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Article

Performance and Reliability of Thermoelectric Conversion Using a Crooked Thermosyphon to Enhance Heat Transfer from Coal Fires

by
Qingfeng Bao
1,
Xiuting Guo
1,
Bo Li
2,
Wuyi Chen
2,
Zhenping Wang
3 and
Yang Xiao
3,*
1
Ventilation Section, Shanxi Lu’an Huanbao Nengyuan Kaifa Co., Ltd., Changzhi 046204, China
2
Changcun Coalmine, Shanxi Lu’an Huanbao Nengyuan Kaifa Co., Ltd., Changzhi 046204, China
3
School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China
*
Author to whom correspondence should be addressed.
Processes 2024, 12(12), 2692; https://doi.org/10.3390/pr12122692
Submission received: 17 October 2024 / Revised: 14 November 2024 / Accepted: 25 November 2024 / Published: 29 November 2024
(This article belongs to the Special Issue Advances in Coal Processing, Utilization, and Process Safety)
Browse Figures
Versions Notes

Abstract

:
A large amount of energy can accumulate and be stored during underground coal fires. As thermal energy cannot be easily removed using the traditional technologies of fire prevention and extinguishment, there is a potential benefit to collecting and utilizing thermal energy from coal fires and converting it to electrical energy. Thus, this work proposes a thermoelectric generator as a solution to convert thermal energy from coal fires to electrical energy. To improve the thermal energy conversion efficiency, an experimental test system was established using a thermosyphon, an electric heating module, a cooling circulation module, a thermoelectric module, and a data acquisition module. Under the condition of ensuring the same input heat and cooling boundary conditions, the influence of three factors, namely the cooling method, the connection method, and the coverage rate of thermoelectric devices, on the performance of the coal fire waste heat conversion system was studied. The results show that, compared with air cooling, water cooling provides a greater temperature difference for the thermoelectric module, and the maximum temperature difference can reach 65.90 °C. Series connection between thermoelectric devices will generate a higher open-circuit voltage and output voltage. The maximum horizontal open-circuit voltage value can reach 3.34 V, and the maximum output voltage is 2.61 V. Compared with the coverage rates of thermoelectric devices of 15.0% and 30.0%, the output power under the coverage rate of 22.5% is the largest at 0.35 W, and its thermoelectric conversion efficiency is also the largest at 0.35%. The optimal combination of thermoelectric modules obtained from the research results can provide ideas for the application of in situ coal fire prevention and control.

1. Introduction

In the context of the rapid development of global industrialization and socialization, heat energy utilization and conversion are a continuous and important concern [1,2,3,4]. In recent years, geothermal development has become a promising approach. Geothermal energy is a new clean energy source and is considered to be one of the largest renewable energy sources. However, it is slower than other power generation methods, owing to the low rate of return in the geothermal conversion process. On the other hand, coal fire disasters are extremely serious in China [5]. The principle of traditional fire prevention methods was to build a barrier, isolate oxygen, and eliminate heat energy. However, the huge heat energy accumulated and stored from a large range of underground coal fires could not be effectively transferred and eliminated through fire extinguishing technologies and could easily lead to the reignition of coal fires [6,7]. Unfortunately, this tremendous heat energy was also unwittingly wasted. According to statistics, globally, coal fires burn about 1.0 billion tons of coal resources every year, generating about 1000 GW of power energy. This is equal to the combined energy of the world’s 500 nuclear power plants and 2.5 times as much as nearly 100 GW of hydroelectric power generation [8].
As efficient heat transfer equipment, thermosiphons have been applied in heat extraction from coal fires, owing to their simple structure, high heat transfer efficiency, and excellent isothermal performance [9]. The working medium, filling ratio, and geometric structure have been varied to improve the heat transfer performance within the thermosiphon. Sarkar et al. [10] found that regardless of the operating conditions, the working fluid enters the condenser in a superheated state and enters the evaporator in a subcooled state. Based on this, they derived expressions for predicting changes in temperature, pressure, specific volume, entropy, and enthalpy in each thermodynamic process, as well as expressions for estimating entropy generation. Machado et al. [11] estimated the thermal performance of thermosyphons through artificial neural networks and found that when the filling rate is 40%, the thermal performance is the best, and a slope of 45° shows better performance than a slope of 90°. Kim et al. [12] studied the relationship between the tilt angle, system charge, and heat transfer performance of a gravity thermosiphon. The optimal working conditions were a tilt angle of 30° and a system charge of 0.5 kg. Solomen et al. [13] found that the heat transmission coefficient of a thermosyphon’s evaporation section increased by 45% when the inclination angle was 45°. Alammar et al. [14] performed a numerical simulation and found that in the positive temperature zone, the minimum thermal resistance of the thermosiphon appeared at a 90° inclination and a 65% filling factor. Khazaee et al. [15] obtained that when the angle of inclination was 40–60°, the evaporation section’s heat transfer coefficient was the largest. In conclusion, changing the inclination angle could effectively improve the heat transfer performance of a thermosiphon. A thermosiphon’s heat transfer capacity is closely related to the heat transfer process at the formation side, so strengthening the structural design of the thermosiphon and increasing the heat transfer coefficient were breakthrough points to realizing the effective use of heat energy from coal fires.
Based on the Seebeck effect, thermoelectric technology was used for the direct conversion of thermal to electrical energy, which could be regarded as a generator without any moving components [16,17,18]. As a result, this generator had higher reliability, low maintenance, and longer service life compared to conventional generators [19,20,21,22]. In addition, it was noiseless, environmentally friendly, and directly converted heat from the environment into electrical energy. Coal fires naturally play the role of thermal engines for semiconductor power generation because of their huge thermal energy. Moreover, thermoelectric conversion efficiency was affected by many factors during its continuous improvement. Under the TE module for TE materials, Hu et al. [23] developed a three-dimensional numerical model and verified it using an experimental device. Shi et al. [24] found a way to obtain higher heights, larger cross-sectional areas, and even smaller TEs under matched load resistances, as well as to build a re-sized T-module under ideal conditions of constant heat input. Hans et al. [25] established a parallel connection to the multi-stage thermoelectric device skewers using the control volume method, which analyzed the output characteristics of the coal fire waste heat conversion system from three different connection modes (all in parallel, all in series, and mixed), and proposed a selection indicator of a string parallel circuit: load resistance, load required voltage, and current. Zhao et al. [26] applied different thermoelectric devices to the coal fire waste heat conversion system and explored the influence of the coverage rate of thermoelectric devices on the heat transfer capacity and the thermosiphon’s thermoelectric conversion efficiency. Li et al. [27] studied the tube-fin heat exchangers in a TEG as the heat and fluid flow of hot side channels. Nugraha et al. [28] utilized experimental and computational studies to research the influence of the heating rate of thermoelectric materials on the hot side on the electrical output. They discovered that when the heating rate was within the range of 0.94 to 5.67 K/s, the voltage, current, and power of the thermoelectric generator increased with the increased temperature.
The aforementioned achievements helped researchers to select and design high-performance thermoelectric generators. However, underground coal fires are complicated events with diverse causes and various constraints. Therefore, this work will consider coal fire prevention and thermoelectric resource utilization and will investigate the conversion performance of thermoelectric generators. A thermosiphon with a curved well structure was designed to remove thermal energy. On this basis, a waste heat conversion system was created by adding thermoelectric modules in order to explore the effect of different combinations. Furthermore, the optimization of the combined thermoelectric modules aimed to improve the thermal energy utilization and conversion capability of the system, inhibiting coal fire spread and achieving resource utilization.

