Solid-state Energy Conversion Research Articles

Flue gas waste heat thermoelectric generator: Laboratory experiment and demonstration application

Promoting the efficiency of internal-combustion engine has great benefits, including low fuel-consumption rate and environmental friendliness. Flue gas waste heat thermoelectric generator (TEG) is such a promising solution owning to its solid-state energy conversion and structural simplicity. In this work, a novel heat collection enhancing strategy is first proposed to improve the uniformity of hot-end temperature distribution, which greatly augments the power generation performance under limited pressure drops. A flue gas waste heat TEG unit with 24 TE modules is able to generate an electric power of 103 W under the pressure drop of 375 Pa. The corresponding heat collection, TE, and overall efficiencies of the TEG unit are 36.53 %, 4.30 %, and 1.58 %, respectively. Several important parameters, including flue gas inlet temperature, cooling intensity, cooling mode, and TEG unit arrangement are explored in detail. A demonstration application of an 8-unit waste heat TEG is installed in an 800-ton inland waterway vessel on China's Grand Canal, and the waterway vessel is put into service for over three months. The generated electric power varies between 23.3 W and 463 W, and the average electric power is 177.6 W during the trip.

The 2023 Nobel Prize in Chemistry: Quantum dots

The 2023 Nobel Prize in Chemistry was awarded to Aleksey I. Ekimov (prize share 1/3), Louis E. Brus (prize share 1/3), and Moungi G. Bawendi (prize share 1/3) for groundbreaking inventions in the field of nanotechnology, i.e., for the discovery and synthesis of semiconductor nanocrystals, also termed quantum dots, that exhibit size-dependent physicochemical properties enabled by quantum size effects. This feature article summarizes the main milestones of the discoveries and developments of quantum dots that paved the road to their versatile applications in solid-state lighting, display technology, energy conversion, medical diagnostics, bioimaging, and image-guided surgery.

Thermoelectric Materials and Applications: A Review

Solid-state energy conversion has been established as one of the most promising solutions to address the issues related to conventional energy generation. Thermoelectric materials allow direct energy conversion without moving parts and being deprived of greenhouse gases emission, employing lightweight and quiet devices. Current applications, main thermoelectric material classes, and manufacturing methods are the topics of this work; the discussion revolves around the crucial need for highly performing materials in the mid-temperature range, and around the development of more scalable fabrication technologies. The different manufacturing methods for thermoelectric bulk materials and films are also discussed. Small-scale technologies are generating increasing interest in research; the high potential of aerosol jet printing is highlighted, stressing the many advantages of this technology. A promising approach to scale the production of miniaturized thermoelectric devices that combines high energy ball milling and aerosol jet printing is proposed in the conclusion.

A skutterudite thermoelectric module with high aspect ratio applied to milliwatt radioisotope thermoelectric generator

