Thermoelectric Conversion Efficiency Research Articles

Optimization of a New Design of Molten Salt-to-CO2 Heat Exchanger Using Exergy Destruction Minimization.

One of the ways to make cost-competitive electricity, from concentrated solar thermal energy, is increasing the thermoelectric conversion efficiency. To achieve this objective, the most promising scheme is a molten salt central receiver, coupled to a supercritical carbon dioxide cycle. A key element to be developed in this scheme is the molten salt-to-CO2 heat exchanger. This paper presents a heat exchanger design that avoids the molten salt plugging and the mechanical stress due to the high pressure of the CO2, while improving the heat transfer of the supercritical phase, due to its compactness with a high heat transfer area. This design is based on a honeycomb-like configuration, in which a thermal unit consists of a circular channel for the molten salt surrounded by six smaller trapezoidal ducts for the CO2. Further, an optimization based on the exergy destruction minimization has been accomplished, obtained the best working conditions of this heat exchanger: a temperature approach of 50 °C between both streams and a CO2 pressure drop of 2.7 bar.

Temperature characteristics of a copper/zinc thermally-regenerative ammonia battery

Bimetallic thermally-regenerative ammonia battery (B-TRAB) is a new way to harvest low-grade waste heat as high-power electricity. Among B-TRABs, copper/zinc thermally-regenerative ammonia battery (Cu/Zn-TRAB) shows the most cost-effective and a relatively high discharge voltage. Within a temperature range of 10–40 °C, the maximum power density of Cu/Zn-TRAB is promoted from 389 W m−2 to 723 ± 45 W m−2 with a linear slope of 12.25 W m−2 °C−1. There is an intense drop in power generation as the operating temperature reaches 45 °C, while ion diffusion and reaction kinetics are not negatively affected by high temperature, indicating that self-discharge caused by ion cross-contamination is the main reason. The cyclic voltammetry tests show that the cathode reaction during discharging is more susceptible to ion cross-contamination at high temperatures. The net energy density first increases and then decreases with the increase of temperature and varies with discharge current, and a maximum of 635 W h m−3 is achieved at 35 °C and a current density of 100 A m−2, with a thermoelectric conversion efficiency ηt of 0.37% (the Carnot-relative efficiency ηt/C is 3.52%). From the simulation results, the condenser temperature has a crucial influence on the energy conversion efficiency, and the ηt can be improved from 0.37% to 1.28% (ηt/C = 12.3%) by reducing the condenser temperature from 43 °C to 21 °C. The highest ηt/C of 14.6% is received at 45 °C.

Reduced Hopping Barrier Potential in NiO Nanoparticle-Incorporated, Polypyrrole-Coated Graphene with Enhanced Thermoelectric Properties

In view of the limiting factor in the thermoelectric conversion efficiency, an attempt has been made in this work with molecular-levelenergy barrier engineering in NiO-incorporated, polypyrrole-coa...

Waste heat recovery, utilization and evaluation of coalfield fire applying heat pipe combined thermoelectric generator in Xinjiang, China

Abstract Coalfield subsurface fires can result in ecological disasters of global dimensions. These fires are difficult to control therefore can cause colossal wastage of resources, negative impacts on the environment and health problems for large populations. However, if the heat can be effectively recycled and utilized, the combustion energy would be recovered to promote fire suppression and pollution reduction. This paper presents the results of feasibility analysis and overall evaluation for the harvest and utilization of underground thermal resources, assisted with an invented system of Waste Heat Recovery and Utilization System (WHRUS) that combines thermosyphon and thermoelectric technologies. Both thermal equivalent model and numerical assessment have been proposed. A series of realistic-scale field experiments initially conducted in the Xinjiang’s fire zone for an extended period are discussed. Using the local geothermic assessment, the heat recovered from subsurface coal fire can be estimated as the summation of convective and conductive type of geothermal energy. The average heat flow through the fire district is about 495 W/m2 and the average utilization efficiency is 58%. The WHRUS shows excellent performance on heat transfer with lower resistance of 0.0049 W/°C and maximum thermal recovery rate greater than 90%. Even though the thermoelectric conversion efficiency is low, the power generated remains stable and every wellbore produces more than 960 W averagely. The majority of the heat extracted is used to heat water for regional heating in a delivering rate of 38%. Through this process approximately 105 MW/a of otherwise wasted heat could be recovered and reused from the entire investigated fire zone.

