Cooling stack

Stack-and-draw process, 22 444 Stack cooling, PAFC, 12 218 Stacking fault energy (SFE), 13 486 Stacking fault interactions, 13 498-499 Staebler-Wronski (SW) effect, 23 42 in hydrogenated amorphous silicon, 22 139... 

Good thermal conductivity to achieve stack cooling and satisfactory temperature distribution between cells... 

The design of BP for PEMFCs is dependent on the cell architecture, on the fuel to be used, and on the method of stack cooling (e.g., water or air-cooling). To date, most of the fuel cells have employed traditional filter-press architecture, so that the cells are planar and reactant flow distribution to the cells is provided by the bipolar plate. The bipolar plate therefore incorporates reactant channels machined or etched into the surface. These supply the fuel and oxidant and also provide... 

These sections include hydrogen feeding, air feeding with humidification, and stack cooling systems. The stack specifications and detailed technical characteristics of the auxiliary components, with all the mechanical and electric devices necessary for stack operation, are reported in Table 7.1. 

Fig. 7.1 The three sections of the fuel cell system experimental apparatus a hydrogen feeding, b air feeding and humidification, and c stack cooling...

Figure 8-11. Stack cooling model for forced convection cooling (heat sink front side open only for visibility)...

The main conclusion from this study is that the waste heat can be transferred from the stack up to a full load of 8-10 W under forced air cooling conditions without exceeding 80 C maximum MEA temperature. One individual microfan can be used for both fuel cell cathode air supply and stack cooling. Table 8-6 compares both models in terms of overall performance. Since the fan consumes energy, the system efficiency reduces... 

Using the stack geometry and stack cooling described in the previous section, the fuel cell stack occupies ca. 100 cm The space requirement for the stack would therefore increase by 50-100% to fulfill the cooling requirements, so that the fuel cell stack and fan cooler would account for ca. 20% of the notebook s volume. Thus, even if the energy density of the stack can be increased by further miniaturization, the space necessary for heat removal will continue to pose a serious obstacle to miniaturization. 

In addition to the individual polymer electrolyte membrane fuel cells and their bipolar plates, special heat-exchanger plates must also be included in the battery stack. Cooling fluid is circulated through these plates in order to eliminate the heat produced during battery operation. At least one such plate must be provided for any two cells when the battery is to be operated with high current densities. These plates could also be used to warm up the battery for a cold start-up. 

We will assume that the partial pressure of water vapour in a stack is negligible p = 0. Under this assumption the rate of stack cooling due to evaporation is maximal and is controlled by a single parameter s. The effect of finite pyg on the results is discussed in Section 5.4.4. 

The terms in square brackets represent stack heating by reversible heat of the useful electrochemical reactions (the term with AS), by irreversible heat (the term with r]), by direct methanol-oxygen combustion (the term with AS cross), and stack cooling due to evaporation (the term with AHevap)-The product p v Cp in the denominator of (5.134) describes stack cooling by the anode flow. 

Sugano et al. [46] reported the analysis of the dynamic behavior of a PAFC stack cooling systems. Miki and Shimizu [47] reported the results of the dynamic characteristics of a fuzzy control based stack cooling system. An analytical, exergetic, and thermoeconomic analysis of a 200 kWel PAFC power plant was presented by Kwak et al. [48]. In [49], a novel optimization tool was developed that realistically described and optimized the performance of a PAFC system. Zhang et al. [50] presented an analytical model to optimize several parameters using a thermodynamic-electrochemical analysis. In [51], a dynamic model was developed to simulate a PAFC system and associated components. 

Sugano N, Ishiwata T, Kawai S et al (1994) Investigation on dynamic characteristics of fuel cell stack cooling system. Trans Jpn Soc Mech Eng 60 1597-1601... 

Miki H, Shimizu A (1998) Dynamic characteristics of phosphoric-acid fuel-cell stack cooling system. Appl Energy 61 41-56... 

With this simplified model the percentage of heat to be removed (<2) by the stack cooling system can be identified by solving 20.1, where in is the enthalpy of component i at the inlet of the control volume, //, out is the enthalpy of component i at the outlet, Q is the heat to be removed by the cooling medium, and f ei is the produced electrical power output of the stack. 

Regarding the temperature distribution between the single cells in a stack, cooling of each cell is advantageous but stack concepts where only every second or third cell is in contact with a cooling cell can also be foimd. For the chosen active cell area of 200 cm a cooling of every third cell is possible [22], However, this increases both the temperature gradient across the active cell area, as well as from ceU to cell. 

In addition to the individual cells and their bipolar plates, special heat-exchanger plates must be included in the battery stack. Cooling fluid is circulated... 

Emergency core cooling system stack, cooling water piunps building... 

The heat flux related to water evaporation in the CCL, q ap, is obtained by multiplying Qyap by the CCL thickness Icl- Unfortunately, the evaporation constant Kyap is poorly known, which makes numerical evaluation of q ap rather unreh-able. Figure 1.11 shows the dimensionless heat flux due to water evaporation in the CCL, normalized with respect to this flux at lOO C, assuming = 0, that is, fast water vapor removal. As can be seen, the heat flux qyap is a strong function of the cell temperature. Evaporation is an important mechanism of cell/stack cooling, and Figure 1.11 provides another argument in favor of cell operation at an elevated temperature. 

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