• Journal of the Korean Institute of Gas
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• v.22 no.3
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• pp.32-37
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• 2018

Recently, the distributed power generation market using natural gas is expected to expand gradually according to the government's future energy conversion policy. Distributed power generation means small power generation source near the power demand site, which has the advantage of reducing the construction costs of the transmission and distribution infrastructure, operating cost and power loss. A typical example of distributed generation using natural gas is the trigeneration system. In this study, we conducted a basic study on the performance analysis of trigeneration desiccant system for dehumidifying / cooling / heating in the air conditioner room by using the cold and engine waste heat energy generated in the trigeneration system. It shows that the system efficiency increases and the energy consumption decreases as the temperature difference between the inlet and outlet of the trigeneration system increases compared with the general air conditioning system.

• Journal of Power Electronics
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• v.15 no.5
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• pp.1305-1317
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• 2015

Distributed generation systems (DGSs) have been getting more and more attention in terms of renewable energy use and new generation technologies in the past decades. The self-excited induction generator (SEIG) occupies an important role in the area of energy conversion due to its low cost, robustness and simple control. Unlike synchronous generators, the SEIG has to absorb capacitive reactive power from the outer device aiming to stabilize the terminal voltage at load changes. This paper presents a novel static VAR compensator (SVC) called a magnetic energy recovery switch (MERS) to serve as a voltage controller in SEIG powered DGSs. In addition, many small scale SEIGs, instead of a single large one, are applied and devoted to promote the generation efficiency. To begin with, an expandable mathematic model based on a d-q equivalent circuit is created for parallel SEIGs. The control method of the MERS is further improved with the objective of broadening its operating range and restraining current harmonics by parameter optimization. A hybrid control strategy is developed by taking both of the stand-alone and grid-connected modes into consideration. Then simulation and experiments are carried out in the case of single and double SEIG(s) generation. Finally, the measurement results verify that the proposed DGS with SVC-MERS achieves a better stability and higher feasibility. The major advantages of the mentioned variable reactive power supplier, when compared to the STATCOM, include the adoption of a small DC capacitor, line frequency switching, simple control and less loss.

• The Korean Journal of Ecology
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• v.28 no.4
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• pp.181-188
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• 2005

Methods used to study carbon sequestration by soil aggregates have often excluded the concentric spatial variability and other dynamic processes that contribute to resource accessibility and solute transport within aggregates. We investigated the spatial gradients of carbon (C) and nitrogen (N) from the exterior to interior layers within macroaggregates, $6.3\sim9.5$ mm, sampled from conventional tillage (CT) and no tillage (NT) sites of a Hoytville silt clay loam. Spatial gradients in C accumulation within macroaggregates were related to the differences in C dynamics by determining the sizes and the turnover rates of fast C and slow C pools in the concentric layers of aggregates. Aggregate exteriors contained more labile C and were characterized by greater C mineralization rates than their interiors in both management systems. In contrast, C in the interior layers of aggregates was more resistant in both systems. These results indicated the spatial differentiation of C dynamics within macroaggregates, i.e., exterior layers as a reactive site and interior layers as a protective site. Greater total C distribution in the exterior layers of NT aggregates indicated more influx of C from the macropores in interaggregate space than C. mineralization (net gain of C), whereas lower C distribution within the exterior layers of CT aggregates indicated net loss of C by greater C mineralization than C influx. We found total C increased approximately 1.6-fold by the conversion of CT soils to NT management systems for a period of 36 years. Differences in total accumulation and the spatial distribution of C within aggregates affected by management were attributed to the differences in aggregate stability and pore networks controlling the spatial heterogeneities of resource availability and microbial activity within aggregates.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.43 no.3 s.309
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• pp.51-59
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• 2006

In this paper, we propose an excellent ultrasonic transceiver system based on a QPSK modulation technique for underwater communication. The transmitter sends a still image at the level of 187dB re $1{\mu}Pa/V@1m$ through a power amplifier by driving an ultrasonic sensor. The receiver performs digital conversion at the 100kHz sampling frequency, demodulation and decoding process for the image sent from the transmitter through the underwater communication. We have shown that the processed image at the receiver is almost the same as the orignal one. The maximum detection distance of the system proposed in this paper is approximately 1.17km. To cope with the difficulties of transmission loss, this paper proposes, implements and analyzes important parameters of sensors and circuits used in the system. Most of the underwater communication has focused on the transmission of audio signal, but this paper suggests an efficient underwater communication system for still image transmission.