Performance analysis of a multilayer thermoelectric generator for exhaust heat recovery of a heavy-duty diesel engine

Thermoelectric generators can be used for low-grade energy conversion. However, the design of compact and highly efficient thermoelectric generators is difficult. In this study, the performance characteristics of a multilayer thermoelectric generator are investigated for the exhaust heat recovery of a heavy-duty diesel engine. First, a mathematical model based on the finite volume method is established according to a designed thermoelectric generator. Subsequently, the working performance of the thermoelectric generator is analysed at the rated engine point. Next, the effects of some key parameters for the heat transfer process are evaluated, and the performance of the thermoelectric generator under various engine conditions is studied. Finally, the potential for the power output is estimated based on the world harmonised transient cycle. The results indicate that the output power increases with the engine load, whereas it is insensitive to the variation in the engine speed. The effects of the fin spacing and the fin height of the plain-fin heat exchanger are more significant compared with that of the fin thickness. The working parameters of the thermoelectric generator fluctuate significantly under transient conditions, and the thermoelectric conversion efficiency varies from 1.41% to 4.12%. The development of thermoelectric materials with high efficiency is critical. Furthermore, the results of this study indicate that the improvement in heat transfer between fluids and solid thermoelectric materials is important. The methodology and outcomes of this study can serve as a reference for the design and application of thermoelectric generators.

High thermoelectric performance in excitonic bilayer graphene

We consider the excitonic effects on the thermal properties in the AB-stacked bilayer graphene. The calculations are based on the bilayer generalization of the usual Hubbard model at the half-filling. The full interaction bandwidth is used without any low-energy assumption. We obtain the unusually high values for the electronic figure of merit even at the room-temperatures which is very promising for the thermoelectric applications of the AB-bilayer structure. We discuss the effects of the interlayer Coulomb interaction and temperature on different thermal parameters in the bilayer graphene and we emphasize the role of the charge neutrality point in the thermal properties and within the excitonic insulator transition scenario. The calculated values of the rate of thermoelectric conversion efficiency suggest the possibility of high-performance device applications of AB-bilayer graphene.

Thermoelectric properties of two-dimensional magnet CrI3

The thermoelectric, phonon transport, and electronic transport properties of two-dimensional magnet CrI3 are systematically investigated by combining density functional theory with Boltzmann transport theory. A low lattice thermal conductivity of 1.355 W m−1K−1 is presented at 300 K due to the low Debye temperature and phonon group velocity. The acoustic modes dominate the lattice thermal conductivity, and the longitudinal acoustic mode has the largest contribution of 42.31% on account of its relatively large phonon group velocity and phonon lifetime. The high band degeneracy and the peaky density of states near the conduction band minimum appear for the CrI3 monolayer, which is beneficial for forming a significantly increased Seebeck coefficient (1561 μV K−1). Furthermore, the thermoelectric figure of merit is calculated reasonably, and the value is 1.57 for the optimal n-type doping level at 900 K. N-type doping maintains a higher thermoelectric conversion efficiency than p-type doping throughout the temperature range, while the difference gradually increases as the temperature rises. Our investigation may provide some theoretical support for the application of the CrI3 monolayer in the thermoelectric field.

Thermoelectric composite with enhanced figure of merit via interfacial doping

In order to improve the thermoelectric conversion efficiency and figure of merit, ZT, composite materials of organic or inorganic constituents often are considered. The limitation of this approach is set by the effective medium theory, which states that the ZT in a composite material cannot exceed the greatest value of any single constituent, if the constituents do not interact. Here, we describe a method that circumvents this limit, based on the introduction of interfacial doping. An electrically and thermally insulating medium is distributed into a conventional thermoelectric host material but is coated with an aliovalent acceptor that is allowed to diffuse locally into the host matrix, thereby doping it locally. The thermal conductivity decreases when the insulating material is added, but the more electrically conducting region around the insulator prevents an equally large increase in electrical resistivity. Employing this method in p-type (Bi1-xSbx)2Te3 compounds results in a maximum figure of merit zT = 1.3, an over 10% improvement compared to the host material alone. We report synthesis and measurement techniques in addition to thermoelectric transport properties. While we report on one material system, the concept is not specific to that system and may be used to provide functionality in other thermoelectric composites.

