Application of Water Tank Employing Smart Sensor for Thermal–Electric Energy Conversion on Vehicles

1College of Mechanical and Automotive Engineering, Zhaoqing University, Zhaoqing Avenue, Zhaoqing City, Guangdong Province 526061, China 2Yulin Normal University, Guangxi 537000, China 3School of Life Sciences, Zhaoqing University, Zhaoqing City, Guangdong Province 526061, China 4Department of Environmental Engineering, Da-Yeh University 51591, Taiwan 5Chaoyang Experimental School of Shantou City, Guangdong 515000, China


Introduction
To realize the recycling of thermal energy, the traditional heat-dissipating water tanks of heavy-duty locomotives are redesigned in this research. Specifically, a shield is mounted outside the water tank, and a detachable number plate is installed in the shield as a spare locomotive number to prevent the loss of the number plate due to theft or other reasons. At the same time, thermoelectric power generation chips are installed at the four corners of the front and rear panels of the shield to generate electric energy using the thermal energy generated by the water tank, thereby achieving energy recycling.
In this research, a thermal power generation device with a simple structure is designed. The device is expected to increase power generation capacity without using additional energy for cooling the thermoelectric power generation chips. Therefore, it is of great importance to cool the surface of the chips. During our experiment, a thermoelectric power generation chip, a flat panel heater, and a radiator with heat pipes and cooling fins are used to generate a temperature difference between the heater and the radiator. Then, electric energy is generated using the thermoelectric power generation chip. An actual situation where the designed thermal power generation device is applied is simulated. An anemometer and an infrared thermometer are also used to monitor the air flow rate around the cooling fins and the temperature of the experimental equipment. (1) The heat generation effect is a phenomenon in which electrons/holes in a heated object move to generate current or accumulate electric charge due to a temperature gradient from a hightemperature region to a low-temperature region. Using a thermoelectric power generation chip, as shown in Fig. 1, thermal energy can be converted into electric energy using the temperature difference between the cold side and the hot side of the chip. (2) The thermoelectric power generation module can generate electric energy via the temperature difference between the two ends of the module. In addition to large-scale thermoelectric systems, an energy hunting technology that collects small amount of energy is also a possible direction of future technological development. By collecting small amounts of energy in the environment, such a module can power portable electronic products, or be used in cases where wires cannot be used or batteries cannot be replaced conveniently. In this way, the ambient temperature or human body temperature can be converted into electric energy using thermoelectric materials. Therefore, the thermoelectric power generation technology is of great significance for future energy reuse. (3)

Thermoelectric effect
A thermoelectric material directly converts thermal energy into electric energy without the application of an external force, and the conversion process is reversible. Thermoelectric materials have three main effects, namely, the Seebeck effect and Peltier effect, as shown in Figs. 2 and 3, respectively, and the Thomson effect. (4)

Thermoelectric module principle and equivalent circuit
The thermoelectric module used in this research is based on the Seebeck effect; the upper part of the module is the hot side and the lower part is the cold side. In the case of a temperature difference, the majority of the carriers on the hot side of the P-type semiconductor will be moved toward the cold side, just like the current flowing from the hot side to the cold side. The majority of the carriers on the hot side of the N-type semiconductor will move from the hot side to the cold side, just like the flow of electrons from the hot side to the cold side, in the opposite direction to the current. (5,6) As shown in Fig. 4, the temperature of the hot side is T h , the temperature of the cold side is T c , and the particle length and cross-sectional area of the semiconductor are L and A, respectively. For P-type thermoelectric particles, the Seebeck constant is α p and the resistivity  is ρ p ; for N-type thermoelectric particles, the Seebeck constant is α n and the resistivity is ρ n . The generated open-circuit voltage V s can be expressed as According to the resistance formula, the equivalent internal resistance R s can be written as The current generated is The power obtained by the load is In the case of impedance matching, the maximum power obtained from the load is given by From the aspect of heat conduction, the thermal energy flowing into the hot side and the cold side, respectively represented by Q h and Q n , can be written as Equations (6) and (7) are composed of three parts, where (α p − α n ) × IT h represents the magnitude of the thermoelectric effect, (I 2 R s )/2 represents the Joule heat, and ( (6) and (7) are the thermal conductance of P-type and N-type thermoelectric particles in the thermoelectric module, respectively. According to the conservation of energy, the difference between Eqs. (6) and (7) is the energy P produced by the thermoelectric module: The relationship between current and load R L is Substituting Eq. (8) into Eq. (9), we have In addition, the power generation efficiency is defined as the ratio of the generated power to the thermal energy flowing into the hot side: The maximum power generation efficiency is given by where T m is the average temperature of the hot and cold sides, and the thermoelectric figure of merit is defined as According to Eqs. (4) and (10), the same behavior of the thermoelectric module is obtained, and the correctness of the model is verified. It can be seen from the above equations that the thermoelectric power generation module includes the Seebeck voltage source V s as a function of temperature and the equivalent internal resistance R s , as shown in Fig. 5. The module is very similar to the equivalent model of the battery. Although Fig. 5 shows the actual equivalent model, it is usually used as the equivalent model of the thermoelectric module to simplify the construction of the built-in Seebeck electromotive force. (7) Because the equivalent model of the thermoelectric module is equivalent to a DC voltage source connected in series with an equivalent resistance, the module is simular to a linear power supply. On the basis of the equivalent model of the thermoelectric module, the relationship between the output voltage and the output current is derived. Firstly, the relationship between its output voltage and current can be written as The output power can be written as Therefore, when V s and R s are known, the output characteristic curve can be drawn using Eqs. (14) and (15), as shown in Fig. 6. According to Eq. (1), increases in the temperature and open-circuit voltage will accordingly increase the Seebeck constant α. According to the relationship between the output power and the output voltage shown in Eq. (14), the maximum power point is obtained when ΔP/ΔV= 0; thus, the differential result is expressed as It can be seen from Eq. (16) that when the impedance is matched, R L = R s , the operating voltage corresponding to the maximum power is half of the open-circuit voltage V s . Many studies have developed maximum power tracking methods for thermoelectric modules using this characteristic of thermoelectric power modules. All these methods require periodic measurement of the open-circuit voltage V s to ensure that the module is currently operating at the maximum power, but the measurement requires the thermoelectric power generation module to be disconnected from the back-end circuit. Thus, periodic measurement not only loses energy when measuring the open-circuit voltage, but also easily causes the instability of the system output. The power generation behavior of thermoelectric power generation modules has been extensively investigated in more detail. These studies on the modeling and analysis of thermoelectric modules have improved the effectiveness in predicting the expected power generation results when designing thermoelectric power generation systems. (8,9)

