Design, Construct And Performance Evaluation Of A Thermometric Generator

ABSTRACT

Energy crisis and environment deterioration are two major problems for 21st century. Thermoelectric device is a promising solution for those two problems. Thermoelectric generator (TEG), is device that convert heat energy into electric energy. This generator accomplish this task by using arrays of specialised circuits known as thermoelectric modules, each of which consists of semiconductor materials — known as p-type and n-type — sandwiched between insulating ceramic substrates. This work deals on thermoelectric power electricity.

TABLE OF CONTENTS

 TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

GLOSSARY OF THERMOELECTRIC TERMS

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• AIM OF THE PROJECT
• OBJECTIVE OF THE PROJECT
• PURPOSE OF THE PROJECT
• SIGNIFICANCE OF THE PROJECT
• STATEMENT OF THE PROBLEMS
• APPLICATION OF THE PROJECT
• PROJECT ORGANIZATION

CHAPTER TWO

2.0      LITERATURE REVIEW
2.1      OVEVIEW OF THE STUDY
2.2     REVIEW OF MATERIALS FOR THERMOELECTRIC GENERATOR
2.3     EFFICIENCY OF THERMOELECTRIC GENERATORS
2.4    REVIEW OF GENERAL USES OF THERMOELECTRIC GENERATORS
CHAPTER THREE

3.0      CONSTRUCTION METHODOLOGY

3.1      COMPONENTS OF THE WORK

3.2      DESCRIPTION OF SYSTEM COMPONENTS

3.3      SYSTEM WORKING PRINCIPLE

3.5      COMPONENTS USED

3.6      WHY USE A DIODE FOR MAKING A TEG

CHAPTER FOUR

RESULT ANALYSIS

4.0      CONSTRUCTION PROCEDURE AND TESTING

4.1      CASING AND PACKAGING

4.2      ASSEMBLING OF SECTIONS

4.3      TESTING AND RESULT OF SYSTEM

4.4      DESIGN CALCULATION

4.5      PRACTICAL LIMITATIONS

CHAPTER FIVE

5.1      CONCLUSION

5.2      RECOMMENDATION

5.3      REFERENCES

 GLOSSARY OF THERMOELECTRIC AND THERMAL TERMS

AMBIENT TEMPERATURE: Temperature of the air or environment surrounding a thermoelectric cooling system; sometimes called room temperature.

BISMUTH-ANTIMONY: A thermometric semiconductor material that exhibits optimum performance characteristics at relatively low temperatures.

BISMUTH TELLURIDE: A thermoelectric semiconductor material that exhibits optimum performance in a “room temperature” range. An alloy of bismuth telluride most often is used for thermoelectric cooling applications.

BTU: British Thermal Unit: The amount of thermal energy required to raise one pound of water by one degree Celsius at a standard temperature of 15°C.

CALORIMETER: A scientific apparatus used to measure the evolution or absorption of heat. Thermoelectric modules, when used in a calorimeter, may exhibit much higher sensitivity than conventional thermopiles.

CASCADE MODULE (MULTI-STAGE MODULE): A thermoelectric module configuration whereby one module is stacked on top of another so as to be thermally in series. This arrangement makes it possible to reach lower temperatures than can be achieved with a single-stage module.

CFM: (Cubic Feet per Minute): The volgenerallyumetric flow rate of a gas, typically air, expressed in the English system of units. For thermoelectric applications, this refers to the amount of air passing through the fins of a forced convection heat sink.

COEFFICIENT OF PERFORMANCE (COP): A measure of the efficiency of a thermoelectric module, device or system. Mathematically, COP is the total heat transferred through the thermoelectric device divided by the electric input power. COP sometimes is stated as COPR (Coefficient of Performance as a Refrigerator) or as COPH (Coefficient of Performance as a Heater).

COLD SIDE OF A THERMOELECTRIC MODULE: The side of a module that normally is placed in contact with the object being cooled. When the positive and negative module leads are connected to the respective positive and negative terminals of a DC power source, heat will be absorbed by the module’s cold side. Typically, the leads of a TE module are attached to the hot side.

CONDUCTION (THERMAL): The transfer of heat within a material caused by a temperature difference through the material. The actual material may be either a solid, liquid or gas (or a combination) where heat will flow by means of direct contact from a high temperature region to a lower temperature region.

CONVECTION (THERMAL): The transfer of heat by means of air (gas) movement over a surface. Convection actually is a combined heat transfer process that involves elements of conduction, mixing action, and energy storage.

COUPLE: A pair of thermoelectric elements consisting of one N-type and one P-type connected electrically in series and thermally in parallel. Because the input voltage to a single couple is quite low, a number of couples normally are joined together to form a “module.”

