고분자 전해질막 연료전지의 무가습 운전 조건에서 성능 향상에 관한 연구

학위논문 (석사)-- 서울대학교 대학원 : 기계항공공학부, 2014. 2. 김민수.As depleting fossil fuels and environmental problems like global warming have been arisen, needs for eco-friendly and renewable energy have been increased. Fuel cell is one of the most famous renewable energy. And it is considered as an alternative energy to fossil fuels. Fuel cell generates electricity by chemical reaction of fuel and oxidant. There are different types of fuel cells such as Polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and Solid oxide fuel cell (SOFC). Above all, polymer electrolyte membrane fuel cell (PEMFC) is considered as an alternative to internal combustion engine because its advantages like relatively low operation temperature, short start-up time, and quick response etc. However, there are some problems to replace internal combustion engine with PEMFC. Too many auxiliary components (Balance of plant) are one of the crucial problems of fuel cell systems, because it takes a large volume of the system. Balance of plants (BOP) is composed of hydrogen provide system, air provide system, cooling system and water management system. Removing the external humidifier is one of the solutions to simplify the BOP system. In PEM fuel cell reaction, pure water is generated. To remove the external humidifier, fuel cell has to use this generated water to humidify membrane of fuel cell because the hydration level of membrane is crucial to the performance of fuel cell. Water inside the membrane transmit proton from anode to cathode. So, hydration level of membrane is directly related to the proton conductivity of membrane. Therefore, the ohmic loss of fuel cell increases when the hydration level of membrane decreases. Generally, when dry air and fuel is fed to the fuel cell, performance of fuel cell deteriorates because the ohmic loss of fuel cell increases. So, generated water management is very important to enhance the performance of fuel cell when dry air and fuel is fed to the fuel cell. So far, lots of studies about fuel cell with non-humidified are conducted by many researcher. Because the deterioration of performance which is caused by low hydration level of membrane is the main issue, many researches are conducted to enhance the performance of fuel cell when it performs under non-humidified conditions. To improve the performance of fuel cell without external humidifier, many researchers conduct experimental studies about hydrophilic micro porous layer (MPL). However, improving the hydrophilicity can cause serious problem, flooding, because MPL is very tiny and thin porous material. In this study, by stacking two gas diffusion layers (GDL), we suggest new type of GDL assembly. Because this double layer GDL has tiny water trap between two GDL, it can retain more generated water than typical GDL. And also, we set relatively low porous GDL to channel side. This method makes generated water hard to penetrate from MPL to channel. By using this double layer GDL, water retention of GDL increases, then hydration level of membrane also increases. Such results improve the performance of fuel cell by enhancing proton conductivity of membrane. As mentioned above, the performance of fuel cell when we supply dry inlet gas is improved by applying double layer GDL. However, the performance of non-humidified fuel cell is lower than fully humidified fuel cell despite double layer GDL is applied to non-humidified fuel cell. So, pressurized fuel cell system is applied to non-humidified conditions. By using back pressure regulator, increases the pressure of inlet gas which is fed to fuel cell. When we applied pressurized system to non-humidified conditions, the results show that the performance of fuel cell increases. As the pressure of the inlet gas of fuel cell increases 1 atm to 2 atm, the performance improve to fully humidified system. As a result, as we applied double layer GDL and pressurized system, we could get a satisfactory performance of fuel cell. From this result, fuel cell could operate without external humidifier by applying double layer GDL and pressurized system.Abstract i Contents iv List of Tables vi List of Figures vii Nomenclatures ix Chapter 1. Introduction 1 1.1 Background of the study 1 1.2 Literature review 5 1.3 Objectives and scope of the study 9 Chapter 2. Experimental set up and test conditions 11 2.1 Introduction 11 2.2 Single fuel cell 11 2.3 Novel gas diffusion layer 13 2.4 Experimental set up and test conditions 15 Chapter 3. Experimental study on the PEMFC performanc under Non-humidified conditions with novel GDL 19 3.1 Introduction 19 3.2 Experimental conditions and method 21 3.2.1 I-V performance of non-humidified PEMFC 21 3.2.2 Effect of novel gas diffusion layer 25 3.2.3 Reason of performance improvement 28 3.2.4 Additional experiment for novel GDL 31 3.3 Comparison of I-V performance 41 3.4 Summary 46 Chapter 4. Experimental study on the PEMFC performanc with non-humidified and pressurized conditions 48 4.1 Introduction 48 4.2 Experimental conditions and method 48 4.2.1 I-V performance of pressurized PEMFC under non humidified condition 49 4.2.2 I-V performance with operating condition 53 4.2.3 HFR measurement 56 4.2.4 Final results of experiment 60 4.3 Summary 63 Chapter 5. Conclusion 64 References 66 Abstract (Korean) 69Maste

