Fuel cell load following

Electrical management, or power conditioning, of fuel cell output is often essential because the fuel cell voltage is always dc and may not be at a suitable level. For stationai y applications, an inverter is needed for conversion to ac, while in cases where dc voltage is acceptable, a dc-dc converter maybe needed to adjust to the load voltage. In electric vehicles, for example, a combination of dc-dc conversion followed by inversion may be necessary to interface the fuel cell stack to a, 100 V ac motor. 

In addition to the requirement for stationary operation mentioned in Figure 3, fuel cell APUs must be able to provide power rapidly after start-up, and must be able to follow loads. While the... 

Connection to the utility grid provides many advantages to on-site power producers such as reliability improvement and increase of load factor, as well as giving the electric utilities a chance to improve the supply capability. When a fuel cell power plant is used for electric utility applications, the inverter is the interface equipment between the fuel cell and the electrical network. The inverter acts as the voltage and frequency adjuster to the final load. The interface conditions require the following characteristics for the inverter ... 

In addition to the way in which the MPL is manufactured, other MPL parameters directly affect fuel cell performance. These include fhickness of fhe MPL, carbon loading, PTFE content, type of carbon parficles, efc. The following subsection will briefly discuss them. 

Catalyst deterioration due to gas poisoning is only avoided by careful gas cleaning. Anodic oxidation followed by dissolution of Pt and transfer to the cathode is a serious cause for Pt loss. It is potential dependent and accelerates as the cathode potential increases, for instance under partial load or in off-time, when the cathode potential drifts toward the oxygen equilibrium potential. Therefore it is of utmost importance that whenever the fuel cell is switched off, the oxygen in the cathode lumen is rapidly exchanged by inert nitrogen and that the cell voltage under operation does not surmount 0.8 V. 

The following example is not as rigorous as the previous model, which had the benefit of some critical validation steps however, it will still show the benefits of a dynamic model, even without such advantage. Here, two events are studied a load loss from the fuel cell, and a load loss from the turbine in the hybrid system. These two events are considered to have the potential for significant damage to the system. 

In a load-following control system, as the demand for current falls, the fuel feed flow is reduced until a predetermined minimum value is reached or until a fuel deficit is detected. If the fuel flow cannot be raised when a fuel deficit is detected, the inverter drawing power from the cell stack will reduce the current draw. If the fuel cell system is connected to a grid, which is also connected to a booster, this is not a problem, because more power can be drawn from the grid to make up the shortfall. If the grid is isolated, it is necessary to shed some of the load on the cells or by making up the shortfall from battery or other storage systems such as capacitor banks in the short term. 

An EFC consists of two electrodes, anode and cathode, connected by an external load (shown schematically in Figure 5.1). In place of traditional nonselective metal catalysts, such as platinum, biological catalysts (enzymes) are used for fuel oxidation at the anode and oxidant reduction at the cathode. J udicious choice of enzymes allows such reactions to occur under relatively mild conditions (neutral pH, ambient temperature) compared to conventional fuel cells. In addition, the specificity of the enzyme reactions at the anode and cathode can eliminate the need for other components required for conventional fuel cells, such as a case and membrane. Due to the exclusion of such components, enzymatic fuel cells have the capacity to be miniaturized, and consequently micrometer-dimension membraneless EFCs have been developed [7]. In the simplest form, the difference between the formal redox potential (F ) of the active site of the enzymes utilized for the anode and cathode determines the maximum voltage (A ) of the EFC. Ideally enzymes should possess the following qualities. 

Fig. 6.28 Experimental results obtained on the fuel cell power train in load following configuration on the R40 driving cycle a battery, input electric drive, and output DC-DC converter powers versus cycle length, b hydrogen, input and output DC-DC converter powers versus cycle length, c battery state of charge versus cycle length...

Emission data will be used to assess on-road emissions from fuel cell vehicles and will be characterized in terms of cold-start on reforming modes. Current fuel processor technologies are not configured to follow a vehicle load and may be... 

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