Net AC Research Articles

Status of SOC Development at Chaozhou Three-Circle

Chaozhou Three-Circle (Group) Co., Ltd. (CCTC) is a leading company in developing and manufacturing of electronic and ceramic components. Since more than 19 years ago, CCTC has been focusing on SOFC R&D and manufacturing, covering the complete value chain from powder to power. Based on anode-supported cell technology (ASC in fuel cell mode), the latest C2-stack in planar design using metallic interconnect exhibits a high efficiency of more than 69% (DC, LHV) at 1.5 kW. Long-term stack tests using pipeline natural gas have been conducted for 20,000 hours, with low degradation rates of below 0.4%/kh. In 2022, several 35 kW SOFC CHP systems were demonstrated with C2 stacks, which showed stable performance over 3,000 hours of operation. According to the verification test by Societe Generale de Surveillance S.A., the net AC efficiency of the system reached 64.1%, and the total efficiency was up to 91.2% after 1,000 hours of continuous operation. In parallel to the continuous progressing in SOFC, optimization in cells and stack designs dedicated for SOEC applications were also conducted in recent years. The status of both SOFC and SOEC development at CCTC will be presented.

Continuous decentralized hydrogen production through alkaline water electrolysis powered by an oxygen-enriched air integrated biomass gasification combined cycle

This research work presents an innovative approach for continuous decentralized production of renewable hydrogen from woody biomass. Alkaline water electrolysis (AWE) is used to produce high-purity hydrogen, while the oxygen by-product is mixed with ambient air and used to fire a biomass-fueled downdraft gasifier in order to produce an upgraded producer gas with a lower heating value (LHV) between 7–8 MJ/Nm3. This fuel gas is then subjected to a conditioning stage and eventually fed to a combined cycle consisting of a recuperative gas turbine as topping unit and a regenerative subcritical organic Rankine cycle as bottoming unit, which together allow for a combined electric power generation efficiency close to 40%. Most of the net AC power from the integrated gasification combined cycle (IGCC) is rectified to DC power and ultimately used to power an alkaline electrolyzer, with a minor share allocated to all the required utilities and ancillary equipment, including hydrogen compression to 200 bar. The results from simulation of the hybrid IGCC-AWE plant under steady-state operating conditions in Aspen Plus V.11 indicate an optimal efficiency of 17.6% based on the LHV of hydrogen. Thus, if sized for a biomass consumption of 1 t/h, the proposed plant is capable of providing around 26 kg/h of compressed hydrogen at 200 bar.

Valley-Hall alternatively changing conductivity in gapped and strained graphene.

We investigated the alternatively changing (ac) valley-Hall effect in a graphene/h-BN structure under the modulations of a static electric field E0, magnetic field B0, and light field EA1. The proximity to the h-BN film leads to a mass gap and strain-induced pseudopotential for electrons in graphene. Starting from the Boltzmann equation, we derive the ac conductivity tensor σ, including the orbital magnetic moment, Berry curvature, and anisotropic Berry curvature dipole. It is found that under B0 ≠ 0, σ for the two valleys can have different amplitudes and even have the same sign, leading to a net ac Hall conductivity. The ac Hall conductivities and the optical gain can be altered by both the amplitude and the direction of E0. These features can be understood from the changing rate of σ with E0 and B0, which is valley-resolved and varies nonlinearly with the chemical potential.

Development of DC Energy Meter for Renewable Energy Application

The world we live in, is becoming more and more dependent on electrical energy, which, sadly, still heavily relies on outdated techniques involving the burning of fossil fuels. Fortunately, more and more people are accepting the fact that this system simply isn’t sustainable in the longer-run. Even if air pollution wasn’t a huge factor, the shortage of these resources is something that is bound to happen in the nearest future, and it would pose an impassable obstacle towards the future growth of mankind. Hence, importance of renewable energy is such an important trend for the future of mankind. At present, net AC meters are available for measurement of four quadrant energy, which is used at output of both on grid and off grid solar inverter. This meter can measure both, energy fed into the grid and energy utilize from the grid. However Energy generated by solar panel cannot be measured directly. Even in off grid inverter the present net meter cannot measure energy generated by solar panel. In case of DC micro grid, where DC energy is used/ consumed directly by the load, the present net meter cannot be used as they can be used in AC energy measurement only. With the launch and implementation of national energy saving and emission reduction policies in relation to distributed power supply by renewable power generation, the DC energy metering technology will become an inevitable trend, breaking the landscape dominated by AC in the field of electric energy metering. This paper describes introduction, block diagram [1], Hardware and software description for development of dc energy meter.