양극 유로 개선을 통한 고분자 전해질막 연료전지의 성능 향상에 관한 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 기계항공공학부, 2018. 2. 김민수.Developing high performance polymer electrolyte membrane fuel cell (PEMFC) is now considered as the one of the most important key to make PEMFC commercialized. Although three losses which are activation losses, ohmic losses, and concentration losses occur when PEMFC operates, mitigating the concentration losses has a great effect on the overall performance of PEMFC because it is dominant losses in high load region. Since the concentration losses of PEMFC are caused by two factors oxygen depletion and flooding at the cathode side, keeping PEMFC away from those problems can be a solution for reducing the concentration losses. PEMFC is composed of many components such as membrane electrode assembly (MEA), gas diffusion layer (GDL), gasket, bipolar plate, and end plate. Among the components, bipolar plate has a great effect on the performance related with concentration losses because it has a flow field where providing gases and produced water flow. Therefore, developing novel flow field on the bipolar plate should be conducted to make performance improved. In this study, two novel flow fields were suggested to get a better performance. The performance improvement was verified and the factors of enhancement were analyzed. The characteristics and analysis of those two flow fields will be discussed. First of all, an inclined channel which has a gradually tapered depth was suggested as a new flow field. Unlike typical channel, the inclined channel has a different inlet and outlet depth. We designed three types of inclined channels which has the same inlet depth but different outlet depth. Therefore, the inclined channels which have different steepness level of channel depth were used as a new flow field at cathode side. Then, the performance of PEMFC was evaluated by using polarization curves and power curves. Moreover, electrochemical impedance spectroscopy (EIS) method also was used to investigate the losses of PEMFC when it operated with the inclined channels. Pressure drop between inlet and outlet of fuel cell is one of parameters we should consider when fuel cell operates because it is related with power consumption of balance of plant (BOP) in PEMFC system. Therefore, pressure drop of PEMFC with inclined channels also measured and compared with conventional channel. Then, we verified that the performance of PEMFC was improved by about 27.9% with inclined channel. Furthermore, numerical analysis was conducted to investigate the effects of inclined channel to performance of PEMFC. To verify the effect of inclined channel on the mitigating flooding, numerical analysis for droplet dynamics was conducted. Moreover, numerical analysis for comparing oxygen concentration along the different channels was also done to explain the effect of inclined channel on the concentration losses decreasing. As a result, we could verify the effect of inclined channel on the PEMFC performance and explain how the inclined channel make PEMFC improved. The other novel flow field suggested for developing high performance PEMFC is porous flow field by using metal foam as a cathode flow field. In case of conventional flow field, gases and liquid water are supposed to flow in certain channel. However, this conventional channel has problems such as flooding and oxygen depletion since produced liquid water can block the channel and diffusion area where gases diffuse to MEA. To solve these problems, several types of metal foams ware inserted into cathode bipolar plate. Then, the effect