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Energy Procedia 75 ( 2015 ) 590 – 596

The 7th International Conference on Applied Energy – ICAE2015 Comparison of segmented and traditional for waste heat recovery of diesel engine Hua Tiana, Na Jianga, Qi Jiab, Xiuxiu Suna, Gequn Shua, Xingyu Lianga*

a State Key Laboratory of Engines, Tianjin University, Tianjin,300072, China b AVIC Shenyang Engine Design and Research Institute, Shenyang, 110015, China

Abstract

This paper established a mathematic model of segmented thermoelectric generator (TEG) based on low-temperature thermoelectric material and medium-temperature thermoelectric material skutterudite. The performance of segmented TEG and traditional TEG has been compared using this model under different conditions, such as heat source temperature, cold source temperature, heat transfer coefficient, and length and cross section area of thermocouple. The results show that the maximum output power and conversion efficiency of segmented TEG are higher significantly using exhaust of diesel engine (DE) as heat source and coolant as cold source. The results also show that the trends of maximum output power and conversion efficiency are contrary with increment of thermocouple length. The trends of maximum output power is linear with increment of cross section area, and the conversion efficiency is constant. Finally, the segmented TEG has a more potential to recover waste heat than tradition TEG. The performance of segmented TEG is decided by the ratio of materials, the optimum design of segmented TEG should base on the crossing point of ZT value for two materials. ©© 20152015 The The Authors. Authors. Published byPublished Elsevier Ltd. byThis Elsevieris an open access Ltd. article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). SelectionPeer-review under and/or responsibility peer-review of Applied underEnergy Innovation responsibility Institute of ICAE Keywords: Waste heat recovery in diesel engine; Segmented thermoelectric generator; Output power; Conversion efficiency

1. Introduction

Diesel engine (DE) is one of the main consumers of petroleum resources, which above 30% of fuel engine become waste heat [1]. If the waste heat can be recovered utilization, the thermal efficiency of DE can be improved. Many methods can be used in recovering waste heat of DE[2].Among them, the TEG can convert directly thermal energy into electric energy, which got more attention by many researchers [3,4].Although the TEG has many advantages, which makes the TEG has been used in recovering waste heat of DE, the low conversion efficiency limits the widely application. In fact, the conversion efficiency of TEG is mainly restricted by thermoelectric material. The ZT values of is change largely for wide temperature range[1]. The exhaust temperature of DE tested by our laboratory is about 523K when operating on low engine load and exceeds to 813K on high load. In general, the temperature of exhaust gas is above 600 K in DE [5,6]. The added ZT value of skutterudite and Bismuth

* Corresponding author. +86-13512277142. E-mail address: [email protected].

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi: 10.1016/j.egypro.2015.07.461 Hua Tian et al. / Energy Procedia 75 ( 2015 ) 590 – 596 591 telluride is higher that of each under the same range of temperature difference, when the exhaust of DE is as heat source. A segmented TEG has been suggested to recover waste heat of DE for improving the thermal efficiency in present paper. Many researched have been focus on the physical properties of segmented TEG [7-9]. However, it is nearly little used in waste heat recovery of DE.

2. Governing equations

Fig.1. shows that segmented TEG and traditional TEG. To simple the calculation progress, the performance of one thermocouple could be compared in present paper. The main purpose is to see that the performance of segmented TEG can be used in waste heat recovery of DE, which can be analyzed in the steady state of DE. Some assumptions are made as follows:(1)The heat conduction is flow along the direction of thermocouple leg. The heat conduction in the axial direction is omitted.(2) No contact thermal resistance and contact electric resistance.(3) Thomason and Fourier heat are ignored.(4) The temperature of heat and cold source are assumed as the surface temperature of two ends in thermocouple.