Progress of SOFC Residential CHP System: Over 50,000 Units Market Experience of Osaka Gas

Osaka Gas began selling the second-generation residential SOFC CHP “ENE-FARM type S” in April 2016, which was developed in collaboration with Aisin Seiki, Kyocera and Noritz. In April 2018, power efficiency when the rated output can be stably is improved 53.5 % (net AC, LHV) from 52.0 % (net AC, LHV). The cumulative sales volume of Osaka Gas’s the residential fuel cell CHP system as in March 2019 was over 100,000. The cumulative number of SOFC-type was more than half. We have been trying to improve its quality and specifications, and reduce its cost. For example, we have improved the merchantability by adding a self-sustaining power generation function and using the Internet. ENE-FARM type S performed at approximately 2,000 customer homes though a power failure occurred mainly in Osaka due to Typhoon in 2018. It was an opportunity that one of the benefits in residential CHP was socially recognized.

Basic Study of Two-Stage Solid Oxide Fuel Cell Stacks with Fuel Regenerator

Two-stage stack technology is a method to generate highly efficient power using fuel cells. To achieve highly efficient power, it is important to regenerate anode gas between the two-stage stacks. The effect on power generation efficiency by removing H2O and/or CO2 from the anode gas using a fuel regenerator has been investigated by simulating the two-stage solid oxide fuel cell stack model. The risk of carbon deposition from regenerated anode gas has also been investigated by calculating the equilibrium state of the anode gas. The maximum efficiency of net AC 71.8% (lower heating value) was obtained when H2O and CO2 were 100% removed from the anode gas, assuming that thermal self-sustainability in the system was achieved.

(Invited) Key Contributions by Dr. H. Russel Kunz in the Fundamental Understanding of Carbonate Fuel Cells

Carbonate fuel cell (MCFC) has progressed since 1960s and matured to a single full-size commercial stack capable of producing 350kW net AC power and 5-year service life. Several research teams worldwide have advanced on the MCFC technology over the past five decades. Among those efforts, contributions by Dr. H. Russell Kunz clearly stand out both in terms of the breadth of areas addressed and the simplicity of the mathematical treatment. Dr. Kunz, at UTC/IFC until his retirement in 1992, made significant contributions on both fundamental understandings and practical solutions in terms of electrochemical performance, cathode stability and electrolyte management, all essential to MCFC stack performance and endurance. This review paper will summarize the fundamental challenges and current state of the MCFC technology and the important contributions by Dr. Kunz.

Development and Commercialization of New Residential SOFC CHP System

Osaka Gas began selling the new version residential SOFC CHP system “ENE-FARM type S” in April 2016, which is developed in collaboration with Osaka Gas, Aisin Seiki, Kyocera and Noritz. Kyocera manufactures SOFC stacks, Aisin Seiki manufactures power generation units, and Noritz manufactures boiler and remote-control units. The electrical efficiency is 52% (net AC, LHV), whereas that of the previous model was 46.5%.The previous system comprised two units: a power generation unit and a heat recovery unit. The latter unit contained the hot-water storage tank and the boiler. The new power generation unit contains the hot-water storage tank. This tank is configured for connection to a normal household boiler. The system size is minimized to facilitate its use as a residential CHP system. Moreover, it is possible to install the new system in a housing complex, where the available space is limited. This feature expands the market for this system because the previous model could only be installed in a single-family house. To achieve the high electrical efficiency of 52% (net AC, LHV) and the cost reduction, three technical issues were addressed. The first was the improvement of the fuel cell performance, the second was the optimization of the fuel utilization rate, and the third was the improvement in durability. Kyocera reduced the internal electrical resistance of the fuel cell stack. In addition, a new coating technology, i.e., electropainting, which is developed by Osaka Gas and Kyocera, was used for current collectors. Consequently, both an increase in power density (reducing the number of cells) and an improvement in the electrical efficiency have been achieved. Current collectors are oxidized at a high temperature because they comprise stainless steel. Their durability has been considerably improved by the electropainting coating technology. Aisin Seiki optimized the system controls, resulting in enhanced electrical efficiency. Osaka Gas conducted field tests in the Kansai area in Japan. We installed the systems in the houses of several employees for evaluating the system behavior. The results made it possible to accelerate product development. In accordance with the liberalization of the power market in Japan in April 2016, if customers use natural gas provided by Osaka Gas, the electrical power from the unit can be sold back to Osaka Gas. We consider that 2016 is a transition period for residential CHP from the market introduction stage to the autonomous market spread stage. We are currently focusing on achieving further developments in the technology, which will enable cost reduction and higher power generation efficiency in order to maximize the target market.

