发表文章毕业论文201709-Cao Qimin-Applied thermal engineering-Perance enhancement of heat pipes assisted thermoelectric generator for automobile exhaust heat r

Research Paper Performance enhancement of heat pipes assisted thermoelectric generator for automobile exhaust heat recovery Qimin Cao a, Weiling Luana, Tongcai Wangb aKey Laboratory of Pressure System and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, PR China bTechnology and Engineering Center for Space Utilization, Chinese Academy of Sciences, Beijing 100094, PR China h i g h l i g h t s ?A heat pipes assisted thermoelectric generator for exhaust heat recovery was proposed. ?The heat pipe insertion depth, the angle between the heat pipe row and the gas discharge direction were confi rmed. ?The output performance of thermoelectric generator were investigated. g r a p h i c a l a b s t r a c t a r t i c l ei n f o Article history: Received 19 August 2017 Revised 23 September 2017 Accepted 26 September 2017 Available online 30 September 2017 Keywords: Heat pipe Thermoelectric generator Automobile exhaust Thermoelectric module a b s t r a c t A new type of heat pipe assisted thermoelectric generator (HP-TEG) for automobile exhaust waste heat recovery is proposed in this paper. To confi rm the optimized confi gurations of the thermoelectric device, an experimental prototype was constructed to obtain the best heat pipes insertion depth (Dhp), and the optimum angle between the heat pipe row and the gas fl ow direction (hhp). A HP-TEG device was designed, constructed after the optimization of heat pipes depth and angle. The operating parameters were further investigated to enhance the performance of thermoelectric generation (TEG). The experi- mental results show that the power output of HP-TEG preferred high exhaust temperature, cold water fl ow rate and mass fl ow rate. The maximum open circuit voltage of 36 thermoelectric modules (TEMs) is measured as 81.09 V. The corresponding power output and pressure drop are 13.08 W and 1657 Pa, respectively, with an optimized thermoelectric power generation effi ciency of the HP-TEG as 2.58%. ? 2017 Elsevier Ltd. All rights reserved. 1. Introduction Nowadays, the rapid development of the automotive market has exacerbated the energy crisis problem. Currently, the average effi ciency of internal combustion engines (ICEs) under driving con- ditions is 25% 1. About 40% of the energy is discharged into the atmosphere through the exhaust, thus reusing the automobile waste heat becomes signifi cantly urgent 2,3. TEMs can directly convert this part of energy to electricity, and have advantages of no moving part, no noise, scalability and being environmentally friendly 4,5. The TEM makes use of what is known as Seebeck effect. It is made up of many N type and P type semiconductor materials which are connected electrically in series but thermally in parallel. Their effi ciency is typically 5% 6. Currently, the most widely used thermoelectric material is Bi2Ti3. Because it has been commercialized, and the cost is low compared to other ther- moelectric materials. However, due to the low thermoelectric /10.1016/j.applthermaleng.2017.09.134 1359-4311/? 2017 Elsevier Ltd. All rights reserved. Corresponding author. E-mail address: luan (W. Luan). Applied Thermal Engineering 130 (2018) 14721479 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: materials performance and poor thermoelectric devices confi gura- tion, the effi ciency of thermoelectric generators is low. To improve the effi ciency of the TEG, optimizing thermoelectric devices struc- ture is a feasible way under current thermoelectric material condi- tions 7. According to the former researches, the using of TEMs can convert waste heat to electric power, thus improve the effi ciency of ICE 811. Thacher et al. 12 built a thermoelectric generator by using a 1080 carbon steel, indicating that decreasing of coolant water can signifi cantly increase the conversion effi ciency of ther- moelectric generator. Crane et al. 13 designed a cylindrical struc- ture with stainless steel clad copper and internal folded fi ns to improve heat transfer rate. The numerical model was built by the design parameters to validate an empirical equation based model and experiment. Liu et al. 14 built a thermoelectric device to recover the waste heat of automobile exhaust. They used exhaust as heat source, and cooled the TEM by water. The maximum output obtained as 944 W in the tests. Thermoelectric