Performance and Reliability Assessment of Thermoelectric Cooling

Project Number : C97-19

Objectives

Assessment of performance and reliability of thermoelectric coolers (TEC) for the thermal control of conventional and high temperature (up to 200 °C) electronics.

Background

Thermoelectric cooling is an active thermal management technique based on the Peltier effect, which describes the generation of a temperature difference across a material in response to an applied potential difference. Heat removal can take place across this temperature gradient. TEC's are solid state semiconductor devices that operate from direct current power sources and have no moving parts. They have been used for the thermal control of laser diodes for fiber optic telecommunication units, cooling of detectors in thermal imaging systems, astronaut cooling units and construction of cold plates. The thermoelectric figure of merit is a measure of the quality of a material to be used as a thermoelement and is defined as Z=S²p/k, where S is the Seebeck coefficient, p the electrical conductivity and k the thermal conductivity. The product of Z and absolute temperature T is nondimensional and is often used to compare the thermoelectric performance of various materials. Current upper limit for ZT in the 100-1500K range is approximately one. Within the last five years several promising thermoelectric materials have been identified, with ZT values as high as 1.4. These advances, along with new challenges such as high temperature electronics require an evaluation of TEC as a thermal control option.

Work Accomplished:

Individual performance tests were conducted from 20^oC to 175^oC on a model HT2-12-30 high temperature TEC module (Melcor Inc.). Performance was characterized for input current vs. generated temperature differential and compared to the manufacturer's quoted performance. The module showed very close agreement with the manufacturer's data.

A two stage TEC test assembly was constructed to assess the modules' ability to meet the requirements of a likely high temperature electronics cooling application. Each stage was operated electrically in parallel to maximize efficiency. Tests showed leakage heat loads to dominate the assembly's performance, leading to the requirement that the TEC's be greatly oversized to obtain the desired temperature differential.

Single stage TECs were operated continuously for periods up to 1500 hrs at temperatures of approximately 170^oC ambient and 190^oC on the TEC's hot side. Temperature differential generated by the modules decreased approximately 5% during the test period, with the majority of the decrease occuring in the first 200 hours of operation.