DEPARTMENT OF MECHANICAL SANDWICH

DEPARTMENT OF MECHANICAL SANDWICH

1. INTRODUCTION

1.1 Introduction to TEG:- In recent years the scientific and public awareness on environmental and energy issues has brought in major interests to the research of advanced technologies particularly in highly efficient internal combustion engines .The number of vehicles (passenger and commercial vehicles) produced from 2005 to 2012 shows an overall increasing trend from year to year despite major global economic down turn in the 20082010 periods. Chinas energy consumption in transportation sector is the lowest (13.5%) although the country produced the highest number of vehicles in 2009 to 2010 as compared to the other countries. Viewing from the socioeconomic perspective, as the level of energy consumption is directly proportional to the economic development and total number of population in a country, the growing rate of population in the world today indicates that the energy demand is likely to increase. It is also expected that the average increase in population growth between 2010 and 2020 is projected to be 10.74%. Energy use in India is given by the chart below. Fig 1.0 Final Energy Usage by the Main Sector Of India In 2005 India is thus presented in Fig.1.1 showing a stable percentage values for transportation sector in 2005 period. As discussed before, the growing number of population puts transportation sector in a very crucial role due to its dependability towards the continuous and rapid development of a nation urban areas and the standard of living for its people.In order to meet the increasing electrical demands of modern automobiles, bigger and bulkier alternators are connected to engines. Bigger and bulkier alternators which operate at an efficiency of 50 to 62% consume around 1 to 5% of the rated engine output. However, due to the expansion of the passenger room and to improve the vehicle aerodynamics, the space for the alternator in engine room is becoming smaller. About 30% of the energy supplied to an IC engine is rejected in the exhaust as waste heat. If approximately 6% heat can be recovered from the engine exhaust, it can meet the electrical requirements of an automobile and it would be possible to reduce the fuel consumption around 10%. Heat is rejected through exhaust gases at high temperature when compared to heat rejected through coolant and lubricating oil. This shows the possibility of energy conversion using a thermoelectric generator (TEG) to tap the exhaust heat energy. TEG is like a heat engine which converts the heat energy into electric energy and it works on the principle of Seebeck effect. Moreover TEG are highly reliable, operate quietly and are usually environmentally friendly. Semi-conducting materials (in conjunction with copper inter-connecting pads), were found to offer the best combination of Seebeck coefficient, electrical resistivity, and thermal conductivity. Semi-conducting materials provide another benefit, the ability` to use electrons or holes (the absence of an electron in a crystal matrix) to conduct current. Thermoelectric module (TEM) has a cold side and a hot side. At the hot side, heat is absorbed by electrons as they pass from a high energy level in the n-type semiconductor element, to a lower energy level in the p-type semiconductor element. The power supply provides the energy to move the electrons through the system. At the cold side, energy is expelled to a heat sink as electrons move from a high energy level element (n-type) to a lower energy level element (p-type). Bismuth telluride-based TEM are designed primarily for cooling or combined cooling and heating applications where electrical power creates a temperature difference across the TEM. By using the modules in reverse, where a temperature differential is applied across the faces of the module, it is possible to generate electrical power. Although power output and generation efficiency are presently low, useful power often may be obtained where a source of heat is available.

Fig 1.1 Vehicle Heat Release Distribution.The Thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. The thermoelectric effect refers to phenomena in which a temperature difference creates an electric potential or electric potential creates a temperature difference. Automotive Thermoelectric Generators (ATEG) recovers heat that escapes from a vehicle powered by an internal combustion engine, and generate electricity with the heat. This leads to the significant increase of demand for electric power in the vehicle which has to be generated by the alternator. It is predicted that if only 6% of the heat contained in the exhaust gases was changed into electric power, it would allow to lower fuel consumption by 10% due to the decreased waste resulting from the resistance of the alternator drive. Power generation system using the thermoelectric generator should generally consist of the following components: heat exchanger, thermoelectric module, cooling system and DC/DC voltage converter. One of the most important design issues related to the construction of the thermoelectric generator TEG is to develop an efficient heat exchanger, which should provide optimal recovery of heat from exhaust gases. Through the application of thermoelectric generators (TEG), in the future a proportion of the energy carried by the exhaust gases that would previously have been lost will be recovered as electrical energy. The German Aerospace Center (DLR) has developed TEG for use in vehicles for doing this and resulting from collaboration with the BMW Group, 200 W of electrical power in a test vehicle was achieved in a demonstration. Given the use of future materials, performances of up to 600 W can be expected, yielding a usage potential of 5%.