of metal foam to the performance of PEMFC and characteristics of each metal foam were investigated. As a result, we found that the maximum power of PEMFC increased about 50.6% when we replaced conventional channel with proper metal foam as a cathode flow field for PEMFC. Moreover, operating characteristics of PEMFC with different metal foams was investigated by polarization curve, EIS method, and stability test. Eventually, novel flow field called mixed metal foam flow field was suggested through the results of investigation. The mixed metal was made by combining two different metal foams which have different cell size. Since metal foams show different advantage with respect to electric conductivity and oxygen diffusion, we placed a metal foam which has a large contact surface area to the upstream region of flow field. Then, metal foam which has a large diffusion region was placed to the downstream region. Finally, we could get about 60.1% of maximum power increase when novel mixed metal foam was used as a flow field to the cathode side of PEMFC. Furthermore, additional experiments were conducted to verify the effect of metal foam on the water management in PEMFC and compare diffusion between conventional channel and metal foam flow field.Chapter 1. Introduction 1 1.1 Background 1 1.2 Literature survey 4 1.2.1 Water management in PEMFC 4 1.2.2 Effects of channel geometry on PEMFC 5 1.2.3 Novel flow field for high performance PEMFC 7 Chapter 2. Experimental study on the PEMFC with inclined channel 9 2.1 Introduction 9 2.2 Experimental setup 12 2.2.1 PEMFC components 12 2.2.2 PEMFC system apparatus 12 2.2.3 Measruing devices 17 2.3 Preparation for experiment 20 2.3.1 Inclined channel design 20 2.3.2 Activation cycle 24 2.3.3 Experimental conditions 25 2.4 Results and discussion 27 2.4.1 PEMFC performance 27 2.4.1.1 Polarization curve 27 2.4.1.2 Power curve 30 2.4.2 Electrochemical impedance spectroscopy 32 2.4.3 Pressure drop 37 2.4.4 Net power 41 2.5 Summary 43 Chapter 3. Numerical analysis of PEMFC with inclined channel 44 3.1 Introduction 44 3.2 Effect of inlcined channel on the oxygen depletion 45 3.2.1 Governing equations 45 3.2.2 Air velocity analysis 49 3.2.3 Oxygen diffusion analysis 51 3.3 Effect of inlcined channel on the liquid water 54 3.3.1 Governing equations 54 3.3.2 Droplet deformation 60 3.3.2.1 Contact angle relationship 60 3.3.2.2 Center of mass 64 3.3.3 Droplet movement in channel 68 3.3.3.1 Droplet detachment 68 3.3.3.2 Droplet velocity 72 3.3.4 Pressure drop 88 3.3.4.1 Pressure drop analysis 88 3.3.4.2 Water film effect 92 3.5 Summary 77 Chapter 4. Experimental study on the performance improvement of PEMFC with metal foam flow field 79 4.1 Introduction 79 4.2 Prepraration for experiement 80 4.2.1 Metal foam 80 4.2.2 Experimental setup 84 4.2.3 Experimental conditions 86 4.3 Results and discussion 88 4.3.1 PEMFC performance 88 4.3.1.1 Polarization curve 88 4.3.1.2 Power density curve 91 4.3.2 Electrochemical impedance spectroscopy 94 4.3.3 Pressure drop 99 4.3.4 Stability comparison 102 4.4 Summary 107 Chapter 5. Advanced experimental study on the investigation of metal foam flow field 108 5.1 Introduction 108 5.2 Mixed metal foam flow field 110 5.2.1 Charateristics of metal foam cell size 110 5.2.2 Fabrication of mixed metal foam 117 5.2.3 Performance of PEMFC with mixed metal foam flow field 119 5.3 Visualization of metal foam flow field 124 5.3.1 Design of transparent cell 124 5.3.2 Experimental conditions 126 5.3.3 Results and discussion 128 5.4 Summary 130 Chapter 6. Concluding remarks 131 References 135 Abstract (in Korean) 147Docto

