A Review On Thermoelectric Generator: Waste Heat ...

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ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962 SPECIAL ISSUE (ICRAME-2015) International Conference on Recent Advances in Mechanical Engineering In collaboration with International Journal of Engineering and Management Research (IJEMR) Page Number: 111-117

A Review On Thermoelectric Generator: Waste Heat Recovery From Engine Exhaust 1, 2

Dipak S. Patil1, Dr. R.R Arkerimath2, Dr. P.V Walke3 Mechanical Engineering Department, G.H. Raisoni College of Engineering & Management, Wagholi, Pune, Maharashtra, INDIA. 3 Mechanical Engineering Department, G.H. Raisoni College of Engineering, Nagpur, Nagpur, Maharashtra, INDIA.

ABSTRACT These Thermal management and energy crisis have been two major problems in this 21st century. Engine exhaust has tremendous amount of energy which can be recovered by waste heat recovery systems and it can be converted in to useful energy such as electric power. The thermoelectric concept is seen as a perfect solution for recovering waste heat from engine exhaust and converts in to electric energy. Since the use of nano thermoelectric materials for thermoelectric applications, there has been a huge quest for improving its figure of merits (ZT) to make it commercially viable. This review starts with thermoelectric concepts and explains briefly properties of thermoelectric materials, preparation of thermoelectric materials, modelling and simulation, experimental investigation and parametric evaluation. The present study focuses on various operating condition i.e. flow rate, temperatures of fluids, heat transfer coefficient of exhaust gas or hot fluid and position of thermoelectric module. The configurations (topology or geometry) of thermoelectric generator play a vital role for increasing effectiveness of the heat recovery system and conversion efficiency of thermoelectric generator. Keywords—Thermoelectric generator (TEG); Thermoelectric material (TEM); Heat exchanger (HE) topology; waste heat recovery (WHR); seebeck effect.

I.

INTRODUCTION

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

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engines. Viewing from the socio-economic 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 [18]. Waste heat is heat, which is generated in a process by way of fuel combustion or chemical reaction, and then “dumped” into the environment even though it could still be reused for some useful and economic purpose. The essential quality of heat is not the amount but rather its “value”. The strategy of how to recover this heat depends in part on the temperature of the waste heat gases and the economics involved. The internal combustion engine exhaust waste heat and environmental pollution has been more emphasized heavily recently. Out of the total heat supplied to the engine in the form of fuel, approximately, 30 to 40% is converted into useful mechanical work; the remaining heat is expelled to the environment through exhaust gases and engine cooling systems, resulting in to entropy rise and serious environmental pollution, so it is required to utilized waste heat into useful work [35]. The latest developments and technologies on waste heat recovery of exhaust gas from internal combustion engines (ICE) include thermoelectric generators (TEG), organic Rankine cycle (ORC), six-stroke cycle IC engine and new developments on turbocharger technology [18]. 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 [18]. A thermoelectric power generator is a solid state device that provides direct energy conversion from thermal energy (heat) due to a temperature gradient into electrical energy based on “Seebeck effect”. The thermoelectric power cycle, with charge carriers (electrons) serving as the working fluid, follows the fundamental laws of thermodynamics and intimately

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ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962

resembles the power cycle of a conventional heat engine [2].

II. BASIC THEORY OF A THERMOELECTRIC POWER GENERATOR Use The basic theory and operation of thermoelectric based systems have been developed for many years. Thermoelectric power generation is based on a phenomenon called “Seebeck effect” discovered by Thomas Seebeck in 1821. When a temperature difference is established between the hot and cold junctions of two dissimilar materials (metals or semiconductors) a voltage is generated, i.e., Seebeck voltage. In fact, this phenomenon is applied to thermocouples that are extensively used for temperature measurements. Based on this Seebeck effect, thermoelectric devices can act as electrical power generators. A schematic diagram of a simple thermoelectric power generator operating based on Seebeck effect is shown in Fig. (1). Heat is transferred at a rate of QH from a high-temperature heat source maintained at TH to the hot junction, and it is rejected at a rate of QL to a lowtemperature sink maintained at TL from the cold junction. Based on Seebeck effect, the heat supplied at the hot junction causes an electric current to flow in the circuit and electrical power is produced. Using the first-law of thermodynamics (energy conservation principle) the difference between QH and QL is the electrical power output We. It should be noted that this power cycle intimately resembles the power cycle of a heat engine (Carnot engine), thus in this respect a thermoelectric power generator can be considered as a unique heat engine [2]. Figure 2 shows a schematic diagram illustrating components and arrangement of a conventional singlestage thermoelectric power generator. As shown in Fig. (2), it is composed of two ceramic plates (substrates) that serve as a foundation, providing mechanical integrity, and electrical insulation for n-type (heavily doped to create excess electrons) and p-type (heavily doped to create excess holes) semiconductor thermoelements.