DEGREES KELVIN: Absolute temperature scale where absolute zero (0K) represents the point where all molecular kinetic energy of a mass is zero. When calculating the temperature dependent properties of semiconductor materials, temperature values must be expressed in degrees Kelvin. On the Celsius scale, 0°C equals 273.15°K; in respect to quantity, one Kelvin degree equals one Celsius degree. Note that the ( ) symbol normally is not used when denoting degrees Kelvin.

EFFICIENCY: For thermoelectric coolers, mathematical efficiency is the heat pumped by a module divided by the electrical input power; for thermoelectric generators, efficiency is the electrical output power from the module divided by the heat input. To convert to percent, multiply by 100. See definition of Coefficient of Performance.

ELEMENT: An individual block of thermoelectric semiconductor material. See definition of DIE.

ENERGY: Energy is the physical quantity which, in the context of thermoelectrics, generally is used to express a unit of heat or electricity. Energy may be stated in British Thermal Units (BTU) or watt-hours. It is important to note the difference between energy and power. Power is the rate at which energy is being used, and power may be stated in BTU/hour or watts. The relationship between power and energy is Power = Energy / Time.

HEAT LEAK: The amount of energy gained or lost by an object being thermoelectrically controlled due to heat transfer to or from external media. Heat transfer may occur due to conduction, convection, and/or radiation.

HEAT LOAD: The quantity of heat presented to a thermoelectric device that must be absorbed by the device’s cold side. The term heat load, when used by itself, tends to be somewhat ambiguous and it is preferable to be more specific. Terms such as active heat load, passive heat load or total heat load are more descriptive and less uncertain as to meaning.

HEAT SINK: A body that is in contact with a hotter object and that expedites the removal of heat from the object. Heat sinks typically are intermediate stages in the heat removal process whereby heat flows into a heat sink and then is transferred to an external medium. Common heat sinks include natural (free) convection, forced convection and fluid cooled.

HOT SIDE OF A THERMOELECTRIC MODULE: The face of a thermoelectric module that usually is placed in contact with the heat sink. When the positive and negative module leads are connected to the respective positive and negative terminals of a DC power source, heat will be rejected by the module’s hot side. Normally, the wire leads are attached to the hot side ceramic substrate.

LATENT HEAT: Thermal energy required to cause a change of state of a substance such as changing water into ice or water into steam.

LEAD TELLURIDE: A thermoelectric semiconductor that exhibits its optimum performance within a temperature range of 250-450°C. Lead telluride is used most often for thermoelectric power generation applications.

LIQUID COOLING: A heat sink method involving the use of water or other fluids to carry away unwanted heat. When comparing alternative heat-sinking methods, liquid cooled heat sinks normally provide the highest thermal performance per unit volume.

MASS FLOW RATE: The weight of a fluid flowing per unit of time past a given cross-sectional area. Typical units include pounds per hour-square foot and grams per second-square centimeter.

MAXIMUM TEMPERATURE DIFFERENTIAL (MAXIMUM DT): The largest difference that can be obtained between the hot and cold faces of a thermoelectric module when heat applied to the cold face is effectively zero. DTmax or Dmax is one of the significant thermoelectric module/device specifications.

MODULE: A thermoelectric cooling component or device fabricated with multiple thermoelectric couples that are connected thermally in parallel and electrically in series.

MULTI-STAGE MODULE (CASCADE MODULE): A thermoelectric module configuration whereby one module is mechanically stacked on top of another so as to be thermally in series. This arrangement makes it possible to reach lower temperatures than can be achieved with a single-stage module.

NATURAL CONVECTION HEAT SINK: A heat sink from which heat is transferred to the surrounding air by means of natural air currents within the environment. No external fan, blower or other appliance is used to facilitate air movement around the heat sink.

N-TYPE MATERIAL: Semiconductor material that is doped so as to have an excess of electrons.

OPTIMUM CURRENT: The specific level of electrical current that will produce the greatest heat absorption by the cold side of a thermoelectric module. At the optimum current, a thermoelectric module will be capable of pumping the maximum quantity of heat; maximum temperature differential (DTmax) typically occurs at a somewhat lower current level.

PELTIER EFFECT: The phenomenon whereby the passage of an electrical current through a junction consisting of two dissimilar metals results in a cooling effect; when the direction of current flow is reversed heating will occur.

POWER SUPPLY: Any source of DC electrical power that may be used to operate a thermoelectric device.

P-TYPE MATERIAL: Semiconductor material that is doped so as to have a deficiency of electrons.

