Effects of Temperautre on Characters of the Thermoelectric Generator using in LNG Energy Recovery

Effects of Temperautre on Characters of the Thermoelectric Generator using in LNG Energy Recovery ZONGZHENG MA*, DACHENG LIU, XINLI WANG, ANJIE YANG Abstract The energy recovery of the liquefied natural gas (LNG)engine using thermoelectric generation technology is good for improving engine thermal efficiency due to its cold energy. One experimental test bed for simulating cold energy recovery of LNG engine is establsihed which the low temperature conditon is realized with liquid nitrogen. Then the relationship between the temperature and characters of thermoelectric generator was researched based on this test bed.the experimental results showe that the output voltage increases with the increase of temperature difference between the hot and cold end of the thermoelectric generator but the relationship is noliner resulting from the seebeck coefficient change. Then the seebeck coefficient is studied when the hot end and cold end is fixed respectively. The results show that the seebeck coefficient has a peak value when the cold end temperature of TEG is -60 and there is different trend on each side of the peak value. The seebeck coefficient reduces with the increase of cold end temperature when the temperature of cold end is higher than -60, while the seebeck coefficient increases with the decrease of cold end temperature when the temperature of cold end is lower than-60. Keywords Thermoelectric Generation; Characters; Seebeck Coefficient; Low Temperature 1. INTRODUCTION With the rapid development of vehicle, it is very convenient to travel and transport. But there are also some problems in energy consumption and environmental pollution because most of the internal combustion engines(ice) are fueled with fossil energy [1-3]. Therefore, the alternative fuels have gained more and more attention because of their abundant resources and low pollution, one of which is natural gas(ng). There are many kinds of application of NG to ICE, such as compressed natural gas (CNG), liquefied natural gas (LNG), adsorbed natural gas (ANG) and gas to liquids (GTL). At present, the latter two technologies are in experimental stage [4].CNG engine has been developed for many years but is still not used widely due to the NG storage problem [5]. For the LNG engine, it is considered as an important alternative due to its cleanness, efficiency, easy storage and fuel economy [6]. However, the ICE s thermal efficiency is only about 40% of the fuel energy. About as much as 60% of the fuel energy has not been effectively utilized and the vast majority of the energy is lost to the air in the form of heat transfer, resulting in a huge waste of energy. Improving the fuel economy and engine thermal efficiency are helpful to energy conservation and environment protection. One of the methods for improving fuel economy and engine efficiency is energy recovery. Thermoelectric 1 Department of Mechanical Engineering, Henan University of Engineering, Zhengzhou, 451191 China; *Corresponding author; Tel: + 86 13526705963, E-mail: zzzhma@126.com. generation technology is one kind of energy recovery technology which converts heat into electricity directly. This kind of energy recovery technology has stable performance, no noise, no wear, small size, light weight, and long life which has attracted more and more attention in recent years. There are two types of engine energy which can be recovered based on thermoelectric generation technology[7]. One is the coolant and the other is the exhaust gas. Currently researches on engine exhaust gas energy recovery has been paid more attention to the relationship of thermo-electric structure form, cooling way, fin length, radiator formal structure and heat transfer efficiency [8,9]. While fewer researches have been done on coolant energy recovery and only one research using thermoelectric generator (TEG) to replace the engine radiator has been reported. The results showed that at 80km/h the energy recovery efficiency was 3.2% while at idle speed the energy recovery efficiency is up to 10%[10,11]. In order to use TEG to achieve energy recovery, the premise is the temperature difference between the both ends of TEG modules, which connectes the cold source and heat source respectively [12]. For the LNG engine, its fuel is LNG which is the natural gas stored in liquid form at about -162 ~-140 and it needs to be heated and become gaseous form before it can be induced into the engine. The gasofication process needs to absorb heat from the surrounding and can produce low temperautre phenomenon which can be reffered as cold energy. The cold energy has many types of applications, such as car cab refrigerator [13] and cabin air-conditioning [14]. So it also an ideal cold source for TEG.The output power can reach 1.97W when the cold end was