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Figure 1: Schematic diagram showing the basic concept of a simple thermoelectric power generator operating based on Seebeck effect [2]

Figure 2: Schematic diagram showing components and arrangement of a typical single-stage thermoelectric power generator [2]. In thermoelectric materials, electrons and holes operate as both charge carriers and energy carriers. The ceramic plates are commonly made from alumina (Al2O3), but when large lateral heat transfer is required, materials with higher thermal conductivity (e.g. beryllia and aluminum nitride) are desired. The semiconductor thermoelements (e.g. silicon-germanium SiGe, leadtelluride PbTe based alloys) that are sandwiched between the ceramic plates are connected thermally in parallel and electrically in series to form a thermoelectric device (module). More than one pair of semiconductors are normally assembled together to form a thermoelectric module and within the module a pair of thermoelements is called a thermocouple. The junctions connecting the thermoelements between the hot and cold plates are interconnected using highly conducting metal (e.g. copper) strips as shown in Fig. (2).


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A. Properties of thermoelectric materials Thermoelectric devices can convert thermal energy from a temperature gradient into electrical energy. This phenomenon was discovered in 1821 and is called the “Seebeck effect,” while the reverse counter part of this phenomenon was discovered by Peltier in1834 [4]. Thermoelectric materials are evaluated by the figure-ofmerit ZT; it is defined in terms of intrinsic material properties of both the N and P type materials and determined by three physical properties Seebeck coefficient (α), electrical conductivity (σ), and thermal conductivity (k). Where α is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. The larger the value of ZT, the better is the thermoelectric material. Obviously, the materials with higher electrical conductivity and lower thermal conductivity have larger value of Z which contributes more to the enhancement of conversion efficiency η [3]. However, ideal thermoelectric materials would have a high electrical conductivity to allow the conduction of electricity, which would create a potential difference across the sample, and a low thermal conductivity to maintain the temperature gradient between the hot and cold side [4].

III.

THERMOELECTRIC MATERIALS

These Due to their low efficiency, the extensive applications of thermoelectric materials have been limited to specialized fields where the reliability rather than the cost is a major consideration. Considering the initial cost of establishing a thermoelectric system, thermoelectric applications with the current conversion efficiency are more suitable for small scale applications. The development of new thermoelectric materials with high efficiency is one of the key factors for expanding the range of thermoelectric applications to the medium/ large scale [2]. A. Conventional thermoelectric materials Thermoelectric materials (those which are employed in commercial applications) can be conveniently divided into three groupings based on the temperature range of operation. Alloys based on Bismuth (Bi) in combinations with Antimony (An), Tellurium (Te) or Selenium (Se) are referred to as low temperature materials and can be used at temperatures up to around 450K. The intermediate temperature range - up to around 850K is the regime of materials based on alloys of Lead (Pb) while thermoelements employed at the highest temperatures are fabricated from SiGe alloys and operate up to 1300K. Although the above mentioned materials still remain the cornerstone for commercial and practical applications in thermoelectric power generation, significant advances have been made in synthesizing new materials and fabricating material structures with improved thermoelectric performance. Efforts have focused

