Modeling of a Thermoelectric Generator Device

Thermoelectric generators (TEGs) are devices that employ Seebeck effect in thermopile to convert temperature gradient induced by waste heat into electrical power. Recently, TEGs have enticed increasing attention as green and flexible source of electricity able to meet wide range of power requirements from thermocouple sensors to power generators in satellites. Thermoelectric generators suffer from low‐conversion efficiency; however, they could be promising solutions, when they are used to harvest waste heat coming from industry processes or central‐heating systems. This chapter covers the working principles behind TEGs, depicts numerous schematics explaining functionality of TEGs, and investigates performance of TEGs. A detailed derivation process, which provides performance expressions dictating operation of TEGs, is exposed in this chapter. In addition, thermal resistance network is shown to explain thermal connection of thermocouples in TEGs in parallel and electrical connection of thermocouples in series. Performance features shown in this chapter are power output, efficiency, and voltage induced within TEG as functions of numerous parameters

Enhancing phonon transmission across a Si/Ge interface by atomic roughness: First-principles study with the Green's function method

Knowledge on phonon transmittance as a function of phonon frequency and incidence angle at interfaces is vital for multiscale modeling of heat transport in nanostructured materials. Although thermal conductivity reduction in nanostructured materials can usually be described by phonon scattering due to interface roughness, we show how a Green's function method in conjunction with the Landauer formalism suggests that interface roughness induced by atomic mixing can increase phonon transmission and interfacial thermal conductance. This is an attempt to incorporate first-principles force constants derived from ab initio density-functional theory (DFT) into Green's function calculation for infinitely large three-dimensional crystal structure. We also demonstrate the importance of accurate force constants by comparing the phonon transmission and thermal conductance using force constants obtained from semiempirical Stillinger-Weber potential and first-principles DFT calculations.United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center Award DE-FG02-09ER46577

Green's function studies of phonon transport across Si/Ge superlattices

Understanding and manipulating coherent phonon transport in solids is of interest both for enhancing the fundamental understanding of thermal transport as well as for many practical applications, including thermoelectrics. In this study, we investigate phonon transmission across Si/Ge superlattices using the Green's function method with first-principles force constants derived from ab initio density functional theory. By keeping the period thickness fixed while changing the number of periods, we show that interface roughness partially destroys coherent phonon transport, especially at high temperatures. The competition between the low-frequency coherent modes and high-frequency incoherent modes leads to an optimum period length for minimum thermal conductivity. To destroy coherence of the low-frequency modes, scattering length scale on the order of period length is required. This finding is useful to guide the design of superlattices to reach even lower thermal conductivity.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-FG02-09ER46577

Phonon Transmission Across Silicon Grain Boundaries by Atomistic Green's Function Method

Nanostructured materials are of great interest for many applications because of their special properties. Understanding the effect of grain boundaries on phonon transport in polycrystals is important for engineering nanomaterials with desired thermal transport properties. The phonon transport properties of Σ3 grain boundaries in silicon are investigated by employing atomistic Green's function method. Results show that similar to electron transport, the perfect grain boundary does not significantly reduce the thermal conductance, while defective grain boundaries can dramatically reduce the thermal conductance. This work may be helpful for the understanding of the underlying thermal transport mechanism across grain boundaries and the design of grain boundaries for energy applications