Simulation of Fuel Economy Effectiveness of Exhaust Heat Recovery System Using Thermoelectric Generator in a Series Hybrid 2011-01-1335 Published 04/12/2011 Masayoshi Mori, Takeshi Yamagami , Mitsumasa Sorazawa , Takatoshi Miyabe , Shunji Takahashi and Tomohide Haraguchi Honda R&D Co., Ltd. Copyright © 2011 SAE International doi:10.4271/2011-01-1335 ABSTRACT Simulation was employed to estimate the fuel economy enhancement from the application of an exhaust heat recovery system using a thermoelectric generator (TEG) in a series hybrid. The properties of the thermoelectric elements were obtained by self-assessment and set as the conditions for estimating the fuel economy. It was concluded that applying exhaust system insulation and forming the appropriate combination of elements with differing temperature properties inside the TEG could yield an enhancement of about 3% in fuel economy. An actual vehicle was also used to verify the calculation elements in the fuel economy simulation, and their reliability was confirmed. INTRODUCTION Methods for optimizing TEG specifications have begun to be proposed in recent years with a view to TEG application in automobiles [1,2]. One such method that has been proposed is simulation capable of optimizing the TEG specifications so that it will yield maximum power or maximum efficiency [2]. When installing exhaust heat recovery systems employing TEG in automobiles, not only are factors that enhance fuel economy engendered by power generation, but at the same time, factors that result in deterioration in fuel economy, such as increased pressure loss in the exhaust system and increased vehicle weight, are also created. Obtaining the optimum fuel economy enhancement requires configuring the system specifications so that these factors are in optimal balance. It is not necessarily the case that optimization of TEG output or efficiency will yield the optimal fuel economy benefit. 1268 SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 For that purpose, it was decided that the object to consider for optimization should be expanded from the TEG alone to the vehicle system. Not only was a simulation created to calculate fuel economy effectiveness with positive and negative factors regarding fuel economy taken into account, but a scheme was also created to optimize the system as installed on a vehicle [1]. A method of use was also proposed to further enhance the fuel economy by installing the system on series hybrid vehicles that have higher exhaust gas temperatures and more stable exhaust gas flow rate than ordinary gasoline vehicles [3]. Meanwhile, measures to insulate the exhaust ports, exhaust manifold, and other such parts so as to reduce dissipation heat loss and increase the amount of heat input to the waste heat recovery unit were also proposed as methods of increasing heat recovery efficiency [4]. For this report, specific series hybrid specifications and engine operating conditions were assumed and fuel economy simulation was employed to estimate the fuel economy enhancement from application of the TEG to the assumed series hybrid. The reduction in fuel consumption that can be obtained by applying insulation to the exhaust system was also examined. The estimation was made by subjecting the properties of multiple sample thermoelectric elements with differing temperature characteristics to self-assessment, and choosing from among them those elements with higher conversion efficiency in each temperature range. Those properties were then taken as the set conditions for the calculation. In order to determine more effective methods for utilizing the thermoelectric elements, estimates were produced for the various cases that arise when the elements selected for each temperature range are used independently and when they are used in combination with elements that have differing temperature ranges. An actual vehicle was also used to conduct accuracy verification of the relationship between pressure loss and the power generation output of the TEG, as well as weight, as computed from the calculation elements in the fuel economy simulation, and the reliability of the calculation elements was evaluated. various elements assessed, those found to have the optimal properties in each category were adopted. Table 1. Assumed specification of series hybrid SET CONDITIONS FOR FUEL ECONOMY SIMULATION In estimating the TEG fuel economy enhancement with a series hybrid as the set conditions, the specifications in Table 1 were assumed. The exhaust temperature at the TEG inlet was set to two types of configuration, one with no particular means of insulation applied and the other with double-layered piping used to insulate the exhaust system. Regarding the effectiveness of insulating the exhaust system, the results of verification tests of insulation effectiveness in exhaust systems conducted in the past were referred to, and it was assumed that an increase in exhaust gas temperature of 120°C could be expected relative to the TEG inlet temperature when no insulation is applied. Regarding the engine operating conditions, two sets of conditions were configured, one a low-power operation with low brake specific fuel consumption (BSFC), and the other a high-power operation with somewhat higher BSFC. The high-power operation is used when a high-load manner of driving is required in which the necessary energy cannot be supplied by the low-power operation. The assumption of the fuel economy simulation carried out here was that priority would be assigned to use of the low-power operation, in response to the driving load, while switching over to the high-power operation as necessary in order to obtain the electric power necessary for operating the vehicle. Figure 1 shows the operating