2. Thermoelectric Generator2.1 Principle of TEG:-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. This effect is called as 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

Fig 2.1 Principle of TEG

2.2 Thermoelectric Materials:-A commonly used thermoelectric material in such applications is Bismuth Telluride (Bi2Te3). Thermoelectric materials have a nonzero thermoelectric effect; in most materials it is too small to be useful. However, low cost materials that have a sufficiently strong thermoelectric effect could be used for applications. Thermoelectric materialsshow thethermoelectric effectin a strong and/or convenient form. Mostly all materials have a nonzero thermoelectric effect in most materials it is too small to be useful. However, low cost materials that have a sufficiently strong thermoelectric effect (and other required properties) could be used for applications includingpower generation,refrigerationand a variety of other applications. The primary criterion for thermoelectric device viability is thefigure of meritgiven by: Which depends on theSee beck coefficient,S,thermal conductivity,, andelectrical conductivity,. The product (ZT) of Z and the use temperature, T, serves as a dimensionless parameter to evaluate the performance of a thermoelectric material. Device efficiency is proportional to ZT, so ideal materials have a large Z value at high temperatures.Materials such asBi2Te3andBi2Se3comprise some of the best performing room temperature thermoelectric with a temperature-independent thermoelectric effect, ZT, between 0.8 and 1.0. Nano structuring these materials to produce a layered super lattice structure of alternatingBi2Te3andBi2Se3layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type).Note that this high value has not entirely been independently confirmed.1) Since the 1950s, the main approach to developing advanced thermoelectric materials was focused on identifying, characterizing, and optimizing bulk degenerate semiconductors2) P-type TAGS materials (GeTe-AgSbTe2 compositions) with peak ZT values around 1.2.3) Solid solutions, such as Bi2xSbxTe3ySey and Si1xGex, have been used to tune the band gap for maximizing ZT values4) A more recent strategy consists of replicating the nanoscale features responsible for enhanced ZT values in bulk materials using advanced synthesis techniques. 5) Significant increase in ZT in some bulk materials, such as silver antimony lead telluride and its alloys, attributed mostly to the formation of self-assembling nano clusters inside a host matrix.2.3 Background of Thermoelectric Generator:-Being one of the promising new devices for an automotive waste heat recovery, thermoelectric generators (TEG) will become one of the most important and outstanding devices in the future one of the most important and outstanding devices in the future production has brought TEG technology into the attention of scientists and engineers.Mori et al studied the potentials of thermoelectric technology in regards to fuel economy of vehicles by implementing thermoelectric (TE) materials available in the market and certain industrial techniques on a 2.0 l gasoline powered vehicle. Hussainetal studied the effects of thermoelectric waste heat recovery for hybrid vehicles. Stobart and Milner explored the possibility of thermoelectric regeneration in vehicles in which they found out that the 1.3kW output of the TE device could potentially replace the alternator of a small passenger vehicle. Stobart etal reviewed the potentials in fuel saving of thermoelectric devices for vehicles. They concluded that upto 4.7% of fuel economy efficiency could be achieved. From these articles, the understanding of TEG technology has been comprehensively discussed as a promising new technology to recover waste heat from internal combustion engines. Studieson thermoelectric devices are still an ongoing matter. TE devices may potentially produce twice the efficiency as compared to other technologies in the current market. TEG is used to convert thermal energy from different temperature gradients existing between hot and cold ends of a semiconductor into electric energy as shown in Fig. 4. This phenomenon was discovered by Thomas Johann Seebeck in 1821 and called the Seebeck effect. The device offers the conversion of thermal energy into electric current in a simple and reliable way. Advantages of TEG include free maintenance, silent operation, high reliability and involving no moving and complex mechanical parts as compared to Rankine cycle system .In regards with the applicability of TEG in modern engines, the ability of ICEs to convert fuel into useful power can be increased through the utilization of the mentioned device. By converting the waste heat into electricity, engine performance, efficiency, reliability, and design flexibility could be improved significantly. The fuel efficiency of gasoline powered, diesel and hybrid electric vehicles (HEVs) that utilize the power generation of IC engine is also was 25% and conversely as much as 40% of fuel energy can be lost in the form of waste heat through an exhaust pipe. An increase of 20% of fuel efficiency can be easily achieved by converting about 10% of the waste heat into electricity. Furthermore, secondary loads from the engine drive trains can be eliminated with the help of TEG system, and as a result torque and horsepower losses from the engine can be reduced. This would help to reduce engine weight and direct the most of the increased power to the drive shaft, which would in turn help to improve the performance and fuel economy. Additionally, the possibility of minimizing the battery needs and exhaustion of vehicle battery life while permitting operation of specific accessories during engine off can be achieved by utilizing TEG.2.4. TEG in the Automotive Industry:-For an automobile engine, there are two main exhaust heat gas sources which are readily available. The radiator and exhaust gas systems are the main heat output of an IC engine. The radiator system is used to pump the coolant through the chambers in the heat engine block to avoid overheating and seizure. Conversely, the exhaust gas system of an IC engine is used to discharge the expanded exhaust gas through the exhaust manifold. Zhang and Chau reported that presently TEG is mostly installed in the exhaust gas system (exhaust manifold) due to its simplicity and low influence on the operation of the engine. Furthermore, TEG system including the heat exchanger is commonly installed in the exhaust manifold suitable for its high temperature region. Basically, a practical automotive waste heat energy recovery system consist of an exhaust gas system, a heat exchanger, a TEG system, a power conditioning system, and a battery pack as shown in Fig. 5; with the operation of the TEG waste heat recovery system described as follows: 1) During the normal operation of an internal combustion engine, the produced waste heat released through the exhaust manifold is captured by the heat exchanger mounted on the catalytic converter of the exhaust gas system.2) Electricity is then generated from the thermal energy captured by the heat exchanger after it is transferred to the TEG system.3) Power conditioning is performed by the power converter to achieve maximum power transfer.

