Thermoelectric Conversion Efficiency Research Articles

Carrier transport model and novel design for micro thermoelectric generator with enhanced performance

The carrier transport processes in thermoelectric materials are significantly correlated with the performance of thermoelectric devices (TEs), which is modeled in this study considering the drift-diffusion equation with thermoelectric contributions for detailing the effects of carrier transport on the yielded current density. A micro thermoelectric generator (μTEG) with boron and arsenic being respectively doped into Si0.7Ge0.3 for p-type and n-type thermoelectric materials is employed as the study case by the three-dimensional numerical simulation to reveal the effects of doping concentrations on the thermoelectric performance. Based on the simulation results a novel μTEG design that incorporates PN junctions to extract minority carriers to reduce the recombination rates of electrons and holes is proposed. The performances for conventional and the present novel μTEGs at different temperature differences are compared. The results show that there is an optimal doping concentration for the conventional μTEG to peak the output power and the thermoelectric conversion efficiency. In particular, the novel μTEG reveals a significant improvement of thermoelectric efficiency by approximately 20% as the hot-end temperature is above 900 K. The transport of minority carriers by the PN junctions demonstrates the expected effects through the analysis on the vector distribution of minority carrier current density.

First-principles investigation of the significant anisotropy and ultrahigh thermoelectric efficiency of a novel two-dimensional Ga2I2S2 at room temperature

Two-dimensional (2D) thermoelectric (TE) materials have been widely developed; however, some 2D materials exhibit isotropic phonon, electron transport properties, and poor TE performance, which limit their application scope. Thus, exploring excellent anisotropic and ultrahigh-performance TE materials are very warranted. Herein, we first investigate the phonon thermal and TE properties of a novel 2D-connectivity ternary compound named Ga2I2S2. This paper comprehensively studies the phonon dispersion, phonon anharmonicity, lattice thermal conductivity, electronic structure, carrier mobility, Seebeck coefficient, electrical conductivity, and the dimensionless figure of merit (ZT) versus carrier concentration for 2D Ga2I2S2. We conclude that the in-plane lattice thermal conductivities of Ga2I2S2 at room temperature (300 K) are found to be 1.55 W mK−1 in the X-axis direction (xx-direction) and 3.82 W mK−1 in the Y-axis direction (yy-direction), which means its anisotropy ratio reaches 1.46. Simultaneously, the TE performance of p-type and n-type doping 2D Ga2I2S2 also shows significant anisotropy, giving rise to the ZT peak values of p-type doping in xx- and yy-directions being 0.81 and 1.99, respectively, and those of n-type doping reach ultrahigh values of 7.12 and 2.89 at 300 K, which are obviously higher than the reported values for p-type and n-type doping ternary compound Sn2BiX (ZT∼ 1.70 and ∼2.45 at 300 K) (2020 Nano Energy 67 104283). This work demonstrates that 2D Ga2I2S2 has high anisotropic TE conversion efficiency and can also be used as a new potential room-temperature TE material.

Dynamic Characteristic Study of Supercritical CO2 Recompression Brayton Cycle System

The supercritical carbon dioxide (SCO2) Brayton cycle has been regarded as the main development direction of future nuclear power generation by more and more scholars, due to its high environmental efficiency and high thermoelectric conversion rate. However, due to fluctuations in the operation of the primary loop of the system with nuclear energy, parameters such as the power of the heat source and the mass flow of the working medium in the system will change, which will affect the dynamic performance and operation of the SCO2 Brayton cycle system. Therefore, it is necessary to study the dynamic response of the system performance under disturbance conditions, analyze the operating characteristics of the SCO2 Brayton cycle system. In this paper, a comprehensive dynamic model of SCO2 recompression Brayton cycle, which analyzes the response curves of critical parameters under the disturbance of heat source heating power and system mass flow rate, is accurately developed based on Simulink software. In order to verify the validity of the proposed model, the simulation results are compared with the experimental results conducted by Sandia Laboratory under the same conditions. The results show that the model has high accuracy, and can reflect the dynamic response of system performance under parameter perturbation. In this paper, the closed-loop simulation is innovatively performed to show the dynamic response to step-change in the heat source power and mass flow rate. And the thermal efficiency is about 31.85%, when the system operates stably at the design point of working condition. If a disturbance is applied to the system, the temperature change will be mainly concentrated near the heat source of the cycle, and the change near the precooler will be relatively small. The change of the heat source power will lead to a large monotonic variation of cycle efficiency. By contrast, an inflection point in cycle efficiency will be resulted in by changing the system mass flow rate. The results of this paper would provide good approaches for the design, control, and improvement of the SCO2 Brayton cycle.