• Journal of the Korean Ceramic Society
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• v.39 no.1
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• pp.86-91
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• 2002

Solid Oxide Fuel Cells (SOFC) are of great interest of next generation energy conversion system due to their high energy efficiency and environmental friendliness. The basic SOFC unit consists of anode, cathode and solid electrolyte. Among these components, anode plays the most important role for the oxidation of fuel to generate electricity and also behaves as a substrate of the whole cell. It is normally requested that the anode materials should have the high electrical conductivity and gas permeability to reduce the polarization loss of the cell. In this study, the effect of pore former on the microstructure of anode substrate was investigated and thus on the electrical conductivity and the gas permeability. According to the results, microstructure and electrical conductivity of anode substrate were greatly influenced by the shape of pore former and especially by the anisotrpy of the pore former. The use of anisotropic pore former is supposed to deteriorate the cell performance by which the electrical conduction path is disconnected but also the effective gas diffusion path for the fuel is reduced.

• Journal of the Korean GEO-environmental Society
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• v.8 no.3
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• pp.35-40
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• 2007

The vertical drain method recently being used in Korea is divided into the sand drain method, the pack drain method, the paper drain method, and the PBD method according to the drainage. However, these methods generate the disturbed zone called the smear zone when the drainage is penetrated into the in-situ ground. The characteristics of the smear zone generated cause the problems that the coefficient of permeability decreases and the consolidation time becomes longer than expected in the design. Although the size of the smear zone is a very important factor directly influencing the degree of consolidation, in the existing studies, the general value for the size of the smear zone proposed has been used in the design. However, the size of the smear zone proposed by the existing studies cause a loss of economical efficiency because of the inaccuracy of the design. Hence, in this study, the characteristics on the size of the smear zone were analyzed by carrying out the three dimensional numerical analysis and the method to determine the conversion size of the smear zone considering the change of the coefficient of permeability was proposed in order to consider the change of the coefficient of permeability in the actual design.

• Journal of Chest Surgery
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• v.27 no.9
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• pp.779-784
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• 1994

Vidio-assisted thoracic surgery[VATS] has recently evolved as an alternative to thoracotomy for several thoracic disorders,and the role of thoracoscopy has expanded with advances in surgical techniques and instruments. From May 1993 to May 1994, 13 patients with mediastinal mass underwent VATS for diagnosis and treatment at Gil General Hospital. There were four males and nine females, and their ages raged from 5 years to 66 years with average 38.8 years. Among 13 patients, 3 were operated for tissue diagnosis,9 for treatment,and 1 for diagnosis and treatment. Pathologic diagnoses were as follows; 5 benign neurogenic tumors, 2 thymoma, 2 sarcoidosis, 1 teratoma, 1 peripheral neuroepithelioma, 1 tbc lymphadenitis, and 1 pericardial cyst. The mean time of operation was 111.7 $\pm$ 30.7 minutes[60-160], mean duration of chest tube drainage was 2.9 $\pm$1.9days[1-9], mean hospital stay was 6.2 $\pm$2.6 days[4-13]. There was no patient needed blood transfusion or conversion to open thoracotomy. Accurate diagnosis was possible in all patients operated for diagnosis and /or treatment.[4/4,100%] Two complications occurred in two patients: 1 transient Horner,s syndrome,1 anhydrosis of left arm. Compared with those of conventional thoracotomy done for mediastinal mass during previous 2 years[May 1991 - April 1993], operative results of VATS were better in all aspects. For mediastinal mass, we concluded that VATS can be done with less morbidity,less complication,less blood loss,shorter operation time and hospital stay,and not more expensive in cost than conventional thoracotomy. Noticeably, we think that VATS is the operation of choice for the diagnosis and palliation of malignant mediastinal mass.

