翻译原文基于热电式发电机的汽车尾气余热发电设计：目标与挑战

International Journal of Automotive Technology, Vol. 9, No. 2, pp. 155?160 (2008) DOI 10.1007/s12239?008?0020?y Copyright 2008 KSAE 1229?9138/2008/039?05 155 THERMAL DESIGN OF AUTOMOBILE EXHAUST BASED THERMOELECTRIC GENERATORS: OBJECTIVES AND CHALLENGES K. M. SAQR1)*, M. K. MANSOUR1) and M. N. MUSA2) 1)Department of Thermo-Fluids, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Malaysia 2)Research Management Center, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia (Received 15 June 2007; Revised 1 December 2007) ABSTRACT?The potential for thermoelectric power generation (via waste heat recovery onboard automobiles) to displace alternators and/or provide additional charging to a vehicle battery pack has increased with recent advances in thermoelectric material processing. In gasoline fueled vehicles (GFVs), about 40% of fuel energy is wasted in exhaust heat, while a smaller amount of energy (30%) is ejected through the engine coolant. Therefore, exhaust-based thermoelectric generators (ETEG) have been a focus for GFV applications since the late 1980s. The conversion efficiency of modern thermoelectric materials has increased more than three-fold in the last two decades; however, disputes as to the thermal design of ETEG systems has kept their overall efficiency at limited and insufficient values. There are many challenges in the thermal design of ETEG systems, such as increasing the efficiency of the heat exchangers (hot box and cold plate), maintaining a sufficient temperature difference across the thermoelectric modules during different operating conditions, and reducing thermal losses through the system as a whole. This paper focuses on a review of the main aspects of thermal design of ETEG systems through various investigations performed over the past twenty years. This paper is organized as follows: first, the construction of a typical ETEG is described. The heat balance and efficiency of ETEG are then discussed. Then, the third section of this paper emphasizes the main objectives and challenges for designing efficient ETEG systems. Finally, a review of ETEG research activities over the last twenty years is presented to focus on methods used by the research community to address such challenges. KEY WORDS : Thermoelectric power generation, Automotive waste heat recovery, Energy management 1. INTRODUCTION Any automobile ETEG typically consists of four compo- nents: a hot box, thermoelectric modules, cold plate, and assembly elements. The hot box is the component where exhaust heat is to be extracted, the cold plate is responsible for dissipating heat after it passes through the thermoelectric modules, and the assembly elements are responsible for applying sufficient compression force to these thermo- electric modules as well as assembling all other compo- nents in the desired combination (Vzquez et al., 2002). A thermoelectric module contains a large number of N-type and P-type semiconductors, arranged in couples, which can convert heat directly into electric power (Rowe and Bhandari, 1983). Throughout this paper, the abbreviation “ TEMs is used to represent thermoelectric modules. Thermal insu- lation is another important component of any ETEG, and is used to decrease the thermal losses across the hot box; also, thermal interface material (i.e., thermal grease) must be applied between the hot and cold surfaces and the TEMs to overcome the thermal contact resistance between the two surfaces and the TEMs. The typical construction and flow direction of working fluids are illustrated in Figure 1. 2. ETEG HEAT BALANCE AND EFFICIENCY In internal combustion engines, approximately 40% of the fuel energy is wasted in exhaust gas, 30% is dissipated in*Corresponding author. e-mail: Figure 1. Typical construction of ETEG. TEMs are assembled between the hot box and two cold plates using assembly elements (compression springs and bolts). 