Freeze-start

In case (1), freezing starts at a high temperature T, and the crystals will be enol crystals, whereas in case (2) the freezing starts at a low temperature T2, and the crystals will be keto crystals. If it is assumed that liquid B represents the equilibrium enol concentration of 1 percent, then it would follow that after a few days the liquid A will have been converted to liquid B, so that such a scenario would explain the observed difference in ease of crystallization between fresh distillate of high enol concentration and aged distillate of low enol concentration. 

Freezing can be viewed as a special case of degradation requiring hundreds of repetitive freeze storage and freeze start-ups to meet automotive requirements. Therefore, this area is subject to the same limitations of costly and time-consuming tests, and relative absence of in situ structural and compositional measurement methods. The result of this situation is a challenging and varied opportunity for fundamental research and discovery (Figure 2). 

Challenges in Operating Control, Efficiency and Freeze Start-Up... 

The freeze start-up is for fuel cell technologies a challenge, that is comparable with the complex task for conventional powertrains to minimize nitrogen dioxide. In the fuel cell stack, respectively in the cathode, pure water is produced, which naturally... 

Fig. 4.29 Power of a series of freeze start-ups in a Mercedes-Benz B-class F-CELL fuel cell system. Source Daimler AG...

In 2007 Daimler achieved a very important milestone concerning fuel-cell technology and its suitability for everyday usage reliable freeze start-ups in series. 

Figure 4.29 shows the power results of several freeze start-ups one after another [35]. 

For both, complex and chemical hydrides, the current status is a similar dis-illusionment as for metal hydride. So far, no substance from one of the three classes is known, which meets the requirements for an application in a vehicle at least approximately. Nonetheless, numerous especially academic research groups still work intensively on the exploration of hydrides. A justification for applications in vehicles could be a combination of a small metal hydride storage system and a compressed gas storage system. The thermodynamics of the hydride could for example support the fuel cell during freeze start. For further details reference to the comprehensive literature is made [41]. 

Increase Pciwer HlghM Rellabibty Freeze Start CapaWlily LI lc>ri Battery... 

Li J, Lee S, Roberts J. Ice formation and distribution in the catalyst layer during freeze-start process—CRYO- SEM investigation. ECS Trans 2007 ll(l) 595-605. 

SchieBwohla E, von Unwertha T, Seyfrieda F and Brilggemann D (2009), Experimental investigation of parameters influencing the freeze start ability of a fuel cell system. Journal of Power Source, 193,107-115. 

During freezing Record the temperature difference (d3"heat transfer medium) in the inlet and outlet of the heat transfer medium in the shelves, frequency optional every 15 s to 3 min in 15 s steps during the main part of freezing. Start the condenser cooling and vacuum pump system. End the freezing when dT eat transfer medium d.2 C fot mOte than 30 min. 

Trends in short- and lOTiger-term directions for key fuel cell components including electrocatalysts/supports, membranes, and bipolar plates have been elaborated in this section improvement of the performance and durability of these components will directly impact the entire automotive fuel cell system requirements, complexity, and cost. Durable catalysts with enhanced ORR activity, durable membranes that perform at very low humidity and durable bipolar plates that have low contact resistance will not only increase the power density and cost of the fuel cell stack but also simplify and lower/eliminate system component costs of the air compressor, humidification systems, recycle pumps, radiator, start-up/shutdown and freeze-start-related components, etc. A combination of advances in all the fuel cell components discussed above, system simplification, governmental policies that are sensitive to sustainable clean energy, and development of a hydrogen infrastructure will enable achieving the projected technical and cost targets needed for automotive fuel cell commercialization. 

Figure 17.1. Cell voltage and equivalent module power vs. time during freeze start-up from -15 °C...

Failure modes studied included fuel starvation and cell reversal, partial fuel starvation, carbon corrosion, membrane thinning and hole formation oxidant starvation excessive drying and flooding, air and fuel contamination, freeze-start and freeze-thaw degradation. 

The number of freeze-thaw or freeze-start-up cycles that the fuel cell mirst survive is of particirlar importance for stack dirrability. The US Department of Energy has set PEM fuel cell stack transportation technical targets of freeze start-up time to 50% of rated power in 30 s from -20 °C, and unassisted start from -40 °C for 2010 and 2015, with 5000 h dirrability. For stationary fuel cell stack systems operating on reformate, a freeze-start-up time of <30 s to rated power at -20 °C, and survivability from -35 °C to +40 °C, with a 40 000 h lifetime was targeted in 2011 (US Department of Energy, 2007). 

Recoverable performance loss, which is performance loss observed after freeze-start that can subsequently be recovered by certain operational conditions. 

Scanning electron microscopy (SEM) MEA cross-section showing an example of catalyst fragmentation after 350 freeze-start cycles from -15 °C, humidified gas purge on shut-down. 

Recoverable performance may occur after freeze-start operation due to water movement into the CCL as freezing occurs in the small pores of the CCL. As the effective pore radius decreases due to ice build-up, the hquid capillary pressure decreases, resulting in movement of water along the thermal gradient towards the freezing region (Patterson et al., 2007). 

The transition region between the reactant manifold ports and the reactant channels is particularly prone to water accirmirlation. Design of this region to either prevent water accumulation during shut-down, or to ensure sufficient channel depth and/or provide alternate paths for gas flow in the event of ice formation, is important to prevent reactant starvation during freeze-start. 

Chiem et al. (2008) reported freeze-start from -20 °C to 50% power in 21 seconds, and 50 freeze- start cycles from -30 with an average start-up time to 50% power... 

Performance at checkpoints between freeze-starts vs cycle number (full load). (Source Haas and Davis, 2009, reproduced by permission of The Electrochemical Society.)... 

FIGURE 9.24 Time evolution of power for freeze start with long purge prior to start-up with an average starting voltage of 0.45 V at freeze climatic chamber temperatures = 6 C (-), Tsu = -8 C ( ), Ts = -10 C (0) and... 

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