used to supply heat for direct uses

such as space heating. Where the geothermal fluid is of low temperatures, binary system

geothermal power plant is used where the geothermal fluids is used to superheat a low boiling

point secondary fluid which is used to drive the turbine.

In modern geothermal power plant, the used geothermal water is eventually re-injected to the

ground for environmental reasons and also as a means to manage the geothermal reservoir. In all

these processes, there are hundreds of equipment and components that need to be maintained to

4

keep the geothermal power plant in good operating conditions. Figure 1 shows a simplified

schematic flow diagram for geothermal power cycle showing the fluid path from a production

well through the plant to a re-injection well.

Figure 1Typical flow diagram for a geothermal power plant

Geothermal fluids contain dissolved and suspended solids, gases and variety of chemical

elements which result from the rock-water interaction that take place during the formation and

movement of the fluids within the geothermal reservoir. The chemical and physical properties

and composition of the fluids affect the way the fluids can be used, the type of design of the

geothermal power plant and the maintenance needs for the power plants. Geothermal power

plants are faced with specific maintenance challenges related to the nature of geothermal fluids.

Unlike in nuclear or fossil-fired steam power plants where the water quality is under control

throughout the cycle, the quality of waters in geothermal power plants depend on the formation

processes in the reservoir. The silica, hydrogen sulphide (H2S), calcites and chlorides among

other chemical constituents put specific maintenance challenges for geothermal power plants

which are not found in the fossil or nuclear steam power plants. Attempts have been made in

recent past to control certain elements of the composition of the fluid in particular silica and

chlorides by chemical dosing procedures which are costly.

5

Geothermal energy and other renewable energy sources have continued to gain greater attention

and importance in the recent years in the world energy sector because of the increased awareness

of the detrimental effects of burning fossil fuels on the environment. The recent increase in the

cost of fossil fuels in particular crude oil (World Press News, 2008) is set to increase the interest

in geothermal energy and other alternative sources of energy. The growing interest in geothermal

energy and the fact that most geothermal power plants are operated as base load stations will put

greater challenges in maintenance team of geothermal power plant to ensure high availability and

reliability of the power plants hence sustainability of geothermal resources thus meeting the

growing expectations. In addition, geothermal power plants have relatively low capital costs

compared to other power plants such as hydropower stations but their operation and maintenance

costs are high. To operate them economically, the maintenance costs have to be minimized.

6

CHAPTER THREE

3.0 METHODOLOGY

The design of a geothermal power plant is an involving task that requires knowledge is various

fields of power generation. Depending on the results from the prospection done before the

drilling works begin, the type of geothermal plant to be developed on any particular location can

be determined.

They are several types of geothermal power plants and they are classified according to the type

of steam in operation. Since pressure and the amount of steam reaching the surface are mainly

the determining factors they help to identify the most productive geothermal plant to be

developed.

There are various types of geothermal power plants around the world; their classification is based

on the type of steam used in a particular plant.

3.1 TYPES OF GEOTHERMAL PLANTS

There are three main designs that a geothermal power plant can take, these are

i. Dry steam power plants

ii. Flash steam power plants

a. Single flashed

b. Double flashed

iii. Binary cycle power plants

3.1.1 Dry steam power plants

Many early geothermal projects, such as The Geysers dry steam power plant in Northern

California, depend on high temperature steam formations to directly provide the energy to drive

power generator turbines. This type of formation is called a "dry steam" power plant because the

7

steam is released from the pressure of a deep reservoir, through a rock catcher, and then past the

power generator turbines

Dry steam reservoirs use the water in the earth's crust, which is heated by the mantle and released

through vents in the form of steam. The dry steam power plant is suitable where the geothermal

steam is not mixed with water. Production wells are drilled down to the aquifer and the

superheated, pressurized steam (180°-350°C) is brought to the surface at high speeds, and passed

through a steam turbine to generate electricity. In simple power plants, the low pressure steam

output from the turbine is vented to the atmosphere, but more commonly, the steam is passed

through a condenser to convert it to water. This improves the efficiency of the turbine and avoids

the environmental problems caused from the direct release of steam into the atmosphere. The

waste water is then re-injected into the ground with reinjection wells.

The underground water reservoirs that feed such a system are refilled when rain falls on the land.

The rainwater eventually soaks back into the crust of the earth. Because this occurs on a

continuous basis, geothermal energy is considered as a renewable resource.

Figure 2 Schematic of a dry steam power plant

This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy where it

has powered electric railroads since 1904. About 6 percent of the energy used in northern

8

California is produced at 28 dry steam reservoir plants found at The Geysers dry steam fields in

northern California. At peak production, these dry steam geothermal power plants are the

world's largest single source of geothermal power producing up to 2,000 megawatts of electricity

per hour. That is about twice the amount of electricity a large nuclear power plant can produce.

These dry steam power plants emit only excess steam and very minor amounts of gases.

3.1.2 Flash steam power plants

Most geothermal sources produce temperatures that are nowhere near the critical point of water.

A single flash resource is typically between 150 °C and 200 °C .This translates to a steam source

that is saturated with vapor and when pressurized will readily condense to the liquid phase. In

traditional Rankine cycle turbines the presence of the liquid phase causes dramatic efficiency

losses. Preventing vapor from entering into the turbine while utilizing a lower temperature

resource is generally done by using a flash process before the steam is sent to the turbine. Due to

the higher frequency of liquid-dominated geothermal fields, single-flash geothermal power

plants are the most commonly installed plants at geothermal fields. A simplified illustration of

the single flash power plant is provided in figure 3 below.

i. Single flash power plant

A single flash geothermal power plant is one that has a single stage at which the super heated

mixture of steam and liquid water is passed through a low pressure well head separator. During

this process most of the fluid vaporizes and flashes into high pressure steam. The steam and

liquid are separated into two distinct phases for processing. The steam is sent to the turbine and

the liquid is sent back to the injection wells. After the steam is used to generate power, it is

condensed back to a liquid in a cooling tower before being re-injected into the reservoir.

9

Figure 3 Schematic of a single flash steam power plant

Because of interaction between the geo-fluid and machinery or piping, care must be taken in

material selection so as to minimize scaling and corrosion. This added design complexity

increases the capital and maintenance costs of the system. Flashing in the pipe carrying the

separated liquid to the injection well due to a pressure drop is also something that must be

avoided as there is chemical precipitation from temperature drop. An environmental concern for

this type of plant is the water vapor plume that will be visible from the cooling tower.

