Yosuke Komatsu

Preparation of YBCO/ZrO2 thin films on Si by MOCVD using a mode converting type of microwave plasma apparatus

Abstract YBCO film has been deposited on (100) silicon substrate via ZrO 2 film as a buffer layer by the use of microwave plasma MOCVD. In the apparatus, the microwave was converted from TM 10 (rectangular) mode to TM 01 (circular) mode by a mode converter to make the plasma more active. ZrO 2 was selected as a buffer layer to prevent the reaction of deposited YBCO film and Si substrate. Although polycrystalline ZrO 2 films are generally obtained by MOCVD, the epitaxial ZrO 2 films with (111) orientation were obtained at below 300°C by the use of this apparatus. The film deposited at 300°C for 2 h had a thickness of 90 nm and consisted of small particles with an average size of 50 nm. According to the ESCA measurements, the films were free from carbon, except for the surface. Epitaxial films of YBCO with c -axis orientation were deposited on the (111) orientated ZrO 2 film at 650–700°C for 1 h. The a -axis orientated film was also obtained at 600°C. The YBCO films consisted of small grains with the average grain size of 0.1 μm and the film thickness was ca. 0.3 μm. The critical temperature ( T c ) and the critical current density ( J c ) of the film obtained at 600°C were 85 K and 1×10 5 A/cm 2 (at 77 K), respectively.

Combining structural, electrochemical, and numerical studies to investigate the relation between microstructure and the stack performance

In this study, the static characteristics of a planar solid oxide fuel cell (SOFC) stack with a standard power output of 300 W were investigated. After a power generation experiment, the microstructure parameters of the tested cell were quantified. SOFC has a complex composite structure for both the anode and the cathode. The electrode microstructure is an important factor determining the electrochemical performance of the entire cell and consequently the entire stack of cells. The most precise information about a cell microstructure can be derived from real structural analysis. A method such as a combination of focused ion beam and scanning electron microscope (FIB–SEM) can provide very detailed information about the electrode microstructure. The presented studies provide a detailed microstructure analysis coupled with stack level electrochemical measurements. The obtained microstructure is implemented into numerical simulation and compared with the stack level simulation. This study proves the import role of microstructure in predicting current–voltage characteristic and the power output of a stack. The presented research can also be used as a benchmark for increasing a number of stack numerical simulations.Graphical Abstract

Numerical analysis on the dynamic behavior of a solid oxide fuel cell with a multivariable control strategy

The paper describes the dynamic analysis of the operation of a solid oxide fuel cell (SOFC) with a multivariable control strategy. Developing a control strategy shall be essential for the practical use of SOFCs in the future. The system needs to be operated flexibly to load change. DC power output control of the SOFC by adjusting the density of the current has been studied previously and the quick response of the SOFC estimated [1]. However, in that study the performance degradation of the SOFC under part-load operation was estimated. To achieve effective operation of the SOFC under the partload condition in the present study, system operation with multivariable control was adopted. The numerical modeling of the SOFC system analysis was done by solving the steady mass balance and the unsteady energy equations including heat transfer (the electrochemical reaction is spontaneous and mass diffusion is a relatively faster transient phenomenon than heat transfer due to the high heat capacity of the system components). Feedback control methodology was used to build the system control strategy. Fig. 1 shows the feedback loop of the SOFC targeted to control of the DC power output. In order to achieve multivariable control of the SOFC as shown in Fig. 2, the DC power output and cell operating, the fuel utilization factor and Steam/Carbon (S/C) ratio were adopted as process variables. For this control purpose, the current density, the air mass flow rate, the fuel mass flow rate and the steam mass flow rate were adopted as the manipulated variables. The relationship between the process variables and the manipulated variables are shown in Fig. 2.

Experimental and analytical evaluation of the drying kinetics of Belchatow lignite in relation to the size of particles

Water removal is a key technology for enhancing efficient utilization of lignite in power generation. An inherent characteristic of lignite, attributed to the large amount of water kept within the fuel, is the factor decreasing the thermal efficiency of lignite-fired power plants. This paper presents the research results on investigating the drying kinetics of Belchatow lignite excavated in the Central Poland in prior to developing a water removal system. Lignite drying test was conducted in superheated steam atmosphere at the temperature range of 110-170 °C. Spherically shaped samples, of which the diameter is 2.5 mm, was used. The experimental results were then analysed with previously conducted measurements of 5, 10, 30 mm samples to investigate the influence of particle size. The presented analysis shows the agreement of the evaluated drying rate at the CDRP to the experimental data. The obtained experimental results were used to predict the drying behaviour of the group of particles. The proposed investigation clarifies the size dependence of the drying characteristics of the multisize group of lignite particles.

An experimental verification of numerical model on superheated steam drying of Belchatow lignite

Due to low production costs, lignite is an important component of energy mixes of countries in its possession. However, high moisture content undermines its applicability as fuel for power generation. Drying in superheated steam is a prospective method of upgrading quality of lignite. The study aimed to validate the drying model of lignite from Belchatow mine in Poland. The experimental investigation on superheated steam drying of lignite was previously conducted. Spheres of 10 mm in diameter were exposed to the drying medium at the temperature range of 110-170oC. The drying behaviour was described in the form of moisture content, drying rate and temperature profile curves against time. With the application of basic coal properties (e.g. density, water percentage, specific heat) as well as the mechanisms of heat and mass transfer in subsequent stages of the process, the numerical model of drying was constructed. It was tentatively verified with reference to experimental results both in terms of drying parameters and temperature. The model illustrated drying behaviour in the entire range of conditions. Nevertheless, further development of numerical model is desirable regarding accuracy of the process parameters.

Influence of geological variations on lignite drying kinetics in superheated steam atmosphere for belchatow deposit located in the central Poland

Lignite-fired coal power plants suffer from a significant heat loss due to the high moisture content in this energy carrier. Water removal from fuel is an indispensable treatment for improving the combustion process, which will foster the efficient utilization of lignite. Superheated steam fluidized bed drying is expected for this purpose in a power generation sector. Understanding drying kinetics of lignite will greatly reinforce design process of a dryer. Physical features as well as the drying behaviour may be divergent among the lignite originated from different depths and positions in a certain mine. To reveal and clarify the influence of the geological features, the drying characteristics of several grades of lignite from the Belchatow mine in Poland were investigated. The attempts to clarify the influence of the divergent properties of the investigated samples on the drying kinetics in superheated steam were presented in this paper.