Future Impact of Thermoelectric Devices for Deriving Electricity by Waste Heat Recovery from IC Engine Exhaust

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1 DOI: /nijesr Future Impact of Thermoelectric Devices for Deriving Electricity by Waste Heat Recovery from IC Engine Exhaust 1 Muhammad Usman Ghani*, 2 Syed Amjad Ahmad, 2 Umair Munir, 2 Aqeel Ur Rehman, 2 M. Aslam Bhutta 1 University of Agriculture, Faisalabad-Pakistan. 2 NFC Institute of Engineering and Fertilizers, Research, Faisalabad, Pakistan. *Corresponding Author: engineer2333@live.com Abstract - Thermoelectric devices are now a days much popular regarding their combined effect of electrical and heat energy. Due to their capability to convert heat energy into electrical energy, they can be used as heat recovery units in engine exhausts. An assembly of thermoelectric modules when placed in the exhaust of engine can produce current. The phenomenon is governed by Seebeck effect and carried out in a single stage. This energy can be utilized in powering electric appliances running in automobiles. It can produce energy from waste heat of engine and thus providing a huge area of interest in automobile industry to extract energy using TEG. Keywords: Thermoelectric Generator, Thermoelectric Material, Pettier Device, Seebeck Effect I. INTRODUCTION: Thermoelectric devices are a hot topic now a days followed by the impact offered by them in their applications, usability, safety, environment friendliness and simple working phenomenon. Thermoelectric devices work by maintaining a temperature gradient on supply of electricity as well as generating electricity when given a heat flow through them. [1]A temperature gradient is maintained by these devices by maintaining one side as cold side and the other as hot side which is connected to an electricity source. Being composed in the solid state they have no complexity in manufacturing followed by a smooth, noise free and working free of any moving mechanical part of working fluid. TEC (Thermoelectric coolers) are superior in this regard of working to generate same effect but in a different and convenient mechanism. [2] They have huge applications at industries where flue gases and waste heat can be used to generate electricity by TEG (thermoelectric generators). In pipes carrying waste gases, chimneys, hot fluid carrying pipes as well as vehicles exhaust pipes. Industrial purposes use them for making extra efficient cars by utilizing waste heat to produce electricity.[3] Military appliances make use of it where other sources are unable to be taken as for their compact size and can be carried easily with a normal Pieter module working at as low as 12 V. Space industry has been in serious concern for past decades to make its applications in space where solar panels are no more able to work as the thermoelectric module will work as long as heat is present. A study was carried out for use of it in producing electric energy from waste heat of an IC engine in order to test the working and viability of TEG in future for automobile industry to be used as waste heat recovery unit and run electrical appliances in automobiles. Fig. 1 Peltier thermoelectric module cut section [Innoveco Australia]. The Proposed effects of Installing TEG in automobile exhaust lines are: Waste heat energy of the fuel can be recovered. 84

2 Increased load on auxiliary components such as AC, headlights, horn, and audio system can be reduced by installing TEG. It helps in increased vehicular electrification to share the electrical load. Reduced heat emissions into atmosphere. Increased fuel efficiency. II. WORKING PHENOMENON OF THERMOELECTRIC DEVICES Where J is the current density, and K is Thomson's coefficient. The three coefficients are related by Thomson relations (Kelvin relations). Where: = - B, and: = TS (2) (3) Thermoelectric devices make use of the thermoelectric modules which work on different principles of conversion between heat energy and electric power. The phenomenon of conversion done by these thermoelectric modules which is governed by following principles. Fig. 3 Thompson Effect B. Seebeck Effect A. Thompson Effect: Fig. 2. Thermo Electric Devices A current carrying conductor with a present temperature gradient has another form of heat absorbed along with the joule heat which is named as Thompson heat. The amount of Thompson heat is given in the formula below in which we can clearly observe that its amount released or absorbed is directly related to the temperature gradient provided. [4] (1) As shown in Figure 3, for a circuit constituted of conductor (or semi-conductor) a or b in series, if there are two connectors 1 and 2 at temperature T h and T c respectively, there will produce a potential difference between y and z at the open-circuit position of b. Under the same material, the thermoelectricity potential is only related to the temperature difference of two connectors, and it can be expressed as (4) The Seebeck effect is invertible. When the temperature difference adds or deletes a minus sign, thermoelectricity 85

