Comparison and parameter optimization of a two-stage thermoelectric generator using high temperature exhaust of internal combustion engine
Introduction
With the decreasing of the oil, energy saving and emission reduction of the automotive have been attracted. Thermoelectric generators can save energy costs and reduce the environment burden [1]. 55–77% (diesel engine) [2] or 70–80% (gasoline engine) [3] of fuel energy in automotive are discharged into the environment in the form of waste heat, with the heat contained in exhaust gas accounting for the major part, which can exceed 50% [4]. If the waste heat contained in exhaust gas could be effectively reutilized, engine thermal efficiency would be improved significantly.
Using the analytical network process, Liang et al. [5] concluded that among all existing waste heat recovery technologies, the thermoelectric generator (TEG) was the most promising method for recovering ICE waste heat in the future. Birkholz et al. [6] conducted an experiment to recover heat by TEG, and the results showed that 90 thermocouples made of FeSi2 could recover 58 W when the temperature difference was 490 K. Nissan Motor Company applied an advanced-type thermoelectric module to gasoline engine vehicles to recover exhaust heat, which could recycle 11% exhaust heat under the climb mode at a speed of 60 km h−1 [7]. Hi-Z Technology detailed the design of a 1.5 kW TEG laboratory prototype unit in 1984 [8]. With funding from the United States Department of Energy, Hi-Z also conducted a study on TEG using exhaust gas as heat source for truck diesel engines. The 72 integrated and improved HZ-14 thermoelectric generator modules (TEMs) can produce 1 kW power at 30 V DC during nominal engine operation [9]. The TEG developed by General Motors, which can generate 350 W for the FTP conditions, can improve fuel economy by almost 3% [10]. These studies show that with the advantages of not having mechanical moving parts and no friction, as well as quiet operation, high stability, and being environment friendly, TEG has become a viable research direction for ICE waste heat recovery.
Most studies on the TEG system have focused on model optimization [11], [12], [13], [14], [15], [16], [17], geometric parameter of thermocouple [18], [19], [20], heat exchanger [21], [22], material [23], [24], and new TEG structure [25], [26]. Kim [15] proposed an experimental method to study the relationship between the Seebeck coefficient and temperature difference of TEG to optimize the TEG mode. Sahin and Yilbas [19] found that when high conversion efficiency of the device was required, decreasing and increasing the shape parameter would be favorable; however, when high power output was required, the shape parameter should be set to zero, which would correspond to the rectangular leg geometry. Meanwhile, increasing both heat transfer area and heat transfer coefficient can improve TEG performance [22]. Anatychuk and Kuz [24] determined which materials were most suitable for the heat recovery of ICE. Numerous new ideas have been proposed to improve TEG performance. Shu et al. [26] proposed TEG combined with organic Rankine cycle to recover the waste heat of ICE. The low ZT value serves a key role in the resulting low conversion efficiency. Riffat and Ma [27] highlighted that the efficiency of TEG is approximately 5% in general, and the main reason it cannot exceed 10% is that the ZT value cannot be increased effectively. Integrating the exhaust waste heat and the ordinary material into a TEG system to improve the engine efficiency has always been a significant topic.
Fig. 1 shows the exhaust temperature of a diesel engine tested by our laboratory. As shown in this figure, the exhaust temperature is about 523 K when operating on low engine load and exceeds to 813 K on high load. Many experiences can also illustrate that the exhaust has high temperature [1], [28]. There is a large temperature difference between heat source and cold source when the TEG is used to recover the exhaust heat. The ZT value changes greatly in this high temperature of heat source and large temperature difference conditions (Fig. 2) [23]. That is to say: the ZT value of these two thermoelectric materials is higher than that of the single material under the same boundary conditions. So, a new high-efficiency two-stage TEG is proposed in this paper. Numerous studies have been conducted on the two-stage thermoelectric cooler [29], [30]. Although the models for thermodynamic analysis are different, the researches of two-stage thermoelectric cooler can give directions on the research of TEG. Xiao et al. [31] compared several types of TEG, such as single-stage, two-stage, and three-stage TEG, using solar energy as heat source. Dai et al. [32] conducted an experiment on serial TEG with two different kinds of material modules as the top and bottom layers. The TEG efficiency does not exceed 2% using the second conversion liquid metal as transient heat source. In this study, a new TEG structure was compared with traditional structures, and then the performance of two-stage TEG using engine exhaust as heat source was discussed by analyzing the influence of heat transfer coefficient and temperature of heat and cold sources, as well as the effect of the number of thermocouples on output work, absorbed heat, and conversion efficiency. The optimum configuration of a basic two-stage TEG was studied in detail. According to the analysis, the best performance of the two-stage TEG is determined, which may provide guidance in the design and application of two-stage TEG in ICE.
Section snippets
System description
A schematic of two types of TEG with an external resistance (RL) is presented in Fig. 3. TEG is composed of a number of TEMs in different forms: serial, parallel, or combination. To simplify the analysis, TEG consists of one TEM. Unlike the single-stage TEM (Fig. 3b), the two-stage TEM consists of m pairs of thermocouples on the top layer and n pairs of thermocouples on the bottom layer, which are connected with a serial wire (Fig. 3a). The total number of thermocouples of the two-stage TEG is M
Governing equations
A new method has been put forward to recover exhaust heat in this paper. The preliminary discussion on performance of two-stage TEG has been researched in the steady state of ICE. The exhaust gas and coolant flow from the surface of two-stage TEG. The heat is transferred from the heat source at temperature of Th, and released at temperature of Tc. A part of the absorbed heat is transformed into electricity through two-stage TEG in this process. The form of the two-stage TEG is electric in
Validation
Numerical results are validated by comparing the value of power and conversion efficiency with that in the research of Chen et al. [25]. The materials are the same at both stages, and the property of the material is set as a constant at different temperatures. For example, the Seebeck coefficient is 2.3 × 10−4 V K−1 and the total internal electrical resistance of the thermocouple is 1.4 × 10−3 Ω m. The temperature of the heat source is 600 K, and the cold side temperature is 300 K. The transfer
Performance comparison of single- and two-stage TEG
Fig. 5, Fig. 6 show a comparison of output power and conversion efficiency for single-stage and two-stage TEG at different heat source temperatures. As shown in Fig. 5, Fig. 6, the maximum output power and conversion efficiency of two-stage TEG are 18.6% and 23.2% higher than that of single-stage TEG, respectively. We can conclude that the two-stage TEG is advantageous over the single-stage TEG when the heat source temperature is between 600 K and 800 K. The external resistances are small when
Conclusion
Based on Newton cooling law, as well as Fourier’s and Seebeck effect, a model of a two-stage TEG was built using the exhaust gas of ICE as heat source. The performance of the generator was analyzed by simulating the effect of relevant factors. The main conclusions are as follows:
- 1.
The output power and conversion efficiency of the two-stage TEG are higher than that of the single-stage TEG when the temperature of heat source varies from 600 K to 800 K.
- 2.
The absorbed heat, output power, and conversion
Acknowledgments
This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB707201), the National Nature Science Foundation of China (No. 51206117), and Natural Science Foundation of Tianjin (No. 12JCQNJC04400). The authors gratefully acknowledge them for support of this work.
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