Harmful gases present in the geo-fluid must be contained in a closed loop system utilizing re-

injection or isolated and treated before the geo-fluid can be released into the surrounding

environment.

ii. Double flashed steam power plant

Double-flash plants may produce 15-25% more power than a single-flash system for the same

geothermal fluid conditions. The increase in efficiency, however, comes at a higher initial capital

cost since the system is far more complicated. A double flash resource is typically between 150

°C and +200°C. It operates in much the same way as a single- flash plant, but instead of sending

the separated liquid directly to the re-injection well it is sent to a second separator to generate

10

additional steam at a lower pressure. The turbine of this type of system must be able to

incorporate the lower pressure steam at an appropriate stage for smooth incorporation.

The principle of operation of a double flash geothermal plant is much the same as that of a single

flash, it is however more expensive owing to the extra equipment associated with the pressure

vessels, piping system for the low pressure steam, additional control valves and a more elaborate

or even extra turbines.

Another option is to use two separate turbines. Due to increased complexity over the single-flash

plant, capital and maintenance costs are significantly higher. As with a single-flash plant, scaling

and corrosion concerns exist due to the direct contact of the well water (which contain dissolved

elements) with the pipes

3.1.3 Binary cycle power plants

A binary power plant flows moderate temperature geo-fluid (150-200°C) through a heat

exchanger heating a secondary working fluid that generally has a lower boiling point than water.

The geothermal fluid is then re-injected into the geothermal reservoir. The heated working fluid

is then expanded through a turbine, which powers an electric generator. A simplified illustration

of the binary power plant is provided below in Figure 3.

Since it is generally understood that each geothermal source is unique in its temperature,

pressure and chemistry, it is advantageous to have flexibility in the process design of a power

plant’s energy conversion system. This is where a binary power plant excels The specific

working fluid used in a particular binary system may be matched to the unique geothermal fluid

temperature. Primarily, the working fluid is selected to maximize the thermodynamic efficiencies

of a particular application, while minimizing the degradation of the system’s materials and

minimizing costs by reducing the need for exotic materials.

The chemical compatibility of the working fluid and the wetted metal surfaces of the binary

power plant effectively extend the system’s lifetime by reducing the amount of maintenance over

the plant’s lifetime. Also the lower boiling points of many of the applicable working fluids allow

for the utilization of low temperatures geothermal source. Spring water is known to contain

11

calcium bicarbonate, iron, manganese, and calcium bicarbonate-sulfate. The thermal water has

also been found to contain 38 picocuries per litre of Radium which is the highest known amount

of any of the thermal waters so far. The total dissolved solids in these springs were

approximately 2,700 mg/l. Waters of these types pose difficulties for a geothermal system.

As water temperature increases, the solubility of minerals also increases. When the temperature

drops, water that was once unsaturated with a mineral now becomes saturated and particulates

precipitate out of the water. These particulates can result in scaling on piping and machinery.

Therefore the less equipment that a geo-fluid comes into contact with will minimize the amount

of equipment that will be damage by scaling, corrosion and abrasion over the lifetime of a

geothermal power plant.

A binary power plant addresses the equipment degradation issues posed by geothermal water by

limiting the temperature drop seen by the geo-fluid in the primary heat exchanger. The high

levels of radium content of the water it poses environmental and safety issues if it were to be

discharged to the ground environment.

Another advantage of the binary power plant system is that it discharges the used geo-fluid

through a re-injection well. This helps to maintain a closed loop geo-fluid reservoir and mitigates

any health and environmental risks posed by the geo-fluid’s chemistry.

Figure 4 Binary-Cycle Power Plants

12

The secondary fluid is flashed to vapor, drives a turbine, and is condensed and re-circulated to do

its job over and over again. Ammonia/water mixtures and hydrocarbons are the working fluids

commonly used in binary cycle plants

3.2 The flash vessel pressure effect

Separator or the flash steam vessel is one of the important parts of a flash steam cycle. It

separates steam from water. Eq. (1) shows steam quality (steam to mixture of steam and water

mass ratio) at the first stage flash vessel X1

(1)

Where:

h0 is the enthalpy of the outlet water and steam mixture from the geothermal well which is

delivered to the flash vessel

hw is the enthalpy of outlet hot water from the flash vessel and

h1fg is the latent heat at the first stage flash vessel pressure.

It is obvious that the output steam mass flow quantity from the flash vessel is the product of X1

and the outlet fluid from the well. Therefore, in order to increase the output steam mass flow rate

it is necessary to minimize the flash vessel pressure. In fact as the pressure decreases the hot

water enthalpy (hw1) decreases. Therefore, according to Equation. (1), X1 increases which causes

the steam mass flow rate to the turbine and the output power to increase. Steam quality in the

second flash vessel (X2) is calculated from Equation. (2)

13

(2)

Where:

hw1 is the enthalpy of outlet hot water from the first separator,

h2fg is the latent heat of outlet hot water from the first flash vessel and

hw2 is the enthalpy of outlet hot water from the second flash vessel.

If the pressure in the second flash vessel decreases, the steam mass flow which is delivered to the

low pressure turbine increases similar to the first flash vessel. It should be noted that the

following limitations arise when reducing the pressure in the first and second flash vessels.

i. By decreasing the pressure in the first flash vessel, hw1 decreases and according to the Eq.

(2), X2 and the inlet steam flow to the low pressure turbine decreases.

ii. Since the output power of the turbine is a function of inlet pressure, by decreasing the

pressure in the first and second flash vessels, the steam inlet pressure to the high pressure

and low pressure turbines decreases. Therefore, the total turbine output power decreases.

If the flash steam pressure decreases to a value less than the allowable threshold, there will be air

leakage in connecting pipes to the separators that finally causes the air into the separators.

According to the above limitations, it is necessary to consider an optimal pressure for the flash

vessels or separators. This value is usually 517 kPa for the first and 138 kPa for the second flash

vessel.