Influence of Operating Parameters and System Design on Efficiency of Biomass and Biogas Based SOFC Systems

With the recent COP 21 declaration many countries have dedicated themselves to a clear path towards decarbonization. This requires major installation of wind and solar power. However, these only provide intermittent electricity. Thus, balance power has to be provided, ideally from renewable resources. Currently only biomass seems to be available in large enough quantity to provide significant balancing power. A wide range of biomass feedstocks can be made available for power generation for example via anaerobic digestion or thermo-chemical gasification. However, resources are still limited. Hence, in order to maximize its potential the biomass has to be used at the highest possible efficiency. Electrochemical conversion, especially in SOFC, in general offers very high efficiency. However, also in SOFC systems the choice of the optimal system design and operating parameters can make a big difference. The presented work investigates utilization of biogas and syngas from biomass gasification in different system designs. This is done using a thermodynamic SOFC model built in Aspen Plus, which has been validated against literature data. A comparison to cell and stack performance of the well known manufacturers Fuel Cell Energy and Forschungszentrum Jülich is presented. It is shown that at identical operating conditions the model shows slightly worse performance than the real stacks. With the model, the influence of different fuel pre-treatment and pre-reforming options are compared at a fixed stack size. Furthermore the effect of anode offgas recirculation, as well as the choice of fuel utilization, oxidant, operating temperature and pressure are investigated. During the analysis operating conditions prone to the risk of carbon deposition are excluded. Also the impact on the balance of plant is taken into account. Results show, that depending on the fuel, system design and parameters the electrical efficiency of the fuel cell alone can vary between 35 and 71%. From this, depending on the system parasitic consumption and losses of up to 8% points have to be subtracted. Thus, net AC efficiencies reach 30 to 63%. Finally a new system design and operating regime is proposed, which can significantly raise the DC efficiency while reducing the parasitic consumption by adjusting the gas treatment and operating parameters. Modelling results of new design show net AC system efficiencies of up to 72%.

Development and Commercialization of New Residential SOFC CHP System

In April 2016, Osaka Gas began the marketing of “ENE-FARM type S,” which was an advanced version of the residential SOFC CHP system. Osaka Gas developed this system in collaboration with Aisin Seiki, Kyocera, and Noritz. The electrical efficiency of the new model was 52 % (net AC, LHV), whereas that of the previous model was 46.5 %. The system size was minimized to facilitate its use as a residential CHP system. We reduced the maintenance, stack, and system costs. In this paper, we describe the developments and features of new residential SOFC CHP system.

Transfer cavity scheme for stabilization of lattice laser in ytterbium lattice clock

For high performance clock, optical lattice is introduced to generate periodic trap for capturing neutral atoms through weak interactions. However, the strong trapping potential can bring a large AC Stark frequency shift due to imbalance shifts for the upper and lower energy levels of the clock transition. Fortunately, it is possible to find a specific “magic” wavelength for the lattice light, at which the first-order net AC Stark shift equals zero. To achieve high stability and accuracy of a neutral atomic optical clock, the frequency of the lattice laser must be stabilized and controlled to a certain level around magic wavelength to reduce this shift.#br#In this paper, we report that the frequency of lattice laser is stabilized and linewidth is controlled below 1 MHz with transfer cavity scheme for ytterbium (Yb) clock. A confocal invar transfer cavity mounted in an aluminum chamber is locked through the Pound-Drever-Hall (PDH) method to a 780 nm diode laser stabilized with modulation transfer spectroscopy of rubidium D2 transition. It is then used as the locking reference for the lattice laser. This cavity has a free spectral range of 375 MHz, as well as fineness of 236 at 780 nm, and 341 at 759 nm. Because neither of the wavelengths of 759 nm and 780 nm is separated enough to use optical filter, they are coupled into the cavity with transmission and reflection way respectively, and the two PDH modulation frequencies are chosen differently to avoid possible interference.#br#The stabilization of the 759 nm lattice laser on transfer cavity can stay on for over 12 hours without escaping or mode hopping. To estimate the locking performance of the system, a beat note with a hydrogen maser-locked optical frequency comb is recorded through a frequency counter at 10 ms gate time for over 3 hours. This beat note shows that the frequency fluctuation is in a range of 10 kHz corresponding to a stability of 2×10-11 level with 0.1 s averaging time, but goes up to 150 kHz corresponding to a stability of 3.6×10-10 at 164 s averaging time. The long-term drift can be the result of air pressure fluctuation on the transfer cavity, or the bad stability of the optical comb in the measurement process. However, current locking performance is still enough for the requirement of 10-17 clock uncertainty.#br#In conclusion, we succeed in realizing frequency stabilization and control for the lattice laser of Yb clock with the transfer cavity scheme. The result shows that the short-term stability is around 10-11 level, though a mid-long-term drift exists. However, the stability of 3.6×10-10 over 164 s can still promise a 10-17 uncertainty for the Yb clock. And, it can be reduced if the averaging time is long enough. The work can be further improved by installing the transfer cavity into vacuum housing for better stability in future.

