Direct Conversion Of Thermal Energy Research Articles

Progress and perspectives in thermoelectric generators for waste-heat recovery and space applications

The direct conversion of thermal energy into electrical current via thermoelectric (TE) effects relies on the successful integration of efficient TE materials into thermoelectric generators (TEGs) with optimized characteristics to ensure either optimum output power density or conversion efficiency. Successfully employed for powering deep-space probes and extraterrestrial rovers since the 1960s, the development of this technology for waste-heat-harvesting applications faces several key issues related to the high temperatures and oxidizing conditions these devices are subjected to. This Perspective provides a brief overview of some prospective thermoelectric materials/technologies for use in radioisotope thermoelectric generators utilized in space missions and highlights the progress made in the field over the last years in the fabrication of TEGs. In particular, we emphasize recent developments that enable to achieve increased power densities, thereby opening up novel research directions for mid-range-temperature applications. In addition to showing how using lower quantities of TE materials may be achieved without sacrificing device performance, we provide an outlook of the challenges and open questions that remain to be addressed to make this technology economically and technologically viable in everyday-life environments.

Self-assembled boron nitride nanosheet-based aerogels as support frameworks for efficient thermal energy storage phase change materials.

Phase change materials (PCMs) are promising in many fields related to energy utilization and thermal management. However, the low thermal conductivity and poor shape stability of PCMs restrict their direct thermal energy conversion and storage. The desired properties for PCMs are not only high thermal conductivity and excellent shape stability, but also high latent heat retention. In this study, the boron nitride nanosheets (BNNSs) were bridged by small amounts of GO nanosheets and successfully self-assembled into BNNS/rGO (BG) aerogels by hydrothermal and freeze-drying processes. The BG aerogels with interlaced macro-/micro-pores have been proven to be ideally suited as support frameworks for encapsulating polyethylene glycol (PEG). The obtained composite PCMs exhibit high thermal conductivity (up to 1.12 W m-1 K-1), excellent shape stability (maintain at 90 °C for 10 min), and high latent heat (187.2 J g-1) with a retention of 97.3% of the pure PEG, presenting great potential applications in energy storage systems and thermal management of electronic devices.

Research Progress on Preparation Methods of Skutterudites

Thermoelectric material is a new energy material that can realize the direct conversion of thermal energy and electric energy. It has important and wide applications in the fields of the recycling of industrial waste heat and automobile exhaust, efficient refrigeration of the next generation of integrated circuits and full spectrum solar power generation. Skutterudites have attracted much attention because of their excellent electrical trGiovanna Latronicoansport performance in the medium temperature region. In order to obtain skutterudites with excellent properties, it is indispensable to choose an appropriate preparation method. This review summarizes some traditional and advanced preparation methods of skutterudites in recent years. The basic principles of these preparation methods are briefly introduced. Single-phase skutterudites can be successfully obtained by these preparation methods. The study of these preparation methods also provides technical support for the rapid, low-cost and large-scale preparation of high-performance thermoelectric materials.

О РАЗРАБОТКЕ ТЕПЛОВЫХ ТРУБ ДЛЯ ЯЭУ

The article discusses the designs of two nuclear reactors cooled by heat pipes with liquid metal coolants. Both designs were developed at IPPE JSC. The first design is a research reactor with a capacity of 1.2 MW. Evaporating sodium is used as a coolant in the core. The evaporating eutectic sodium-potassium alloy is used as a coolant in the secondary loop. The third loop contains gas as working fluid of the Stirling or Brighton cycle. The report presents the results of thermo-hydraulic experiments that confirmed the main design parameters. The second design called RIFMA is an innovative super small NPP with direct conversion of thermal energy into electricity and is supposed, in particular, to be used in the Arctic region. Thermal power is 100 kW, efficiency is not less than 10 %. Molybdenum heat pipes filled with lithium are used to transfer heat from the core. To convert energy, thermophotovoltaic converters are proposed. They are cooled by low-temperature heat pipes that remove residual heat and transfer it to air radiators. A nuclear power plant concept and three versions of the core are presented.

