Thermoelectric Conversion System Research Articles

Optimization of methane partial oxidation for hydrogen production with local free space addition: An efficient combination of porous media reactor and thermoelectric conversion system

Thermoelectric technology plays a pivotal role in the efficient conversion and utilization of exhaust heat. Abundant waste heat resources are present in the tail gas of hydrogen production, significantly influenced by combustion characteristics. To investigate the impact of combustion performance on hydrogen yield and power output, an efficient porous media reactor was designed and integrated with a thermoelectric conversion system. The combustion characteristics of a thermoelectric system incorporating unsteady porous media were examined. Furthermore, the effects of the porous media’s structural parameters on temperature distribution, gas concentration, and power generation were analyzed under various operating conditions. The results revealed that radial free space structures exhibited higher hydrogen and methane conversion efficiencies compared to axial free space structures at verge and toroidal positions. However, the height of the radial free space exerted a more substantial influence on temperature distribution and gas production than the size of the free space itself. At a height of 39 mm, hydrogen concentration and yield reached their peak values of 14.5 % and 53 %, respectively. Combustion was found to significantly impact power generation, with a downward shift of the flame resulting in decreased power output. The inlet velocity was observed to affect the thermoelectric generator power, with a maximum steady-state power of 3.95 W achieved at a velocity of 16 cm/s. These findings provide practical guidance for the efficient utilization of industrial waste heat.

Heat transfer characterization of SCO2 in non-uniformly heated rectangular cooling tubes under large mass flow and heat flux conditions

A Thermoelectric Conversion System (TECS) with SCO2 as the circulating mass can effectively mitigate the fuel heat sink and electrical energy shortage problems of hypersonic vehicles. However, there is still a lack of research on the use of SCO2 in the harsh thermal conditions in scramjet. In this paper, the heat transfer mechanism of SCO2 in rectangular cooling tubes are numerically analyzed for the first time based on the actual working environment of scramjet. The effects of uneven heat flux (q) axial and circumferential allocations and circumferential and axial tilting of the tube are innovatively considered. Meanwhile, we analyze the heat transfer mechanism of SCO2 under different operating parameters. The results show that the uneven allocation of axial q can suppress the heat transfer deterioration (HTD), and the overall heat transfer coefficient (HTC) can be increased by 11.68 % when the q allocation is Gaussian. The increase of the tube inclination angle can improve its heat transfer performance, and the local HTC can be increased by up to 5.64 % when θ2 = 180°.Increasing the mass flow rate (G) and inlet temperature (Tin) can increase the overall HTC of the tube. This paper can provide guidance for the design and improvement of TECS.

Bi2Te3-based flexible thermoelectrics

Bi2Te3-based materials stand out as the top-performing material for thermoelectric applications at room temperature. However, its inherent rigidity has posed challenges for widespread usage in flexible thermoelectric conversion systems. Recent endeavors have focused on achieving Bi2Te3-based flexible films through bulk thinning, physical/chemical deposition, paste casting, as well as the template method. These efforts have led to a surge in research publications. This review aims to offer a comprehensive update on the synthesis approaches, microstructures, thermoelectric performances/flexibility, and the underlying mechanisms. Future research should focus on investigating innovative deposition techniques, exploring new composite phases/templates, refining fabrication parameters, and among others, to enhance the thermoelectric performances as well as flexibility.

Phase-Transition-Promoted Thermoelectric Textiles Based on Twin Surface-Modified CNT Fibers.

With the fast development of new science and technology, wearable devices are in great demand in modern human daily life. However, the energy problem is a long-lasting issue to achieve real smart, wearable, and portable devices. Flexible thermoelectric generators (TEGs) based on thermoelectric conversion systems can convert body waste heat into electricity with excellent flexibility and wearability, which shows a new direction to solving this issue. Here in this work, polyethylenimine (PEI) and gold nanoparticles (Au NPs) twin surface-modified carbon nanotube fibers (CNTFs) were designed and prepared to fabricate thermoelectric textiles (TET) with high performance, good air stability, and high-efficiency power generation. To better utilize the heat emitted by the human body, microencapsulated phase change materials (MPCM) were coated on the hot end of the TET to achieve the phase-transition-promoted TET. MPCM-coated TET device could generate 25.7% more energy than the untreated control device, which indicates the great potential of the phase-transition-promoted TET.