This study focuses on the design and optimization of a high aspect ratio skutterudite (SKD) thermoelectric module (TEM) for a milliwatt Radioisotope Thermoelectric Generator (RTG). The RTG is a solid-state energy conversion device that utilizes the heat generated by radioactive isotope decay to generate electrical energy through the Seebeck effect. The aim of this work is to enhance the output performance of the RTG by investigating the influence of TEM structure on temperature distribution and overall performance using thermal resistance network and finite element analysis (FEA) methods. In this work, the RTG's Radioisotope Heater Unit (RHU) and diameter are kept constant as 4 W and 70 mm, while the effects of leg length (L), number of legs (n), and cross-sectional area (A) on voltage, output power, and conversion efficiency are studied. It is observed that increasing L, selecting an appropriate A, and reducing n result in a larger temperature gradient within the TEM, ultimately improving the output power and conversion efficiency of the milliwatt RTG. For A = 0.64 mm2, L = 25 mm, and n = 36, the milliwatt RTG generates maximum output power (Pmax) of 86.95 mW and maximum conversion efficiency (ηrmax) of 2.17%. To optimize the output power and conversion efficiency of the milliwatt RTG while simultaneously assuring the large voltage and manufacturability, a SKD TEM with 64 legs, each with dimensions of 0.8 × 0.8 × 25 mm3, is chosen. The milliwatt RTG installed with this SKD TEM will provide a voltage output (Vout) of 1.13 V, maximum output power (Pmax) of 63.30 mW, and maximum conversion efficiency (ηrmax) of 1.58%. Furthermore, it exhibits excellent environmental adaptability, capable of operating in ambient temperatures (T∞) up to 538 K. The thermal stress distribution in the high aspect ratio SKD TEM is also analyzed. Due to the varying coefficients of thermal expansion (CTE) between the SKD material and the adhesive, the maximum stress within the SKD TEM occurs inside and around the p-SKD legs. Finally, a SKD TEM of 9.9 × 9.9 × 25 mm3 containing 64 legs with each leg's dimension of 0.8 × 0.8 × 25 mm3 is fabricated. It shows an open circuit voltage (Voc) of 3.51 V, a maximum output power (Pmax) of 194.45 mW, and a maximum conversion efficiency (ηtmax) of 3.4% at the hot side temperature (Th) of 723 K and cold side temperature (Tc) of 296 K.

Modelling and performance analysis of a novel thermophotovoltaic system with enhanced radiative heat transfer for combined heat and power generation

Solid-state energy conversion technologies such as thermophotovoltaics (TPV) have attracted much interest recently in the development of high-performance combined heat and power generation (CHP) systems because of the merits such as vibration-free operation, long working life, and versatile fuel source. However, current work is limited in power density, system efficiency, and power to heat generation ratio that are constrained by the device structure and working principle of existing techniques. In this work, we propose to apply zero-gap TPV (zTPV) to create a new type of CHP system that can potentially address the limitations in existing systems. zTPV, different from conventional TPV with a vacuum or gas-filled gap between the thermal emitter and photovoltaic cell, possesses an infrared-transparent intermediate solid layer. Our theoretical analysis, combining electromagnetic, thermal, and power generation modelling in zTPV, has confirmed that an one-order-of-magnitude higher thermal radiation dictates the energy conversion in zTPV. The significantly higher thermal radiation is due to the energy transfer of low-loss high-wavevector electromagnetic modes. In conventional TPVs, these modes are not allowed to transport in the gap, and thus not able to contribute to energy conversion. In contrast, the inserted infrared transparent layer allows the transport of these high-density modes, leading to much higher power density (over one order of magnitude enhancement), higher power conversion efficiency (nearly 25% improvement), and high power to heat ratio that is relevant to the system efficiency improvement in CHP applications. The novel device configuration of zTPV allows us to propose a hybrid CHP design that integrates zTPV with thermoelectric (TE) arrays into an ultra-compact CHP device. The effect of different system parameters is modelled and analyzed to demonstrate the high performance of the hybrid CHP.

Improving Electrocatalytic Activity of Cobalt-Free Barium Ferrite-Based Perovskite Oxygen Electrodes for Proton-Conducting Solid Oxide Cells via Introducing A-Site Deficiency