generators effi ciency can be also improved through the application of heat transfer enhanced devices. Because of the low thermal resistance of heat pipe, it is widely applied to exhaust waste heat 15. Heat pipes will make the TEM surface temperature be closer to the exhaust temperature when fi ns are employed in the gas fl ow channel. The waste heat recovery system with heat pipes has a series of benefi ts. There are also some studies about HP-TEG according to available literatures. Liu et al. 16 con- structed a HP-TEG system, using a kind of fl at heat pipe with par- allel micro channels. The optimal number of thermoelectric couples was analyzed under design parameters, which showed that the power output increased with inlet air temperature of heat pipes raised. Kim et al. 17 designed a HP-TEGs system for waste heat recovery. In this device, the heat pipes absorbed heat from exhaust and conducted to the aluminum block. TEMs were attached to the aluminum block and cooled by a water cooled sink. 112 TEMs were used in this device. The maximum power output of 350 W was generated in this system. Orr et al. 18 set up an air- cooled exhaust waste heat recovery system using heat pipes to transfer heat to the TEM surface. Eight TEMs were used in this sys- tem. The power output was 6.03 W, which was used to charge the battery. The thermal to electric effi ciency and Carnot effi ciency were 1.43% and 21.48%, respectively. A lot of HP-TEG systems were built in the previous studies. But for enhancing heat transfer, few research focused on the design of the specifi c parameters of the heat pipe distribution. The present work designed a thermoelectric device with heat pipes. 16 round copper heat pipes were employed in this system, with eight heat pipes arranged in a row and the water-cooled cold plates. The Dhpand hhpwere investigated in this study. The results demon- strated that the device had the highest heat transfer rate with the Dhpof 60 mm and the hhpof 15 ?C. For the optimized HP-TEG system, the open circuit voltage and maximum output power of thermoelectric generator were signifi cantly improved with the employment of heat pipes. 2. Experiment setup To study the detailed parameters of thermoelectric generator system, two experimental prototypes were built. Firstly, an exper- imental prototype of TEGs with heat pipes was built to confi rm the best Dhpand hhp. As shown in Fig. 1, four heat pipes were inserted in stainless steel fl ow channel. The size of the exhaust fl ow channel was 240 mm (length) ? 120 mm (width) ? 60 mm (height) ? 1 mm (thickness). Two TEMs were attached to aluminum block sur- face and cooled by coolant, the rectangular wing coolant channel with the dimensions of 190 mm ? 50 mm ? 10 mm was made from aluminum alloy. In this study, the Bi2Ti3TEM (TEP1- 142T300) were sandwiched between alternating hot and cold layer. This type of TEM contains 127 thermocouples and with the approximate dimensions 40 mm ? 40 mm ? 4 mm. The applica- tion heat pipes were copper moderate temperature. Dowtherm was adopted as work fl uid because of the high exhaust tempera- ture. The heat pipes have mesh wick structure, which have advan- tages of simple structure, easy processing and low cost. Heat pipes with the external diameter of 8 mm and length of 120 mm were employed in this paper. For the experimental prototype, the Dhp and hhpwere investigated. Based on the results of Dhpand hhp, a thermoelectric generator system was built. The specifi c structure of HP-TEG is shown in Fig. 2, the main components of the device are exhaust fl ow channel, four wing coolant channels, two main coolant channels, heat pipes, rectangular fi ns and TEMs. The dimensions of exhaust fl ow channel and wing coolant channel are same as those in the previous experimental setup. The alu- minum alloy main cooling channel has a size of 220 mm ? 120 mm ? 10 mm. 36 pieces of TEMs were used in this system. Sixteen pieces of TEMs were installed in two plates of heat pipes, the rest were installed in both sides of exhaust fl ow channel. Sixteen heat pipes at a distance of 8 mm were evenly distributed in two heat pipes plates. The aluminum alloy rectangular fi ns with the dimen- sions of 150 mm ? 30 mm ? 