Model predictive control for automatic operation of space nuclear reactors: Design, simulation, and performance evaluation

Space nuclear reactors possess the inherent benefits of independence from solar energy and the capability to withstand intricate external circumstances. Distinguishing themselves from terrestrial reactors, space nuclear reactors function within the realm of outer space, operating autonomously throughout their entire operational cycles. Consequently, the establishment of a dependable automatic control system assumes utmost significance. In this paper, a high-fidelity nonlinear space thermionic nuclear reactor model is established, encompassing neutron physics, thermal–hydraulic, and thermoelectric conversion system models. An automatic control system with a model predictive controller (MPC) is designed based on the system. A simplified state-space model is established based on system identification as the internal model for the MPC controller, demonstrating good agreement with the dynamic response of the nonlinear system model. The deployment of the MPC controller for system control simulations exhibits excellent control performance under various operating conditions. In the case of a nuclear power step response, the MPC controller achieves an overshoot of 0.07 % and a settling time of 0.65 s. For an electric power step response, the MPC controller yields an overshoot of 0.28 % and a settling time of 36.6 s. These control performances are significantly better than those achieved by the PID controller. In power tracking control, the MPC controller demonstrates virtually error-free tracking performance. When external reactivity disturbances are introduced, the MPC controller outperforms the PID controller in swiftly compensating for the disturbances, maintaining the system's operating state at the desired level.

Numerical research on performance of new structure centrifugal compressor for supercritical CO2 power systems

The supercritical carbon dioxide (sCO2) power cycle system is a potential thermoelectric conversion system with the advantages of high efficiency and compactness, which can be used in the fields of solar thermal power, nuclear power, and other thermal power systems. The compressor is one of the important machines in sCO2 power systems. To improve the performance and reduce the condensation risk of sCO2 centrifugal compressors, a new structure sCO2 centrifugal compressor is proposed in this paper, which has a partial-cover impeller and self-circulation-channel casing (PISC). The internal flow of the compressor with the new structure is analyzed by the CFD method. The flow and condensation in sCO2 centrifugal compressors with the PISC structure and that with the semi-open impeller and traditional casing (SITC) are compared. The results show that the PISC sCO2 compressor has an inhibiting effect on the condensation at the compressor inlet location, reducing the condensation volume to approximately 82% of that of the original structure under design conditions. The total pressure ratio and isentropic efficiency of compressors are simulated, and the result shows that the minimum stable mass flow rate is reduced by about 13% by introducing the new PISC structure into the sCO2 centrifugal compressor, and the maximum isentropic efficiency is increased from 83.5% to 84.4% compared to that of the SITC compressor. A new design idea for the impeller and sCO2 compressor structure is provided in this paper. The research results can be applied to sCO2 compressors and improve the performance and stability of sCO2 power cycle systems.

Fluoride-Salt-cooled high-Temperature Advanced Reactor (FuSTAR): An integrated nuclear-based energy production and conversion system

Based on the analysis of energy structure and the development trends of advanced nuclear energy technology, combining the complementary advantages of nuclear energy and other energy sources in China, the Fluoride-Salt-cooled high-Temperature Advanced Reactor (FuSTAR) system has been proposed. FuSTAR system is designed to address production capacity issues in areas with energy shortages or resource concentration and can be combined with high-temperature process heat to achieve high-value-added processes such as hydrogen production. In this paper, the Tri-structural isotropic fuel combined with helical cruciform elements has been studied in detail on the neutron physics and thermal-hydraulic properties, and the flow and heat transfer experiment is carried out for the first time. The multilayer nonlinear programming method has been applied to the design of thermal transport systems. Then the optimal parameters and configuration of the thermoelectric conversion system are obtained by the superstructure optimization method. The inherent safety of FuSTAR is demonstrated through deterministic safety analysis. In addition, the iodine-sulfur cycle technology and the characteristics of tritium production are also analyzed. This study highlights the significant potential of integrating nuclear energy with other energy sources in an optimized energy-production system.