Fig.1. A structure of the theoretic model Fig.2. The comparison of simulated and tested results The junction temperature will be set, which is validated by following equations. The heat flow equation of P-leg thermoelectric material in high temperature region and low temperature region is, respectively: TT TT q O hJP q O JPc 11l 22l 1 2 ˄1˅ T h O TdT ³T 1 O JP 1 TT Where q1, hJP and l1 stand for the heat flux, average thermal conductivity and length of thermocouple, respectively. Th and Tc are assumed values in calculation process. Where Ȝ1(T) stand for the function that the thermal conductivities vary with temperature of two materials. According to the assumptions, the heat flux is equal in two materials of P-leg. So, the length rate of materials can be obtained from Eqs. (1): l O TT 11 hJP lTT 22O JPc ˄2˅

2.1. The output power and conversion efficiency

The internal resistances, average of P-leg (same to N-leg) is:

ll12 TThJP UU12 ldl ldl DD12 TdT TdT ³³00 ³³TTJP c RP D A P TT p hc (3) Where RP is the internal resistance of P-leg, ȡ1(l) is the function that the electrical conductivity varies with length of material in high temperature region, and Ap is the cross section area of P-leg. Į1(T),Į2(T), stand for the function that the Seebeck coefficients vary with temperature in high and low temperature 592 Hua Tian et al. / Energy Procedia 75 ( 2015 ) 590 – 596

thermoelectric materials of P- leg. So the open circuit voltage (U) and the current (I) of segmented RRR thermocouple can be expressed as equation(4), in which in P N : U I UTT ˄˅DDpnhc  R  R in L ˄4˅ Due to the temperature difference in two ends of ceramic, the absorbed (Qh) and released (Qc) heat can be expressed as: QTTK  QTTK  hhshh cccsc (5) According to the law of conservation of energy, the absorbed and released heats consist of peltier heat, thermal conduction and joule heat. So, Qh and Qc can be expressed as: QITKTTIR D  0.5 2 QITKTTIR D  0.5 2 hh hc in chcinc ˄6˅ Ths and Tcs can be got when the Th and Tc are the assumed values first. The Ths and Tcs are compared with the set values. If the rate is equal, the assumed Th and Tc are reasonable. Otherwise, Th and Tc need to reset and calculated again. K is the thermal conductance of segmented thermocouple. The output power and conversion efficiency is defined as follows, 2 (7) PQ hc QIR= L K P / Qh

2.2. Thermoelectric material properties

A segmented TEG is used in waste heat recovery of DE, with the exhaust gas and coolant as heat and cold source. The TEG is operated under high temperature heat source and large temperature difference conditions. Bismuth telluride is a favorable low-temperature thermoelectric material which has better property at low temperature (300-500K), while skutterudite is a good medium-temperature, which has higher ZT value from 500K to 800K [10]. Ba0.3In0.3FeCo3Sb12/Ba0.4In0.4Co4Sb12 (Skutterudite) are used as materials of P/N in high temperature region, Bi0.48Sb1.52Te3/Bi2Te2.7Se0.3 (Bismuth telluride) are the materials of P/N in low temperature region [11], which are also used as materials of traditional TEG. Table.1 lists the structure parameters of TEG.

2.3. Boundary conditions

The effects of relative factors have been analyzed on the performance of segmented TEG. Table.2 lists some values, which are assumed for simulating the performance of segmented TEG. The assumed temperature of exhaust gas and coolant lie in the range of reasonable. The maximum output power can be obtained when the external resistance is equal to the internal resistance [12]. So, the ratio of both is set 1. The length and cross section area of ceramic are based on the real thermoelectric module (TEP1-12656- 0.6, manufactured by thermonamic) assumed. In other subsequent analysis without special description, the parameters are taken from tables 1 and 2. Table.1 Parameters of PN materials Thermocouple Thermocouple Length rate of length (mm) cross section area (mm2) two materials 3 0.25 1:3/1:1/3:1

3. Model validation

A TEG module with 126 thermocouples was used in validation progress. The TEG module was simulated using mathematical model in paper. The simulated results are obtained by changing the temperature of heat and cold source. Fig. 2 shows comparison of the simulated results and tested results. Hua Tian et al. / Energy Procedia 75 ( 2015 ) 590 – 596 593