156K. M. SAQR, M. K. MANSOUR and M. N. MUSA the engine coolant, 5% is lost as radiation and friction, and 25% is reserved for vehicle mobility and accessories (Yang, 2005). ETEG systems tend to recover heat from the first 40%; however, due to many factors, only 56% efficiency is available for todays ETEG technology (Kushch et al., 2001). The heat balance for the system is explained in Figure 2. Qinand Qoutare the exhaust gas energies at the inlet and exit of the hot box, respectively, and Qo is the effective heat transferred through the TEMs. The heat losses in the system are expressed as QLosses (see Equations (1) and (2), and Q1 is the heat lost from the non- used sides of the hot box by radiation and convection (i.e., heat transferred normal to the drawing plane). Q2 is the heat lost from the leg-sides of the TEMs by convection and radiation, and Q3 is the heat lost by conduction through the assembly structure. The heat lost through gaps between the TEMs is Q4, and Q5 is the heat lost by conduction in the TEMs due to thermal contact resistance. The heat balance of the model in Figure 2 can be expressed as: Qin?Qout=Q0+QLosses(1) QLosses=Q1+Q2+Q3+Q4+Q5(2) and the overall efficiency of the ETEG can be expressed as: (3) where the conversion efficiency of the TEMs is shown as: (4) and Pois the electric output power from the ETEG. The efficiency of the heat exchanger can be expressed as: (5) (6) The percentage of each component of the heat loss, as well as that of the useful amount of heat passing through TEMs to the available heat entering the ETEG, are primarily based on the design of the ETEG. However, a recent case study suggested that Qout represents 45% of the total exhaust heat, while another 45% is represented in Q2, Q5, and Qo together (Rowe, 2005). 3. OBJECTIVES AND CHALLENGES Increasing the overall efficiency is the main objective in any ETEG design; this increase can be achieved by numer- ous techniques. The thermal efficiency can be increased by reducing the heat dissipated due to the thermal contact resistance (Q5), which can be implemented by applying a uniform compression to the TEMs and using highly con- ductive thermal interface material between the hot/cold sides and the modules. Because the hot box and cold plate are normally electrically conductive, electrical insulation should always be applied to the TEMs; ceramic wafers are ideal for fulfilling the thermal interface and electric insu- lation requirements. A temperature drop of about 1015oC can therefore be achieved across these wafers (Hi-Z Inc., 1996). Other solutions, such as silicon- and metal-based grease, might not be suitable for ETEG applications because of the high operating temperature of the modules ( 200oC) as well as the cost considerations. The efficiency of the heat exchanger can be controlled by many methods. When the hot box has a relatively large cross-sectional area, the exhaust gas velocity is damped when entering the hot box channel. This sudden drop in gas velocity forms a thick thermal boundary layer that causes the overall heat transfer coefficient to be sharply decreased. Internal fins, turbulators, and corrugated surfaces are most favorable solutions for eliminating the effect of this bound- ary layer. However, an important factor that should always be considered is the free cross-sectional area for the flow, because it may cause the engine efficiency to decrease. To minimize conduction losses through the assembly, the number of assembly elements contacting the hot box directly should be minimized as much as possible and free surfaces should be strictly insulated. The ratio (?) can also be increased by reducing the heat loss through air gaps between the TEMs. This can be easily achieved by using thermal insulation to prevent convection from the hot box surface. Also, spray insulation with conductivity as low as 0.03 W/m.K is available and also sustainable for high temperature applications. The ability of the cold plate to provide sufficient cooling for TEMs at all operating conditions is another important thermal requirement in ETEG. Generally, the cold plate can be classified to two types according to heat dissipation from the system (Saqr and Musa, 2007): 1. Radiator-based cold plate 2. Heat sink-based cold plate In the first type, the cold plate dissipates exhaust heat to the engine radiator through the engine coolant (i.e., water), while in the second type the cold plate dissipates heat to the ambient air through a heat sink; see Figure 3(a) and Figure 3(b). ?OV=?m?HX? ?m= Generated Power Heat absorbed by TEMs - - ?m=Po Qo - - ?HX= Actual heat transferred Maximum possible heat transfer - - ?HX= Q0Q4+ QinQout - - ?= Q0 Q0Q4+ - - Figure 2. Heat balance of a typical ETEG structure. THERMAL DESIGN OF AUTOMOBILE EXHAUST BASED THERMOELECTRIC GENERATORS157 In the radiator-based cold plate (Figure 3(a), the coolant pump and piping usually require resizing to compensate for the additional cooling load. Heat sink cold plates can be efficient if the ETEG is to operate at low temperature gradients; this technique takes advantage of the movement of the vehicle to create turbulence in the air flow over the cold plate. 