Exergy and Energy Analysis of Wind-Thermal System

Current wind systems are intermittent and cannot be used as the baseload energy source. The research on the concept of wind power using direct thermal energy conversion and thermal energy storage, called wind powered Thermal Energy System (WTES), opened the door to a new energy system called Wind-thermal, which is a strategy for developing baseload wind power systems. The thermal energy is generated from the rotating energy directly at the top of the tower by the heat generator, which is a simple and light electric brake. The rest of the system is the same as the tower type concentrated solar power (CSP). This paper’s results suggest that the energy and exergy performance of the WTES (62.5% and 29.8%) is comparable to that of conventional wind power, which must be supported by the backup thermal plants and grid enhancement. This cogeneration nature of the WTES system makes this system suitable for using wind power as a direct heat source in several heat-demanding processes such as chemical production. Also, the light heat generator reduces some issues of wind power, such as noise and vibration, which are two main bottlenecks of the wind power technology.

Modelling and simulation of a thermoelectric waste heat recovery system – TWRHS

The thermoelectric effect allows the direct conversion of thermal energy into electrical energy without the need for moving parts and its applications are endless, with only the availability of a temperature gradient. Combustion vehicles generate energy losses of up to 62% and it is estimated that efficiency with thermoelectric generators can be increased by up to 8%. In this work we propose a system of waste heat from exhaust gases from the thermoelectric effect. A computational model was developed using the finite element method. Based on the simulations, a set of curves of electrical variables can be made as a function of temperature. There is a good correlation between the model and datasheet data. Based on the modeled system, it was possible to evaluate a generation system with the generation of up to 120W with 5.8% efficiency, representing an increase of up to 3% in overall efficiency.

Review of software solutions and results of computational and experimental studies and performance optimization of systems with thermionic energy conversion

The main objective of this paper is to summarize multiphysical tasks and methods for their solution in the area of space nuclear power systems being solved and developed by specialists of JSC SSC RF–IPPE. The physical and technical justification of the development and operation of space nuclear power systems with direct conversion of thermal energy into electricity includes an electrothermophysical calculation of characteristics of a thermionic electrical power generating cell and electrical power generating channel as a whole, neutronic physical calculations and structural optimization of the core and radiation shielding. To determine the electrothermophysical performance of the electrical power generating channel, its 3D detailed computation (software COMSOL-EGK) is used. For the neutronic calculation of the core and radiation shielding characteristics, unified 3D computational software systems are used that implement the Monte Carlo method (in particular, MMKFK-2, MCNP), making it possible to take into account the heterogeneity and all geometry features. The computational procedures and solutions listed in the review are applied in a systematic and integrated manner at JSC SSC RF–IPPE in justification of performance of space nuclear power systems being currently designed. Key words: thermionic reactor-converter, radiation shielding, electrical power generating channel, MCNP, COMSOL – EGK.

Unileg Thermoelectric Module Comprised by Coated Halide-Perovskite Thin Film

Abstract The direct conversion of thermal energy into electricity is possible by thermoelectric effect. CsSnI3 perovskite has shown a way with its intrinsic ultralow thermal conductivity and large Seebeck coefficient. In this work, CsSnI3 thin films were optimized. Thin films were structurally and chemically characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). Thermoelectric properties such as electrical conductivity, Seebeck coefficient, and thermal conductivity were measured near room temperature (300 K). CsSnI3 thin films unileg thermoelectric modules were fabricated on a glass substrate. The maximum output power is obtained about 0.8 nW for five legs (25 mm × 3 mm × 600 nm) modules for the temperature difference of about 5 °C.

Development of an experimental model of a tourist thermoelectric generator and researching ways to increase its efficiency

The paper describes the development of an experimental model of a tourist thermoelectric generator (TTEG). TTEG is a compact portable thermoelectric device that provides direct conversion of thermal energy into electricity and is designed to boil water and charge mobile devices in camp conditions. This paper describes the model of the developed TTEG and provides an analysis of its design. The operational conditions of the TTEG are determined and the ways of optimizing its design are investigated. The main requirements for the developed TTEG model are identified in order to increase the efficiency of the device. The application of the electric load control unit with the function of the maximum power point tracking (MPPT) is described.