points of low-power operation (Low) and of high-power operation (High) in the BSFC map. The operating point for the highpower operation was selected to yield approximately two times the power of the low-power operation while holding the increase in BSFC down to a minimum. Table 2 shows the operating times of high-power and low-power operation and engine stop time when operating with the assumed engine on US fuel economy driving cycles. Figures 2 and 3 show the respective performance of P-type and N-type elements, set as the conditions for calculating the TEG fuel economy benefit. The thermoelectric elements can be classified under the three general categories of lowtemperature type elements that have high performance at 200°C or below, middle-temperature type elements that demonstrate high performance at around 350°C, and hightemperature type elements that have high performance at around 450°C. Multiple element samples were obtained for the various element categories and self-assessment of the performance of those elements was performed. Among the SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 Figure 1. BSFC map of engine adopted for simulation Table 2. Assumed engine operation schedule 1269 Figure 2. Figure of merit of P type thermoelectric element takes as inputs the generated power, increase in exhaust pressure loss, and weight increase that are obtained from the three upstream calculations, and then uses the fuel economy sensitivity of those factors obtained using the actual vehicle to convert them into fuel economy [1]. The calculation accuracy in the heat exchanger performance calculation, power generating performance calculation, and system weight calculation described above was verified on an actual vehicle. Since the series hybrid assumed as the object for the present fuel economy simulation does not actually exist, the verification was performed using an existing vehicle as a substitute. A simulation conducted in advance was used to predict the state with TEG installed, and then the accuracy of those results was verified by comparing them with the measurement results obtained from the actual vehicle with TEG installed. Table 3 shows the test conditions used in verifying the accuracy of the calculation elements of the fuel economy simulation. The test employed a current production model CIVIC that was placed on a chassis dynamo where loads equivalent to actual driving were applied and measurements of the steady states at multiple vehicle speeds were taken. Table 4 shows the actual measurement results for flow and temperature of the exhaust gas and coolant supplied to the TEG as taken at the measurement points. Figure 5 shows the structure of the TEG used in verifying accuracy, and Fig. 6 shows the TEG as installed. The testing was conducted with the TEG placed in the center tunnel under the floor and downstream from the catalyst. Table 3. Test condition Figure 3. Figure of merit of N type thermoelectric element SET CONDITIONS FOR ACCURACY VERIFICATION OF FUEL ECONOMY SIMULATION CALCULATION ELEMENTS Figure 4 shows the composition of the fuel economy simulation process. The fuel economy simulation is composed of three basic calculation elements, which are heat exchanger performance, power generating performance, and system weight. The heat exchanger performance calculation, which is highest upstream, carries out computation for certain exhaust gas conditions to determine what heat exchanger involves what extent of pressure loss, in order to calculate what extent of heat flux density will be possible. The power generating performance calculation is carried out to determine what extent of power generation is possible using that heat. The system weight calculation estimates how much the resulting TEG will weigh. The fuel economy simulation 1270 SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 Table 4. Measured inlet condition of TEG FUEL ECONOMY SIMULATION RESULTS Figure 7 shows the generated power obtained by the TEG using low-temperature type, middle-temperature type, and high-temperature type elements, with each type used alone as Figure 4. Compositions of fuel economy calculation well as in combinations of low- and high-temperature type elements. It also shows the calculated average fuel economy enhancement under LA4 City/Highway (Urban Dynamometer Driving Schedule and Highway Fuel Economy Dynamometer Driving Schedule) and US06 (Supplemental Federal Test Procedure), in every case both with and without exhaust system insulation. EFFECT OF ELEMENT TYPE ON GENERATED POWER AND FUEL ECONOMY ENHANCEMENT First, the effect of the element type on generated power and fuel economy enhancement was examined. When using the elements alone, the use of elements that perform at higher temperatures can be expected to yield a greater increase in power and a greater fuel economy enhancement. The efficiency of thermoelectric elements in converting heat to electricity can be calculated using equation (1). (1) Attention to the figure of merit ZT will show that when the performance of the P-type and N-type elements shown in Figs. 2 and 3 is averaged, the figures are greater for lowSAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 temperature and middle-temperature types than for hightemperature types. However, the material properties of the low-temperature and middle-temperature types are such that, compared to the high-temperature type, they are unable to maintain a high temperature on the heated side of the element (Th), so that the Carnot efficiency shown in the first term of equation (1) is lower. This depresses the efficiency of conversion into electricity (ηMat) so that the high-temperature type element obtains higher generated power and greater fuel economy enhancement. EFFECT