Regulation of Electrode–Electrolyte Interactions for Improved Heat Recovery of a Thermo-Induced Electric Double-Layer Capacitor

Low-grade heat recovery through ion redistribution (i.e., Soret effect) in electric double-layer capacitors is a recent emergence for the conversion of thermal energy into electrical energy. A popular approach to improving the performance of such devices is optimizing the composition of the electrolyte. However, in addition to the thermodiffusion behavior of electrolyte ions, the electrode–electrolyte interactions also play a vital role in determining the ionic Seebeck coefficient, although it has not been clearly investigated. In this work, the crucial role of electrode–electrolyte interactions in thermoelectric conversion efficiency is demonstrated by employing vertical graphene as a base material for surface modification. After the surface modification via ozone treatment, oxygen-containing functional groups are enriched on graphene sheets. After improving the interfacial interactions between vertical graphene electrode and electrolyte, the ionic Seebeck coefficient is largely enhanced by 4.6-fold (up to 1 mV K–1) while the capacitance is improved by 1.29 times. Density functional theory calculations reveal that such a high thermal response originates from the stronger ion–electrode interaction and the higher charge transfer efficiency via oxygen-containing functional groups. In addition, the energy stored in supercapacitors can be extracted by discharging through a resistor. The findings of this study can help us to better understand the role of electrode–electrolyte interactions in increasing the ionic Seebeck coefficient, providing a new strategy for improved heat-to-electric conversion.

High thermoelectric performance of PNP abrupt heterostructures by independent regulation of the electrical conductivity and Seebeck coefficient

Improving thermoelectric conversion efficiency is a hotspot in the current field of thermoelectricity. In this work, a novel scheme for the PI-Bi0.5Sb1.5Te3/N-Bi2Te2.97Se0.03/PII-Bi0.5Sb1.5Te3 abrupt heterostructure vertical to temperature gradient is presented, resulting in theoretical results of high output power and conversion efficiency. Under the effect of temperature gradient, the hole concentrations in the PNP heterostructure change linearly and the build-in voltages also vary. As a result, the PNP heterostructure can possess the amplifying characteristic of the bipolar transistor, namely, the forward conduction occurs at the cold end in PI-N junction and the reverse bias occurs at the hot end in N-PII junction. The calculation results show that the optimal output power can reach 48.38 mW and conversion efficiency can reach 14.56% at ΔT = 50 K. This work offers a new idea and method for the realization and application of high-performance thermoelectric materials.

Numerical analysis of segmented thermoelectric generators applied in the heat pipe cooled nuclear reactor

The power system of heat pipe reactor is divided into dynamic conversion and static conversion. Segmented thermoelectric generator (STEG) is a typical static conversion, but there is not enough literature to study its application in heat pipe reactor. Therefore, we have to design the STEG used in the heat pipe reactor. Firstly, we successfully simulated the STEG in COMSOL 5.5 and optimized the geometry and performance. The numerical simulation under steady-state conditions is carried out to determine the optimal STEG geometry. Then, the optimal STEG is connected with a heat pipe to form a single-channel model for simulation to explore the performance. The maximum thermoelectric performance can reach 15.75% for a single STEG, and the maximum stress is about 270 MPa. In the single-channel model, thermoelectric conversion efficiency reduces from 15.75% to 15.63%. This work provides a preliminary basis for numerical simulation in the combination between the STEG and heat pipe reactor.

