**2. Materials and Methods**

Before discussing the experiment, thermoelectric generation phenomena will be briefly explained. The solid-state direct energy conversion is based on the thermoelectric phenomena which cover the Seebeck, Peltier, and Thompson effects. For the thermoelectric generation case, the dominating thermoelectric phenomenon is the Seebeck effect of which the main principle is as follows: When the junctions of two dissimilar semiconductors, as shown in Figure 1, are maintained at different temperatures, the current flows through the closed-loop. The temperature difference between the two junctions and the electromotive force has the following linear relation where is the Seebeck coefficient.

$$E = \alpha\_{AB} \Delta T \tag{1}$$

where: *E*—open circuit voltage (V); α—Seebeck coefficient (V/K); ∆*T*—hot/cold side temperature difference [17]. The figure of merit ZT (figure of merit) determines the generation performance of a specific thermoelectric material and is characterized by the thermal/electrical conductivity, Seebeck coefficient, and the hot/cold side average temperature as described by Equation (2).

$$\text{ZT} = \frac{\alpha^2 \sigma}{\kappa} \overline{T} \tag{2}$$

where: ZT—figure of merit (-); κ—thermal conductivity (W/m<sup>2</sup> ·K); σ—electrical conductivity (Ω)**;** *T*—mean temperature (K) [17].

ZT is a measure that determines the performance of thermoelectric material itself. Higher ZT is desirable in general. Looking at Equation (2), we can find why higher ZT is desirable. Low thermal conductivity between the hot and cold side of the module ensures ∆*T* is relatively well preserved so that electricity is well-produced according to Equation (1). High electrical conductivity and the Seebeck coefficient implies the material has more capability to produce electricity. Higher mean operating temperature implies the material is capable of operating at high temperature, and consequently higher ∆*T*.

Seebeck coefficient.

**2. Materials and Methods** 

where: *E—*open circuit voltage (V); *α—*Seebeck coefficient (V/K); Δ*T—*hot/cold side temperature difference [17]. The figure of merit ZT (figure of merit) determines the generation performance of a

coefficient, and the hot/cold side average temperature as described by Equation (2).

generators for automobile exhaust waste heat recovery [17]. Four main factors that control the thermal efficiency of the thermoelectric generator were presented: heat exchanger geometry, heat exchanger materials, the installation site of the thermoelectric generator within the car, and the

Regarding the above works, the present study tried to hold focus on the other aspects of optimized conditions. As an example, thermoelectric generation performance was evaluated depending on the compressive force applied to the module. The compressive force is, in specific, a crucial means not only to lock TEG device to the heat source, but also to minimize the heat resistance, so TEG can perform maximum power. However, some optimal force exists where, putting more compressive force can reduce the performance because of heat transfer convergence to its maximum level, and putting more stress on the module can reduce the power because of the material's deformation. Moreover, putting more stress may result in mechanical failure. Additionally, the effect of the thermoelectric module surface temperature uniformity on the overall power generation performance was evaluated quantitatively. On the other hand, the experiment on the thermoelectric module power performance depending on the heat sink was conducted in a similar but partially different way from what Esarte et al. did [16]. The main difference was that in the present study, various types of heat sinks with different geometry (fin height, fin array) were applied whereas in Esarte's work, heat sinks were almost similar to one other with the same size except that fin thicknesses and numbers were varied from one another. After all, the main purpose of the present study was to simulate the waste heat recovery TEG system in automobiles and small-sized industrial facilities and thus, find the design factor that enhances the overall performance of the TEG system.

Before discussing the experiment, thermoelectric generation phenomena will be briefly explained. The solid-state direct energy conversion is based on the thermoelectric phenomena which cover the Seebeck, Peltier, and Thompson effects. For the thermoelectric generation case, the dominating thermoelectric phenomenon is the Seebeck effect of which the main principle is as follows: When the junctions of two dissimilar semiconductors, as shown in Figure 1, are maintained at different temperatures, the current flows through the closed-loop. The temperature difference between the two junctions and the electromotive force has the following linear relation where is the

coolant system of the thermoelectric generator.

