Lim S.,Republic of Korea Naval Academy |
Lim S.,Seoul National University |
Jang C.J.,Korea Ocean Research and Development Institute |
Oh I.S.,Seoul National University |
Park J.,Woods Hole Oceanographic Institution
Journal of Marine Systems | Year: 2012
A climatology for the mixed layer depth (MLD) in the East/Japan Sea was produced using temperature profile data collected from 1931 to 2005. MLD is defined as the depth at which the temperature differs from that at 10. m depth by 0.2. °C. It varies seasonally with a range of about 20. m, a minimum, near the subpolar front (SPF, 38°-41°N), 60-100. m in south of 38°N and north of 41°N, and about 200. m near the winter convection region (132°-135°E & 41°-43°N). The weaker seasonality near the SPF seems to result from year-round strong stratification sustained largely by advected warm water of the East Korean Warm Current and a result of complex dynamic process of frontogenesis including lateral dynamics or wind-induced friction. The temperature-based MLD does not show any significant difference (mostly less than 20. m) from the density-based values over most of the East/Japan Sea except a few localized regions near the Russian and Japanese coasts where barrier layers form from late fall through early spring. This study confirms that atmospheric forcing largely dominates the overall magnitude of MLD seasonal variability in most of the East/Japan Sea. © 2012 Elsevier B.V.
PubMed | Republic of Korea Naval Academy, Kookmin University, Kangwon National University and Seoul National University
Type: | Journal: Scientific reports | Year: 2015
Although numerous reports on nonprecious metal catalysts for replacing expensive Pt-based catalysts have been published, few of these studies have demonstrated their practical application in fuel cells. In this work, we report graphitic carbon nitride and carbon nanofiber hybrid materials synthesized by a facile and gram-scale method via liquid-based reactions, without the use of toxic materials or a high pressure-high temperature reactor, for use as fuel cell cathodes. The resulting materials exhibited remarkable methanol tolerance, selectivity, and stability even without a metal dopant. Furthermore, these completely metal-free catalysts exhibited outstanding performance as cathode materials in an actual fuel cell device: a membrane electrode assembly with both acidic and alkaline polymer electrolytes. The fabrication method and remarkable performance of the single cell produced in this study represent progressive steps toward the realistic application of metal-free cathode electrocatalysts in fuel cells.
Oh H.,KAIST |
Song S.,Republic of Korea Naval Academy |
Ahn S.,KAIST |
2015 IEEE 4th Global Conference on Consumer Electronics, GCCE 2015 | Year: 2015
There has been a dramatic increase in the use of XML data to deliver information over the Web. Many Web sites are now publishing feeds (most commonly known as RSS; often called really simple syndication) for their subscribers. To improve performance of content syndication system, history based pattern learning approaches are mainly used. This paper proposes a new feed querying scheme which is used in web feed aggregator. Heuristic approach is adopted to handle various feed publishing pattern. The simulation confirms that proposed scheme shows performance improvements in both item missing probability and post-it delay compared with conventional studies. © 2015 IEEE.
PubMed | Republic of Korea Naval Academy, Kangwon National University, Pohang University of Science and Technology and Korea Basic Science Institute
Type: Journal Article | Journal: ACS applied materials & interfaces | Year: 2015
Core-shell structure nanoparticles have been the subject of many studies over the past few years and continue to be studied as electrocatalysts for fuel cells. Therefore, many excellent core-shell catalysts have been fabricated, but few studies have reported the real application of these catalysts in a practical device actual application. In this paper, we demonstrate the use of platinum (Pt)-exoskeleton structure nanoparticles as cathode catalysts with high stability and remarkable Pt mass activity and report the outstanding performance of these materials when used in membrane-electrode assemblies (MEAs) within a polymer electrolyte membrane fuel cell. The stability and degradation characteristics of these materials were also investigated in single cells in an accelerated degradation test using load cycling, which is similar to the drive cycle of a polymer electrolyte membrane fuel cell used in vehicles. The MEAs with Pt-exoskeleton structure catalysts showed enhanced performance throughout the single cell test and exhibited improved degradation ability that differed from that of a commercial Pt/C catalyst.
Kim S.-Y.,Republic of Korea Naval Academy |
Choe S.,Seoul National University |
Ko S.,Samsung |
Sul S.-K.,Seoul National University
IEEE Electrification Magazine | Year: 2015
Mechanical machinery has dominated the field of ship propulsion systems in the last century. Since fuel accounts for a large portion of the ship life-cycle cost, the current trend of moving toward fuel economy and environmental regulations in marine engineering has led to the efficient use of onboard energy. Incorporating power electronics into ship propulsion systems has been a very common method of improving fuel efficiency since approximately 1980. One high-profile example is the 1968-built Queen Elizabeth 2, whose steam turbines were replaced with a diesel-electric propulsion plant in 1986. Some commercial ships are now being built with electric drives, such as passenger ships, car ferries, shuttle tankers, cable-laying ships, icebreakers, and floating offshore platforms. Navies around the world have been actively exploring integrated power systems (IPSs) for use on future surface combatant ships since the late 20th ?century. Although the ship IPS has many distinct advantages, such as increased survivability and maneuverability, reliability inhibits further fuel economy improvement. The onboard battery energy storage system (BESS) was recently suggested to increase fuel economy and ensure reliability at the same time. Furthermore, an active front-end (AFE) converter can provide improved grid quality at transient states such as fault and future mission load in addition to steady state as with normal sea sailing. © 2015 IEEE.
Lee H.,Seoul National University |
Lee H.,Republic of Korea Naval Academy |
Kim G.,Seoul National University |
Kim J.,Seoul National University |
And 2 more authors.
