• Proceedings of the Korean Information Science Society Conference
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• 2006.06d
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• pp.217-219
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• 2006

4G mobile networks are expected to support various multimedia services over IP networks and also satisfy high spectral efficiency requirement. In cellular systems including 4G networks, hotspot cells can occur when available wireless resources at some location are not enough to sustain the needs of users. The hotspot cell can potentially lead to blocked and dropped calls, which can deteriorate the service quality for users. In a 4G mobile network, a band of users enjoying multimedia services can move around, which may generate heavy flows of traffic load. This situation can generate the hotspot cell which has a short life span of only a few minutes. In this paper, we propose a handover-based scheme which can effectively manage hotspot cells in 4G mobile networks. With the scheme, the current serving cell can recognize the load status of the target cell in advance before handover execution. Adaptive handover time control according to the amount of traffic load of cells can effectively and flexibly manage the hotspot cell in the network. And, through our hotspot cell management scheme, acceptable service quality can be supported as users continuously maintain connections with the network. In the simulation results, we find that our scheme generates smaller number of hotspot cells and supports higher service quality than the compared schemes.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2007.11a
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• pp.279-285
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• 2007

Since the residential load is an AC load and the output of solar cell is a DC power, the photovoltaic system needs the DC/AC converter to utilize solar cell. In case of driving to interact with utility line, in order to operate at unity power factor, converter must provide the sinusoidal wave current and voltage with same phase of utility line. Since output of solar cell is greatly fluctuated by insolation, it is necessary that the operation of solar cell output in the range of the vicinity of maximum power point. In this paper, DC/AC converter is three phase PWM converter with smoothing reactor. And then, feedforward control used to obtain a superior characteristic for current control and digital PLL circuit used to detect the phase of utility line.

• 한국신재생에너지학회:학술대회논문집
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• 2005.06a
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• pp.273-276
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• 2005

Performance analysis of a solid oxide fuel cell/micro gas turbine hybrid system is conducted at design-point and part-load conditions and its results are discussed in this study. With detailed considerations of the heat and mass transfer phenomena along various flow streams of the SOFC, the analysis based on a quasi-2D model reasonably predicts its performance at the design-point operating conditions. In case of part-load operations, performance of the hybrid system to three different operation modes(fuel only control, speed control, and VIGV control) is compared. It is found that the simultaneous control of both supplied fuel and air to the system with a variable MGT rotational speed mode is the optimum choice for the high performance operation. And then, the dynamic characteristics of a solid oxide fuel cell are briefly introduced.

• 한국신재생에너지학회:학술대회논문집
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• 2008.05a
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• pp.78-81
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• 2008

Residential fuel cell cogeneration systems have gained much interest due to its high efficiency. In the present study, we have performed numerical simulation of residential fuel cell cogeneration system which includes a fuel cell/battery hybrid system. The cogeneration system consists of 1kW PEFC, two 60Ah batteries, inverter/converter and reformer. Several empirical models have been employed for respective components to improve the accuracy of the simulations. The load varies seasonally. The present simulations can successfully predict the characteristics of the hybrid cogeneration system and thus it can be utilized for establishing an optimal operating strategy of the system.

• Proceedings of the Korean Geotechical Society Conference
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• 2006.10a
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• pp.593-602
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• 2006

대구경 현장타설말뚝에 대한 시험하중의 증가와 함께 적용이 활성화되고 있는 오스터버그 셀(Osterberg cell)방식이 적용된 국외의 4 개소 시험결과에 대하여 소개하였다. 사례연구의 대상이 된 4 개소 시험말뚝은 일반적인 시험보다는 결과상의 특징이 있어 현재 및 향후 동일한 시험방법이 적용될 경우 고려될 수 있는 내용을 중심으로 기술하였다. 이들 말뚝시험에서는 가장 전형적으로 지지력의 균형이 이루어져 말뚝의 극한현상이 발생되지 않아 설계하중을 충분히 확인한 경우뿐 아니라 선단지지력이 부족하여 선단 그라우팅 후 재시험을 한 경우, O-cell을 말뚝의 선단에 가장 근접하여 설치 한 후 시험한 경우, 그리고 다단면(multi-level test)시험이 수행되었던 예 등을 살펴보았다.

