*3.3. Experimental Results of Power Generation Performance with Various Heat Sinks*

*3.3. Experimental Results of Power Generation Performance with Various Heat Sinks*  The generation performance revealed different characteristics depending on the fin shapes of the heat sinks labeled in Figure 5. Figure 8 represents the hot/cold side temperature difference of the thermoelectric module depending on the hot side temperature for natural and forced convection cooling, respectively. Similarly, Figure 9 shows the electric power performance depending on the hot side temperature. It could be seen that the distributions of the six cases show a resemblance between Figure 6 and Figure 7. Higher temperature difference showed higher power generation performance. For the natural convection cooling condition, as shown in Figures 8a and 9a, Case 6 (extruded fin of height 120 mm) showed the best power generation performance whereas Case 3 (bonded fin, height 45 mm) showed the lowest performance. This could be interpreted as the heat sink, having higher fins and wider fin pitches with less thermal contact resistance were best suited for the natural convection. It would have been relatively easy for natural convection induced buoyancy flows to pass over the path with enough space with reduced boundary layer blockage (Case 6) [21]. On the contrary, the heat sinks, having relatively low fins, narrow fin pitches with high thermal contact resistance, showed poor performance during the natural convection. The main reason for the low performance could be thought of as insufficient space between fins for the air to be circulated. To summarize, Case 6 achieved the highest electric power value of 2.4 W at the hot side temperature of 230 °C in natural convection mode. In the forced convection cooling case, however, the case distributions were relatively different from the previous natural convection cooling case. Case 3 showed the highest power generation performance and the most significant increase than the previous case, although Case 6 achieved relatively higher performance than others as well. This is because unlike the previous natural convection cooling condition, the air circulation between the fins in Case 3 would have been relatively easy by external means, in this case, a cooling fan, and with the contribution of a higher heat transfer area as well due to many numbers of fins [21]. Additionally, forced convection can have enough power to overcome boundary layer blockage within the passage. The highest electric power value at hot side temperature 230 °C was measured 10.5 W in this case which is 15 times the previous power value at the natural convection cooling. The heat sinks with relatively lower fins than others, except for Case 3, showed relatively low performance as expected. The generation performance revealed different characteristics depending on the fin shapes of the heat sinks labeled in Figure 5. Figure 8 represents the hot/cold side temperature difference of the thermoelectric module depending on the hot side temperature for natural and forced convection cooling, respectively. Similarly, Figure 9 shows the electric power performance depending on the hot side temperature. It could be seen that the distributions of the six cases show a resemblance between Figures 6 and 7. Higher temperature difference showed higher power generation performance. For the natural convection cooling condition, as shown in Figures 8a and 9a, Case 6 (extruded fin of height 120 mm) showed the best power generation performance whereas Case 3 (bonded fin, height 45 mm) showed the lowest performance. This could be interpreted as the heat sink, having higher fins and wider fin pitches with less thermal contact resistance were best suited for the natural convection. It would have been relatively easy for natural convection induced buoyancy flows to pass over the path with enough space with reduced boundary layer blockage (Case 6) [21]. On the contrary, the heat sinks, having relatively low fins, narrow fin pitches with high thermal contact resistance, showed poor performance during the natural convection. The main reason for the low performance could be thought of as insufficient space between fins for the air to be circulated. To summarize, Case 6 achieved the highest electric power value of 2.4 W at the hot side temperature of 230 ◦C in natural convection mode. In the forced convection cooling case, however, the case distributions were relatively different from the previous natural convection cooling case. Case 3 showed the highest power generation performance and the most significant increase than the previous case, although Case 6 achieved relatively higher performance than others as well. This is because unlike the previous natural convection cooling condition, the air circulation between the fins in Case 3 would have been relatively easy by external means, in this case, a cooling fan, and with the contribution of a higher heat transfer area as well due to many numbers of fins [21]. Additionally, forced convection can have enough power to overcome boundary layer blockage within the passage. The highest electric power value at hot side temperature 230 ◦C was measured 10.5 W in this case which is 15 times the previous power value at the natural convection cooling. The heat sinks with relatively lower fins than others, except for Case 3, showed relatively low performance as expected. It could be concluded that heat sinks with higher fins, low fin pitches, and high heat transfer areas were well suited for forced convection cooling.

It could be concluded that heat sinks with higher fins, low fin pitches, and high heat transfer areas

**Figure 8.** Hot/cold temperature difference side by case corresponding to the hot side temperature for (**a**) natural convection; (**b**) forced convection (*n* = 6). **Figure 8.** Hot/cold temperature difference side by case corresponding to the hot side temperature for (**a**) natural convection; (**b**) forced convection (*n* = 6). **Figure 8.** Hot/cold temperature difference side by case corresponding to the hot side temperature for (**a**) natural convection; (**b**) forced convection (*n* = 6).