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primarily on improving the material’s figure-of-merit, and hence the conversion efficiency, by reducing the lattice thermal conductivity. B. Preparation method of thermo-electric materials Owing to the fact that optimal properties can only be obtained at very narrow temperature region for each homogeneous material, which greatly limits its efficient utilization of many dispersed energy sources. Therefore, the concept of functionally graded material (FGM) has been widely accepted. Bi2Te3 is the best materials discovered till now used in ambient temperature, and βFeSi2 is one of the proper materials used in intermediate temperature region (600/900 K) with the characteristics of cheapness, oxidization-resistance as well as environmental friendliness. If these two materials are deliberately selected and arranged along the axis from low temperature side to higher one, they could work efficiently in their respective optimal temperature regions, thus much higher performance of a FGM could be expected than that of homogeneous materials. However, many problems will occur, if inhomogeneous materials are jointed together. Firstly, the contamination of semiconductors due to inter diffusion across the interfaces is significantly detrimental to the properties of FGM discussed. Secondly, delamination crack will take place resulting from thermal expansion coefficients mismatch of the materials coupled, for example, the thermal expansion coefficient of Bi2Te3 is almost two times that of FeSi2 [5].

a) Dip Coating Method Graded structure FeSi2/Bi2Te3 has been prepared by dip coating procedure using Sn-based alloy Sn95Ag5 with the melting point of 245 degree C as the soldering material. They measured the thermoelectric properties (Seebeck coefficients, electrical conductivity as well as power output) using a self-assembled apparatus. They also examined characteristics of graded materials. The maximum power output is approximately 2.5- 3 times that of monolithic material β-FeSi2 at the same temperature difference whether a Ni layer was sandwiched in between Sn95Ag5 and Bi2Te3 or not. Combined with the results of Seebeck coefficient measurement, it can be concluded that not only are the properties of Bi2Te3 at low temperature side benefited, but also the superior performance of β FeSi2 at higher temperature region are made fully use [5].

b) Spark Plasma Sintering Method The compacts of each homogeneous material of FeSi2 and Bi2Te3 were prepared by spark plasma sintering (SPS), and their electrical properties of Seebeck coefficient and electrical conductivity were measured in vacuum. The densities of the alloys by SPS, measured in Archimedes method, are 4.72x103 kg m3 for FeSi2 and 7.51x103 kg m3for Bi2Te3, respectively, and those by pressure less sintering (PS) are 4.27x103 kg m3 for FeSi2 and 6.15x103 kg m3 for Bi2Te3, which are all higher than 96% of theoretical ones. J.L. Cui optimized the segmented thermoelectric material. They calculated thermoelectric


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properties and transport properties of the homogeneous material β-FeSi2. They measured optimal junction temperature and the length ratio. When the segment length ratio for the material FeSi2/Bi2Te3 is kept at approximately 10:1, the junction temperature is 195 degree C. The maximum power outputs both through calculation and measurement are all about 1.4 times those of the homogenous material β-FeSi2 if the optimized segmented material is used [6]. c) Silicon Moulding Method This method consists of three major steps: (1) micromachining a silicon mould; (2) filling the mould with thermoelectric materials; (3) connecting P- and N-type elements and assembling the whole module. A novel micro-materials process using micro machined silicon moulds has been proposed to fabricate thermoelectric micro-modules with extremely high density of fine and high P–N elements. Silicon moulds with 10,000 holes of 300 micro m deep and 40 m2 in a 100mm2 area were successfully prepared, and Bi–Sb–Te alloy element arrays with dimensions equal to the holes were fabricated using the micro machined Si mould [7]. d) Explosive Consolidation Method Novel mixture fabrications such as shock consolidation, involving the production of a high density of interfaces, including sub-micron grain sizes, were also found to be inadequate. Rapid prototyping/controlled layer rapid manufacturing, ultrasonic and microwave consolidation or sol–gel/aerogel technologies may offer more efficient fabrication alternatives if inefficient scattering of electrons and phonons can be avoided. These micro indentation hardness values are essentially twice the values for the corresponding n- and p-type, melt-grown materials [8]. e) Cryogenic Grinding Method Bi2Te3 nano-sized powders with an average particle size of about 70 nm are successfully prepared by a cryogenic grinding (CG) in the condition of liquid nitrogen. Compared with high-energy ball milling (HEBM), spark plasma sintering (SPS) showed that cryogenic grinding (CG) can produces much finer and good sinter ability Bi2Te3 powders [9]. C. Promising thermoelectric materials Several general rules to increase the thermoelectric efficiency by decreasing the lattice contribution to the thermal conductivity were launched (or recovered from ideas earlier developed on the 1950s), the most important ones being (i) the use of compounds with complex crystal structures, (ii) the presence of heavy atoms weakly bounded to the structures, (iii) the existence of inclusions and/or impurities, (iv) the formation of solid solutions and (v) the existence of a large number of grain boundaries. The following are the thermoelectric materials are used for thermoelectric generator [12].

