Investigation of Thermoelectric Generators

Methods of Experimental Physics II

Ethan Bock & Marcelo Herrera

Introduction

This experiment investigated the thermoelectric figure of merit for two thermoelectric generators that contained either Bismuth Telluride (Bi2Te3) or Bismuth Tin (BiSn). Thermoelectric generators convert temperature differences across the hot and cold reservoirs directly into electricity, known as the Seebeck effect. This effect, along with the Peltier effect, which converts an applied current through the device into a temperature difference between the hot and cold reservoirs, were explored in this experiment. In order to maximize the output power of the device, the internal impedance of the device had to be matched with a load resistance. Applications include converting waste heat into electric power.

Theory

Seebeck Effect S=ΔV/ΔT

ΔT is the temperature difference

ΔV is the voltage difference

S is the Seebeck coefficient (V/K)

Peltier Effect

Q=ΠI-(I^2 r)/2

Q is the heat absorbed or generated

&Pi is the Peltier coefficient (V)

I is the current applied

r is the internal impedance

Kelvin’s Relation:

Π=S·T

T is the cold reservoir’s temperature

Figure of Merit:

Z=(8·η)/(T_h (1-3·η)-T_c (1+η))=S^2 σ/κ

η is the efficiency, which is equal to the ratio of power out to heat in

Z is temperature independent and compares η to the Carnot efficiency

σ and κ are the electrical and thermal conductivities, respectively

Semiconductors in Thermoelectric Generators

Thermoelectric generators are made of heavily doped semiconductors because obtaining the maximum power output requires a balance between electrical conductivity and the Seebeck coefficient. P-type and n-type semiconductors are connected in series to avoid thermally shorting the device and increase the total Seebeck coefficient.

Experimental Setup

Equipment:

•Type J thermocouples •Hot plate •Hollow copper cylinder for cold reservoir •Multimeter

Seebeck effect: •Copper block for hot reservoir •Various load resistors

Peltier effect:

•Stainless steel block for hot reservoir •Variable power supply

To collect data for the Seebeck effect, the temperature of the hot plate was consistently increased while the cold reservoir was kept at a constant temperature. 𝛥T between the reservoirs led to 𝛥V read by a DVM. To collect data for the Peltier effect, the reservoirs were held at thermal equilibrium. When a current was applied across the device, the hot side of the TEG would heat the cold reservoir while the cold side of the TEG would cool the hot reservoir. To calculate the heat drawn from the hot reservoir by the TEG, a hot plate was used to supply heat into a metal block with known thermal conductivity such that the hot reservoir temperature matched the cold reservoir temperature.

Results

Conclusion

The goal of this experiment was to determine the internal impedance of each thermoelectric generator, test both Seebeck and Peltier modes, determine the Seebeck coefficient and Peltier coefficient, and to verify Kelvin's relation. The Bismuth Tin thermoelectric generator was found to have a higher figure of merit, (4.56±0.38)·10-4 , than the Bismuth Telluride device, (6.47±0.43)·10-4. The figure of merit describes the ability of a given material to effectively produce thermoelectric power and is directly correlated to the devices’ efficiency, as seen in Equation (5). As expected from the figure of merit measurements, the Bismuth Tin device was also calculated to have a higher efficiency, ranging from 0.674 to 0.77 whereas the efficiency of the Bismuth Telluride device ranged from 0.482 to 0.569. This experiment concludes that a Bismuth Tin device should be used in place of Bismuth Telluride devices, and, to exclude heat loss to surroundings, future attempts to perform this experiment should be conducted in a vacuum.

References

-Kraftmakkher, Yaakov. Simple Experiments with a Thermoelectric Module. European Journal of Physics, August 2005.

-O’Halloran, Steve. Power and Efficiency Measurement in a Thermoelectric Generator. American Society of Engineering and Education, 2012

-Matsch, Leander W. Electromagnetic and Electromechanical Machines. John Wiley & Sons Inc. Third Edition, Pages 525-531.

-Ashcroft/Mermin. Solid State Physics. Brooks/Cole Cengage Learning. Pages 256-259.

-Kittel, Charles. Introduction to Solid State Physics. John Wiley & Sons Inc. Eight Edition, Pages 214-219.