2. Theoretical Consideration

The thermoelectric generator (TEG) is mainly composed of four parts, namely the heat pipe serving as a heat source, the copper block enhancing the heat conduction ability, the thermoelectric device for thermoelectric conversion, and the water-cooled heat exchanger. The heat exchanger on the hot side of the TEG has the function of absorbing heat from the outside and transferring it to the thermoelectric module. When there is a temperature difference between the hot and cold sides of the TEG, the combination of p-type and n-type side semiconductor pairs has the function of generating electricity. The schematic diagram of the TEG system is shown in Figure 1. The thermoelectric devices of TEG components are connected in series with the thermally conductive copper block. Heat is transferred to the thermally conductive copper block through the heat pipe. The thermally conductive copper block is connected to the hot side of the thermoelectric device and serves as a heat exchanger. The cold side is connected to the water-cooled heat exchanger and serves as a radiator. If the hot side of the TEG absorbs a large amount of heat, the electron energy on the hot side inside the thermoelectric device will be higher than that on the cold side. This will cause the electrons on the hot side of the TEG to enter the cold side at a higher speed than that with which the electrons on the cold side enter the hot side [4]. At this time, the hot side end of the thermoelectric device will carry a positive charge and the cold side end will carry a negative charge. Equation (1) shows the relationship between the induced voltage difference and the temperature difference.
α = − Δ V Δ T
Among them, α refers to the Seebeck coefficient of the semiconductor material and ΔV and ΔT represent the potential difference and temperature difference in the TEG, respectively. Equation (2) can be used to calculate the voltage generated by an open circuit when no current is flowing.
V K = α Δ T T E G = α T h − T c
Tc and Th represent the temperatures of the cold side and the hot side of the TEG, respectively. VK represents the voltage generated by an open circuit when no current is flowing, and ΔTTEG represents the temperature difference between the two sides of the TEG.
At present, the equivalent thermal conductivity (effective thermal conductivity, equivalent-value thermal conductivity, or comparable thermal conductivity) is used to evaluate the heat transfer performance of a thermosyphon. Specifically, the thermosyphon is considered to be equivalent to a solid-metal pipe with the same external dimensions as the thermosyphon. Although it reflects the quality of the heat transfer performance of the thermosyphon to a certain extent, it is greatly affected by geometric factors, which weakens the influence of the boiling heat transfer coefficient of the evaporation section and the condensation heat transfer coefficient of the condensation section on the heat transfer performance. Considering the convective heat transfer process of the working medium inside the thermosyphon, the equivalent convective heat transfer coefficient, as a new calculation method, is used to quantitatively evaluate the heat transfer ability of the thermosyphon. Equation (3) is as follows:
α e f f = 2 λ Q i n L e + L c 2 π λ d i T w , h , o − T w , c , o − Q i n d i L e + L c ln d o / d i
In the formula, αeff represents the heat transfer performance of the thermosyphon; Qin is the input heat, in watts (W); Le and Lc are the lengths of the evaporation section and the condensation section of the thermosyphon, respectively, in meters (m); di and do are the inner and outer diameters of the thermosyphon, respectively, in meters (m); λ is the thermal conductivity coefficient of carbon steel as the material, 48 W/(m·K); and Tw,h,o and Tw,c,o are the wall temperatures of the evaporation section and the condensation section, respectively, in degrees Celsius (°C).

3. Materials and Methods

3.1. Experimental System

The experimental test system comprised five elements: the thermosiphon, the electric heating device, the water-cooled cycle device, the thermoelectric device, and the data acquisition device. The physical picture is shown in Figure 1. Recent studies have shown that thermal resistance is mainly controlled at the formation side in thermal energy devices such as thermosiphons, and inclined or curved structures revealed better heat transfer performance than vertical structures. When the angle was 30°, the pipette’s heat transfer performance could be improved by increasing the evaporation section’s boiling heat transfer coefficient [8]. Therefore, the angle of the evaporation section to the coalbed was optimized and a thermosiphon with an evaporation section inclination angle of 30° was designed in this work. In the experiment, carbon steel, which is commonly used in normal-temperature and medium-temperature thermosyphons, was selected as the material for processing thermosiphon, and the processed thermosiphon was further cleaned. The carbon steel has an outer diameter of 22 mm, an inner diameter of 19 mm, and a height of 0.8 m. Conventional distilled water was selected as the internal working medium of the thermosiphon with a 30% filling ratio.
The electric heating device was composed of flexible heating wire (HTC-060, Omega, Norwalk, CT, USA) and a power regulator (STG-1000, Xishuo, Beijing, China). The wire’s diameter, total length, and power were 4.80 mm, 1.80 m, and 125 W, respectively. A power regulator with an auto-regulator range of 0–300 V was used with the heating wire.
The water-cooled cycle device mainly includes a chiller (DIC006ASS-LA2, Ruitou, Xi’an, China) and a water-cooled heat exchanger. The chiller can provide constant heat transfer boundary conditions for the thermoelectric system by setting the cooling flow rate and water temperature. The size and thickness of the thermoelectric device (TEP-1263-2.4, Nanchang, China) were 30 × 30 mm and 2.4 mm, with 126 pairs of P-N sections internally. The thermoelectric conversion performance parameters of the thermoelectric devices under a constant cold surface temperature of 30 °C are shown in Table 1. To enhance heat conduction, thermal copper blocks were chosen as the material. It has an inner diameter of 23 mm, and the external dimensions of the thermal copper blocks are length × width × height: 30 × 30 × 30 mm, as shown in Figure 2.
The heating system is composed of a flexible heating wire and a power regulator. In the experiment, the HTC-060 flexible heating wire manufactured by OMEGA was chosen. This wire has a diameter of 4.8 mm, a total length of 1.8 m, and a power of 125 W. During the experiment, the heating wire was evenly and densely wound around the evaporation section of the thermosyphon to obtain a stable and uniform heat-flux density. Given that the power in the experimental design was relatively low, the power regulator needed to reduce the voltage for the purpose of adjusting the power of the heating wire.
Therefore, in combination with the selected range of input power values, in the experiment, a power regulator of model STG-1000W with auto-coupling voltage regulation from 0 to 300 V manufactured by Xishuo Technology Co., Ltd. (Suzhou, China) was selected for use in conjunction with the heating wire. A physical picture of the heating wire and the power regulator is shown in Figure 3.
The data acquisition device mainly consists of a multi-function data conversion module, a data acquisition switch unit, a PC computer, special processing software, and thermocouples. The data acquisition device can directly measure the thermocouples, resistance, AD/DC voltage, and other parameters. The thermocouples used mainly included ETAGK30K high-temperature thermocouples (Aiyong, Suzhou, China) (up to 480 °C, accuracy 0.3 °C) and ETA1006 T thermocouples (Aiyong, Suzhou, China) (up to 150 °C, accuracy 0.1 °C).
The data acquisition device is shown in Figure 4, andit will be used in conjunction with the Bench Vue Data Acquisition (DAQ) software (Keysight DAQ970A). In the experiment, the data acquisition unit is used in conjunction with the corresponding module. It can directly measure parameters such as thermocouples, resistances, AC/DC voltages, etc. The characteristics are as follows:
(1)
The scanning rate supports up to 450 readings per second at most.
(2)
Each channel can be set with Ax + B to calibrate thermocouples and support over-temperature alarms.
(3)
The configured Bench Vue DAQ data analysis software supports a graphical network interface, which is convenient for the real-time display and analysis of measured values.
(4)
With a built-in signal conditioning function, it minimizes the noise from external wiring entering the system and avoids subjective speculation in error analysis.

