The high-temperature fuel cell (MCFC, SOFC) is expected to be very effective power generation system in the future, due to its high efficiency, high-temperature waste heat utilization, quiet operation and low emission of pollutants to the environment. In this study, the molten carbonate fuel cell with large active area was analyzed to increase the stack stability and durability for the commercialization. In addition, the solid oxide fuel cell was simulated and experimented to investigate the transport phenomena and resolve the high temperature issues (flow uniformity, sealing, and stack technology). The followings are described with the respective subjects.
In the second chapter, three-dimensional numerical simulation of conservation equations for mass, momentum, energy, species and electrochemical reaction has been carried out to compare the performance characteristics of a Molten Carbonate Fuel Cell (MCFC) with three different flow types, co-flow, counter-flow and cross-flow. Depending on the flow types, distributions of pressure difference, temperature and current density in the electrolyte matrix were examined and the fractions of various losses were scrutinized. The simulation results show that the co-flow type has the lowest pressure difference across the matrix and the distributions of temperature and current density are more uniform than other types. However, it was found that since irreversible losses due to ohmic resistance, anode activation and cathode activation are smallest in the counter-flow type, best performance can be expected by the MCFC with the counter-flow type.
And in the third chapter, the present work is carried out to compare the temperature and thermal stress distributions in external and internal reforming MCFC systems, and to compare structural stability. A three-dimensional numerical analysis model, describing fluid flow, heat and mass transfer, and the chemical and electrochemical reaction processes, has been developed for the purpose of this study. The temperature distributions generated by the CFD code with the proposed analysis model are applied to calculate the thermal stress distribution in an MCFC unit cell by using finite element method (FEM). Through the present analysis, it was found that the heat flux pattern is significantly different between each other. The rate of heat transfer toward the anode side is slightly less than that toward the cathode sides in the external reforming(ER) MCFC. However, most of the internally generated heat is transferred toward the anode side due to the highly intensive endothermic process occurring in the internal reforming(IR) MCFC. The electric power of the IR-MCFC was 3.2% lower than that of the ER-MCFC, but its efficiency was 11% higher. Although IR-MCFC has slightly lower electrical power, since it has more uniform temperature and thermal stress distributions than the ER-MCFC, the IR-MCFC is mechanically more stable and thus extended lifespan can be expected for high temperature operation.
In the fourth chapter, Computational fluid dynamics (CFD) simulation are presented in this work for investigating the performance of a planar anode-supported solid oxide fuel cell (SOFC). This work is to simulate the current-voltage (I-V) characteristics. Compiled with the geometry of cell test housing, a 3D numerical model and test conditions were established to analyze the anode-supported cell (ASC) performance including current density and temperature distributions, fuel and oxidant concentration. The temperature profiles after calculating heat and mass transfer with the electrochemical model were applied to calculate the thermal stress distributions in a unit cell by using a three-dimensional (3D) FEA model. The constructed 3D FEA model consists of the complete components used in a practical SOFC cell, including anode electrode-electrolyte-cathode electrode (PEN) assembly and separator with plenum. This study would enhance the reliability of predicting potential failure locations in an SOFC cell. The effects of SOFC support condition, contact behavior of the components (anode-electrolyte-cathode-separator), temperature gradient, and thermal expansion mismatch between components were characterized. Based on the results, this modeling work will be implemented to analyze the distributions of fuel and oxidant gases for the SOFC stack, to minimize the thermal gradients inside the stack, and to optimize the manifold/flow passage in the future.
Next in the fifth chapter, Sealing of high-temperature fuel cells is a critical issue for maintaining the electrical performance of the fuel cell over a long period of time. Especially, thermal stresses due to repeated heating and cooling cycles can degrade or fracture seals. Long-time material interactions and corrosion can also degrade seals. These are ongoing problems for developers of high-temperature fuel cells that are used to generate electricity from hydrogen or hydrocarbon fuels. In this study, sealing efficiency and factor using hydrodynamics were induced for sealing design and control parameters. Two kinds of high-temperature gasket were evaluated to apply solid oxide fuel cell test. The gasket of Fuelcellmaterials made in U.S.A. had a better performance with respect to the sealing efficiency and sealing factor. Therefore, the SOFC unit cell test was performed with this gasket made in Fuelcellmaterials. The SOFC Cell made in Komico. Inc. was used for the SOFC unit cell test. In the experimental results, the deviation of temperature distribution was under 5 ℃, therefore the inner temperature distribution of the SOFC cell frame was subject to the furnace heat control. In addition, the experimental results with the high flow rate had a high performance compared with the low flow rate because the high flow rate could have an enough reduction process of the electrodes. Comparing with before and after the experimental observation of the SOFC cell, the appearance of SOFC cell after the experiment had fracture lines due to the residual stresses induced by the thermal cycles as mentioned above.
