The experimental study on the performances of the thermoelectric generator unit affected by heat rate of exhaust system

Use your smartphone to scan this QR code and download this article ABSTRACT This study examines the effect of heat rate transferring from the exhaust system of motorcycle to the environment on the performance of the Thermoelectric Generator Unit (TGU). This heat rate is changed by attaching thermal insulationmaterial on the outside of the exhaust system and changing the heat absorbing area of the TGU. The TGU consists of 8 thermoelectric generator modules and collects heat energy from the exhaust to produce electricity. It is attached on the custommuffler of the Suzuki Sapphire 125 and tested in the speed range from 20km/h to 50km/h. The results show that this heat rate affects both the temperature and output power generated by the TGU. The reduction of this heat rate reduces the cool side temperature by limiting the warming effect of cooling air and raise the hot side temperature of the TGU by decreasing heat loss. These two effects lead to the increase of the temperature difference between both sides of the TGU and therefore the output power increases. The difference in output power between test cases can reach up to 54%. Moreover, the heat loss at exhaust tube affects both temperature and output power of TGU from low to mid vehicle speed. However, at high speed, this heat loss at the exhaust tube does not considerably affect the output power of the TGU. To summarize, by reducing the heat rate between exhaust system and environment, the TGU can reach its stable working condition faster and produces more output power. Nonetheless, reducing this heat rate too much may lead to the excess of the hot side temperature of TGU, therefore damaging the thermoelectric generatormodules and reducing the conversion efficiency of the TGU.


INTRODUCTION
Today, environment pollution, traditional resource depletion, and energy security are important issues for all countries in the world. This has required people to find alternative energy sources or use efficiently for existing energy sources. In an internal combustion engine, about 30 -40% of the energy of fuel generated from combustion is in exhaust gas 1 and most of it is wasted directly into the environment. The recovery and reuse of this energy source have attracted the attention of researchers and many scientific works that have been carried out on the application of Thermoelectric Generator (TEG)  using heat source from the exhaust gas of internal combustion engines. In previous studies [23][24][25][26] , the authors conducted research and development of Thermoelectric Generator Unit (TGU) using heat source from motorcycle exhaust based on surveying the change in hot side temperature and cool side temperature through changing the structure of the Hot Side Heatsink (HSH) and the Cool Side Heatsink (CSH). In this study, the authors investigated the influence of heat rate on the operating characteristics of the TGU through the insulation of the exhaust system. The purpose of this study is to show the influence of heat rate on the hot side temperature, the cool side temperature, the temperature difference and the output power of the TGU. The research results have important value as a basis for optimizing the structure and improving the conversion efficiency of the TGU.

BASIS OF THEORY
Heat transferring through the exhaust system wall On the exhaust system of the vehicle, there are two main types of heat transfer: conduction and convection. Convection takes place between the inner surface of the exhaust system and the exhaust gas; and between the outer surface of the exhaust system and the ambient air. Conduction takes place on the exhaust system wall. Insulating the outer surface of the exhaust system changes the characteristics of the thermal convection between the exhaust system and the environment. Figure 1 shows the heat transfer process through the exhaust system wall when an external insulation layer   The heat rate Q (W) is calculated according to the formula: With: Where: k (W/m 2 .K) is the overall heat transfer coefficient; α 1 and α 2 (W/m2.K) respectively are the convection heat transfer coefficient between the inner surface of the exhaust system and the exhaust gas, and between the outer surface of the exhaust system and the ambient air; δ 1 and δ 2 (m) respectively are the thickness of the exhaust system wall and the heatinsulation layer; λ 1 and λ 2 (W/m.K) respectively are the thermal conductivity coefficient of the material of the exhaust system and the insulation material; F (m 2 ) is the heat transfer area; t f 1 (K) is the temperature of the exhaust gas; t f 2 (K) is the temperature of the ambient air.

Seebeck effect
Seebeck effect is the phenomenon that the thermal energy is converted directly into electrical energy in a closed -circuit consisting of two metallic conductors being weld together. It is called a closed -circuit thermocouple. The current is generated by the temperature difference between the two welds ( Figure 2). The voltage of this electrical current is calculated using the formula: Where: S (V/K) is Seebeck coefficient; T h (K) and T c (K) are the temperatures of the hot side and the cool side, respectively; △T (K) is the temperature difference between the hot side and the cool side.

