ME 381R Fall 2003 Micro-Nano Scale Thermal-Fluid Science and Technology Lecture 15: Introduction to Thermoelectric Energ

1. ME 381R Fall 2003 Micro-Nano Scale Thermal-Fluid Science and Technology Lecture 15: Introduction to Thermoelectric Energy Conversion (Reading: Handout) Dr. Li Shi Department of Mechanical Engineering The University of Texas at Austin Austin, TX 78712 www.me.utexas.edu/~lishi lishi@mail.utexas.edu

2. References Thermo-electrics: Basic principles and New Materials Development by Nolas, Sharp and Goldsmid Thermoelectric Refrigeration by Goldsmid Thermodynamics by Callen. Sections 17-1 to 17-5

3. Outline • Thermoelectric Effects • Thermoelectric Refrigeration • Figure of Merit (Z) • Direct Thermal to Electric Power Generation

4. Applications Cryogenic IR Night Vision Beer Cooler Thermally-Controlled Car Seat Electronic Cooling

5. Basic Thermoelectric Effects • Seebeck effect • Peltier Effect • Thomson effect

6. Seebeck Effect • In 1821, Thomas Seebeck found that an electric current would flow continuously in a closed circuit made up of two dissimilar metals, if the junctions of the metals were maintained at two different temperatures. S= dV / dT; S is the Seebeck Coefficient with units of Volts per Kelvin S is positive when the direction of electric current is same as the direction of thermal current

7. Peltier Effect • In 1834, a French watchmaker and part time physicist, Jean Peltier found that an electrical current would produce a temperature gradient at the junction of two dissimilar metals. П <0 ; Negative Peltier coefficient High energy electrons move from right to left. Thermal current and electric current flow in opposite directions.

8. Peltier Cooling П >0 ; Positive Peltier coefficient High energy holes move from left to right. Thermal current and electric current flow in same direction. q=П*j, where q is thermal current density and j is electrical current density. П= S*T (Volts) T is the Absolute Temperature

9. Thomson Effect • Discovered by William Thomson (Lord Kelvin) • When an electric current flows through a conductor, the ends of which are maintained at different temperatures, heat is evolved at a rate approximately proportional to the product of the current and the temperature gradient. is the Thomson coefficient in Volts/Kelvin Seebeck coeff. S is temperature dependent Relation given by Kelvin:

10. Thermoelectric Refrigeration The rate of heat flow from one of the legs (i=1 or 2) : (1)

11. The rate of heat generation per unit length due to Joule heating is given by: (2) Eqn 2 is solved using the boundary conditions T= Tc at x=0 and T= Th at x= l. Thus it is found that: (3) The total heat removed from source will be sum of q1 and q2 qc= (q1 + q2 )|x=0 (4) Eqs. 1, 3, 4 K: Thermal conductance of the two legs R: Electrical Resistance of the two legs

12. The electrical power is given by: COP is given by heat removed per unit power consumed Differentiating w.r. to I we get max. value of COP where and Tm=(Th+Tc)/2 A similar approach can be used to obtain the maximum degree of cooling and maximum cooling power.

13. It is obvious that z will be maximum when RK will have minimum value. This occurs when: When this condition is satisfied z becomes: Further, if S2=-S1 = S, k1 = k2 = K, s1 =s2 =s

14. ZTm vs. COP

15. Criteria • For greatest cooling efficiency we need a material that: • conducts electricity well (like metal) • conducts heat poorly (like glass) • Bismuth telluride is the best bulk TE material with ZT=1 • To match a refrigerator, ZT= 4 - 5 is needed • To efficiently recover waste heat from car, ZT = 2 is needed

16. Fundamental limitations: k and s, S and s are coupled. Progress in ZT

17. Thermoelectric Power Generation • Used in Space shuttles and rockets for compact source of power. • Diffusive heat flow and Peltier effect are additive i.e. both reduce the temperature gradient. • The efficiency of power generation is given by: where: w is the power delivered to the external load QH is the positive heat flow from source to sink