3.2. Design Scheme

The experiments were carried out by arranging a thermoelectric module in the condensing section based on the determined tilted structure of the thermosiphon’s evaporating section to ensure the same conditions of heat input. The thermoelectric conversion performance of the system was investigated by examining the effects of three factors, namely the thermoelectric device coverage (15.0%, 22.5%, and 30.0%), connection mode (series and parallel), and cold-end heat dissipation mode (air-cooled and water-cooled), on the heat transfer process. Figure 5 shows the schematic diagram of the covering principle of thermoelectric devices. The optimum operating parameters for each factor were determined to obtain the optimum thermoelectric conversion efficiency of the coal fire waste heat conversion system at constant heat flow, where the thermoelectric device coverage was the area of the thermoelectric unit covered to the overall area of the condensing section.

3.3. Experimental Conditions

Taking the experimental test system with a thermoelectric device coverage rate of 22.5% under the water-cooled mode as an example, as shown in Figure 6, the input power was set to 100 W and a thermosiphon with an inclination angle β = 30° was chosen as heat extraction device. The pipeline and heating section of the cooling circulation system were wrapped in thermal cotton to reduce heat loss. Based on the measuring point of the external wall surface of the heat exchanger, the heating was increased in the four-way outlet, obtaining a fixed difference in temperature at the cold and hot ends of the thermoelectric devices at different levels of the thermosiphon. T-type thermocouples were arranged at the hot and cold ends of each of the thermoelectric devices and fixed with high-temperature thermal conductive tape to measure the temperature change at the cold end. The operation process was as follows:
  • The cooling circulation system. The circulation water temperature was set to 10 °C (with the ambient temperature as a reference), and the flow rate was 8.3 L/min. Subsequently, the water temperature was stabilized.
  • The heating system. The input power was adjusted to be stable near 100 W, and the thermosiphon was heated for a period until the coal fire waste heat conversion system ran stably and the temperature of the relevant measuring point did not fluctuate greatly.
  • The data acquisition system. During the stable operation of the coal fire waste heat conversion system, the horizontal open-circuit voltage and total open-circuit voltage of the thermoelectric devices were measured and recorded by a multimeter. In addition, the load coil was added, forming a loop to measure the output voltage and current.
It can be concluded from the test that during the experiment, a thermosiphon with an inclination angle β = 30° should be chosen as the heat extraction device, the cooling circulation pipeline and heating section need to be wrapped with thermal insulation cotton, and the cold and hot sections of each thermoelectric device should be arranged with T-type thermocouples to measure temperature changes. The difference between air-cooled and water-cooled modes was that in the air-cooled mode, there was natural air convection, and the condensing section and thermoelectric module carried out heat exchange with the surrounding environment of the experiment without increasing the cooling circulation system. The measuring points are distributed on the outer wall of the thermosiphon at the four-way water inlet and outlet points.

3.4. Data Processing and Uncertainty

Under the premise of a single material length scale and module, the thermoelectric device’s power performance was consistent with the heat source’s hot end temperature and the cold end’s heat dissipation mode. The temperature difference between the hot and cold ends of the thermoelectric devices can be calculated as follows:
∆ T = T T E M , h ¯ − T T E M , c ¯
where ∆T is the temperature difference between the hot and cold ends of the thermoelectric devices. TTEM,h and TTEM,C are the average temperatures of the hot and cold ends for the coal fire waste heat conversion system under stable operation, respectively, in °C.
The nominal power calculation formula is as follows:
P = U 2 R
where P is the nominal power, W. U is the output voltage, V. R indicates the load resistance, Ω.
The formula for the thermoelectric conversion efficiency of the coal fire waste heat conversion system is as follows:
η = P Q i n
where η is the thermoelectric conversion efficiency, %. Qin is the input power, W.

4. Results and Discussion

4.1. Compatibility Analysis of Thermosiphon and Thermoelectric Module

The thermoelectric conversion performance is mainly affected by the heat source temperature and cold end temperature. When the boundary conditions of heat transfer at the cold end are determined, the fit between the thermosiphon and the thermoelectric module determines the heat transmission. The heat of the thermosiphon absorption transfer in the coal fire waste heat conversion system was taken as the thermoelectric device’s converted electrical energy. The other part is the heat loss generated during the transfer, including the heat loss of the heat transfer medium (conductive copper block) and the loss to the environment. Figure 7 describes the variation in the wall temperature of the condensing section of the thermosiphon and the thermoelectric device’s hot end temperature with the thermoelectric device’s coverage rate under air-cooled and water-cooled modes. The compatibility of the thermosiphon and the thermoelectric module was explored by comparison.
As Figure 7a shows, under air-cooled conditions, there is a similarity between the thermoelectric device hot end temperature trend and the condensation section temperature trend. The heating system was initiated, but due to the short heating time of the thermosiphon, it failed to make any changes. When the internal working medium of the thermosiphon was heat-evaporated, the system started to work immediately. The sharp rise in temperature in the condensing section also caused the hot end temperature of the thermoelectric device to rise rapidly. After a period of time, the heat input and output of the system were in balance, and the temperatures of the condensing section and the thermoelectric device also tended to stabilize. When the thermoelectric device had a coverage of 15.0%, the condensing section’s wall temperature was similar to the temperature of the hot end of the thermoelectric device, but the latter was slightly lower. When the coverage of the thermoelectric devices increased to 22.5%, the gap between the two increased, and the thermoelectric devices’ temperature decreased compared with previous values; this is mainly due to the increased number of thermoelectric devices, which caused the volume of the heat transfer medium to increase and the heat loss to also increase. As the number of thermoelectric devices continued to increase, the gap between the two decreased, and since the coverage of the thermoelectric devices increased, the heat exchange area was reduced from the condensate section to the external environment, thus delivering more heat to the thermoelectric devices. Therefore, the temperature of the hot end of the thermoelectric device did not change based on the amount of heat transfer medium. The wall temperature of the condensing section presented very similar changes under the three coverage rates. It could be seen that, compared with 15.0% coverage, 22.5% coverage had a higher utilization rate, although the heat loss increased, while 30% coverage was allocated by the same amount of heat due to the number of media; therefore, its utilization rate was also less than that of 22.5%.
As presented in Figure 7b, the water-cooled mode had a more pronounced impact on the coal fire waste heat conversion system than the air-cooled mode. With the increase in the coverage rate of thermoelectric devices, the air-cooled temperature difference gradually rises, with the maximum temperature difference increasing from 5 °C to 20 °C and the wall temperature of the condensation section reaching a maximum close to 160 °C. Under water-cooled conditions, however, as the coverage rate of thermoelectric devices increases, the maximum temperature difference does not change much, and the wall temperature of the condensation section reaches a maximum close to 100 °C. The temperature difference between the condensing section and the hot end of the thermoelectric device continued to grow over time as they were both trending in the same direction. The thermoelectric device’s hot end temperature dropped initially and then abruptly rose during the early stages. The thermal conductivity of liquid water was roughly 0.59 W/m/K, which was significantly higher than that of air (0.02 W/m/K) at standard pressure and temperature [29]. The water-cooled heat exchanger’s cooling force was greater than that of the thermoelectric device at first, causing a temporary fall in the temperature of the thermoelectric device’s hot end. The latter, which was consistent with that of the air-cooled mode, had a rapid increase to a stable change. Under the water-cooled mode, the temperatures of both the condensing section and the thermoelectric element were lower than under the air-cooled mode, and the temperature difference between the two significantly increased. The reason for this phenomenon was that, on the one hand, the water-cooled heat exchanger had a significant impact on the system and more heat was absorbed by the internally circulating water after the thermal copper block and the thermoelectric device, which accelerated heat loss in the condensing section. On the other hand, the thermoelectric device made direct contact with the heat exchanger. Due to the low temperature of the hot end, the condensing section and the thermoelectric device’s hot end had an increased temperature difference. The temperature difference between the condensing section and the thermoelectric device’s hot end when the coverage rate was 22.5% was relatively minor when compared to the temperature difference between the thermoelectric device coverage rates of 15.0% and 30.0%. Compared with the 22.5% air-cooled rate, the trend flattened earlier and the temperature differential grew because of the difference in the cooling method. The increase in heat dissipation caused by the increased volume of the heat transfer medium led the condensation section temperature and the thermoelectric device hot end temperature to range between 15% and 30% coverage. Simultaneously, the distance between the two lines gradually increased with the number of thermoelectric devices.
In summary, under both air-cooled and water-cooled methods, there is a huge difference in the temperature difference between the wall temperature of the condensation section and the hot end of the thermoelectric device. The temperature difference in the air-cooled mode is concentrated within 0–30 °C, and that in the water-cooled mode is concentrated within 0–40 °C. Meanwhile, under the air-cooled method, the maximum wall temperature of the condensation section reaches nearly 160 °C, while under the water-cooled method, the maximum temperature is only around 100 °C. This indicates that when water is used as a medium, it will store a portion of heat by itself, and the effect of using the water-cooled method on the coal fire waste heat conversion system is more significant.