1 mm was designed to enhance heat transfer. Two sets of rectangular fi nned heat pipes that are attached to aluminum blocks. Between the TEMs and the exhaust fl ow channel, the graphite paper was employed to improve the heat transfer rate. Moreover, the bolts were used to minimize the internal resistance of heat transfer device and TEMs. The total weight of the device is about 4.31 Kg (see Fig. 3). The details of experimental HP-TEG system are illustrated in Fig. 4. In this system, the exhaust gas was simulated by hot air. The electric heater was used to heat the incoming air from the sur- roundings. The air was supplied by using an air blower. Cold water was used as coolant, the inlet temperature of the cooling water was Nomenclature Abbreviation HP-TEGheat pipe assisted thermoelectric generator TEGthermoelectric generator TEMthermoelectric module ICEsinternal combustion engines Symbols Dhpheat pipe insertion depth mm hhpangle between the heat pipe row and the gas discharge direction ? DTtemperature different ?C _ Qcold waterheat transfer rate of the cold water W _ Qinheat transfer ratio to the thermoelectric modules hot side W Poutmaximum output power of thermoelectric generator W _m cold water mass fl ow rate Kg=h Cp specifi c heat at constant pressure J/(Kg ?C) q,cold water density Kg/m3 _ V cold water fl ow rate L=h gteg power generation effi ciency % Q. Cao et al./Applied Thermal Engineering 130 (2018) 147214791473 about 18.4 ?C. The fl ow rate of the air and outlet of air blower to control the pressure were mounted by back pressure and global value. The fl ow rates of cold water were measured by rotor fl ow meters. K-type thermal couples were used to measure tempera- tures of cold water and hot air. Finally, the voltage, electric current and electric resistance values were collected by data acquisition system (Agilent 34970A). 3. Results and discussion 3.1. The determination of the Dhpand hhp Heat pipes, which consist of evaporating, adiabatic, and con- densing sections, were used in this thermoelectric generator. As the pressure inside the pipe is approximate vacuum, the working liquid vapors at low temperatures and transfers heat from evapo- rating section to condensing section. In order to take advantages of heat pipes and improve the heat transfer ability of the HP-TEG system, an experimental prototype was built and tested to confi rm the Dhpand hhpbefore the system design. There are two steps in this experiment. Firstly, the hhpwere changed in a certain Dhp. After that, the best Dhp was investigated at the fi xed angle. The cold water fl ow rates and hot air inlet temperature were set at 40 L=h and 200 ?C, respectively. The TEMs were connected in series. The experiment results are shown in Figs. 4 and 5. As shown in Fig. 4, the open circuit voltage and the maximum output power increased signifi cantly when the Dhpwas less than 60 mm. However, when the Dhpexceeded 60 mm, the increasing trend of output power became slow. It was increased less than 5%. It can be interpreted that the length of evaporator section enlarged with the rising of insertion depth. Nevertheless, the length of evaporator section will increase unconspicuous when the Dhpis over 60 mm. It can be also seen that the power output with fi ns was higher than these without fi ns. At the Dhpof 60 mm, the open circuit voltage with fi ns was 5.21 V, about 43% higher than the results without fi ns. The maximum output power of each TEM was about 0.38 W, which increased nearly 105% with the employment of fi ns. According to circuit theory, when road resistance is equal to inner resistance, the output power of the sys- tem will go up to the maximum value 19. As can be seen from Fig. 5, the open circuit voltage and maximum output power increased with the rise of hhp. It can be explained that when the hhpincreased, the contact area between the hot air and the heat pipe rose. In this sense, the heat exchange rate was improved suf- fi ciently. The power output at the hhpof 15? was higher than the output at the hhpof 10?. The maximum output power of one single TEM was 0.411 W, which is about 10% higher than that of the angle of 10?. Furthermore, when the hhpexceeded 15?, the output will not change obviously. The Dhpand hhp were demonstrated that had infl uences on HP- TEGs power output. To further enhance the heat transfer rate, the Dhpand hhpshould be at least 60 mm and 15?, respectively. How- ever, in order to control the size of the device, 60 mm depth and 15? angle were selected to design the HP-TEG device. 