443 SOFC-AMTECの複合モジュール化に関する性能解析(燃料電池,電力貯蔵,環境保全型エネルギー技術)

The alkali metal thermal to electric converter (AMTEC) used in the Advanced Radioisotope Power System (ARPS) program has been developed around a specific system configuration that was derived from the radioisotope thermoelectric generator (RTG) used for the last 20 years. This configuration, and the restrictions it places on the converter technology, limits the overall generator performance. An advanced AMTEC system design, that is not based on RTG heritage, using AMTEC chimney cells, is better suited for the AMTEC technology and provides substantially better power density and efficiency. It also provides system scale-ability from 10 to 100 Watts. While technology development continues to proceed with the ARPS cell design in order to validate the technology, efforts are also underway to develop the advanced technology to meet the more aggressive mass, efficiency, life and electromagnetic performance requirements of future missions. INTRODUCTION The alkali metal thermal to electric converter (AMTEC) has been considered to be a potential candidate for small radioisotope space power systems since the early 1980's. Recent converter technology advances and system design work by Hendricks et al. and Schock et al.' have shown these systems to be very attractive relative to the state of the art radioisotope thermoelectric generators (RTG's), currently used in all spacecraft radioisotope power system. The AMTEC technology not only reduces the system mass relative to RTG technology, it also requires significantly less radioactive material than the RTG's for any given power requirement. From mid-1997 to mid-1999, the Advanced Radioisotope Power System (ARPS) program, was focused on developing the first AMTEC power system for the Europa and Pluto missions. This power system, which has been described in the literature, is the first generation of AMTEC space systems. It is derived from the RTG system technology and has several features in common with the RTG systems. • The cells are mounted to the inside of a cylindrical housing. This The general purpose heat source (GPHS) is stacked, and the stack is held from each end in much the same way as the RTG. The cells use the same mounting feature used in the RTG unicouple. The cell must support the weight of the multilayer insulation package, system is 14% efficient and 21 kg for approximately 100-120 W, end of mission power. The use of RTG system reduced the system level design effort, however it also increased the AMTEC cell development risk by requiring that the technology be configured for the system as opposed to configuring the system to meet the technology needs. The combination of developing the technology under the constraint of the RTG heritage requirements and a number of changes in the spacecraft power requirements made AMTEC technology development very difficult. During the course of the two years of development, there were two major changes in system power level and one change in configuration. This resulted in 3 different cell revisions that were actually designed and tested. The final E-revision cell is shown in Figure 1. Figure 1. ARPS AMTEC Cell, Revision E. American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. With the challenges of developing the materials and fabrication processes for the refractory metal cell (previous designs were stainless steel), these design changes made it very difficult to mature the fabrication process. In mid-1999 DOE and NASA made the decision that AMTEC would not be ready for the first planned ARPS mission in 2003 or 2004. Cell testing had not yet reached the point of demonstrating reliable performance and was not yet ready for life testing at that time. Cell development has continued since mid-1999 and substantial progress has been made, with cells now capable of producing power at or near predicted levels and, in most cases, to operating reliably at nominal and elevated temperatures. These cells are still designed for the RTG heritage system, and while they are not the design that would provide the highest efficiency or power, they are a baseline to fully develop and prove the technology for mission use. Life testing could begin early next fiscal year. ADVANCED SYSTEM DESIGN An AMTEC system design that is significantly lower mass than the current ARPS design has been described in the literature. This design is not constrained by RTG heritage. It places the cells at only two ends of the heat source stack. A new method is used to support only two GPHS. It eliminates the cell mounting features used in the ARPS cell. This system is over 20% efficient and 8.9 kg for approximately 100 W, end of mission. Closure Plate Cell Interconnects

Design optimization and integration of nickel/Haynes-25 AMTEC cells into radioisotope power systems