Proton-conducting solid oxide cells are regarded as promising solid-state energy conversion devices to realize the high-efficiency conversion between electrical energy and chemical energy. However, the high cost, easily reducible ion characteristics, and large thermal expansion coefficient related to the valence state of the traditional high-catalytic cobalt-based oxygen electrode are unavoidable problems. Cobalt-free barium ferrite-based oxides with triple-conducting properties are considered the most promising prospective candidates for highly active oxygen electrode materials because of the increased electrochemical reaction active sites, excellent thermal stability, and low thermal expansion coefficient. In this work, by introducing 5% A-site deficiency at the atomic scale, the oxygen vacancy concentration is increased so that the catalytic reaction kinetics and proton-conduction ability of ABO3-type perovskite oxygen electrode material BaFe0.5Sn0.2Bi0.3O3−δ are improved. The effects of non-stoichiometry on the phase composition, microstructure, hydration capacity, thermal expansion characteristics, conductivity, and chemical stability are investigated. In both fuel cell and electrolysis cell operation modes, the proton-conducting solid oxide cells with the A-site-deficient Ba0.95Fe0.5Sn0.2Bi0.3O3−δ oxygen electrode demonstrate excellent electrochemical performance, giving a peak power density of 0.69 W cm–2 (increased by 19.0%) and an electrolysis current density of 1.64 A cm–2 (increased by 30.2%) at 700 °C. Moreover, even at 600 °C with 50% high steam partial pressure, electrolysis for hydrogen production is maintained for about 100 h without significant attenuation, confirming the vital role of A-site defect engineering in the design of advanced oxygen electrode materials.

Optimal Design of PV Inverter Using LCOE Index

This work uses design optimization of a power electronics converter to achieve the best levelized cost of energy in a PV application. The methodology uses detailed models of power electronics’ active and passive components to determine the cost and performances of the solid-state energy conversion and connect them to the system-level vision. The deterministic algorithm used for converter sizing allows taking into account a large number of variables and constraints. Methodology, models, and some illustrations of the results are provided in this paper. A sensitivity analysis was also conducted on the cost model.

Highly active cobalt-free perovskites with Bi doping as bifunctional oxygen electrodes for solid oxide cells

Cobalt-containing perovskite materials have been intensively investigated as bifunctional oxygen electrodes for solid oxide cells because of their mixed ionic-electronic conducting nature. However, the direct application of these materials to the most widely used zirconia-based electrolytes has been limited because of their thermo-chemical incompatibility. Herein, we introduce a cobalt-free manganite-perovskite with significantly improved electrocatalytic activity by doping with Bi which possesses stereochemically active lone pair electrons. Replacing Bi with Pr in Pr0.8Sr0.2MnO3−δ increases the oxygen vacancy concentration and promotes oxygen ion incorporation and charge transfer processes at the electrode. Consequently, we apply Pr0.5Bi0.3Sr0.2MnO3−δ, to the interlayer-free yttria-stabilized zirconia electrolyte-based cells, which exhibits record high performance in the fuel cell (1.91 W∙cm−2) as well as electrolysis (1.90 A∙cm−2) modes at 700 °C along with remarkable long-term stability (450 h). Thus, our findings demonstrate that Pr0.5Bi0.3Sr0.2MnO3−δ is a highly promising bifunctional catalyst for solid-state energy storage and conversion applications.

Advances in Ag2Se-based thermoelectrics from materials to applications

Thermoelectric materials and their devices can realize the solid-state energy conversion between thermal and electrical energy, therefore serving as a promising alternative to conventional fossil fuels for energy supply.

Nanophotonic control of thermal emission under extreme temperatures in air.

Nanophotonic materials offer spectral and directional control over thermal emission, but in high-temperature oxidizing environments, their stability remains low. This limits their applications in technologies such as solid-state energy conversion and thermal barrier coatings. Here we show an epitaxial heterostructure of perovskite BaZr0.5Hf0.5O3 (BZHO) and rocksalt MgO that is stable up to 1,100 °C in air. The heterostructure exhibits coherent atomic registry and clearly separated refractive-index-mismatched layers after prolonged exposure to this extreme environment. The immiscibility of the two materials is corroborated by the high formation energy of substitutional defects from density functional theory calculations. The epitaxy of immiscible refractory oxides is, therefore, an effective method to avoid prevalent thermal instabilities in nanophotonic materials, such as grain-growth degradation, interlayer mixing and oxidation. As a functional example, a BZHO/MgO photonic crystal is implemented as a filter to suppress long-wavelength thermal emission from the leading bulk selective emitter and effectively raise its cutoff energy by 20%, which can produce a corresponding gain in the efficiency of mobile thermophotovoltaic systems. Beyond BZHO/MgO, computational screening shows that hundreds of potential cubic oxide pairs fit the design principles of immiscible refractory photonics. Extending the concept to other material systems could enable further breakthroughs in a wide range of photonic and energy conversion applications.