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A) Interfaces In Bulk Thermoelectric Materials The grain and interfacial structure of nanostructured bulk thermoelectric materials are rapidly advancing. Embedded interfaces can enhance the electronic and thermal transport properties of TE materials [10]. b) Ceramics Thermoelectric properties of La1-xSrxCuO3-δ (LSCuO) samples have been prepared by sol–gel method in the temperature range between 100 and 290 K. The transport properties of the samples are modified by changing La atoms for strontium atoms. The obtained samples exhibit a rhombohedral (Hex) structure and high Seebeck coefficient values, which decrease with Sr level. The studied transport properties yield maximum values for PF close to 60 μW/K2 cm [11]. c) Semiconductors CuxAyTez (A=Ge, As, Ga and Si) general composition glasses point to chalcogenide semiconducting glasses as a new family of potential high performance thermo electric materials. The studies made on telluriumbased semiconducting glasses containing high concentrations of copper have shown interesting ZT values, up to 0.2 at 300K [12].

IV.

THERMOELECTRIC GENERATOR

Thermoelectric generators (also called Seebeck generators) are devices that convert heat (temperature differences) directly into electrical energy, using a phenomenon called the Seebeck effect (a form of thermoelectric effect). In 1821, Thomas Johann Seebeck discovered that a thermal gradient formed between two dissimilar conductors produces a voltage. At the heart of the thermoelectric effect is the fact that a temperature gradient in a conducting material results in heat flow; this results in the diffusion of charge carriers. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference. In 1834, Jean Charles Athanase Peltier discovered the reverse effect that running an electric current through the junction of two dissimilar conductors could, depending on the direction of the current, bcause it to act as a heater or cooler 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. 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.


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Basically, a practical automotive waste heat energy recovery system consists of an exhaust gas system, a heat exchanger; a TEG system, a power conditioning system, and a battery pack [18]. A. Experimental investigation of thermoelectric generator The experiments are carried out to examine the influences of the main operating conditions, the hot and cold fluid inlet temperatures, flow rates and the load resistance, on the power output and conversion efficiency. Xing Niu et al. designed and constructed parallel-plate heat exchanger and performed experimentation/test on thermoelectric generator unit incorporating the commercially available thermoelectric modules with the parallel-plate heat exchanger. The results show that both the maximum power output and the corresponding conversion efficiency are greatly affected by the operating conditions, especially the hot fluid inlet temperature and flow rate [26]. Mohd Izam Abd Jalil et al. selected four types of Thermoelectric Module (TEM) and experimentally tested on different cooling system techniques. Testing was conducted using a candle flame as a heat source to produce a suitable temperature with the maximum temperature of 200°C. This study is presented to identify type of TEMs and cooling system that can produce the higher, stable and efficient power energy. As the result, the TEM 19811-9L31-02CN1 is the most stable performance and efficient because it has a large number of thermo elements, tall element type and has high thermal resistance. The proposed water based cooling system has advantages in term of stability as well as capability to maintain the temperature different between hot and cool side of the TEM [28]. Hongliang Lu et al. proposed heat exchanger as 1-inlet 2-outlet and 2-inlet 2outlet based on the referenced structure: empty cavity. A test bench was developed for finding various operating conditions. The symmetrical 1-inlet 2-outlet increased hydraulic disturbance and enhances heat transfer, resulting in the more uniform flow distribution and higher face temperature than the 2-inlet 2-outlet and empty cavity. 2Inlet 2-outlet among the three structures was always the largest in pressure drop and the most dependent to temperature and mass flow rate [29]. B. Parametric evaluation of thermoelectric generator Sumeet Kumar et al. used various type of topology of thermoelectric generator for testing/ analysis. The computational tool was also adapted to model other topologies such as transverse and circular configurations (hexagonal and cylindrical) maintaining the same volume as the baseline TEG. The transverse design is found to be an improvement over traditional, longitudinal designs. A transverse TEG with a single module length is found to be the most favorable design among all the designs studied, with the highest electrical power generation and lower pressure drops [31]. Chien-Chou Weng et al. investigate a thermoelectric generator which extracts heat from an