Dynamic model of a novel thermogravity rotary actuator for solar energy harvesting

The direct conversion of thermal energy to mechanical energy is always been a challenging but demanding problem. In this article, we presented a nonconventional direct conversion mechanism of the readily available solar thermal and gravity energy to rotational mechanical energy. The developed rotating assembly affixed concentrically with a plurality of actuating arms harnesses the integrated solar-gravity energy. Each actuating arm shifts the center of gravity of the attached solid mass with concentrated solar heating. The alternate shift in the center of gravity of each actuating arm with exposure to solar radiation produces a continuous revolution of rotating assembly around its axis of rotation. With a particular set of the system parameters with Al/Si3N4 bimetallic strip the thermal efficiency of the proposed thermogravity bimetallic strip heat actuator/engine is 12.41% which is 100 times more efficient compared to a gravity-less thermal engine with same set up. The proposed thermogravity system can generate mechanical power ranges from less than 1 W to 40 kW with a particular set of system parameters. This solar-gravity rotary actuator is a novel work in this domain which has a huge impact and contribution in the available rotary actuator or energy harvester knowledge.

Performance evaluation and optimization of the cooling system of a hybrid thermionic-photovoltaic converter

Hybrid thermionic-photovoltaic (TIPV) converters are efficient and clean solutions for the direct conversion of thermal energy to electricity, taking advantage of both the photovoltaic and thermionic phenomena. An important hurdle for their efficient operation is the overheating of the PV cell integrated within the TIPV anode, due to partial conversion of the emitted electron and photon fluxes to thermal heat. This obstacle needs to be overcome with an efficient, yet practical, cooler. In this work, a copper plate heat spreader is experimentally tested for TIPV cathode temperatures up to 1450 °C, whilst its performance is also assessed using a validated CFD model for temperatures up to ~2000 °C. A multi-parametric analysis is conducted testing two coolants: i) a water/ethylene glycol mixture at various temperatures (−5–40 °C) and mass flow rates (0.05–0.4 kg·s−1), and, ii) cryogenic liquid nitrogen at a temperature of −196 °C and mass flow rate of 0.074 kg·s−1. Numerical results reveal that with water/ethylene mixture the PV can withstand heat fluxes up to 360 W·cm−2, without its temperature exceeding 100 °C. For higher thermal fluxes (360–600 W·cm−2), cryogenic liquid nitrogen is found to prevent the PV overheating and, therefore, is an attractive coolant; however, it poses safety concerns due to its possible boiling. Finally, two additional cooling system designs are proposed, a heat sink with straight fins and another with copper pipes, which offer higher heat transfer areas, but are more difficult to manufacture, than the copper plate heat spreader.

Effect of Co content on [Ca2CoO3−δ]0.62[CoO2] thermoelectric properties

The use of thermoelectric materials, which is a clean energy conversion method, can realize the direct conversion of thermal energy and electrical energy. Ceramic samples of the [Ca2CoO3−δ]0.62[CoO2] system with different contents of Co were prepared by sol–gel method combined with cold-press technology. X-ray diffraction and field emission scanning electron microscopy were used to confirm the single-phase composition and plate grain morphology of the samples. Analysis of the thermoelectric properties of [Ca2CoO3−δ]0.62[CoO2] (300–1026 K) shows that the electrical conductivity of the CCO–Co3 sample with low Co content is increased by 12% compared with that of the original sample CCO–Co1 at 1026 K. The thermal conductivity is decreased to 2.99 Wm−1 K−1. The ZT value of the samples with low Co content reaches 0.09 at 1026 K, which is 50% higher than that of the original sample CCO–Co1. Therefore, reducing Co content is an effective method to improve the thermoelectric properties of [Ca2CoO3−δ]0.62[CoO2] ceramics.