Thermoradiative devices enabled by hyperbolic phonon polaritons at nanoscales

Theromoradiative (TR) devices, though proposed a decade ago, have seen little investigation due to their low-performance compared to other solid-state energy conversion technologies. Herein, we propose an InSb-hBN TR device with a nanoscale vacuum gap down to 10 nm that has the potential to achieve efficient waste heat recovery using solid-state technologies. By coupling the hyperbolic phonon polaritons of hBN with the interband transition of InSb, this TR device design can achieve an output power nearly 4 orders-of-magnitude higher than far-field scenarios and an upper bound efficiency up to 80% of the Carnot limit corresponding to only considering radiative loss. A detailed balance model combined with fluctuational electrodynamics for a multilayered anisotropic structure is developed to theoretically estimate the performance of TR devices from far- to near-field regime. Nonradiative losses are also investigated to elucidate their effects on performance. This work helps deepen the understanding of TR devices and provides a promising pathway to efficient low-grade (< 600 K) heat recovery.

Bismuth telluride based efficient thermoelectric power generator with electrically conducive interfaces for harvesting low-temperature heat

Nearly 50% of the heat released by various sources in the environment is found in the temperature range below 573 K. Thermoelectric generators (TEGs) offer a robust solid-state energy conversion mechanism to harvest this waste heat into useful electrical energy. To develop efficient TEGs, a prerequisite is to couple n - and p -type thermoelectric materials through metallic interconnect, with low specific contact resistivity ( ρ c ). The realization of low ρ c involves several critical factors, such as the chemical and mechanical stability, which should be carefully optimized to achieve consistent TEG performance under long-term operation. In the present work, we have demonstrated the concept of a graded interface layer to realize electrically conducive contacts in mechanically alloyed n -Bi 2 Te 2.7 Se 0.3 and p -Bi 0.5 Sb 1.5 Te 3 thermoelectric material (TM) having moderate figure-of-merit ( zT ). We have reproducibly achieved electric contacts with low ρ c in these materials (∼59-71 μΩ-cm 2 ) using a graded layer of Ni. Using the established approach, the conversion efficiency ( η exp ) of 6.5% (±10%) is demonstrated for the temperature difference (Δ T ) of 270 K. Achieved η exp in the present work matches well with the simulated conversion efficiency ( η sim )∼7.0%, and is one amongst the best-reported efficiencies surpassing the state-of-the-art commercial modules.

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

Harvesting low-grade heat by coupling regenerative shape-memory actuator and piezoelectric generator

Low-grade thermal energy is ubiquitous in nature with different scales. Being concise in nature, solid-state energy conversion technologies based on ferroic materials experienced fast development over the years, yet their efficiencies are still not as competitive as the conventional Rankine cycle. To improve the efficiency of solid-state generators, this study proposed a novel composite device leveraging the strain coupling between the regenerative shape-memory actuator and the piezoelectric generator. Taking the realistic losses into account, our experimentally validated numerical model indicated that the new concept has 14.4% efficiency, equivalent to 57% second-law efficiency, when the length and cross-sectional area of the shape-memory actuator and the piezoelectric generator is properly matched. It was also found that relaxor ferroelectric materials are more efficient than the baseline PZT ceramics. The impact and design guideline of other parameters were also investigated, including the utilization factor, transmission factor, and transition temperature. The proposed concept provides a new path for harvesting the low-grade thermal energy, and could potentially be integrated with caloric cooling counterparts to provide power and refrigeration simultaneously as a next-generation co-generation and tri-generation technology.