Study on load tracking characteristics of closed Brayton conversion liquid metal cooled space nuclear power system

It is vital to output the required electrical power following various task requirements when the space reactor power supply is operating in orbit. The dynamic performance of the closed Brayton cycle thermoelectric conversion system is initially studied and analyzed. Based on this, a load tracking power regulation method is developed for the liquid metal cooled space reactor power system, which takes into account the inlet temperature of the lithium on the hot side of the intermediate heat exchanger, the filling quantity of helium and xenon, and the input amount of the heat pipe radiator module. After comparing several methods, a power regulation method with fast response speed and strong system stability is obtained. Under various changes in power output, the dynamic response characteristics of the ultra-small liquid metal lithium-cooled space reactor concept scheme are analyzed. The transient operation process of 70 % load power shows that core power variation is within 30 % and core coolant temperature can operate at the set safety temperature. The second loop's helium-xenon working fluid has a 65K temperature change range and a 25 % filling quantity. The lithium at the radiator loop outlet changes by less than ±7 K, and the system's main key parameters change as expected, indicating safety. The core system uses less power during 30 % load power transient operation. According to the response characteristics of various system parameters, under low power operation conditions, the lithium working fluid temperature of the radiator circuit and the high-temperature heat pipe operation temperature are limiting conditions for low-power operation, and multiple system parameters must be coordinated to ensure that the radiator system does not condense the lithium working fluid and the heat pipe.

Multi-objective optimization and configurations comparison of closed-air Brayton cycle

The microreactor power system is distinguished by its efficiency, compactness, and remarkable environmental adaptability. In this study, closed-air Brayton cycle (CABC) is proposed as the microreactor thermoelectric conversion system, and thermodynamic model of CABC with different configurations are established. The design of cycle parameters and component structures is accomplished through a combined system-component design approach. To obtained the optimal system design, the genetic algorithm and nondominated sorting genetic algorithm II are separately employed for single-objective and multi-objective optimization. In accordance with the results of multi-objective optimization, the intercooling reheat recuperative cycle attains the highest cycle efficiency at 35.4% and a power to mass ratio of 59.1 kW t−1. Conversely, the reheat recuperative cycle achieves the highest power density at 179.6 kW m−3 and its simpler structure renders it the optimal configuration for CABC. In comparison to single-objective optimization, the design results of multi-objective optimization exhibit a more balanced performance across three key indicators, signifying a superior overall performance of the optimized design scheme.

Effect analysis on energy conversion enhancement of porous medium micro-combustor and thermoelectric system and its optimization

The energy conversion of micro combustion system has become the main research direction at present. However, due to the small size of micro-combustors, they face challenges such as unstable combustion and difficulty in effectively converting generated thermal energy into usable energy. This work mainly studies the thermoelectric conversion system based on porous medium micro-combustor, which mainly uses the high energy density of micro-combustor to replace the traditional battery energy and improve the working time of small equipment. And this work also establishes the coupling model of porous medium micro combustor and thermoelectric module. By analyzing the performance parameters of the micro thermoelectric system in different flow rate, the results show that at low flow rates, semiconductor materials with better thermoelectric performance, such as Bi2Te3, can be used to improve the energy conversion efficiency of the system. However, the low heat release from combustion leads to lower maximum output power. At high flow rates, there is a significant improvement in the thermoelectric conversion performance of the system, but it is limited by the temperature of the thermoelectric materials. To optimize the impact of these factors on micro-thermoelectric systems, the optimization methods were analyzed as: (1) The temperature uniformity of micro-combustor wall can be improved by using mixture with low equivalence ratio and high flow rate; (2) enhancing the thermal conductivity of the micro-combustor and porous medium to improve the thermal conductivity of micro thermoelectric system; (3) Using CH4-H2 mixed combustion to reduce combustion temperature and ensure that thermoelectric materials work within the normal temperature range; (4) The segmented thermocouple is used to improve the thermoelectric performance of thermoelectric module. The results show that using a mixture with low equivalence ratio and high flow rate can achieve a maximum conversion efficiency of 5.51%. Using mixed combustion to reduce combustion temperature can increase the maximum output power of the system to 57.8 W.