Finally in the sixth chapter, It is necessary that the fuel cell as an available actual power system develop the stack technology. Therefore, in this study an 100 Watt Planar SOFC Stack Module(PSSM) was designed and investigated. The PSSM was consisted of the stack with the SOFC unit cells and the stack structure with base station, manifold, end plate, and vessel. The 100 Watt class PSSM composed of 4 unit cells was induced by the experimental results extrapolated from the SOFC unit cell test. To investigate the conceptual design of the PSSM, the heat and flow analysis was executed. The separator, designed for both a current collector and a gas channel, is comprised of repeated shapes. Since the boundary of the flow passage is periodic both in stream-wise and transverse directions, only a small part of the flow channel was simulated. The computational results for the flow resistance can be expressed by the following Darcy’s Law in the case of simple homogeneous porous media. These calculation results from the separator flow analysis was used in the vessel flow and stack analysis. The vessel flow in the cathode flow region was analyzed to verify the flow uniformity in the cathode channel of the separator. In addition, the stack analysis with electrochemical reaction model was executed to investigate the performance and the transport phenomena of the PSSM. Due to the flow and temperature uniformity, the respective SOFC cell had similar contours of reactant gases, temperature, and current density. In the case of two different flow rates, It was expected that the high flow rate had a little higher performance comparing with two fold higher pressure drop than the low flow rate.
대면적(1mx1m) MCFC 단위전지에 대한 3가지 유동 패턴에 따른 연료전지 성능과의 관계를 분석하여 Counter-flow가 온도 및 연료의 변화방향이 같은 원인으로 비가역 손실이 보다 균일하게 분포하여 전지 전압이 보다 높음을 알수 있었으며, 개질 방식에 따른 단위전지 해석 결과는 내부 개질이 다소 전압이 떨어지지만 구성요소의 Thermal Stress에 의한 안정성 면에서 보다 안정적임을 알 수 있었다.
Planar SOFC Cell Frame에 대한 설계 및 해석 측면에서 Flow Uniformity (2% 이내)가 확보 가능한 유로를 설계하였으며, 실제 실험 조건을 모사하기 위하여 Furnace 온도를 벽면 온도 조건으로 설정하여 열유동 해석을 수행시 온도차가 약 10 ℃ 정도로 작은 온도 편차를 보임을 확인하였다.
위의 낮은 온도 편차에 대한 작동온도에서의 열응력 해석에서는 온도편차에 의한 열응력과 면압에 의한 구속조건보다는 소결된 구성요소가 상온으로 온도가 떨어지면서 생기는 잔류응력에 의한 Damage가 더 중요하다는 점을 SOFC 실험후 사진을 통하여 확인하였으며, 이에 SOFC의 가장 큰 단점인 Thermal Cycle에에 취약하다는 점을 확인하였다.
SOFC 단위전지 실험을 수행하기 위하여 Sealing Test를 수행하였으며, 무엇보다도 정량적인 비교를 위하여 유체역학적 수식을 전개하여 실험값을 비교하였으며, 이에 가스켓에 대한 평가를 수행하였다.
평판형 SOFC 단위전지 실험을 통하여 셀 내부 온도 분포 및 연료전지 성능 테스트를 수행하엿으며, 온도 편차는 5 ℃미만으로 열/유동 해석 결과인 10 ℃와 비슷하게 10cm x 10cm의 소면적 단위전지의 온도 분포 특성을 확인하였으며, 해석결과보다 높은 전지 성능을 확인하였다.
약 40 Watt급 SOFC Short Stack Module에 대한 설계 및 해석을 통하여 Vessel 에서의 Cathode Channel로의 유량 공급이 균일한 점을 확인하였으며, In-cell Stack 해석을 통하여 약 40 Watt의 출력에 대한 온도, 가스조성 및 전류 밀도 분포를 예측하였으며, 소면적으로 인하여 온도 편차는 약 4 ℃로 미미하다는 점과 유량을 2배 올려 Utilization을 낮췄을 경우에는 압력이 약 2배 정도 상승하는데 비해 전지 및 출력 상승률은 6%에 그침을 확인하였다.
앞에서 수행한 단위전지 성능해석한 결과와 실험한 결과에 대한 Validation을 통하여 신뢰성을 확보하고, 고온연료전지에서 가장 큰 이슈인 Thermal Cycle에 대한 정량적인 분석을 위하여 각 구성재료에 대한 열 및 기계적 물성치 실측 데이터 확보가 필요하며, 마지막으로 100 Watt급 Stack에 대한 제작/실험 및 평가는 향후 과제로 남아 있다.