OF EXHAUST SYSTEM INSULATION ON GENERATED POWER AND FUEL ECONOMY ENHANCEMENT Next, the effect on generated power and fuel economy enhancement due to the presence or absence of insulation will be examined. There are constraints on the thermoelectric elements due to their physical properties, and the heat exchanger specifications and element shapes are therefore adjusted so that the temperature on the heated side of the elements will not change because of the presence or absence of insulation. Consequently, the efficiency of conversion into electricity by the thermoelectric elements remains almost entirely unchanged in the various types of element. As shown in Fig. 7, however, the application of insulation results in an increase in generated power and fuel economy enhancement relative to when insulation is not applied. That enhancement occurs at a more conspicuous rate in the high-temperature type elements compared to the low-temperature type elements. The increase in generated power and fuel economy enhancement takes place because the insulation has increased the heat input (Q) to the TEG in equation (2), which 1271 Figure 5. Structure of TEG expresses generated power, and the enhancement is conspicuous in the high-temperature type elements because the rate of increase in the heat input from before insulation to after insulation is greater in the high-temperature type elements than it is in the low-temperature type elements. (2) As shown in Table 5, the heat exchanger with hightemperature type elements in the optimally designed TEG has a smaller fin pitch than the heat exchanger with lowtemperature type elements and so it has greater heat transfer performance. The reason for this is that the high-temperature 1272 SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 type elements generate more power than the low-temperature type elements and this power can compensate for the deterioration in efficiency of the internal combustion engine resulting from TEG pressure loss. This kind of heat exchanger performance differential results in an increase in the capacity of the TEG to absorb the greater heat of the exhaust gas, in which the temperature is raised higher because of the insulation. That increase in the heat input in the high-temperature type elements, which have higher heat transfer performance, is about 100 W greater than in the lowtemperature type elements. The high-temperature type elements also need to maintain the element temperature at a higher level compared to the low-temperature type elements because, as shown in Fig. 2 and Fig. 3, their power generating efficiency is lower in the low element temperature range. Consequently, the TEG exhaust gas outlet temperature is higher with the high-temperature type elements than with the Figure 6. Installation image of TEG Figure 7. Results of generated power and fuel economy enhancement low-temperature type elements, and the heat input to the high-temperature type element TEG is smaller than to the low-temperature type element TEG. The rate of increase in the heat input to the various elements after application of insulation will be relatively greater in the high-temperature type elements, which have smaller heat input to the TEG before insulation is applied and which have a greater increase in the heat input after insulation is applied, than in the lowtemperature type elements, which have the opposite tendency. SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 1273 Table5. Optimized heat exchanger specification and thermal conditions of TEG (Low-power operation) RESULTS FROM VERIFICATION OF ACCURACY OF CALCULATION ELEMENTS FOR FUEL ECONOMY SIMULATION Figure 8 shows the results from verification of the heat exchanger performance calculations. The relationships of the heat flux density of heat recovered by the heat exchanger to the pressure losses that occur before and after the heat exchanger were compared for the actual measurements and the simulation. The predictions made in the simulation largely agreed with the results of actual measurement. EFFECT OF COMBINATIONS OF ELEMENTS WITH DIFFERING TEMPERATURE PROPERTIES ON GENERATED POWER AND FUEL ECONOMY ENHANCEMENT As described above, the high-temperature type elements can be expected to have greater recovery performance due to their higher Carnot efficiency. There are issues, however, in that the TEG exhaust gas outlet temperature is higher, more heat is disposed of without being recovered by the TEG, and the amount of heat input to the TEG is smaller. An effective approach to overcoming these issues is to place lowtemperature type elements, which perform even at low gas temperatures, downstream from the high-temperature elements inside the TEG, increasing the heat input to the TEG. This combined use of high-temperature and lowtemperature type elements can increase recovery efficiency by around 30% compared to when the high-temperature type elements are used alone, as shown in Fig. 7. In terms of performance, the application of thermoelectric elements subjected to self-assessment, the application of insulation in the exhaust system, and the appropriate combination of hightemperature and low-temperature type elements in the TEG can be expected to yield a fuel economy enhancement of around 3%. Apart from the method of combining the appropriate elements upstream and downstream in the TEG, it has also been proposed that a segmentation and cascading method be used in which the heated side of the element has high-temperature type elements arranged in an overlapping manner, and the cooled side of the element has lowtemperature type elements [5]. As distinct from the method of combining the proper elements upstream and downstream in the TEG, the application of this segmentation and cascading method to upstream elements can be expected to yield still greater increases in generated power and greater fuel economy enhancement. 