The simulated results can agree with the tested results. The maximum relative error is under 3.8%. So, the mathematical model is applicable to simulate for segmented TEG. Table.2 Some assumed parameters Exhaust gas temperature (K) 650 Cooling water temperature (K) 300 Length of ceramic (mm) 0.5 Cross section area of ceramic (mm2) 1 Ratio of external resistance to internal resistance 1

4. Results and discussion

4.1. The effect of heat source temperature and cold source temperature

Five types of thermoelectric generators can be compared in present paper, traditional TEG made of Bismuth telluride thermoelectric materials. segmented TEG made of skutterudite and bismuth telluride, the ratio of two materials are 1:3, 1:1 and 3:1, respectively, which can be named as segmented 1, segmented 2 and segmented 3. Fig.3 shows the maximum output power with temperature of heat source. The segmented TEG has a remarkable advantage when the temperature changes from 600 to 750 K. the more ratio of skutterdite in segmented TEG, the more output power with the increment of exhaust temperature. The trends of conversion efficiency are different from these of output power (Fig. 4). The conversion efficiency of traditional TEG is highest when the heat source temperature low 600 K. That of traditional TEG is lowest, which is made of skutterudite. The conversion efficiency of segmented1, 2 and 3 are higher than that of traditional TEG when the heat source temperature high 600, 650 and 700 K respectively. The output power and conversion efficiency of segmented 2 are higher 41.7%, 31.7% and 12.8%, 79% than those of traditional TEG made of bismuth telluride and skutterudite.

Fig.3. Dependence of maximum output power Fig.4. Dependence of conversion efficiency Fig.5.Dependence of maximum on cold on heat source temperature on heat source temperature source temperature The variation of performance with cold source temperature is shown in Fig.5. The maximum output power decreases with increment of cold source temperature. The output powers of segmented TEGs are higher than that of traditional TEGs when the cold source temperature below 400 K. However, the conversion efficiency of traditional TEG is higher than that of segmented 3 with the same range of temperature (Fig. 6). The output power and conversion efficiency of segmented TEGs 1 are higher 16.5%, 14.9% and 1.98%, 75.2% than those of traditional TEG made of bismuth telluride and skutterudite; 28.6%, 27% and 3.62%, 73% for the segmented 2; 29.8%, 28% and -8.2%, 53.2% for segmented 3 respectively, when the cold source temperature is 350 K.

4.2. The effect of length of thermocouple and cross section area of thermocouple 594 Hua Tian et al. / Energy Procedia 75 ( 2015 ) 590 – 596

The variation of performance with length of thermocouple is shown in Fig.7, which can be compared in different heat source temperature. The maximum output power decreases with increment of thermocouple length, which is irrelative with heat source temperature. The segmented TEGs have more output power than traditional TEG when the length of thermocouple is higher 1.0 mm, which can be calculated when the heat source temperature is less 700 K. the maximum output power of skutterudite traditional TEG increases with the increment heat source temperature, which can be higher than that of segmented 1 when the length of thermocouple is more 0.5 mm. The maximum output power of segmented 3 is more than others when the heat source temperature high 700 K, which are 1.12 W, 1.42 W and 2.35 W and more 33.8%, 29.2% and 25% than those of skutterudite traditional TEG when the thermocouple length is 3 mm. Fig. 8 shows the conversion efficiency of TEGs with increment of thermocouple length. The conversion efficiency increases with the increment of thermocouple length. The conversion efficiency of segmented 2 is more than others when the heat source temperature high 700 K, which are more 12.8%, 27.8% and 44.4% than those of bismuth telluride traditional TEG when the thermocouple length is 3 mm.

Fig.6. Dependence of conversion efficiency Fig.7. Dependence of maximum output power on thermocouple length on cold source temperature at (a)Ths=650 K. (b) Ths=700 K.

Fig.8. Dependence of conversion efficiency on thermocouple length at (a)Ths=650 K. (b) Ths=700 K.

