• Transactions of the Korean hydrogen and new energy society
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• v.23 no.1
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• pp.8-18
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• 2012

This paper presents a three-dimensional, transient cold-start polymer electrolyte fuel cell (PEFC) model to numerically evaluate the effects of membrane electrode assembly (MEA) design and cell location in a PEFC stack on PEFC cold start behaviors. The cold-start simulations show that the end cell experiences significant heat loss to the sub-freezing ambient and thus finally cold-start failure due to considerable ice filling in the cathode catalyst layer. On the other hand, the middle cells in the stack successfully start from $-30^{\circ}C$ sub-freezing temperature due to rapid cell temperature rise owing to the efficient use of waste heat generated during the cold-start. In addition, the simulation results clearly indicate that the cathode catalyst layer (CL) composition and thickness have an substantial influence on PEFC cold-start behaviors while membrane thickness has limited effect mainly due to inefficient water absorption and transport capability at subzero temperatures.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.43 no.5
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• pp.693-702
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• 1994

This paper deals with direct voltage stability analysis using a power system energy function. The structure preserved energy function is proposed as an energy function for voltage stability analysis. With the use of the proposed energy function voltage collapse conditions are derived, which yields the exactly same results with the Jacobian matrix approach. The voltage collapse phenomenon is analyzed by several methods, which shows that all of the methods produce the same voltage condition. This study also investigates the voltage collapse dynamics by using the proposed energy function. As a result, it has been found that the voltage collapse can be classified into two categories: static and dynamic instablilties which have quite different behaviors. In addition a new method is presented to calculate the power capacity limit of transmission lines with respect to voltage stability. The proposed method is tested for a 2-bus sample system, which shows the characteristics of voltage collapse phenomenon via the energy function.

• Nuclear Engineering and Technology
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• v.46 no.5
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• pp.681-688
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• 2014

Hydride reorientation behaviors of PWR cladding tubes under typical interim dry storage conditions were investigated with the use of as-received 250 and 485ppm hydrogen-charged Zr-Nb alloy cladding tubes. In order to evaluate the effect of typical cool-down processes on the radial hydride precipitation, two terminal heat-up temperatures of 300 and $400^{\circ}C$, as well as two terminal cool-down temperatures of 200 and $300^{\circ}C$, were considered. In addition, two cooling rates of 2.5 and $8.0^{\circ}C/min$ during the cool-down processes were taken into account along with zero stress or a tensile hoop stress of 150MPa. It was found that the 250ppm hydrogen-charged specimen experiencing the higher terminal heat-up temperature and the lower terminal cool-down temperature generated the highest number of radial hydrides during the cool-down process under 150MPa hoop tensile stress, which may be explained by terminal solid hydrogen solubilities for precipitation, and dissolution and remaining circumferential hydrides at the terminal heat-up temperatures. In addition, the slower cool-down rate generates the larger number of radial hydrides due to a cooling rate-dependent, longer residence time at a relatively high temperature that can accelerate the radial hydride nucleation and growth.

• Proceedings of the IEEK Conference
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• 2001.10a
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• pp.742-745
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• 2001

Plastics are one of difficult materials for recycling due to their characteristics in use. Recycling ratio of waste plastics was around 40% in last year in Japan, which includes energy recovery. Feed stock recycling and mechanical recycling are not easy because of additives in commercial plastics. Then, pyrolysis treatments have been done to recovery energy. Although plastics are easy to fire, complete combustion of them is not easy if anti-firing agents are added especially. Therefore, researches on pyrolysis or combustion behaviors of plastics containing additives are important from a view point recycling of plastics. Direct observation of popular plastics like polystyrene (PS), polycarbonate (PC), polyphenyle ether (PPE) and polyvinyl chloride (PVC) to investigate their pyrolysis behaviors in the present study. In case of PS, melting and gas evolution started at 9$0^{\circ}C$ and 39$0^{\circ}C$ respectively. And combustion finished at 445$^{\circ}C$. On the other hand, more than $600^{\circ}C$ and sufficient oxygen are required for complete combustion of PC and PPE.

• Computers and Concrete
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• v.10 no.2
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• pp.173-196
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• 2012

Experimental results of cyclic reversed lateral force test on a two-story reinforced concrete shear wall sub-assemblage are simulated analytically by using the PERFORM-3D program. A comparison of experimental and analytical results leads to the following conclusions: (1) "Shear Wall" and "General Wall" models with "Concrete shear" cannot simulate the pinching phenomena due to shear and show larger amounts of inelastic energy absorption than those in the experiment. (2) Modeling a story-height wall by using two or more "General Wall" elements with "Diagonal shear" in the vertical direction induces the phenomenon of swelling-out at the belly, leading to the erroneous simulation of shear behaviors. In application to tall building structures, it is recommended to use one element of "General Wall" with "Diagonal shear" for the full height of a story. (3) In the plastic hinge area, concrete deformations of analytical models overestimate elongation and underestimate shortening when compared with experimental results.

