The rapid increase in the usage of the fossil-fuel-derived energy after the industrial revolution has resulted in many environmental problems, such as abnormal climate change, environmental pollution, poorer air quality, destruction of ecosystem, etc. To restore and preserve the ecological equilibri...
The rapid increase in the usage of the fossil-fuel-derived energy after the industrial revolution has resulted in many environmental problems, such as abnormal climate change, environmental pollution, poorer air quality, destruction of ecosystem, etc. To restore and preserve the ecological equilibrium, it is inevitable to transit to the clean power system which does not emit the green house gases, CO2, NOx and SOx, etc. There are many clean alternative energy sources such as solar cells, wind power, geothermal energy, hydraulic energy, wave and tidal energy. However, those clean energy sources are quite dependent on time, climate condition and geographical position. So, we need a clean power generator which can be used regardless of the environmental limitations.
Fuel cell is a clean energy converter which can produce electricity irrespective of the surrounding environmental conditions, as long as the fuel, eg. hydrogen, is supplied. Therefore, the use of fuel cells has come into the spotlight and been a promising one of the alternative clean power systems due to its high efficiency and low pollutant emission characteristics. Recently, the hybrid system composed of both fuel cell and natural energy sources is popular, which can make up for each other's weakness with its strengths. Hydrogen fuels for fuel cell can be obtained by water electrolysis using the energy from the natural energy sources, such as solar cells, wind power, hydraulic energy. Water can be split into hydrogen and oxygen by natural energy sources and they are recombined into water in fuel cells. So water loop can be made by the combination of water electrolysis system using an intermittent natural energy sources and fuel cell system. The hybrid combination of natural energy sources and fuel cells is expected to produce a clean and sustainable power system by the water loop. Fuel cell systems have further advantages of silence and high energy conversion efficiency. The silent operation of fuel cells can provide extremely low noise during operation of unmanned aerial vehicles (UAVs).
In this thesis, we deal with the polymer electrolyte fuel cell (PEFC) among the various types of fuel cells, such as direct methanol (DMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC). This thesis especially focused on the fabrication of electrode, membrane electrode assembly (MEA), and stacks for system applications using the homemade PEFC stacks.
The stacking process is important to provide stable and uniform performances of unit cells in a fuel cell stack. The performance of the unit cells showed significant difference not only by stack operating conditions such as gas flow rate, back pressure and cooling of the stack but also by the fabrication methods of electrodes, membrane electrode assembly (MEA) and stack. The sealing and uniform pressure between the MEA and bipolar plates in the fabrication of the stack were found to be crucial factors to get the uniform and stable I-V performances. The performance of a PEFC stack was also affected by the gasket thickness in unit cells and we could obtain low cell voltage variations of the unit cells in the stack less than 3% by selecting an optimal gasket thickness for the homemade PEFC stack.
The homemade stack with 16 cells was made using the homemade MEAs with 300 ㎠ of apparent active area and bipolar plates designed in the laboratory and it was used to fabricate a 1 kW fuel cell system for hybridization with solar cell/water electrolysis system. The PEFC stack and system were operated at dead-end conditions for both the anode and cathode lines using pure hydrogen and oxygen gases, and unreacted gases were re-circulated to increase stack and system efficiencies. The purging of unreacted gas was periodically conducted at both the anode and cathode sides every 60 seconds to remove water, and the amount of purged gas was measured quantitatively using a graduated cylinder. 99.7% of fuel utilization was achieved in the fuel cell system by re-circulating the hydrogen and oxygen with the periodic purging. In the system performance test, energy flow diagram analysis revealed electric efficiencies of 60% and 49% for the fuel cell stack and system, respectively, at an operating condition of 1.1 kW electric output. The durability test of the fuel cell system operated for one month at the daily start and stop (DSS) condition showed no performance degradation of the PEFC system which verifies that the homemade PEFC system is quite stable and applicable for practical use.
Another application of a PEFC system for the propulsion of a UAV was also studied by a homemade hybrid propulsion system composed of 36-unit cells and balance of plant (BOP), and a lithium battery (LiPo) in parallel connection. Characteristics and performance of the homemade PEFC system were evaluated in view of dynamic load responding capability and energy efficiency. The LiPo battery was only switched on when high power is required for take-off, accelerating and landing. The independent use of a homemade PEFC system and a battery for a UAV showed good load responding capability and high fuel cell system efficiency of about 45% during cruising. Parasitic loss and amount of unreacted hydrogen gas discharged outside of PEFC were about 3.91% and 0.89%, respectively. Flight tests confirmed that the hybrid propulsion system, based on parallel connection of a PEFC system and a battery pack, is very effective in the operation of a UAV.
