Ancillary Power Generation from Exhaust Heat Using ...

International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.92 (2015) © Research India Publications; http/www.ripublication.com/ijaer.htm

Ancillary Power Generation from Exhaust Heat Using Thermoelectric Technology D.Muralidar1*,M.Varun Kumar2, V.G.Navaneeth3, P.S.S.Srikar4,A.Yugesh Chandra5 1,2

Assistant Professor, TIFAC-CORE,VIT University, India. 3 Student,SITE,VIT VIT University, India. 4,5 Student,TIFAC-CORE,VIT VIT University, India. *Corresponding author’s Email: [email protected], Mobile:+91-9894335864 Abstract In the recent years waste heat recovering techniques are peaking up as automobile engines reached their maximum efficiency due to the thermodynamic constraints, as in a petrol driven vehicle only about 25% of the fuel energy is used for vehicle movement and accessories [1]; the rest is lost in the form of excess heat and coolant, as well as friction and parasitic losses. TEGAE (thermoelectric generators using automotive exhaust) were tried and tested to capture the rest of the unused heat and results appear to be unsatisfactory. However, the idea seems to be promising even though efficiencies for TEGAE are as low as 3-5% with existing technology. The TEGAE with certain design improvements can be found interesting and can provide better results. The conversion efficiencies of the thermoelectric materials has increased more than three times in the past twenty years, nevertheless some design issues of the TEGAE’s have kept the efficiency at insignificant values there by reducing the output power. Instead of cascading more thermoelectric coolers, a single thermoelectric generator is utilized to reach the desired power levels. Keywords Friction, parasitic losses, single thermoelectric generator, TEGAE, Thermoelectric effect. I. Introduction In modern days, the word heat recovery has become a prevalent term in automotive world, as conventional power sources such as petrol and diesel are either facing worldwide scarcity or becoming costly day by day. The benefits of waste heat recovery includes reduction in the fuel consumption, decrease equipment sizes and pollution levels, and also reduction in ancillary power consumption [2].

Energy distribution of an internal combustion petrol engine

40% loss in exhaust gas

25% utilized for vehicle's movement

30% loss in cooling system

5% lost due to friction and radiation

Fig.1. The typical energy distribution of a petrol engine. Although there are a numerous devices to accomplish heat recovery, thermoelectric generator (TEGAE) can be most suitable automotive applications, which becomes principal target of this paper. TEGAE’s are devices which convert thermal gradient i.e. temperature differences directly into electrical energy, using a phenomenon called the "Seebeck effect" or "thermoelectric effect". Now let us try to understand the fundamental physics behind the seebeck effect on which the construction of TEGAE’s is based, Consider a piece of metal and the one side of the metal is heated and the other end is cooled, the electrons at the hot end have the high energy than the equivalent electrons at the cool end and the hot side electrons tend to move faster towards the cool end than the cool side electrons which move towards the hot side. The hot side of the metal forms the positive and cool side of the metal forms the negative this creates the voltage and this effect is called thermoelectric effect and proposed by “seebeck”. The voltage produced by this metal is small so we need

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tocascade more metal pieces and interconnected through wires but an opposite voltages may be formed between the wires.

Fig.2. Demonstration of forming hot side and cold side on thermoelectric material.

Fig.3. Demonstration of forming voltage as a result of creating a thermal difference. So we go for the semi conducting materials which are also capable of conducting with positive particles, for a ptype material at the hot side the positive particles have high energy and move towards cold side. Now the hot side becomes negative and cold side becomes positive and if we interconnect both p-type and n-type materials alternatively a reasonable amount of voltage can be generated. The problem with these devices is that the metals conduct both electrons and heat, to increase efficiency of the devices heat conduction should be prevented. Therefore, we require materials with high electrical conductivity and low thermal conductivity and this is achieved by using metal alloys, which have different sized atoms that increases the electrical conductivity and slows down the thermal conductivity and helps to maintain the temperature difference for a longer period to generate power. Chalcogenides family materials are the major ingredients in the manufacturing of thermoelectric modules among them bismuth telluride and its alloys are very decent thermoelectric constituents suitable for the operating temperature ranges of exhaust gas at the tail pipe. II. Efficiency of the thermoelectric modules The efficiency of the thermoelectric module is quantified by the “Seebeck coefficient” of that particular material, which is a measure of the degree of an induced thermoelectric voltage in response to a thermal gradient created across that material. The unit of Seebeck coefficient is V/K, The Seebeck coefficient for a thermoelectric material is denoted by ‘β’ and it depends on the parameters like type of molecular arrangement, absolute temperature and its relationship with the temperature is non-linear. The efficiency of the thermoelectric modules is defined as‘η’and it is given as



(Th -Tl )   1  Kt   1 Th  1  Kt   Tl / Th 

Where This the hot side temperature and Tl is the cold side temperature obtained on the thermoelectric modules and Kt is the figure of merit of the device and it is denoted by

Kt 

(  2T )



and it is unit less. Where



is the

thermal conductivity and  is the Seebeck coefficient of the device. Figure of merit of the device directly gives the conversion efficiencies for the particular material, the commercially available thermoelectric modules have