Self-powered and flexible integrated solid-state fiber-shaped energy conversion and storage based on CNT Yarn with efficiency of 5.5%

Self-powered and flexible integrated solid-state fiber-shaped photo capacitors (SS-FPCs), which combine energy-harvesting and energy-storage functions in a single device, have drawn a lot of attention as potential applications in wearable electronics. Herein, a flexible carbon nanotube yarn (CNTY) with improved mechanical and electrical properties is used as a shared electrode to create SS-FPCs. In addition, CNTY was doped into n- and p-type using polyethyleneimine (PEI) and iron (III) chloride (FeCl3) to improve the electrocatalytic behavior of CNTY, respectively. The optimized SS-FPC based on p-type material-doped CNTY, which integrates the high specific capacitance of the solid-state fiber-shaped electrochemical energy storage (SS-FES) unit with 78.26 mF cm−2 and a high power conversion efficiency (PCE) of the solid-state fiber-shaped dye-sensitized solar cells (SS-FDSSCs) unit with 5.50%, exhibits a remarkable overall energy conversion efficiency of up to 4.69% and excellent discharging time of about 6 min owing to the synergistic action of the two units. Compared to traditional integrated power systems, our self-powered and flexible integrated SS-FPCs are lightweight, flexible, affordable, and suitable for special applications, especially fabric for wearable electronics.

Achieving Enhanced Thermoelectric Performance in Multiphase Materials

ConspectusThermoelectric (TE) devices enable direct solid-state energy conversion from heat to electricity and vice versa, thereby showing great potential in warranting the supply of sustainable energy and mitigating the potentially catastrophic effects of climate change. Therefore, as a clean-energy-generation technology, TE materials have received tremendous research efforts in both industrial and academic communities for applications in the recovery of ubiquitous low-grade waste heat. Achieving high efficiency in TE materials is an ongoing pursuit of the TE research community, considering approximately 90% of all waste heat in the USA comes from medium-temperature (e.g., from 573 to 873 K) heat sources. Hence, synergistic enhancements in the figures-of-merit (ZT) are still highly desired and remain a key task for improving commercial applications of TE materials.Over the past decades, by navigating the interplay between heat and electronic transport, different advanced paradigms in current state-of-the-art TE materials, as well as emerging TE materials, have been adopted to obtain high ZT values, especially for the average ZT (ZTavg), as the power conversion efficiency (PCE) of a TE device depends on the ZTavg over a wide temperature range. Yet, now there are few reviews or perspective reports on methods to synergistically boost both the peak ZT (ZTpeak) and ZTavg, Herein, our recently developed strategies for optimizing and designing high-performance TE materials through advanced multiphase nanocomposite paradigms and material design are presented. Multiphase composites with distributed nanostructures (e.g., nanoplates and/or nanoparticles) provide effective phonon scattering through the nanoscale secondary phases. On the basis of the physical mechanism (the charge carrier transfer, phonon transfer/scattering, phonon-charge carrier interaction, and interface phenomenon), one can derive the optimum direction for the improvement of ZTpeak and ZTavg of TE semiconductors.In this Account, we summarize and discuss the recent advances and insights from our work in designing TE materials with both high ZTpeak and ZTavg by combining precisely engineered doping and nanostructuring. This Account is organized as follows: First, we summarize the advances of representative state-of-the-art TE materials in chronological order, including the advancement of performance-improving mechanisms. Second, we highlight the application of our effective multiphase material design strategies to leading Pb-based (PbTe, PbSe, and PbS) TE materials, with an emphasis on ZTavg and SnSe-based TE materials. Third, we introduce Sb2Si2Te6, which possesses an ultralow thermal lattice conductivity, as a promising new TE material, which was enhanced through a cellular nanostructure strategy. Finally, we outline an entropy-driven paradigm to optimize current SnSe-based and MnTe-based materials, thereby improving their TE performances.