3 potential would remain its absolute value with or without a minus sign. (7) C. Pettier Effect: The Peltier effect is the opposite of the Seebeck effect. In Figure 3, if an electromotive force is added at y and z, electric current I is produced in the circuit composed of a and b. At the same time, one connector of the conductors absorbs the heat while the other releases the heat. The experiment found that the heat absorption (release) rate q is in proportion to electric current I, namely: In real practice, the dimensionless optimum value Z of electric material with single temperature difference is expressed by its multiplying with absolute temperature. q=π ab I (5) (8) Where, π ab is Peltier coefficient, W/A. D. Basic principles of thermoelectric generator: A p-type thermoelectricity component and an n-type thermoelectricity component were connected with metallic electrodes at the hot-end, which is called thermoelectricity couple or temperature difference uncouple. As is shown in Figure 2, the open-end of the thermoelectricity couple is connected with an external load whose resistance is rl. When the electric current moves through the circuit, the electric power of the load is I2rL. Consequently, a generator that converts thermal energy to electricity is obtained. [5] III. MATERIALS AND METHODS A. Selecting standard for thermoelectric materials In order to increase the efficiency of the thermoelectric generator, it is necessary to increase the optimum value Z of thermoelectricity leg. (6) Optimum value Z is the standard of evaluating the quality of a certain materials in the research of thermoelectric materials. From the equation, it is found that to find materials with high optimum value, the only method is to enhance the Seebeck coefficient and electrical conductivity and reduce thermal conductivity of the material. [6] B. Major thermoelectric materials At the present, common thermoelectric materials are bismuth telluride (Bi 2 Te 3 ) and its alloys, plumbous telluride (PbTe) and its alloys and silicon germanium (SiGe) alloys. C. Thermoelectric Module It is semiconductor which is highly doped by pollutants to increase the electric conductivity of the semiconductor. Good semiconductor has electric conductivity in between 2µV/K - 3µV/K. [7] When choosing semiconductor it has to withstand that much high operating temperature. Some of the good thermoelectric module semiconductors are Bi 2 Te 3, CaMnO, Ca 3 Co 4 O 9, Sb 2 Te 3, and PbTe Bi 2 Te 3 based materials shown to have seebeck coefficient (voltage per unit temperature difference) of 287 μv/k at 328K, However, one must realize that Seebeck Coefficient and electrical conductivity have a tradeoff; a higher Seebeck coefficient results in decreased carrier concentration and decreased electrical conductivity.[8] In another case bismuth telluride has high electric conductivity of S m/m 2 with its very low lattice thermal conductivity of 1.2 W/(m K). For 1K temperature difference and 2 modules it produces 4.2% efficiency and 15volts it is commercially available TEG. [9] CaMnO 3 bulks were prepared by a solid state reaction. They show metallic 86