Experimental investigation of a novel heat pipe thermoelectric generator for waste heat recovery and electricity generation

Waste heat recovery helps reduce energy consumption, decreases carbon emissions, and enhances sustainable energy development. In China, energy-intensive industries dominate the industrial sector and have significant potential for waste heat recovery. We propose a novel waste heat recovery system assisted by a heat pipe and thermoelectric generator (TEG) namely, heat pipe TEG (HPTEG)，to simultaneously recover waste heat and achieve electricity generation. Moreover, the HPTEG provides a good approach to bridging the mismatch between energy supply and demand. Based on the technical reserve on high-temperature heat pipe manufacturing and TEG device integration, a laboratory-scale HPTEG prototype was established to investigate the coupling performances of the heat pipes and TEGs. Static energy conversion and passive thermal transport were achieved with the assistance of skutterudite TEGs and potassium heat pipes. Based on the HPTEG prototype, the heat transfer and the thermoelectric conversion performances were investigated. Potassium heat pipes exhibited excellent heat transfer performance with 95% thermal efficiency. The isothermality of such a heat pipe was excellent, and the heat pipe temperature gradient was within 15°C. The TEG's thermoelectric conversion efficiency of 7.5% and HPTEG's prototype system thermoelectric conversion efficiency of 6.2% were achieved. When the TEG hot surface temperature reached 625°C, the maximum electrical output power of the TEG peaked at 183.2 W, and the open-circuit voltage reached 42.2 V. The high performances of the HPTEG prototype demonstrated the potential of the HPTEG for use in engineering applications.

Trace bismuth and iodine co-doping enhanced thermoelectric performance of PbTe alloys

Lead telluride (PbTe) is an excellent thermoelectric material in the intermediate temperature zone and has been applied to deep space exploration, waste heat recovery and other fields. However, the low thermoelectric conversion efficiency of the n-type PbTe alloys limits its applications. Here, the thermoelectric performances have been enhanced in n-type PbTe alloys through trace bismuth (Bi) and iodine (I) co-doping. The Pb1−xBixTe1−xIx (x = 0.00%, 0.05%, 0.10%, 0.20% and 0.50%) alloys are synthesized in the single phase compounds by a stepwise synthesis method. The carrier concentration has reached an optimal concentration range within the order of 1019 cm−3. The highest absolute Seebeck coefficient of 244 μV/K is obtained for 0.05% doped alloy at 730 K. The highest absolute Seebeck coefficient leads to high power factor for 0.05% doped, especially in low- and middle-temperature range. The highest power factor ∼25 μW K−2 cm−1 has been obtained at 329 K. Complex micro-scale grain boundaries and point defects strongly increase the phonon scattering and then lead to the lowest lattice thermal conductivity of 0.64 W mK−1 at 674 K for x = 0.50%, which is 26% lower than that of pristine PbTe. As a result, the highest figure of merit, zT ∼ 0.9 has been determined in 0.20% doped samples at 725 K. Moreover, the highest average figure of merit, zTave ∼ 0.7 has been achieved in 0.05% doped samples in the 323–723 K temperature range, which is about two or three times higher than reported for single Bi or I doped PbTe samples.

Spin-mediated charge-to-heat current conversion phenomena in ferromagnetic binary alloys

Spin-mediated charge-to-heat current conversion phenomena, i.e., the anomalous Ettingshausen effect (AEE) and the anisotropic magneto-Peltier effect (AMPE), have been investigated in various ferromagnetic Ni-Fe, Ni-Pt, Ni-Pd, and Fe-Pt binary alloys at room temperature. When a charge current is applied to a ferromagnetic conductor, the AMPE modulates the Peltier coefficient depending on the angle between the directions of the charge current and magnetization, while the AEE generates a heat current in the direction perpendicular to both the charge current and magnetization. We observed the strong material dependence of the thermoelectric conversion coefficients and figures of merit of these phenomena. Among the ferromagnetic alloys used in this study, Ni$_{95}$Pt$_{5}$ exhibits the largest AMPE of which the anisotropy of the Peltier coefficient is $\sim 12\%$. In contrast, the magnitude of the AEE signals is moderate in Ni$_{95}$Pt$_{5}$ but largest in Ni$_{75}$Pt$_{25}$ and Ni$_{50}$Fe$_{50}$. We discuss these behaviors by exploring the relations between these charge-to-heat current conversion phenomena and other transport as well as magnetic properties. This systematic study will provide a clue for clarifying the mechanisms of the AMPE and AEE and for enhancing the thermoelectric conversion efficiency of these phenomena.