4.2. Temperature Difference Between Cold and Hot Ends in Thermoelectric Modules

Considering the non-uniformity of the temperature distribution on the wall surface of the condensing section, the thermocouple devices’ cold and hot end temperature differences at different levels were further compared and discussed in detail. The two thermoelectric devices were arranged on the same horizontal plane. The position close to the evaporation section was called horizontal plane 1 (including the two thermoelectric semiconductor cooling plates). The other positions were named horizontal plane 2, horizontal plane 3, and horizontal plane 4 in the vertical direction, and each horizontal plane was equally spaced. Figure 8a,b present the temperature difference between the thermoelectric devices at different levels under air-cooled and water-cooled conditions, respectively. The trend in the different levels of temperature differences between thermoelectric-cooled pieces was observed under air-cooled and water-cooled modes. Although the coverage of the thermoelectric devices was different in the air-cooled mode, the temperature difference between thermoelectric devices increased sharply at first, then decreased rapidly, and finally tended to be stable.
Due to the complexity of the exothermic process of the condensation of the working medium in the thermosiphon, the thermoelectric components had different temperature differences at the same level. When the air-cooled mode was selected and the thermocouple coverage was 15.0%, the thermocouple temperature difference fluctuated in the range of 5–15 °C. When the thermocouple coverage increased to 22.5%, the thermocouple temperature difference at all levels was significant and fluctuated in the range of 10–17 °C. With the same input power and convective heat transfer, the increase in temperature difference indicated that more heat released from the condensing section was transferred to the thermoelectric device for thermoelectric conversion and less heat was dissipated to the environment. The overall temperature difference showed a decreasing trend as the coverage of the thermoelectric device increased. This was because the increased coverage of the thermoelectric module in the condensing section distributed the same heat more uniformly to the individual thermoelectric devices, which led to a reduction in the hot ends’ temperature, resulting in a decrease in the hot and cold ends’ temperature difference. The size of the difference in temperature was almost determined by the distance of the thermosiphon evaporation section: the closer the distance, the greater the difference in temperature, regardless of the coverage rate. In the scenario where horizontal plane 1 was greater than horizontal plane 2 and horizontal plane 3, temperature differences almost always existed. This was mainly caused by the higher temperature of the inner wall of the condensing section at that location. Nonetheless, the temperature difference between the left and right thermal electric devices was irregular. The temperature difference at level 2 was maximized at a coverage of 30%, which was due to the uneven heat distribution.
According to Figure 8b, when the water-cooled mode is selected, regardless of the thermoelectric device coverage rate, the temperature difference in the level 1 thermoelectric device is always greater than or equal to that of other levels. When the thermoelectric device coverage rate is 15%, the left–right temperature difference in level 1 is approximately 20 °C, and that of level 2 is approximately 3 °C. When the thermoelectric device coverage rate is 22.5%, the left–right temperature difference in level 1 is approximately 15 °C, while that of level 2 is still approximately 3 °C and that of level 3 is approximately 15 °C. When the thermoelectric device coverage rate is 30%, the left–right temperature difference in level 1 remains approximately 15 °C, while that of level 2 also increases to 15 °C, the left–right temperature difference in level 3 remains unchanged at 15 °C, and there is almost no temperature difference at level 4. When the temperature coverage was 22.5%, the overall difference in the temperature of the thermoelectric cooler increased, the maximum difference in temperature decreased, and the difference in temperature of the thermoelectric cooler was concentrated within 30–50 °C. With a further increase in the coverage of the thermoelectric device, more heat was dispersed toward the thermal device’s hot end, the thermoelectric device’s temperature difference was significant, and the fluctuation range of the change was large. Due to the lower coverage heat transfer efficiency, the rapid increase in the difference in temperature was delayed for 15% coverage of horizontal plane 2. As the coverage continued to increase, the maximum temperature difference progressively decreased, and the temperature differences at each level stage became closer and more likely to overlap.
The trends in thermoelectric equipment were different when choosing between air-cooled and water-cooled modes of heat dissipation. This was attributed to the superiority of forced convection heat transfer, which accelerated the heat loss from the cold end of the thermal power plant. In addition, the air-cooled mode was more unstable than the water-cooled mode.
The temperature profile became more and more compact as the coverage increased, and the ends of the air-cooled level 1 tended to be far apart. In addition, the overlap of cooling tended to be higher than that under the water-cooled mode at the same coverage. According to the principal analysis, the thermoelectric conversion relied on the cold and hot ends’ temperature difference for power generation, and the larger the temperature difference was, the more it improved the power generation performance of the system. Therefore, compared with the air-cooled mode, the water-cooled mode can enable the system to achieve better working conditions and output performance.