4 behavior at temperatures higher than about 4 K and electrical resistivity Ω is lower than 12 mωcm at 1K in air. For CaMnO 3, S value reaches -13 muv/k at 973 K. Both thermoelectric properties are dominated mainly by crystallographic structure. [1]Thermal conductivity samples is as low as 1.5 W/m-K 2 and dimension-less figure of merit ZT reaches.16 at 973 K for CaMnO 3 in air. It generate 3.9% efficiency 2.6 V for 2 modules and 1K temperature difference. This TEG is preferred for High operating temperature. We have Ca 3 Co 4 O 9 semiconductor for high temperature withstanding property. Ca3Co4O9 has some good thermo electric properties. It can withstand 8ºc. [11] Seebeck property 26µV/K, Electrical resistance 11.6 mωcm. figure of merit ZT=.23. Thermal conductivity 1.2Wm -1 K -1. This property taken for 88K. This TEG generates 4.2 % efficiency 4.2 volts for 2 modules and 1K temperature difference. F. Design of thermoelectric generator : The thermoelectric generator was constructed by using 16 thermocouples. These thermoelectric modules were bismuth telluride. In terms of electrical design, the thermoelectric modules were connected in series. Each when connected in series generated current depending on the temperature difference between the hit and cold parts. The hot part was suspended for reducing heat sink. [14]Cold water was used for cooling the sink for maximum difference. Dimensions of each module were 4 mm in width, 4 mm in length with thickness of 4 mm. These thermoelectric modules were connected with thermoelectric couples connected electrically in series and thermally in parallel. D. Thermoelectric shield : It is a material which protects the modules damage due to high Temperature. Mostly Ceramics material for this which is Al 2 O 3. It also transfers temperature to the modules from hot side. It should be thick. [12] E. Thermal fin: It is used here for increase the thermal gradient value. When we increase the Thermal gradient value it increase the seebeck voltage generated by TEG. This FIN also transfers the heat from Thermoelectric Module. It is made by Aluminum metal. When we include Thermal fin it increase the efficiency of the TEG. [13] Fig. 5 Bismuth Telluride thermoelectric device (TEC1-1276) This allowed the thermoelectric generator to be under direct influence of all gases emitted from the engine exhaust. The thermoelectric power generator is a solid state device that provides direct energy conversion from thermal energy due to temperature gradient into electrical energy based on the Seebeck effect. IV. RESULTS AND DISCUSSION: Fig. 4 Thermal fins [Electronics-cooling China] The TEG was tested in a couple of experiments attached to the exhaust of a four stroke petrol engine run at 87

5 RPM of 1, 15 and 2 with varying temperature gradients with 1 minutes intervals. Then by finding the current and voltage, power was calculated using formula: P= V X I Watt. The resulting trends between different parameters were shown graphically. The results in figure 6 shows that the increase in temperature difference results in increased power. Since power produced is directly related to the temperature difference provided to the TEG, it increases as more temperature gradient is provided. By taking Power on Y-axis and Temperature Difference on X-axis the graph is plotted as showed in figure 6. Current (micro Imp) T vs Current T ( C) 1 rpm 15 rpm 2 rpm T Vs Power Fig.7 Graph between ΔT and Current Power (W) rpm 15 rpm The graph plotted between temperature difference and voltage in the figure 8 shows different trends at different speeds of engine. It increased as the temperature gradient increased at 1 rpm engine speed and at higher speeds the voltage range changes with the change in temperature difference. 1 2 rpm T( C) T Vs Voltage Fig. 6 Graph between ΔT and power By taking values of current on Y-axis and temperature deference on X-axis, a graph is plotted which indicated that the increase in current as temperature difference increases. Current is the desired power to run the electrical appliances. Voltage (V) rpm 15 rpm 2 rpm T( C) Fig. 8 Graph between ΔT and Voltage As power increases system efficiency also increases in a linear way as showed in the figure 9. When engine was run at different RPMs with gradual increase with the 88

6 increasing RPMs. RPMs increased so resulting in an increase in the system efficiency which was a positive sign for the readings at hand. Power Vs System Efficiency The relationship between current and voltage produced during experiment run at different speeds of the engine showed in figure 11. Current and voltage also came out to be increasing when RPM of the engine was increased from 1 to 2. The comparison showed different trends in the outcome of effect of current and voltage on each other. 4 System Efficiency (%) Power (W) 1 rpm 15 rpm 2 rpm Voltage (V) Current Vs Voltage rpm 15 rpm 2 rpm Fig. 9 Graph between power and system efficiency Current (micro Imp) Increase in power also results in the increase in the Carnot efficiency of the engine as a result of using TEG at the exhaust. Carnot efficiency is regarded as the factor relating to mechanical efficiency of engine which is affected by the waste heat recovery done by the TEG. Fig. 11 Graph between current and voltage at different engine speeds. V. CONCLUSIONS: Carnot Efficiency (%) Power Vs Carnot Efficiency Power (W) Fig. 1 Graph between power and Carnot efficiency 1 rpm 15 rpm 2 rpm A detailed study on Thermoelectric Generator was carried out with the following conclusions being made. Electric voltage was produced between two points of electric conductors if they were at different temperatures. The charge carriers migrated from the hot side to the cold side. The voltage is proportional to the temperature difference and depends on the material and its Seebeck coefficient. The maximum temperature difference tested was 48 C at 2 rpm and it produced a thermal efficiency of 3.58% and an output power of 48.9 watts. The lab view based experimental setup presents results for a particular Bismuth Telluride thermoelectric device (TEC1-1276) and ZT = 1. The maximum difference tested (48 C) was fairly modest, higher differences would result in higher 89

