Design and Control of High Temperature PEM Fuel Cell System
Publication: Research › Ph.d. thesis
E-cient fuel cell systems have started to appear in many dierent commercial applications and large scale production facilities are already operating to supply fuel cells to support an ever growing market. Fuel cells are typically considered to replace leadacid batteries in applications where electrical power is needed, because of the improved power and energy density and the removal of long charging hours.
The primary focus of this dissertation is the use of high temperature polymer electrolyte membrane (HTPEM) fuel cells that operate at elevated temperatures (above 100oC) compared to conventional PEM fuel cells, that use liquid water as a proton conductor and thus operate at temperatures below 100oC. The HTPEM fuel cell membrane in focus in this work is the BASF Celtec-P polybenzimidazole (PBI) membrane that uses phosphoric acid as a proton conductor. The absence of water in the fuel cells enables the use of designing cathode air cooled stacks greatly simplifying the fuel cell system and lowering the parasitic losses. Furthermore, the fuel impurity tolerance is signicantly improved because of the higher temperatures, and much higher concentrations of CO can be endured without performance or life time losses.
In order to evaluate the performance of using HTPEM fuel cells for electricity production in electrical applications, a 400 W fuel cell system is initially designed using a cathode air cooled 30 cell HTPEM stack. The stack runs on pure hydrogen in a deadend anode configuration at a pressure of 0.2 bar with a combined PI and feedforward air ow control strategy. Some of the problems involved in using fuel cells running at high temperatures is longer start-up times, therefore different heating strategies are examined in order to minimize the heating time for systems with critical demands for this. A 1kW fuel cell stack with optimized ow plates was heated in 5 minutes using the introduction of an electrical air pre-heater.
Using pure hydrogen in compressed form is problematic due to the very small density of hydrogen, even at high pressures. Hydrogen is a very energy e-cient gas, but large investments are required for a full production and distribution system before the fuel is available for general purpose use in consumer applications. Using liquid renewable fuels that can be produced and transported using existing techniques is benecial if the fuel cell systems are adapted. Converting a liquid renewable fuel such as methanol in a chemical reactor, a reformer system, can provide the high temperature PEM fuel cells with a hydrogen rich gas that e-ciently produces electricity and heat at similar e-ciencies as with pure hydrogen. The systems retain their small and simple configuration, because the high quality waste heat of the fuel cells can be used to support the steam reforming process and the heat and evaporation of the liquid methanol/water mixture. If e-cient heat integration is manageable, similar performance to hydrogen based systems can be expected.
In many applications benets can be gained from operating fuel cells together with batteries. In automotive applications and small utility vehicles large power peaks are experienced for accelerations. Very large and expensive fuel cell systems are needed in order to supply these peak powers, which do not occur that often during a normal driving cycle. The combination of batteries and super capacitors together with fuel cells can improve the system performance, lifetime and cost. Simple systems can be designed where the fuel cells and batteries are directly connected, but the introduction of power electronics can increase the degrees of freedom for the system when determining control strategy.
The high temperature PEM fuel cell is a promising alternative for converting renewable fuels into electricity and heat in it's simplicity in systems design and reliability during operation.
|Place of publication||Aalborg|
|Publisher||Institut for Energiteknik, Aalborg Universitet|
|Number of pages||223|
- fuel cell, renewable energy, hydrogen, methanol, system design, control