Cell transient operation

System integration involves numerous miscellaneous development activities, such as control software to address system start-up, shut-down and transient operation, and thermal sub-systems to accomplish heat recovei y, heat rejection and water recoveiy within the constraints of weight, size, capital and operating costs, reliability, and so on. Depending on the application, there will be additional key issues automotive applications, for example, demand robustness to vibrations, impact, and cold temperatures, since if the water freezes it will halt fuel cell operation. 

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. 

R. Bomp, J. Davey, F. Garzon, D. Wood, and K. More. REM fuel cell durability with transportation transient operation. ECS Transactions 3 (2006) 879-886. 

As with the other reactor configurations, vaporizers, heat exchangers, and a heat source are also needed for microreactors.Unless the hydrogen is 99.999% pure, the PEM fuel cell typically will utilize 70—80% of the diluted hydrogen fed to it. The unreacted hydrogen from the fuel cell anode, augmented with additional fuel as needed, can be used as fuel for the combustor. The use of anode off-gas requires special controls for transient operating conditions for example, a mechanism is needed to... 

Recent kinetic studies indicate that carbon corrosion can be significant under normal transient operation.56,57,60-62 The rate of voltage change, common in the automotive application, enhances cathode carbon-support corrosion.16 Hence, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling. Currently, the relationship between fuel cell performance decay and accumulated carbon-support loss is only empirical.22 More effort has to be made to incorporate mechanisms that can accurately quantify voltage decay with carbon-support loss.31,32... 

The construction may have a single measuring system, or a twin or differential measuring system. Simple solution calorimeters have a single cell, while a DSC has twin cells and operates in the scanning mode. The use of twin cells reduces the effects of internal and external noise and transient fluctuations. 

In the left side, gas A flows in the direction as shown and picks up B due to the diffusion of B from the other side of the porous medium. Similarly, B at the other side will pick up A from its diffusion through the porous medium. The flow rates of the two sides can be carefully adjusted to give zero pressure gradient across the media (that is the total pressure is uniform throughout the porous medium). The concentrations of gases A and B are analysed by detectors, such as the thermal conductivity cell, and then the diffusive fluxes of A and B can be calculated. This is the steady state method. Recently, this method was extended to allow for transient operation such as a step change or square pulse in one chamber and the response is monitored in the other chamber. With this transient operation, the contribution from the dead end pore can be studied. This contribution is not seen by the steady state method, but its advantage is the ease of operation under isothermal operations. Detailed analysis of diffusion cell under steady state and transient conditions is provided in Chapter 13. 

To overcome these problems, the operation of the diffusion cell in the transient mode will reveal the contribution of the dead end pores. It is important to obtain the information of these dead end pores as they are usually the pores providing most of the adsorption capacity in the pellet. The principles of the steady state and transient operations of the diffusion cell are very similar to the principles of the time lag presented in the last chapter. The dead end pores are not reflected in the time lag information. Their information must be obtained from the analysis of the transient curve describing the approach of the receiving reservoir s pressure towards steady state. Thus, in order to understand the diffusion characteristic of a pellet, both the steady state and transient operations should be carried out. 

In the transient operation of the diffusion cell, an inert gas is usually passed through the two chambers and their total pressures are adjusted to be the same. At time t = 0+ an impulse of tracer, either non-adsorbing gas or adsorbing gas, is injected into one chamber and its concentration is monitored at the other chamber. Perfect mixing is assumed in both chambers. This is usually satisfied with a proper design of the system, for example having the influent stream injected directly to the flat face of the particle. 

The transient operation of the diffusion cell depends on the shape of this input versus time. Usually the following three inputs in concentration are normally used ... 

Cold start is essentially a transient operation. Research on cold start is relatively new. Hishinuma et al. [35], Wilson et al. [36], McDonald et al. [37], and Cho et al. [38] conducted early studies on cold start. Wang [10] presented theoretical analyses, identifying three stages of cold start. Considering a fuel-cell lumped system, the cell temperature is then proportional to the waste heat production rate. The time scale that it takes to heat the cell to 0 °C can be evaluated by... 

Figure 31.12 Transient response of cell voltages and HFR during sub-freezing operations. All cells are operated at 0.02Acm 2 and -10°C [39].

Development of more durable ORR and HOR catalysts, which are resistant to the voltage-cycles occurring during the transient operation of fuel cell vehicles (owing to the variable power demand during typical vehicle drive cycles) [2,3]. 

F. Garzon, etal., Micro and Nano X-Ray Tomography of PEM Fuel Cell Membranes After Transient Operation , PCS Transaction, 11, 1139 (2007). 

Borup, R. L., Davey, J. R., Garzon, F. H., Wood, D. L., Welch, P. M., and More, K. 2006b. PEM fuel cell durability with transportation transient operation. ECS Trans. 3 879-886. 

Garzon F. H. et al. 2007. Micro and nano x-ray tomography of PEM fuel cell membranes after transient operation, ECS Transactions 11 1139-1149. 

A fuel cell power plant is usually operated with variable loads, including the periodic connection and disconnection of different power consumers. This leads to periodic changes in the load resistance / ext and the discharge or operating current. Any such act gives rise to a transient state where one parameter (e.g., current) changes and other parameters (e.g., heat removal) have to acconunodate to the new conditions. For normal operation of fuel cell power plants, it is important that the time spent under transient operating conditions be as short as possible. 

Furube A, Wang Z-S, Sunahara K, Kara K, Katoh R, Tachiya M (2010) Femtosecond diffuse reflectance transient absorption for dye-sensitized solar cells under operational conditions effect of electrolyte on electron injection. J Am Chem Soc 132(19) 6614— 6615... 

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