7 efficiency. Typical thermoelectric devices require a temperature of approximately 5 C to achieve an efficiency of up to 15 %. The development, validation and demonstration of TEG is the futuristic approach towards coping up with alternate energy sources is needed in order to maintain the green environment by enabling commercial viability in waste heat energy recovery on large scale. Automobile industry will be looking forward to the implementation of TEG in order to decrease the waste heat emission into atmosphere by using it for auxiliary electrical devises in automobiles. [11] U. Erturun, K. Erermis, K. Mossi, Effect of various leg geometries on thermo-mechanical and power generation performance of thermoelectric devices, Applied Thermal Engineering 214, vol. 73, pp [12] D. Zhou and S. Chu-ping, Study on thermoelectric material and thermoelectric generator Journal of Chemical and Pharmaceutical Research, 215, vol. 7(3), pp [13] F.J. DiSalvo, Thermoelectric cooling and power generation, Science, 1999, vol. 285, pp [14] T.M. Tritt, Holey and unholey semiconductors, Science, 1999, vol. 283, pp REFERENCES [1] H. K. Lyeo, A. A. Khajetoorians, Li Shi, P. Kevin, Pipe, J. R. Ram, S. Ali, C. K. Shih, Profiling the thermoelectric power of semiconductor junctions with nanometer resolution, Science, 24, vol. 33, pp [2] K. F. Hsu, S. Loo, F. Guo, Wei Chen, J. S. Dyck, C. Uher, T. Hogan, E. K. Polychroniadis, M. G. Kanatzidis, Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit, Science, 24, vol. 33, pp [3] T. C. Harman, P. J. Taylor, M. P. Walsh, B. E. LaForge, Quantum dot superlattice thermoelectric materials and devices, Science, 22, vol. 297, pp [4] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O Quinn, Thin-film thermoelectric devices with high room-temperature figures of merit, Nature, 21, vol. 413, pp [5] D. Chung, T. Hogan, P. Brazis, M. R. Lane, C. Kannewurf, M. Bastea4, C. Uher, M. G. Kanatzidis, CsBi4Te6: A High- Performance Thermoelectric Material for Low-Temperature Applications, Science, 2, vol. 287, pp [6] Il-Ho Kim, S. M. Choi, W. S. Seo, D. I. Cheong, Thermoelectric properties of Cu-dispersed bi.5 sb 1.5 te 3, Nanoscale Research Letters 212, 7:2 [7] D. Kenfaui, G. Bonnefont, D. Chateigner, G. Fantozzi, M. Gomina, J. G. Noudem, Ca 3 Co 4 O 9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties, Materials Research Bulletin 21, vol. 45, pp [8] B. I. Ismail, W. H. Ahmed, Thermoelectric Power Generation Using Waste-Heat Energy as an Alternative Green Technology, Recent Patents on Electrical Engineering 29, vol. 2, pp [9] M. Rezaei, W.K. Muzammil, M.H. Hassan, S. Paria, M. Hasanuzzaman, Technologies to recover exhaust heat from internal combustion engines, Renewable and Sustainable Energy Reviews, 212, vol. 16, pp [1] D.Y. Chu ng, T. Hogan, P. Brazis, M. R. Lane, C. Kannewurf, M. Bastea, C. Uher, M.G. Kanatzidis, CsBi 4 Te 6 : A High- Performance Thermoelectric Material for Low-Temperature Applications, Science, 2, vol. 287, pp