fixed at -120 C and the temperature difference between the two ends maintained at 120K and the TEG is made up with 26 thermoelectric modules[15]. Meawhile, the output power can rise to 414.8 W theoretically when the hot end is used with the engine cooling water and the multi-layer material is also used[16]. And the energy recovery efficiency reaches 0.9% and the TEG power density reaches 8.0. W/m when the new finned tube method was used[17]. The TEG efficiency can reach 9% when the cold end was fixed at 130K and the temperature difference between the two ends maintained at 130K[18]. It also showed that the TEG efficiency is affected by the materials of TEG modules and the effeciency is good when the P-type material is Bi 2 Te 3 Sb 2 Te 3 while the N-type material is Bi 2 Te 3 Bi 2 Se 3.So the TEG efficiency is related to the characters of the TEG[19]. The characters of the TEG are mostly characterized by seebeck coefficient which is defined as the magnitude of an induced thermoelectric voltage in response to a temperature difference across two ends of TEG. In actual application the seebeck coefficient is set as a constant value and [20-23]. However, there is error when the output voltage was calculated byv T, (where: V is output voltage, T is the temperature difference, is seebeck coefficient) [24].Moreover, the effective seebeck coefficient was presented by Cheng-Ting Hsu and it is suggested that the effective seebeck coefficient should be used to calculate the output voltage [25]. And the previous research also indicated that seebeck coefficient was reduced with the increase of temperature difference of TEG no matter which end of the TEG [26]. The changes of TEG characters should be revealed when the TEG was applied to LNG engine energy recovery. So one test bed was established to study the characters of TEG at low temperature. 2. TEST BED SETS The real LNG cold energy system is installed in the LNG engine intake pipe and it is between the LNG tank and carburetor which is shown in figure 1. This system can maintains its original structure and keeps its performance. And the structure of the LNG cold energy recovery is shown in figure 2. It includes three parts, LNG channel, engine coolant channel, and TEG modules. The engine coolant channel is filled with engine coolant and its temperature is nearly 90 which can be refer as hot end of TEG system. While, the LNG channel is filled with LNG which has cold energy and can be refer as cold end of TEG system. Fig.1 Diagram of LNG cold energy recovery system installation position Fig.2 Diagram of LNG cold energy recovery system However, the experiment based on real LNG engine is difficulty and hard to control the influence facts. So one test bed is designed to simulate the LNG cold energy recovery system working progress. And the schematic diagram of the TEG test bed is shown in figure 3. The system can be divided into four parts: the cold end, the hot end, the TEG modules and the measurement system. Among them, the cold end is made of aluminum container which is cooled with liquid nitrogen. The hot end is one box with water and can be heated with external power. The TEG modules are F30345 and the size is 40 mm 40 mm 3.8mm whose internal resistance is 2.4Ω, seebeck coefficient is 0.000375V/K, thermal conduction coefficient is

Vol. XX (YEAR) Running head/short title maximum 80 characters pp 0.5W/K. And its material is Bi 2 Te for N type semiconductor and Sb 2 Te 3 for P type semiconductor. It produced by Orient Electric Co.,Ltd, China. Fig.3 Diagram of TEG experiment test bed In order to measure the temperature of the cold and hot end the Pt100 thermo resistor was applied and its accuracy is ±0.2 and reaction time is 2.50s. The relationship of temperature and output resistance is shown in equation (1). R( t ) R (0 C ) [1 At Bt Bt C ( t 100 C) t (1) In which the R( t ) is the output resistance, t is the temperature, R ( 0 C ) is the resistance at 0, A is one constant parameter, and the value is 3.9083 10-3, B is one constant parameter, and the value is -5.775 10-7, C is one constant parameter, and the value is -4.183 10-12. Then the processing circuit was applied for thermo resistor and Altai's USB2002 acquisition system was also used for data acquisition. The sample frequency was calculated by main frequency and 2 3 ] sample channels and the relationship is shown in equation (2). F F / N sample main channel (2) In which the F sample is sample frequency, F is main the main frequency, N channel is the number of sample channels. In order to reduce the thermal resistant the silicone is used to connect the four parts. Meanwhile, some copper are applied in the test bed to control the temperature of both ends by changing the number which is shown in figure 1. The hot end is filled with water which can be heated. This water temperature control system is shown in figure 4 and it is based on the micro controller. The target temperature can be set in the micro controller and actual temperature is measured with thermocouple sensor. In order to control the water temperature the thermocouple signal should be converted with the processing circuit chip MAX6675. Fig.4 Diagram of the water temperature control system The bias x between the target temperature Y and actual temperature X is calculated in the micro controller and the heater is turned on if the actual temperature is lower than the target temperature or else the heater is turned off. And the function of the control system is represented by f. Y f ( x X ) (3) Based on the diagram of the control unit, the control circuit schematic is shown in figure 5 which includes the AT89C51 microcontroller, clock circuit, power supply circuit, a rectifier filter circuit, voltage