Enhanced thermoelectric performance by lone-pair electrons and bond anharmonicity in the two-dimensional Ge2Y2 family of materials with Y=N , P, As, or Sb

Using density functional theory combined with the Boltzmann transport equation, the charge, thermal transport, and thermoelectric properties in two-dimensional (2D) ${\mathrm{Ge}}_{2}{Y}_{2}$ ($Y=\text{N}$, P, As, or Sb) monolayers characterized by two structural phases, i.e., $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Y}}_{2}$ and $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Y}}_{2}$, have been studied systematically. Our theoretical results demonstrate that the lone-pair electrons have remarkable influences on their lattice thermal conductivity. By performing comparative studies on the two different structures of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$, we uncover that the above influences not only originate from the interactions between the lone-pair electrons around Sb atoms and the bonding electrons of the adjacent Ge atom, but also from the interlayer Coulomb repulsive forces of lone-pair electrons distributed in different layers. The latter leads to a strong anharmonicity, which greatly suppresses the lattice thermal conductivity. Thus, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$ monolayer has an ultralow thermal conductivity with 0.19 W/mK, while $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$ monolayer with 5.1 W/mK at the temperature 300 K. Owing to the ultralow lattice thermal conductivity induced by lone-pair electrons, the predicted maximum value of the thermoelectric figure of merit ($ZT$) reaches 1.2 for $p$-type and 1.18 for $n$-type doping $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}$. Our theoretical results put forward another effective mechanism to design and optimize 2D thermoelectric materials with high thermoelectric conversion efficiency.

Mixed “ionic-electronic” thermoelectric effect of reduced graphene oxide reinforced cement-based composites

Incorporating functional fillers can provide electronic carriers to enhance the electronic thermoelectric effect of cement-based materials. However, the enhancement of the thermoelectric effect by incorporating multiple kinds and high amounts of such functional fillers might increase the cost, degrade the mechanical properties, and thus hinder the wider applications of the thermoelectric cement-based materials. It may be beneficial to develop a new strategy to improve the thermoelectric effect and at the same time to reduce the content of the functional fillers. In this study, the mixed “ionic-electronic” thermoelectric effect is proposed and evaluated for the cement-based composite. The cement pastes at the reduced graphene oxide (rGO) contents of 0.00 wt%, 0.05 wt%, 0.10 wt%, 0.15 wt% were fabricated, of which the thermoelectric effect is measured and analyzed under the conditions of after sealed curing, after drying, and after leaching. It is found that after the leaching process, the cement-based composite incorporated with rGO (rGO/C) becomes a mixed “ionic-electronic” thermoelectric cement-based composite (M-TECC) due to the cooperative work between the metal cations and the holes carriers provided by pore solution and rGO, respectively. The power factor representing the thermoelectric conversion efficiency is significantly improved as well. The findings of this study provide a new perspective for improving the thermoelectric effect of cement-based composites.

Properties of Nitrous Oxide and Helium mixtures for space nuclear recompression Brayton cycle

Space nuclear reactors are the research foundation of space nuclear power and nuclear propulsion. Thermoelectric conversion efficiency and mass of high-power nuclear reactors have always been essential factors that restrict aerospace design. Supercritical nitrous oxide (S-N2O) Brayton cycle is becoming hot research due to its high-power conversion efficiency, low energy loss, compact and simple system structure, making it widely used in space nuclear reactor application. The recompression cycle is proposed to improve the thermal efficiency of the S-N2O cycle and effectively weaken the “pinch point” phenomenon that may occur in the regenerators. The thermodynamic of the S-N2O Brayton cycle has been studied but has little research on the nitrous oxide (N2O) and helium (He) mixtures as the working medium for the Brayton cycle. In this paper, physical properties were studied on the mixture of supercritical nitrous oxide and helium (S-N2O+He) as the working medium of the space power system based on a recompression Brayton cycle. Consider the thermodynamic properties of the mixture at seven different temperature nodes, analyze the variation trend of compressibility factor, specific heat, specific heat ratio, thermal conductivity, and dynamic viscosity with temperature, respectively. Finally, determined the mixing ratio of the working medium at the maximum thermoelectric conversion efficiency of the cycle and estimated the mass and specific mass of the Brayton rotating unit.