Fig 2.2 A Typical Waste Heat Energy Recovery System

3. Design of the Thermoelectric GeneratorThe TEG consists of an exhaust gas heat exchanger, counter flow coolant cooling chamber and 18 TEM connected in series. The amount of heat transferred from the exhaust gas to the TEM depends on the design of the TEG and the critical parameter is the heat flux which crosses the TEM. In order to achieve this, the TEM should work close to its best conditions of power, and it is also necessary to reduce the thermal resistance which includes thermal resistance from the exhaust gases to the inner wall of heat exchanger, and the thermal resistance from the inner wall of the heat exchanger to the hot surface of the module. The heat transmission from the inner wall to the hot surface of the TEM is basically a heat conduction problem. Thermal resistance exists between the surfaces of the hot plate and TEM because of the surface roughness. Hence, care was taken to ensure high degree of smoothness between the surfaces by polishing it. The most common materials used in the construction of the support structure of the TEG are steel, stainless steel, and in one case haste alloy, and aluminum. Table 3.1 shows the thermal properties of these alloys. According to these properties, the best material to be used for the contact of the TEG with the exhaust pipe is copper because it is 6 times and 25 times better as a thermal conductor than carbon steel and stainless steel, respectively, while the maximum gain in weight compared to the lightest steel is only 14.3%.3.1 Properties of different materials:-Another possibility, could be an aluminum alloy, that is 3.5 times lighter than copper. In this case it is necessary to have a very good control of the temperatures because the melting point of duralumin is only approximately 500 C, and in some cases is surpassed by the exhaust gas temperatures. As the TEG has to be fabricated for a small car, aluminum was the only choice for the use of hot plate due to its high conductivity and light weight. Cast iron of 5 mm thickness was used to make the frame of the heat exchanger. Figure 6 shows the cast iron frame.

