Fuel Cells

PAFC

Researchers are currently working on fuel cell technologies that combine the benefits of PEMFC and PAFC into a single membrane that operates at intermediate temperatures of 90 - 160°C. While often referred to as a "high temperature PEM", the devices can also be described as low temperature PAFCs, as the membranes include phosphoric acid residues bound up in solid phase organic polymers.

It is still too early to tell what the long term impacts of this technology could be, but there are several compelling factors that could encourage its success:

• Anode CO tolerance increases with operating temperature. This factor alone could greatly simplify reformer design and increase the lifetime of reformate-fueled stacks.
• The moderate stack temperatures are still low enough to allow rapid start-up and shutdown, thereby maintaining many of the advantages of PEMFC systems.
• By increasing the temperature difference between the stack and the environment, thermal and water management functions of the fuel cell system are greatly simplified.
• Preliminary tests indicate that these higher temperature stacks may not need to be saturated with water during operation, thus (slightly) decreasing the system water requirements and increasing the flexibility of operation.

MCFC

MCFC technology has now been through several generations of field testing, and additional testing continues. Future development is focusing on:

• Extending stack life
• Increasing the power density
• Reducing the cost

SOFC

With extensive demonstration experience accumulated on tubular SOFC technology, the primary development challenges relate to cost reduction. Siemens-Westinghouse is focusing their current development efforts on:

• Identifying configurations that require less stringent material purity specifications
• Identifying routes to reduce the amount of insulation in the system. Large volumes of intricately shaped material are presently required to maintain high operating temperatures.
• Moving manufacturing processes towards net-forming rather than machining to minimize scrap production.
• Use of less exotic alloys, which is directly related to the high operating temperature in conventional designs.

Additionally, as with all novel technologies, there remains a need to demonstrate the reliability and operating cost of these technologies prior to their commercialization.

For planar SOFC technologies, the primary challenges still relate to the difficulties maintaining the structural integrity of the units at their high operating temperatures. These issues are focused particularly on:

• Maintenance of seals and manifolds under severe thermal stresses
• Long-term mechanical integrity of planar systems in (as yet undemonstrated) field tests of integrated fuel cell stacks
• Long-term material compatibility of planar systems in (as yet undemonstrated) field tests of integrated fuel cell stacks.

PEMFC

Outstanding challenges for PEMFC systems are as follows:

• For those systems operating at pressures > 1.5 atm, there is no compressor/expander product that can operate at appropriate airflows and efficiency to provide power to a PEMFC stack without imposing an unacceptable burden on system parasitic loads. This limitation has led many developers of stationary power systems to focus on near-ambient pressure operation.
• Long-term operation has yet to be demonstrated. Much of the current experience even on synthetic reformate has witnessed decays in power output over time, suggesting that more testing and product development will be critical.
• Long-term demonstration of CO control technology that can reliably produce acceptably low levels of CO remains to be done. While manufacturers have developed systems capable of producing 10 - 20 ppm CO, these results still need to be accomplished in long-term, real-world test environments.
• Operation of fully integrated systems in a broad range of thermal environments with adequate water recovery over extended periods has not yet been demonstrated.
• Operation of fully integrated systems in environments where freezing temperatures occur over extended periods has not yet been demonstrated.