Alkali Metal Thermal To Electric Converter Research Articles

Mathematical model establishment and optimum design of a novel cesium thermionic–thermoelectric hybrid generator

Static power sources, such as thermionic energy converters (TEC), thermoelectric generators (TEG), thermophotovoltaic converters (TPV), and alkali metal thermal to electric converters (AMTEC), have enormous potential application in aerospace, military, electronic devices, and low-grade waste heat recovery and utilization. Aiming to further improve the power output and conversion efficiency, any two of them can be cascaded according to the difference in operating temperature. In this study, a novel mathematical model of the static hybrid power generator composed of a cesium thermionic energy converter (CTEC) and a TEG was proposed. The model considered the irreversible heat loss and the additional energy barrier of the CTEC module, and the Thomson effect of the TEG module, synchronously. To obtain the temperature distribution of CTEC and TEG modules, energy balance equations were determined by nonequilibrium thermodynamic analysis. Then, the effects of the output voltage and anode work function of the CTEC, the heat source temperature, and the electric current of the TEG on the output performance were investigated systematically by means of MATLAB. The results indicate that the maximum efficiency of the hybrid generator reaches 26.82% at heat source and heat sink temperatures of 1800 K and 300 K, respectively, which is significantly higher than that of the individual CTEC module (21.28%). This reveals that the waste heat released by the CTEC is utilized effectively in the TEG module, and the increased heat source temperature will further improve the hybrid generator’s power output and efficiency. The output power and efficiency are obviously improved by reducing the anode work function. When ϕC = 1.0 eV, i = 0.005, the power reaches a maximum of 21.61 W at 0.37 V, and the efficiency reaches a maximum of 25.90% at 0.55 V. Moreover, the curve of the output power as a function of conversion efficiency could be used to determine the optimal operating region of the hybrid generator, i,e., Pη < P < Pmax, ηP < η < ηmax, thereby obtaining the thresholds of the hybrid system parameters, i.e., VP < V < Vη, T1,P < T1 < T1,η, T2,P > T2 > T2,η, T3,P > T3 > T3,η and TCs,P > TCs > TCs,η. This study provides a reference for the optimal design and application of a CTEC–TEG hybrid generator in the future.

Parametric study and performance assessment of a biomass based alkali metal thermoelectric converter hybrid system

A novel system was developed by combining the alkali metal thermal to electric converter (AMTEC) and biomass gasification. The primary purpose of this study was to examine the feasibility of utilizing the emitted gas caused by biomass gasification as the heat source of the AMTEC device. Key features were studied at constant temperatures under the influence of changes in biomass. These studies were performed on the effects of the equivalence ratio and the fuel flow rate of the biomass system on the total output power, efficiency, and effective voltage of the intended AMTEC. Due to coupling circumstances, ER changes were limited between 0.3 and 0.4. The findings revealed that the equivalence ratio affected the output power, efficiency, and total current of the system reversibly while directly impacting the effective voltage. Increasing the fuel flow rate initially enhanced the efficiency and total output power, but they both were reduced above a specific fuel flow rate. A maximum efficiency of 18.5% was obtained at the optimum fuel flow rate of biomass. More tubes in the cell improved the overall performance of the developed system. However, design and construction cost limit the number of tubes. The results obtained reveal the advantages of the hybrid model.

Optimization of vapor anode multi-tube alkali metal thermoelectric converter based on an integrated model

The alkali metal thermal to electric converter (AMTEC) is a promising technology for direct conversion of heat to electricity. Design optimization and cell fabrication have been attracting wide interest to improve the cell performance. An integrated analysis model has been developed in this paper. The model includes three coupled parts of a thermal model, an electric model, and a pressure loss model, which are solved by numerical iteration method. All radiation view factors in the cell are calculated by the thermal radiation analysis software RadCAD based on Monte-Carlo ray-tracing technique to accurately and stably simulate the heat flux between the radiation surfaces to support design optimization. The integrated analysis model’s predictions are in good agreement with experimental results of PX-3A cell for a wide range of external loads, which confirms the accuracy of the integrated analysis model and the rationality of the solution processes. After model validation, the effects of various parameters on the cell performance are investigated, including the cold end temperature, geometry of the heat shield, number of BASE tubes, heat loss through the cell wall, and electrode material. The optimization suggestion and optimal parameter range are given in the analyses. At last, an integrative optimization is performed using achievable parameters under current technology. The peak power and conversion efficiency have been improved from 4 W to 6.7 W and 13.5% to 18.1%, respectively.

