Yongtaek Lee

Effects of External Humidification on the Performance of a Polymer Electrolyte Fuel Cell

The dryness of the membrane of a polymer electrolyte fuel cell (PEFC) decreases the ionic conductivity, resulting in performance reduction. In this study, the effects of external humidification to the membrane were investigated by varying the humidification side such as anode humidification, cathode humidification, and both anode and cathode humidification (called as both-side humidification). The amount of required water vapor into the gas was increased rapidly to maintain the relative humidity constant with the increase of cell temperature. The best performance of the cell was achieved by both-side humidification. However, as the humidity condition approached saturation state, anode humidification yielded comparable performance to both-side humidification. In anode humidification, the increase of the cell temperature degraded the performance, even though the amount of water supply to the membrane remained constant. At constant relative humidity conditions with anode humidification, the polarization curves of the PEFC were almost the same, regardless of the cell temperature when the relative humidity was higher than 60%.

Variation of Porosity and Gas Permeability of Gas Diffusion Layers Under Compression

Key Words: Gas Diffusion Layer(가스확산층), Proton Exchange Membrane Fuel Cell(PEMFC 고분자전해질료전지), Compression(압축), Porosity(공극률), Gas Permeability(기체투과율), Pore Network(공극네트워크) 초록: 본 연구는 높은 압력으로 체결되어 있는 고분자전해질연료전지(PEMFC) 스택의 구성부품중 가장 크게 변형되는 가스확산층(GDL)의 공극률과 기체투과율의 변화를 제시하였다. 압축하중에 따른 체적변화를 실험을 통하여 측정하고 기존에 제시된 관계식을 이용하여 공극률과 기체투과율의 변화을 예측하였다. 또한 물의 배출을 향상시키기 위하여 첨가되는PTFE 가 압축상태의 GDL 의 공극률과 기체투과율에 미치는 영향을 연구하였다. 물질전달에 직접 영향을 미치는 기체투과율은 PTFE 가 많이 포함된 GDL에서 급격하게 감소하였다. 결과적으로 같은 압축하중으로 체결하는 경우 GDL 의 PTFE 함량에 따라서 공극네트워크를 통한 물질전달은 크게 달라질 수 있다. 본 결과를 이용하면GDL 에서의 전달현상에 대한 개선된 상관식을 개발할 수 있고 그로 인하여 모델링의 정확성을 향상시킬 수 있다. Abstract: This study suggested the variations of porosity and gas permeability of gas diffusion layers (GDLs), which are easily deformed among the components of a highly compressed PEMFC stack. The volume change owing to compression was measured experimentally, and the variations in the porosity and gas permeability were estimated using correlations published in previous literature. The effect of polytetrafluoroethylene (PTFE) which is added to the GDLs to enhance water discharge was investigated on the variations of porosity and gas permeability. The gas permeability which strongly affects the mass transport through GDL, decreases sharply with increasing compression when the GDL has high PTFE loading. As a result, the mass transport through the pore network of GDL can be changed considerably according to the PTFE loading even with the same clamping force. The accuracy of modeling of transport phenomena through GDL can be improved due to the enhanced correlations developed based on the results of this study.

Condensation Heat Transfer Characteristics of R-134a with Wall Thickness and Surface Roughness on Stainless Steel Horizontal Plain Tubes

The filmwise condensation heat transfer coefficients of R-134a on the horizontal copper and stainless steel tubes were measured and analyzed. The outside diameter of the tubes was 15.88 mm, and the tube thickness ranged from 0.89 to 1.65 mm. The polished stainless steel tube had an RMS surface roughness() of 0.37 m, and commercial stainless steel tubes had an surface roughness() of 1.855 m. The tests were conducted at the saturation temperatures of 20 and , and the liquid wall subcoolings from 0.4 to . The measured condensation heat transfer coefficients were significantly lower than the predicted data by the Nusselt analysis. This trend in the stainless steel tube was explained by the effects of thermal resistance of tube material and surface roughness. Based on the experimental data with respect to wall thickness and surface roughness, it was suggested that the existing correlation on external condensation should be modified by considering material and surface roughness factors. The revised correlation was developed by introducing the effects of wall thickness and surface roughness into the Nusselt equation. The average deviation of the revised correlation was 13.0 %.

Numerical Study on the Cooling Characteristics of a Passive-Type PEMFC Stack

Key Words: PEMFC(고분자전해질연료전지), Passive Type Fuel Cell(수동공기공급형연료전지), TemperatureDistribution(온도분포), Air Flow Rate(공기유량), Channel Configuration(채널형상)초록: 수동공기공급형고분자전해질연료전지는팬을이용하여주변의공기를스택에공급한다. 공급된공기는연료로쓰이는동시에스택의냉각에도사용된다. 이러한방식은시스템에서가습기, 공기압축기, 냉각수설비를제거할수있어서시스템을단순화시키고경량화시킬수있는반면냉각성능은기존의냉각수를이용하는방식에비하여떨어진다. 따라서시스템의신뢰성확보를위하여최적의냉각성능을낼수있도록스택을설계하는것이중요하다. 본연구에서는고분자전해질연료전지스택의냉각성능향상을위하여다양한채널형상, 공기극의유량분포, 외부대류열전달계수의변화가스택의온도분포에미치는영향에대한전산해석을수행하였다. 그결과, 채널의rib이두꺼운경우에냉각성능이가장뛰어났으며유량을중앙부에집중시킨경우에고온집중현상이감소하였다.Abstract: In a passive-type PEMFC stack, axial fans operate to supply both oxidant and coolant to cathode side of thestack. It is possible to make a simple system because the passive-type PEMFC stack does not require additional coolingequipment. However, the performance of a cooling system in which water is used as a coolant is better than that ofthe air-cooling system. To ensure system reliability, it is essential to make cooling system effective by adopting anoptimal stack design. In this study, a numerical investigation has been carried out to identify an optimum coolingstrategy. Various channel configurations were applied to the test section. The passive-type PEMFC was tested byvarying airflow rate distribution at the cathode side and external heat transfer coefficient of the stack. The best coolingperformance was achieved when a channel with thick ribs was used, and the overheating at the center of the stack wasreduced when a case in which airflow was concentrated at the middle of the stack was used.

Study on the Steady-State and Dynamic Performance of Polymer Electrolyte Fuel Cells with the Changes of External and Self-Humidification Conditions

The performance characteristics of the polymer electrolyte fuel cells (PEFCS) were investigated under various humidification conditions at steady-state and transient conditions. The PEFC studied in this study was characterized by I-V curves in the potentiostatic mode and EIS (electrochemical impedance spectroscopy). The I-V curves representing steady-state performance were obtained from OCV to 0.25 V, and the dynamic performance responses were obtained at some voltages. The effects of anodic external humidification were measured by varying relative humidity of hydrogen from 20% to 100% while dry air was supplied in the cathode. At the high voltage region, the performance became higher with the increase of the temperature, while at the low voltage region, the performance decreased with the increase of temperature. The EIS showed that ohmic losses were larger at the dry condition of membrane and the effects of mass transport losses increased remarkably when the external and self-humidification were high. The dynamic responses were also monitored by changing the voltage of the PEFC instantly. As the temperature increased, the current reached steady-state earlier. The self-humidification with the generated water delayed the stabilization of the current except for low voltage conditions.