Energy and economic analysis of thermoelectric generator on wood fuel

The advantages of thermoelectric generator (TEG) are the absence of moving parts, reliability, long service life, the possibility of fully automatic operation without maintenance. At the same time, TEGs also have disadvantages - low efficiency and high cost. In this regard, it is advisable to use them in autonomous installations of small power, as well as in devices intended mainly for heat supply, when the low efficiency of thermal energy conversion into electrical energy does not play a role. It is the latter case that is realized in furnaces that burn biomass. In this paper, we consider the main physical effects due to which the direct conversion of thermal energy into electrical energy occurs in semiconductor thermoelements. The conjugate problem of heat and electric charge transfer with temperature-dependent properties of a semiconductor is reduced to a problem with constant properties. Based on the literature data, the boundary conditions for the TEG were selected, its sizes were optimized, current-voltage characteristics were constructed, and the maximum power and efficiency were determined. Estimates of the cost and economic efficiency of using TEG are made.

Modeling a thermocell with proton exchange membrane and hydrogen electrodes

Direct conversion of thermal energy to electric energy with thermoelectric generators is an attractive technique to recover low-temperature heat. Thermoelectric generators based on galvanic cells (thermocells) provide promising results with respect to the Seebeck coefficient. In this study, based on the theory of non-equilibrium thermodynamics, we simulated a thermocell with hydrogen gas electrodes and a proton exchange membrane. We calculated a maximum power density of 1461 mW/m2 and a thermal efficiency of 2% relative to the Carnot efficiency for a cell operating with the same gas composition at both the anode and the cathode, but fully saturated at the anode. We predict a Seebeck coefficient in the range of 0.7–1.8 mV/K, higher than those of classical thermoelectric generators. The thermocell presented here provides promising values regarding the Seebeck coefficient.

In-situ synthesis of flexible hybrid composite films for improved thermoelectric performance

Thermoelectric (TE) materials and devices, which enable direct conversion of thermal energy into electricity and vice versa, are a favorable technology for waste heat recovery and flexible energy generators and coolers. Here, we report the enhanced thermoelectric properties of flexible hybrid films composed of carbon nanotubes (CNTs) with good electrical and mechanical properties and inorganic nanowires with high Seebeck coefficient synthesized through an in-situ facile solution method. A two-step interface engineering process is applied to the flexible composite aiming to enhance thermoelectric properties: (1) Bi2Te3 nanowires are grown from the surface of uniformly dispersed CNTs by in-situ synthesis, and (2) a mechanical pressing and a post heat-treatment were employed to form tight interface bonding and adjust the nanowires close to stoichiometric composition. Impressively, the CNTs/Bi2Te3 nanowires composite films exhibited a peak TE power factor of approximately 0.74 mW m−1K−2 at room temperature along with admirable flexibility. The Seebeck coefficient and electrical conductivity were improved by a factor of 30 and 2.2, respectively, through mechanical pressing and post-annealing. This increase is attributed to the phase homogenization of inorganic nanowires to its stoichiometric phase and electron filtering effects due to enhanced interfaces both qualitatively and quantitatively. The flexibility and retention of the conductivity were evaluated via two different methods. This composite with high TE performance will be utilized to create flexible thermoelectric devices for energy harvesting. In addition, the findings from this study can be applied to other flexible thermoelectric materials systems to enhance thermoelectric properties.

Enhanced thermoelectric properties of SiC nanoparticle dispersed Cu1.8S bulk materials

Thermoelectric materials are a relatively new functional material that can be used for the direct conversion of thermal energy to electrical energy through the transmission and interaction of carriers and phonons in the solid. Polycrystalline p-type Cu1.8S dispersed with x wt% SiC nanoparticle (x = 0.1, 0.5, 1.0, 3.0) thermoelectric materials were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). The effects of the SiC nanoparticles on the thermoelectric properties of the material were investigated. The phase structure and microstructure of all the bulk samples were checked by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. Compared to pristine Cu1.8S, Cu1.8S polycrystals dispersed with nano-SiC showed increased Seebeck coefficients, decreased electrical conductivities, and reduced thermal conductivities in the temperature range of 323–773 K. The maximum dimensionless figure of merit (ZT) was improved from 0.49 for pristine Cu1.8S samples to 0.87 for the 1.0 wt% SiC dispersed Cu1.8S sample at 773 K.