• Rapid Communication in Photoscience
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• v.2 no.1
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• pp.9-17
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• 2013

Visible light-sensitive $TiO_2$ and ZnO nanostructure materials have attracted great attention as the promising material for solar energy conversion systems such as photocatalysts for water splitting and environmental purification as well as nano-biosensors. Success of their applications relies on how to control their surface state behaviors related to the exciton dynamics and optoelectronic properties. In this paper, we briefly review some recent works on single nanoparticle photoluminescence (PL) technique and its application to observation of their surface state behaviors which are raveled by the conventional ensemble-averaged spectroscopic techniques. This review provides an opportunity to understand the temporal and spatial heterogeneities within an individual nanostructure, allowing for the potential use of single-nanoparticle approaches in studies of their photoenergy conversion and nano-scale optical biosensing.

• Journal of the Korean Solar Energy Society
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• v.30 no.4
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• pp.71-78
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• 2010

Investigation of the effective soil thermal conductivity(k) is the first step in designing the ground loop heat exchanger(borehole) of a geothermal heat pump system. The line source method is required by New and Renewable Energy Center of Korea Energy Management Corporation in analyzing data obtained from thermal response tests. Another important factor in designing the ground loop heat exchanger is the borehole thermal resistance($R_b$). There are two methods to evaluate $R_b$ : one is to use a line source method, and the other is to use a shape factor of the borehole. In this study, we demonstrated that the line source method produces better results than the shape factor method in evaluating $R_b$. This is because the borehole thermal resistance evaluated with the line source method characteristically reduces the temperature differences between an actual and a theoretical thermal behaviors of the borehole. Evaluation of $R_b$ requires soil volumetric heat capacity. However, the effect of the soil volumetric heat capacity on the borehole thermal resistance is very small. Therefore, it is possible to use a generally accepted average value of soil volumetric heat capacity($=2MJ/m^3{\cdot}K$) in the analysis. In this work, it is also shown that an acceptable range of the initial ignoring time should be in the range of 8~16hrs. Thus, a mean value of 12 hrs is recommended.

• Journal of information and communication convergence engineering
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• v.5 no.2
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• pp.104-108
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• 2007

Ad hoc networks are a type of mobile network that function without any fixed infrastructure. One of the weaknesses of ad hoc network is that a route used between a source and a destination is to break during communication. To solve this problem, one approach consists of selecting routes whose nodes have the most stable link cost. This paper proposes a method for improving the low power distributed MAC. This method is based on the context awareness of the each nodes energy in clustering. We propose to select a new scheme to optimize energy conserving between the clustering nodes in MANET. And this architecture scheme would use context-aware considering the energy related information such as energy, RF strength, relative distances between each node in mobile ad hoc networks. The proposed networks scheme could get better improve the awareness for data to achieve and performance on their clustering establishment and messages transmission. Also, by using the context aware computing, according to the condition and the rules defined, the sensor nodes could adjust their behaviors correspondingly to improve the network routing.

• Composites Research
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• v.12 no.5
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• pp.54-64
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• 1999

This study investigated the impact behaviors of the stitched sandwich composites under the low energy impact by the use of drop weight impact tester. These sandwich composites condidted of the glass fabric faces with a urethane foam core. The upper face and the lower face were stitched to combinr through the core thickness direction using the polyester reinforcements. Four types of the stitched sandwich composites, each having a different core thickness, were tested to determine the effects of the core thickness. The impact conditions were changes with the variations of the mass and drop height of the impact tup. The test results showed that the core thickness and the impact condidtions such as the drop height and the mass of the impact tup affected the impact force, the contact time, and the strain behaviors of the stitched sandwich composites. The stitched sandwich composites are able to avert the damage and also maintain the structural integrity even thouth the presence of the damage owing to the through-the-thickness reinforcements. However, it is important to improve the wetting ability of the stitched reinforcements so that the conventional structures are substituted for the stitched sandwich composites effectively.

• ETRI Journal
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• v.37 no.1
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• pp.197-202
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• 2015

A smart grid is a modernized electrical grid that uses information about the behaviors of suppliers and consumers in an automated fashion to improve the efficiency, reliability, economics, and sustainability of the production and distribution of electricity. In the operation of a smart grid, demand side management (DSM) plays an important role in allowing customers to make informed decisions regarding their energy consumption. In addition, it helps energy providers reduce peak load demand and reshapes the load profile. In this paper, we propose a new DSM scheduling scheme that makes use of the day-ahead pricing strategy. Based on the Rubinstein-Stahl bargaining model, our pricing strategy allows consumers to make informed decisions regarding their power consumption, while reducing the peak-to-average ratio. With a simulation study, it is demonstrated that the proposed scheme can increase the sustainability of a smart grid and reduce overall operational costs.