Stack Performance

Given the factors which influence the design of the stack, it is logical to proceed with the question of relating these factors to stack performance. It has already been proposed that a Gaussian distribution of pollutant... 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Figure 14.16 the shows fuel cell stack performance of a 1 kWe atmospheric PEMFC stack using PtRu anodes, operating on various gas compositions. As can be clearly seen, already small concentrations of CO lead to a large decrease of fuel cell performance. An air-bleed of 1.5% air in hydrogen is able to mitigate this ef-... 

Eastman Chemical Company, Kingsport, TN, Courtesy of G. Stack, Performance Plastics, March 2001. 

Figure 3-3 Multi-Cell Stack Performance ou Dow Membrane (31)...

Furthermore, bypassing or isolating a problematic stack in a network could be a difficult control process. In the conventional parallel configuration, stack performance is not so interrelated. 

The results show that, at temperatures below 60 °C and an air feed stoichiometry below three, the cathode exhaust is fully saturated (nearly fully saturated at 60 °C) with water vapor and the exhaust remains saturated after passing through a condenser at a lower temperature. In order to maintain water balance, all of the liquid water and part of the water vapor in the cathode exhaust have to be recovered and returned to the anode side before the cathode exhaust is released to the atmosphere. Because of the low efficiency of a condenser operated with a small temperature gradient between the stack and the environment, a DMFC stack for portable power applications is preferably operated at a low air feed stoichiometry in order to maximize the efficiency of the balance of plant and thus the energy conversion efficiency for the complete DMFC power system. Thermal balance under given operating conditions was calculated here based on the demonstrated stack performance, mass balance and the amount of waste heat to be rejected. 

The newly assembled 30-cell stacks (one is shown in Figure 2.2) were immediately run in the DMFC mode at a fixed stack voltage at 60 °C with a 0.5 M methanol solution fed at the anode manifold and dry ambient air fed at the cathode manifold, without subjecting the stacks to any H2/air break-in conditions. The stacks gradually reached the reported levels of performance within a few hours and remained stable for at least the initial week of testing in our laboratory before they were sent to our partner for system integration. An extended life test over 1000 h on a five-cell stack built identically revealed stable stack performance. During the initial run, the... 

Figure 2.9 Steady-state V-l curves for a 30-cell DMFC stack operated at various temperatures with a 0.5 M methanol solution feed at 125 ml min at the anode and with 0.76 atm dry air feed at 7.35 SLPM at the cathode. The steady state of the stack performance was verified by comparing the V—l curves with that of stack performance over 30 min for each given stack voltage listed in Table 2.2.

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

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.

The 60-cell stack performance at 40 °C is extrapolated from that of a 30-ceU stack operated at the same temperature. 

From 200 to 1150 h, the air feed stoichiometry was reduced to 2.2. The stack current during the entire life test period is plotted in Figure 2.16. These test results shows that a stable DMFC stack performance can be achieved over an extended period with a low air feed stoichiometry with our current stack design. 

However, we should point out that for real-world application, the environmental conditions and the stack operation mode of recirculating the anode fiuid may pose significant challenges to the stack performance stability because of greater possibilities of exposing the stack to contamination and accumulation of contaminants within the stack. Certain precautionary means must be exercised to mitigate degradation of the stack performance caused by contamination. 

As a second example, results from fully three dimensional simulations of a 5 cell fuel cell stack performed using FLUENT s SOFC module are presented. The geo-... 

Burt, A.C. (2005) Refinement of numerical models and parametric study of SOFC stack performance, Dissertation, West Virginia University, Morgantown. 

This section examines some experimental evidence for the transient behavior of individual cells evaluated on single-cell test stands and stacks contained within full systems. Results from models developed to help understand some of the detailed physical effects that influence cell performance are also examined. The goal is to introduce, albeit in brief, some of the principle dynamic characteristics of single cell and full stack performances. Single cell studies are important since they isolate the cell in a well understood and controlled environment thereby removing the effects of other external processes (e g., reforming) which may have their own transient behavior that affects cell operation (e g., controlling cell input fuel composition). Such... 

SRU and stack performance were evaluated through current density-voltage (i-V) measurements. I-V curves were plotted until a maximum cell voltage of 1.5 V. Short-term durability tests were conducted on the stack over a few hundreds of hours, the stack being galvanostatically controlled to reach an initial average voltage around 1.3 V per cell. 

Stack performances and short-term durability test... 

These results on SRU and single cell underline that the use of optimised cells integrating advanced new materials and coatings is very important to increase stack performances. 

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