Geophysical Research Letters | Year: 2014
We measured Ra isotopes (223Ra and 228Ra) in surface seawater of the northwestern Pacific marginal seas to trace the flow rate and mixing of the Changjiang diluted water (CDW) in the summer of 2012. Based on the horizontal distribution of 223Ra activities, the arrival time of CDW from the river mouth to 450 km offshore northeast was estimated to be 20-35 days, which is similar to that determined in previous studies. Moreover, we successfully calculated the relative contribution of CDW at each sampling station using a salinity and 228Ra diagram. Using this unique method, we found that the relative contribution of CDW was more than 30% in most surface seawaters of the northern East China Sea, the Yellow Sea, and the southern sea off Korea. Our results suggest that CDW is of high significance in the biogeochemistry of surface seawater of these northwestern Pacific marginal seas during the summer monsoon period. Key Points The flow rate of CDW is determined using 223Ra as a tracer The relative contribution of CDW is determined using 228Ra and salinity CDW is dominant in large surface areas of northwestern Pacific marginal seas ©2014. American Geophysical Union. All Rights Reserved.
Lim T.,University of Texas at Austin |
Kim O.-H.,Republic of Korea Naval Academy |
Sung Y.-E.,Seoul National University |
Kim H.-J.,Korea Institute of Industrial Technology |
And 3 more authors.
Journal of Power Sources | Year: 2016
Onion-like Pt-terminated Pt-Cu bimetallic nano-sized electrocatalysts (Pt/Cu/Pt/C) were synthesized by using an electroless deposition method. The synthesized Pt/Cu/Pt/C consisted of a Pt-enriched shell, a sandwiched Pt-Cu alloy layer, and a Pt core. The Pt/Cu/Pt/C showed higher electrocatalytic activity toward oxygen reduction reaction in half-cell test than that of commercial Pt/C due to an electronic structure change in the Pt-enriched shell, resulting from the sandwiched Pt-Cu alloy layer underneath. The stability of the Pt/Cu/Pt/C was examined by using both half-cell and single-cell degradation tests. In both tests, the Pt/Cu/Pt/C exhibited stronger resistance to catalyst degradation than that of the commercial Pt/C. It is notable that cell performance with the Pt/Cu/Pt/C was fully recovered by N2 purging after single-cell degradation testing, indicating there was no permanent damage to the electrocatalyst during the test. It is suggested that thermodynamically-stable structure of the Pt/Cu/Pt/C contributed to the improved stability. © 2016 Elsevier B.V.
Lee S.-H.,Republic of Korea Naval Academy |
Joo Y.,Seoul National University |
Back J.,Kwangwoon University |
Seo J.-H.,Seoul National University |
Choy I.,Kwangwoon University
Journal of Power Electronics | Year: 2011
We propose a torque and pitch control scheme for variable speed wind turbines with permanent magnet synchronous generator (PMSG). A torque controller is designed to maximize the power below the rated wind speed and a pitch controller is designed to regulate the output power above the rated wind speed. The controllers exploit the sliding mode control scheme considering the variation of wind speed. Since the aerodynamic torque and rotor acceleration are difficult to measure in practice, a finite time convergent observer is designed which estimates them. In order to verify the proposed control strategy, we present stability analysis as well as simulation results.
Kim Y.K.,Korea Institute of Intellectual Property |
Lee K.,Seoul National University |
Park W.G.,American University of Washington |
Choo K.,Republic of Korea Naval Academy
Research Policy | Year: 2012
This paper examines how the role of patents and utility models in innovation and economic growth varies by level of economic development. Using a panel dataset of over 70 countries, we find that patent protection is an important determinant of innovation and that patentable innovations contribute to economic growth in developed countries, but not in developing. Instead, in developing economies, a minor form of intellectual property rights (IPRs) - namely utility models - is conducive to innovation and growth, controlling for other factors. Using Korean firm level data as a case study, we find that utility model innovations contribute to firm performance when firms are technologically lagging and that those minor innovations can be a learning device and thus a stepping stone for developing more patentable inventions later on. Upon reaching higher levels of technological capabilities, firms become more reliant upon patents and less on utility models. Thus the lesson here is that patent protection enhances innovation and economic growth in countries where the capacity to conduct innovative research exists. Where this capacity is weaker, a system that provides incentives to conduct minor, incremental inventions is more conducive to growth. The significance of this paper is to emphasize the importance not just of the strength of IPRs but of the appropriate type of IPRs for economic development. © 2011 Elsevier B.V.
Lee H.,Republic of Korea Naval Academy |
Jeong Y.,Republic of Korea Naval Academy
International Journal of Naval Architecture and Ocean Engineering | Year: 2012
This study presents the nano-sized particle emission characteristics from a small turbocharged common rail diesel engine applicable to prime and auxiliary machines on marine vessels. The experiments were conducted under dynamic engine operating conditions, such as steady-state, cold start, and transient conditions. The particle number and size distributions were analyzed with a high resolution PM analyzer. The diesel oxidation catalyst (DOC) had an insignificant effect on the reduction in particle number, but particle number emissions were drastically reduced by 3 to 4 orders of magnitude downstream of the diesel particulate filter (DPF) at various steady conditions. Under high speed and load conditions, the particle filtering efficiency was decreased by the partial combustion of trapped particles inside the DPF because of the high exhaust temperature caused by the increased particle number concentration. Retarded fuel injection timing and higher EGR rates led to increased particle number emissions. As the temperature inside the DPF increased from 25 °C to 300 °C, the peak particle number level was reduced by 70% compared to cold start conditions. High levels of nucleation mode particle generation were found in the deceleration phases during the transient tests. © SNAK, 2012.