Thermoelectric degrees of freedom determining thermoelectric efficiency

Abstract

Thermal energy can be directly converted to electrical energy as a result of thermoelectric effects. Because this conversion realises clean energy technology, such as waste heat recovery and energy harvesting, substantial efforts have been made to search for thermoelectric materials. Under the belief that the material figure of merit $zT$ represents the energy conversion efficiencies of thermoelectric devices, various high peak-$zT$ materials have been explored for half a century. However, thermoelectric properties vary greatly with temperature $T$, so the single value $zT$ does not represent device efficiency accurately. Here we show that the efficiency of thermoelectric conversion is completely determined by \emph{three} parameters $Z_{\mathrm{gen}}$, $\tau$, and $\beta$, which we call the \emph{thermoelectric degrees of freedom}. The $Z_{\mathrm{gen}}$, which is an average of material properties, is a generalisation of the traditional figure of merit. The $\tau$ and $\beta$, which reflect the gradients of the material properties, are proportional to escaped heat caused by the Thomson effect and asymmetric Joule heat, respectively. Our finding proposes new directions for achieving high thermoelectric efficiency; increasing one of the thermoelectric degrees of freedom results in higher efficiency. For example, thermoelectric efficiency can be enhanced up to 176\% by tuning the thermoelectric degrees of freedom in segmented legs, compared to the best efficiency of single-material legs.

For consistency, the title is updated considering the published article. Main articles with 9 pages, 4 figures, supplementary information with 35 pages, 9 figures, 6 tables

References in corpus (6)

• BoltzTraP. A code for calculating band-structure dependent quantities
• Distribution of phonon lifetime in Brillouin zone
• New bulk p-type skutterudites DD0.7Fe2.7Co1.3Sb12-xXx (X = Ge, Sn) reaching ZT>1.3
• Counterintuitive example on relation between ZT and thermoelectric efficiency
• Thermoelectric power factor of Bi-Sb-Te and Bi-Te-Se alloys and doping strategy: First-principles study
• Transport Properties of Ni, Co, Fe, Mn Doped Cu0.01Bi2Te2.7Se0.3 for Thermoelectric Device Applications

Cited by in corpus (5)

• Best Thermoelectric Efficiency of Ever-Explored Materials
• Thermoelectric Algebra Made Simple for Thermoelectric Generator Module Performance Prediction under Constant Seebeck-Coefficient Approximation
• Unique Temperature Distribution and Explicit Efficiency Formula for One-Dimensional Thermoelectric Generators under Constant Seebeck Coefficients
• Wiedemann-Franz Law and Thermoelectric Inequalities: Effective ZT and Single-leg Efficiency Overestimation
• Reference compositions for bismuth telluride thermoelectric materials for low-temperature power generation