CHANGING OF THE STRUCTURE OF THE CONTACT REGION OF THERMOELECTRIC MATERIALS ON THE BASIS OF BISMUTH TELLURIDE AT ELEVATED TEMPERATURES

Destruction of the thermoelectric material on the basis of bismuth telluride under high temperatures, which prevents the use of these materials for a direct conversion of thermal energy into electrical energy, are revealed. Mechanisms of the occurring process are proposed. It is shown that the industrial technology of spark cutting leads to appearance of the damaged layer, which facilitates the penetration of solder into the volume of the thermoelectric material in subsequent soldering of thermoelements.

Thermosalient crystals – new materials for clean energy conversion

Thermosalient compounds, colloquially known as "jumping crystals", are promising materials for fabrication of actuators that are also being considered as materials for clean energy conversion because they are capable of direct conversion of thermal energy into mechanical motion. During heating and/or cooling, these materials undergo rapid phase transitions accompanied by large and anisotropic change in their unit-cell dimensions at relatively small volume change, causing the crystals to jump up to height of several centimeters. Although the list of about a dozen reported thermosalient materials has been expanded recently, this extraordinary phenomenon remains poorly understood. The main practical burden with the analysis of these crystals is their propensity to disintegrate during the transition. By using a combination of structural, microscopic, spectroscopic, and thermoanalytical techniques, we have investigated the thermosalient effect in a prototypal example of a thermosalient solid, the anticholinergic agent oxitropium bromide, and we proposed the mechanism responsible for the effect. We found that heating/cooling over the phase transition causes conformational changes in the oxitropium cation, which are related to increased separation between the ion pairs in the lattice. On heating, this change triggers rapid anisotropic expansion by 4% of the unit cell, whereby the b axis increases by 11% and the c axis decreases by 7%. The phase transition is reversible, and shows a thermal hysteresis of approximately 20 K. Additional interesting observations were that the high-temperature phase of this material can also be obtained by short exposure of the room temperature phase to UV light or with grinding.

Design of an MOM diode‐coupled frequency‐selective surface

AbstractThis article presents the design of a slot‐antenna‐based frequency selective surface coupled with metal‐oxide‐metal diodes integrated into the structure. This design takes advantage of a single self‐aligned patterning step using shadow evaporation. The structure is optimized at 10.6 μm to have less than 2% reflection with 70% of the incident energy dissipated into the oxide layer. Initial experimental results conducted with e‐beam lithography are presented. The fabricated structure is shown to produce a polarization sensitive unbiased DC current. This design will be useful for both infrared sensing and imaging as well as direct conversion of thermal energy. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:489–493, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27363

Direct Thermoelectric Microgeneration Using Residual Heat of Photovoltaic System

All photovoltaic panel heats up when exposed to sunlight and this heating reduces the electrical power output of the same. This work presents the use of this unwanted waste heat, converting it into thermal energy directly by means of the Seebeck effect, which is the direct conversion of thermal energy into electrical energy by means of an arrangement of semiconductor materials that when exposed to temperature gradients generate electric current. In this work emphasis was placed on the influence of temperature on generation processes involved. Thus, the theoretical evaluation, it presents the mathematical models of thermoelectric and photovoltaic systems by raising the curves of voltage, current and electric power generated, and analyses the influence of temperature in each model. To obtain the simulation curves it uses MATLAB ® 5.3, taking into account the parameters of thermoelectric modules and real photovoltaic cells. In practical evaluation, a prototype was assembled containing thermoelectric module attached to the bottom of a photovoltaic panel in order to use the heat energy absorbed by the panel. The data were stored and analyzed, where we observed the influence of temperature in both systems, validating the mathematical modeling. It is the applicability of the mathematical model given the results obtained with the prototype system.