4.3. Horizontal Open-Circuit Voltage and Output Performance

Based on the good matching degree between the above systems and the advantages of the water-cooled mode, the effects of the coverage of thermoelectric devices on the thermoelectric module under the water-cooled mode were further explored. The open-circuit voltage of a battery is calculated as the difference between the electrode potential of the positive terminal and the electrode potential of the negative terminal when the battery is disconnected (when no current is flowing through the terminals). Therefore, the open-circuit voltage can directly reflect the ability of thermoelectric conversion. Just as the connection mode of the circuit elements would affect the current pass method, thus impacting the performance of the circuit elements, the connection mode between the thermoelectric devices affected the thermoelectric module’s power generation performance. Therefore, based on the cooling mode and the coverage rate of the thermoelectric device, series and parallel connection methods were added, and then the influence of these three factors on the thermoelectric conversion performance of the coal fire waste heat conversion system was also analyzed. Figure 9 depicts the open-circuit voltage at each level in different thermoelectric device coverages. Although the thermoelectric device had different positions, the open-circuit voltage at different levels showed the same change trend with the change in connection mode. With the increase in coverage at all levels, both series and parallel voltages gradually decreased. Under the same cooling conditions, when the thermoelectric devices were connected in series with each other, the open-circuit voltage was greater than when they were connected in parallel. For the same coverage, the increase multiple in each horizontal plane was similar, the open-circuit voltage at horizontal plane 1 was higher than that at the other horizontal planes, and the open-circuit voltage gradually decreased as the distance from the evaporation section increased. When the thermoelectric device coverage was 15.0%, the maximum horizontal open-circuit voltage could reach 2.34 V. When the coverage of the thermoelectric devices increased to 30.0%, the open-circuit voltage at level 2 was slightly lower than that at other levels. This was mainly due to the uneven heat distribution on the wall of the condensing section, which led to differences in heat distribution at different levels. As the coverage of the thermoelectric devices increased, the heat of the thermoelectric device allocated to different locations tended to the average value, leading to an overall decrease in the horizontal open-circuit voltage.
Based on the above comparison and the analysis of each open-circuit voltage under different connection modes, the experiment added a load to form a complete loop on this basis, taking the voltage and nominal power as the visual data. Then, the influence of the connection mode and load value selection on the thermoelectric module’s output performance was investigated, and the connection mode and the best matching load value were optimized. Based on the empirical values in previous research [21], 3, 7, 11, 15, and 20 Ω were chosen as the experimental values to measure the horizontal output voltage and the nominal power of different resistance values. To make the obtained rules more general and convincing, the horizontal output voltage and nominal power under the coverage of three kinds of thermoelectric devices were measured and calculated, as presented in Figure 5 and Figure 6, respectively. By observing the value of the output voltage and nominal power, it could be judged that the loop’s output voltage and nominal power in series were better than those in parallel connection, and the maximum output voltage’s corresponding load value was different from the maximum nominal power’s corresponding load value. As shown in Figure 10, the load resistance and its output voltage were approximately linearly related when connected in series and in parallel. The temperature of the voltage value at both ends of the load increased linearly with the increase in the load value when it was farther from the evaporation section. When farther away from the evaporating section, the voltage value at both ends of the load increases linearly with the increase in the load value. However, when closer to the evaporating section, when the load resistance increased, the output voltage also appeared to increase slowly. In addition, with more levels, the output voltage difference between the levels would decrease relatively.
The trends in the nominal power under the two connections were very different, as illustrated in Figure 11. As the load resistance value increased, the series connection nominal power presented an approximate parabolic variation. As the resistance value increased, the power increased at the beginning. In the parallel connection, the nominal power varied linearly with the increase in load resistance, and the load resistance corresponding to the maximum nominal power was 3 Ω. According to the maximum power transfer theorem, the maximum nominal power could be obtained when the value of the load resistance was equal to the value of the internal resistance of the DC circuit. In addition, it was estimated that the value of a single thermal device’s internal resistance was 5.5 Ω.

4.4. Heat Transfer and Power Generation Performance

The thermoelectric conversion performance of the coal fire waste heat conversion system was closely related to the heat transfer process between the various components of the system. Figure 12 plots the temperature distribution and trends in each part of the system during stable operation. As the coverage of thermoelectric devices increased, the temperature in different regions of the system also changed. The horizontal coordinates Evap, cond, TEMh, TEMc, Win, and Wout represent the evaporation section, the condensing section, the thermoelectric module’s hot end, the thermoelectric module’s cold end, and the water-cooled heat exchanger’s inlet and outlet in the system, respectively. For the thermosiphon and the thermoelectric module’s hot end, the temperature decreased as the thermoelectric device’s coverage increased. However, the trend in the temperature of the thermoelectric module’s cold end was the opposite, and the increased coverage under the same cooling conditions added to the heat transfer area for the heat exchanger and thermoelectric module, thus increasing the heat exchanger’s work force. Considering the heat exchanger’s higher cooling water flow rate, the water-cooled heat exchanger’s inlet and output temperatures varied less.
Based on the system’s temperature distribution in different areas, the heat transmission thermal resistances within the system can be further calculated to evaluate the coal fire waste heat conversion system’s heat transfer performance. The thermal resistance between the thermosiphon, the thermoelectric module, and the cold end of the thermoelectric module and the cooling water was calculated as follows [21]:
R h p = T h p − T t e m , h Q i n R t e m = T t e m , h − T t e m , c Q i n R w = T t e m , c − T w , i n + T w , o u t 2 Q i n
where Rhp is the thermal resistance of the thermosiphon tube, Rtem is the thermoelectric module’s thermal resistance, and Rw is the thermal resistance between the cold end of the thermoelectric module and the heat exchanger, K/W; Thp is the wall temperature of the thermosiphon tube, Ttem,h and Ttem,c are the temperatures of the thermoelectric module’s hot and cold ends, and Tw,in and Tw,out are the inlet and outlet water temperatures of the heat exchanger, °C.
From Figure 13, it can be seen that when the thermoelectric components’ coverage increases, the coal fire waste heat conversion system’s total heat transfer thermal resistance increases, and the maximum heat transfer thermal resistance of the system is 1.64 K/W. Comparing the thermal resistance of the three parts of the coal fire waste heat conversion system, the thermosiphon had the smallest thermal resistance, followed by the thermoelectric module’s cold end temperature and the heat exchanger, and the thermoelectric module had the largest thermal resistance. The heat transfer thermal resistance of the three parts of the system (Rhp, Rtem, and Rw) increased with the increase in coverage. The thermoelectric device’s coverage had little effect on the thermosiphon’s thermal resistance, which remained almost unchanged for the three coverage rates. The increase in coverage caused the thermoelectric module’s capacity and thermal resistance to increase accordingly. Meanwhile, the heat transfer area between the water-cooled heat exchanger and the thermoelectric module increased, and their contact increased thermal resistance. The temperature difference between the evaporation and condensation sections also had great influence on the thermoelectric module, and the thermosiphon’s thermal resistance could not be ignored. For the thermoelectric module, the coverage rate had a greater influence on the thermoelectric module’s thermal resistance, the thermal resistance of the cold end of the module, and the heat exchanger’s thermal resistance. In the selection of the coverage rate, researchers should consider the combined influence of heat transfer thermal resistance and system output performance.
Figure 14 describes the open-circuit voltage value of the coal fire waste heat conversion system under the coverage of three types of thermoelectric devices. When the coverage rate increased from 15.0% to 22.5%, the system’s open-circuit voltage significantly increased, and the growth rate was 24%. It was found that the system’s open-circuit voltage with 22.5% coverage and 30.0% coverage was approximately equal and did not increase significantly because of the increase in coverage. The results showed that the thermoelectric conversion performance of the coal fire waste heat conversion system did not increase with the increase in the coverage of the thermoelectric devices; the coverage had a peak value, and if this was too large, it caused resource waste.
Figure 15a,b depict that in the load resistance range of 3–20 Ω, when the load resistance increased, the output voltage increased, and the nominal power also rose. When the coverage was 15.0%, the output voltage could reach the maximum value, and the nominal power was also maximized, with the next highest values achieved at 22.5% coverage, whereas the minimum output voltage and nominal power values were observed when the coverage was 30.0%. When the load resistance was 20 Ω, the maximum output voltage and nominal power were 2.61 V and 0.34 W. However, the variation in the output voltage shows that the range of the load resistance selection was too small to achieve the optimum output performance for the system. Due to the limitation of the load coil, the system could not obtain the optimal nominal power for different coverage ranges directly by measurement. Therefore, the open-circuit voltage measured values and the thermoelectric device internal resistance estimated values were used to calculate the output voltage’s theoretical values, and then they were compared with the measured values, as presented in Table 2. Equation (8) is the calculation formula for the theoretical value of the output voltage. Among them, U’ represents the theoretical value of the output voltage, Uk is the open-circuit voltage value, and Rin and Rout represent the internal resistance and external resistance, respectively. The internal resistance is estimated to be 5.5 Ω. When comparing the theoretical and measured values of the output voltage under different coverage rates, the difference between the two was small, which in turn allowed us to determine the correctness of the estimated value of 5.5 Ω for the internal resistance of the thermoelectric device.
U ′ = U k R i n + R o u t
Based on an internal resistance value of 5.5 Ω, the optimal matching load value, the system’s maximum nominal power, and its maximum thermoelectric conversion efficiency under different coverages were determined, according to the maximum power transmission theorem. As seen in Table 3, at a 22.5% coverage, the optimal matching load value was 33 Ω, the optimal nominal power of the system was 0.35 W, and the optimal thermoelectric conversion efficiency was 0.35%. The above detailed discussion shows the impact of each factor on the thermoelectric conversion performance of the coal fire waste heat conversion system, including the cooling mode, the connection mode of the thermoelectric devices, and their coverage rate. The optimal combination of thermoelectric modules, which could ensure the optimal operation of the system, is the water-cooled mode, series connection, and a thermoelectric device coverage rate of 22.5%.