1274 SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 Figures 9 and 10 show the results of verification of the thermoelectric element temperature in power generating performance calculation. Figure 9 shows the average temperature for the heated side of the elements shown in Fig. 11 for the entire 32 modules. Figure 10 likewise shows the average values for the temperature difference between the heated side and the cooled side. Both show a positive agreement between measurement and simulation. Figure 12 shows the electromotive force generated between the anode and cathode shown in Fig. 11, integrated for the entire modules. Figure 13 shows the results for the electromotive force converted into the module's generated power. The conversion to power output was derived from the results of precise measurements of the generated power relative to the temperature difference that occurred in the module prepared separately using specialized instruments. In both the electromotive force and the generated power, the simulation shows a positive reproduction of the actual device. Figures 9 and 10 also indicate a match between the simulation and actual measurements of the temperature at every location, derived from the power generating performance calculation process. Finally, Table 6 shows the results for verification of the system weight calculations. The TEG prototype made for this research, as shown in Fig. 6, was designed to allow various different measurements to be made. Its volume is therefore about two times that of a TEG not made for instrumentation. When this difference in volume is taken into account, the weight difference seen here between the estimate and measurement is generally appropriate. The weight derived from calculations is close to the weight of a TEG that is envisioned for practical use and not for the purpose of measurement. The validity of the three calculation elements of heat exchanger performance calculation, power generating performance calculation, and system weight calculation in the fuel economy simulation has been verified. Therefore, the approximately 3% fuel economy enhancement achieved by the TEG as calculated in the fuel economy simulation is credible. Figure 8. Comparison of heat exchanger performance Figure 11. Structure and measured properties of thermoelectric module Figure 9. Comparison of thermoelectric element temperature at heated side Figure 12. Comparison of electromotive force Figure 10. Comparison of temperature difference created to thermoelectric element Figure 13. Comparison of converted generated power SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1 1275 Table6. Comparison of TEG weight and volume CONTACT INFORMATION Masayoshi Mori Honda R&D Co., Ltd. Automobile R&D Center [email protected] SUMMARY/CONCLUSIONS The reduction in fuel consumption that would occur with a TEG installed in a series hybrid and using thermoelectric elements with the performance shown in self-assessment was estimated, leading to the conclusion that about 3% fuel economy enhancement could be expected. By using hightemperature type elements on the upstream side of the TEG, where exhaust gas temperatures are high and low-temperature type elements on the downstream side, where the temperatures are lower, the recovery efficiency was increased by about 30% compared to when high-temperature elements are used alone. The appropriate combination of elements upstream and downstream in the TEG was effective in fuel economy enhancement. The application of insulation in the exhaust system yielded a fuel economy enhancement, particularly in the high-temperature type elements where heat input can be effectively increased. A verification of the accuracy of the calculation elements in the fuel economy simulation was also conducted and the credibility of those elements was confirmed. REFERENCES 1. Mori, M., Yamagami, T., Oda, N., Hattori, M. et al., “Current Possibilities of Thermoelectric Technology Relative to Fuel Economy,” SAE Technical Paper 2009-01-0170, 2009, doi:10.4271/2009-01-0170. 2. Crane, D. T., “An introduction to system level steady-state and transient modeling and optimization of high power density thermoelectric generator devices made of segmented thermoelectric elements”, 29th international conference on thermoelectrics, abstracts book ICT 2010, p61 Takeshi Yamagami Honda R&D Co., Ltd. Automobile R&D Center [email protected] Mitusmasa Sorazawa Honda R&D Co., Ltd. Automobile R&D Center [email protected] Tomohide Haraguchi Honda R&D Co., Ltd. Automobile R&D Center [email protected] Takatoshi Miyabe Honda R&D Co., Ltd. Automobile R&D Center [email protected] Shunji Takahashi Honda R&D Co., Ltd. Automobile R&D Center [email protected] DEFINITIONS/ABBREVIATIONS BSFC Brake Specific Fuel Consumption FP Fin Pitch 3. Mori, M., Yamagami, T., Oda, N., Hattori, M., Haraguchi, T., Sorazawa, M., “Heat Recovery of Automobile Using Thermoelectric Element and Its Effect on Fuel Economy”, Honda R&D Technical Review, October 2009, Honda Motor Co., Ltd., p.73-79 LA4 City Urban Dynamometer Driving Schedule 4. Ibaraki, S., Endo, T., Kojima, Y, Takahashi, K., Baba, T., Kawajiri, S., “Research of a Rankine Cycle On-Board Heat Waste Recovery System”, JSAE paper 20065602 LA4 Highway Highway Fuel Economy Dynamometer Driving Schedule 5. LaGrandeur, J., Crane, D., Hung, S., Mazar, B., Eder, A., “Automotive Waste Heat Conversion to Electric Power using Skutterudite, TAGS, PbTe and BiTe”, 2006 DEER Conference US06 Supplemental Federal Test Procedure TE Thermoelectric Element TEG Thermoelectric Generator 1276 SAE Int. J. Mater. Manuf. | Volume 4 | Issue 1