**Figure 1.** Schematic of the basic thermoelectric generation circuit. A temperature difference between two dissimilar semiconductors produces a voltage difference between the two substances, which can be applied to thermal-to-electrical energy conversion. **Figure 1.** Schematic of the basic thermoelectric generation circuit. A temperature difference between two dissimilar semiconductors produces a voltage difference between the two substances, which can be applied to thermal-to-electrical energy conversion.

The conversion efficiency, described by Equation (3) could be derived by using energy balance and is shown as a function of ZT and the hot/cold side temperature in the thermoelectric material.

$$\varepsilon = \frac{T\_H - T\_C}{T\_H} \frac{\sqrt{\text{ZT} + 1} - 1}{\sqrt{\text{ZT} + 1} + \frac{T\_C}{T\_H}} \tag{3}$$

=∆ (1)

where: *TH*—hot side temperature (K); *TC*—cold side temperature (K) [17].

As seen in Equation (3), *T<sup>H</sup>* and *T<sup>C</sup>* are fixed with varying ZT. We can see conversion efficiency increases with higher ZT.

Current commercial thermoelectric materials usually have ZT of 0.8–1 and conversion efficiency of 4–5%. However, thermoelectric material Bi2Te3, GeTe, PbTe are being developed continuously with each material being doped with Bi and In, and due to the recent developments, it has been reported ZT as high as 2.4 and that conversion efficiency as high as 15% can be achieved [18–20].

Now, looking at the thermoelectric generation system for exhaust gas waste heat recovery, the overall system efficiency cannot exactly match the thermoelectric conversion efficiency due to heat loss and thermal contact resistance which occurs at the interfaces. Considering such irreversible effects, the overall system efficiency of the TEG can be analyzed as follows:

$$
\eta\_{\text{ov}} = \varepsilon \times \eta\_{\text{HX}} \times \rho\_{\prime} \tag{4}
$$

where: ε—conversion efficiency (-); η*HX*—heat exchanger efficiency; ρ—heat flux efficiency [16].

η*HX* and ρ indicates the heat exchanger efficiency and heat flux efficiency, respectively. Here, heat exchanger efficiency, η*HX*, is defied the ratio of ∆*T*co-ci to ∆*T*hi-ci, which depends on fin geometry, convection type, and flow condition. Heat flux efficiency can be defined as a ratio of actual heat flux to maximum theoretical heat flux. Thermal resistance indeed exists within the interface region of the TEG system such as heat source to a thermoelectric module, and heat sink. Since the conversion efficiency, ε is an uncontrollable value at this moment, the focus should be on improving the other two values, by either designing a highly efficient heat exchanger or reducing thermal contact resistance.

In the present study, three different experiments were conducted, independently. In the first experiment, the thermoelectric module generation performance was evaluated using a high-performance thermoelectric module with heat source and sink block. The general specifications of the thermoelectric module are specified in Table 1. In the second experiment, the TEG system for exhaust gas waste heat recovery was constructed and the power performance characteristics were compared to those of the first experiment. In the last experiment, various types of heat sinks were applied to the thermoelectric module to determine the power generation performance. For the measurement devices, a K-Type thermocouple was used to measure the hot/cold side temperature of the thermoelectric module as well as the inlet/outlet gas temperature inside the TEG system. The voltage was measured by extending the lead wire of the thermoelectric module to the multimeter. The airflow rate in the TEG system was measured using the TESTO portable sensing probe. Overall, such data were converted into digitalized formats by using the Agilent 34970A data logger. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 5 of 12


