Molten Semiconductors for High Temperature Thermoelectricity

Abstract

High temperature (>900 • C) industrial waste heat recovery remains a key challenge for thermoelectric materials. The unique combination of high temperature, low heat-flux, and large surface area of waste heat generation as analyzed herein shows that active materials cost is the main metric inhibiting application. Molten compounds with semiconducting properties are therefore proposed as a cost-effective addition to solid-state materials for these conditions. A review of prior experimental results is presented, after which we demonstrate the performance of a laboratory-scale device based on molten SnS. The results allow reporting, for the first time, the Figure of Merit (Z T ) and the conversion efficiency of the candidate materials. In addition, the Seebeck coefficient of molten SnS is reported. The results confirm the opportunity offered by molten thermoelectric compounds and allow discussion of the remaining materials and engineering challenges that need to be tackled in order to envision the future deployment of thermoelectric devices based on molten semiconductors. Industrial waste heat represents an important source of energy (around 5% of the total US annual energy production, i.e. 1,000,000 GWh), a significant fraction of it being generated at high temperature (around 20% of the industrial waste heat is at T > 650• C).1 Because of such high exergy, there is an interest in directly recovering this energy in the form of electricity, potentially via thermoelectric conversion.Though exceptional progress on solid-state materials for thermoelectrics has been accomplished, including extending their maximum range of operating temperature, there are to date no material systems and devices that have been implemented in the factories which generate this high quality waste heat.We herein first analyze the cost and materials properties required to potentially enable such deployment. The analysis reveals that for large-scale (i.e. large surface of heat dissipation), low heat-flux and high temperature applications such as primary steel or glass production, the cost of the thermoelectric materials is the primary driver for materials choice. In addition, the heat-generating locations in the existing processes call for materials compatible with very high temperature. Finally, the chosen materials system will have to exhibit mechanical stability when subject to a large temperature difference at high temperature.Additionally, a survey of the prior art and available data show that molten semiconductors are likely to fulfill the majority of the proposed criteria, suggesting the opportunity to complement solid-state materials to extend thermoelectricity to high-temperature industrial waste heat recovery.Finally, the practical opportunity offered by molten thermoelectrics is demonstrated, and both experimental materials property data as well as performance of a laboratory-scale device are reported. The results enable the discussion of the remaining materials science and engineering challenges to allow further developments and fut...

Recent progress in nanostructured silver sulfide: from synthesis and nonstoichiometry to properties

“…Tellurides, selenides, suldes, and their solid solutions are the most attractive thermoelectric materials. [58][59][60][61] Silver chalcogenides Ag 2 Se and Ag 2 S have attracted much interest due to reversible transformation between semiconductor and superionic phases. In study, 58 it has been shown that the Seebeck coefficient and ZT for Ag 2 S nanocrystal are equal to À76 mV K À1 and 0.12 at the temperature of the transition from a-Ag 2 S to b-Ag 2 S. For nanocrystalline Ag 2 Se and Ag 4 SeS, values of ZT are 0.23 and 0.33, respectively.…”

Radiative thermal conduction of molten tin sulfide estimated from its optical emission spectrum

“…Estimate of radiative thermal conductivity. The temperature of operation for SnS tested in a thermoelectric test cell is between 900 to 1050°C, 1 where a black body emits most photons with wavelength from 2.19 to 2.47 based on Wien's Displacement Law.…”

“…Sample preparation. Preparation of SnS powder has been described in details by Zhao et al 1 In summary, elemental sulfur (99.5%, Sigma Aldrich) and tin (99.85% metals basis, <150…”

“…Industrial high temperature heat management is an essential challenge for materials extraction and processing. In the context of high temperature (>1000°C) waste-heat recovery using molten thermoelectric materials, 1 the question of the nature of heat-transfer in such materials systems remains critical.…”

“…These studies found that the thermoelectric properties of molten semi-conductors are promising especially in certain metal-sulfur compounds such as Cu2S which has a thermoelectric figure of merit zT close to 1. A recent study 1 investigated the thermoelectric performance of molten SnS with a laboratory-scale device which not only measured the materials-level thermoelectric properties such as Seebeck coefficient but also the device-level thermoelectric power output and conversion efficiency. One conclusion was that thermal conductivity is the missing link to fully understanding the performance of molten thermoelectric materials, a link necessary to achieve future connection between materials-level property study and large-scale deployment.…”