5. Conclusions

This work focused on the influence of the coverage of thermoelectric devices, the connection mode, and the cooling mode of the cold end on the thermoelectric generator. The main conclusions can be drawn as follows:
(1) Through changing the temperature of the condensation section and the thermoelectric module’s hot end under the air-cooled mode, it was found that the addition of thermal conductive media was beneficial for increasing the heating area of the thermoelectric modules. Compared to the air-cooled mode, the water-cooled mode had a more significant impact on the system in terms of improving the thermal energy utilization rate. It can provide a larger temperature difference for thermoelectric modules, with a maximum temperature difference of 65.90 °C.
(2) When the water-cooled method is selected and the thermoelectric device coverage rate is 15.0%, the temperature difference in the level 1 thermoelectric device is significantly higher than that of level 2, and the maximum left–right temperature difference in the thermoelectric device can reach 20 °C. When the temperature difference coverage rate is 22.5%, the overall temperature difference in the thermoelectric module increases, but the maximum temperature difference decreases, and the left–right temperature difference in the thermoelectric device is concentrated within the range of 15 °C. When the coverage rate is 30%, the overall temperature difference shows a downward trend and the left–right temperature difference in the thermoelectric device is still concentrated within the range of 15 °C.
(3) The horizontal open-circuit voltage was higher in series than in parallel. When the load was added to form a loop, it could be found that the output voltage was greater in series than in parallel, the maximum horizontal open-circuit voltage value can reach 3.34 V, and the maximum output voltage is 2.61 V. The thermoelectric device’s internal resistance was estimated to be 5.5 Ω.
(4) The system thermal resistance increases with the increase in the coverage range of the thermoelectric device. The thermal resistance of the thermoelectric module, the cold end of the thermoelectric module, and the heat exchanger are more affected by the coverage range. Comparing the open-circuit voltage, output voltage, and output power under three thermoelectric device coverage ranges of 15.0%, 22.5%, and 30.0%, when the coverage range is 22.5%, the output performance of the system is better, with an output power of 0.35 W and a thermoelectric conversion efficiency of 0.35%. The optimal combination of the thermoelectric modules obtained from the research results can provide ideas for the application of waste heat utilization in in situ coal fire disasters.

Author Contributions

Conceptualization, Q.B., Z.W. and Y.X.; methodology, Q.B. and X.G.; validation, B.L., W.C. and Z.W.; investigation, Q.B. and X.G.; resources, B.L. and W.C.; data curation, X.G.; writing—original draft preparation, Q.B. and X.G.; writing—review and editing, Z.W. and Y.X.; visualization, B.L.; supervision, Y.X.; project administration, Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Qingfeng Bao, Xiuting Guo, Bo Li, Wuyi Chen were employed by the company Shanxi Lu’an Huanbao Nengyuan Kaifa Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The Shanxi Lu’an Huanbao Nengyuan Kaifa Co., Ltd. had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Nomenclature

α Seebeck coefficient of the semiconductor materialdiinner diameters of the thermosyphon [m]
ΔVpotential difference in the TEG [V]doouter diameters of the thermosyphon [m]
ΔTtemperature difference in the TEG [°C]λthermal conductivity coefficient of carbon steel [48 W/(m·K)]
Tctemperatures of the cold side [°C]Tw,h,owall temperatures of the evaporation section [°C]
Thtemperatures of the hot side [°C]Tw,c,owall temperatures of the condensation section [°C]
VKvoltage generated by open circuit [V]TTEM,haverage temperatures of the hot ends [°C]
ΔTTEGtemperature difference between the two sides of the TEG [°C]TTEM,caverage temperatures of the cold ends [°C]
αeffheat transfer performance of the thermosyphon Pnominal power [W]
Qininput heat [W]Uoutput voltage [V]
Lelengths of the evaporation section [m]Rload resistance [Ω]
Lclengths of the condensation section [m]ηthermoelectric conversion efficiency [%]
Rhpthermal resistance of the thermosiphon tube [K/W]Rtemthermal resistance of thermoelectric module [K/W]
Rwthermal resistance between the cold end of the thermoelectric module and the heat exchange [K/W]Thpwall temperature of the thermosiphon tube [°C]
Tw,ininlet water temperatures of the heat exchanger [°C]Tw,outoutlet water temperatures of the heat exchanger [°C]
U’theoretical value of the output voltage [V]Ukopen-circuit voltage value
Rininternal resistance [Ω]Routexternal resistance [Ω]

