Project Report 2013

SSET, KARUKUTTY DEPARTMENT of EEE

TABLE OF FIGURES

FIG.NO TITLE PAGE NO

2.1 Seebeck arrangement 5

2.2 Generation of electricity 7

2.3 Peltier effect 8

2.4 Thermocouple measuring circuit 10

2.5 Block diagram 12

2.6 Thermoelectric module 13

2.7 Thermoelectric generator 14

2.8 Principle of thermoelectric generator 14

2.9 Schematic diagram of thermoelectric generator 16

2.10 Basic inverter schematic 18

4.1 Performance characteristics 22

4.2 Wooden stove 23

4.3 Heat sink and cooling system 24

4.4 Module connection 25

4.5 Battery 25

4.6 Circuit diagram of inverter 26

4.7 Pin configuration of IC3525 27

4.8 MOSFET 29

4.9 MOSFET working 30

4.10 Transformer 31

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CHAPTER 1

INTRODUCTION

Electricity is no longer a luxury; it has become a necessity in our everyday lives. Have you ever

imagined the life without electricity for an extended period of time? Every year thousands, even

millions have been in this position when a winter storm knocked out power over large areas.

Solar panels are a great alternative energy source, but they only produce electricity during

daylight hours and their output is significantly reduced during winter months and cloudy days.

Using a TEG can provide your home’s energy needs and depending on what state you live in,

you will begetting a check from the electric company instead off a bill in the future!!

A thermoelectric generator is a system that exploits the Seebeck effect to convert heat energy into

electricity. Utilizing several semi-conductors in series, a thermoelectric module exploits a

difference in temperature to capture energy from thermally excited electrons. It is an attractive

option for providing small amounts of electricity to homes in the developing world that

are not connected to the power grid. Generally, household’s use biomass stoves for their

cooking needs. Such stoves can be equipped with a thermoelectric. Subsequently, the generator

converts waste heat to electricity, providing a household with enough electricity.

The generator system consists of a thermoelectric module, a hot sink, and a cold sink. Heat from

the stove is captured by the hot sink and transferred to the hot side of the thermoelectric module.

The thermoelectric module converts a fraction of this heat energy into electricity. The remainder

of the heat is rejected to the environment, by the cold sink. The electricity produced by the

module flows to the load via two leads on the thermoelectric module.

The ultimate aim of this project is to convert the waste heat into the useful electrical energy. The

Existing wooden stove is modified into multifunctional wooden stove which can be used both for

cooking and also for generating of the electricity.

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CHAPTER2

LITERATURE SURVEY

Paper1:‘Study of thermoelectric generator incorporated onto a

multifunction wooden stove’

Author: Doctor Daniel Champier, Associate Professor, UPP

Abstract: Replacing traditional open firestoves, characterized by low efficiency, with improved

ones is an important challenge for the developing countries. Adding thermoelectric generators

can provide electricity that permits not only the use of an electric fan increasing the ratio of air to

fuel to achieve a complete combustion in the stoves but also the satisfaction of the basic needs:

lights, phones and the other electronic devices. T test the thermoelectric modules, an

experimental device has been carried out in the laboratory where gas heater simulates stove. The

performance of the generator mainly depends on the heat transfer through the modules and

especially on thermal contact resistances. The study of temperature and electrical power

measurements is compared to a theoretical analysis using thermoelectric and heat transfer

equations. The thermo electric generator has produced up to 9.5w.

Paper2:‘Performance of Stove Mounted Thermoelectric Generator’

Author: Harry O’Hanley. Prof. Derek Rowell

Abstract: A prototype cook stove has been fabricated with a thermoelectric generator mounted in

the chimney. This device produces both heat and electricity and is intended for use in

developing-world households that are not connected to the electrical grid. It is equipped with a

hot sink located in the flue to collect heat, a thermoelectric module to generate electricity, and

a cold sink with a cooling fan to reject heat to the environment. Three aspects of this

configuration were investigated: the correlation between temperature difference across the

module and power output, the impact of the cooling fan on the thermal resistance of the cold

sink, and the effect of matching load impedance to source impedance. Results confirmed

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the theoretical correlation, in which power output is approximately proportional to the

square of temperature difference. The thermal resistance of the cold sink was determined

to decrease exponentially with an increase of impinging airflow. Lastly, maximum power

was extracted from the thermoelectric module when the load and source impedances were

equal. These results were used to optimize the stove configuration and it was experimentally

calculated that the stove will operate best with matched impedances and the cooling fan

sub-maximally powered with 10.9V. These findings can be used to better design a power

generating cook stove. Additionally, the experimental procedure is repeatable for other cook

stove prototypes.