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3.2.1 The turbine inlet pressure effect

The turbine inlet enthalpy is a function of steam inlet pressure. Therefore, by increasing the

steam inlet pressure, the turbine output power increases. The turbine output work is calculated

from Equation (3)

(3)

Where h1 and h2 are the inlet and outlet steam enthalpies respectively. The turbine outlet

enthalpy is a function of condenser pressure

The only thermodynamic parameter that can be changed in the steam turbine is the inlet

enthalpy. It can be enhanced by increasing the inlet pressure. The turbine pressure is usually

designed for the final pressure at the end of its design life which is around 20-30 years. The

optimal pressure design is based on the geothermal turbine operational experience. For example,

the existing experience shows that for a turbine with the initial 1.3 MPa inlet pressure, the

selection of very high values for the inlet pressure is not suitable.

The high pressure causes the geothermal resource to exhaust before the machine life ends up

which is not economical. Therefore, the pressure limitations should be considered for the

geothermal turbine. This is usually 448 kPa for high pressure and 103 kPa for low pressure

turbines. Choosing low pressures will also increase the electricity generation cost.

3.2.2 The condenser pressure effect

The turbine outlet enthalpy is a function of condenser pressure. Therefore, decreasing the turbine

outlet enthalpy causes the turbine output work to increase (Equation 3). Also by increasing the

condenser pressure, the Turbine Steam Rate (T.S.R) increases. This can be calculated using

Equation. (4).

(4)

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Where:

T.S.R. is the ratio of the required steam mass flow rate for delivering 1 kWh of electrical power,

A.H.D is the adiabatic heat drop between the inlet and outlet turbine pressure and

W is the mass ratio of water to the steam and water mixture at the turbine outlet.

Figure 5 Turbine Steam Rate versus condenser pressure (Pc)

Figure 3.5 shows that by increasing the condenser pressure, the turbine steam rate increases

which causes the output turbine power to decrease.

(5)

The above relationship is more tangible at the higher inlet pressures. In fact at Pt = 400 kPa, by

increasing the condenser pressure T.S.R increases rapidly. Therefore in order to increase the

turbine work, condenser pressure should be reduced. However, there are the some limitations in

the reduction of condenser pressure:

1. The turbine outlet loss increases by reduction of the condenser pressure. In fact reduction

16

of condenser pressure decreases the steam quality or increases the water droplets at the

turbine exhaust. These droplets will create a drag force which tends to reduce the turbine

output power.

2. The droplets will be also responsible for erosion of the turbine last stage blades.

3. Further reduction of the condenser pressure, will cause the water outlet from condenser to

freeze.

Therefore based on the above limitations the condenser optimal pressure range is selected from

6.9 kPa to 13.8 kPa.

3.2.3 The geothermal fluid enthalpy effect

The geothermal fluid enthalpy (h0) is one of the important parameters in flash steam cycle

operation. If one chooses the well outlet mass flow rate as 10 t/h and the pressures according to

the data in Figure 8, the turbine output power is calculated as the following:

(6)

Where:

W1 is the product of X1 and well outlet mass flow rate,

W2 is the product of X2 and well outlet mass flow rate,

(T.S.R)1 and (T.S.R)2 are high pressure and low pressure turbines’ steam rate respectively.

The power potential is calculated from the following equation:

(7)

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The power potential is defined as the amount of electricity generation per geothermal well outlet

fluid mass flow rate.

The thermal efficiency is also calculated from the following equation:

(8)

3.3 Turbine DesignThe design of a steam turbine is very critical for the efficient production of electrical power.

Several characteristics of steam turbines cause design problems. Steam turbines must be operated

at high rotational speeds, so the blades must be designed to withstand a tremendous amount of

centrifugal force. The rotor and blade assemblies for steam turbines are usually machined from a

forged piece of chromium and steel alloy. This assembly must be very precisely balanced before

the machine is put into operation. The leakage of steam from the enclosed rotor and blade

assembly must be prevented. Solid seals cannot be used along the rotor shaft, so-called “steam”

seals are used to provide a minimum clearance between the seals and the shaft. The bearings of a

steam turbine must be carefully designed to withstand both axial and end pressures of high

magnitudes.

Figure 6 Diagram of an impulse turbine

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The motive power in a steam turbine is obtained by the rate of change in momentum of a high

velocity jet of steam impinging on a curved blade which is free to rotate. The steam from the

boiler is expanded in a nozzle, resulting in the emission of a high velocity jet. This jet of steam

impinges on the moving vanes or blades, mounted on a shaft. Here it undergoes a change of

direction of motion which gives rise to a change in momentum and therefore a force.

Steam turbines are mostly 'axial flow' types; the steam flows over the blades in a direction

parallel to the axis of the wheel. 'Radial flow' types are rarely used.

The steam turbine is a device for obtaining mechanical work from the energy stored in steam.

Steam enters the turbine with high energy content and leaves after giving up most of it. The high-

pressure steam from the boiler is expanded in nozzles to create a high-velocity jet of steam.

The nozzle acts to convert heat energy in the steam into kinetic energy. This jet is directed into

blades mounted on the periphery of a wheel or disc. The steam does not ‘blow the wheel around'.

The shaping of the blades causes a change in direction and hence velocity of the steam jet. Now

a change in velocity for a given mass flow of steam will produce a force which acts to turn the

turbine wheel, for example, mass flow of steam (kg/s) x change in velocity (m/s) = force

(kgm/s2).

This is the operating principle of all steam turbines, although the arrangements may vary

considerably. The steam from the first set of blades then passes to another set of nozzles and then

blades and so on along the rotor shaft until it is finally exhausted. Each set comprising nozzle

and blades is called a stage.

3.3.1 Steam turbines control

The valves which admit steam to the turbines are known as 'maneuvering valves'. There are

basically three valves, the ahead, the astern and the guarding or guardian valve. The guardian

valve is an astern steam isolating valve. These valves are hydraulically operated by an

independent system employing a main and standby set of pumps. Provision is also made for hand

operation in the event of remote control system failure.

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Operation of the ahead maneuvering valve admits steam into the main nozzle box. Remotely

operated valves are used to open up the remaining nozzle boxes for steam admission as increased

power is required. A speed-sensitive control device acts on the ahead maneuvering valve to hold

the turbine speed constant at the desired value. Operation of the astern maneuvering valve will

admit steam to the guardian valve which is opened in conjunction with the astern valve. Steam is

then admitted to the astern turbines.