Thermoelectric power generation chip
A thermoelectric cooler, also called a refrigerating sheet or Peltier device, refers to a twosided sheet, with one side absorbing heat and the other side dissipating heat. Such heatconductive sheets do not generate cold themselves. The parameters of the thermoelectric cooler in this study are shown in Table 1.

Temperature feedback
A Pt100 platinum thermistor is used to feed back the tank temperature to a cellphone app in real time. The resistance of Pt100 is proportional to the temperature: when the temperature of Pt100 is 0 °C, its resistance is 100 Ω, and at 100 °C the resistance is about 138.5 Ω.
The circuit is designed to connect the three-wire Pt100 to the bridge and then amplify the signal through an LM358 operational amplifier to obtain a reference voltage value (i.e., the temperature corresponding to the current contact surface of Pt100). The circuit diagram and the resistance index table of Pt100 are shown in Fig. 7 and Table 2, respectively, where the circuit includes a Wheatstone bridge.
The resistance of Pt100 can be calculated by where R t is the resistance, R 0 is the resistance when Pt100 is at 0 °C, t is the temperature, A = 3.940 × 10 −3 , B = −5.802 × 10 −7 , and C = −4.274×10 −12 . (10) Temperature conversion is achieved using the formula where V is the AD input, R t is the resistance connected in series with the thermal resistance in the bridge, c is the amplification factor of the operational amplifier, and the AD precision is 2 10 = 1024. The magnification of the operational amplifier is given by