Paper3: ‘High Efficiency Thermoelectric Generator: Integration’

Author: Dr. Lon E. Bell, BSST LLC.25 February 2011

Abstract: Thermoelectric generator efficiency is one of the key metrics for assessment of

technology viability in particular applications. It provides an important indication of the

progress made in development of thermoelectric materials having improved performance

compared to state of the art materials. BSST has conducted a worldwide survey of

thermoelectric material development efforts, identifying and sampling the most promising

developments. These materials were assessed by their assembly and incorporation in

demonstration power generation devices. Experimental methods were employed to assembly

device components consisting of multiple types of thermoelectric material to optimize

performance. Experimental generator designs were developed and employed to incorporate the

thermoelectric components and provide a demonstration of the best possible performance that

could be obtained from the materials selected. Bulk segmented materials were used in both n-

and p-legs of tested couples and

prototype generators. A compact cylindrical TEG, comprised of an axial heat source (electrical

heater in a demo device), radial segmented TE elements, and a cold side with air or water

cooling was developed and used to demonstrate material performance. The inner volume of the

device operated in an Argon atmosphere. The design was adaptable to a variety of practical heat

sources.

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2.1 THERMOELECTRICITY

Thermoelectricity is a two-way process. It can refer either to the way a temperature difference

between one side of a material and the other can produce electricity, or to the reverse: the way

applying an electric current through a material can create a temperature difference between its

two sides, which can be used to heat or cool things without combustion or moving parts.

The fundamental problem in creating efficient thermoelectric materials is that they need to be

good at conducting electricity, but not at conducting thermal energy. That way, one side can get

hot while the other gets cold, instead of the material quickly equalizing the temperature. But in

most materials, electrical and thermal conductivity go hand in hand. New nano-engineered

materials provide a way around that, making it possible to fine-tune the thermal and electrical

properties of the material.

2.2 THERMOELECTRIC EFFECT

The thermoelectric effect is the direct conversion of temperature differences to electric voltage

and vice-versa. A thermoelectric device creates a voltage when there is a different temperature

on each side and vice versa. At the atomic scale, an applied temperature gradient causes charge

carriers in the material to diffuse from the hot side to the cold side. This effect can be used to

generate electricity, measure temperature or change the temperature of objects. Because the

direction of heating and cooling is determined by the polarity of the applied voltage. The term

"thermoelectric effect" encompasses three separately identified effects: the Seebeck effect,

Peltier effect and Thomson effect.

2.2.1 SEEBECK EFFECT

The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar

electrical conductors or semiconductors produces a voltage difference between the two

substances.When heat is applied to one of the two conductors or semiconductors, heated

electrons flow toward the cooler one. If the pair is connected through an electrical circuit, direct

current (DC) flows through that circuit.

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The voltages produced by Seebeck effect are small, usually only a few microvolts (millionths of

a volt) per kelvin of temperature difference at the junction. If the temperature difference is large

enough, some Seebeck-effect devices can produce a few millivolts (thousandths of a volt).

Numerous such devices can be connected in series to increase the output voltage or in parallel to

increase the maximum deliverable current. Large arrays of Seebeck-effect devices can provide

useful, small-scale electrical power if a large temperature difference is maintained across the

junctions.

The Seebeck effect is responsible for the behavior of thermocouples, which are used to

approximately measure temperature differences or to actuate electronic switches that can turn

large systems on and off. This capability is employed in thermoelectric cooling technology.

Commonly used thermocouple metal combinations include constantan/copper, constantan/iron,

constantan/chromel and constantan/alumel.

Fig 2.1: Seebeck arrangement

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The voltage V developed can be derived from:

Where   and   are the Seebeck coefficients of metals A and B as a function of temperature

and   and   are the temperatures of the two junctions. The Seebeck coefficients are non-linear

as a function of temperature, and depend on the conductors' absolute temperature, material, and

molecular structure. If the Seebeck coefficients are effectively constant for the measured

temperature range, the above formula can be approximated as:

The Seebeck effect is caused by charge-carrier diffusion. Charge carriers in the materials will

diffuse when one end of a conductor is at a different temperature from the other. Hot carriers

diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold

end of the conductor, and vice versa. If the conductor were left to reach thermodynamic

equilibrium, this process would result in heat being distributed evenly throughout the conductor.

The movement of heat (in the form of hot charge carriers) from one end to the other is a  heat

current and an electric current as charge carriers are moving.