Research on safety characteristics of heat pipe reactor with semiconductor thermoelectric conversion system

The heat pipe reactor exhibits notable characteristics of inherent safety and a compact configuration, rendering it a favored choice for space and ocean reactor designs. This study focuses on investigating the safety characteristics of the SAIRS-C space heat pipe reactor by employing a self-developed TAPIRS-D code under three selected typical accidents. The obtained results demonstrate that the heat pipe reactor exhibits inherent safety features, enabling it to sustain normal operation even in the event of partial heat pipe failure(2 out of 60 heat pipes fail) and partial heat sink loss(cooling rate decreasing from 100% to 10% in 10 s). The reactivity-insertion accident(0.358$ is introduced into the core) is found to exert the most significant influence on the heat pipe reactor. However, the crucial parameters of the reactor during the accident, such as fuel temperature, remain within the established safety limits. The outcomes of this research not only contribute to a comprehensive understanding of safety characteristics of heat pipe reactor, but also provide a crucial guidance for further design enhancements.

Sustainable utilisation and transformation of the thermal energy from coalfield fires: A comprehensive review

Coalfield fires provide high-temperature heat storage at relatively shallow depths, their overburden and upper strata contain immense amounts of thermal energy, which is identical to the reservoir condition of shallow geothermal energy. Therefore, heat waste resources produced by coalfield fires can be developed and utilised as low-medium grade geothermal resources. By comparing the available methods for thermal energy extraction and conversion, it was found that the heat transfer technology using two-phase closed thermosyphon and thermoelectric conversion using thermoelectric conversion device were the most suitable for use in coalfield fire areas. As a highly effective heat conduction scheme, the two-phase closed thermosyphon was proposed as a feasible means of thermal energy extraction. Meanwhile, thermoelectric conversion device was utilised to transform thermal energy directly into electrical energy to realise the reuse of the waste heat. Some influencing factors on the heat transfer properties of two-phase closed thermosyphon were analysed, and efficiency of the thermoelectric conversion device was investigated. The thermoelectric parameters for these thermoelectric materials of interest were evaluated, and several effective methods for the improvement of thermoelectric conversion efficiency were exposed. In addition, an enhanced thermoelectric conversion and utilisation system with increased conversion efficiency was proposed. The results presented in the current study can provide a theoretical basis and technical support information for the feasible management and adequate control of coalfield fires and utilisation of the resulting thermal energy.

Study on Heat Transfer of Carbon Dioxide in Airborne Thermoelectric Conversion System

To design more effective heat exchange ducts for thermoelectric conversion systems on aircraft, the heat transfer process of a working fluid in a nonuniformly heated square duct was simulated in this study and the influence of the heated wall position was further investigated. Due to the effects of the two main vortex structures in the duct, the highest wall temperature was found in the upper-wall heating case. With increasing working pressure, the influence of the heated wall position on the heat transfer process also increased. When the working pressure was 30 MPa, the wall temperature trend exhibited significant differences in cases with different heated walls, and the maximum wall temperature difference along the duct could be up to 110 K. With an increasing inlet temperature, the influence of the heated wall position on the heat transfer process decreased. Meanwhile, for cases under different pressures (specifically from 8 to 30 MPa), if the inlet temperature was higher than the value at which was , the influence of the heated wall position on the yield strength of the duct also decreased with an increasing inlet temperature.