• The Transactions of the Korean Institute of Electrical Engineers P
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• v.63 no.3
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• pp.212-217
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• 2014

The electric power output characteristics of magnesium fuel cell were investigated with regard to internal resistance. A equivalent circuit with the series-connected three internal resistance was introduced to analyze of the response to change of power. The power output analysis was employed in order to investigate the effect of internal resistances for the electrolyte concentration, air electrode area, Mg electrode area and distance between the electrodes. It was confirmed that internal resistance is generated by the electrolyte, air electrode and metal electrode, then those Internal resistances had a significant effect on the power output decrease. The power output was a maximum when the load resistance maches the internal resistance of the magnesium fuel cell. The fuel efficiency was only 50% at maximum power output. Higher fuel efficiency was achieved when the load resistance is greater than the internal resistance.

• 한국신재생에너지학회:학술대회논문집
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• 2007.06a
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• pp.288-291
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• 2007

Since the residential load is an AC load and the output of solar cell is a DC power, the photovoltaic system needs the DC/AC converter to utilize solar cell. In case of driving to interact with utility line, in order to operate at unity power factor, converter must provide the sinusoidal wave current and voltage with same phase of utility line. Since output of solar cell is greatly fluctuated by insolation, it is necessary that the operation of solar cell output in the range of the vicinity of maximum power point. In this paper, DC/AC converter is three phase PWM converter with smoothing reactor. And then, feedforward control used to obtain a superior characteristic for current control and digital PLL circuit used to detect the phase of utility line.

• Journal of Aerospace System Engineering
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• v.15 no.2
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• pp.1-9
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• 2021

In this paper, a thickness compensation function is introduced to consider the shear deformation and warping effect resulting from increased thickness in the composite multi-cell wing box. The thickness compensation function is used to perform the structure optimization of the multi-cell. It is determined by minimizing the error of an analytical formula using solid mechanics and the Ritz method. It is used to define a structural performance prediction expression due to the increase in thickness. The parameter is defined by the number of spars and analyzed by the critical buckling load and the limited failure index as a response. Constraints in structural optimization are composed of displacements, torsional angles, the critical buckling load, and the failure index. The objective function is the mass, and its optimization is performed using a genetic algorithm.

• 한국신재생에너지학회:학술대회논문집
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• 2011.11a
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• pp.89.2-89.2
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• 2011

In automotive applicatons or water vehicles, the polymer electrolyte membrane fuel cell(PEMFC) stack is keep moving while their operation. Especially the inclination environment can take a effect to fuel cell stack perfromance, because this condition can cause a bad effect to water exhaust of fuel cell stack. In this study, a large scale stack(over 100kW power) is inclined upto 30 degree in lengthwise and crosswise using stack lift equipment. And the stack is operated in 10~100% load. No significant change has appeared in crosswise inclined condition and lenthwise low angle. But in lenthwise large angle tilting condition, the fuel cell performance has significantly decreased. And this performance decrease is aggravated in low load. An active water exhaust device is applied to the stack to prevent the performance decrease. And in lenthwise large angle tilting condition, this device cause a good effect to fuel cell stack performance.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.10 no.5
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• pp.1976-1997
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• 2016

This paper investigates the problem of co-tier interference mitigation for dynamic small- cell networks, in which the load of each small-cell varies with the number of active associated small-cell users (SUs). Due to the fact that most small-cell base stations (SBSs) are deployed in an ad-hoc manner, the problem of reducing co-tier interference caused by dynamic loads in a distributed fashion is quite challenging. First, we propose a new distributed channel allocation method for small-cells with dynamic loads and define a dynamic interference graph. Based on this approach, we formulate the problem as a dynamic interference graph game and prove that the game is a potential game and has at least one pure strategy Nash equilibrium (NE) point. Moreover, we show that the best pure strategy NE point minimizes the expectation of the aggregate dynamic co-tier interference in the small-cell network. A distributed dynamic learning algorithm is then designed to achieve NE of the game, in which each SBS is unaware of the probability distributions of its own and other SBSs' dynamic loads. Simulation results show that the proposed approach can mitigate dynamic co-tier interference effectively and significantly outperform random channel selection.