In a system where both ends are kept at a constant temperature difference, there is a constant

diffusion of carriers. If the rate of diffusion of hot and cold carriers in opposite directions is

equal, there is no net change in charge. The diffusing charges are scattered by impurities,

imperfections, and lattice vibrations or phonons. If the scattering is energy dependent, the hot

and cold carriers will diffuse at different rates, creating a higher density of carriers at one end of

the material and an electrostatic voltage.

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Fig2.2: Generation of electricity

This electric field opposes the uneven scattering of carriers, and equilibrium is reached where the

net number of carriers diffusing in one direction is canceled by the net number of carriers

moving in the opposite direction. This means the thermo power of a material depends greatly on

impurities, imperfections, and structural changes that vary with temperature and electric field;

the thermo power of a material is a collection of many different effects. Early thermocouples

were metallic, but many more recently developed thermoelectric devices are made from

alternating p-type and n-type semiconductor elements connected by metallic connectors.

Semiconductor junctions are common in power generation devices. Charge flows through the n-

type element, crosses a metallic interconnect, and passes into the p-type element. When a heat

source is provided, the thermoelectric device functions as a power generator. The heat source

drives electrons in the n-type element toward the cooler region, creating a current through the

circuit. Holes in the p-type element then flow in the direction of the current. Therefore, thermal

energy is converted into electrical energy.

2.2.2PELTIER EFFECT

The Peltier effect is the presence of heat at an electrified junction of two different metals. The

Peltier effect is a temperature difference created by applying a voltage between two electrodes

connected to a sample of semiconductor material. This phenomenon can be useful when it is

necessary to transfer heat from one medium to another on a small scale. In a Peltier-effect

device, the electrodes are typically made of a metal with excellent electrical conductivity. The

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semiconductor material between the electrodes creates two junctions between dissimilar

materials, which, in turn, creates a pair of thermocouple voltage is applied to the electrodes to

force electrical current through the semiconductor, thermal energy flows in the direction of

the charge carriers.

Peltier effect,  the cooling of one junction and the heating of the other when electric current is

maintained in a circuit of material consisting of two dissimilar conductors; the effect is even

stronger in circuits containing dissimilar semiconductors. In a circuit consisting of a battery

joined by two pieces of copper wire to a length of bismuth wire, a temperature rise occurs at the

junction where the current passes from copper to bismuth, and a temperature drop occurs at the

junction where the current passes from bismuth to copper.

Fig 2.3: Peltier effect

Peltier-effect devices are used for thermoelectric cooling in electronic equipment and computers

when more conventional cooling methods are impractical.

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2.3THERMOELECTRIC GENERATOR

Thermoelectric generators (also called Seebeck generators) are devices which convert heat

(temperature differences) directly into electrical energy, using a phenomenon called the

"Seebeck effect" (or "thermoelectric effect"). Their typical efficiencies are around 5-8%. Older

Seebeck-based devices used bimetallic junctions and were bulky while more recent devices use

semiconductor p-n junctions made from bismuth telluride (Bi2Te3), lead telluride (PbTe),calcium

manganese oxide, or combinations depending on temperature. These are solid state devices and

unlike dynamos have no moving parts, with the occasional exception of a fan or pump.

2.3.1THERMOCOUPLES

A thermocouple consists of two conductors of different materials (usually metal alloys) that

produce a voltage in the vicinity of the point where the two conductors are in contact. The

voltage produced is dependent on, but not necessarily proportional to, the difference of

temperature of the junction to other parts of those conductors. Thermocouples are a widely used

type of temperature sensor for measurement and control and can also be used to convert a

temperature gradient into electricity. Commercial thermocouples are inexpensive,

interchangeable, are supplied with standard connectors, and can measure a wide range of

temperatures. In contrast to most other methods of temperature measurement, thermocouples are

self powered and require no external form of excitation. The main limitation with thermocouples

is accuracy; system errors of less than one degree Celsius (C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to temperature.

Thermocouples for practical measurement of temperature are junctions of specific alloys which

have a predictable and repeatable relationship between temperature and voltage. Different alloys

are used for different temperature ranges. Properties such as resistance to corrosion may also be

important when choosing a type of thermocouple. Where the measurement point is far from the

measuring instrument, the intermediate connection can be made by extension wires which are

less costly than the materials used to make the sensor. Thermocouples are usually standardized

against a reference temperature of 0 degrees Celsius; practical instruments use electronic

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methods of cold-junction compensation to adjust for varying temperature at the instrument

terminals. Electronic instruments can also compensate for the varying characteristics of the

thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature

measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Fig 2.4: A thermocouple measuring circuit with a heat source, cold junction and a measuring

instrument.

