B. V. K. Reddy 1 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261 e-mail: bvkreddy680@gmail.com Matthew Barry Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261 e-mail: mmb49@pitt.edu John Li Adjunct Professor Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261 e-mail: johnli407@yahoo.com Minking K. Chyu Leighton and Mary Orr Chair Professor Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261 e-mail: mkchyu@pitt.edu Convective Heat Transfer and Contact Resistances Effects on Performance of Conventional and Composite Thermoelectric Devices The performance of P shaped conventional and composite thermoelectric devices (TEDs) applied to waste heat recovery by taking the Fourier heat conduction, Joule heat- ing, and the Peltier and Thomson effects in TE materials is investigated using analytical solutions. The TE legs built with semiconductor materials bonded onto a highly conduc- tive interconnector material in a segmented fashion is treated as the composite TED, whereas the legs merely made from semiconductors is treated as the conventional TED. The top and bottom surfaces of TEDs are subjected to convective heat transfer conditions while the remaining surfaces exposed to ambient are kept adiabatic. The effects of con- tact resistances, convective heat transfer coefﬁcients, and TE leg heights L on TEDs’ per- formance are studied. An increase in electrical and/or thermal contact resistance and a decrease in heat transfer coefﬁcients are resulted in a decrease in power output P 0 and conversion efﬁciency g. Depending on the contact resistances and convective heat trans- fer loads, the optimum L where a maximum P o occurs is obtained typically in the range of 1–4 mm. For TE leg size greater than optimum L and TED operating under higher con- vective heat transfer conditions, the composite design exhibited better power output and lower conversion efﬁciency compared to conventional design. The effects of interconnec- tor lengths and cross-sectional area on the composite TED’s characteristics are also investigated. An increase in a length and a decrease in a cross-sectional area of the inter- connector decreases the composite TED’s performance. However, based on the increase of the interconnector’s electrical resistance in relation to the device’s total internal resist- ance, the composite TED exhibited both negligible and signiﬁcant change behavior in P 0 . [DOI: 10.1115/1.4028021] Keywords: composite, contact resistances, convective heat transfer, conventional, waste heat recovery, performance, thermoelectrics 1 Introduction Fossil-fuel power plants, automobiles, and industrial equipment, as well as renewable energy systems such as photovoltaic and fuel cells, reject a majority of the input energy as a waste heat. Recover- ing a small quantity of this waste heat (5–10%) can increase the system efﬁciency and mitigate the environmental impacts of fossil- fuel based power generation. A promising and viable technology that recovers waste heat and directly converts it into electricity are thermoelectric devices (TEDs). TEDs work as power generators via the Seebeck effect when the junctions of the thermoelectric mate- rial, typically semiconductors, are exposed to a temperature gradi- ent. Conversely, using the Peltier effect, TEDs act as refrigerators when a voltage potential is applied across the terminals, creating a temperature gradient at the thermoelectric material junction. Research on TEDs and their application for automobile exhaust waste heat recovery, power generating units in space applications and speciﬁc purposes in aerospace, military, instrumentation, and industrial products have been reported extensively [1,2]. TEDs are compact, scalable to most applications, noise free and have inherent low operation costs. Although the conversion efﬁciency of state-of- the-art TEDs is moderate (5–15%), researchers are continuously exploring novel methods to enhance efﬁciency by improving intrin- sic TE material properties and optimizing system design. The TE material’s performance is measured by the ﬁgure of merit (ZT ¼ ra 2 T=j, where r, a, T, and j are the electrical con- ductivity, Seebeck coefﬁcient, absolute temperature, and thermal conductivity, respectively). The ZT has been enhanced via meth- ods of nano-structuring and -fabrication and scattering short- and long-wavelength phonons across multiple length scales [3,4]. To date, the most efﬁcient materials reported for waste heat recovery at room-temperature applications are bismuth–telluride based alloys [5,6], and for high-temperature applications, lead–tellurides [7–9], and clathrates [10]. Further, the TED’s efﬁciency has been improved by increasing the hot and cold junction temperature dif- ferential (T h -T c ) via novel designs [11–14], optimization of TE leg geometries [15–17], and the minimization of internal and external resistances [18–20]. Caillat et al. [11] showed skutterudites in a segmented fashion can achieve a conversion efﬁciency of 15%. El-Genk et al. [12] demonstrated a maximum conversion efﬁ- ciency of 7.4% for segmented SiGe TEDs. Furthermore, using cascading TEDs, Kaibe et al. [13] reported a conversion efﬁciency of 12.1%. Recently, Crane et al. [14] fabricated a full-scale cylindrical-shaped TED using segmented and high-power density TE elements. They applied this design to an automobile exhaust 1 Corresponding author. Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 2, 2013; ﬁnal manuscript received July 3, 2014; published online August 5, 2014. Assoc. Editor: Wilson K. S. Chiu. Journal of Heat Transfer OCTOBER 2014, Vol. 136 / 101401-1 Copyright V C 2014 by ASME Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 12/04/2014 Terms of Use: http://asme.org/terms