Fuel cell system efficiency

Heat rejection is only one aspect of thermal management. Thermal integration is vital for optimizing fuel cell system efficiency, cost, volume and weight. Other critical tasks, depending on the fuel cell, are water recovery (from fuel cell stack to fuel processor) and freeze-thaw management. 

Assuming a theoretical efficiency of the fuel-cell system of around 60% and an electric-drive-train efficiency of 90%, the overall fuel-cell system efficiency is about 55%. The theoretical efficiencies for a fuel cell cannot be realised in practice. The efficiency of the system (including fuel treatment, air supply and others) is already lower than that of the pure fuel-cell stack on its own the overall efficiency of the FC drive train falls to less than 40% as a result of additional components, such as compressors, control electronics and others, see Fig. 13.6. 

Integrated Fuel Cell System Efficiency, Dynamics, Costs... 

Fig. 4.11 Stack and fuel cell system efficiency as function of current density [46]...

Fig. 6.6 Stack and fuel cell system efficiency versus DC-DC converter inlet electric power under the experimental conditions of Fig. 6.4...

An important point to consider about the stack management, with reference to an electric power train operating in dynamic conditions, as determined by road requirements, is the regulation of the stack temperature together with the other control parameters of water and reactants to avoid mass transfer limitations and membrane drying out or flooding. Moreover, the interaction between stack and auxiliaries has to be balanced taking into account the optimization of fuel cell system efficiency and reliability (see Sect. 4.6). 

Fig. 6.29 Stack and fuel cell system efficiency versus cycle length in hard hybrid configuration on the R47 driving cycle...

The air stoichiometry is another parameter which influences the stack performance. The air stoichiometry is defined as ratio between the supplied oxygen flow and the used oxygen from the stack. The fuel cell stack performance can be increased by increasing the stoichiometry as depicted in Fig. 4.26. The increase of the supplied air from the air module is feasible by increasing the electric power of the compressor. This however leads to a decrease the overall fuel cell system efficiency due to the necessary higher electric power needs of this compressor. 

Both parameters, pressurized air and air stoichiometry are important for the development of the air system. An overall optimum needs to be found between increase of the fuel cell stack power and the electrical power consumption of the air system. This relation is important to optimize the influence of the auxiUaries, specially the air system, and the fuel cell system efficiency. 

Gemmen R S and Johnson C D (2006), Evaluation of fuel cell system efficiency and degradation at development and during commercialization, Journal of Power Sources, 159,646-655. 

However, for automobile applications, H2 has to be highly pressurized due to space limitations. If the overall electrolysis plus pressurization efficiency is 70% to generate 700 bars H2, and the overall fuel cell system efficiency is 40%, then the entire electrolyzer-fuel cell system can achieve a well-to-wheel electrical efficiency of aroimd 28%. This is still about twice of the tank-to-wheel efficiency achieved by conventional internal combustion engines (ICEs). 

Ellis, M. W., Von Spakovsky, M.R. Nelson D.J. (2001). Fuel cell systems efficient, flexible energy conversion for the 21st century. IEEE Proceedings. Vol. 89, pp. 1808. 

So, what would be a reasonable fuel cell system efficiency Table 9-7 lists some ranges of components efficiency and the resulting system efficiency. 

Tjsys = the vehicle efficiency, which is a product of the fuel cell system efficiency (including the fuel processor, if any, fuel cell, and power converter), traction efficiency (typically about 93%), and electric drive efficiency (typically 90% or higher). 

FIGURE 10-9. Capacity factor and efficiency of residential fuel cell power system as a function of nominal power (for a fuel cell system efficiency in Figure 10-7 and a load profile in Figure 10-8). 

A load profile of a household may be approximated by a power distribution curve as shown in Table El. The cost of grid electricity is 0.15 per kWh and the price of natural gas is 0.32 per m. Calculate the simple payback time for a fuel cell that is sized to cover the maximum load and compare it with the payback time for a fuel cell that is sized for an average power. The fuel cell cost is 1000 per kW. Fuel cell system efficiency is as shown in Figure 10-7. 

FIhv = hydrogen heating value (kWh kg ) lower or higher heating value depending on how the fuel cell system efficiency is expressed... 

A regenerative fuel cell is installed to provide constant 2-kW power output from a PV array. The fuel cell system efficiency is 50% (LHV) and the electrolyzer system efficiency is 75% (also LHV). The fuel cell operates 12 hours during the night. During the day the PV array provides 2kW for the load and it runs the electrolyzer. During these 12 hours the electrolyzer operates with 22% capacity factor. Calculate ... 

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