PropertiesHastelloySteelStainless SteelCopperDuralium

Type-AISI 1010AISI 30299.9Cu+ Ag-

Melt Point (k)1533167016701293923

Density (kgm^-3 )83007830805589502770

K(Wm-1 K-1)T=294K9.1----

T=300K-6415386174

T=473K14.1----

T=500K-5419-188

Kp(Jkg-1K-1)T=294K486----

T=300K-434480385875

Table 1.1 Properties of Different MaterialsTwo plates were used for the TEM: hot plate and cold plate. Modules were placed on the top side of the hot plate and the exhaust gas (flowing through the frame) was in direct contact with the bottom side of the plate. The cold plate was assembled with the cooling chamber and was placed over the modules with spacer block in between them. The spacer block was used to increase the distance between the hot and cold plate, so as to maintain the temperature difference. Silicon foam was used around the modules to provide necessary insulation and also to eliminate the problem of water condensation. Aluminum plates of 5 mm thickness were used as hot plate and copper plates of 3 mm thickness were used as cold plates. Thermal grease was used to increase the thermal conductivity between the hot plate and the modules. Figure 3.2 shows the aluminum plate with TEM attached to it. Asbestos gasket was placed in between the hot plate and the frame to arrest the exhaust gas leakage at the junction. The total weight of the TEG was 14.6 kg which is 0.5 kg higher than that of the predicted. The excess weight may be due to welded joints and additional bolts and nuts used during fabrication.

Fig 3.1 Cast Iron Frame

Fig 3.2Aluminum Plate With ModulesFigure 3.4 shows the complete assembly of TEG. The coolant used in engine cooling system is circulated into the TEG by using a small split joint. Figure 3.3 shows the schematic of the TEM sand witched between hot and cold plate and the flow direction of the hot and cold fluids. This avoids the requirement of a separate cooling system for modules. The power required to pump the coolant to the TEG is one of the losses as compared to the power produced by the TEG. As the coolant chamber has to be designed to accommodate maximum flow of coolant at higher speeds and loads, the maximum flow of coolant was estimated by taking exhaust gas properties at higher speeds and loads. The specific heat capacity of the exhaust gas was calculated using an exhaust gas calorimeter.

Fig 3.3 Schematic Diagram of the Tem Sandwitched between The Hot Plate And Cold Plate

Fig 3.4 TEG with the Stand

3.2 Experimental Set-Up:-The test was carried out on an engine dynamometer, details of which are given in tab.1 2. No modifications were made on the engine. The exhaust pipe was insulated on the upstream side of the exhaust chamber up to the catalytic converter to minimize heat loss. 0-1200 C K-type thermocouples with digital measuring unit were used to measure exhaust gas temperature, hot and cold side temperatures of TEM. Backpressure of the exhaust gas was measured using a U- -tube mercury manometer. DC voltmeter and ammeter were used to measure the voltage and current produced by the TEG. A Rota meter was used to find the flow rate of the engine coolant flowing through the cooling chamber. An additional coolant circuit (by-pass) was provided for TEG using solenoid valve in order to overcome the burn out of the TEM during the engine warm up period.

MakeMaruti 800

TypeThree Cylinder $ Stroke SI Engine Water Cooled SOHC

Displacement796cm3

Maximum Power37 Bhp at 5000 rpm

Maximum Torque59Nm at 2500 rpm

Dynamometer MakeDynaspede

Maximum Torque80 Nm at 3000 rpm

Maximum Power25K

ControllerPC Manual Based

Table 1.2 Specification of the Test Setup

Fig4.1 Assembly of Thermoelectric GeneratorOnce the engine thermostat valve opens, the by-pass valve closes and coolant circuit resumes with engine coolant circuit. Three bulbs of 25 W each and one bulb of 15 W were used as load for the TEM. Dynamometer torque, engine speed, TEG output, TEG coolant inlet and exit and surface temperatures were measured. The engine was operated at various loads using eddy current dynamometer. Engine load was varied by changing the engine speed keeping the torque constant. Horiba exhaust gas analyzer was used to measure the exhaust gas toxicity. Table 3 shows the range, accurary, and uncertainties of the instruments used in this study. To achieve higher exhaust gas temperature the thermoelectric generator was located just down- stream of the exhaust headers next to catalytic converter. The only concern is that the genera-tor, if located upstream of the catalytic converter, would de-crease the efficiency and in-crease the warming time of the catalytic converters. Schematic diagram of the experimental setup is shown in fig.4.2