References

  1. Jouhara, H.; Żabnieńska-Góra, A.; Khordehgah, N.; Doraghi, Q.; Ahmad, L.; Norman, L.; Axcell, B.; Wrobel, L.; Dai, S. Thermoelectric generator (TEG) technologies and applications. Int. J. Thermofluids 2021, 9, 100063. [Google Scholar] [CrossRef]
  2. Luo, D.; Yan, Y.; Wang, R.; Zhou, W. Numerical investigation on the dynamic response characteristics of a thermoelectric generator module under transient temperature excitations. Renew. Energ. 2021, 170, 811–823. [Google Scholar] [CrossRef]
  3. Nozariasbmarz, A.; Poudel, B.; Li, W.; Kang, H.B.; Zhu, H.; Priya, S. Bismuth telluride thermoelectrics with 8% module efficiency for waste heat recovery application. IScience 2020, 23, 101340. [Google Scholar] [CrossRef] [PubMed]
  4. Riyadi, T.W.B.; Utomo, B.R.; Effendy, M.; Wijayanta, A.T.; Al-Kayiem, H.H. Effect of thermal cycling with various heating rates on the performance of thermoelectric modules. Int. J. Therm. Sci. 2022, 178, 107601. [Google Scholar] [CrossRef]
  5. Deng, J.; Qu, G.Y.; Ren, S.J.; Wang, C.P.; Xiao, Y.; Wang, J.R.; Zhao, X. Investigation on high temperature point detection of spontaneous combustion of loose coal based on optimal acoustic signal. Process Saf. Environ. 2024, 185, 423–434. [Google Scholar] [CrossRef]
  6. Qu, G.Y.; Deng, J.; Ren, S.J.; Wang, C.P.; Song, Z.Y.; Wang, J.R.; Xu, Y.; Zhao, X.-Q. Research on temperature measurement model of loose coal considering humidity using acoustic wave method. Fuel 2024, 373, 132317. [Google Scholar] [CrossRef]
  7. Xiao, Y.; Yin, L.; Tian, Y.; Li, S.G.; Zhai, X.W.; Shu, C.M.; Ren, S.J. Sustainable utilisation and transformation of the thermal energy from coalfield fires: A comprehensive review. Appl. Therm. Eng. 2023, 233, 121164. [Google Scholar] [CrossRef]
  8. Liu, Y.; Wen, H.; Chen, C.; Guo, J.; Jin, Y.; Zheng, X.; Cheng, X.; Li, D. Research status and development trend of coal spontaneous combustion fire and prevention technology in China: A review. ACS Omega 2024, 9, 21727–21750. [Google Scholar] [CrossRef]
  9. Gomaa, A.; Rady, W.A.; Youssef, A.Z.; Elsaid, A.M. Thermal performance of heat pipe at different internal groove ratios and working fluids: An experimental investigation. Therm. Sci. Eng. Prog. 2023, 41, 101827. [Google Scholar] [CrossRef]
  10. Sarkar, D.; Savory, E.; DeGroot, C. Thermodynamic modeling of thermosyphons and heat pipes. Phys. Fluids 2023, 35, 123307. [Google Scholar] [CrossRef]
  11. Machado, P.L.; Pereira, T.S.; Trindade, M.G.; Biglia, F.M.; Santos, P.H.; Tadano, Y.S.; Alves, T.A. Estimating thermal performance of thermosyphons by artificial neural networks. Alex. Eng. J. 2023, 79, 93–104. [Google Scholar] [CrossRef]
  12. Kim, Y.; Shin, D.; Kim, J.; You, S.; Lee, J. Boiling and condensation heat transfer of inclined two- phase closed thermosyphon with various filling ratios. Appl. Therm. Eng. 2018, 145, 328–342. [Google Scholar] [CrossRef]
  13. Solomon, A.B.; Danie, V.A.; Ramachandran, K.; Pillai, B.C.; Singh, R.R.; Sharifpur, M.; Meyer, J.P. Performance enhancement of a two-phase closed thermosiphon with a thin porous copper coating. Int. Commun. Heat Mass Transf. 2017, 82, 9–19. [Google Scholar] [CrossRef]
  14. Alammar, A.A.; Al-Dadah, R.K.; Mahmoud, S.M. Numerical investigation of effect of fill ratio and inclination angle on a thermosiphon heat pipe thermal performance. Appl. Therm. Eng. 2016, 108, 1055–1065. [Google Scholar] [CrossRef]
  15. Khazaee, I.; Hosseini, R.; Kianifar, A.; Noie, S.H. Experimental consideration and correlation of heat transfer of a two-phase closed thermosyphon due to the inclination angle, filling ratio, and aspect ratio. J. Enhanc. Heat Transf. 2011, 18, 31–40. [Google Scholar] [CrossRef]
  16. Tong, Z.; Fang, C.; Zang, G.; Wu, H. Experimental study on the passive regulation of a CO2 two-phase thermosyphon loop. Appl. Therm. Eng. 2023, 219, 119457. [Google Scholar] [CrossRef]
  17. Zhong, K.Q.; Xiao, Y.; Lu, X.; Deng, J.; Yin, L.; Tian, Y.; Shu, C.-M. Nanofluidic two-phase closed thermosyphon-assisted thermoelectric generator for heat recovery from coal spontaneous combustion. Appl. Therm. Eng. 2021, 197, 117397. [Google Scholar] [CrossRef]
  18. He, W.; Guo, R.; Liu, S.; Zhu, K.; Wang, S. Temperature gradient characteristics and effect on optimal thermoelectric performance in exhaust power-generation systems. Appl. Energy 2020, 261, 114366. [Google Scholar] [CrossRef]
  19. Chen, Y.; Hou, X.; Ma, C.; Dou, Y.; Wu, W. Review of Development Status of Bi2Te3-Based Semiconductor Thermoelectric Power Generation. Adv. Mater. Sci. Eng. 2018, 2018, 1210562. [Google Scholar] [CrossRef]
  20. Luo, Y.; Zhang, L.; Li, J.; Li, C.; Xie, L.; Liu, Z.; Meng, F.; Qing, X. Study on thermal conductance allocation ratio of heat sink of thermoelectric cooler for electronic device in cold region. Energy Procedia 2015, 75, 603–607. [Google Scholar] [CrossRef]
  21. Alegria, P.; Catalan, L.; Araiz, M.; Rodriguez, A.; Astrain, D. Experimental development of a novel thermoelectric generator without moving parts to harness shallow hot dry rock fields. Appl. Therm. Eng. 2021, 200, 117619. [Google Scholar] [CrossRef]
  22. Ju, Y.S.; Lee, C. Evaluation of the energy efficiency of the shell coal gasification process by coal type. Energy Conver. Manag. 2017, 143, 123–136. [Google Scholar] [CrossRef]
  23. Hu, X.K.; Takazawa, H.; Nagase, K.; Ohta, M.; Yamamoto, A. Three-dimensional finite element simulation for a thermoelectric generator module. J. Electron. Mater. 2015, 44, 3637–3645. [Google Scholar] [CrossRef]
  24. Shi, Y.; Zhu, Z.; Deng, Y.; Zhu, W.; Chen, X.; Zhao, Y. A real-sized three-dimensional numerical model of thermoelectric generators at a given thermal input and matched load resistance. Energy Conver. Manag. 2015, 101, 713–720. [Google Scholar] [CrossRef]
  25. Hans, R.; Manikandan, S.; Kaushik, S.C. Performance Optimization of Two-Stage Exoreversible Thermoelectric Converter in Electrically Series and Parallel Configuration. J. Electron. Mater. 2015, 44, 3571–3580. [Google Scholar] [CrossRef]
  26. Zhao, Y.; Fan, Y.; Li, W.; Li, Y.Z.; Ge, M.H.; Xie, L.Y. Experimental investigation of heat pipe thermoelectric generator. Energy Conver. Manag. 2022, 252, 115123. [Google Scholar] [CrossRef]
  27. Li, W.; Paul, M.C.; Siviter, J.; Montecucco, A.; Knox, A.R.; Sweet, T.; Min, G.; Baig, H.; Mallick, T.; Han, G.; et al. Thermal performance of two heat exchangers for thermoelectric generators. Case Stud. Therm. Eng. 2016, 8, 164–175. [Google Scholar] [CrossRef]
  28. Nugraha, N.A.; Baskoro, D.C.; Riyadi, T.W.B.; Wijayanta, A.T. Thermal characteristics of thermoelectric generator at various heating rates: Experimental and numerical assessments. Therm. Sci. Eng. Progress 2024, 52, 102671. [Google Scholar] [CrossRef]
  29. Liley, P.E. Thermophysical properties of fluids. In Mechanical Engineers’ Handbook, Volume 4: Energy and Power; John Wiley & Sons: Hoboken, NJ, USA, 2015; p. 1. [Google Scholar]
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Figure 1. Physical picture of thermoelectric performance test system of coal fire waste heat conversion system.
Figure 1. Physical picture of thermoelectric performance test system of coal fire waste heat conversion system.
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Figure 2. Physical picture of the sleeve-type heat-conducting copper block.
Figure 2. Physical picture of the sleeve-type heat-conducting copper block.
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Figure 3. The physical pictures of the heating wire and the power regulator. (a) The heating wire. (b) The power regulator.
Figure 3. The physical pictures of the heating wire and the power regulator. (a) The heating wire. (b) The power regulator.
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Figure 4. The data acquisition device Keysight DAQ970A.
Figure 4. The data acquisition device Keysight DAQ970A.
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Figure 5. Schematic diagram of the thermoelectric device coverages. (a) 15.0% coverage rate. (b) 22.5% coverage rate. (c) 30.0% coverage rate.
Figure 5. Schematic diagram of the thermoelectric device coverages. (a) 15.0% coverage rate. (b) 22.5% coverage rate. (c) 30.0% coverage rate.
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Figure 6. Schematic diagram of thermoelectric performance test system of coal fire waste heat conversion system.
Figure 6. Schematic diagram of thermoelectric performance test system of coal fire waste heat conversion system.
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Figure 7. Variation in wall temperature in the condensing section and the thermoelectric device’s hot end temperature. (a) Air-cooled mode and (b) water-cooled mode.
Figure 7. Variation in wall temperature in the condensing section and the thermoelectric device’s hot end temperature. (a) Air-cooled mode and (b) water-cooled mode.
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Figure 8. Temperature difference diagram of thermoelectric module with (a) air-cooled and (b) water-cooled modes.
Figure 8. Temperature difference diagram of thermoelectric module with (a) air-cooled and (b) water-cooled modes.
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Figure 9. Open-circuit voltage under different thermoelectric device coverage rates. (a) 15.0% coverage rate. (b) 22.5% coverage rate. (c) 30.0% coverage rate.
Figure 9. Open-circuit voltage under different thermoelectric device coverage rates. (a) 15.0% coverage rate. (b) 22.5% coverage rate. (c) 30.0% coverage rate.
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Figure 10. Output voltage at different levels under (a) series connection and (b) parallel connection.
Figure 10. Output voltage at different levels under (a) series connection and (b) parallel connection.
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Figure 11. Nominal power at different levels under (a) series connection and (b) parallel connection.
Figure 11. Nominal power at different levels under (a) series connection and (b) parallel connection.
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Figure 12. Temperature distribution of components of coal fire waste heat conversion system.
Figure 12. Temperature distribution of components of coal fire waste heat conversion system.
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Figure 13. Heat transfer resistance distribution of coal fire waste heat conversion system.
Figure 13. Heat transfer resistance distribution of coal fire waste heat conversion system.
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Figure 14. Open-circuit voltage of thermoelectric module.
Figure 14. Open-circuit voltage of thermoelectric module.
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Figure 15. System’s output voltage and nominal power with different coverage rates. (a) Output voltage, and (b) output power.
Figure 15. System’s output voltage and nominal power with different coverage rates. (a) Output voltage, and (b) output power.
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Table 1. Operating performance parameters of thermoelectric device.
Table 1. Operating performance parameters of thermoelectric device.
ModelOpen-Circuit Voltage (V)Matched Load Value (Ω)Matching Load Value (Ω)Nominal Power (W)Heat Flow Through the Module (W)
TEP1-1263-2.410.85.45.45.496
Table 2. Theoretical and measured values of system output voltage under different coverage rates.
Table 2. Theoretical and measured values of system output voltage under different coverage rates.
Load Resistance (Ω)C = 15.0%, Output Voltage (V)C = 22.5%, Output Voltage (V)C = 30.0%, Output Voltage (V)
Theoretical ValueMeasured ValueRelative ErrorTheoretical ValueMeasured ValueRelative ErrorTheoretical ValueMeasured ValueRelative Error
30.660.630.0480.560.520.0770.430.420.024
71.321.290.0231.191.160.0260.930.880.057
111.831.800.0171.71.670.0181.361.340.015
152.322.310.0042.132.080.0241.731.700.018
202.612.6102.572.530.0162.132.080.024
Table 3. Maximum system nominal power and thermoelectric conversion efficiency corresponding to different coverage of thermoelectric devices.
Table 3. Maximum system nominal power and thermoelectric conversion efficiency corresponding to different coverage of thermoelectric devices.
Coverage of Thermoelectric Devices (%)Optimal Load Matching Value (Ω)Maximum Nominal Power (W)Thermoelectric Conversion Efficiency (%)
15.0%220.340.34%
22.5%330.350.35%
30.0%440.260.26%
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MDPI and ACS Style