Thermoelectric generator -TEG
The relationship between the output power P (W) and the heat rate Q (W) transferring through the TEG module is presented by the formula: Thermal -electrical conversion efficiency η is determined by the formula: Where: △T (K) is the temperature difference between the hot side and the cool side of TEG module; T (K) is the average temperature of the hot side and cool side; and Z (1/K) is the quality factor of the thermoelectric material, presented by the formula: is the thermal conductivity coefficient of the material of the TEG module; and ρ (Ω.m) is resistivity.
Considering the circuit provides load ( Figure 3) of TEG, we have the following equation:

Science & Technology Development Journal -Engineering and Technology, 3(SI2):SI24-SI36
Where: E S (V) is the electromotive force of TEG; U S (V) is the voltage generated by the Seebeck effect; S (V/K) is Seebeck coefficient; R i (Ω) is the resistance of the TEG; R L (Ω) is the load resistor; T h (K) is the temperature of the hot side; T c (K) is the temperature of the cool side; △T (K) is the temperature difference between the hot and cool side; k t (W/K) is the total thermal conductance of TEG; P L (W) is the output power with load; Q in (W) is the heat input; Q out (W) is the heat output; I L (A) is the amperage; U L (V) is the voltage.
The TEG modules used in the model are type TEP1-142T300 with dimensions of 40 mm x 40 mm x 3.8 mm; and the maximum working temperature is 300 0 C. In this study, two aluminum HSHs are used with different quantity of fins: 21 fins and 12 fins; the CSH is made of aluminum with 11 fins. The structure of the HSHs and CSH are shown in Figures 5, 6 and 7.  To change the heat rate through the TGU, the authors change the HSH and insulate the exhaust system by fiberglass fabric (maximum working temperature: 500 0 C; thermal conductivity coefficient λ : 0.027 -0.04 W/m.K) to investigate the characteristics of the TGU.