Fig 4.2 Schematic Diagram of Experimental Set-Up3.3 Result and Discussion:-In order to observe the differential change in exhaust back pressure and exhaust emissions due to the addition of TEG to the exhaust system, experiments were conducted on the test engine with and without TEG and results were compared. Figures 1.1 to 1.4 shows the variation of exhaust back pressure, engine coolant flow, HC, and CO emission with respect to engine brake power (BP).

Fig 1.1 Variation of Back Pressure with Brake Power

Fig 1.2.Variation of Engine Coolant Flow Rate With Brake Power

Fig 1.3 Variation of Hydrocarbon Emission With Brake Power

Fig 1.4 Variation of Carbon Monoxide Emission with Brake PowerIt was very clear that the addition of TEG to the engine exhaust has very little effect on exhaust back pressure and exhaust emissions. Moreover at maximum load the amount of coolant circulated in TEG is 0.45 kg/min. which is around 2.1% of the coolant circulated in the engine cooling system at the same load. Figure 1.5 shows the power generated by TEG during the tests. The power output of TEG increased with increase in the temperature difference (T), this is due to increase in the exhaust gas temperature with increase in engine power output. The increase in exhaust mass flow rate also plays an important role in the power produced by TEG. A thermoelectric generators power increases with the square of the temperature difference applied across it. Figure 1.6 shows the variation of exhaust gas temperature (EGT) with the power produced by the engine.

Fig 1.5 Variation Of Power With Temperature Difference

Figure 1.6 Variation Of EGT With Brake PowerTo extract more power, the length of TEG can be increased and more modules can be added. Due to constraints like space availability and weight it becomes cumbersome in this study. The specific power of the present TEG was 0.0048 kW/kg (power out of the TEG/total weight of the TEG). The temperature drop across the exhaust in TEG is mainly due to convective heat transfer to the walls. At higher engine power the effectiveness of the heat exchanger decreases due to higher exhaust gas velocity and this result in the offset of T vs. power curve from linear trend. Figure 1.7 shows the variation of temperature difference between the hot plate and the cold plate with respect to power developed by the engine. If internal fins were used in the TEG, the heat transfer would have been augmented. Since the maximum hot side temperature of the module limits to 220 C and the possible increase in exhaust back pressure it is not possible to implement fins in our study. Small change in exhaust back pressure was regulated by means of a by-pass line fitted with the gate valve. This same arrangement will not be effective for larger back pressure variations. Since, opening of the gate valve will affect the effectiveness of the heat exchanger. The range of the efficiency is between 1.5 at 10 kW to 3% at 25 kW. The conversion efficiency of the TEG can improved if the operating region of the same is situated in the parts of power and efficiency curve with negative slope as given in 1.7 Variations in both the bulk and surface temperature on the coolant side were much smaller than in the exhaust side and thus decreasing the coolant side temperatures would be beneficial.