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automotive exhaust pipe and turns the heat into electricity. The influences of the number and the coverage rate on the heat-exchanger of the TEGs were explored via simulations. It is found the heat sinks attached to the downstream TEGs possibly cause the downstream wall cooler which thus loots heat from the upstream hotter wall, resulting in a degradation of the performance of the upstream TEGs. Consequently implementing more TE couples may not necessarily be worthwhile economically. Leaving some downstream part of the heat exchanger uncovered with the TE couples on the other hand results in a hotter downstream wall, more heat transferred to the upstream TE couples, and consequently a larger power generation rate [32]. Jiin-Yuh Jang et al. optimized TEG module spacing and its spreader thickness as used in a waste heat recovery system is investigated and solved numerically using the finite difference method along with a simplified conjugate-gradient method. A search for the optimum module spacing (S) and spreader thickness (Hsp), ranging from 40 mm < S < 300 mm and 1 mm< Hsp < 30 mm, respectively, is performed. The optimum TEG module spacing (S) and its spreader thickness (Hsp) are found to be strongly dependent on the waste gas heat transfer coefficient, h; while they were weakly dependent on the temperature difference between the waste gas and the cooling water. The results obtained in this study should provide useful information for the industrial TEG module design for waste heat recovery [33]. Ahmet Z. Sahin et al. carried out theoretical analysis of thermoelectric power generator and influence of thermoelectric leg geometry on the device efficiency and the power generation is formulated. The geometric configuration of the legs in the device is associated with the shape parameter and incorporated in the analysis. The influence of the shape parameter on the device efficiency and power generation is examined for various temperature and external load resistance ratios. It is found that increasing or decreasing of the shape parameter (µ) has a favourable effect on the device efficiency; however, the shape parameter (µ) has an adverse effect on the thermoelectric power generation. [34].

V.

CONCLUSION

These n which charge carriers serve as the working fluid. There are many methods are available for preparation of thermoelectric materials, such as Dip Coating, Spark Plasma Sintering, Silicon Molding, Explosive Consolidation, Cryogenic Grinding and so on. Semiconductor, Ceramic and Poly (3, 4ethylenedioxythiophene) thermoelectric materials have very law figure of merits. The grain and interfacial structure of nanostructured bulk thermoelectric materials are rapidly advancing. The transport properties of the


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samples are modified by changing La atoms for strontium atoms. Tellurium-based semiconducting glasses containing high concentrations of copper have shown interesting ZT values, up to 0.2 at 300K. Various approaches will continue to study thermoelectric nanomaterials with narrowband gaps, heavy elements doping, point defects loading and nano structuring. Especially, for practical thermoelectric applications, the synthetic approaches of thermoelectric nanomaterials. By proper architectural design, the efficiency of thermoelectric system can be increased nowadays; researchers have used analytical models to develop thermoelectric structures, obtaining optimal results. Other researchers have used numerical methods to simulate the behaviour of thermoelectric modules. There is several technologies are available for recovering waste heat from engine exhaust such as thermoelectric generator, rankine bottoming cycle, turbocharger and six-stroke internal combustion engine. Thermoelectric generator is extremely and silent in operation since they have no mechanical moving parts and require considerably less maintenance. It can be converted law grade heat energy in to electric energy. Mathematical model of a thermoelectric generator (TEG) device uses the exhaust gas of vehicles as heat source. The model simulates the impact of relevant factors, including vehicles exhaust mass flow rate, temperature and mass flow rate of different types of cooling fluid, convection heat transfer coefficient, height of PN couple, the ratio of external resistance to internal resistance of the circuit on the output power and efficiency. The models are also used in optimization studies of thermoelectric waste heat recovery with air cooling in a cross flow heat exchanger. The symmetrical 1-inlet 2-outlet increased hydraulic disturbance and enhances heat transfer, resulting in the more uniform flow distribution. The transverse design is found to be an improvement over traditional, longitudinal designs. A transverse TEG with a single module length is found to be the most favourable design.

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