4. REVIEW OF ETEG RESEARCH The first ETEG for automobile applications was built in 1963 (Neild, 1963), but the most important research in this area has been performed over the last twenty years. In this section, three examples of ETEG research are discussed to explain how the research community has addressed differ- ent thermal design challenges, and to highlight the most important developments in ETEG technology. 4.1. 1 KW ETEG Hi-Z Inc. presented an ETEG for installation on diesel trucks in 1992 (Bass et al., 1992). A total number of 72 HZ-13 modules were used to produce power in this ETEG. Each of these modules contained 49 couples of hot pressed bismuth telluride-based materials. The module size was 5.3 cm square by 0.5 cm thick and 82 g in weight. The hot and cold side design temperatures were 230oC and 30oC, respectively (Hi-Z Inc., 1996). The total length of the ETEG was 48.26 cm with a total diameter of 22.86 cm and a total weight of 13.6 kg. A hexagonal cross-section hot box was used with aluminum radiator-based cold plates used as a heat dissipater. The hot box was fabricated by welding flat steel plates together to form a hexagonal shape; these steel plates had a total number of 90 internal longitudinal fins to enhance the heat transfer from the exhaust gases to the modules. The TEMs were held against the hot box under a compressive uniform load of 14 kg/cm2. This compressive loading was applied using Belleville springs to ensure minimum thermal contact resistance between TEMs and heat exchanger surfaces (see Figure 4). The generator was designed to produce 1 KW of electricity, but when the experimental tests were performed on a 14 L Cummins NTC 325 engine, the ETEG produced only 400 W, which was less than half of the goal. It was concluded from the temperature profile measurements that the reduction in generated power was caused by a boundary layer problem in the gas side, where the exhaust gas velocity decreases according to the enlargement in the flow cross-section (Bass et al., 1995). In order to resolve this problem, the number of fins was reduced from 90 to 32 fins, while the length of each fin was Figure 3. (a) Radiator based ETEG (b) ETEG with a heat sink cold plate. Figure 4. Cross section of Hi-Z ETEG. 158K. M. SAQR, M. K. MANSOUR and M. N. MUSA increased to maintain heat transfer area. A discontinuity was introduced to the fin geometry by adding 0.953 cm gaps at 3.81 cm intervals. In addition, swirl fins were installed in the hot box to ensure full turbulence of exhaust gas. A summary of the Hi-Z 1 KW available experimental data is presented in the following table: 4.2. 35.6 W ETEG In 1998, Nissan Motors Inc published the result of experi- mental testing for a SiGe-based ETEG. The generator contained 72 SiGe-based modules; each module produced 1.2 W at a temperature difference of 563 K between the hot and cold surfaces of the module (Ikoma et al., 1998). The ETEG overall dimensions were 440?180?70 mm3 and the overall weight was 14.5 kg. The modules were mounted between a hot box made of SUS304 alloy2 and two water- cooled jackets made of aluminum, as seen in Figure 5. The inner shell had a rectangular cross-section and two smooth faces with which the hot side of the module came into contact. In order to transfer as much heat as possible through the modules, fins for heat exchange were formed inside the inner shell in parallel with the gas flow. The modules were arranged in twelve blocks (M1 to M12), with each block of three modules separated by 10 mm; conse- quently, the proportion of area occupied by modules to the faces of the inner shell was only 55%. Measurements were performed under conditions corre- sponding to the 60 km/hr hill climb mode of a 3000cc gasoline engine vehicle. A combustor bench was utilized for simulating the engine exhaust at the desired operating conditions, measuring temperature distribution in the gene- rator, and evaluating the electric power of the generator. The exhaust gas flow produced by the combustor was divided into two lines. One line was connected to the gene- rator, and the other to the bypass line. The temperature and the flux of exhaust gas from the combustor is controlled