RADIATION (THERMAL): The transfer of heat energy by electromagnetic waves as a result of a temperature difference between two bodies. In thermoelectric cooling applications, radiation losses are quite small and usually have to be considered only for multi-stage coolers operating near a DTmax condition.

RESISTIVITY (ELECTRICAL): Resistivity is a bulk or inherent property of a material that is unrelated to the physical dimensions of the material. Electrical resistance, on the other hand, is an absolute value dependent upon the cross-sectional area (A) and Length (L) of the material. The relationship between Resistivity (r) and Resistance (R) is: r = (A/L) (R)

SEEBECK EFFECT: The phenomenon whereby an electrical current will flow in a closed circuit made up of two dissimilar metals when the junctions of the metals are maintained at two different temperatures. A common thermocouple used for temperature measurement utilizes this principle.

SI: An abbreviation for System International, the international standard metric system of units.

SILICON-GERMANIUM: A high temperature thermoelectric semiconductor material that exhibits its optimum performance within a temperature range of 500-1000 C. Silicon-Germanium material most often is used for special thermoelectric power generation applications that utilize a radioisotope/nuclear heat source

SINGLE-STAGE MODULE: The most common type of thermoelectric cooling module using a single layer of thermoelectric couples connected electrically in series and thermally in parallel. Single-stage modules will produce a maximum temperature differential of approximately 70°C under a no-load condition.

THERMAL CONDUCTIVITY: The amount of heat a material will transmit per unit of temperature based on the material’s cross-sectional area and thickness.

THERMAL RESISTANCE (HEAT SINK): A measure of a heat sink’s performance based on the temperature rise per unit of applied heat. The best heat sinks have the lowest thermal resistance.

THERMOELECTRIC DEVICE: A general and broad name for any thermoelectric apparatus. The term Thermoelectric Device has recently been modified to exclude thermoelectric modules in favor of thermoelectric assemblies.

THERMOELECTRIC GENERATOR: A device that directly converts energy into electrical energy based on the Seebeck Effect. Bismuth telluride-based thermoelectric generators have very low efficiencies (generally not exceeding two or three percent) but may provide useful electrical power in certain applications.

THERMOELEMENT: Another name for a thermoelectric element or die.

THERMOPILE: When a thermoelectric module is used in a calorimeter application it is frequently called a thermopile. Some have used the word thermopile as a synonym for thermoelectric module regardless of application, but such use is unusual.

THOMSON EFFECT: The phenomena whereby a reversible evolution or absorption of heat occurs at opposite ends of a conductor having a thermal gradient when an electrical current passes through the conductor.

 CHAPTER ONE

1.0                                                        INTRODUCTION

A thermoelectric generator is a solid-state device that works similar to solar panels but converts heat, rather than sunlight, directly into electricity. A thermoelectric generator is usually made of bismuth telluride semiconductor junctions that are only several millimeters thick. This differs drastically from the bimetallic junctions that were previously used, which were much thicker. Thermoelectric generators generally do not have any moving parts, except for a fan, and can be used in a wide variety of applications to generate electricity.

In a thermoelectric generator, heat is transferred through a piece of metal such as bismuth telluride, that has a high resistance to heat and low thermal conductivity. As the heat travels through the metal, it is converted into electricity and can then be transferred to a conductor or directly to an electronic device. Generally, many thermoelectric generators are connected to each other in a series in order to generate as much electricity as possible.

Thermoelectric generators are advantageous because they allow systems to retrieve heat that is otherwise wasted in the form of exhaust or mechanical waste. Thermoelectric generators also recover heat that occurs naturally, such as the heat that geothermal vents, volcanoes, hot springs, or high-atmosphere applications produce. Generally, heat is not intentionally produced for a thermoelectric generator, as this would lead to an overall loss of electricity or other resources. Thermoelectric generators are most efficient when retrieving heat over 250 degrees Celsius.

Thermoelectric power generators consist of three major components: thermoelectric materials, thermoelectric modules and thermoelectric systems that interface with the heat source. However, this work discuses the design, construction and performance evaluation of the thermoelectric power generator.

1.1                                         BACKGROUND OF THE PROJECT

In the last decade, problems related to energy factors (oil crisis), ecological aspects (climatic change), electric demand (significant growth) and financial/regulatory restrictions of wholesale markets have arisen worldwide. These difficulties, far from finding effective solutions, are continuously increasing, which suggests the need of technological alternatives to ensure their solution. One of these technological alternatives is known as distributed generation (DG), and consists of generating electricity as near as possible of the consumption site, in fact like it was made in the beginnings of the electric industry, but now incorporating the advantages of the modern technology [1]. Here it is consolidated the idea of using clean nonconventional technologies of generation that use renewable energy sources (RESs) that do not cause environmental pollution, such as wind, solar (photovoltaic and thermal), hydraulic, among others.