**Table 1.** Specifications for thermoelectric (TE) modules.

#### *2.1. Module Performance Experimental Setup 2.1. Module Performance Experimental Setup*

As described in Table 1, the thermoelectric module is originally produced from Hi-Z Inc., USA, and was applied to the experimental device to evaluate its performance as seen in Figure 2. The HZ20 model was specified as having a maximum power performance of 19 W. A copper plate with four cartridge heaters inserted inside the plate was used as a heating device to ensure the temperature uniformity of the hot side surface of the thermoelectric module. An aluminum water cooling jacket with a thickness of 2 mm, was attached on the cold side surface of the thermoelectric module. Using a PID temperature controller, the module hot side temperature was controlled from 40–230 ◦C for the HZ20 model. On the other hand, the cooling jacket was constantly supplied with water of 8LPM and an approximate temperature of 17–18 ◦C. The hot side temperature was increased by 10 ◦C for each step, and the open-circuit voltage was measured. When each module achieved its maximum operating temperature, the electric power was measured with variable load resistance which ranges from 0.1–100 Ω. The reason for such measurement was to find the matching load in which the maximum power could be achieved. Additionally, by putting the load cell above the heat sink, the whole pile was compressed by the C-clamp. After so, the module generation performance depending on the compressive force was measured as well. As described in Table 1, the thermoelectric module is originally produced from Hi-Z Inc., USA, and was applied to the experimental device to evaluate its performance as seen in Figure 2. The HZ20 model was specified as having a maximum power performance of 19 W. A copper plate with four cartridge heaters inserted inside the plate was used as a heating device to ensure the temperature uniformity of the hot side surface of the thermoelectric module. An aluminum water cooling jacket with a thickness of 2 mm, was attached on the cold side surface of the thermoelectric module. Using a PID temperature controller, the module hot side temperature was controlled from 40–230 °C for the HZ20 model. On the other hand, the cooling jacket was constantly supplied with water of 8LPM and an approximate temperature of 17–18 °C. The hot side temperature was increased by 10 °C for each step, and the open-circuit voltage was measured. When each module achieved its maximum operating temperature, the electric power was measured with variable load resistance which ranges from 0.1–100 Ω. The reason for such measurement was to find the matching load in which the maximum power could be achieved. Additionally, by putting the load cell above the heat sink, the whole pile was compressed by the C-clamp. After so, the module generation performance depending on the compressive force was measured as well.

**Figure 2.** Experiment setup for measuring module performance. **Figure 2.** Experiment setup for measuring module performance.

### *2.2. TEG Setup and Power Performance Testing 2.2. TEG Setup and Power Performance Testing*

measured.

A waste-heat recovering TEG system was constructed by the assembly of a duct, a heat exchanger, a water cooling jacket, and a thermoelectric module. The details of such assembly are described in Figure 3. The exhaust gas was simulated by a hot air blower which was capable of discharging air at a temperature ranging from 25–250 °C. Making indirect contact with hot air, the module was attached to the upper wall of the duct. Unlike the previous experiments, the module hot side temperature was controlled by using a hot air blower which was attached to the duct inlet. Figure 4 shows the schematic of the TEG system experimental setup. Strictly, it was the center of the module hot side temperature which was measured, but it will be abbreviated as just module hot side temperature as a convenience from next on. Additionally, the hot air inlet/outlet temperature and air flowrate at the normal temperature were measured. The module hot side temperature was controlled from 40–230 °C for the HZ20 model. As the whole system stabilized, the power performance was A waste-heat recovering TEG system was constructed by the assembly of a duct, a heat exchanger, a water cooling jacket, and a thermoelectric module. The details of such assembly are described in Figure 3. The exhaust gas was simulated by a hot air blower which was capable of discharging air at a temperature ranging from 25–250 ◦C. Making indirect contact with hot air, the module was attached to the upper wall of the duct. Unlike the previous experiments, the module hot side temperature was controlled by using a hot air blower which was attached to the duct inlet. Figure 4 shows the schematic of the TEG system experimental setup. Strictly, it was the center of the module hot side temperature which was measured, but it will be abbreviated as just module hot side temperature as a convenience from next on. Additionally, the hot air inlet/outlet temperature and air flowrate at the normal temperature were measured. The module hot side temperature was controlled from 40–230 ◦C for the HZ20 model. As the whole system stabilized, the power performance was measured.

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 6 of 12

**Figure 3.** Assembly of waste heat recovering thermoelectric generator (TEG) system. pipe system, a commercial thermoelectric module is attached.