Metal–organic framework coated porous structures for enhanced thermoelectric performance

Self-powered sensors/transmitters can be operated by thermoelectric generators if the temperature difference across the device is maximized. Here, we demonstrate a novel strategy to increase the overall thermoelectric conversion efficiency near room temperature (≈30 °C) through enhancement of the heat transfer between the thermoelectric generator and the atmosphere by utilizing the latent heat of atmospheric water, radiative cooling, and an enhanced surface area. To maximize the sorption and emissivity, we coat porous copper substrates with metal-organic frameworks. Thermoelectric generators interfaced with these heat sinks exhibit a 50%–70% enhancement in the overall heat transfer coefficient owing to the increased surface area due to particle binding and the material properties of metal–organic frameworks. Furthermore, proof-of-concept experiments reveal an ≈100% increase in the total electromotive force generated by the thermoelectric generator within 30 min. Our study not only introduces a novel method to enhance the thermoelectric conversion efficiency, but also provides physical insight into the link between sorption and thermal processes.

Thermal-hydraulic analysis of heat pipe reactor experimental device with thermoelectric generators

Heat pipe reactor relies on heat pipes to transfer the fission energy generated in the core to the secondary loop or thermoelectric conversion system. In our group's design, thermoelectric generators (TEGs) are selected to convert fission energy to electricity. In this paper, we established a heat pipe reactor experimental device with TEGs. To verify the heat transfer performance of the heat pipes and the thermoelectric conversion efficiency of TEGs, steady-state experiments were carried out. The minimum temperature drop between the evaporator and condenser sections of heat pipes can reach 6.4 °C. The maximum output power of TEGs can go 74.4 W. The thermoelectric conversion efficiency is about 8.0%. The Computational Fluid Dynamics (CFD) method is used to simulate the steady-state operation of the experimental device. After comparing the simulation results with the experimental data, the temperature error of the heat pipes is less than 26.8 °C, and the temperature error of the TEGs is less than 17.3 °C. A CFD simulation method for heat pipe reactor steady-state operation is presented based on the verification and validation.

Experimental study on the influence of bionic channel structure and nanofluids on power generation characteristics of waste heat utilisation equipment

• Snake-shaped bionic surface is applied to waste heat utilization equipment. • Influence of bionic structures on power generation characteristics is studied. • Thermoelectric conversion efficiency is studied. • Nanofluids increase temperature of the hot end by 7.19% at the maximum. To recover waste heat resources and improve the utilisation rate of waste heat, a cogeneration system based on bionics was designed. To ma x imise the utilisation rate of waste heat, the effects of different bionic channel structures (angular frequency ω = 20, 25, and 30 rad/s; amplitude A = 1, 2, and 3 mm; and phase shift α = 0°, 90°, and 180°) on the power generation characteristics were studied experimentally. The innovation of this study lies in designing bionic structures with different shapes and replacing traditional working fluids with nanofluids. The experimental results showed that, when the working medium was a CuO–H 2 O nanofluid, the temperature at the hot end of the thermoelectric plate increased by 7.19%. For different bionic structures, the thermoelectric conversion efficiency of waste heat utilisation equipment with ω = 30 rad/s, A = 3 mm, and α = 180° was the highest, being 15.29%, 18.79%, and 6.97% higher than those with ω = 20 rad/s, A = 1 mm, and α = 0°, respectively. The results of this study can provide guidance for the design and operation of the power generation characteristics of waste heat utilisation equipment.