A series of previous OSC papers described: 1) a novel methodology for the coupled thermal, fluid flow, and electrical analysis of multitube AMTEC (Alkali Metal Thermal-to-Electric Conversion) cells; 2) the application of that methodology to determine the effect of numerous design variations on the cell’s performance, leading to selection and performance characterization of an OSC-recommended cell design; and 3) the design, analysis, and characterization of an OSC-generated power system design combining sixteen of the above AMTEC cells with two or three GPHS (General Purpose Heat Source) radioisotope heat source modules, and the applicability of those power systems to future space missions (e.g. Pluto Express and Europa Orbiter) under consideration by NASA. The OSC system design studies demonstrated the critical importance of the thermal insulation subsystem, and culminated in a design in which the eight AMTEC cells on each end of the heat source stack are embedded in Min-K fibrous insulation, and the Min-K and the GPHS modules are surrounded by graded-length Mo multifoil insulation. The present paper depicts the OSC-recommended AMTEC cell and generator designs, and identifies the need for an electrically heated (scaled-down but otherwise prototypic) test assembly for the experimental validation of the generator’s system performance predictions. It then describes the design of an OSC-recommended test assembly consisting of an electrical heater enclosed in a graphite box to simulate the radioisotope heat source, four series-connected prototypic AMTEC cells of the OSC-recommended configuration, and a prototypic hybrid insulation package consisting of Min-K and graded-length Mo multifoils. Finally, the paper describes and illustrates an OSC-recommended detailed fabrication sequence and procedure for the above cell and test assembly. That fabrication procedure is being implemented by AMPS, Inc. with the support of DOE’s Oak Ridge and Mound Laboratories, and the Air Force Phillips Laboratory (AFPL) will test the performance of the assembly over a range of input thermal powers and output voltages. The experimentally measured performance will be compared with the results of OSC analyses of the same insulated test assembly over the same range of operating parameters.

The alkali metal thermal to electric converter (AMTEC) system which utilizes the sodium ion conductivity of a beta″‐alumina solid electrolyte (BASE) is expected to have high conversion efficiency above 30% including practical heat losses. However, the achieved experimental efficiencies have been around 15%. In this paper, current–voltage characteristics and heat and mass transfer processes on a single cell have been examined experimentally and thermal electrode conversion efficiency has been discussed. Measured electrode conversion efficiency without thermal losses showed that it was about 40% at a power density of 0.3 W/cm2. A theoretical analysis on the thermal losses has also been conducted and these losses are estimated to be 0.3 W/cm2 in a practical tube type cell, so that an actual cell system efficiency of 30% is expected. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 234–244, 2001

The Alkali Metal Thermal to Electric Converter (AMTEC) is a static energy conversion device that operates at high thermal to electric conversion efficiencies that are essentially independent of size, have reached 19% and are expected to reach 25% to 30% in 1997. AMTEC systems have been chosen by NASA and DOE for spacecraft applications and have considerable promise for a wide variety of terrestrial applications. Reduction of parasitic heat losses in AMTEC systems related to radiative heat transfer from the hot side to the condenser can make a substantial contribution to system efficiency. Through design, analysis and the fabrication and testing of cells and systems, the proposed program to develop a Directed Reflectivity Condenser (DRC) has investigated the feasibility of an improved AMTEC condenser component. Phase 1 work showed the potential for adding from 4% to 7% to overall system efficiency for identical operating conditions using the concept. A detailed thermal analysis of several DRC capped cell designs was carried out and some of the conditions under which a DRC, used as the condenser at an end cap of a cylindrical converter, can reduce thermal radiation related losses were determined. A model experimental converter was built and tested to compare DRC and planar condenser surfaces. The results of both analysis and experiment indicate that for moderate aspect ratios of a cylindrical, end condensed converter, the DRC can reduce overall thermal losses by up to 4%. The initial effort in Phase 2 extended the analysis to a novel 150 watt radial AMTEC cell design. This analysis indicated that for the effective aspect ratio of this new converter design, the system performance at the 100+ watt level was not significantly improved by use of a DRC type condenser surface. Further analyses however showed that for cylindrical, end-condensed converters, optimized for use with internal radiation shields, the use of DRC surfaces on the side walls of the converter could be more effective than on the condenser end surface itself. The experimental work in Phase 2 was intended to incorporate a DRC into this cell design and use its measured performance to refine the state-of-the-art AMTEC analytical models. Because the analysis had indicated that the new radial converter design, which may be useful for systems at the {approx} 100 watt level was not much assisted by the DRC properties, this program was redirected toward the simpler cylindrical converter design with the corner cube surfaces on the side walls. The Phase II program was proposed and planned with a funding level substantially below the maximum potentially available for Phase II programs at that time. At the time, there were two other funded government sponsored programs at AMPS for which positive results of the analyses described in this report were expected to lead to incorporation of the DRC concept into converters scheduled to be built for these programs. The programs of interest were the Air Force program titled ''Radiation Tolerant, Eclipse Compatible, Solar AMTEC System'' (F29601-99-C-0132) and the DOE/NASA Advanced Radioisotope Power System (ARPS) program. Shortly after its start, the Air Force program was canceled due to elimination of AF SBIR funds at AFRL and the ARPS program was reduced to a level that could not support introduction of novel concept testing. As a result of these two circumstances, the direct testing of the DRC concept in a full up converter was not completed in the Phase II period.