Thermal Designing of an Inner Cathode Type Alkali Metal Thermal to Electric Conversion (AMTEC) Cell

Thermal Designing of an Inner Cathode Type Alkali Metal Thermal to Electric Conversion (AMTEC) Cell

Condensation heat transfer characteristics of the alkali metal thermal to electric converter (AMTEC) porous wick condenser

An axisymmetric 2-D mathematical model was established and the volume of fluid +method was adopted to numerically investigate the condensation heat transfer process of alkali metal in the alkali metal thermal to electric converter (AMTEC) porous wick condenser. The steady-state physical fields of working medium were obtained and the impacts of some relevant parameters including geometrical and operational parameters on condensation heat transfer characteristics were discussed. The results show that the thickness of the liquid sodium increases along the radial direction. The temperature distribution in liquid phase is approximately linear. The condensation rate at the phasechange interface decreases along the radial direction, and the cold end heat flux, q, increases initially and then decreases along the radial direction. The porous wick condenser has very limited self-adaptive ability once the mass flow rate, M , exceeds in some critical value. In addition, there is a corresponding T for a certain M to ensure the w in stable phase-change interface. The performance of AMTEC can be enhanced by reducing the distance between artery and cell wall.

A new AMTEC/TAR hybrid system for power and cooling cogeneration

In this paper, a new AMTEC/TAR hybrid system was proposed to utilize condensation heat of the alkali metal thermal to electric converter (AMTEC) as heat source for the triple-effect absorption refrigeration (TAR). Based on the first and second laws of thermodynamics, a theoretical analysis has been undertaken to evaluate the performance of the hybrid system. Meanwhile, the effects of the electrode current density, the condenser temperature in AMTEC, the pressure of medium pressure generator, and the temperatures of absorber, medium pressure generator and low pressure generator on the performance of the hybrid system were investigated and discussed. The results show that the AMTEC condensation heat can be effectively utilized by the TAR. When the evaporation temperature of AMTEC is 1200 K, the maximum value of exergetic efficiency of hybrid system is 42.7%, the input heat flow rate is 166.4 kW, and the output electric power and cooling load are 48.8 kW and 110.2 kW, respectively. The maximum exergy destruction rate is generally from the heat loss of AMTEC, but when the electrode current density is relatively high or the temperature of AMTEC evaporator is relatively low, the maximum exergy destruction rate comes out of the BASE.

Phase change heat transfer characteristics of porous wick evaporator with bayonet tube and alkali metal as working fluid

Porous wick evaporator is an important power source of alkali metal thermal to electric converter (AMTEC) available for different kinds of energy sources including renewable energy. A bayonet tube structure is adopted in porous wick evaporator which is based on the principle of capillary pumped loop (CPL). The phase change heat transfer characteristics in the evaporator were numerically studied and the impact of the bayonet tube was investigated. The working fluid temperatures on two feature lines were selected to analyze the effect of bayonet tube on temperature of porous wick evaporator. The results show that for the evaporator using alkali metal as working fluid, a stable vapor-liquid interface is maintained at wick-groove interface. The bayonet tube divides the working fluid flow in porous wick into two parts, and the temperature change rule inside the bayonet tube is different from that outside the bayonet tube. The bayonet tube structure in the evaporator causes the temperature distribution in liquid channel to become more uniform and the maximum temperature to be decreased. The fluid temperature in the evaporator, especially in the liquid channel, can effectively be affected by the thermal resistance per unit area, length and diameter of bayonet tube. These parameters for bayonet tube must be limited within acceptable ranges in a specific operating condition, and there are optimum values to guarantee the best performance of the evaporator.