Recently, a rising interest on thermal generation based on solid-state devices such as thermoelectric generators (TEG) has emerged as a feasible option of generation of clean energy, mainly because of the development of new semiconductor materials and of their commercial availability in the existing open markets [3-4]. TEGs allow generating electricity directly and with no moving parts from a temperature difference held across the junction of two dissimilar semiconductor materials. These devices share the major characteristics of photovoltaic (PV) systems, being their advantages the possibility of generating electricity continually while they are provided of heat and the significant reduction of costs, reaching today the sixth part of a PV system. Consequently, TEGs are presently arising as a new option inside the portfolio of renewable energy sources and are becoming serious candidates for applications in DG.

Based on the stated above, the present work proposes the application of this novel technological alternative in distributed generation systems. The development of the TEGs integrated into the distribution power grid is presented and the analysis of the dynamic performance of the device and the impact of its use in electric system are included. This work comprises the detailed modeling of TEGs and the power electronic interface with the electric system, as well as the design of the control scheme of the global system.

1.2                                             OBJECTIVE OF THE PROJECT

The main objective of this work is to design an electrical device could be used in power plants in order to convert waste heat into electrical power and in automobiles as automotive.

1.3                                              PURPOSE OF THE PROJECT

The purpose of this work is to have a type of generator that function similar to heat engines and are less bulky and also are less efficient than heat engines. This device converts heat energy produced from a heat source directly into electrical energy.

1.4                                                 SCOPE OF THE PROJECT

Thermoelectric generators, also known as TEGs, are devices that convert heat energy into electric energy. These generators accomplish this task by using arrays of specialized circuits known as thermoelectric modules, each of which consists of semiconductor materials — known as p-type and n-type — sandwiched between insulating ceramic substrates. While thermoelectric generators have several benefits, they also have their downsides.

1.5                                         SIGNIFICANCE OF THE PROJECT

One of the greatest importances of thermoelectric generators lies in the fact that they can derive their power from heat that would otherwise just dissipate into its surroundings. Unlike the case with a standard gasoline or diesel generator, purchasing fuel for a thermoelectric generator is unnecessary, as the generator can “steal” its fuel from any device or machine that creates and releases substantial amounts of heat. These devices can include ovens, burners and furnaces, as well as machines — such as automobiles — that produce heat as a by-product of creating power for other functions, such as propulsion.

The thermoelectric modules that make up thermoelectric generators have solid-state constructions, which make the generators highly durable. “Solid-state” refers to the fact that the modules consist entirely of solid, fixed materials and do not rely on gases or vacuums. In contrast, other modules use tube construction, wherein they pass electrical currents through glass tubes filled with gasses or containing vacuums. Unlike tube modules, solid-state thermoelectric modules are robust and are not prone to cracking or shattering — even when faced with turbulent conditions.

1.6                                              PROBLEM OF THE PROJECT

Cost: One of the main problems of thermoelectric generators, which as of 2011 has prevented their adoption on a wider scale, lies in their cost. Single thermoelectric module capable of producing 1 watts of electrical power cost more than what it takes to produce 1W of electricity from other means.

Efficiency:  most thermoelectric generators have an average efficiency of 4 per cent, which means the generators cannot pass on 96 per cent of the energy they obtain from heat sources. Thermoelectric generator will only operate efficiently when supplying electrical current to a device that has a similar electrical resistance. For example, a 100-watt thermoelectric generator could theoretically power a 100-watt light bulb efficiently but would ultimately waste energy if attempting to power a 30-watt bulb.

1.7                                         APPLICATIONS OF THE PROJECT

Thermoelectric generators are used in a wide variety of applications, but generally in situations in which there are few energy sources or a great deal of heat is otherwise wasted. Thermoelectric generators are most notably used in the aerospace industry to power space shuttles, space probes, and satellites with the heat generated from the radioactive decay that radioactive substances produce. This type of thermoelectric generator is known as a Radioisotope Thermal Generator (RTG). Experiments are currently being undertaken to also use thermoelectric generators in automobiles in order to utilize the wasted heat that exhaust fumes and cooling agents produce.

1.8                                        PROJECT WORK ORGANIZATION

The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows:

Chapter one of this work is on the introduction to the study. In this chapter, the background, significance, objective, purpose, and problem of the study were discussed.

Chapter two is on literature review of this study. In this chapter, all the literature pertaining to this work was reviewed.

Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed.

Chapter four is on testing analysis. All testing that result accurate functionality was analyzed.

Chapter five is on conclusion, recommendation and references.