The principle of operation of the thermocouple is the seebeck effect i.e. the voltage is developed

based on the temperature difference between the hot and the cold junctions of the thermocouple.

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Any attempt to measure this voltage necessarily involves connecting another conductor to the

"hot" end. This additional conductor will then also experience the temperature gradient, and

develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the

effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a

circuit in which the two legs generate different voltages, leaving a small difference in voltage

available for measurement. That difference increases with temperature, and is between 1 and 70

microvolts per degree Celsius (µV/°C) for standard metal combinations.

The voltage is not generated at the junction of the two metals of the thermocouple but rather

along that portion of the length of the two dissimilar metals that is subjected to a temperature

gradient. Because both lengths of dissimilar metals experience the same temperature gradient,

the end result is a measurement of the difference in temperature between the thermocouple

junction and the reference junction.

2.3.2PROPERTIES OF THERMOCOUPLE CIRCUITS

The behavior of thermoelectric junctions with varying temperatures and compositions can be

summarized in three properties:

Homogeneous material—a thermoelectric current cannot be sustained in a circuit of a single

homogeneous material by the application of heat alone, regardless of how it might vary in cross

section. In other words, temperature changes in the wiring between the input and output do not

affect the output voltage, provided all wires are made of the same materials as the thermocouple.

Intermediate materials—the algebraic sum of the thermoelectric EMFs in a circuit composed

of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature.

So if a third metal is inserted in either wire and if the two new junctions are at the same

temperature, there will be no net voltage generated by the new metal.

Successive or intermediate temperatures—if two dissimilar homogeneous materials produce

thermal EMF1 when the junctions are at T1 and T2 and produce thermal EMF2 when the

junctions are at T2 and T3, the EMF generated when the junctions are at T1 and T3 will be

EMF1 + EMF2, provided T1<T2<T3

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Thermocouples can be connected in series to form a thermopile, where all the hot junctions are

exposed to a higher temperature and all the cold junctions to a lower temperature. The output is

the sum of the voltages across the individual junctions, giving larger voltage and power output.

Fig 2.5: Thermoelectric wooden stove block diagram

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COLD

HOT

TEM

BATTERY

OSCILLATOR

CIRCUIT POWER CIRCUIT TRANSFORMER

LOAD

WOODEN STOVE

INVERTER

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2.4THERMOELECTRIC MODULE

Thermoelectric modules are solid-state devices (no moving parts) that convert electrical energy

into a temperature gradient, known as the "Peltier effect" or convert thermal energy from a

temperature gradient into electrical energy, the "Seebeck effect." Thermoelectric modules used

as TE generators or TEGs are rather inefficient and little power is produced.

Fig 2.6: Thermoelectric module

With no moving parts, thermoelectric modules are rugged, reliable and quiet heat pumps,

typically 1.5 inches (40 x 40mm) square or smaller and approximately ¼ inch (4 mm) thick. The

industry standard mean time between failures is around 200,000 hours or over 20 years for

modules left in the cooling mode.

Because the cold side of the module contracts while the hot side expands modules with a

footprint larger than 1.5 - 2 inches square usually suffer from thermally induced stresses, at the

electrical connection points inside the module causing a short, so they are not common. Long,

thin modules want to bow for the same reason and are also rare. Larger areas than an individual

module can maintain are cooled or have the temperature controlled by using multiple modules.

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Since the energy available from a single thermocouple is very small, arrays of thermocouples

must be used to construct thermoelectric devices capable of handing practical amounts of power.

Higher power devices can be made by connecting thermocouples in series to increase the voltage

capability and in parallel to increase the current capacity. Such an array of thermocouples is

called a thermopile. The thermocouples are connected using the conducting material and the

thermopile is given a covering using ceramic substances in order to withstand the temperature

difference applied .The unit as a whole is called as the thermoelectric module.

2.5 PRINCIPLE

OF THERMOELECTRIC GENERATION

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Fig 2.7: Thermoelectric generator (TEG)

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Fig2.8: Principle of TEG

In the thermoelectric module the metal cannot be used since the interconnections between the

thermocouples using metal may cause the voltage to be developed in the opposite direction and

as a result the net voltage produced id reduced and the output is drastically reduced.

In order to avoid the problems while using the metallic conductors we use p and n type

semiconductors connected together so that the voltage developed is aiding and the net output

produced is increased.

