Study on Efficiency Enhancement Method for Polymer Electrolyte Membrane Fuel Cell (PEMFC) System

Abstract

Due to oil depletion and environmental issues, there is a growing demand for eco-friendly energy. Conventional fossil fuel, which composes of carbon, emits harmful carbon dioxide when it is converted to a usable energy source such as electrical and mechanical power. Many researchers suggested a clean energy source to substitute fossil fuel, and hydrogen was emerged as one of promising clean energy source because it doesnt release carbon dioxide and only makes water by combining oxygen. Polymer electrolyte membrane fuel cell (PEMFC) is an electrochemical energy conversion device that converts electrochemical potential to useful electric energy. The chemical potential of hydrogen and oxygen is changed into electrical potential through PEMFC. PEMFC has many sub-systems except fuel cells called a balance of plant (BOP). The BOP consists of an air processing system (APS), a thermal management system (TMS), a fuel processing system (FPS). These sub-systems consume the power of the fuel cell to operating components such as coolant pump, air compressor, and radiator fan. The parasitic power of BOP lowers the efficiency of PEMFC and reduces available energy from the fuel cell. Therefore, to increase the efficiency of PEMFC, parasitic power from the BOP should be decreased while maintaining the performance of fuel cells. In this study, two methods were suggested to improve the efficiency of PEMFC by reducing parasitic BOP power. Whether consumed power from the BOP was lowered, the performance of the fuel cell should be maintained. With metallic micro porous media, called metal foam, supplied air can be reduced. Metal foam can effectively diffuse air through the reaction surface. Usually, the supplied air stoichiometric ratio is 2. By providing sufficient air to the cathode side, water from the reaction is removed following the channel. However, in the metal foam flow field, porous structure effectively removes water and enhance diffusion. For this reason, supplied air can be lowered without any power loss. Performance and loss are investigated using I-V current curves and electrochemical impedance spectroscopy (EIS), respectively. Using two-phase cooling, we can reduce the coolant flow rate. The single-phase cooling method is widely used because of its simple system composition. However, it takes away the heat by sensible heat, thereby increasing coolant temperature. For uniform temperature cooling, a two-phase cooling method is used. Two-phase cooling takes heat by latent heat, which has a constant temperature. By two-phase cooling, it is possible to keep the temperature of the cooling system, and the required coolant flow rate can be reduced. At the experiment, the temperature of cells was measured, and we verified uniform temperature distribution. Furthermore, the temperature profile of the inside of the stack is analyzed by numerical modeling. In conclusion, system efficiency can be enhanced using a porous media flow field and a two-phase cooling method at APS and TMS, respectively. This research will contribute to the automotive industry and power plant industry by demonstrating the possibility of efficiency enhancement.

연료전지는 친환경적인 특징과 높은 에너지 변환 효율로 차세대 에너지 변환 장치로 각광받고 있다. 높은 에너지 변환 효율에도 불구하고, 연료전지를 작동시키기 위해 운전장치에서의 동력 소비는 필수적이다. 본 연구에서는 연료전지 운전장치에서의 소비 동력 감소를 통해 연료전지 시스템의 효율을 높이는 연구를 진행하였다. 다공성 유로인 메탈폼을 이용하여 연료전지의 공기공급부에서 소비 동력을 감소시키는 연구를 진행하였다. 메탈폼을 적용한 연료전지에서 사형 유로와 비교하여 공급 기체의 유량이 감소함에도 불구하고 성능이 유지됨을 실험적으로 확인하였으며, EIS 분석을 통해 메탈폼의 높은 기체 확산성이 유량 감소에도 성능을 유지하는 원인임을 이끌어내었다. 비등 냉각을 이용하여 연료전지 냉각시스템에서의 소비 동력을 감소시키는 연구를 진행하였다. 수냉각시스템과 비교하여 연료전지 냉각 채널에서 HFE-7100의 비등을 통하여 연료전지의 셀 온도가 균일해짐을 확인하였으며, 실험과 모델링을 통한 연료전지의 온도 분포를 도출하였다. 잠열을 이용하기 때문에 적은 유량으로도 효과적인 냉각을 할 수 있었으며, 여러 운전조건에서 온도 측정을 통해 최적 유량을 이끌어 내었다. 메탈폼과 HFE-7100을 이용하여 연료전지의 효율 향상 방법에 대한 연구를 진행하였으며, 이를 이용하여 성능을 유지하면서 운전장치에서 소비되는 소비 동력을 감소시켜 시스템 효율을 증가시킬 수 있음을 제시하였다. 본 연구는 차량용, 발전용 등 다양한 연료전지 시스템에서 효율을 높이기 위한 방법으로 활용될 것으로 기대된다.