중복을 허용하는 분산 데이터베이스에서 데이터 분할의 일관성 유지에 관한 연구

학위논문(석사)--서울대학교 대학원 :산업공학과,1997.Maste

新都市 單獨住宅地 都市設計의 成果에 관한 硏究 : 盆唐 新都市를 中心으로

학위논문(석사)--서울大學校 環境大學院 :環境造景學科,1995.Maste

A study on the development of a flat-panel X-ray imaging system

의공학과/박사[한글] 평판 검출기를 기반으로 하는 디지털 방사선 촬영 시스템은 기존의 스크린-필름 시스템 보다 영상의 질이 우수하고, 촬영 즉시 출력이 가능하며, 환자의 피폭량을 감소시켜 주고, 방사선과의 작업 흐름을 개선 시켜 주는 등의 많은 장점을 가지고 있다.본 연구에서는 비정질 실리콘을 기반으로 하는 평판 X-선 검출기를 사용하여 평판 X-선 영상 시스템을 통합하고, 시스템을 운용할 수 있는 소프트웨어를 개발하였다. 평판 검출기를 캘리브레이션 하기 위한 소프트웨어를 개발하고, 평판 X-선 영상 보정을 위한 알고리듬들을 구현하였다. 영상 보정의 정확도를 높이기 위하여 flat-field 보정 방법에 옵셋의 비선형 보정을 추가하고 이를 위한 알고리듬을 개발하였고, 평판 검출기 및 영상 보정 알고리듬의 성능 평가를 위하여 modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE) 등을 측정하였다. 또한 영상의 대조도 향상을 위하여 unsharp masking을 기반으로 하는 다중스케일 비선형 증폭 알고리듬을 제안하였다. 이 알고리듬은 라플라스 피라미드를 기본으로 하여 하나의 영상을 7개 이상의 층으로 분해하고, 각 층의 계수들을 비선형 함수를 사용하여 적정한 수준으로 강화시키며, 잡음 억제 알고리듬을 포함하고 있어서 낮은 농도 영역의 잡음을 적정 수준으로 제한시켜 준다. 이를 임상 데이터에 적용한 결과 과다한 잡음 증가 없이 세부 구조의 가시화가 향상되었다.개발된 평판 X-선 영상 시스템을 통하여 영상을 획득한 결과 피라미드 기반의 다른 알고리듬들과 유사한 수준의 훌륭한 질의 영상을 얻을 수 있었고, 임상적으로 사용이 가능할 것으로 기대된다. [영문]A digital radiography imaging system based on flat-panel detector (FPD) has some advantages compared with the conventional screen-film system. It provides equal or superior image quality, an instant image display, reduced radiation dose for the detection of a patient, and a facilitation of the work flow in the radiology department.In this study, a flat-panel detector based on amorphous silicon was integrated into a projection radiography system and a Windows application that can operate the system was developed. A calibration software to the flat-panel detector was developed and algorithms for the correction was implemented. A nonlinear offset correction was added to the flat-field correction way in order to raise accuracy of the correction. Basic image quality parameters such as modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) were measured for the system evaluation. Also, a multi-scale nonlinear amplification algorithm based on unsharp masking was proposed for an enhancement of the contrast. The proposed algorithm based on the Laplacian Pyramid, decomposed one image to the layers more than seven and the coefficients of the each layer was amplified with a nonlinear function. It also imported a noise containment algorithm to limit noise amplification. As a result of having applied to some clinical data, a detail visibility was improved significantly without unacceptable noise boosting.Images that acquired with the developed system are superior quality to conventional screen-film systems as those that applied other algorithms based on the pyramid methods, and expected to be accepted in the clinical applications.ope

Determination of crystal structures and conformational analyses of the (4S, 6S) and (4S*,6R*) forms of 3-(4-Chlorophenyl)-6-methyl-6-(phenoxymethyl)-4-(1H-1,2,4,-triazol-1-yl)-5,6-dihydro-4H-1,2-oxazine

Thesis (master`s)--서울대학교 대학원 :화학과 물리화학전공,1998.Maste