A critical component of alkali metal thermal-to electric converter (AMTEC) devices for long duration space missions is the sodium beta-alumina solid electrolyte ceramic (BASE), for which there exists no substitute.

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).

The alkali metal thermal to electric converter (AMTEC) is an electrochemical device for the direct conversion of thermal energy to electrical energy with efficiencies potentially near Carnot. The future usefulness of AMTEC for space power conversion depends on the efficiency of the devices. Systems studies have projected from 15% to 35% thermal to electric conversion efficiencies, and one experiment has demonstrated 19% efficiency for a short period of time. A recent experiment in a recirculating test cell (RCT) has demonstrated sustained conversion efficiencies as high as 13.2%. The cell was operated at lower current and 12% efficiency for over 1700 hours at the time of this writing. The cell required a maturation period of 355 hours at high temperature. During this period, the cell was operated once at 12% efficiency but was generally operated at lower powers. The maturation period ended with the formation of a reflective sodium film on the condenser surface which reduced the parasitic thermal losses in the cell. After maturation, the cell demonstrated the first experimental demonstration of the maximum efficiency occuring at a lower current than the maximum power. The cell also demonstrated an unexpected decrease in parasitic loss with increasing cell current. The decrease in parasitic loss resulted from the development of a more reflective sodium film at higher sodium fluxes.

Optimization of the TIEC/AMTEC cascade cell for high efficiency

In this paper the operation and performance of a high temperature solid oxide fuel cell (SOFC) stack on biomass syn-gas from a demonstration biomass gasification combined heat and power (CHP) plant is investigated. The objective of this work is to develop a computer simulation model of a biomass-SOFC CHP system, flexible enough for use in industry, capable of predicting system performance under various operating conditions and using diverse fuels. The biomass gasifier is of the dual fluidised bed (DFB) type with steam as the gasifying agent and is operated at atmospheric pressure. The tubular SOFC configuration, developed by Siemens Power Generation Inc (SPGI), is selected. It is considered to be the most advanced design and is approaching commercialisation. The SOFC stack model, developed using the chemical process flowsheet simulator Aspen Plus, is of equilibrium type and is based on Gibbs free energy minimisation. The SOFC model performs heat and mass balances and considers the ohmic, activation and concentration losses for the voltage calculation. Data available in the literature on the SPGI SOFC operating on natural gas is used to validate the model. The system model predicts thermodynamic condition and composition of all internal flow streams, the heat generated by the SOFC stack, voltage (V), current (I) and efficiency. Operating parameters are varied over a wide range, parameters such as fuel utilisation factor (Uf), current density (j) and steam to carbon ratio (STCR) have significant influence. The results indicate that there must be a trade-off between voltage, efficiency and power with respect to j and the SOFC stack should be operated at low STCR and high Uf, within certain limits. SOFC stack operation on biomass syn-gas is compared to operation on natural gas and as expected there is a drop in performance, which is attributed to increased input fuel and air flow due to the lower quality of the fuel gas. The optimum realistic operating conditions with regard to SOFC stack performance are identified. High electrical efficiencies are predicted making these systems very attractive for CHP applications.

The Sodium Heat Engine (SHE) is a direct thermal to electrical energy conversion device using sodium beta''-alumina solid electrolyte (BASE) as an ion selective membrane and operating on the basic principle of a thermally regenerative concentration cell. SHE cells have demonstrated high output power densities and should be capable of high efficiency even in units of relatively small output power. The work performed under this subcontract was directed toward gaining an understanding of the problem of electrode stability as related to the Sodium Heat Engine and of the detailed manner in which this high power electrode-electrolyte system functions. Substantial progress toward this objective was made.

The Phillips Laboratory Power and Thermal Management Division (PL/VTP), in conjunction with ORION International Technologies, initiated the Alkali Metal Thermal to Electric Conversion (AMTEC) Protosystem Test Program to evaluate the performance of AMTEC cells in a system configuration. Advanced Modular Power Systems (AMPS) and the Jet Propulsion Laboratory (JPL) fabricated six geometrically identical cells for the program. Three different electrode/current collector material combinations appear in the six cells ‐ two of each type. The six cells were tested individually in air using cartridge heaters. Three of the six cells were then wired in series and tested in air using cartridge heaters. This test sequence is being repeated in vacuum.

