Efficiency voltage

Substituting the expressions for actual work and reversible work in terms of voltages. Equation 4.59 leads to the definition of cell voltage efficiency as the ratio of actual voltage and the ideal or reversible voltage 

For example, the reversible voltage for a hydrogen-oxygen cell at the STP is E , = 1.229 V with water in liquid form. Considering a fuel cell with an operating cell voltage of 0.7 V, the cell voltage efficiency can be estimated as 

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By changing the device architecture e.g. by building multi- instead of single layer structures the physical and chemical processes in the LED can be greatly altered. For that reason the fundamental properties of the LED, such as threshold voltage, efficiency, emission color, brightness, and lifetime can be optimized in multilayer structures [43J. 

The fuel gas composition also has a major effect on the cell voltage of SOFCs. The performance data (33) obtained from a 15 cell stack (1.7 cm active electrode area per cell) of the tubular configuration (see Figure 8-1) at 1000°C illustrate the effect of fuel gas composition. With air as the oxidant and fuels of composition 97% H2/3% H2O, 97% CO/3% H2O, and 1.5% H2/3% CO/75.5% CO2I2OV0 H2O, the current densities achieved at 80% voltage efficiency were -220, -170, and -100 mA/cm, respectively. The reasonably close agreement in the current densities obtained with fuels of composition 97% H2/3% H2O and 97% CO/3% H2O indicates that CO is a useful fuel for SOFCs. However, with fuel gases that have only a low concentration of H2 and... 

From the insights presented, conclusions can be drawn about reserves for improvements in catalyst effectiveness, voltage efficiency, water handling capabilities, stability through optimized operating conditions, and advanced structural design. Due to recent comprehensive coverage, these topics will be revisited here with appropriate brevity. 

The macrohomogeneous model was exploited in optimization studies of the catalyst layer composition. The theory of composifion-dependent performance reproduces experimental findings very well. - The value of the mass fraction of ionomer that gives the highest voltage efficiency for a CCL with uniform composition depends on the current density range. At intermediate current densities, 0.5 A cm < jo < 1.2 A cm , the best performance is obtained with 35 wt%. The effect of fhe Nation weight fraction on performance predicted by the model is consistent with the experimental trends observed by Passalacqua et al. ... 

The current density of fhe fransition from ideally wetted sfafe to transition region or fully saturated state is a key parameter for opfimizafion of CCLs in view of fheir water-handling capabilities. A larger value of fhis critical current density allows extracting higher voltage efficiencies and power densities from PEFCs. Critical current densities depend on structural parameters and operating conditions. 

For a PEMFC, fed with reformate hydrogen and air, the working cell voltage is typically 0.8 V at 500mAcm, which leads to a voltage efficiency Eg given by... 

An additional problem arises from ethanol crossover through the proton exchange membrane. It results that the platinum cathode experiences a mixed potential, since both the oxygen reduction and ethanol oxidation take place at the same electrode. The cathode potential is therefore lower, leading to a decrease in the cell voltage and a further decrease in the voltage efficiency. 

Figure 2.11 shows the stack energy conversion efficiency as a function of stack voltage and stack power output. The stack energy conversion efficiency is calculated by the voltage efficiency (= average cell voltage of a stack/1.21 V) times its... 

Electrolyzers Eligh voltage efficiency High parasitic power consumption... 

Thermodynamic analysis was performed to determine the equilibrium redox potential for the Cu(I) Cu(II) conversion in the HCl(aq) solution. These data are important for estimating the voltage efficiency of the electrolyser and understanding the phase equilibria in the anolyte over the experimental ranges of temperature and applied potential. 

Current efficiency is defined as the fraction of the total current participating in the desired reaction. The portion of the current that produces undesired products is usually a function of the current density generally, parasitic reactions are more likely to be favored at higher current densities. Overall energy efficiency is the product of the voltage efficiency times the current efficiency. In an optimization it is useful to examine the magnitudes of the losses from the various sources and to determine whether the major losses can be minimized. 

It is clear that most of the applied voltage is required for the decomposition process. The voltage efficiency is 2.2 V/3.7 V = 0.6, (i.e., 60% of the potential drop is required to carry out the reaction). The remaining 40% of the potential is converted into heat by various irreversible processes. When this reaction is carried out in a modern cell, the current efficiency is quite high, usually more than 95%. Overall energy efficiency for this process is just under 60%. 

This deviation from the thermodynamic reversible behavior leads to a decrease of the cell voltage by ca. 0.4-0.6 V, which reduces the energy efficiency of the fuel cell by a factor eE = E(j)/ Eeq, called voltage efficiency (eE = 0.80/1.23 = 0.65 for a cell voltage of 0.80 V), so that the overall energy efficiency at 25°C for the H2/02 fuel cell becomes 0.83 x 0.65 = 0.54. 

Calling ratio Et/E, the voltage efficiency y)B and taking into account also the equation (II-9), we can give to the equation (11-11) the following simple form ... 

This means that energy efficiency is a product of current efficiency and voltage efficiency. 

Overall Energy Consumption. The voltage efficiency is the ratio of the thermodynamic decomposition voltage to the actual cell voltage ( cell) eiieigV efficiency (nener )> which is a product of the current and voltage efficiencies, can be expressed for a diaphragm cell as... 

The voltage efficiency i)v = C/cell/ theoretic reflects voltage losses due to polarization phenomena at the electrodes and ohmic potential drops at the interfaces in the electrolyte, and in the interconnectors (the theoretic cell voltage for water production is 1.23 V, voltages between 0.6 and 0.9 V are usually observed in real systems depending on current densities). 

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