The effect of two main vortexes generator on heat transfer enhancement in mini rectangular ducts of airborne thermoelectric conversion system

To enhance the heat transfer process of supercritical pressure fluid in rectangular channels of the airborne thermoelectric conversion system, the combined effect of the Reynolds stress, properties variation and longitudinal fins on convective heat transfer process in ducts under supercritical pressure is investigated. Then, a method to induce the two main vortexes structure in rectangular ducts is proposed. Results show that, in smooth ducts, the two main vortexes structure is formed due to the effect of density variation. By comparing the flow and heat transfer process in the smooth duct and ridged duct with trapezoidal fins attached on the lower wall, it is found that, due to the effect of new vortexes induced by trapezoidal fins, the two main vortexes structure is inhibited, which makes local wall temperature increases about 50 K. Accordingly, the thermal performance enhancement factor of rectangular duct with trapezoidal fins is lower than 1. To enhance the two main vortexes structure, a method that using triangular fins to induce two main vortexes structure is proposed, and the duct with one triangular longitudinal fin attached on the upper wall is designed, which has better thermal performance than ducts with trapezoidal fins. Due to the effect of triangular fins, the dominant distance of two main vortexes structure increases by 416%. Accordingly, in ducts with triangular fins, local heat transfer coefficient is enhanced about 23.5%, and the maximum of thermal performance enhancement factor is about 1.29.

System-component combined design and comprehensive evaluation of closed-air Brayton cycle

A microreactor power system possesses characteristics such as efficiency, compactness, and high adaptability to the environment. This paper proposes a microreactor thermoelectric conversion system based on a closed-air Brayton cycle, where the basic parameters of the cycle parameters and components are designed. Herein, the system-component combined design method is applied, which combines the cycle thermodynamic model with the component design method to iteratively correct the cycle thermodynamic calculations and component design results. A system power density index, i.e., the net output power per unit system volume, is proposed to comprehensively evaluate the efficiency and compactness of the system. This study investigates the impact of cycle pressure ratio, compressor inlet temperature, cooler exhaust temperature, and recuperator effectiveness on the thermodynamic performance index (cycle efficiency, system power density, and components exergy destruction rate). It is observed that the system power density increases initially and decreases subsequently within a certain range of the design parameters. Finally, a genetic algorithm tool is employed to optimize the cycle parameters using system power density and cycle efficiency as optimization objectives. The optimization results demonstrate that the maximum system power density is 402.34 kW/m3 and the corresponding cycle efficiency and system volume are 24.86% and 3.09 m3, respectively. When the system total volume is limited under 5 m3, the maximum cycle efficiency is 31.09% and the corresponding power density is 310.9 kW/m3.

Temperature dependent phase transition and negative thermal expansion of Hg2Cl2 compound: insights from first-principle DFT and Born-Oppenheimer on the fly molecular dynamics calculations

ABSTRACT The paper is aimed to understand the key phonon modes that are responsible for the temperature dependent structural phase transition and negative thermal expansion of Hg2Cl2 compound for the first time with the aid of density functional theory and Born-Oppenheimer on the fly molecular dynamics calculations. The phonon dispersion spectra, phonon density of states and Grüneisen parameters for the body-centered tetragonal and base-centered orthorhombic phases of the compound have been explored in detail. The order parameter associated with the phase transition and its nature has also been reported herewith. We believe that the present study will not only help for futuristic designs of improved functionalized systems with Hg2Cl2 compound but also can augment their applications in thermoelectric conversion systems, fibre-optic communications, thermal expansion compensators and in fuel cells.

UV-LIG-Based Perfect Broadband Absorber for Solar Thermoelectric Generation

Thermoelectric technology is gaining paramount importance for solar energy conversion and electricity production to increase green energy resources with high efficiency. An enormous amount of research is being carried out for engineering various solar absorbers using tailored materials and structures for solar energy harvesting. However, the methods to achieve cost-effective light absorbers are still challenging. Here we present a perfect broadband solar absorber for efficient photothermal conversion of sunlight employing a low-cost ultraviolet laser-induced graphene (UV-LIG) prepared on a polymer material using the conventional direct laser writing method. Recently the LIG generation employing the direct laser writing method has been recognized as a straightforward and low-cost technique to generate graphene foam. We have patterned the UV-LIG to produce a two-dimensional grid pattern to limit the reflection losses. The resulting UV-LIG surface exhibits very high absorption (>99%) for the entire spectral range of sunlight. Significantly, the absorber attains a temperature of 90.4 °C under 1 sun irradiation within a response time of less than 60 s. Further, we have exploited the extraordinary photothermal property of the patterned UV-LIG together with a commercially available thermoelectric generator (TEG) to make a solar thermoelectric generator (STEG) device with excellent output performance. Our investigation perceived that the structured UV-LIG absorber-based STEG has excellent power generation capability compared to the STEGs with other absorbing materials. Remarkably, we could achieve an output voltage of 273.9 mV under 1 sun irradiation, one of the highest among the recent STEGs, primarily due to the enhanced absorption of absorbing material. These results suggest that the proposed versatile and robust solar energy harvesting technology can be employed for solar–thermoelectric conversion systems.