by changing the ratio of air to fuel (A/F) and the flow rate of air blowing into the combustor. The flux of exhaust gas through the generator is controlled by adjusting the orifice diameter at the inlet of each line, as seen in Figure 7. The experimental results are explained in the following table: Table 1. Available experimental results from the 1 KW Hi- Z ETEG. Hot-side heat exchanger Material Thermal Conductivity Working fluid Inlet Temperature Outlet temperature Carbon Steel 50 W/m.K Exhaust gas ?1 ? Cold-Side heat exchanger Material Thermal Conductivity Working Fluid Inlet Temperature Outlet Temperature Aluminum 204 W/m.K Water ? ? Heat Transferred from the hot side to the cold side of the ETEG (Q0+QLosses) ? Test engineCommins 14 L Maximum power (Po)1068 W Engine operating conditions at maximum ETEG power 1700 rpm 300 hp ETEG overall efficiency at maximum power test (?OV) 1.3% TEM conversion efficiency (?m)4.5% Temperature difference across TEM surfaces at maximum ETEG power 250oC 1(?) Refers to non-available data 2Chemical composition of SUS304: Fe, 0.08% C, 17.520% Cr, 811% Ni, 2% Mn, 1% Si, 0.045% P, 0.03% S Figure 5. Schematic structure of Nissan ETEG. Figure 6. Schematic showing the combustor test bench used to simulate engine exhaust for the Nissan ETEG testing. THERMAL DESIGN OF AUTOMOBILE EXHAUST BASED THERMOELECTRIC GENERATORS159 In their report, Ikoma et al. reported that if and could be increased from 510% to 50%, and from 12% to 5%, respectively, then the same ETEG would produce 950 W at the same testing conditions. They have emphasized that the key element in enhancing ETEG efficiency is the thermal effectiveness of the heat ex- changers. 4.3. 300 W ETEG The experimental results from a 300 W ETEG were report- ed in 2004. The project joined Clarkson University and Delphi Systems and was funded by NYSERDA and the DOE in order to develop a 300 W ETEG to be mounted on a GM Sierra pickup truck. In order to achieve the power goal, a total of 16 HZ-20 thermoelectric modules were used. Eight modules were mounted on each side of a carbon steel hot box, and all modules were connected electrically in series and thermally in parallel. The HZ-20 bismuth-tullerdie based modules generated 19 W at minimum if the temperature difference between the two sides of the modules was 200oC. The module dimensions were 75755 mm, and the weight was 115 gm. The TEMs were assembled at a preload pressure of 200 psi between the hot box and two aluminuim water jackets. The overall dimensions of the ETEG were 330273216 mm with a weight of 39.1 kg (Tacher et al., 2007). See Figure 7 (Tacher et al., 2006). A PCU (power conditioning unit) was used to adopt the generated voltage to match the vehicle electrical system at 12 and 24 V. Testing of the ETEG was performed on a V8 270 hp gasoline engine from a GM Sierra pickup truck. A pre-cooling heat exchanger was used to lower the inlet ?HX? ?m Table 2. Experimental results from the 35.6 W ETEG combustor bench testing. Hot-side heat exchanger Material Thermal Conductivity Working fluid Inlet Temperature Outlet temperature SUS304 21.5 W/m.K Exhaust gas 592oC 527oC Cold-Side heat exchanger Material Thermal Conductivity Working Fluid Inlet Temperature Outlet Temperature Aluminum 204 W/m.K Water 35oC 39.5oC Heat Transferred from the hot side to the cold side of the ETEG (Q0+QLosses) 4 KW Test engine3.0 L Gasoline Maximum power (Po)35.6 W Engine operating conditions at maximum ETEG power 60 km/hr Hill Climb Mode ETEG overall efficiency at maximum power test (?OV) 0.1% TEM conversion efficiency (?m)12% Temperature difference across TEM surfaces at maximum power 123oC Figure 7. The 300 W ETEG before testing. Table 3. Experimental results from the 300 W ETEG road testing at 112.65 km/h. Hot-side heat exchanger Material Thermal Conductivity Working fluid Inlet Temperature (Case 1) Outlet temperature (Case 1) Inlet Temperature (Case 2) Outlet Temperature (Case 2) Carbon Steel 50 W/m.K Exhaust gas 530.8oC 421.2oC 617.3oC 484.6oC Cold-Side heat exchanger Material Thermal Conductivity Working Fluid Inlet Temperature (Case 2) Outlet Temperature (Case 2) Inlet Temperature (Case 3) Outlet Temperature (Case 3) Aluminum 204 W/m.K Water 86.7oC 93.9oC 77.6oC 87.8oC Heat Transferred from the hot side to the cold side of the ETEG (Q0+QLosses) ? Test engine Maximum power (Po)255.1 W Engine operating conditions at maximum ETEG power 112.65 km/h Climb up a grad