Studies On Performance-evaluation And Heat-dissipation Problems Of High-efficiency Thermoelectric Conversion Devices

With global energy crisis and ecological deterioration, seeking for alternativeenergy sources or new approaches of fossil energy utilization is becoming more andmore urgent. Due to irreplaceable features compared with traditional refrigeration waysuch as simple structure, no emissions of toxic gases, no moving parts, no mechanicalfriction, long working life, high reliability etc., novel refrigeration systems based onthermoelectric effects are given increasing attention which have been applied to variousindustrial fields. In a complete thermoelectric conversion micro-energy system, theconversion efficiency is largely influenced by heat dissipation conditions at the hot endof thermoelectric device as well as the thermoelectric properties of materials themselves.Hence, it is rather critical to find new high-efficiency methods for dissipating heat anddevelop overall property characterization system for testing thermoelectric device’sperformance, both of which have been the key thermophysics problems inthermoelectric conversion micro-energy systems.This paper concentrates on experimental work regardingthe two keythermophysics problem mentioned above.In order to solve the problem ofproperty characterization for thermoelectric mateirals, an overallcharacterization system of thermoelectric device’s performance is establishedbased on direct-current transient Harman method. The overall properties ofinnovative thermoelectric modules are experimentally investigated, includingSeebeck coefficient, value of merit (ZT value), and equivalent electrical andthermal conductivities. The measurement accuracy is verified by the testingresults of commercial bismuth telluride thermoelectric modules. In addition,novel thermoelectric modules with sandwiched structures are fabricated, onwhich testing experiments are conducted. Experiment results reveal thatZTvalues of novel modules are smaller than that of commercial Bi2Te3modules,while Seebeck coefficients of novel modules with sandwiched structures areunexpectedly larger. Meanwhile, both electrical and thermal conductivities ofnovel modules are larger than those of traditional modules.Moreover, the heat transfer enhancement of nanofluid in a micro-tube is studied for the sake of solving heat dissipation demands in high-efficiencythermoelectric conversion system with large heat flux in its hot ends by cuttingrectangular channels at the end face of thermoelectric devices then pumpingaqueous suspensions of carbon nanotubes (CNTs) through these channels. A3-omega method is adopted to measure thermal conductivities of nanofluids, andno odd increase of effective thermal conductivity is observed. It is found thatthe enhancement is less than13%for the nanofluid at CNTs volumeconcentration of1.26vol.%.In contrast, considerable enhancement in theaverage convective heat transfer of nanofluids is observed compared with that ofdistilled water, while the corresponding friction factors of nanofluids show goodconsistency with distilled water’s value, which could be well predicted by theHagen-Poiseuille flow theory, indicating that the CNT nanofluids couldsignificantly enhance heat transfer performance with little extra penalty in pumppower, thus have great potential for applications in future heat transfer systems.