June 22, 2008
June 22, 2008
June 25, 2008
13.199.1 - 13.199.10
An Optimized Humidity Control System for PEM Fuel Cells Abstract
Hydrogen Fuel Cells require humidity to function efficiently and cost effectively. There is an optimum range of humidity for any given load condition and cell design. Hydrogen Fuel Cells inherently produce water, thus creating some of the necessary humidity for the cell to function. However, this self humidification through the back diffusion from the cathode side provides a limited range of operation for the fuel cells.
Without external control of the humidity, fuel cells do not operate at optimum condition. They generally have a warm up time lasting many minutes, in which they operate at limited output power. During operation, flooding could occur when there is an excessive amount of moisture built up in the fuel cell when the cell temperature is relatively low and inlet reactant gases are externally humidified. The humid air could condense to water inside the cell. The water then limits the flow of air through the reactant flow conduits and isolates the catalyst surface from the reactant gases and the electrolyte. Air carries oxygen to the active sights in the Membrane Electrode Assembly (MEA). Oxygen flow should be adjusted at the exact stoichiometric ratio otherwise the fuel cell will starve for its reactant fuel, thus the output power efficiency is reduced to unsatisfactory levels.
As the temperature of the fuel cell rises to 50 ºC or higher, another phenomenon occurs that limits a hydrogen fuel cell’s output power. The internal heat generated by the fuel cell electrochemical reaction is enough to evaporate any water or moisture built up. This dries the conductive membranes, which in turn reduces their ionic conductivity thus curtailing the output power. This condition does not allow fuel cells to reach maximum allowable operating temperatures.
This paper presents an optimized humidity control system, which monitors vital data from humidity sensors and makes necessary adjustments to the external humidification apparatus at all given load conditions. This method ensures maximum power efficiency at all load and operating conditions.
Humidity is one of the critical parameters which affect the performance of the fuel cells. Humidity is often referred to as a water management problem. Properly hydrated membranes maximize the performance and extend their lifetime, but poorly dehydrated ones can reduce the performance dramatically and shorten the life of the membranes. However, excessive humidity causes water flooding inside the fuel cell that blocks the flow of gases and covers the catalyst as a result. Most of the research work related to relative humidity is analyzed in fuel cell stacks. Various approaches are addressed to understand the phenomena of water management inside the fuel cell.
Understanding the transport phenomena of water inside the membrane and exploring the characteristics and affecting factors were investigated by some researchers1-4. Channel designs
ASEE holds the copyright on this document. It may be read by the public free of charge. Authors may archive their work on personal websites or in institutional repositories with the following citation: © 2008 American Society for Engineering Education. Other scholars may excerpt or quote from these materials with the same citation. When excerpting or quoting from Conference Proceedings, authors should, in addition to noting the ASEE copyright, list all the original authors and their institutions and name the host city of the conference. - Last updated April 1, 2015