# Heat Transfer in Thermoelectric Generators for Waste Energy Recovery in Piston Engines
## 5 Conclusions  
A thermoelectric prototype for exhaust gases from a piston engine was designed and studied following a low pressure drop approach. The design and analysis are valid for piston engines working on the ground or in aircraft. Different issues are studied: the influence of the replacement of the exhaust pipe for the TEG, the recirculation produced, and the influence of fins. The following conclusions can be drawn from this work:  
* The typical sudden cross-sectional area expansion at the TEG inlet causes an important recirculation region;
* The recirculation increases the pressure drop and has mild but not unimportant effects on the heat transferred and surface temperature;
* An enlarged inlet cone reduces the recirculation and the pressure drop of the TEG, but more heat is lost across the cone walls, reducing the heat available for the thermoelectric modules;
* Splitting the inlet with flow separators in one direction did not result in a decrease in recirculation, but increased it;
* Internal straight fins aligned with the flow do not significantly increase the pressure drop in this type of device;
* In addition, the fins reduced the recirculation, lowering the overall pressure drop;
* An energy production of 176 W with 16.2 W of pressure drop losses resulted in a net energy production of 159.7 W;
* A significant decrease in pressure drop (\(-61.8\%\)) was achieved compared to a flat-type TEG design.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Design** & **Squared** & **Flat Type**[26] & **Variation** \\ \hline Power output & 175.9 W & 106.2 W & + 65\% \\ \hline Pressure drop & 292.7 Pa & 766.6 Pa & \(-61.8\%\) \\ \hline Number of & & & \\ thermoelectric & 72 & 80 & \(-10\%\) \\ modules & & & \\ \hline \end{tabular}
\end{table}
Table 7: A comparison with a flat-type thermoelectric generator in literature with similar heat source conditions. Percentages refer to the flat-type design values.  
Figure 15: Flat-type heat exchanger of a thermoelectric generator [9; 23] for comparisons with the squared design proposed here. The water circuit and thermoelectric modules are not shown for better visualization of the main geometry.  
**Author Contributions:** Conceptualization, P.F.-Y.; methodology, P.F.-Y.; software, J.J. and F.J.M.; validation, P.F.-Y. and O.A.; formal analysis, P.F.-Y. and F.J.M.; resources, F.J.M. and O.A.; data curation, J.J.; writing--original draft preparation, P.F.-Y. and J.J.; writing--review and editing, P.F.-Y., F.J.M. and O.A.; visualization, J.J.; supervision, P.F.-Y. and O.A.; funding acquisition, O.A. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Spanish Ministry of Science, Innovation and Universities, grant number RTI2018-095923-B-C21 (project RECUPERA).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors wish to thank the technical support provided by the Nissan Europe Technology Centre, Spain.  
**Conflicts of Interest:** The authors declare no conflict of interest.