Enhancing the Thermoelectric Performance of Polycrystalline SnSe by Decoupling Electrical and Thermal Transport through Carbon Fiber Incorporation.

Thermoelectric (TE) materials have attracted extensive interest because of their ability to achieve direct heat-to-electricity conversion. They provide an appealing renewable energy source in a variety of applications by harvesting waste heat. The record-breaking figure of merit reported for single crystal SnSe has stimulated related research on its polycrystalline counterpart. Boosting the TE conversion efficiency requires increases in the power factor and decreases in thermal conductivity. It is still a big challenge, however, to optimize these parameters independently because of their complex interrelationships. Herein, we propose an innovative approach to decouple electrical and thermal transport by incorporating carbon fiber (CF) into polycrystalline SnSe. We show that the incorporation of highly conductive CF can successfully enhance the electrical conductivity, while greatly reducing the thermal conductivity of polycrystalline SnSe. As a result, a high TE figure-of-merit (zT) of 1.3 at 823 K is obtained in p-type SnSe/CF composite polycrystalline materials. Furthermore, SnSe samples incorporated with CFs exhibit superior mechanical properties, which are favorable for device fabrication applications. Our results indicate that the dispersion of CF can be a good way to greatly improve both TE and mechanical performance.

Enhanced thermoelectric performance in GaxCo4Sb12.3-yTey skutterudites via suppressing bipolar effects for commercial application

The n-type filled and doped skutterudites GaxCo4Sb12.3 and Ga0.2Co4Sb11.3Te composites with the well-distributed GaSb nanoinclusions are synthesized through the manipulating of metastable Ga fillers and enrichment of Sb by an in-situ method with a proper annealing procedure. Ga atoms can fill the icosahedron cages of skutterudite at high temperature, but at low temperature, they are driven out from the lattice voids and form the second phase of GaSb at grain boundaries. The presence of GaSb second phases reduces the thermal conductivity effectively. Te doping leads to a significant increase in carrier concentration of Ga0.2Co4Sb11.3Te, thus largely suppresses the bipolar effect of GaxCo4Sb12.3, resulting in a great enhancement in power factor. Moreover, Te doping induces mass and strain fluctuation, which decreases the lattice thermal conductivity further. Consequently, the maximum ZT is increased from 0.56 for Ga0.2Co4Sb12.3 at 573 K to 1.48 for Ga0.2Co4Sb11.3Te at 873 K, which is advantageous to improve the thermoelectric conversion efficiency for commercial application.

Thermoelectric properties of monolayer α-Te: Low lattice thermal conductivity and extremely high dimensionless figure of merit

We have investigated the thermoelectric properties of monolayer α-Te by the first-principles and the Boltzmann transport theory. The results show that monolayer α-Te is an indirect bandgap semiconductor with a moderate bandgap of 0.63 eV. The value of the Seebeck coefficient for monolayer α-Te is larger than 200 μVK−1, the electrical conductivity has the magnitude of 1×106. The lattice thermal conductivity is smaller than 4 Wm−1K−1, which is extremely low. Instructively, the dimensionless figure of merit for monolayer α-Te is more than 4, which is markedly high. The above phenomena imply that monolayer α-Te can be served as a considerable thermoelectric material with excellent thermoelectric conversion efficiency.

Theoretical and Experimental Evaluation of a Thermoelectric Generator Using Concentration and Thermal Energy Storage

To improve the thermoelectric conversion efficiency of solar thermoelectric power, a concentration solar thermoelectric generator (CTEG) unit based on concentrating and storing energy is designed. A Fresnel lens is used to concentrate thermal energy, and a phase change material (PCM) is used to store thermal energy to increase the temperature difference between the hot and cold ends of thermoelectric (TE) sheets. The heat stored in the PCM container will help to generate continuous solar energy at night and improve the thermal power conversion efficiency of the TEGs. The energy conversion equilibrium equation is established for the CTEG unit. By numerical calculation, we conclude that the absorption rate of the coating surface is reduced by 0.1 and the maximum thermoelectric conversion efficiency is reduced by approximately 0.4%. By selecting different heights of the heat exchanger, the unit supplies 1.025 v, 0.4204 v and 0.299 v open circuit voltages on average for approximately 30, 44 and 53 minutes, respectively. Therefore, the height of the heat exchanger affects the rate of heat energy absorption and release. The reversible operation of the CTEG-PCM unit is conducive to the day and night operation of solar power generation and is more suitable to solving the self-supplying power problem for wireless temperature sensors in forests.