Conceptual design for critically-high power generation efficiency by using multi-stage solid oxide fuel cells (SOFCs) or proton-conducting ceramic fuel cells (PCFCs)

Recently we have developed the conceptual design for realizing a critically-high electrical efficiency with solid oxide fuel cells (SOFCs). SOFCs are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Recently we developed and reported a conceptual design that has a potential to realize a critically-high fuel-to-electricity conversion efficiency of up to as high as 85% (LHV, gross DC), in which a high-temperature multi-stage solid oxide fuel cells (SOFCs) is combined with a proton-conducting solid oxide electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 77% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC). In the conceptual design the proton-conducting electrolyte was assumed to have a protonic transport number of 1. However, the protonic transport number of the proton-conducting solid oxide electrolyte depends on the material and operating conditions such as temperature, oxygen partial pressure, kinds of fuel and so on, and would affect the electrical efficiency. Therefore, to realize the critically-high electrical efficiency by using the multi-stage electrochemical oxidation, proton-conducting solid oxide having a high ionic transport number should be developed.

Effect of proton-conduction in electrolyte on electric efficiency of multi-stage solid oxide fuel cells.

Solid oxide fuel cells (SOFCs) are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Therefore, evolved technologies are required to achieve considerably higher electrical efficiencies. Here we present an innovative concept for a critically-high fuel-to-electricity conversion efficiency of up to 85% based on the lower heating value (LHV), in which a high-temperature multi-stage electrochemical oxidation is combined with a proton-conducting solid electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 76% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC).

Numerical Simulation of SOFC System Performance at 90% Fuel Utilization with or without Anode Off-Gas Recycle for Enhancing Efficiency

To investigate possibility to enhance electrical efficiency, performance of methane-fueled 10 kW SOFC systems was numerically simulated with calculated voltage-current-density curves at 750ºC of a single cell with a bipolar plate, which approximated a stack performance. Operation at 90% fuel utilization (U f) with anode off-gas recycle (AGR) and with steam reforming (no recycle) led to 75-67%-LHV and 72-66%-LHV DC electrical efficiency at 0.05-0.40 A/cm2, respectively. System simulations considering heat loss from a cell stack clarified that 750ºC stack temperature can be maintained at 0.12-0.40 A/cm2 and 64-58% and 63-59% net AC electrical efficiency will be attainable at U f = 90% with and without AGR, respectively. Furthermore, enhancement of net AC efficiencies was found to be 11-10 percentage points at those current densities against ones of a Ene-farm-like conventional system using steam reforming at U f = 73%.

Numerical Simulation of SOFC System Performance at 90% Fuel Utilization with or without Anode Off-Gas Recycle for Enhancing Efficiency

To investigate possibility to enhance electrical efficiency, performance of methane-fueled 10 kW SOFC systems was numerically simulated with calculated voltage-current-density curves at 750ºC of a single cell with a bipolar plate, which approximated a stack performance. Operation at 90 % fuel utilization (U f) with anode off-gas recycle (AGR) and with steam reforming (no recycle) led to 75-67 %-LHV and 72-66 %-LHV DC electrical efficiency at 0.05-0.40 A/cm2, respectively. System simulations considering heat loss from a stack clarified that 750ºC stack temperature can be maintained at 0.12-0.40 A/cm2 and 64-59 % and 63-59 % net AC electrical efficiency will be attainable at U f = 90% with and without AGR, respectively. Furthermore, enhancement of net AC efficiencies was found to be 10 percentage points at those current densities against ones of a conventional system using steam reforming at U f = 73 %.

Development of Anode Off-Gas Recycle Blowers for High Efficiency SOFC Systems

Prototype of efficient anode-off gas recycle blowers was developed with a 3D turbo impeller and a DC brush-less built-in motor for high efficiency SOFCs. Performance test of the blowers clarified 26% and 52% total blower efficiency for the type SSR-70 for 10 kW-class SOFC and SSR-140 for 250 kW, respectively. System performance simulation of an SOFC system with the SSR-70 blower elucidated that 59% net AC electrical efficiency can be expected. When the blower efficiency is improved to 50% as of SSR-140, electrical efficiency will become 60%. We will continue developing more efficient recycle blowers.