**Figure 4.** Schematic of a TEG experimental setup. The experimental setup is divided into two parts: (1) hardware for the experiment—hot air blower, exhaust pipe, a thermoelectric system with heat exchanger, water chamber to supply cold water; (2) data acquisition system: desktop computer, **Figure 4.** Schematic of a TEG experimental setup. The experimental setup is divided into two parts: (1) hardware for the experiment—hot air blower, exhaust pipe, a thermoelectric system with heat exchanger, water chamber to supply cold water; (2) data acquisition system: desktop computer, sensors to measure electricity, temperature, data logger.

**Figure 4.** Schematic of a TEG experimental setup. The experimental setup is divided into two parts: *2.3. Power Generation Performance of TEM with Various Heat sink Types*  For the present experiment, a total of six types of heat sinks were applied to the module, and each heat sink had its belonging case as shown in Figure 5. The experiment was conducted in two Imitating exhaust pipe systems capable of waste heat recovery, the porous copper heat exchanger is stored inside the pipe system to maximize the heat transfer. The water jacket attached to the other side of the thermoelectric module was designed and aluminum is selected due to anti-corrosion characteristics and high heat conductivity. Between the water cooling jacket and exhaust pipe system, a commercial thermoelectric module is attached.

#### (1) hardware for the experiment—hot air blower, exhaust pipe, a thermoelectric system with heat exchanger, water chamber to supply cold water; (2) data acquisition system: desktop computer, steps. For natural convection cooling, only the heat sink was attached to the cold side of the module in the first step. In the second step, a cooling fan was attached to the heat sink, for forced convection *2.3. Power Generation Performance of TEM with Various Heat sink Types*

sensors to measure electricity, temperature, data logger.

temperature of the module stabilized, the power was soon measured.

sensors to measure electricity, temperature, data logger. *2.3. Power Generation Performance of TEM with Various Heat sink Types*  For the present experiment, a total of six types of heat sinks were applied to the module, and each heat sink had its belonging case as shown in Figure 5. The experiment was conducted in two steps. For natural convection cooling, only the heat sink was attached to the cold side of the module in the first step. In the second step, a cooling fan was attached to the heat sink, for forced convection cooling. The cooling fan had a maximum flowrate of 10 CFM and consumed 5 W of electrical power. cooling. The cooling fan had a maximum flowrate of 10 CFM and consumed 5 W of electrical power. Additionally, it was attached to the heat sink in a way so that the air could flow in a parallel direction to the fins. As the module performance experiment, the module hot side temperature was controlled by the copper heating plate from 50–230 ° C and was increased by 30 °C for each step. To decrease the thermal contact, the thermal compound (silicone grease) was injected at the interface and the compressive force was applied on the whole pile by using the C-clamp. As the hot/cold side temperature of the module stabilized, the power was soon measured. For the present experiment, a total of six types of heat sinks were applied to the module, and each heat sink had its belonging case as shown in Figure 5. The experiment was conducted in two steps. For natural convection cooling, only the heat sink was attached to the cold side of the module in the first step. In the second step, a cooling fan was attached to the heat sink, for forced convection cooling. The cooling fan had a maximum flowrate of 10 CFM and consumed 5 W of electrical power. Additionally, it was attached to the heat sink in a way so that the air could flow in a parallel direction to the fins. As the module performance experiment, the module hot side temperature was controlled by the copper heating plate from 50–230 ◦ C and was increased by 30 ◦C for each step. To decrease the thermal contact, the thermal compound (silicone grease) was injected at the interface and the

the thermal contact, the thermal compound (silicone grease) was injected at the interface and the compressive force was applied on the whole pile by using the C-clamp. As the hot/cold side

Additionally, it was attached to the heat sink in a way so that the air could flow in a parallel direction

compressive force was applied on the whole pile by using the C-clamp. As the hot/cold side temperature of the module stabilized, the power was soon measured. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 7 of 12


**Figure 5.** Heat sink specifications. Basic specification of the six types of heat sinks used in the **Figure 5.** Heat sink specifications. Basic specification of the six types of heat sinks used in the thermoelectric experiment (size, fin numbers, and thickness of fin/pitch).