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the figure of merit close to unity. The effectiveness of TEGAE’s are mostly affected by the sort of materials used. So, the right selection of materials becomes crucial for the design of a good TEGAE. The typical TEGAE consists of a hot chamber which consists of a thermoelectric modules on which the heat sinks are mounted. The hot chamber is the section where the exhaust is captured the heat sinks are employed to dissipate heat from the thermoelectric modules. Traditionally many types of heat exchanging methods were proposed such as ribbing, grooving, and protrusions since the first automotive thermoelectric generator was invented by Neild et al in 1963 [3]. III.critical thermal investigations The exhaust gas distribution in the TEGAE has three important aspects firstly it determines the maximum operating temperature for the thermoelectric modules; secondly it determines the energy conversion efficiency of the thermoelectric modules from heat to electrical energy; thirdly it determines the thermal stress between the thermoelectric modules and the hot chamber [4]. If the hot chamber has the big cross sectional area the velocity of exhaust gas is reduced when entering the TEGAE and this abrupt drip in the velocity of exhaust gas leads to the formation of thick thermal boundary that decreases the heat transfer according to Bass et al [5], so for maintaining the engines efficiency the free cross sectional area must be ensured in hot chamber. And to diminish the conduction losses the assembly components used in the construction should be lessened and the unused area must be firmly insulated.Additional crucial thermal requisite in TEGAE is design of suitable cold plate and these are classified into two types by Saqret al [6] 1.Radiator-based cold plate 2.Heat-sink based cold plate.In the first type the cold plate dissipates heat through the engine coolant, the problem with this type of technique is that the coolant pump and the piping needs to be resized to recompense for the added cooling burden. Whereas in the second type heat sinks can be operated at even low temperatures with good efficiency and this technique also takes the advantage of the air flow to dissipate heat during the movement of the vehicle. Another major advantage of the thermoelectric technology is it has no moving parts, requires no additional power requirements, compact in size, lightweight and runs for a longer duration without maintenance [7]. IV. Teagae Design Considering all the above implications, the construction of the TEGAE plays a key role in determining its efficiency to convert thermal energy into electrical energy. Thus the thermoelectric module, aluminum pad and copper pad are polished and cleaned thoroughly to eliminate the microscopic gaps that trap the heat energy and form insulation layer between the two surfaces and a small amount of thermal grease with high thermal conductivity is utilized to achieve perfect interface between the surfaces. The heat sink, thermoelectric module, and copper pad are tightly compressed and clamped with screws by applying gently pressure gradually on both sides alternatively. The entire setup is mounted on to the tail pipe of the exhaust and thermoelectric module is subjected to the thermal gradient, by dissipating the heat through the heat sinks mounted above the thermoelectric modules voltage is developed, and the output voltage can be regulated to suit the desired load or application.

Fig.4. Designed TEGAE model V. Results And Discussions The experimental results were plotted in fig5 & fig6 for output voltage and output power with respect to output current and it can be interpreted that in order to achieve maximum power the current has to be in the range of around 2-2.5 amps which can give the power up to 18 watts by maintaining the hot side temperature in the range of 250-300 degrees and cold side temperature around 30-35 degrees.

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Fig.5. Plot for output voltage vs. output current

Fig.6. Plot for output power vs. output current VI. Conclusions The literature survey reveals that many heat recovery systems were implemented, but they suffered from the low conversion efficiencies, through this novel approach to design TEGAE the productive power can be generated by using less number of thermoelectric modules. The efficiency of the TEGAE is mainly dependent on the thermal difference created between the modules, geometry, type of heat exchange method used, type of materials used for construction. This idea can be further enhanced by employing innovative designs by using superior thermoelectric materials, heat sinks, assembly components, and by investigating better installation sites for better exhaust flow. VII. Acknowledgement The authors would sincerely thank Prof Dr.K.Ganesan Director, TIFAC-CORE, VIT University, for Providing an environment for excellent R&D which helped a lot in carrying our project successfully and also Mr.K.Pradeep Chandra, SMBS, VIT for giving his valuable inputs in designing the TEGAE design without their contribution this outcome could not be possible. References [1] Saqr, K. M., Mansour, M. K., & Musa, M. N. (2008). Thermal design of automobile exhaust based thermoelectric generators: Objectives and challenges. International Journal of Automotive Technology, 9(2), 155-160. [2] Jadhao, J. S., &Thombare, D. G. (2013). Review on exhaust gas heat recovery for IC engine. International Journal of Engineering and Innovative Technology (IJEIT), 2(12). [3] Neild, A. B. (1963). Portable thermoelectric generators (No. 630019). SAE Technical Paper. [4] Liu, X., Deng, Y. D., Chen, S., Wang, W. S., Xu, Y., & Su, C. Q. (2014). A case study on compatibility of automotive exhaust thermoelectric generation system, catalytic converter and muffler. Case Studies in Thermal Engineering, 2, 62-66. [5] Bass, J. C., Elsner, N. B., & Leavitt, F. A. (1995, August). Performance of the 1 kW thermoelectric generator for diesel engines. In AIP Conference Proceedings (pp. 295-295). IOP INSTITUTE OF PHYSICS PUBLISHING LTD. [6] Saqr, K. M., Mansour, M. K., & Musa, M. N. (2008). Thermal design of automobile exhaust based thermoelectric generators: Objectives and challenges. International Journal of Automotive Technology, 9(2), 155-160. [7] Remeli, M. F., Kiatbodin, L., Singh, B., Verojporn, K., Date, A., &Akbarzadeh, A. (2015). Power Generation from Waste Heat Using Heat Pipe and Thermoelectric Generator. Energy Procedia, 75, 645-650.

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