Three-Dimensional Analysis of the Influence of the Magnetic Flux Density on Minimum PR in a Faraday-Type MHD Channel

The magnetohydrodynamic (MHD) behavior and performance characteristic have been revealed by 3-D numerical simulation of the Faraday-type MHD generator. Numerical results show that the generator inlet–outlet pressure ratio (PR) will affect the plasma behaviors and generator performance in the channel. At high PR, the plasma flows at supersonic, and higher electric power [enthalpy extraction rate (EE)] can be obtained. However, at low PR, boundary layer separation occurs with deflected flow of velocity streamlines and vortex in the separated flow region, owing to the oblique shock wave. Therefore, there exists a minimum PR to maximize power generation performance. Under the strong MHD interaction, the Lorentz force causes the loss of Mach number and the increase of static pressure, but it can alleviate the effect of oblique shock for a lower PR. Furthermore, suppressing the boundary layer separation effect gradually evolves into a normal shock wave. When PR is higher, the thermoelectric conversion efficiency increases, and then a weak shock wave will be generated in the channel, which will propagate upstream and evolve into a normal shock wave. Consequently, under a higher magnetic flux density, the minimum PR can be effectively reduced, and the best generator performance can be obtained. In addition, an “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> ”-shaped velocity profile will be generated due to the significantly increased MHD interaction. The results calculated in the present simulation are valuable and important for setting working gas conditions and evaluating generator performance.

Enhancement of thermoelectric properties and first-principles calculations of Ag / Ti co-substituted Ca3Co4O9

Thermoelectric materials of Ca3Co4O9 with excellent comprehensive performance have stagnated the research of thermoelectric conversion devices due to their low thermoelectric conversion efficiency. Ca3-xAgxCo4-yTiyO9 was synthesized by double substitution at Ca and Co sites via a traditional solid-state method to change the microstructure and improve the thermoelectric properties in this paper. Ag and Ti co-substituted increase the porosity, reduce the concentration of Co4+ ions, and improve the Seebeck coefficient and power factor. The optimum substitute amount is x = 0.2, y = 0.2, and the Seebeck coefficient and power factor are 238.4 μV/K and 1481.9 μW/K2m. The first-principles calculation results show that the addition of Ti increases the covalent bond strength between Co–O atoms, and the addition of Ag narrows the gap, which is beneficial to the improvement of thermoelectric properties.

Two-dimensional layered architecture constructing energy and phonon blocks for enhancing thermoelectric performance of InSb

InSb is a narrow-bandgap semiconductor with a zinc blende structure and has been wildly applied in photodetectors, infrared thermal imaging, and Hall devices. The facts of decent band structure, ultrahigh electron mobility, and nontoxic nature indicate that InSb may be a potential mid-temperature thermoelectric material. The critical challenges of InSb, such as high thermal conductivity and small Seebeck coefficient, have induced its ultrahigh lattice thermal conductivity, and thus low ZT values. In view of this, we have developed a competitive strategy typified by the cost-efficient nanocompositing of z wt% QSe2 (Q = Sn, W). Specifically, the QIn+ and SeSb+ point defects were introduced in the InSb system by nanocompositing the vested two-dimensional layered QSe2. In addition, the enlarged valence band maximum of intrinsic WSe2 acted as ladders can scatter a fair number of hole carriers, resulting in the relatively enhanced Seebeck coefficient of high temperature. Moreover, the disorderly distributed nanosheets/particles, and dislocations acting as obstacles can effectively delay the heat flow diffusion, inducing the strong scattering of thermal phonons. Consequently, an enhanced power factor of ∼33.3 µW cm−1 K−2 and ZT value of ∼0.82 at 733 K have been achieved in the 3% WSe2 sample, companied with the engineering output power density ωmax ∼233 µW cm−2 and thermoelectric conversion efficiency η ∼5.2%. This artificially designed approach indicated by suited nanocompositing can integrate several engineering strategies such as point defects, nanoengineering, and energy filtering into one, providing a reference to optimize the thermoelectric performance of other thermoelectric systems.