Output voltage/v regulator circuit, a display device circuit and reset circuit, et al. Fig.5 Schematic circuit of the water control system 3. RESULTS AND DISCUSSION It should be noticed that too low temperature is hard to reach for the test bed and the lowest temperature is -64 in this experiment. Figure 6 shows the vibration of the temperature difference and output voltage when the liquid nitrogen is poured into cold end container and the hot end temperature is fixed at 100. It can be seen from the figure that the maximum output voltage is 6.7V and minimum is only 0.9V.And the reason is that the temperature difference is getting big. It also indicates the output voltage increases with the increase of temperature difference. 7 6 5 4 3 2 1 0 20 30 40 50 60 70 80 90 100 110 Temperature difference/ Fig.6 Relationship between the temperature difference and output voltage It reveals that the output voltage has no linear relationship with the temperature difference especially when the temperature difference is larger than 70. The increase rate of the output voltage is higher when the temperature difference is large. The temperature difference change can be implied by the changes of hot

Temperature/ Vol. XX (YEAR) Running head/short title maximum 80 characters pp end and cold end temperature which are shown in figure 7. It indicates that both of the hot and cold end temperature decrease when the liquid nitrogen is poured into cold end container. But the decrease rate of cold end is bigger than the hot end. And it also can be seen from figure 7 that the cold end temperature is lower than 0 when the temperature difference is larger than 70. Then the temperature difference is almost steady. 120 100 80 60 40 20 0-20 -40 Hot end Cold end Temperature difference -60-80 0 2 4 6 8 10 12 14 Time/s Fig.7 Changes of temperature for different parts It is well known that the seebeck coefficient is calculated in the following equation: V 0 (6) H C ( T T ) Where: is seebeck coefficient; V is Output 0 circuit voltage, V, T H is temperature of hot end,, T C is temperature of cold end,. So the slope of the curve in figure 6 is the seebeck coefficient of the TEG. Therefore, it can be concluded that the seebeck coefficient is changed at different temperature condition which means the temperature has an effect on seebeck coefficient. In the further analysis the temperature of the hot end and cold end is fixed respectively. And the influence factor is researched in the following part. Effect of cold end temperature on seebeck coefficient In order to study the relationship between the seebeck coefficient and cold end temperature, some experiments have done when the hot end is fixed at 51. It can be seen from the figure 8 that the seebeck coefficient varies with the cold end temperature. When the cold end temperature is -57.5, the seebeck coefficient is about 0.0606 and the seebeck coefficient reaches a maximum 0.0612 when the cold end temperature is -59.5. But the seebeck coefficient decreases when the cold end temperature reduced and the value is only 0.0597 when the temperature is -61.5. So it can be concluded that the seebeck coefficient increases firstly and then decreases with the decrease of cold end temperature when the hot end temperature is fixed at normal temperature and the cold end temperature is at low temperature.