Fig.9. Dependence of maximum output power on thermocouple cross section at (a)Ths=650 K. (b) Ths=700 K. The variation of performance with cross section area of thermocouple is shown in Fig.9, which can be compared in different heat source temperature. The maximum output power increases with increment of cross section area, which is irrelative with heat source temperature. The maximum output power of segmented TEGs is higher than that of traditional TEG when the cross section area is larger 9 mm2. And, Hua Tian et al. / Energy Procedia 75 ( 2015 ) 590 – 596 595 this difference value will be increases with increment of cross section area. Fig. 10 shows the conversion efficiency of TEGs with increment of cross section area. The conversion efficiency of TEGs has no variations when the cross section area changes from 1 to 81 mm2. The conversion efficiency of segmented TEG is more than that of traditional TEG except 650 K. the performance of segmented 2 has better than others with increment of heat source temperature.

Fig.10. Dependence of conversion efficiency on thermocouple cross section area at (a)Ths=650 K. (b) Ths=700 K.

5. Conclusions

The performance of TEG has been effect by the temperature of heat and cold source. The heat source temperature is a main factor on comparing the performance of segmented and traditional TEG with the range of coolant temperature in DE. The trends of maximum output power and conversion efficiency are contrary with increment the length of thermocouple. The trend of maximum output power is linear with increment of cross section area, but the conversion efficiency is constant. The performance of segmented TEG is better than that of traditional TEG for the exhaust gas as heat source and coolant as cold source. Of course, the ratio of materials should be designed reasonable. Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB707201), the authors gratefully acknowledge them for support of this work. References: [1]J. Yang.Potential applications of thermoelectric waste heat recovery in the automotive industry.Thermoelectrics, 2005. ICT 2005. 24th International Conference on, 2005, p.170-174. [2]X. Liang, X. Sun, G. Shu, K. Sun, X. Wang, X. Wang. Using the analytic network process (ANP) to determine method of waste energy recovery from engine.Energ Convers Manage, 2013;66:304-311. [3]S.B. Riffat, X. Ma, Thermoelectrics. a review of present and potential applications.Appl Therm Eng, 2003;23: 913-935. [4]A. Patyk.Thermoelectric generators for efficiency improvement of power generation by motor generators – Environmental and economic perspectives.Appl Energ, 2013;102:1448-1457. [5]H.G. Zhang, E.H. Wang, B.Y. Fan. A performance analysis of a novel system of a dual loop bottoming organic Rankine cycle (ORC) with a light-duty diesel engine.Appl Energ, 2013;102:1504-1513. [6]V. Dolz, R. Novella, A. Garcia, J. Sanchez.HD Diesel engine equipped with a bottoming Rankine cycle as a waste heat recovery system. Part 1: Study and analysis of the waste heat energy. Appl Therm Eng, 2012;36: 269-278. [7]W.S. El-Genk, H.H. Saber. High efficiency segmented thermoelectric unicouple for operation between 973 and 300 K, Energ Convers Manage, 2003;44:1069-1088. [8]L.N. Vikhor, L.I. Anatychuk. Generator modules of segmented thermoelements.Energ Convers Manage, 2009;50:2366-2372. [9]C. Hadjistassou, E. Kyriakides, J. Georgiou, Designing high efficiency segmented thermoelectric generators, Energ Convers Manage, 2013;66:165-172. [10]J. Xiao, T. Yang, P. Li, P. Zhai, Q. Zhang.Thermal design and management for performance optimization of solar thermoelectric generator.Appl Energ, 2012;93:33-38. [11]W. Zhao, H. Zhou. The Design and Properties of Bi2Te3/CoSb3 thermoelectric generator with wide temperature range,Wuhan University of Technology, 2012, p.82. [12]S. Kim.Analysis and modeling of effective temperature differences and electrical parameters of thermoelectric generators.Appl Energ, 2013;102:1458-1463. 596 Hua Tian et al. / Energy Procedia 75 ( 2015 ) 590 – 596

Biography LIANG Xingyu received his MS from Shenyang Institute of Chemical Technology, China, in 2003, and his PhD from Department of Thermal Energy Engineering, Tianjin University, China, in 2006. He is currently an associate professor at State Key Laboratory of Engines, Tianjin University, China. His research interests include modern design of engines and waste heat recovery.