The operation is as follows:

The temperature at the hot side is more at the hot side than at the cold side and as a result

the electrons at the hot side of the n type semiconductor is having more energy and tends

to flow towards the cold side. The movement of the electrons from the hot side is more

faster than that from the cold side. Hence the flow of electron is from the hot side to the

cold side

Whereas when the temperature at the hot side is increased for the p type semiconductor

the holes or the positive charge particle gets more energy and flow from the hot side to

the cold side.

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Thus in effect the cold side portion of the n type semiconductor contains the negative

charge and act as the negative terminal and the cold side portion of the p type

semiconductor has the positive charge hence it will be acting as the positive terminal.

By connecting the adjacent n type and p type semiconductors at each side alternately the

effective output increases since they are aiding.

2.6THERMOELECTRIC MATERIALS

Bismuth Telluride (BizTe3) is the most semiconductor material that often used in the

thermoelectric module construction. The crystals of bismuth telluride have the layers of atoms

forming the sequence as:

The bismuth and tellurium layers are connected by strong covalent ionic bonds. There are 2 ways

to fabricate thermoelectric materials which are either by using directional crystallization from a

melt or pressed powder metallurgy. By using the directional crystallization, typically Bismuth

Telluride material is fabricated in ingot or boule form and then sliced into wafers of various

thicknesses. Then the wafer is diced into blocks that may be assembled into thermoelectric

cooling modules after the wafer's surfaces have been properly prepared. The blocks of Bismuth

Telluride material, usually are called elements or dice.

Beside Bismuth Telluride (Bi2Te3), other thermoelectric materials including Lead Telluride

(PbTe), Silicon Germanium (SiGe), Bismuth-Antimony (Bi-Sb), Mercury Telluride and Silver

Telluride alloys may be used in specific situations.

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2.7 WOOD BURNING STOVE

A wood-burning stove is a heating appliance capable of burning wood fuel and wood-

derived biomass fuel, such as wood pellets. Generally the appliance consists of a solid metal

(usually cast iron or steel) closed fire chamber, a fire brick base and an adjustable air control.

The appliance will be connected by ventilating stove pipes to a suitable chimney orflue, which

will fill with hot combustion gases once the fuel is ignited. The chimney or flue gases must be

hotter than the outside temperature to ensure combustion gases are drawn out of the fire chamber

and up the chimney. Many wood-burning stoves are engineered such that they can be converted

to multi-fuel stoves with the addition of a grate.

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Fig 2.9: Schematic diagram of thermoelectric generator

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2.8 INVERTER

An inverter, is an electrical power converter that changes direct current (DC) to alternating

current (AC); the converted AC can be at any required voltage and frequency with the use of

appropriate transformers, switching, and control circuits.

Solid-state inverters have no moving parts and are used in a wide range of applications, from

small switching power supplies in computers, to large electric utility high-voltage direct

current applications that transport bulk power. Inverters are commonly used to supply AC power

from DC sources such as solar panels or batteries.

The inverter performs the opposite function of a rectifier. The electrical inverter is a high-

power electronic oscillator. It is so named because early mechanical AC to DC converters were

made to work in reverse, and thus were "inverted", to convert DC to AC.

The square wave output has a high harmonic content, not suitable for certain AC loads such as

motors. Square wave units were the pioneers of inverter development.

2.8.1 BASIC DESIGNS

In one simple inverter circuit, DC power is connected to a transformer through the centre tap of

the primary winding. A switch is rapidly switched back and forth to allow current to flow back to

the DC source following two alternate paths through one end of the primary winding and then

the other. The alternation of the direction of current in the primary winding of the transformer

produces alternating current (AC) in the secondary circuit.

The electromechanical version of the switching device includes two stationary contacts and a

spring supported moving contact. The spring holds the movable contact against one of the

stationary contacts and an electromagnet pulls the movable contact to the opposite stationary

contact. The current in the electromagnet is interrupted by the action of the switch so that the

switch continually switches rapidly back and forth. This type of electromechanical inverter

switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios. A similar

mechanism has been used in door bells, buzzers and tattoo guns.

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Fig 2.10: Basic inverter schematic

As they became available with adequate power ratings, transistors and various other types

of semiconductor switches have been incorporated into inverter circuit designs. Certain ratings,

especially for large systems (many kilowatts) use thyristors (SCR). SCRs provide large power

handling capability in a semiconductor device, and can readily be controlled over a variable

firing range.