3.3.2 Classification of steam turbines

On the basis of operation, steam turbines can be classified as:

(i) Impulse turbine and

(ii) Impulse-reaction turbine.

a. Impulse turbine

In impulse turbine, the drop in pressure of steam takes place only in nozzles and not in moving

blades. This is obtained by making the blade passage of constant cross-sectional area

The impulse arrangement is made up of a ring of nozzles followed by a ring of blades. The high-

pressure, high-energy steam is expanded in the nozzle to a lower-pressure, high-velocity jet of

steam. This jet of steam is directed into the impulse blades and leaves in a different direction.

The changing of steam direction and therefore velocity produces an impulsive force which

mainly acts in the direction of rotation of the turbine blades. There is only a very small end thrust

on the turbine shaft.

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Figure 7 photo of an impulse steam turbine

These high velocity steam jets contain significant kinetic energy, which the rotor blades, shaped

like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop

occurs across only the stationary blades, with a net increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure

Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a

very high velocity. The steam leaving the moving blades has a large portion of the maximum

velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity

is commonly called the "carry over velocity" or "leaving loss".

b. ReactionTurbines

The reaction arrangement is made up of a ring of fixed blades attached to the casing, and a row

of similar blades mounted on the rotor, i.e. moving blades. The blades are mounted and shaped to

produce a narrowing passage which, like a nozzle, increases the steam velocity. This increase in

velocity over the blade produces a reaction force which has components in the direction of blade

rotation and also along the turbine axis. There is also a change in velocity of the steam as a result

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of a change in direction and an impulsive force is also produced with this type of blades. The

more correct term for this blade arrangement is 'impulse-reaction'.

Figure 8 A diagram showing both Impulse and reaction turbines

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In the reaction turbine, the blades are arranged to form convergent nozzles. This type of turbine

makes use of the reaction force produced as the steam accelerates through the nozzles formed by

the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a

jet that fills the entire circumference of the rotor. The steam then changes direction and increases

its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the

rotor, with steam accelerating through the stator and decelerating through the rotor, with no net

change in steam velocity across the stage but with a decrease in both pressure and temperature,

reflecting the work performed in the driving of the rotor.

3.3.3 Compounding Effect

Compounding is the splitting up, into two or more stages, of the steam pressure or velocitychange through a turbine. Pressure compounding of an impulse turbine is the use of a number ofstages of nozzle and blade to reduce progressively the steam pressure. This results in lower ormore acceptable steam flow speeds and better turbine efficiency.

Velocity compounding of an impulse turbine is the use of a single nozzle with an arrangement ofseveral moving blades on a single disc. Between the moving blades, are fitted guide bladeswhich are connected to the turbine casing. This arrangement produces a short lightweight turbinewith a poorer efficiency which would be acceptable in, for example, an astern turbine.

The two arrangements may be combined to give what is called 'pressure-velocity compounding'.The reaction turbine as a result of its blade arrangement changes the steam velocity in both fixedand moving blades with consequent gradual steam pressure reduction. Its basic arrangementtherefore provides compounding.

The term 'cross-compound' is used to describe a steam turbine unit made up of a high pressureand a low pressure turbine. This is the usual main propulsion turbine arrangement. Thealternative is a single cylinder unit which would be usual for turbo-generator sets.

3.3.4 Operation and maintenance

When warming up a steam turbine for use, the main stream stop valves have a bypass line to

allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the

system along with the steam turbine. A turning gear is engaged when there is no steam into the

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turbine, it slowly rotate the turbine to ensure even heating thus prevents uneven expansion. After

first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight

plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first

to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to

slowly warm the turbine.

Figure 9 A modern steam turbine generator installation

Problems with turbines are now rare and maintenance requirements are relatively small. Any

imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go

and punching straight through the casing. It is, however, essential that the turbine be turned with

dry steam - that is, superheated steam with minimal liquid water content. If water gets into the

steam and is blasted onto the blades (moisture carryover), rapid impingement and erosion of the

blades can occur leading to imbalance and dangerous failure. Also, water entering the blades will

result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with

controls and baffles in the pressurizes to ensure high quality steam, condensate drains are

installed in the steam piping leading to the turbine.

3.3.5 Starting up the turbine

When starting up the turbine for the first time, or after any extended period of idleness, special

care must be taken to see that everything is in good condition and that all parts of the machine

are clean and free from injury. The oil piping should be thoroughly inspected and cleaned out if

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there is any accumulation of dirt. The oil reservoirs must be very carefully wiped out and

minutely examined for the presence of any grit. (Avoid using cotton waste for this, as a

considerable quantity of lint is almost sure to be left behind and this will clog up the oil passages

in the bearings and strainer.)The pilot valves should be removed from the barrel and wiped off,

and the barrels themselves cleaned out by pushing a soft cloth through them with a piece of

wood. In no case should any metal be used.

If the turbine has been in a place where there was dirt or where there has been much dust

blowing around, the bearings should be removed from the spindle and taken apart and

thoroughly cleaned. With care this can be done without removing the spindle from the cylinder,

by taking off the bearing covers and very carefully lifting the weight of the spindle off the

bearings, then sliding back the bearings. It is best to lift the spindle by means of jacks and a rope

sling, as, if a crane is used; there is great danger of lifting the spindle too high and thereby

straining it or injuring the blades. After all the parts have been carefully gone over and cleaned,

the oil for the bearing lubrication should be put into the reservoirs by pouring it into the governor

gear case. Enough oil should be put in so that when the governor, gear case, and all the bearing-

supply pipes are full, the supply to the oil pump is well covered.

Special care should be taken so that no grit gets into the oil when pouring it into the machine.

Considerable trouble may be saved in this respect by pouring the oil through cloth. A very

careful inspection of the steam piping is necessary before the turbine is run. If possible it should

be blown out by steam from the boilers before it is finally connected to the turbine. Considerable

annoyance may result by neglecting this precaution, from particles of scale, red lead, gasket, etc.,

out of the steam pipe, closing up the passages of the guide blades.

When starting up, the first thing is to revolve the spindle without vacuum being on the turbine.