Figure1.7 Variation of Temperature Difference With Brake Power

4. Analysis of Thermoelectric Generator4.1 Benefits of TEG:-1) The primary goal of ATEGs is to reduce fuel consumption. Forty percent of an IC engines energy is lost through exhaust gas heat. By converting the lost heat into electricity, ATEGs decrease fuel consumption by reducing the electric generator load on the engine. ATEGs allow the automobile to generate electricity from the engine's thermal energy rather than using mechanical energy to power an electric generator. Since the electricity is generated from waste heat that would otherwise be released into the environment, the engine burns less fuel to power the vehicle's electrical components, such as the headlights. Therefore, the automobile releases fewer emissions.2) Decreased fuel consumption also results in increased fuel economy. Replacing the conventional electric generator with ATEGs could ultimately increase the fuel economy by up to 4%.3) The ATEGs ability to generate electricity without moving parts is an advantage overmechanical electric generatorsalternative.4.2 Problems of TEG:- The use of an ATEG presents new problems to consider:1) Since the exhaust has to flow through the ATEGs heat exchanger, kinetic energy from the gas is lost, causing increased pumping losses. This is referred to asback pressure, which reduces the engines performance.2) To make the ATEGs efficiency more consistent, coolant is usually used on the cold-side heat exchanger rather than ambient air so that the temperature difference will be the same on both hot and cold days. This increases the radiators size since piping must be extended to the exhaust manifold. It also adds to the radiators load because there is more heat being transferred to the coolant.3) ATEGs are made primarily of metal and, therefore, contribute a significant weight to the vehicle. An ATEG designed for small cars and trucks weighs about 125lb (57kg), while for large trucks and SUVs, it can contribute up to 250lb (110kg) to the vehicle. The added weight causes the engine to work harder, resulting in lower gas mileage.4) Cost is a prevalent issue in ATEGs. Thermoelectric generators with higher efficiencies require higher quality, more expensive thermoelectric materials. With thethermal cyclingand vibration of the vehicle, the generators longevity is a concern. Although high quality thermoelectric materials could produce more electricity, the cost of replacing them could outweigh the savings in fuel economy.4.3 Current & Future Developments for TEG:-Recently, an increasing concern of environmental issues of emissions, in particular global warming and the constraints on energy sources has resulted in extensive research into innovative technologies of generating electrical power and thermoelectric power generation has emerged as a promising alternative green technology. In addition, vast quantities of waste heat are discharged into the earths environment much of it at temperatures which are too low (i.e. low-grade thermal energy) to recover using conventional electrical power generators. Thermoelectric power generation offers a promising technology in the direct conversion of waste-heat energy, into electrical power. In this paper, a background on the basic concepts of thermoelectric power generation is presented and recent patents of thermoelectric power generation with their important and relevant applications to waste-heat energy are reviewed and discussed. Currently, waste heat powered thermoelectric generators are utilized in a number of useful applications due to their distinct advantages. These applications can be categorized as micro- and macro-scale applications depending on the potential amount of heat waste energy available for direct conversion into electrical power using thermoelectric generators. Micro-scale applications included those involved in powering electronic devices, such as microchips. Since the scale at which these devices can be fabricated from thermoelectric materials and applied depends on the scale of the miniature technology available. Therefore, it is expected that future developments of these applications tend to move towards nano technology. The macro-scale waste heat applications included: domestic, automobiles, industrial and solid waste. Currently, enormous amounts of waste heat are discharged from industry, such as manufacturing plants and power utilities. Therefore, most of the recent research activities on applications of thermoelectric power generation have been directed towards utilization of industrial waste heat. Future developments in this area might focus onto finding more suitable thermoelectric materials that could handle higher temperatures from various industrial heat sources at a feasible cost with acceptable performance. Another future direction is to develop more novel thermoelectric module geometries and configurations. The developments of more thermoelectric module configurations by developing novel flexible thermoelectric materials will make them more effective and attractive in applications where sources of waste heat have arbitrary shapes.4.4 Advantages:-1) They are extremely reliable (typically exceed 100,000 hours of steady-state operation) and silent in operation since they have no mechanical moving parts and require considerably less maintenance.2) They are simple, compact and safe;3) They have very small size and virtually weightless;4) They are capable of operating at elevated temperatures;5) They are suited for small-scale and remote applications typical of rural power supply, where there is limited or no electricity;6) They are environmentally friendly;They are not position-dependent; andThey are flexible power sources4.5 Disadvantage: 1) Low energy conversion efficiency rate 2) Slow technology Progression 3) Limited Applications 4) Requires relatively constant heat source 5) Lack of customer/industry education about thermoelectric generators

4.6 Application of TEG in Various Fields:-1) Industrial Waste Heat Applications:- Most of the recent research activities on applications ofthermoelectricpower generation have been directed towards utilization of industrial waste heat. Vast amounts of heat are rejected from industry, manufacturing plants and power utilities as gases or liquids at temperature which are too low to be used in conventional generating units (