Bao, Q.; Guo, X.; Li, B.; Chen, W.; Wang, Z.; Xiao, Y. Performance and Reliability of Thermoelectric Conversion Using a Crooked Thermosyphon to Enhance Heat Transfer from Coal Fires. Processes 2024, 12, 2692. https://doi.org/10.3390/pr12122692

AMA Style

Bao Q, Guo X, Li B, Chen W, Wang Z, Xiao Y. Performance and Reliability of Thermoelectric Conversion Using a Crooked Thermosyphon to Enhance Heat Transfer from Coal Fires. Processes. 2024; 12(12):2692. https://doi.org/10.3390/pr12122692

Chicago/Turabian Style

Bao, Qingfeng, Xiuting Guo, Bo Li, Wuyi Chen, Zhenping Wang, and Yang Xiao. 2024. "Performance and Reliability of Thermoelectric Conversion Using a Crooked Thermosyphon to Enhance Heat Transfer from Coal Fires" Processes 12, no. 12: 2692. https://doi.org/10.3390/pr12122692

APA Style

Bao, Q., Guo, X., Li, B., Chen, W., Wang, Z., & Xiao, Y. (2024). Performance and Reliability of Thermoelectric Conversion Using a Crooked Thermosyphon to Enhance Heat Transfer from Coal Fires. Processes, 12(12), 2692. https://doi.org/10.3390/pr12122692

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