Approaches to thermal isolation with thin-film superlattice thermoelectric materials
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Approaches to thermal isolation with thin-film superlattice thermoelectric materials Pratima Addepalli, Anil J. Reddy, Rama Venkatasubramanian Abstract We propose the use of hetero-structured high ZT materials in thermally isolated thermoelectric couples for the purpose of improving overall device efficiency. Use of high ZT material incorporated in convective flow devices will yield a two-fold improvement in COP over isothermal TE devices. The analytical approach presented in this paper requires optimization of TE element aspect ratio according to available ∆T at location in the device. We discuss the limitations on thermoelectric element characteristics and geometry to achieve high COP. We also utilize moderate temperature differentials to meet the target of high COP. If space requirements are restrictive, or materials processing is limited, the materials described herein provide the unique ability to alter Seebeck coefficient, thermal conductivity, and electrical such that the required aspect ratio is suitable. Improvement in Seebeck coefficient and electrical resistivity independently increase COP by as much as 25% or more when applied to thermally isolated device considered.
1. Introduction Solid-state thermoelectric (TE) devices have found limited advantage over commercial refrigerants for large thermal capacity cooling applications. In order for these TE device to compete, enhancement of the dimensionless figure of merit (ZT), device coefficient of performance (COP), and optimization of the operational current is essential. The figure of merit, ZT, for thermoelectrics in turn involves significant improvement in material properties of Seebeck coefficient, lattice and electronic components of thermal conductivity, and electrical resistivity. Results from Venkatasubramanian et al.[7] and Harman et al. [5] represent notable leaps in thermoelectric materials technology. For any refrigeration system, the system efficiency, defined by the coefficient of performance or COP, is the ratio of the amount of energy the system is capable of extracting to the total power input to the system. High COP ensures improved overall efficiency and reduced cost of operation. One way of improving COP is by implementing the concept of thermal isolation in conjunction with counter flow heat exchange developed by Fenton et al. [3]. Thermal barriers have been suggested that preclude longitudinal heat conduction (thus isothermal condition) along the outer surfaces of the thermoelectric modules. This results in the creation of sectioned thermoelectric heat pumps that would function individually, each operating at a different temperature gradient. Bell has suggested enhancing the overall efficiency by utilizing thermal isolation and manipulation of temperature differentials with convective heat transfer of materials (to be cooled or heated) through portions of the thermoelectric devices [1,2]. Utilization of these strategies offer the possibility of improving the COP of a thermoelectric device by a factor of two over its non-isolat
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