Power stack

Alternatively, the fuel cell stack can he operated at ambient pressure. Although this simplifies the system considerably and raises overall efficiency, it docs reduce stack power and increase thermal management challenges. 

Figure 2.10 The corresponding stack power output for the stack performance shown in Figure 2.9.

Figure 2.11 shows the stack energy conversion efficiency as a function of stack voltage and stack power output. The stack energy conversion efficiency is calculated by the voltage efficiency (= average cell voltage of a stack/1.21 V) times its... 

Figure 2.11 Stack energy conversion efficiency plotted against stack current (a) and stack power output (b) from the results of steady-state stack performance obtained at selected operating conditions listed in Table 2.2.

Stack Power generation efficiency over 55-65% (LHV-HHV) Stack Power generation efficiency over 55%... 

The overall efficiency of a fuel processor/fuel cell system is commonly defined as the ratio of the fuel cell stack power output P to the LHV of the fuel ... 

Figure 3.22. Temperature field along the length x of a tubular SOFC cylinder (air/Oj inlet to the right, tube end with flow bending at left, radially from r = 0 to 0.5 cm), as simulated by a dynamic flow model. The middle horizontal band is the electrodeelectrolyte assembly, and the top part (above r = 0.72 cm) is the fuel channel. (Reprinted from P. Li and M. Chyu (2003). Simulation of the chemical/electrochemical reactions and heat/mass transfer for a tubular SOFC in a stack. /. Power Sources 124, 487-498. Used by permission from Elsevier.)...

Li, P-W. and M. Chyu, M. (2003). Simulation of the chemical/electrochemical reactions and heat/mass transfer for a tubular SOFC in a stack. /. Power Sources 124, 487-498. 

Conversion from areal power output of an MEA, Pout, expressed typically as W cm-2, to stack power density ex-... 

Fig. 56 System volume for a 1-W DMFC power source versus number of watt-hours stored (= hours of use time at 1-W to the load per single fuel cartridge) compared to the volume of a prismatic Li-ion battery of the same number of watt-hours. The three cases shown are for three different volumes of the nonfuel-containing components of the DMFC system, that is, stack + auxiliaries. Assumptions for the DMFC system are stack power density of 50-100 W L-1 (0.05-0.1 W cm-3), auxiliaries volume = stack volume, and system conversion efficiency = 30%.

You may wish to build a fuel cell stack power supply with greater voltage and current output. 

H2O molecules are dragged by protons during stack power requirement... 

The main ancillary components that concur to reduce the gross stack power are reported in Fig. 4.10. The auxiliaries are actuated electrically, and the total DC power produced by stack is partially consumed by the motors that move the compressors and pumps, and by the electricity necessary to supply the radiator fan and the controller. 

The Fig. 6.4 shows the characteristic curves of the stack, in terms of voltage and power versus stack current. The stack output voltage decreases from 32 V, at low load, to about 22 V, at the highest load tested (115 A), when the stack power reaches the peak value of 2.5 kW. The polarization curve presents a linear relationship between voltage and current in the working range of 10-90 A, with a voltage decrease from 27 to 24 V. 

The Fig. 6.5 shows the relation between R and the stack power, which is used for all the tests reported in this chapter. This figure evidences that the selected air management strategy determines R values decrease from R = 9 at open circuit to about R — 2 for a stack power between 500 and 2000 kW. These values of R are specifically selected to minimize the air compressor energy losses (see Sect. 4.3). 

Fig. 6.5 Stoichiometric ratio values utilized in experimental tests as function of stack power...

The dynamic behavior of the FCS is firstly verified starting from the analysis of the energy lost during the start-up phases, evaluating the performance as function of acceleration rates [2]. In particular, warm-up tests are performed starting from two different initial stack temperatures, 15 and 30°C. For each one of these temperatures, two accelerations of 20 and 200 Ws are used up to the stack power of 1200 W. At the end of each acceleration phase, a steady-state operation follows until the stack temperature reaches the value of 45°C. 

Other dynamic tests are affected varying the stack power and evaluating the stack response to hydrogen purge, external humidification, and air management... 

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...

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