Design and construction of the ATEFA facility for experimental investigations of AMTEC test modules

The Alkali Metal Thermal-to-Electric Converter (AMTEC) is an electrochemical cell that requires a high temperature heat source to generate electricity. At KIT the AMTEC technology is being investigated focusing on the use of concentrating solar energy as heat source. First a review on AMTEC technology is given. Further, the design and realization phases of the AMTEC Test Facility (ATEFA) and AMTEC test cell are presented, including the data acquisition and control system and two key technology developments: a ceramic to metal joint for high temperatures (800 – 1000 °C) and the magnetron sputtering of cathode layers on the ceramic electrolyte. The sheet resistance of several electrode samples has been analyzed using the 4-point probe technique and the microstructure of the cathode layer has been examined using the scanning electron microscopy (SEM).

Development of high temperature liquid metal test facilities for qualification of materials and investigations of thermoelectrical modules

Three classes of experimental liquid metal facilities have been completed during the LIMTECH project aiming the qualification of materials, investigation of thermoelectrical modules, investigation of sodium transitional regimes and fundamental thermo-dynamical flows in concentrating solar power (CSP) relevant geometries. ATEFA facility is dedicated to basic science investigation focussed on the alkali metal thermal-to-electric converter (AMTEC) technology. Three SOLTEC facilities are aimed to be used in different laboratories for long term material investigation sodium environment up to a 1000 K temperature and for long term tests of AMTEC modules. The medium scale integral facility KASOLA is planned as the backbone for CSP development and demonstration.

Thermal behavior of heat-pipe-assisted alkali-metal thermoelectric converters

The alkali-metal thermal-to-electric converter (AMTEC) changes thermal energy directly into electrical energy using alkali metals, such as sodium and potassium, as the working fluid. The AMTEC system primarily consists of beta-alumina solid electrolyte (BASE) tubes, low and high-pressure chambers, an evaporator, and a condenser and work through continuous sodium circulation, similar to conventional heat pipes. When the sodium ions pass through the BASE tubes with ion conductivity, this ion transfer generates electricity. The efficiency of the AMTEC directly depends on the temperature difference between the top and bottom of the system. The optimum design of components of the AMTEC, including the condenser, evaporator, BASE tubes, and artery wick, can improve power output and efficiency. Here, a radiation shield was installed in the low-pressure chamber of the AMTEC and was investigated experimentally and numerically to determine an optimum design for preventing radiation heat loss through the condenser and the wall of AMTEC container. A computational fluid dynamics (CFD) simulation was carried out to decide the optimum size of the low-pressure chamber. The most suitable height and diameter of the chamber were 270 mm and 180 mm, respectively, with eight BASE tubes, which were 150 mm high, 25 mm in diameter, and 105 mm in concentric diameter. Increasing the temperature ratio (T Cond /T B ) led to high power output. The minimum dimensionless value (0.4611) for temperature (T Cond /T B ) appeared when the radiation shield was made of 500-mesh nickel. Simulation results for the best position and shape for the radiation shield, revealed that maximum power was generated when a stainless steel shield was installed in between the BASE tubes and condenser.

Parametric Analysis of Condensation Heat Transfer Characteristics of AMTEC Wick Condenser

By using theoretical analysis method, effects of different parameters on condensation heat transfer characteristics of porous wick condenser of alkali metal thermal to electric converter (AMTEC) are discussed. The results show that, along the radius direction r of the condenser, the condensate film thickness δ(r) increases gradually, and the Nusselt number Nu(r) decreases. With increasing of the permeability K or decreasing of the effective thermal conductivity λe and the heat transfer temperature difference ΔT, the condensate film thickness δ(r) decreases significantly, and the Nusselt number Nu(r) increases. However, the BASE tube temperature TB and condensate film temperature TS have little impact on condensation heat transfer characteristics, which indicates that for AMTEC condenser, the latent heat releasing dominates the total heat transfer while the radiation heat transfer almost has no influence.

