Water and Air Management for Fuel Cells

From Roads2HyCom Hydrogen and Fuel Cell Wiki - A Reliable Source of Information - Edited by Technology Experts Only

Metrics Table

Air Management

Water Management

Summary

Water management for anode and cathode streams is a requirement coming from the necessity to keep certain water content into stack's membranes in order to keep ionic conductivity and mechanical resistance of the membrane itself. A double task is therefore required:

• Humidification of reactants
• Water recovery at stack outlet

Concerning reactants' humidification, according to stack’s design, different opportunities are available for humidification:

• External humidification
• Internal / integrated humidification

In external humidification scheme, streams are humidified in a section outside the stack. There are different possible technical solutions:

• Water injection (into compressor, into cathode stream)
• Humidifier based on membranes (plates and frame or tubes and bundle - Figure a)
• Humidifier based on ceramic materials (Figure b)

Electric heaters are not taken into consideration because of electric power that is required for water’s phase change. It would have a detrimental impact on the whole fuel cell system efficiency.

Humidifiers for fuel cell application

Internal humidification means that dry reactants are humidified within the stack, with an integrated humidifier or with a mixing between water, which is used for cooling, and reactants themselves. This later solution is used for applications that do not require high electric power (up to 5 kW).

Water recovery at the outlet is required in order to keep a zero water balance. Devices like membrane and ceramic composites humidifiers are particularly interesting since the same device can be used to humidify reactants with saturated streams at the stack's outlet.

As alternative, mechanical devices or heat exchangers may be used. Mechanical devices include centrifugal separators (suitable for liquid water recovery), while heat exchangers (gas to gas or liquid to gas) seem more suitable for condensing water vapour in streams at stack's outlet.

Technology for all these components is well known in different industrial fields, but its use with fuel cells requires additional features due to particular requirements coming from them (see next section).

The air management of a fuel cell has to control the air flow into the fuel cell in order to maintain the fuel cell stoichiometry (lambda) between 1 and 1,5 for the reaction and between 4 and 10 for cooling if the fuel cell is air-cooled.

Key Issues

Beyond the requirement of a zero water balance between humidification and water recovery, some other key issues have to be taken into consideration with fuel cell water management.

Fuel cell technology, at the present, is mainly involved with deionized water. Use of this fluid introduces some key issues that have to be solved:

• Chemical compatibility, since deionized water is very active from a chemical point of view
• Behaviour of the device during freezing ambient temperature.

Chemical compatibility is an hard issue for heat exchangers: coating technologies are evaluated in order to avoid chemical attack from deionized water.

Water volume changes during freezing-defreezing operations may be critical for membrane and (partially) for ceramic materials humidifiers.

Fuel cells require vapour state of water in reactants at stack’s inlet: liquid water, infact, may have two negative effects on stack:

• electrodes' flooding, which prevents gas diffusion
• shorten fuel cell catalyst’s life.

Humidifiers, therefore, must be designed in order to keep moist hydrogen and moist air (or oxygen) without any water droplets It is a hard control issue keeping a humidified (at least saturated) stream at different operating pressure and temperatures without any water drop. An example of routine to evaluate proper humidification is shown in figure below.

Routine to evaluate proper humidification of a stack

For automotive applications, volumes and weight have a relevant role.

Data Lacking

Less amount of data was compiled for water and management for two main reasons:

• Data is not available
• Metric is not appropriate for these devices

Costs and life data are not available since, at the moment, there is no market for fuel cell humidifiers and experimental applications are more interested towards humidification performances rather than life and durability.

More information is desired regarding most of the metrics.

References

• N.N.
  Manual: Dpoint D X 5 Fuel Cell Humidifier
  Dpoint technologies

• N.N.
  Manual: Hydrogen Products
  Linde AG

• D. Stolten
  Grundlagen und Technik der Brennstoffzellen
  RWTH Aachen

• Shizhong Chen, Yuhou Wu, Hong Sun, Jia Sun
  Experimental Investigation of Gravity Effect on Performance of PEM Fuel Cells
  WHEC 16, Lyon France, 13 - 16 June 2006

• C. Gondrand , JY. Laurent , J. Pauchet , M. Prat , M. Quintard , L. Rouillon, P. Tafforeau
  Characterization of Polymer Electrolyte Fuel Cell (PEFC) Active Layer and Experimental Study of the Water Behaviour in a Micro fuel cell
  WHEC 16, Lyon France, 13 - 16 June 2006

• J. Haubrock, G. Heideck, Z. Styczynski
  Electrical Efficiency Losses occurred by the Air Compressor for PEMFC
  WHEC 16, Lyon France, 13 - 16 June 2006

• E.J. Carlson, P. Kopf et al.
  Cost Analysis of PEM Fuel Cell Systems for transportation
  National Renewable Energy Laboratory, 2005

Notes

1. ↑ E.J. Carlson, P. Kopf et al., Cost Analysis of PEM Fuel Cell Systems for transportation, National Renewable Energy Laboratory, 2005