3.2. Advantages of the heat pipe assisted thermoelectric power generation system Heat pipes have high thermal conductivity, which can transfer heat with a small thermal resistance and over a signifi cant dis- tance. The thermal conductivity of heat pipes can be even higher than copper. In order to demonstrate the advantages of heat pipes applications, the comparative experiment of TEG with heat pipes and without heat pipes were carried out. The same numbers of TEMs were used in this two devices. The output effi ciency of HP-TEG mainly depends on two key variables: the temperature and the mass fl ow rate of exhaust. In this experiment, the power output of the devices with different mass fl ow rate and exhaust temperature were investigated. Fig. 1. The schematic of the experimental prototype by heat pipes. 1474Q. Cao et al./Applied Thermal Engineering 130 (2018) 14721479 Figs. 6 and 7 show the comparison of open circuit voltage and max- imum output power of the TEG at different operating parameters. It can be seen that the open circuit voltage and maximum output power increased with the increase of temperature and mass fl ow rate of exhaust augment. This is because the increase of hot air temperature and mass fl ow rate will come up the temperature dif- ference between both sides of TEMs. At the same time, the high temperature difference will benefi t to the output performance of TEG. The best working temperature of Bi2Ti3TEM is between 150 ?C and 220 ?C, so the power output of thermoelectric generator increased the most signifi cant when the exhaust temperature went up from 150 ?C to 250 ?C. It can be seen from Fig. 6, when the mass fl ow rate was less than 120 Kg/h, the output voltage of the TEG without heat pipe was slightly larger than that with heat pipes application. With the increase of the mass fl ow rate, the output voltage of the HP-TEG increased more obviously. Under the opera- tion condition at maximum exhaust temperature of 300 ?C and cold water fl ow rate of 40 L=h, the maximum open circuit voltage of HP-TEG reached to 81.09 V. It increased by nearly 7.5% than the output of TEG without heat pipe. The power density expresses the average output power per unit area, which is an important parameter to measure the performance of TEG. As can be seen from Fig. 7, with the increase of exhaust temperature and mass fl ow rate, the maximum output power and power density have the same trend as the open circuit voltage. The maximum output power of HP-TEG and TEG without heat pipes were 13.08 W and 11.34 W, respectively, and their power densities were 229.80 W=m2and 198.98 W=m2. The maximum Fig. 2. Schematic of the HP-TEG structure: (a) top view, (b) front view, (c) experimental picture of HP-TEG. Q. Cao et al./Applied Thermal Engineering 130 (2018) 147214791475 power output and power density of HP-TEG were 10.17% and 15.49% greater than the TEG without heat pipes, respectively. It can be concluded that the TEG with heat pipes has better out- put performance than that without heat pipes. This is due to the high thermal conductivity coeffi cient and low thermal resistance of the application heat pipes. When the temperature of the heat and cold source are fi xed, the temperature difference between two sides of TEM will increase by using heat pipes. Furthermore, when fi ns are employed, heat pipes can be used to increase the fi n effi ciency which consequently lower the thermal resistance between the TEM and gases. 3.3. Thermoelectric power generation performance 3.3.1. Infl uence of cold water fl ow rate Cold water fl ow rate is an essential operating parameter for the TEG optimization. Figs. 8 and 9 present the open circuit voltage and Fig. 3. Schematic of the experimental thermoelectric waste heat recovery system. Fig. 4. Experiment results for open circuit voltage and maximum output power of experiment (a, b) prototype. (In the case of hhpis 10?). Fig. 5. Open circuit voltage and maximum power output of experiment prototype with fi ns. (In the case of Dhpis 60 mm). 1476Q. Cao et al./Applied Thermal Engineering 130 (2018) 14721479 maximum output power in different exhaust temperature with the increase of cold water fl ow rate. In this experiment, the mass fl ow rate remained stable at 180 Kg/h. It was shown that both open cir- cuit voltage and maximum output power went up with the rising of cold water fl ow rate. The increases of cold water fl ow rate could enlarge the temperature difference between TEMs cold and hot sides. Exhaust temperature is also a very important parameter for HP-TEG. When the exhaust temperature is higher, more heat is transferred to the hot side of TEMs.