Comprehensive thermal-hydraulic performance and thermoelectric conversion efficiency of bionic battery waste heat recovery system

To recover battery waste heat, the influence of different groove arrangement (symmetrical and staggered), groove depths (H = 1, 2 and 3 mm) and nanofluids mass fractions (ω = 0.0–0.5%) on the heat transfer performance and power generation characteristics of the battery waste heat recovery device were studied by experiments. The innovation of this paper is: replacing the traditional heat exchange working fluid with a new type of heat exchange working fluid; designing grooves of different shapes based on a sinusoidal curve, and studying the influence of different structural parameters on the thermal-hydraulic performance and power generation characteristics of the battery waste heat recovery device. The experimental results showed that when the mass fraction is 0.5%, the groove depth is 2 mm and the groove arrangement is staggered, the thermal-hydraulic performance and power generation characteristics of the waste heat recovery device are the best. Compared with symmetrical arrangement, the Nusselt number and power generation efficiency of staggered grooves are increased by 1.24 and 5.98% at most.

A prototype thermoelectric module based on p-type colusite together with n-type nanostructured PbTe for power generation

Thermoelectric power generation from the prototype π-shaped module composed of p-type colusite (Cu26Cr2Ge6S32) and n-type nanostructured PbTe (Pb0.98Ga0.02Te-3% GeTe) was demonstrated in this study. The thermoelectric figure of merit zT of Cu26Cr2Ge6S32 and Pb0.98Ga0.02Te-3% GeTe was ∼0.8 and ∼1.2 at 665 K, respectively. In PbTe, transmission electron microscopic images and energy-dispersive x-ray elemental maps reveal the insertion of nanoscale precipitates induced by the GeTe alloying. Contact layers based on Au and Co-Fe were used for p- and n-type thermoelectric legs, respectively, which allow the low electrical specific contact resistances of ≤10 × 10−10 Ω m2 at room temperature. Maximum thermoelectric conversion efficiency ηmax of ∼5.5% was obtained for the Cu26Cr2Ge6S32 and Pb0.98Ga0.02Te-3% GeTe-based two-pair module when the hot-side Th and cold-side Tc temperatures were maintained at 673 and 283 K, respectively. A three-dimensional finite-element simulation predicts the ηmax of ∼7.1% for the module at Th and Tc of 673 and 283 K, respectively.

Train a correct BP neural network as the input layer and as the training sample

For this problem, we train a correct BP neural network with the number of layers, material and thickness of each layer, emission spectrum pollution index as the input layer, as the training sample, and comprehensive thermal efficiency as the output layer, to obtain the network mapping relationship between comprehensive thermal efficiency and design parameters of multilayer structure. Then, through particle swarm optimization algorithm and genetic algorithm, six layers are obtained, which adopt silica silicon silica germanium silicon germanium structure, with refractive index of 21.7832 and thickness of 130.819. Currently, the corresponding thermoelectric conversion efficiency is the highest.

The first power generation test of hot dry rock resources exploration and production demonstration project in the Gonghe Basin, Qinghai Province, China

Hot dry rock (HDR) is a kind of clean energy with significant potential. Since the 1970s, the United States, Japan, France, Australia, and other countries have attempted to conduct several HDR development research projects to extract thermal energy by breaking through key technologies. However, up to now, the development of HDR is still in the research, development, and demonstration stage. An HDR exploration borehole (with 236 °C at a depth of 3705 m) was drilled into Triassic granite in the Gonghe Basin in northwest China in 2017. Subsequently, China Geological Survey (CGS) launched the HDR resources exploration and production demonstration project in 2019. After three years of efforts, a sequence of significant technological breakthroughs have been made, including the genetic model of deep heat sources, directional drilling and well completion in high-temperature hard rock, large-scale reservoir stimulation, reservoir characterization, and productivity evaluation, reservoir connectivity and flow circulation, efficient thermoelectric conversion, monitoring, and geological risk assessment, etc. Then the whole-process technological system for HDR exploration and production has been preliminarily established accordingly. The first power generation test was completed in November 2021. The results of this project will provide scientific support for HDR development and utilization in the future.©2022 China Geology Editorial Office.

Significantly Enhanced Power Factor for Superior Thermoelectric Conversion Efficiency in Snte by Doping Elemental Indium

Significantly Enhanced Power Factor for Superior Thermoelectric Conversion Efficiency in Snte by Doping Elemental Indium