• Journal of Chest Surgery
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• v.48 no.2
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• pp.105-111
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• 2015

Background: The aim of this study was to establish whether pulmonary lobectomy using video-assisted thoracic surgery (VATS) can be safely performed by trainees with limited experience with open lobectomy. Methods: Data were retrospectively collected from 251 patients who underwent VATS lobectomy at a single institution between October 2007 and April 2011. The surgical outcomes of the procedures that were performed by three trainee surgeons were compared to the outcomes of procedures performed by a surgeon who had performed more than 150 VATS lobectomies. The cumulative failure graph of each trainee was used for quality assessment and learning curve analysis. Results: The surgery time, estimated blood loss, final pathologic stage, thoracotomy conversion rate, chest tube duration, duration of hospital stay, complication rate, and mortality rate were comparable between the expert surgeon and each trainee. Cumulative failure graphs showed that the performance of each trainee was acceptable and that all trainees reached proficiency in performing VATS lobectomy after 40 cases. Conclusion: This study shows that trainees with limited experience with open lobectomy can safely learn to perform VATS lobectomy for the treatment of lung cancer under expert supervision without compromising outcomes.

• Advances in pediatric surgery
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• v.10 no.1
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• pp.31-34
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• 2004

The laparoscopic splenectomy (LS) became popular over the last 10 years. The advantage of LS over open splenectomy (OS) includes short hospital stay, improved cosmesis, less development of postoperative intestinal ileus, and less analgesics required. The purpose of this study is to evaluate the outcome of LS at Asan Medical Center from January 1999 to January 2003. The records of 14 consequent children who underwent splenectomy were reviewed retrospectively. Patients characteristics, morbidity, mortality, operative time, blood loss, and hospital stay were analyzed., Seven patients age 5 to 15 years underwent LS under the indications: idiopathic thrombocytic purpura (ITP, n=3), hereditary spherocytosis (n=3), and myelodysplastic syndrome (n=l). Seven patients, age 7 to 16 years, underwent OS during the same period for ITP (n=7). Median operative time was 120 mInutes (80 to 170 mins.) in OS, and 270 minutes (110 to 480 mins,) in LS (p<0.05). Median length of hospital stay was 6 days (3 to 8) in OS, and 4 days (3 to 6) in LS (p>0.05). Median splenic length was 12.0cm (9.2 to 18.0) in OS, 14.0 cm (10.0 to 19.5) (p>0.05). Accessory spleens were identified in 3 of 7 LS and 1 of 7 OS cases. In the LS group, there was no conversion to open surgery. Two patients in LS required blood transfusion postoperatively. LS in children can be performed as effectively and safely as OS.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.4 no.3
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• pp.527-536
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• 2000

This paper discusses the design of self-oscillating mixer type low noise down converter using the microwave field effect transistor. The mixer is consists of local oscillator in which high stability dielectric resonator and band pass filter to get rid of spurious oscillation at intermediate frequency stage. The microstrip antenna was integrated in the same substrate which generate 12.3GHz and low noise amplifier was also added after antenna using 3 stage of high electron mobility transistors. The output frequency from the local oscillator was chosen as 11.3GHz for the Ku-band application. The measured phase noise was -804dBc/Hz at 100kHz offset frequency, and the gain was 7～12dB in frequency range from 12.0GHz to 12.7GHz. The noise figure at intermediate frequency stage was 64H. The designed model shows less conversion loss than previous diode type mixer. The proposed mixer can be used in digital satellite broadcasting and communication system and expected to use in next generation low noise block design.