Experiment description
The diagram of the experimental system is shown in Figure 8.   of the TGU, the data collection system will record the values including hot side temperature, cool side temperature, voltage, current of the TGU; and speed of the vehicle. The temperature sensors used are RTD PT100 type B (4 mm in probe diameter, 30 mm in probe length). In that, 02 sensors are located on 02 different points of the HSH to measure the temperature of the HSH; the other 02 sensors are located on the CSH, which is symmetrical with 02 sensors on the HSH, to measure the temperatures of the CSH. The average temperature of the two sides is calculated by averaging the temperature value of the two sensors on those sides. Voltage and current are measured by an integrated circuit (power circuit) operating in the ADC method (Analog-to-digital converter, the input voltage is converted to the numerical value to represent the magnitude of voltage, the current is calculated based on the initial resistance value and the voltage value). Vehicle speed is measured by an infrared sensor (V1) by SI27 measuring the speed of the front wheel. Characteristic values obtained by the data collection system are transmitted directly to the laptop via Bluetooth connection in real-time. The parameters of the temperature sensors and the infrared sensors are shown in Table 1.
The TGU installed on Suzuki Sapphire 125 (Figure 9) is tested on urban roads at stable speed modes of 20, 30, 40 and 50 km/h with a load of 2 peoples. The time duration for investigating at each mode is 15 minutes, of which the first 10 minutes is used to achieve the temperature stabilization state and the 5 remain minutes is used to collect the device's parameters.
The test cases are shown in Table 2. The actual images of the TGU before and after insulation are shown in Figures 10 and 11. Figure 12 and Figure 13 compare the hot side temperature, the cool side temperature, the temperature difference between the hot and cool side, and the output power of cases (1) and (3).
The results in Figure 12 shows that the hot side temperature in the case of heat-insulating is always higher than in the case of non-heat-insulating. This is explained by the fact that insulating the exhaust system helps reduce the heat loss and increase the heat rate which is absorbed by the HSH. At 50 km/h, the hot side temperature of case (3) and case (1) are 215 0 C and 185.7 0 C, respectively. In the speed range from 20 km/h to 40 km/h, the cool side temperature of case (3) does not seem to change. Meanwhile, the cool side temperature of case (1) increases gradually. This is due to insulating the exhaust system helps reduce the warming phenomenon of the cooling air which flows through the CSH by the effect of heat loss. Therefore, the cooling capability of the CSH increases. Figure 13 shows that the output power of case (3) is always higher than that of case (1) due to the higher temperature difference. At 50 km/h, the maximum output power of case (1) and case (3) are 5.37W and 8.28W, respectively. As the speed increases, the output power of case (3) increases faster than that of case (1). Specifically, as the speed increases from 20 km/h to 50 km/h, the output power of case (1) increases up to 233.5% (from 1.61W to 5.37W) and the output power of case (3) increases up to 314% (from 2W to 8.28W). This is explained by the fact that the higher increase in the temperature difference of case (3), resulting from absorbing more heat and better cooling capability when insulating the exhaust system. Figure 14 and Figure 15 compare the hot side temperature, the cool side temperature, the temperature difference between the hot and cool side, and the output power of cases (2) and (3).
The results in Figure 14 show that the hot side temperature of case (2) is lower than that of case (3) due to heat loss at the exhaust tube. From 40 km/h, the hot side and cool side temperature of case (3) increase slower than those of case (2). This is explained by the fact that the heat rate, which is absorbed by the HSH, increases considerably when insulating all the exhaust system. Therefore, the TGU reaches the heat saturation state faster and the temperature increases slower. Figure 15 shows that the output power of case (3) is higher than that of case (2) due to the higher temperature difference. In the speed range from 30 km/h to 40 km/h, the average difference of output power between 2 cases is 0.6W. However, as the speed increases, this difference decreases to 0.28W at 50 km/h. Figure 16 and Figure 17 compare the hot side temperature, the cool side temperature, the temperature difference between the hot and cool side, and the output power of cases (3) and (4).
The results in Figure 16 show that the hot side temperature of case (3) is considerably higher than that of case (4) due to larger fin quantity (21 fins and 12 fins, respectively). At 50 km/h, the difference in hot side temperature between 2 cases is 18.8 0 C. Meanwhile, the cool side temperature of the 2 cases is not considerably different because the cooling capability of the CSH is substantially larger than the heat rate which is absorbed by the HSH. Figure 17 shows that the temperature difference between the hot side and cool side of 2 cases are considerably different. At 50 km/h, the temperature difference of case (3) and case (4) are 116 0 C and 103 0 C, respectively. In the speed range from 20 km/h to 40 km/h, the output power of case (3) is higher than that of case (4). However, as the speed is greater than 40 km/h, the output power of case (4) is higher. At 50 km/h, the output power of case (3) and case (4) are 8.28W and 8.87W, respectively. This is explained by the fact that if the hot side temperature is too high, it may lead to a decrease in conversion efficiency of TEGs. Then the output power will increase slower, especially in the case (3).

CONCLUSIONS
This study examines the effect of heat rate transferring from the exhaust system to the environment on the performance of the thermoelectric generator unit (TGU) by insulating the exhaust system. The work results in the conclusion as follows: • The heat transfer between the surface of the exhaust system and the environment affects the characteristics of the hot side and the cool side of  (3) Insulate the exhaust system 21 1mm (4) Insulate the exhaust system 12 1mm Figure 10: The TGU before insulating Figure 11: The TGU after insulating the exhaust system the TGU. By insulating the outside surface of the exhaust system, the heat loss reduces. Therefore, the hot side heatsink absorbs more heat energy and it leads to the increase of hot side temperature. Besides that, insulating the exhaust system helps reduce the warming phenomenon of the cooling air by the effect of heat loss. Thus, the cooling capability of the cool side heatsink increases. The output power of the TGU increases considerably when insulating the exhaust sys-tem. Specifically, at 50 km/h, the difference in output power between the case of insulating and non-insulating is 54.2%.
• By insulating the exhaust system, the heat loss reduces. Therefore, the TGU will reach to the heat saturation state faster.
• The output power does not only depend on the temperature difference between the hot side and the cool side of the TGU. If the hot side temperature is too high, the conversion efficiency of thermoelectric generators will decrease. Thus, the output power may increase slower despite the high-temperature difference.

CONFLICTS OF INTEREST
The authors declare that they have no conflicts of interest.

AUTHOR CONTRIBUTIONS
The contributions of each author in this paper are listed as below: • Hong Duc Thong contributes in the supervision, project administration, supporting the experiment process and writing & editing the paper. • Nghiem Phan Thien Quan contributes in conducting the research and investigation process, analyzing the data and writing the manuscript of paper. • Mai Van Tinh contributes in conducting the research and investigation process, analyzing the data and writing the manuscript of paper.