Electronic Structure and Thermoelectric Properties of Mg2Sn Films Fabricated by Using Co-Sputtering Process With Stoichiometric Modification

Colossal values of the Seebeck coefficient were first discovered in Mg-poor Mg2Sn films in the temperature range 600–700 K. Cubic and orthorhombic structures of the Mg2Sn films for solid-state thermoelectric energy conversion devices were prepared by radio frequency (RF) magnetron co-sputtering. The Mg-Sn films showed a structural phase transition between the metastable orthorhombic and the stable cubic structures as the atomic ratio Mg/Sn varied near the stoichiometric ratio. The Mg-poor orthorhombic Mg-Sn films transited from p-type to n-type conductivity at 500 K and exhibited colossal values of the Seebeck coefficient and power factor in the temperature range 600–700 K. The colossal Seebeck coefficient, which exceeded -10 mVK-1, indicates long-term stability over a wide temperature range. Furthermore, the energy band structure and density-of-state were calculated for the cubic and the orthorhombic Mg2Sn using the first-principles calculation with the Perdew-Burke-Ernzerhof (PBE0) hybrid functionals, which yielded results that were consistent with the experimental data.

Quantum dot photolithography using a quantum dot photoresist composed of an organic-inorganic hybrid coating layer.

Quantum dots (QDs) have emerged as an important class of materials for diverse applications such as solid-state lighting, energy conversion, displays, biomedicine, and plasmonics due to their excellent photonic properties and durability. Soft lithography, inkjet printing, nanoimprinting, and polymer deep-pen lithography are primary lithography techniques employed to implement micro-patterns with QDs, however, there are limited reports on QD photolithography using conventional photolithography processes suitable for mass production. This study reports a QD photolithography technique using a custom-developed QD photoresist made of an organic–inorganic hybrid coating layer. Using this QD photoresist, various QD micro-patterns, including red or green micro lines, RGB color filters for smartphone displays at 340 ppi, and atypical micro logo patterns of the Korea University, were successfully fabricated. Furthermore, various process parameters were developed for the QD photolithography with this custom QD photoresist, and the optical properties of the QD films were also investigated. To demonstrate its applicability in contemporary smartphone displays, the color coordinates of the QD films were compared to those of the BT.2020 standard. The chromaticity of the QD photoresist in CIE 1931 color space covered 98.7% of the NTSC (1987) area while providing more expansive color space. Overall, the QD photoresist and its photolithography techniques reported in this study hold great promise in various fields of QD-based applications, including bio-labeling, optical detectors, and solar cells.

Assessment of solid-state power generators for low enthalpy geothermal resources

Before producing electricity, most modern thermal power generation devices must convert thermal energy to mechanical work. Thermoelectric generation technology is a solid-state energy conversion process that uses thermoelectric transformation materials to convert thermal energy into electricity. This study discusses an innovative method to utilize thermoelectric technology in renewable energy. Heat may be converted directly to electricity using thermoelectric technology which is quiet, compact and environmentally friendly. In recent years, new and intriguing materials that have the potential to improve the efficiency of this technology have been found. This research focuses on generating power using solid-state generators such as thermoelectric generators (TEG) modules made from Bismuth telluride material composition. The output power was measured in an experimental research under various situations, including varied input temperatures and temperature differential between the hot and cold sides. It has been determined that by combining several TEG modules, we can generate up to 500-W of electrical power from low enthalpy geothermal resources.

Understanding Thermoelectrics Through Carrier and Phonon Transport

Solid state energy conversion using thermoelectric (TE) phenomena has attracted great interest in power generation by using waste heat and active cooling/heating from electricity. Since first observation of the Seebeck and Peltier effect in the early 1820s, the TE phenomena has been applied in limited fields due to difficulties during the last two centuries in controling performance, which is related to both the carrier and the phonon transport behaviors. This article briefly introduces not only technological research issues for universal use of thermoelectrics but also the latest strategy for applications from the viewpoint of materials.