HYDROGEN COOLING FOR FUEL CELLS AND THE ACCOMPANIED WATER MANAGEMENT IMPROVEMENT
It is widely accepted that using fuel cell systems in a civil aircraft can improve the design in multiple ways, such as reduced noise, lowered emissions and higher fuel economy and thus is better suited than traditional aircrafts. To further strengthen their competences in the aviation sector, the weight reduction and reliability improvement of these aviation fuel cell system designs are of great importance.
Fuel cells are electricitygenerating devices that electrochemically convert the energy stored in fuels into electricity.
Water balance is one of the greatest challenges associated with the PEM fuel cell. Depending on the load and the operating conditions, the electrolyte membrane can flood or dry out. There are various strategies to achieve the correct water balance, such as humidifiers, hydrophobic coating or microporous layers. For small scale fuel cell applications, the fuel cell can be cooled by the inlet air stream but this is not practical for larger applications, as the required air flow is comparatively high and would lead to a dry-out of the fuel cells. The consequence is a considerable reduction in the performance and lifetime of the cell. The risk of dehydration was reduced by a microporous layer, but this impairs the cell-internal species transport. In addition, the required pumping power is high. The power density of an air-cooled cell is only about one fifth of that of a liquid-cooled cell.
Exhaust gas recirculation is a common practice in PEM fuel cell operation. It is commonly used for membrane humidification, flooding control and nitrogen accumulation prevention to increase the efficiency of the PEM fuel cell stack. Recirculation is performed, for example, by recirculation pumps or ejectors. To the knowledge of the inventor, anode recirculation is not currently used for cooling purposes.
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Hydrogen cooling is proposed to optimise the heat and water management of fuel cells. According to the invention, the anode exhaust gas is recirculated to improve cooling and the cathode exhaust gas is recirculated to improve water management. The anode exhaust gas consists mainly of hydrogen, which has a higher cooling capacity compared to air. In addition, cathode exhaust gas recirculation also provides a certain amount of cooling to the fuel cell stack. Cathode exhaust gas in particular is very humid and provides more effective humidification compared to anode exhaust gas. In one possible implementation, the invention comprises an air-cooled PEM fuel cell in which the anode and cathode exhaust gas is recirculated. The anode exhaust gas passes through a heat exchanger, whereby the gas is cooled. The invention also includes at least one implementation form in which a dynamic water separator and a nitrogen separator are implemented in the anode exhaust gas recirculation. Furthermore, the invention includes a control system that can dynamically adjust the exhaust gas rates to achieve the desired operating condition of the fuel cell stack. The control system can actively change the amount of water separated from the anode exhaust gas stream depending on the humidity in the fuel cell stack. In another possible implementation, only the anode exhaust gas is recirculated for cooling purposes. The membrane humidification is controlled by a humidification device. The control system in this variant includes, among other things, the control of the anode gas mass flow and the adjustment of the water separation ratio. The control takes place depending on the operating state of the fuel cell stack.
- Reduction in system size while maintaining power density
- Reducing system complexity, thereby reducing the likelihood of system failure
- Increase of the overall efficiency
- System dynamics can be improved
- Fuel cell lifetime can be increased
- The start-up capability of the fuel cell system is improved
Scope of application
The invention relates to fuel cells that can be used in particular in the aviation industry, but also in the automotive industry. Furthermore, the invention can be used in stationary applications for power supply in remote areas.
License for commercial exploitation / Research & Development cooperation
- EP pending
Keywordsanode exhaust gas, cathode exhaust gas, cooling, cooling system, exhaust gas recirculation, fuel cells, heat management, Hydrogen cooling, improvement, recirculation, water management
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