CHAPTER3

OBJECTIVES

The main objectives of this project are:

The main objective is to find a substitute for general source of electricity

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Remedy for rural area electrification

Incorporation of thermo electric module onto the wooden stove

Generation of electricity from the waste heat of the stove

Study of the inverter design

CHAPETR4

PROJECT PHASES

Collection and

analysis of TEM

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Making of the

wooden stove

Incorporation of the

TEM on wooden

stove

Electrical

connections

Inverter

Design

Observations

4.1 PHASE1: COLLECTION AND ANALYSIS OF THE TEM

4.1.1COMPARING DIFFERENT MODULES

Different thermoelectric modules characteristics were compared and the availability of the

modules were studied. The modules compared include TEC12704,TEC12705,TEC12706 etc.

Table 4.1: Comparison of different modules

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Among the different thermo electric modules the available one was the TEC12704.

The following are the specifications of the TEC12704

Table 4.2: TE module dimension

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Table 4.3: TE module parameter

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The performance characteristics of the TEC12704 with the variation in the

temperature are given below:

Fig 4.1: Performance characteristics

4.2 PHASE2: MAKING OF THE WOODEN STOVE

The wooden stove is to be built in order to integrate the thermo electric module to it. The

following steps were used in the construction of the multifunctional wooden stove:

The wooden frame for the construction of the stove was built. The frame was built in two

layers.

5 holes were made on either side of the frame in order provide the provision for the

incorporation of the thermo electric modules to the stove.

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4 bolts were used in order to make the frame a rigid structure.

The 0.25 inch iron rods were made into L shape and were placed inside the frame.

The plantain stem was placed inside the holes before filling the cement.

The cement, sand and water were made into a mixture and was filled in to the frame

made earlier.

The stove was covered with cloths to prevent the loss of the moisture. The arrangement

was made wet regularly for 4 days.

The cover for the stove consist of 2 layers:- the topmost layer is of aluminium and the

bottom layer made of particular type of ceramic material made to withstand high

temperature.

After 4 days the plantain stem shrinked and was taken out.

A wooden layer was provided on both sides in order to prevent the flames from coming

through the holes.

Fig 4.2: Wooden stove

4.3 PHASE3: INCORPORATION OF TEM ON WOODEN STOVE

4.3.1HEAT SINK

The heat applied to all the thermo electric modules must be equal and the modules must not be

directly subjected to the flames. In order to provide the equal heat to all the cells and to keep it

away from the flame heat sink was provided. Al was chose to build the heat sink since it

provides equal distribution of the heat on the surface.

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The construction and placement of the sink is as follows:

The 5 pieces of the Al with length 15cm and width 4cm were bend into L shape.

These 5 pieces were riveted onto a thick rod of Aluminium.

The arrangement of the heat sink is placed onto the wooden stove by inserting the 5 fins

to the five holes of the stove.

4.3.2COOLING SYSTEM

The cooling system is made in order to provide the cooling at the cold junction of the

thermocouples in the thermo electric module. The cold water was supplied through the cooling

system in order. The steps involved are:-

The hollow rod of aluminium was cut into the 2 pieces of the required length.

Four small hollow tubes were cut and they were placed at the ends of the hollow rod.

The two ends of the tubes were connected using a plastic pipe and among the remaining

tubes one was given to the water inlet and the other for the outlet of the water.

Fig 4.3: Heat sink and cooling system

4.4 PHASE4: ELECTRICAL CONNECTIONS

4.4.1 MODULE CONNECTION

The output voltage obtained from each cell was 2.25volt and to get an output voltage of about

20volt the cells were all connected in series so that the voltage developed by each cell gets added

up. The series connection was made by connecting the red and the blue wires of the consecutive

modules.

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The 20 volt was desired in order to withstand the voltage drop occurring while loading to the

battery. The connections are made as shown in the figure below.

Fig 4.4: Module connection

4.4.2 BATTERY

A battery is a device consisting of one or more electrochemical cells that convert stored

chemical energy into electrical energy. The secondary batteries (rechargeable batteries), which

are designed to be recharged and used multiple times are used. The 12v 7Ah battery is used in

order to store the electric charge generated from the stove.

Fig 4.5: Battery

4.5 PHASE5: INVERTER DESIGN

The inverter was made in order convert the DC energy stored in the battery to the AC to make it

suitable for the house hold appliances.

The inverter was made for the conversion from 12v DC to 230v AC with a frequency of 50Hz.