After the spindle is turning slowly, the vacuum is brought up. The reason for this is, that when

the turbine is standing still, the glands do not pack and air in considerable quantity will rush

through the glands and down through the exhaust pipe. This sometimes has the effect of unequal

cooling. In case the turbine is used in conjunction with its own separate condenser, the

circulating pump may be started up, then the turbine revolved, and afterward the air pump put in

operation; then, last, put the turbine up to speed. In some cases, however, where the turbine

exhausts into the same condenser with other machinery and the condenser is therefore already in

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operation, the valve between the turbine and the condenser system should be kept closed until

after the turbine is up revolving, the turbine in the meantime exhausting through the relief valve

to atmosphere.

Care must always be taken to see that the turbine is properly warmed up before being caused to

revolve, but in cases where high superheat is employed the turbine is rotated up until it is just

moderately hot, and before it has time to become exposed to superheat. In the case of highly

superheated steam, it is not undesirable to provide a connection in the steam line by means of

which the turbine may be started up with saturated steam and the superheat gradually applied

after the shaft has been permitted to revolve. For warming up, it is usual practice to set the

governor on the trigger and open the throttle valve to allow the entrance of a small amount of

steam. The turbine is operated at a reduced speed for a time, until there is assurance that the

condenser and auxiliaries are in proper working order, that the oil pump is working properly, and

that there is no sticking in the governor or the valve gear.

After the turbine is up to speed and on the governor, its speed is recorded by counting the strokes

of the pump rod, as it is possible that the adjustment of the governor may have changed while the

machine has been idle. It is well at this time; while there is no load on the turbine, to be sure that

the governor controls the machine with the throttle wide open. It might be that the main poppet

valve has sustained some malfunctioning not evident during inspection, or was badly leaking.

Should there be such defect; steps should be taken to regrind the valve to its seat at the first

opportunity.

On the larger machines an auxiliary oil pump is always furnished. This should be used before

starting up, so as to establish the oil circulation before the turbine is resolved. After the turbine

has reached speed, and the main oil pump is found to be working properly, it should be possible

to take this pump out of service, and start it again only when the turbine is about to be shut down.

3.3.6 Running turbine

While the turbine is running, it should have a certain amount of careful attention. This, of course,

does not mean that the engineer must stand over it every minute of the day, but he must

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frequently inspect such parts as the lubricators, the oiling system, the water supply to the glands

and the oil-cooling coil, the pilot valve, etc. He must see that the oil is up in the reservoir and

showing in the gage glass provided for that purpose, and that the oil is flowing freely through the

bearings, by opening the pet cocks in the top of the bearing covers. An ample supply of oil

should always be in the machine to keep the suction in the tank covered.

Care must be taken that the pump does not draw too much air. This can usually be discovered by

the bubbling up of the air in the governor case, when more oil should be added.

It is well to note from time to time the temperature of the bearings, but no alarm need be

occasioned because they feel warm to the touch; in fact, a bearing is all right as long as the hand

can be borne upon it even momentarily. The oil coming from the bearings should be preferably

about 120 degrees Fahrenheit and never exceed 160 degrees. It should generally be seen that the

oil-cooling coil is effective in keeping the oil cool. Sometimes the cooling water deposits mud on

the cooling surface, as well as the oil depositing a Vaseline-like substance, which interferes with

the cooling effect. The bearing may become unduly heated because of this, when the coil should

be taken out at the first opportunity and cleaned on the outside and blown out by steam on the

inside, if this latter is possible. If this does not reduce the temperature, either the oil has been in

use too long without being filtered, or the quality of the oil is not good.

Should a bearing give trouble, the first symptom will be burning oil which will smoke and give

off dense white fumes which can be very readily seen and smelled. However, trouble with the

bearings is one of the most unlikely things to be encountered, and, if it occurs, it is due to some

radical cause, such as the bearings being pinched by their caps, or grit and foreign matter being

allowed to get into the oil. The oil strainer should also be occasionally taken apart and

thoroughly cleaned, which operation may be performed, if necessary, while the turbine is in

operation. The screens should be cleaned by being removed from their case and thoroughly

blown out with steam. In the case of a new machine, this may have to be done every two or three

hours. In course of time, this need only be repeated perhaps once a week. The amount of dirt

found will be an indication of the frequency with which this cleaning is necessary.

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The proper water pressure, about five pounds per square inch, must be maintained at the glands.

Any failure of this will mean that there is some big leak in the piping, or that the water is not

flowing properly. The pilot valve must be working freely, causing but little kick on the governor,

and should be lubricated from time to time. Should it become necessary, while operating, to shut

down the condenser and change over to non-condensing operation, particular care should be

observed that the change is not made too suddenly to non-condensing, as all the low-pressure

sections of the turbine must be raised to a much higher temperature. While this may not cause an

accident, it is well to avoid the stresses which necessarily result from the sudden change of

temperature.

3.3.7 Shutting Down

When shutting down the turbine the load may be taken off before closing the throttle; or, as in

the case of a generator operating on an independent load, the throttle may be closed first,

allowing the load to act as a brake, bringing the turbine to rest quickly. In most cases, however,

the former method will have to be used, as the turbine generally will have been operating in

parallel with one or more other generators. When this is the case, partially close the throttle just

before the load is to be thrown off, and if the turbine is to run without load for some time, shut

off the steam almost entirely in order to prevent any chance of the turbine running away. There is

no danger of this unless the main valve has been damaged by the water when wet steam has been

used, or held open by some foreign substance, when, in either case, there may be sufficient

leakage to run the turbine above speed, while running light. At the same time, danger is well

guarded against by the automatic stop valve, but it is always well to avoid a possible danger. As

soon as the throttle is shut, stop the condenser, or, in the case where one condenser is used for

two or more turbines, close the valve between the turbine and the condenser. Also open the

drains from the steam strainer, etc. This will considerably reduce the time the turbine requires to

come to rest. Time may be saved by leaving the field current on the generator.

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3.3.8 Speed regulation

Steam turbines used in electrical power production must be rotated at a constant speed. If turbine

speed changes, the frequency of the generator output voltage will be changed from the standard

50-Hz value. Therefore, a system of governors is used in a steam turbine to regulate its speed.

The governor system adjusts the turbine speed by compensating for changes in generator power

demand. As more load is placed on the generator (increased consumption of electrical power),

the generator offers an increased resistance to rotation. Thus, power input to the turbine must be

increased accordingly. The governor system of the turbine automatically adjusts the steam input

to the turbine blades to compensate for increases and decreases in the load demand placed upon

the generator that it drives.