Recent Progress on the Application of BASE in Energy Storage and Environmental Protection

Beta-alumina solid electrolyte (BASE) is a fast ion conductor material which shows high conductivity of sodium ions and low electronic conductivity. Recent Progress on the Application of BASE in energy storage such as Na Based Batteries (NBBS) and Alkali metal thermal to-electric converter (AMTEC), and Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) in environmental protection were reviewed.

Novel Mo/TiN composites for an alkali metal thermal-to-electric converter (AMTEC) electrode

Core–shell composite powders composed of molybdenum (Mo) coated on titanium nitride (TiN) were synthesized using a Pechini-type polymerizable complex method to improve the microstructural stability and electrochemical performance of alkali metal thermal-to-electric converter (AMTEC) electrodes. The coverage of porous Mo shells was controlled by adjusting the concentration of resin-embedded molybdenum sources around TiN particles. Each composite powder was characterized by microstructural analysis (SEM, TEM) and X-ray spectroscopy (XRD, EDX and XPS). During the synthesis process, thin oxide layers (i.e., TiOx, with x=1.0–2.0) were formed on the TiN surface. Each Mo and TiN phase shares a common intermediate oxide layer, and there are strong interactions between oxygen anions and Mo atoms at the interface. As a result, there are cohesive networks between Mo and TiN at the core–shell interface, and these networks control the grain growth of Mo at high temperatures (>700°C). A single cell composed of Mo/TiN powder (Mo 1.0mol%) was observed to perform well (180mW/cm2) and exhibit a low area specific resistance (ASR at 0.7V) of 0.4Ωcm2 at 700°C.

The Structure and Properties of Pulsed dc Magnetron Sputtered Nanocrystalline TiN Films for Electrodes of Alkali Metal Thermal-to-Electric Conversion Systems

Titanium nitride films used as an important electrode material for the design of alkali metal thermal-to-electric conversion (AMTEC) system have been prepared using dc (direct current) and asymmetric-bipolar pulsed dc magnetron sputtering. The pulse frequency and the duty cycle were varied from 5 to 50 kHz and 50 to 95%, respectively. The deposition rate, grain size and resistivity of pulsed dc sputtered films were decreased when the pulse frequency increased, while the nano hardness of titanium nitride films increased. We present in detail coatings (e.g., deposition rate, grain size, prefer-orientation, resistivity and hardness). Our studies show that titanium nitride coatings with superior properties can be prepared using asymmetric-bipolar pulsed dc sputtering.

Effect of Mass Flow Rate of Working Fluid on Performance of Capillary Porous Wick Evaporator with Bayonet Tube for AMTEC

An axial symmetric invariable temperature phase change interface model is established to investigate numerically the effect of mass flow rate of working fluid on performance of capillary porous wick evaporator with bayonet tube for alkali metal thermal-to-electric converter (AMTEC) system. The numerical results reveal that the temperature of working fluid in the wick and liquid channel decreases with increasing the mass flow rate. The suitable structure of bayonet tube is dependent on the mass flow rate of working fluid. The bayonet tube having larger wall thermal resistance is selected for the smaller mass flow rate of working fluid, and vice versa. Also, the flow in the liquid channel is greatly affected by the mass flow rate of working fluid in comparison with the capillary porous wick evaporator without bayonet tube.

Parametric study on flow and heat transfer characteristics of porous wick evaporator based on AMTEC

By establishing an axial symmetric invariable temperature phase change interface model of porous wick evaporator based on alkali metal thermal-to-electric converter (AMTEC), a parametric study has been implemented to explore the impact of effective thermal conductivity model, working fluid, wick material, porosity and particle size of porous wick on the flow and heat transfer characteristics. The results indicate that the inner surface temperature profile of wick is closely dependent on effective thermal conductivity models; it is the minimum for series model while the highest for parallel model. The inner surface temperature of wick with Na-K alloy as working fluid is obviously lower than that with sodium or potassium as working fluid. The wick made of refractory ceramics makes the minimum of inner surface temperature, while the one with nickel appears the highest temperature. Increasing the porosity can reduce the pressure drop in the wick, but also increases the temperature of evaporator.