For the simplicity square wave inverter was made. The inverter circuit has mainly 3 parts :

The oscillatory part using IC3525

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The power part

The transformer and load part

Fig 4.6: Circuit diagram of inverter

4.5.1THE OSCILLATORY PART: IC 3525

The oscillatory part consists mainly of the IC3525. The oscillatory part is to provide the

necessary frequency of the output square wave.

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Fig4.7: Pin configuration of IC3525

The IC3525series of pulse width modulator integrated circuits are designed to offer improved

performance and lowered external parts count when used in designing all types of switching

power supplies. The input common-mode range of the error amplifier includes the reference

voltage eliminating external resistors. A sync input to the oscillator al lows multiple units to be

slaved or a single unit to be synchronized to an external system clock. A single resistor between

the CT and the discharge terminals provide a wide range of dead time adjustment. These devices

also feature built in soft-start circuitry with only an external timing capacitor required. A

shutdown terminal controls both the soft-start circuitry and the output stages, providing

Instantaneous turn off through the PWM latch with pulsed shut down, as well as soft-start recycle

with longer shut down commands. These functions are also control led by an under voltage

lockout which keeps the out puts off and the soft-start capacitor discharged for sub-normal input

voltages. This lockout circuitry includes approximately 500 mV of hysteresis for jitter free

operation. Another feature of these PWM circuits is a latch following the comparator. Once a

PWM pulses has been terminated for any reason the outputs will remain off for the duration of the

period. The latch is reset with each clock pulse. The output stages are totem-pole designs capable

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IC 3525

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SSET, KARUKUTTY DEPARTMENT of EEE

of sourcing or sinking in excess of 200 mA. The IC3525Aoutput stage features NOR logic, giving

a LOW output for an OFF state.

4.5.2 POWER PART: IRF44 (MOSFET)

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is

a transistor used for amplifying or switching electronic signals. Although the MOSFET is a four-

terminal device with source (S), gate (G), drain (D), and body (B) terminals, the body (or

substrate) of the MOSFET often is connected to the source terminal, making it a three-terminal

device like other field-effect transistors. Because these two terminals are normally connected to

each other (short-circuited) internally, only three terminals appear in electrical diagrams. The

MOSFET is by far the most common transistor in both digital and analog circuits, though

the bipolar junction transistor was at one time much more common.

In enhancement mode MOSFETs, a voltage drop across the oxide induces a conducting

channel between the source and drain contacts via the field effect. The term "enhancement

mode" refers to the increase of conductivity with increase in oxide field that adds carriers to the

channel, also referred to as the inversion layer. The channel can contain electrons (called an

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Table 4.4: Maximum ratings

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SSET, KARUKUTTY DEPARTMENT of EEE

nMOSFET or nMOS), or holes (called a pMOSFET or pMOS), opposite in type to the substrate,

so nMOS is made with a p-type substrate, and pMOS with an n-type substrate (see article

on semiconductor devices). In the less common depletion mode MOSFET, detailed later on, the

channel consists of carriers in a surface impurity layer of opposite type to the substrate, and

conductivity is decreased by application of a field that depletes carriers from this surface layer.

Fig 4.8: MOSFET

The traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of

silicon dioxide (Si O 2) on top of a silicon substrate and depositing a layer of metal or

polycrystalline silicon (the latter is commonly used). As the silicon dioxide is a dielectric

material, its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a

semiconductor. When a voltage is applied across a MOS structure, it modifies the distribution of

charges in the semiconductor. If we consider a p-type semiconductor a positive voltage,  ,

from gate to body creates a depletion layer by forcing the positively charged holes away from the

gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile,

negatively charged acceptor ions. If   is high enough, a high concentration of negative charge

carriers forms in an inversion layer located in a thin layer next to the interface between the

semiconductor and the insulator. Unlike the MOSFET, where the inversion layer electrons are

supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much

more slowly by thermal generation through carrier generation and recombination centers in the

depletion region. Conventionally, the gate voltage at which the volume density of electrons in the

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inversion layer is the same as the volume density of holes in the body is called the threshold

voltage. When the voltage between transistor gate and source (VGS) exceeds the threshold voltage

(Vth), it is known as overdrive voltage.

Fig 4.9: MOSFET working

4.5.3 TRANSFORMER AND THE LOAD

A transformer is a static electrical device that transfers energy by inductive coupling between its

winding circuits. A varying current in the primary winding creates a varying magnetic flux in the

transformer's core and thus a varying magnetic flux through the secondary winding. This varying

magnetic flux induces a varying electromotive force (EMF), or "voltage", in the secondary

winding.