**Figure 9.** Electric generation performance based on the electric power of the thermoelectric module side by case depending on the hot side temperature for (**a**) natural and (**b**) forced convection cooling **Figure 9.** Electric generation performance based on the electric power of the thermoelectric module side by case depending on the hot side temperature for (**a**) natural and (**b**) forced convection cooling (*n* = 6). **Figure 9.** Electric generation performance based on the electric power of the thermoelectric module side by case depending on the hot side temperature for (**a**) natural and (**b**) forced convection cooling (*n* = 6).

(**a**) (**b**)

#### (*n* = 6). **4. Conclusions 4. Conclusions**

**4. Conclusions**  From the present study, the potential for adopting a thermoelectric system in the ISS environment, especially in the biomedical field, has been investigated by conducting various experiments. In conclusion, the following facts could be found out through the experiments. First, there was a maximum point where the thermoelectric power performance deteriorated when more compression was applied. Second, one of the important factors of a TEG system performance was the temperature uniformity of a thermoelectric module surface and the necessity of a heat spreader was important in a TEG system. Third, when using fins as a heat sink, forced convection insured much higher thermoelectric generation performance than that of natural convection, in general. However, each heat sink revealed different performance in each condition. With fulfilling optimum force, highperformance heat sink, and temperature uniformity in the ISS environment, the PCR system will be From the present study, the potential for adopting a thermoelectric system in the ISS environment, especially in the biomedical field, has been investigated by conducting various experiments. In conclusion, the following facts could be found out through the experiments. First, there was a maximum point where the thermoelectric power performance deteriorated when more compression was applied. Second, one of the important factors of a TEG system performance was the temperature uniformity of a thermoelectric module surface and the necessity of a heat spreader was important in a TEG system. Third, when using fins as a heat sink, forced convection insured much higher thermoelectric generation performance than that of natural convection, in general. However, each heat sink revealed different performance in each condition. With fulfilling optimum force, highperformance heat sink, and temperature uniformity in the ISS environment, the PCR system will be provided with stable electric energy to operate. From the present study, the potential for adopting a thermoelectric system in the ISS environment, especially in the biomedical field, has been investigated by conducting various experiments. In conclusion, the following facts could be found out through the experiments. First, there was a maximum point where the thermoelectric power performance deteriorated when more compression was applied. Second, one of the important factors of a TEG system performance was the temperature uniformity of a thermoelectric module surface and the necessity of a heat spreader was important in a TEG system. Third, when using fins as a heat sink, forced convection insured much higher thermoelectric generation performance than that of natural convection, in general. However, each heat sink revealed different performance in each condition. With fulfilling optimum force, high-performance heat sink, and temperature uniformity in the ISS environment, the PCR system will be provided with stable electric energy to operate.

provided with stable electric energy to operate. **Author Contributions:** Conceptualization, J.H.C. and S.J.; methodology, J.H.C.; software, J.H.C.; validation, J.H.C. and S.J.; formal analysis, J.H.C. and S.J.; investigation, J.H.C. and S.J.; data curation, J.H.C. and S.J.; writing—original draft preparation, J.H.C. and S.J.; writing—review and editing, J.H.C. and S.J.; visualization, **Author Contributions:** Conceptualization, J.H.C. and S.J.; methodology, J.H.C.; software, J.H.C.; validation, J.H.C. and S.J.; formal analysis, J.H.C. and S.J.; investigation, J.H.C. and S.J.; data curation, J.H.C. and S.J.; writing—original draft preparation, J.H.C. and S.J.; writing—review and editing, J.H.C. and S.J.; visualization, J.H.C.; supervision, S.J.; funding acquisition, S.J. All authors have read and agreed to the published version of **Author Contributions:** Conceptualization, J.H.C. and S.J.; methodology, J.H.C.; software, J.H.C.; validation, J.H.C. and S.J.; formal analysis, J.H.C. and S.J.; investigation, J.H.C. and S.J.; data curation, J.H.C. and S.J.; writing—original draft preparation, J.H.C. and S.J.; writing—review and editing, J.H.C. and S.J.; visualization, J.H.C.; supervision, S.J.; funding acquisition, S.J. All authors have read and agreed to the published version of the manuscript.

J.H.C.; supervision, S.J.; funding acquisition, S.J. All authors have read and agreed to the published version of the manuscript. the manuscript. **Funding:** This research received no external funding.

**Funding:** This research received no external funding.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2018R1C1B5086455).

**Conflicts of Interest:** The authors declare no conflict of interest.
