Stack efficiency

The formation of such complexes apparently involves a delicate balance of binding forces, since a-phenyl-ethylamine 30 shows only modest tendencies to form 2 1 complexes and its stacking efficiency is reduced. The structural details of these complexes are not known, but intermoleeular NOE experiments favor structures such as shown in 31. The distance between the aromatic and amine recognition sites in the... 

For fuel cells, a small amount of leakage does not cause a large drop in the stack efficiency and is generally tolerated. DC electricity is the valued product, and any unconverted fuel, such as H2 and CO, that is leaked out of the anode chamber is combusted and contributes heat to the cells. Leakage hydrodynamics is covered later in this chapter. 

The system efficiency is lower than the stack efficiency due to power requirements for auxiliary components and due to power conversion. A well-designed system should not use more than 10% of the fuel cell output power for auxiliary components. The efficiency of DC/DC or DC/AC converters is relatively high (typically >90%) but their number and configurations must be optimized for given application. 

The overall cell-stack efficiency (relative to the lower heating value for hydrogen) has been found to be typically around 40%, with a maximum of 45% around nominal current. These values are very low compared to the values in excess of 60% found for other cases like the HYSOLAR Electrolyser 2 at DLR. Operation of the DLR electrolyser with intermittent loads proved that ... 

The system efficiency from AC power to compressed hydrogen is of the order of 50% (HHV), where under variable power input the electrolyser stack efficiency varied between 70 and 80%, which was reduced to 55-65% when the AC conversion efficiency was taken into consideration. 

Specifically speaking, membrane conductivity represents only the membrane s resistance to flow of protons (H+) and is highly dependant on its thickness (cp) and water content. Electrical resistance of electrodes, cell interconnects, and the formation of any insulating layer on the electrode surface are all bundled under the conductivity term. Voltage decreases for a given current as temperature increases and can be controlled to improve stack efficiency. 

A. Murphy, G. Dubois, T. D. P. Stack, Efficient epoxidation of electron-deficient olefins with a cationic manganese complex, J. Am. Chem. Soc. 125 (2003) 5250. 

The operative parameters which can be regulated to optimize the stack performance are MEA humidification, reactant pressure, stack temperature, and stoichiometric ratio. While the role of membrane humidification, already partially discussed in Sect. 3.2, is closely considered in Sect. 4.5 and in case studies (Chaps. 6 and 7), the influence of the other parameters is examined here with reference to the stack of Fig. 3.5. These effects have already been described from a thermodynamic point of view (see Sect. 3.1), while kinetic implications are considered in this section for their importance in determining the stack efficiency. 

Since the electrochemical reaction (3.13) is exothermic (Table 3.1), a potential loss should be expected by thermodynamics however, the results reported in Fig. 3.7 evidence that kinetic implications are prevalent in determining the global effect of temperature on stack efficiency, in particular the increase in the exchange current density and the improvement of mass transport properties can be invoked to explain the behavior represented. 

In Fig. 4.3 two schemes of the air supply sub-system are reported, related to both low and high pressure fuel cell plants. As the air supply sub-system implies the highest power consumption among all BOP components, and could heavily affect the overall system efficiency (see Sect. 4.6), the utilization of blowers (Fig. 4.3a) would permit to limit power losses, even if their impact on the overall efficiency would not be negligible. In fact, blowers not only requiie a not negligible electiic power consumption especially at minimum load, but they can provide low air pressure values, then limiting cell voltages and thus stack efficiency (see Sect. 3.3). In any case the low cost and simplicity make this solution more appropriate for small size power trains (1-10 kW). 

Stack needs to operate in this last range in most part of driving conditions in order to guarantee high conversion efficiency. Efficiency loss due to fuel cells is then limited to the resistance to proton flow through the polymeric membrane. The system efficiency profile at low current density can be explained taking into account that fuel cells are more efficient at part load (see stack efficiency curve), and the impact of parasitic component consumption on net produced power... 

The performance of the air compressor affects both stack and system efficiency by means of the impact of flow rate level and working pressure on both cell voltage and compressor consumption [48, 49]. Low pressures and low stoichiometric ratios minimize the compressor consumption but stack efficiency results negatively affected (see also Sect. 3.3). 

The voltage data of Fig. 6.4 are presented also in terms of stack efficiency curve in Fig. 6.6, where t/stack is plotted against the electric power produced by the FCS and measured at the DC-DC converter inlet. In the same hgure, the actual and theoretical curves of FCS efficiency (j/fcs) are also reported, they refer to the real consumptions of ancillary components or to the ideal losses expected for that same system after optimization of all individual components (maximum FCS efficiency in Fig. 6.6). The stack efficiency varies from 0.7 at 0.1 kW to 0.56 at... 

Fig. 6.8 FCS warm up from 15 to 45°C, at 20 W up to 1.2 kW stack power. FCS efficiency, stack efficiency, and temperature versus time...

The Figs. 6.8 and 6.9 show stack and FCS efficiencies versus time for the tests starting from the temperature of 15°C. In both these two tests, the selected value of final stack temperature of 45°C is reached in about 10 min, while the stack efficiency achieves a value (0.53) a little bit lower than that maximum (0.59) already at the end of the accelerations ramps (1200 W). 

When the stack temperature reaches the value of 45°C, the stack efficiency increases from 0.53 to 0.59, and when power reaches its set maximum value (1200 W) the FCS efficiency increases from 0.45 to 0.50 in both tests. From Figs. 6.8 and 6.9, it is possible to evaluate the energy losses due to the warm-up period of 600 s, they resulted about 5% of the FCS steady-state maximum efficiency. Moreover, the energy losses are not significantly affected by the power acceleration variation from 20 to 200 Ws Start-up operations are also verified at the starting temperature of 30°C and 200 W s (Fig. 6.10), the results evidence... 

Experiment No. Stack temperature (K) Stack power (W) Stack efficiency FCS efficiency Coefficient of variation Cv (%)... 

The instantaneous efficiency of the stack and FCS are shown in Fig. 6.29 for R47 cycle in hard hybrid conhguration and in Fig. 6.30 for R40 cycle in soft hybrid configuration. According to the results obtained in steady-state operation, during the low load phases the stack efficiency reaches the value of about 0.7, whereas during the power variations required by the electric drive the efficiency decreases down to the lowest value of about 0.6, in correspondence with the most... 

If the stack efficiency is calculated using the equation already shown in Sect. 6.4 ... 

Differently from stack efficiency the values are clearly dependent on the profile of R utilized, due to compressor consumptions connected to different motor speed regulations. In particular, highest j/sc values decrease from 0.60 to 0.53 passing from management curve 4 to 1, with major differences in the field of lower loads (under 100 A). 

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