The transformer used is a step up transformer. The transformer steps up the voltage to 230 volt,

which is the required voltage. The transformer is having number of turns in the secondary greater

than that at the primary.

Fig4.10: Transformer

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12v

12v

Primary 12v-0-12v centre tapped secondary230v

Project Report 2013

SSET, KARUKUTTY DEPARTMENT of EEE

4.5.4: INVERTER COMPONENTS DESIGNING

The designing of the oscillatory circuit is governed by the following equation:

Fosc = 1

C(0.7RT+3RD)

To get an ac signal of ‘f’ frequency the input to the oscillatory circuit of the IC3525 should be

‘2f’.therefore to get an 50HZ signal at the output the input frequency to the oscillator should me

100HZ.

Let C=100nf and RD=470ohm

100 = 1

100*10-9(0.7RT +3*470)

Hence RT = 140.857Kohm (150Kohm standard value)

The 104micro farad is connected between Vref and Inv.input in order to give a constant input to

the error amplifier.

The capacitor connected between the soft start and the ground serves the function of controlling

the starting current and the gradual increase of the duty cycle to the desired value.

The equation governing the soft start is:

Tsec = Css *Vout

I

I = 51 micro amperes

Vout = 2.5V

To get soft start time as 0.1sec the capacitor of 2.2micro farad is chosen.

A 2.2Kohm and a 104 micro farad is connected in parallel across the ground and the shut down

pin as a filter for the noises. It should be kept low in order to generate the gate pulses for the

mosfet to get triggered.

The 33ohm resistor is connected between the output pins and the mosfets to limit if any high

current flow through it.

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4.6 OBSERVATIONS

The firewood was placed in the stove and the ignition was given. The temperature began to rise

slowly. At the same time the cold water was passed through the cooling system which was

arranged at the cold side of the modules. After sometime the temperature difference at the

thermo electric module began to build up. The multimeter showed a deflection. The voltage

began to increase rapidly for about 5 minutes. Then the rising of the voltage become slow. After

reaching a particular temperature the voltage rise stopped and it began to decrease slowly.

In order to prevent the decrease of the voltage the heat sink fins are slowly released from the

stove. The electric energy from the modules were stored in a battery. The inverter was driven

from the battery. The output from the inverter is 230volt and the wattage was 150w.

CHAPTER5

ECONOMIC ANALYSISTABLE 5.1: Economic analysis

Sl no. Item Quantity Price per Total

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unit

1 TEC12704 12 550 6600

2 Inverter

2.1 Transformer 1 625 625

2.2 MOSFET(IRF44) 2 15 30

2.3 IC 3525 1 15 15

2.4 Other

components

80

3 Battery 1 700 700

4 Stove 1 1500 1500

5 Miscellaneous

expense

520

Total expense 10070

Comparison of the LPG stove and the thermoelectric wooden stove

A study was done with the ordinary LPG gas stove and the Thermo Electric wooden stove .The

comparison period was taken as one year. The cost for the operation of the two systems were

compared and tabulated.

Table 5.2: cost comparison with LPG

ITEM QUANTITY RATE AMOUNT

LPG STOVE 1 5500 5500

THERMOELECTRIC

WOODEN STOVE

1 8100 8100

LPG WITH SUBSIDY 9 460 4140

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LPG WITHOUT

SUBSIDY

3 960 2880

HARD WOOD 12*90 KG 4 4320

By the above comparison the total cost for the LPG stove for one year is obtained as 12520 Rs.

Where as in the case of the wooden stove it costs only 12420 Rs.

The electricity generated per day while burning for 3.5 hrs in an average =105watts

Electricity generated per month = 105*30 = 3150watts

Electricity generated per year = 3000*12 = 37800 watts

Thus the thermo electric wooden stove acts as the secondary source of energy for the households.

Table 5.3: Advantage and disadvantages

ITEM LPG THERMO ELECTRIC

STOVE

ADVANTAGES Ease of use

Quick heating

Clean burning

Renewable

Low cost

Readily available

DISADVANTAGES Non renewable

High cost

Not easily available

Pollution

Slow heating

CHAPTER6

CONCLUSION

From this project we came to the conclusion that thermoelectric wooden stove can be used as a

substituent for general source of electricity for households. The construction of the

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multifunctional wooden stove is very useful since its efficiency is increased by utilizing its waste

heat to generate electricity. The thermo electric wooden stove is very useful in the rural area

electrification where electricity has not reached yet. The components used in this project are very

less and hence can be built easily and fast which is an added advantage to the thermoelectric

wooden stove.

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