Role of Thermal Strategies in Thermoelectric Power Generation

1. Role of Thermal Strategies in Thermoelectric Power Generation Troy J. Dent Jr. and Ajay K. Agrawal Department of Mechanical Engineering The University of Alabama, Tuscaloosa

2. Motivation • Portable power generation • Thermoelectric power generation • No moving parts or noise • Poor performance due to low heat transfer rate between working fluids and TE module • TE research focus primarily been on improving TE materials.

3. Thermoelectric (TE) Effects • TE power generation from a temperature differential across the TE elements • TE module formed by a series of TE elements • TE effects • Joule heating • Peltier effect • Thomson effect

4. Thermoelectric (TE) Effects • Joule Heating • Thomson Effect • dV due to temperature difference • Peltier Effect • dV due to material difference

5. Thermoelectric Efficiency • Thermoelectric efficiency based on • Thermoelectric figure-of-merit • Seebeck coefficient - a (V/K) • Electrical resistivity - re (W·m) • Thermal conductivity - k (W/m·K) • TE module efficiency

6. System Efficiency • System efficiency • Heat input, no heat recirculation • Heat input with heat recirculation

7. Objectives • Comparison of thermal strategies • No fins, Finned, Water-cooled • Effect of thermal strategies on: • Fluid and TE module temperature • Heat transfer rate between TE module / fluids • Heat input ratio, QR • Thermoelectric module efficiency, ηm • System efficiency, ηs

8. Model Layout No fins Finned Water-cooled

9. CFD Parameters • Laminar Flow • Hot Fluid - ReD = 211 • Cold Fluid - ReD = 643 • Fluid flow inlets • Uniform temperature • Tc = 300 K, Th = 1500 K • Uniform mass flux • Air - 1.8 kg/m²∙s • Water - 84.53 kg/m²∙s • Temperature-dependent material properties • Silicon-Germanium TE material properties • TE elements insulated • No axial conduction heat transfer • DO Radiation Model

10. CFD Governing Equations • Conservation of Mass • Conservation of Momentum • Conservation of Energy • Source Terms • Mass - Sm; Momentum - Sx, Sy & Sz; Energy - SE

11. Thermoelectric Module • Thermoelectric junctions • Hot junction • Cold junction • Thermoelectric legs • p-type • n-type • Jp = I/Ap; Anp = An/Ap • Jp and Anp optimized for ηs

12. Absolute Axial Velocity Profiles No fins Finned Water-cooled

13. Temperature Profiles No fins Finned Water-cooled

14. Heat Flux Vector Plot No fins Finned Water-cooled • Significant axial conduction in the metal conductor

15. Axial Mean Temperature Profile Hot Fluid Cold Fluid

16. Axial Mean Temperature Profile Mean Junction Temperature Hot Junction - Cold Junction

17. TE Module Performance Heat Input Rate, Qh Thermoelectric Efficiency, ηTE TE Power Generation Rate

18. TE Module Performance Carnot Efficiency TE material parameter, γ

20. Individual Thermoelectric Effects Joule Heating Thomson Effect Peltier Effect • Heat source rate of individual TE effects • Joule heating & Thomson effect generate heat - Power loss • Peltier effect absorbs heat - TE power generation

21. Conclusions • Reduction of thermal resistance between TE module and fluid can significantly improve system efficiency. • Good thermal strategies will result in improved system efficiency, even with poor thermoelectric performance as in the water-cooled case. • Improved system efficiency is possible through better understanding of the interaction of heat transfer, fluid flow, and thermoelectric power generation. • Research of thermal strategies in combination with thermoelectric material research can yield better thermoelectric power generation.

22. Acknowledgments • Troy Dent is supported by • Graduate Assistance in Areas of National Need (GAANN) Fellowship program of the • US Department of Education