Fuel cells meaning defined

The system electrical efficiency of a fuel cell is defined as the ratio of the electrical power sent out by the fuel cell system to the chemical energy (i.e., the enthalpy) of the fuel (e.g., H2) received by the fuel cell system. This is the efficiency a user cares abouf fhe mosf, and a fuel cell developer tries every means to achieve the highest number. 

Any governing model equations have to be supplemented by initial and boundary conditions, all together called side conditions. Their definition means imposing certain conditions on the dependent variable and/or functions of it (e.g. its derivative) on the boundary (in time and space) for uniqueness of solution. A proper choice of side conditions is crucial and usually represents a significant portion of the computational effort. Simply speaking, boundary conditions are the mathematical description of the different situations that occur at the boundary of the chosen domain that produce different results within the same physical system (same governing equations). A proper and accurate specification of the boundary conditions is necessary to produce relevant results from the calculation. Once the mathematical expressions of all boundary conditions are defined the so-called properly-posed problem is reached. Moreover, it must be noted that in fuel cell modeling there are various... 

Apart of this traditional meaning, recently the term of membrane electrode (assembly) is used. It is defined as two electrodes (the anode and the cathode) with a very thin layer of catalyst, bonded to either side of an ion-exchange membrane. It is an element of polymeric membrane fuel cell. 

Consider, for example, a catalyst layer with thickness / = 10 pm and I = 1 A cm-2 (corresponding to D tv 10-4 cm2s 1 at T = 300 KPo2 = 1 atm), which obeys the phase diagram in Fig. 14 (at defined composition). The optimum current density range of operation for this electrode would be 0.5-1.0 A cm-2. If, however, for example, for reasons of maximum efficiency, the target current density of fuel cell operation is 0.25 A cm-2 then the electrode thickness should be increased to at least l = 30 pm. This would mean that (since composition is kept fixed) the material costs of the catalyst layer would increase as well by a factor of 3. It is thus obvious that a phase diagram of the catalyst layer is a powerful tool for a rational choice of the optimal thickness of the layer. 

Novel concepts of electrode fabrication focus on ways to realize well-defined 3D porous electrode architectures, in which the distribution of TPB is fine-tuned for maximum fuel cell performance. In principle, this means electrode manufacture... 

Fuel cell electrodes are porous gas diffusion electrodes, which are usually described in modeling using homogenization [144]. This means that the pore electrolyte and the electrode material share the same geometrical domain and the electric potential in the electronic and ionic conductors are present in the same geometrical domain. Also the concentration variables for the species in the gas phase, the species dissolved in the electrolyte, and the constituents of the electrolyte may be present in the same geometrical domain defined by the gas diffusion electrodes. The electrochemical reactions that occur at the interface between the pore electrolyte and the electrode are introduced as sources or sinks in the material and current balances. To calculate the chemical composition in the electrolyte and in the gas phase in every point in space in a geometrical domain, material balances for each of the species in the solution as well as a conservation of mass for the whole have to be defined. The conservation of mass for the whole solution may eliminate one of the species material balances, which for a dilute solution usually is the solvent s material balance. The constitutive relations in the electrolyte may be the... 

What a fuel cell is not is a battery or an internal combustion engine. With a fuel cell as long as there is a fuel supply it will keep producing electricity, whereas a battery only has a defined unit of fuel, which once exhausted stops producing electricity. In a fuel cell, the conversion to power is through electrochemical means, whilst in an engine it is by combustion. 

In many of the sections that follow, the term stoichiometric is used. Its meaning could be defined as just the right amount . So, for example, in the simple fuel cell reaction... 

Stoichiometric Ratio In fuel cell parlance, the term stoichiometry is defined as the inverse of the Faradic efficiency. Smdents may be confused with this terminology, since the stoichiometric condition typically describes a balanced chemical reaction equation with no excess oxidizer. Here, the term stoichiometry is used slightly differently, and its meaning is similar to the definition of equivalence ratio used in combustion. Unlike chemical reactions, the reduction and oxidation reactions are separated by electrolyte, so each electrode can have a discrete stoichiometry ... 

In a fuel cell work is done in the isothermal process by ions which flow from one side of the electrolyte to the other. The flow of ions is possible due to their concentration gradient occurring on both sides of the cell. In the case of SOFC, the gradient is equivalent to the pressure differential, which means isothermal expansion. Maximum work during isothermal expansion is defined by the following equation [3, 4] (see Sect. 2.1.3 for details) ... 

In this model, the drop of voltage due to a variation of fuel utilization is modelled also considering the effect on the cathode diffusion. The effect of diffusion on the cell voltage sensitivity to fuel utihzation has a significant effect when the limiting current density is approached, or for low value of the air stoichiometry. It should also be noticed that in the equations (34) and (35), as posed in [3], the variable L has not the meaning of excess of oxidant with respect to the stoichiometric requirement, and so it has a different meaning compared to the air stoichiometry Xax as defined and used in this Chapter. 

Another example is the systematic analysis undertaken by Palsson et al. on combined SOFC and gas turbine cycles [36]. In combination with a robust and accurate 2-D SOFC model, the system-level model attempts to provide an unbiased evaluation of performance prospects and operational behaviours of such systems. The 2-D SOFC model was integrated into a process simulation tool. Aspen Plus , as a user-defined model, whereas other components constituting the system are modelled as standard unit operation models. Parametric studies can be carried out to gain knowledge of stack and system behaviour such as the influence of fuel and air flow rate on the stack performance and the mean temperature and the effects of cell voltage and compressor pressure on the system efficiency. The pressure ratio is shown to have a large impact on performance and electrical efficiencies of higher than 65% are possible at low-pressure ratios. 

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