Design, Analyses, and Fabrication Procedure of Amtec Cell, Test Assembly, and Radioisotope Power System for Outer-Planet Missions

Solid oxide fuel cells (SOFCs) are electrochemical conversion devices which directly convert chemical energy of the fuel into electrical energy with high efficiency. SOFC comprises of an anode, electrolyte and cathode layer that are fabricated individually upon each other, usually one at a time. Additive manufacturing (AM) is a new fabrication technique that utilizes a layer by layer fabrication technique, which can potentially be suitable for use in SOFC, since it is also building different materials in a layer-wise manner. Furthermore, the use of laser-based AM techniques can eliminate the issue of long sintering hours by directly densifying the materials using laser as the heat source. In addition, SOFCs possessing newer designs can be fabricated with ease using this technique, gradient-structure electrodes can also be created either by design or varying the laser parameter and material composition. Therefore, this thesis aims to develop a process as well as study the feasibility for fabricating SOFC using laser-based AM techniques. First, a step-by-step process was conducted to identify significant factors affecting the building of Nickel metal support structures using Selective Laser Melting (SLM) process. Laser power, scanning speed and hatch spacing were found to be the dominant factors affecting the printability of the parts. Next, a porous structure was designed and printed which acted as a metal support for SOFC. A functional cell was further built upon the metal support and the electrochemical performance was investigated. Although a low open circuit voltage (OCV) of 0.53 V was obtained with a maximum power density of 11.0 mW/cm2 from the cell, it demonstrated potential of this technique as a new fabrication process. In order to realize the fabrication of the entire SOFC by laser-based AM, the influence of laser on SOFC materials were conducted. The effect of two different laser sources, Nd:YAG fibre laser and CO2 laser was studied on NiO-YSZ anodic material, which showed that CO2 xvi laser was better absorbed by the oxide material. In addition, dendrite features were formed on the surface, which were influenced by the rapid solidification process and could be modified by adjusting laser processing parameters. Although the laser-modified SOFC did not perform as well as the conventional one, it was found that significant improvement was made on the charge transfer kinetics. Lastly, the influence of laser on YSZ electrolyte surface showed that the polarization resistance of the cell was reduced due to better oxygen adsorption/dissociation process. These experiments demonstrated that laser-based AM is a viable fabrication tool for SOFC.

The development of highly efficient modular power generation units is of great importance. Solid oxide fuel cells (SOFC) are currently considered as the most efficient devices for the conversion of hydrocarbons, (natural gas, LPG, JP or diesel) into electric power. To provide stable SOFC operation, initial fuel, before feeding to the SOFC anode, should be converted to synthesis gas with high hydrogen content. Catalytic steam, combined steam-dry, partial oxidation or autothermal reforming are the most appropriate reactions for this purpose. The optimum temperature for these reactions is 550 − 900 °C, which falls within the SOFC working temperature range. The development of a catalyst system that can act as a combined catalytic reformer and heat exchanger (to remove or supply heat at the reaction zone) is very promising for efficient power generation. The combined catalytic reformer and heat exchange catalyst should have the following characteristics: high intrinsic activity with respect to its weight and volume (due to size limitations of the reformer); coking resistance; high intrinsic heat conductivity (in order to provide uniform temperature and reaction rate distribution across the reformer); low pressure drop. The present report summarizes the results obtained during a systematic study of several new nickel- and rhodium- based catalyst compositions supported on metal foams, their performance, and fundamental principles of the above mentioned reactions for conversion of various common hydrocarbon fuels. A procedure for deposition of active catalyst components onto metal foams has been developed. It was systematically verified on Ni, Ni-Fe, FeCrAlY-steel foams with porosity from 10 to 70 pores per inch (ppi), obtained from various manufacturers. The catalytic performance of the coated metal foam substrates was studied in a fixed-bed continuous flow reactor. Total feed flow rate (GHSV − gas hourly space velocity) ranged from 3,000 to 30,000 hr-1. The reaction temperature varied from 500 to 800 °C. The catalysts (as-synthesized and used) were characterized by BET, XRD, XPS, TEM, EDX, SEM, TPR and TPD techniques.