Influence Na in GeTe for thermoelectric applications

Germanium telluride (GeTe), a semiconductor material, is a complex solid state system with interesting fundamental properties and it serves as a base for high-efficiency thermoelectric materials. Generally, thermoelectric conversion efficiency can be enhanced with appropriate strategies such as doping, alloying, and nanostructuring in the thermoelectric materials. Since there is a significant discrepancy between the electronic and thermal transport data for GeTe-based materials reported in the literature obscuring the baseline knowledge and preventing a clear understanding of the effect of alloying GeTe with various elements, we introduce pentavelant ions into GeTe to enhance the Seebeck coefficient and reduction of thermal conductivity to overcome the discrepancy between the electronic and thermal transport data. A complete study including XRD, Seebeck coefficient, electrical resistivity, and thermal conductivity on Ge rich Ge1-xNaxTe (x = 0.01, 0.02 &0.03) samples was conducted. All compounds were prepared by solid state reaction and hot press method. From the results, Na doping enhances the Seebeck coefficient in the low temperature range and drastically decreased at high temperature. Our data show that at 723 K, Ge0.99Na0.01 Te & Ge0.98Na0.02Te has power factor about 25 µWcm-1K−2. In addition the influence of Na in GeTe to enhance the electrical conductivity.

Thermoelectric phenomena and thermodynamic laws

The paper deals with thermoelectric phenomena from the standpoint of nonequilibrium thermodynamics of irreversible processes. The consideration is based on the fact that thermoelectric effects can occur if there is electrical and thermal contact of electrically conductive media. The necessary condition for the effect is the subsystems’ different ordering degree of charge and heat carriers or different entropy and heat capacity per each carrier particle. The presented approach gives the possibility to understand physics of thermoelectric phenomena deeply. Its development can help to establish the theoretical limits of the efficiency of thermoelectric energy conversion imposed by fundamental physical laws.

Achieving high room-temperature thermoelectric performance in cubic AgCuTe

Average ZT of near unity provides a competitive thermoelectric conversion efficiency of ∼12% at low temperature difference of 400 K.

Improvement in the thermoelectric performance of highly reproducible n-type (Bi,Sb)2Se3 alloys by Cl-doping.

(Bi,Sb)2Se3 alloys are promising alternatives to commercial n-type Bi2(Te,Se)3 ingots for low-mid temperature thermoelectric power generation due to their high thermoelectric conversion efficiency at elevated temperatures. Herein, we report the enhanced high-temperature thermoelectric performance of the polycrystalline Cl-doped Bi2−xSbxSe3 (x = 0.8, 1.0) bulks and their sustainable thermal stability. Significant role of Cl substitution, characterized to enhance the power factor and reduce the thermal conductivity synergetically, is clearly elucidated. Cl-doping at Se-site of both Bi1.2Sb0.8Se3 and BiSbSe3 results in a high power factor by carrier generation and Hall mobility improvement while maintaining converged electronic band valleys. Furthermore, point defect phonon scattering originated from mass fluctuations formed at Cl-substituted Se-sites reduces the lattice thermal conductivity. Most importantly, spark plasma sintered Cl-doped Bi2−xSbxSe3 bulks are thermally stable up to 700 K, and show a reproducible maximum thermoelectric figure of merit, zT, of 0.68 at 700 K.

Influence of Channel Diameter on the Efficiency of Permeable Thermoelements From Bi-Te-Se-Sb Materials

The results of studies of permeable thermoelements are presented. The physical model, method of calculation and design of a permeable thermocouple in which the fluid is pumped through the branches of the semiconductor material are described. For materials based on Bi-Te-Se-Sb, the influence of structural parameters (channel diameter and number, branch height and number of segments) in optimal conditions of thermoelement efficiency on energy characteristics is calculated. The features of methods of solving problems in 3-D and 1-D cases for different models of permeable thermocouples are described. 3-D simulation of the generator thermocouple based on COMSOL Multiphysics was used for the calculations. On the basis of computer-aided design, optimal values of such parameters are calculated, which allow to establish the necessary material science requirements for the creation of thermocouple and indicate the possibility of increasing the efficiency of thermoelectric energy conversion by 1.2 - 1.5 times compared to traditional thermocouples.