The Role of Bayonet Tube on Heat and Mass Transfer Characteristics of Capillary Porous Wick Evaporator Based on AMTEC

An axial symmetric 2-D invariable temperature phase change interface model of capillary porous wick evaporator with bayonet tube for alkali metal thermal-to-electric converter (AMTEC) was established to simulate flow and heat transfer characteristics in capillary porous wick and liquid channel by solving the mass and heat transport controlling equations. The effects of length, radius and thermal resistance of the bayonet tube on the distributions of pressure, velocity and temperature in capillary porous wick and liquid channel were analyzed. The results show that the performance of evaporator can be improved by inserting a bayonet tube into the liquid channel, including better averaging function of temperature, reduction in the peak temperature on the inner surface of capillary porous wick, and better adaptability in working environment. However, bayonet tube parameters should be carefully chosen depending on specific working conditions.

Concept design of solar thermal receiver using alkali metal thermal to electric converter (AMTEC)

A preliminary design of solar thermal generating system using alkali metal thermal to electric converter (AMTEC) is proposed and discussed in this paper. High conversion efficiency, compactness, lightweight, and maintenance free are its characteristics. A vapor-condense-type AMTEC cell incorporated in a solar receiver of about 10 kWe in size was designed. The thermal energy flow for the designed receiver has been calculated and examined. The analysis has been conducted under two operating conditions, maximum efficiency mode and maximum output power mode. On the maximum efficiency mode, the calculated conversion efficiency is 27% for the AMTEC system and 19% for the overall receiver system with the output power is 10.3 kWe at 1000 K. On the maximum output power mode, the conversion efficiency is 20% for the AMTEC system and 17% for the overall receiver system with the output power is 19.1 kW e at 1000 K. The preliminary calculated result has shown that the solar-AMTEC generating system has performance advantage in the solar power generating systems.

A parabolic dish/AMTEC solar thermal power system and its performance evaluation

This paper proposes a parabolic dish/AMTEC solar thermal power system and evaluates its overall thermal–electric conversion performance. The system is a combined system in which a parabolic dish solar collector is cascaded with an alkali metal thermal to electric converter (AMTEC) through a coupling heat exchanger. A separate type heat-pipe receiver is selected to isothermally transfer the solar energy from the collector to the AMTEC. To assess the system’s overall thermal–electric conversion performance, a theoretical analysis has been undertaken in conjunction with a parametric investigation by varying relevant parameters, i.e., the average operating temperature and performance parameters associate with the dish collector and the AMTEC. Results show that the overall conversion efficiency of parabolic dish/AMTEC system could reach up to 20.6% with a power output of 18.54 kW corresponding to an operating temperature of 1280 K. Moreover, it is found that the optimal condenser temperature, corresponding to the maximum overall efficiency, is around 600 K. This study indicates that the parabolic dish/AMTEC solar power system exhibits a great potential and competitiveness over other solar dish/engine systems, and the proposed system is a viable solar thermal power system.

A review on advances in alkali metal thermal to electric converters (AMTECs)

The alkali metal thermal to electric converter (AMTEC) is one of the most promising technologies for direct conversion of thermal energy to electricity and has been receiving attention in the field of energy conversion and utilization in the past several decades. This paper aims to present a comprehensive review of the state of the art in the research and development of the AMTEC, including its working principles and types, historical development and applications, analytical models, working fluids, electrode materials, as well as the performance and efficiency improvement. The current two major problems encountered by the AMTEC, the time-dependent power degradation and relatively low efficiency compared to its theoretical value, are discussed in depth. In addition, a brief comparison of the AMTEC with other direct thermal to electric converters (DTECs), such as the thermoelectrics converter (TEC), thermionics converter, and thermophotovoltaics converter, is given, and combinations of different DTECs to further improve DTECs' power generation and overall conversion efficiency are demonstrated. Future research and development directions and the issues that need to be further investigated are also suggested. It is believed that this comprehensive review will be beneficial to the design, simulation, analysis, performance assessment, and applications of various types of AMTECs. Copyright © 2009 John Wiley & Sons, Ltd.