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to

prevent damage while some applications (such as the generation of alternating current electricity)

require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an over

speed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If

this fails then the turbine may continue accelerating until it breaks apart, often spectacularly.

Turbines are expensive to make, requiring precision manufacture and special quality materials.

During normal operation in synchronization with the electricity network, power plants are

governed with a five percent droop speed control. This means the full load speed is 100% and the

no-load speed is 105%. This is required for the stable operation of the network without hunting

and drop-outs of power plants. Normally the changes in speed are minor. Adjustments in power

output are made by slowly raising the droop curve by increasing the spring pressure on a

centrifugal governor. Generally this is a basic system requirement for all power plants because

the older and newer plants have to be compatible in response to the instantaneous changes in

frequency without depending on outside communication

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3.3.8 Turbine protection

A turbine protection system is provided with all installations to prevent damage resulting from an

internal turbine fault or the malfunction of some associated equipment. Arrangements are made

in the system to shut the turbine down using an emergency stop and solenoid valve. Operation of

this device cuts off the hydraulic oil supply to the maneuvering valve and thus shuts off steam to

the turbine. This main trip relay is operated by a number of main fault conditions which are;

1. Low lubricating oil pressure.

2. over speed.

3. Low condenser vacuum.

4. Emergency stop.

5. High condensate level in condenser.

6. High or low boiler water level.

Other fault conditions which must be monitored and form part of a total protection system are:

1. high pressure and low pressure rotor eccentricity or vibration.

2. High pressure and low pressure turbine differential expansion, i.e. rotor with respect to casing.

3. High pressure and low pressure thrust bearing wear down.

4. Main thrust bearing wear down.

5. Turning gear engaged (this would prevent starting of the turbine).

Such 'turbovisory' systems, as they may be called, operate in two ways. If a tendency towards adangerous condition is detected a first stage alarm is given. This will enable corrective action tobe taken and the turbine is not shut down. If corrective action is not rapid, is unsuccessful, or a

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main fault condition quickly arises, the second stage alarm is given and the main trip relay isoperated to stop the turbine.

3.3.9 Thermodynamics of steam turbines

The steam turbine operates on basic principles of thermodynamics. Superheated vapor (or dry

saturated vapor, depending on application) enters the turbine, after it having been exited at high

temperature and high pressure. The high heat/pressure steam is converted into kinetic energy

using a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type

turbine). Once the steam has exited the nozzle it is moving at high velocity and is sent to the

blades of the turbine. A force is created on the blades due to the pressure of the vapor on the

blades causing them to move.

A generator is attached onto the shaft, and the energy that was in the vapor can now be converted

to electrical power and used. The gas exits the turbine as a saturated vapor (or liquid-vapor mix

depending on application) at a lower temperature and pressure than it entered with and is sent to

the condenser to be cooled. Looking at the first law of thermodynamics, the equation compares

the rate at which work is developed per unit mass. Assuming there is no heat transfer to the

surrounding environment and that the change in kinetic and potential energy is negligible when

compared to the change in specific entropy, the following equation is arrived at:

(9)

Ẇt is the rate at which work is developed per unit time

ṁ is the rate of mass flow through the turbine

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3.4 Calculating turbine efficiency

The efficiency of the steam turbine can be calculated by using the Kelvin statement of the

Second law of Thermodynamics.

(10)

Where:

Wcycle is the Work done during one cycle

QH is the Heat transfer received from the heat source

On considering the Carnot cycle the maximum efficiency of a steam turbine can be calculated.

This efficiency can never be achieved in the real world due to irreversibility during the process,

but it does give a good measure as to how a particular turbine is performing.

(11)

Where:

TL is the absolute temperature of the vapor moving out of the turbine

TH is the absolute temperature of the vapor coming from the steam separators

3.4.1 Isentropic turbine efficiency

To measure how well a turbine is performing isentropic efficiency is considered. Isentropic

efficiencies involve a comparison between the actual performance of a device and the

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performance that would be achieved under idealized circumstances. When calculating the

isentropic efficiency, heat to the surroundings is assumed to be zero. The starting pressure and

temperature is the same for both the isentropic and actual efficiency. Since state 1 is the same for

both efficiencies, the specific enthalpy h1 is known. The specific entropy for the isentropic

process is greater than the specific entropy for the actual process due to irreversibility in the

process. The specific entropy is evaluated at the same pressure for the actual and isentropic

processes in order to give a good comparison between the two.

The isentropic efficiency is given to us as the actual work divided by the maximum work that

could be achieved if there were no irreversibly in the process.

(12)

Where:

h1 is the specific enthalpy at state one

h2 is the specific enthalpy at state two for an actual process

h2s is the specific enthalpy at state two for an isentropic process

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Figure 10 steam cycle with superheat

Process 1-2: The working fluid is pumped from low to high pressure.

Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by anExternal heat source to become a dry saturated vapor.

Process3-3' The vapor is superheated.

Process 3-4 and 3'-4': The dry saturated vapor expands through a turbine, generating power.

This decreases the temperature and pressure of the vapor, and some condensation may occur.

Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant pressure

3.5 The Generator

Electric generators are devices that convert energy from a mechanical form to an electrical form.

This process, known as electromechanical energy conversion, involves magnetic fields that act as

an intermediate medium. The input to the machine can be derived from a number of energy

34

sources. For example, in the geothermal power generation its the steam that drives the shaft of

the machine.

The generator’s operation is based on Faraday’s law of electromagnetic induction. In brief, if a

coil (or winding) is linked to a varying magnetic field, then an electromotive force, or voltage,

electromagnetic forces, is induced across the coil. Thus, generators have two essential parts: one

creates a magnetic field and the other where the electromagnetic forces’s are induced. The

magnetic field is typically generated by electromagnets (thus, the field intensity can be adjusted

for control purposes), whose windings are referred to as field windings or field circuits. The coils

where the electromagnetic forces are induced are called Armature windings or armature

circuits. One of these two components is stationary (stator), and the other is a rotational part

(rotor) driven by an external torque. Conceptually, it is immaterial which of the two components

is to rotate because, in either case, the armature circuits always “see” a varying magnetic field.