Results and Discussion
In accordance with the expected functions of the product, structure design and size calculation are preliminarily carried out, and then a manual drawing is performed in accordance with the design data. After that, CAD software is used for drawing, and the design results are shown in Figs. 8 and 9.
After the sketch design is completed, the three-dimensional modeling software UG is used for modeling. The front and rear panels of the shield are as shown in Figs. 10 and 11, respectively. The thermoelectric power generation chip is installed at the four corners of the panel, and a spare number plate is installed in the middle of the front panel. The vehicleʼs rear shield is shown in Fig. 11, and a motion simulation is performed on the vehicle model. Amplifying circuit with Wheatstone bridge. The main alloying elements of 6061 aluminum alloy are magnesium and silicon, which form a Mg 2 Si phase. If a certain amount of manganese or chromium is added to the alloy, the anti-fatigue and corrosion-resistance performances of the alloy can be improved. Sometimes a small amount of copper or zinc is added to increase the strength of the alloy without significantly reducing the corrosion-resistance performance. There is a small amount of copper in the conducting material to offset the adverse effects of titanium and iron on conductivity. Zirconium or titanium can refine grains and control the recrystallized structure. To improve the machinability, lead and antimony can be added. When Mg 2 Si is solid-dissolved in aluminum, the alloy will possess an artificial age-hardening function. The 6061 aluminum alloy is an Al-Mg-Si alloy with medium strength, good plasticity, and excellent corrosion-resistance performance. In particular, the alloy does not tend to crack due to stress corrosion and also has excellent weldability and cold workability, making it a promising alloy with a wide range of applications.
The 6061 aluminum alloy can be anodized and colored, or painted with enamel, making it suitable for use as a building decoration material. It contains a small amount of Cu, giving it a higher strength alloy than 6063 aluminum alloy, but its quenching sensitivity is also higher than that of 6063 alloy. However, after compression, wind quenching cannot be achieved. Under  this condition, re-solution treatment and quenching aging are required to obtain higher strength.
The mechanical and physical properties of the 6061 aluminum alloy are a yield strength of 5.51485 × 10 7 N/m 2 , a tensile strength of 1.24084 × 10 8 N/m 2 , an elastic modulus of 6.9 × 10 10 N/m 2 , a Poisson ratio of 0.33, a mass density of 2700 kg/m 3 , a shear modulus of 2.6 × 10 10 N/m 2 , and a thermal expansion coefficient of 2.4 × 10 −5 /K, as shown in Figs. 12 and 13. Through the above static finite element analysis, it is found that the front shield is subjected to stress between 25.4982 and 19911.9 Pa, and its maximum stress is much smaller than the yield stress of 5.515 × 10 7 Pa. Also its displacement varies from 0 to 3.478902 × 10 −5 mm, and the strain is in the range of 2.8742 × 10 −10 -2.04853 × 10 −7 , which are both within the allowable range. Thus, the requirements for displacement and strain are met.
An analysis of the heat flow is conducted in the simulation module of SolidWorks software using the relationship between temperature changes and thermodynamic or physical parameters. The heat flow analysis can visualize the distribution and variation of heat on the shield, enabling the thermal energy to be more fully utilized. The heat flow analysis is performed on the front and rear panels. Using the engine as the heat source, the heat is transferred to the water tank. The water tank is fixed to the panel at a certain distance. The heat is generated by the engine and then transferred to the power generation chip through the water tank. Then electric energy is generated via the desired temperature difference. Considering the actual heat generation of the engine and the heat dissipation due to the wind, the temperature of the heat source is set at 60 °C.
The chip material is silicon with a thermal conductivity of 124 W/(mK) and a mass density of 2330 kg/m 3  Using the Peltier effect of the semiconductor material, when direct current passes through a galvanic couple of two different semiconductor materials in series, heat can be absorbed and released at the corresponding side of the galvanic couple, thus achieving the purpose of cooling. This is a refrigeration technology that produces negative thermal resistance and has no moving parts and high reliability. The results of heat flow analysis are shown in Figs. 14 and 15.   Figure 16 shows the self-designed water tank. It uses 6061 aluminum alloy, which has good plasticity and excellent corrosion resistance, no stress corrosion cracking tendency, excellent weldability, and good cold workability. The operation range is from 120.512 to 145.333 °C, where the maximum temperature is less than the melting point of aluminum of 660.37 °C. Thus, the requirements for the heat flow are met. The temperature difference is 24.821 °C, which also satisfies the requirement of the temperature difference for power generation. Figure 17 shows the temperature-measuring device and its circuit. The circuit is controlled by an Arduino chip. Figure 18 shows the results of the test using the app.
The Pt100 is connected with a Wheatstone bridge, an operational amplifier, and a Bluetooth module. The temperature change induces a voltage difference between the two ends of the bridge. The voltage is amplified by a factor of 4 through an operational amplifier and acts as the AD input of the Arduino chip. The current temperature value is calculated and sent to the computer via Bluetooth.
The main points of this study are summarized as follows: 1. Common vehicles driven by fuels (gasoline, diesel, and aviation oil) are equipped with water tanks, which are used to dissipate heat. 2. The thermal energy on a water tank is lost as waste heat and is not fully utilized. 3. In this research, a multifunctional cooling water tank that generates electric energy from thermal energy is designed. It also has anti-theft and power storage functions. 4. The production process is simple, fast, and cost-effective. Moreover, the cost can be reduced by mass production, and its prospects for commericalization are promising. 5. Resources from universities and industry can be integrated to jointly develop new technologies and products.

Conclusions
This paper describes an innovative design that recycles thermal energy into electric energy for vehicles. In the proposed design, a shield is mounted on the outside of the water tank of a traditional heavy-duty locomotive to protect the water tank. Such a shield is installed with a detachable number plate as a spare locomotive number to prevent the loss of a number plate due to theft or other reasons. On the other hand, thermoelectric power generation chips are mounted at the four corners of the front and rear panels of the shield and use the heat from the water tank to generate electric energy. By employing the proposed design, energy can be recycled and reutilized to reduce environmental impact and conserve energy. Furthermore, the  research results of this paper can be used for talent cultivation and research and development. In summary, it provides a foundation for integrating a wide range of resources for vocational education in practical technology.