thermoelectric experiment (size, fin numbers, and thickness of fin/pitch). **3. Results and Discussion**

#### **3. Results and Discussion**  *3.1. Module Performance Experimental Results*

*3.1. Module Performance Experimental Results*  Figure 6a describes the open-circuit voltage and electrical power of the HZ20 model, depending on the module's hot side temperature. Since Δ*T* has a linear relationship with voltage according to Equation (1), the power has a double square relationship with Δ*T* accordingly. For the hot side temperature condition, the model achieved the maximum power output of 15.5 W. Figure 6b shows the closed-circuit voltage and electrical power depending on the load resistance value. The measurements were done when the hot side temperature of the module was at the highest, which is 230 °C. It could be seen that the electric power of the thermoelectric module achieved its maximum at the load resistance value of 0.3 Ω. From such fact, it could be induced those load resistance value matched with the internal resistances of the thermoelectric module. Additionally, the values were the same as the internal resistances labeled in Table 1. In Figure 6c, the open-circuit voltage was measured depending on the compressive force applied to the thermoelectric module at hot side temperature 230 °C. From the graphs, it could be known that there existed an optimal point of the compressive force where the maximum generation performance could be achieved. The open-circuit voltage was Figure 6a describes the open-circuit voltage and electrical power of the HZ20 model, depending on the module's hot side temperature. Since ∆*T* has a linear relationship with voltage according to Equation (1), the power has a double square relationship with ∆*T* accordingly. For the hot side temperature condition, the model achieved the maximum power output of 15.5 W. Figure 6b shows the closed-circuit voltage and electrical power depending on the load resistance value. The measurements were done when the hot side temperature of the module was at the highest, which is 230 ◦C. It could be seen that the electric power of the thermoelectric module achieved its maximum at the load resistance value of 0.3 Ω. From such fact, it could be induced those load resistance value matched with the internal resistances of the thermoelectric module. Additionally, the values were the same as the internal resistances labeled in Table 1. In Figure 6c, the open-circuit voltage was measured depending on the compressive force applied to the thermoelectric module at hot side temperature 230 ◦C. From the graphs, it could be known that there existed an optimal point of the compressive force where the maximum generation performance could be achieved. The open-circuit voltage was maximized when 200 kgf of compressive forces had been applied. Applying more compressive force resulted in the generation performance decrease. One possible reason could be the internal crack within the thermoelectric module because of the excess compressive force.

within the thermoelectric module because of the excess compressive force.

maximized when 200 kgf of compressive forces had been applied. Applying more compressive force resulted in the generation performance decrease. One possible reason could be the internal crack

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 8 of 12

**Figure 6.** (**a**) Open circuit voltage and electrical power of HZ20 corresponding to the hot side temperature. (**b**) Closed-circuit voltage and electrical power corresponding to load resistance (@*TH* = 230 °C). (**c**) Measurement of the open-circuit voltage depending on the compressive force applied to the thermoelectric module (@*TH* = 230 °C) (*n* = 6). **Figure 6.** (**a**) Open circuit voltage and electrical power of HZ20 corresponding to the hot side temperature. (**b**) Closed-circuit voltage and electrical power corresponding to load resistance (@*T<sup>H</sup>* <sup>=</sup> <sup>230</sup> ◦C). (**c**) Measurement of the open-circuit voltage depending on the compressive force applied to the thermoelectric module (@*T<sup>H</sup>* <sup>=</sup> <sup>230</sup> ◦C) (*<sup>n</sup>* <sup>=</sup> 6).

#### *3.2. Experimental Results of TEG Power Generation Performance 3.2. Experimental Results of TEG Power Generation Performance*

Based on the experimental results of the thermoelectric module performance test, the experimental results of the present TEG test were analyzed. Figure 7 describes the power performance of the TEG model respectively when attached to the exhaust gas duct. As mentioned earlier, since Δ*T* has a linear relationship with voltage according to Equation (1), it is reasonable that power came out as double square relation with Δ*T*. Although Figure 7 reveals quite a similar pattern with Figure 6a from the module performance test, it could be seen that the overall performance of the TEG system was declined. The maximum electric power was 8.2 W which was 2.3 W less than the previous module performance test. This was due to the presence of temperature gradient along the axial direction of the exhaust gas duct which affected the module surface temperature distribution as well. As a result, the possibility of the performance decline arises from the uniform temperature Based on the experimental results of the thermoelectric module performance test, the experimental results of the present TEG test were analyzed. Figure 7 describes the power performance of the TEG model respectively when attached to the exhaust gas duct. As mentioned earlier, since ∆*T* has a linear relationship with voltage according to Equation (1), it is reasonable that power came out as double square relation with ∆*T*. Although Figure 7 reveals quite a similar pattern with Figure 6a from the module performance test, it could be seen that the overall performance of the TEG system was declined. The maximum electric power was 8.2 W which was 2.3 W less than the previous module performance test. This was due to the presence of temperature gradient along the axial direction of the exhaust gas duct which affected the module surface temperature distribution as well. As a result, the possibility of the performance decline arises from the uniform temperature distribution on the module surface.

distribution on the module surface. Since the hot side of the TEG is capable of operating up to 230 °C, and it is a device that only needs to be adjusted within 100 °C when developing a PCR device, it will not be difficult to get such a heat source in the ISS environment, whether it can be radiation energy or other heat devices. As mentioned earlier, in the process of selecting the gene to be amplified from the DNA sample of interest through PCR, the temperature change inside the unit between 60 °C and 95 °C is important; the temperature is raised to 95 °C so that all double-stranded DNA would melt into a single strand, and then lowered to 60 °C so that the primers could bind to the gene of interest. Additionally, the optimal temperature for the polymerase to operate the replication of DNA strands is 72 °C. The TEG may have great potential not only for providing power to the real-time PCR device, but also for using Since the hot side of the TEG is capable of operating up to 230 ◦C, and it is a device that only needs to be adjusted within 100 ◦C when developing a PCR device, it will not be difficult to get such a heat source in the ISS environment, whether it can be radiation energy or other heat devices. As mentioned earlier, in the process of selecting the gene to be amplified from the DNA sample of interest through PCR, the temperature change inside the unit between 60 ◦C and 95 ◦C is important; the temperature is raised to 95 ◦C so that all double-stranded DNA would melt into a single strand, and then lowered to 60 ◦C so that the primers could bind to the gene of interest. Additionally, the optimal temperature for the polymerase to operate the replication of DNA strands is 72 ◦C. The TEG may have great potential not only for providing power to the real-time PCR device, but also for using it as a heat. Preheating the real-time PCR device can be possible by utilizing the TEG cold side as a heat source. By using an

it as a heat. Preheating the real-time PCR device can be possible by utilizing the TEG cold side as a

thermoelectric module (*n* = 6).

were well suited for forced convection cooling.

adequate heat-resisting ceramic material, PCR can be controlled within 100 ◦C. This would effectively save energy for operating real-time PCR in the ISS environment where electric supply may be limited. °C. This would effectively save energy for operating real-time PCR in the ISS environment where electric supply may be limited.

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**Figure 7.** Open circuit voltage and power for the TEG depending on the hot side temperature of the **Figure 7.** Open circuit voltage and power for the TEG depending on the hot side temperature of the thermoelectric module (*n* = 6).