However, practical considerations lead to the common design that for ac generators, the field

windings are mounted on the rotor and the armature windings on the stator.

3.5.1 Synchronous Generators

In this chapter the most elementary principles of operation of synchronous machines will be

presented.

It is convenient to introduce the fundamental principles describing the operation of a

synchronous machine in terms of an ideal cylindrical-rotor machine connected to an infinite bus.

The infinite bus represents a bus bar of constant voltage, which can deliver or absorb active and

reactive power without any limitations. The ideal machine has zero resistance and leakage

reactance, infinite permeability, and no saturation, as well as zero reluctance torque. The

production of torque in the synchronous machine results from the natural tendency of two

magnetic fields to align themselves. The magnetic field produced by the stationary armature is

denoted as φs. The magnetic field produced by the rotating field is φf.

The resultant magnetic field is:

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φr = φs + φf

The flux φr is established in the air gap of the machine. (Bold symbols indicate vector

quantities.) When the torque applied to the shaft equals zero, the magnetic fields of the rotor and

the stator become perfectly aligned. The instant torque is introduced to the shaft, either in a

generating mode or in a motoring mode; a small angle is created between the stator and rotor

fields. This angle (λ) is called the torque angle of the machine.

Figure 11 Generator stator windings

The design is based on a horizontally split two-piece frame, which supports the stator core

flexibly. This flexibility isolates the double frequency vibrations from the foundation. The

laminated stator core is build up of magnetic sheet segments made of high quality silicon steel

coated on both sides with a heat-resisting varnish. The winding bars are insulated by mica paper

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as the actual high-grade insulation material. At the slot ends, a corona-protection sleeve is added

to provide the necessary stress grading between the slot end and the increasing voltage in the

overhang region.

The rotor windings consist of hollow rectangular conductors of hard-drawn copper, which is

alloyed with 0.1% silver to increase its strength at high temperatures. The winding is installed in

the slots in such a way that it can expand uniformly outwards from the middle towards each end.

This ensures smooth running during thermal changes with minimum sensitivity to fast load

changes. All of the insulation materials used in the rotor complies with Class F specifications.

A damper winding is formed with winding slot wedges. Centrifugal force binds them together in

the area of the end-bell seat to form a complete damper cage.

The generators in a geothermal power plant are operated at a constant speed to ensure the

frequency remains constant. This is achieved by running the turbine at a constant speed too. By

regulating the steam pressure coming in contact with the turbine fins, a constant smooth speed is

achieved. A governor is put in place to regulate the steam pressure and hence the constant speed.

3.6 Power Transformer

The power tapped from the generators is not at a high enough voltage for transmission, therefore

step up power transformers are installed at the power plant to step up voltage.

The transformer is based on two principles: first, that an electric current can produce a magnetic

field (electromagnetism), and, second that a changing magnetic field within a coil of wire

induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in

the primary coil changes the magnetic flux that is developed. The changing magnetic flux

induces a voltage in the secondary coil.

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CHAPTER FOUR

DISCUSSION

4.1 Advantages of geothermal powerGeothermal power production has many advantages over other forms of power generation

including:

i. Noise pollution

During operation a geothermal power plant does not cause any noise pollution to the

surrounding population, Because turbine-generator buildings are usually designed are

typically well-insulated acoustically and thermally, and equipped with noise absorptive

interior walls. Noise from normal power plant operation at the site boundary would

occupy a range of 15 to 28 dBA below the level of a whisper.

ii. Water Quality

In a geothermal facility, geothermal water is isolated during production, injected back

into the geothermal reservoir, and separated from groundwater by thickly encased pipes,

making the facility virtually free of water pollutants. Most geothermal reservoirs are

found deep underground, well below groundwater reservoirs. As a result, these deep

reservoirs pose almost no negative impact on water quality and use. Because the

geothermal water in a binary, air cooled plant is contained in a closed system, binary

power plants do not consume any water.

iii. Renewable energy

Geothermal resources are sustainable because of the heat from the earth and water

injection, and thus will not diminish like fossil fuel reserves. As time progresses and

technology improves, the ability to extract geothermal resources with ease will increase,

not decrease, geothermal is a renewable energy technology that can offer base load or

intermediate power, and can achieve high capacity factors. Geothermal represents a

plentiful resource that has not been utilized to its full potential, Geothermal is an

indigenous source of energy.

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iv. Reliability

Geothermal resources are available 24 hours a day regardless of changing weather,

geothermal energy is therefore a reliable and not even weather changes will affect it as it

happens with hydropower plants during dry periods.

v. Environmental impacts.

Wastewater that would otherwise damage surface waters is being used to recharge the

wells and irrigate local land. In addition, electricity generation from geothermal resources

eliminates the mining, processing and transporting required for electricity generation

from fossil fuel resources. The construction of a geothermal power plant does not require

vast piece of land hence does not cause damage to vegetation or require human or

wildlife population to be relocated.

vi. Affordability

Since geothermal is a natural source of energy, it produces affordable power since the

generation process does not incur any cost of fuels to burn the water to steam, therefore

making power affordable to all classes of life.

vii. Emissions

Unlike fossil fuel power plants, no smoke is emitted from geothermal power plants,

because no burning takes place: only steam is emitted from geothermal facilities. This

makes geothermal a green source of energy in that no carbon or any other harmful gases

are emitted into the atmosphere. It is advisable the world adopt such green sources of

power to counter the carbon quantities being emitted which in turn contribute to global

warming.

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4.2 Disadvantages of geothermal powerGeothermal has very few disadvantages, they include:

i. High initial capital

Geothermal power generation is an intensive project that requires a high initial investment to

construct; some of the most involving activities that raise the cost are prospection and drilling.

They require high qualified professionals to undertake prospection before drilling can

commence. This is necessary to avoid drilling wells that will not produce sufficient steam to run

the turbines.

ii. Piping area

A geothermal plant in some site may require large areas to lay pipes running from various

production wells to the well top separators. This may arise due to wells being far apart. This also

raises the cost of installing such a plant due to extra piping material requirement.

iii. Harmful gases

It is important to take care of a geothermal site because if the holes were drilled improperly,

then potentially harmful minerals and gas could escape from underground. These hazardous

materials are nearly impossible to get rid of properly. Pollution may occur due to improper

drilling at geothermal stations.

iv. Location

Perhaps the biggest drawback when it comes to geothermal energy is that it cannot be set up just

anywhere, First of all, location that offers just the right kind of hot rocks is the major necessity.