Modeling of a 5 kWe tubular solid oxide fuel cell based system operating on desulfurized JP-8 fuel for auxiliary and mobile power applications

An onboard autothermal reformer (ATR) integrated with a SOFC stack offers potential for high energy efficiency and utilization, low emission and quiet operation avoiding cost associated with hydrogen storage and infrastructure. Such a system can be a viable and attractive option especially for military's need for quiet and less pollutant Auxiliary Power Unit (APU) and Mobile Electric Power (MEP) units in temporary and permanent base camps [1,2]. A 5 kWe Solid Oxide Fuel Cell (SOFC) system operating on desulfurized JP-8 fuel was modeled using Aspen Plus process simulation software to examine the effects of oxygen to carbon ratio (O2/C) and steam to carbon ratio (H2O/C) at different ATR operating temperatures (700–850 °C), while keeping the SOFC stack temperature constant at 910 °C. Anode recycle steam and heat have been used to reform the desulfurized JP-8 fuel which would make the system lighter and compact for mobile application. The system modeling revealed a maximum net AC efficiency of 39.5% at 700 °C and a minimum of 32.6% at 850 °C ATR operating temperatures, respectively. Sensitivity analysis with respect to fuel utilization factor (Uf) and current density (j) were also conducted to identify the optimum operating window.

Performance of a 10 kW SOFC Demonstration Unit

Experimental results of the performance of a solid oxide fuel cell (SOFC) unit are presented. The unit was designed, manufactured and tested at the VTT Technical Research Centre of Finland. The 10 kW power class SOFC stack and stack module were designed and manufactured by Versa Power Systems (VPS). A successful commissioning test of the unit was conducted in 2010. Long term testing of the unit was started in November 2010. The unit has been operated with natural gas for over 1500 hours supplying electricity to the local grid. The unit has shown robust and uninterrupted performance. Stack DC efficiency of 60% and system net AC efficiency of 43% has been measured during the operation.

Cracks Detection in Mg Alloy by Electro-Ultrasonic Spectroscopy

New non-destructive testing (NDT) method is based on the effect of the ultrasonic vibrations on the electron transport in samples with macroscopic defects as cracks or defect centers affecting electrical conductivity. On the frequency given by the subtraction of exciting frequencies new intermodulation signal appears. Its value is given by electric resistance modulation by the defects and un-homogeneities in the sample structure. In our experiment we used the ultrasonic actuator with frequency fU when the period of wave is longer than the dielectric relaxation time in analyzed sample. In this case the effects of electron bunching by ultrasonic wave are negligible. The ultrasonic wave length is much larger than the electrons mean free path and the wave period is much longer than the mean free time among the electrons collision with scattering centers and defects. Then the electron transport is described by the quasi-steady state transport equation in one-electron approximation. Because of the requirement of charge neutrality, no net AC electric current with the ultrasonic wave frequency fU can be carried by the wave. Similar situation exists for samples excited by standing ultrasonic waves. The electrical conductivity varies with time due to that the cracks geometry is changed with frequency of the ultrasonic vibration. The sample conductivity is affected mainly by the presence of cracks and defects boundaries perpendicular to the electric current density vector. In our experiment the amplitude of ultrasonic vibration is so low that no new cracks are generated and then the proposed testing method belongs to NDT.

Performance evaluation of different configurations of biogas-fuelled SOFC micro-CHP systems for residential applications

Three configurations of solid oxide fuel cell (SOFC) micro-combined heat and power (micro-CHP) systems are studied with a particular emphasis on the application for single-family detached dwellings. Biogas is considered to be the primary fuel for the systems studied. In each system, a different method is used for processing the biogas fuel to prevent carbon deposition over the anode of the cells used in the SOFC stack. The anode exit gas recirculation, steam reforming, and partial oxidation are the methods employed in systems I–III, respectively. The results predicted through computer simulation of these systems confirm that the net AC electrical efficiency of around 42.4%, 41.7% and 33.9% are attainable for systems I–III, respectively. Depending on the size, location and building type and design, all the systems studied are suitable to provide the domestic hot water and electric power demands for residential dwellings. The effect of the cell operating voltage at different fuel utilization ratios on the number of cells required for the SOFC stack to generate around 1 kW net AC electric power, the thermal-to-electric ratio (TER), the net AC electrical and CHP efficiencies, the biogas fuel consumption, and the excess air required for controlling the SOFC stack temperature is also studied through a detailed sensitivity analysis. The results point out that the cell design voltage is higher than the cell voltage at which the minimum number of cells is obtained for the SOFC stack.