Applying a Multiple-Input Single-Output Interleaved High Step-Up Converter with a Current-Sharing Device Having Different Input Currents to Harvest Energy from Multiple Heat Sources

In this paper, a thermoelectric conversion system for multiple heat sources is proposed. For design convenience, the overall system employs only a single-stage converter. Such a converter uses coupling inductors and switched capacitors to increase the voltage gain. In order to reduce the high-frequency voltage oscillation of the turn-off of the main switches created from leakage inductors, two active clamp circuits with zero voltage switching (ZVS) turn-on are employed. By doing so, although the current-sharing device with interleave control is embedded in the proposed converter, the input currents can be unequal. Therefore, the inputs of the converter can operate under individual maximum power points and transfer the energy from different thermoelectric generators (TGs) to a single load. Furthermore, the main switches have low voltage stress during the turn-off period. As for the maximum power point tracking (MPPT) method, it utilizes a three-point-weighting method to improve the tracking stability. In addition, the number of inputs of this converter can be extended. The MPPT simulation is presented to verify the feasibility as well as several experimental waveforms to demonstrate the effectiveness. The field programmable gate array (FPGA) is used as a digital control kernel to control the thermoelectric conversion system.

Thermal design of standalone miniature thermo-electric power conversion system using variable area liquid fuel combustor

The novel standalone miniature thermo-electric power conversion system using variable area liquid fuel combustor is proposed in the present work. There is a need to provide an alternative energy source for portable electronic devices to cater to its need for high energy density requirements, and the proposed system is a feasible option for the same. The various critical parameters required for designing a thermoelectric power conversion system are identified as per the requirement of standalone nature and flame stabilization. The 15 W thermo-electric power conversion system is designed based on these parameters. The variable area combustor and the concentric fuel tube helps for the natural circulation of fuel and air inside the combustor and flame stability. The temperature of liquid fuel film over the fuel orifice exit is calculated by equating mass diffusion transfer number with thermal diffusion transfer number at steady state condition assuming the Lewis number unity. The Thin metallic porous media of ∼ 10 mm diameter is suggested based on the thermal analysis over the fuel orifice exit to enhance the fuel evaporation rate.

Ridge regression and artificial neural network to predict the thermodynamic properties of alkali metal Rankine cycles for space nuclear power

The alkali metal Rankine cycle system has great advantages in space nuclear power system. The investigation of thermodynamic properties of alkali metals is fundamental to develop thermodynamic model of thermoelectric conversion system. This paper build models for predicting the thermodynamic properties of alkali metals (potassium, sodium, cesium, rubidium) by statistics and machine learning. Pearson correlation analysis is performed to explain the correlation between measurable variables (temperature, pressure, specific volume, specific heat capacity) and unmeasurable variables (enthalpy, entropy). Ridge regression and radial basis artificial neural network models are established for the enthalpy and entropy of alkali metals respectively. Five evaluation indicators: residual, SSE, MSE, ME and R2 are selected to evaluate the accuracy of different types of prediction models. The practicability and extensibility of the prediction model are proved by case analysis. The results show that the prediction model of enthalpy and entropy based on radial basis neural network has high accuracy, and R2 can reach above 0.9. The entropy prediction model can even be stable at 0.999 when applied to different operating conditions and alkali metals. The new method proposed in this work solves the difficulty of experimental measurement in high temperature section to some extent, and opens a new idea for studying the properties of working medium. At the same time, the validation of the extrapolation feasibility of this method is of great significance to improve the thermodynamic properties database of alkali metals in the whole temperature range.