Successful cold start

Knowing that 1 mg/cm2 of product water is a threshold, how much water can be stored at maximum within each component of the fuel cell, and how much can be removed to the outside For the cathode catalyst layer (CCL) with typical thickness of 10 p,m and 50% pore volume fraction, the CCL water storage capacity is approximately 0.5 mg/cm2. A 30- un-thick membrane can store 1.5 mg/cm2 of water, but its actual water storage capacity depends on the initial water content, A., and therefore is proportional to (ks.where A.sa, denotes the water content of a fully hydrated membrane. The escape of water into the GDL is unlikely due to the very low vapor pressure at cold-start temperatures (Pv>sa, = 40 Pa at —30°C). For reference, the GDL with 300 pm thickness and 50% porosity would store about 15 mg/cm2 of water, if it could be fully utilized. This capacity is too large to be used for cold start. From this simple estimation we can conclude that the CCL water storage capacity alone is not sufficient for successful cold start and that a successful strategy is to store water in the membrane. 

The freeze-resistant 2005 FCX models can operate at -20°C. Other companies, including DaimlerChrysler and GM have also had success with cold-starting cells. The technique used is to keep all water present as a vapor and not allow water droplets to occur. 

As mentioned earlier, cold start of the fuel cell system is considered successful when the cell temperature increases above 0°C before the cell shuts down due to oxygen starvation. Next, how long must the cell be able to run under such a condition A simple, back-of-envelope calculation can be made. Fuel cell operation generates both heat and water. Heat generation from the fuel cell operation is calculated with the relation... 

Combining Eqs. (3) and (4), we can conclude that product water of approximately 1 mg/cm2 is a criterion for successful self-startup. As shown with Eqs. (3) and (4), the temperature increase is proportional to the product water, and therefore, the product water from the fuel cell operation, Am o, is considered a quantitative index to measure the cold-start performance. 

Jiang et al.16 developed a multiphase, three-dimensional model to describe non-isothermal cold start and to study the effect of temperature rise. Due to the temperature rise during cold start, more water was transported into the membrane and less ice formation occurred in the catalyst layer. It was also found that a lumped thermal analysis significantly overestimated the overall thermal requirement for successful self-cold start. In addition, pre-startup conditions such as gas purge had significant impact on cold start that implied the importance of the shutdown process. 

Simple calculations of heat and water balance indicate that for successful self-cold start from — 30°C, heat release corresponding to approximately 1 mg/cm2 of product water from ORR reaction is required. Therefore, the key challenge is the removal or storage of 1 mg/cm2 water. The cathode catalyst layer of common design can store 0.5 mg/cm2 water, at most. The vapor phase removal of water from the catalyst layer to the gas channel is negligibly small at this temperature. Hence, it is necessary to store some water in the electrolyte membrane. 

Two sets of combustor components were successfully engine tested for 10 h each by Williams International. The engine tests consisted of operating at temperatures mostly between 1175 and 1400°C with over 5 cold starts in each test. The components experienced peak temperatures as high as 1500°C. 

The proof of concept that SC-SOFCs can be considered for portable power generation was provided by Shao etal. [19], who successfully powered a 1.5 V MP3 player with a miniature two-ceU SC-SOFC stack. The heat generated from the exothermic reactions allowed rapid start-up from cold start to stable power in less than 1 min and thermally self-sustained the system. However, the low cell efliciency of current SC-SOFCs questions their practical implementation, and energy-harvesting appHcations where energy is generated from waste gases and efficiency plays a secondary role seem more adequate. Hibino and co-workers... 

PEMFCs must have the ability to survive and start up from sub-freezing temperatures, also called cold start, to be deployed successfully in automobiles. Under freezing environmental conditions, water produced has a tendency to freeze in open pores in the catalyst layer and GDL, rather than being removed from the fuel cell, thus creating mass transport limitations that eventually result in the shutdown of PEFC operation. Figure 31.11 shows the accumulation of solid water in a fuel cell in sub-freezing operation detected by neutron imaging [34]. 

The next two sections of this chapter deal with the impacts of freezing and cold start on PEMFCs. For each section fundamental mechanisms, effects on performances and mitigation strategies are successively presented. 

Vitamin C (ascorbic acid) is probably the most known vitamin in the world. Its legendary fame is based on the two events its exceptionally important role in the treatment of scurvy and Linus Pauling s proposal to use the huge doses of ascorbic acid for the prevention of common cold. The latter proposal, based obviously on the antioxidant properties of ascorbic acid, generated numerous studies and was frequently disputed, but many people (me including) successfully apply ascorbic acid for the treatment of starting stage of common cold. 

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