The rapid increase in the usage of the fossil-fuel-derived energy after the industrial revolution has resulted in many environmental problems, such as abnormal climate change, environmental pollution, poorer air quality, destruction of ecosystem, etc. To restore and preserve the ecological equilibrium, it is inevitable to transit to the clean power system which does not emit the green house gases, CO2, NOx and SOx, etc. There are many clean alternative energy sources such as solar cells, wind power, geothermal energy, hydraulic energy, wave and tidal energy. However, those clean energy sources are quite dependent on time, climate condition and geographical position. So, we need a clean power generator which can be used regardless of the environmental limitations.
Fuel cell is a clean energy converter which can produce electricity irrespective of the surrounding environmental conditions, as long as the fuel, eg. hydrogen, is supplied. Therefore, the use of fuel cells has come into the spotlight and been a promising one of the alternative clean power systems due to its high efficiency and low pollutant emission characteristics. Recently, the hybrid system composed of both fuel cell and natural energy sources is popular, which can make up for each other's weakness with its strengths. Hydrogen fuels for fuel cell can be obtained by water electrolysis using the energy from the natural energy sources, such as solar cells, wind power, hydraulic energy. Water can be split into hydrogen and oxygen by natural energy sources and they are recombined into water in fuel cells. So water loop can be made by the combination of water electrolysis system using an intermittent natural energy sources and fuel cell system. The hybrid combination of natural energy sources and fuel cells is expected to produce a clean and sustainable power system by the water loop. Fuel cell systems have further advantages of silence and high energy conversion efficiency. The silent operation of fuel cells can provide extremely low noise during operation of unmanned aerial vehicles (UAVs).
In this thesis, we deal with the polymer electrolyte fuel cell (PEFC) among the various types of fuel cells, such as direct methanol (DMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC). This thesis especially focused on the fabrication of electrode, membrane electrode assembly (MEA), and stacks for system applications using the homemade PEFC stacks.
The stacking process is important to provide stable and uniform performances of unit cells in a fuel cell stack. The performance of the unit cells showed significant difference not only by stack operating conditions such as gas flow rate, back pressure and cooling of the stack but also by the fabrication methods of electrodes, membrane electrode assembly (MEA) and stack. The sealing and uniform pressure between the MEA and bipolar plates in the fabrication of the stack were found to be crucial factors to get the uniform and stable I-V performances. The performance of a PEFC stack was also affected by the gasket thickness in unit cells and we could obtain low cell voltage variations of the unit cells in the stack less than 3% by selecting an optimal gasket thickness for the homemade PEFC stack.
The homemade stack with 16 cells was made using the homemade MEAs with 300 ㎠ of apparent active area and bipolar plates designed in the laboratory and it was used to fabricate a 1 kW fuel cell system for hybridization with solar cell/water electrolysis system. The PEFC stack and system were operated at dead-end conditions for both the anode and cathode lines using pure hydrogen and oxygen gases, and unreacted gases were re-circulated to increase stack and system efficiencies. The purging of unreacted gas was periodically conducted at both the anode and cathode sides every 60 seconds to remove water, and the amount of purged gas was measured quantitatively using a graduated cylinder. 99.7% of fuel utilization was achieved in the fuel cell system by re-circulating the hydrogen and oxygen with the periodic purging. In the system performance test, energy flow diagram analysis revealed electric efficiencies of 60% and 49% for the fuel cell stack and system, respectively, at an operating condition of 1.1 kW electric output. The durability test of the fuel cell system operated for one month at the daily start and stop (DSS) condition showed no performance degradation of the PEFC system which verifies that the homemade PEFC system is quite stable and applicable for practical use.
Another application of a PEFC system for the propulsion of a UAV was also studied by a homemade hybrid propulsion system composed of 36-unit cells and balance of plant (BOP), and a lithium battery (LiPo) in parallel connection. Characteristics and performance of the homemade PEFC system were evaluated in view of dynamic load responding capability and energy efficiency. The LiPo battery was only switched on when high power is required for take-off, accelerating and landing. The independent use of a homemade PEFC system and a battery for a UAV showed good load responding capability and high fuel cell system efficiency of about 45% during cruising. Parasitic loss and amount of unreacted hydrogen gas discharged outside of PEFC were about 3.91% and 0.89%, respectively. Flight tests confirmed that the hybrid propulsion system, based on parallel connection of a PEFC system and a battery pack, is very effective in the operation of a UAV.
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