Output voltage/v Seebeck coefficient 0.0612 0.0610 0.0608 0.0606 0.0604 0.0602 0.0600 0.0598 Hot end temperature was fixed at 51 0.0596-62 -61-60 -59-58 -57 Cold end temperature/ Fig.8 Relationship between the seebeck coefficient and cold end temperature It is known that the seebeck coefficient is based on the property of the semiconductor, which is related to the crystal structure and chemical composition of the material. In general, it increases with the temperature decrease of both ends of TEG at normal temperature area [26]. However, there is a peak value for certain material and the seebeck will decrease when the temperature decreases [27]. As mentioned before its material is Bi 2 Te for N type and Sb 2 Te 3 for P type semiconductor for the TEG applied in this experiment. So the peak value appears and there is different trend at each side of the peak. It should be noted that the output voltage will also appear a peak value, and the trend of output voltage is the same as seebeck coefficient which is shown in figure 9. But the slope of output voltage curve is small than the seebeck coefficient when the temperature of cold end is lower than the -60 due to the influence of temperature difference which can be seen in equation (6). 6.85 6.80 6.75 6.70 6.65 6.60 6.55 Hot end temperature was fixed at 51 6.50-63 -62-61 -60-59 -58-57 -56 Cold end temperature/ Fig.9 Relationship between the output voltage and cold end temperature Effect of hot end temperature on seebeck coefficient The figure 10 shows the relationship between the seebeck coefficient and hot end temperature when the cold end is fixed at -57.5. It can be found that the maximum value of seebeck coefficient appears at 41.7 and the value is 0.0649 while the minimum value is 0.0567at 46.7. And the seebeck coefficient is decreased with the increase of hot end temperature. From the foregoing analysis, the seebeck coefficient of the semiconductor material is reduced with the temperature increase of both ends of TEG. So

Output voltage/v Seebeck coefficient Vol. XX (YEAR) Running head/short title maximum 80 characters pp when the hot end temperature is fixed, the seebeck coefficient is reduced with the increase of cold end temperature. However, the output voltage shows a different trend when the hot end temperature increases which is shown in figure 11. And the output voltage increases with the increase of hot end temperature. This can also be explained by the temperature difference increase and can be calculated by equation (6). 0.066 0.064 Cold end temperature is fiexed at -57.5 0.062 0.060 0.058 0.056 41 42 43 44 45 46 47 Hot end temperature/ Fig.10 Relationship between the see-beck coefficient and hot end temperature 6.3 6.2 Cold end temperature is fiexed at -57.5 6.1 6.0 5.9 40 41 42 43 44 45 46 47 48 Hot end temperature/ Fig.11 Relationship between the output voltage and hot end temperature Coefficient difference at normal and low temperature condition It can be seen from the analysis above that the seebeck coefficient is nearly 0.06 when the cold end temperature is lower than -50 and hot end temperature is higher than 40. But when the both ends temperature is higher than 22 the seebeck coefficient is only 0.04 [26] which is only 67 percentage of 0.06. So this is good for LNG engine energy recovery which can supply low temperature for the cold end. Meanwhile, it is also helpful to achieve higher efficient at low temperature in other application conditions. 4. CONCLUSION In order to recover the cold energy of LNG engine, the relationship between the temperature and characters of TEG at low temperature was studied based on the test bed and the following conclusions can be reached. (1) At low temperature condition, there is a peak value of seebeck coefficient when the cold end temperature and the hot end is fixed at certain temperature.

(2) The seebeck coefficient reduces with the increase of cold end temperature when the temperature of cold end is higher than certain temperature. (3) The seebeck coefficient increases with the decrease of cold end temperature when the temperature of cold end is lower than certain temperature. (4) The seebeck coefficient of TEG at low temperature condition is higher than normal temperature condition. Acknowledgement The work is supported by Young college teachers funded projects in Henan Province (No.2014GGJS-120) and local college national college students innovation and entrepreneurship training program 2017 (NO.201711517006). The authors would like to express appreciation of financial support by the Reliability Engineering Center of Henan Institute of Engineering and the Power-driven Machinery and Vehicle Engineering Research Center of Henan Institute of Engineering. References [1] A Matthew. A. B. Kromera, E. Christopher, 2010. 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