Just any hot rocks will not do, since some rocks might prove too strong to drill through. These

rocks also need to be within a reasonable depth to make drilling down to them a feasible option.

Volcanic areas often provide the most geothermal efficiency

v. Drying up of wells

Unbelievably, it is also possible for a specific geothermal area to run dry or lose steam.

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4.3 Plant maintenance practices

The maintenance practices in geothermal power plants vary from vary from one field to another

depending on the nature of field, the plant design and the inherent practices. Each plant has its

own method of doing maintenance based on experience and unique problems in the plant in

addition to recommendations by manufacturers of equipment. Visits and interviews were

contacted from Olkaria geothermal power plants in Kenya. An overview of maintenance

practices in these power plants in relation to properties of the geothermal fluids is discussed

A typical geothermal power plant has hundreds of operating equipment that have to be

maintained to preserve their functionality, maintain plant safety and improve plant efficiency. A

generalized flow diagram for a typical electricity producing geothermal power plant is shown in

Figure 4.1 below. Only major processes and equipment are shown. A complete assembly of a

geothermal power plant consists of thousands of components that make it a complex. In a typical

electricity producing geothermal power plant, the main processes are steam gathering and

transmission, turbine and its auxiliaries, generator and electrical, Gas extraction, cooling

processes and instrumentation and controls.

Figure 12 A simplified process flow diagram for a GPP

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A summary of the main components in the processes is shown in Table 8. Only the major

components under each system are presented

Table 1 summary of main parts in a GPP

Most equipment and systems at Olkaria 1 power plant are based on analogue signal system and

are largely manually operated. Olkaria 2 power plant operations are automated and most

equipment monitored and controlled via SCADA system with most control signal being digital.

The main method of maintenance used in Olkaria is time based preventive maintenance (PM).

The PM programs include major overhauls every five years, annual inspections, semi annual

maintenance and monthly tasks. There are 5-year major overhauls for turbines, two year

overhauls for auxiliary equipment, annual inspection for the whole plant, semiannual, quarterly

and monthly maintenance for auxiliaries. Maintenance actions can also be based on observed

signs of deterioration.

Besides the preventive maintenance, the power plants are regularly inspected and any potential

failure indications addressed .The management for the maintenance procedures is based on

management maintenance subsystem (MMS). Monitoring in Olkaria 2 are done online using the

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distributed control system (DCS) and the SCADA system but in Olkaria 1monitoring is mainly

done manually.

OTHER RELATED SUMMARIES

Table 2 geothermal power growth since 2005 in various countries

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Table 3 a summary showing various aspects in geothermal development

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CHAPTER FIVE

5.1 CONCLUSIONS

This report achieved its objectives by looking at various geothermal power plant components

that constitute various designs of power generation by use of geothermal energy

Abundant geothermal resources throughout the nation can provide an environmentally friendly

source of energy. Data compiled from a variety of sources point to geothermal energy as an

environmental option for new power generation that is far better than other energy sources such

as fossil fuels. In addition, geothermal remains as environmentally friendly as most other

renewable sources, while simultaneously offering reliability and a source of base load power that

is unique among most other renewable options available. Geothermal Development Company is

conducting research on a regular basis to improve the already minimal environmental impacts of

geothermal energy and to decrease the associated costs.

While currently used at only a fraction of its potential, geothermal energy can substantially

contribute to the energy needs of the twenty-first century. With increased federal research and

development funding in conjunction with supportive renewable energy policies

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5.2 RECOMMENDATIONS

The outlook for geothermal energy use depends on at least three factors: the demand for energy

in general; the inventory of available geothermal resources; and the competitive position of

geothermal among other energy sources. Focusing on the demand for energy in the developing

countries like Kenya, then there is need to carry out vigorous and extensive search for more

green energy to ensure that the development that comes with affordable energy does not come

with demerits such as air and noise pollution. Geothermal power is therefore recommended.

The Demand for energy will continue to grow. Economies are expanding, populations are

increasing and energy-intensive technologies are spreading. All mean greater demand for energy.

At the same time, there is growing global recognition of the environmental impacts of energy

production and use from fossil fuel and nuclear resources. Public polls repeatedly show that most

people prefer policies in support for renewable energy. In this case geothermal is a green

renewable source of energy hence highly encouraged.

The Inventory of accessible geothermal energy is sizable. Using current technology geothermal

energy from already-identified reservoirs can contribute as much as 40% of Kenyas energy

supply. And with more exploration, the inventory can become larger. The entire world resource

base of geothermal energy has been calculated in government surveys to be larger than the

resource bases of coal, oil, gas and uranium combined. The geothermal resource base becomes

more available as methods and technologies for accessing it are improved through research and

experience.

With proper and economic designs of geothermal power plants, it can been seen that with the

potential not yet harnessed, then a greater world population can rely on green energy that is

environment friendly.

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REFERENCES

1. DiPippo, R., Geothermal Power Plants, Second Edition, Elsevier Ltd., 2008

2. http://www.digtheheat.com/geothermal/dry_steam_plant.html

3. http://www.gutenberg.org/files/27687/27687-h/27687-h.htm

4. Hubert E. Collins Steam Turbines FIRST EDITIONSecond Impression McGRAW-HILL BOOK COMPANY, INC

5. http://www.geothermalpowerplant.com/6. Geoff Klempner and Isidor Kerszenbaum Operation and Maintenance of Large Turbo

Generators

7. Alyssa Kagel, Diana Bates, & Karl Gawell , Geothermal Energy Association Guide to

Geothermal Energy and the Environment

8. Seiki Kawazoe - Geothermal Japan History and Status of Geothermal Power

Development and Production

9. Ormat Technologies Inc. (http://www.ormat.com)

10. John Bird, Electrical and electronic principles and technology,2nd Edition

11. The 2nd Joint International Conference on “Sustainable Energy and Environment”

21-23 November 2006, Bangkok, Thailand The Study of Key Thermodynamic Parameters

Effects on the Performance of a Flash Steam Geothermal Power Plant

12. Clety Kwambai Bore,Kenya Electricity Generating Co. Ltd – KenGen Olkaria GeothermalProject

.