Theoretical energy conversion efficiency

Fuel Cell Efficiency The theoretical energy conversion efficiency of a fuel cell ° is given by the ratio of the free energy (Gibbs function) of the cell reaction at the cell s operating temperature AG to the enthalpv of reaction at the standara state AH°, both quantities being based on a mole of fuel ... 

Electromotive force E° = -AG"/rtF = 1.23 V Theoretic energy conversion efficiency... 

Table 1.1 Number of electrons, standard potential, theoretical specific energy and density energy, pure compound capacity, and theoretical energy conversion efficiency for alcohol oxidation in DAFC...

The pure compound capacity accounts for the amount of charge that can be released by the fuel, it is independent of E° and proportional to the ratio njM. Therefore exhibits the same trend as E p. The theoretical energy conversion efficiency is the ratio between the reversible (maximum) electric work that can be obtained by electrochemical oxidation of the fuel and the heat released by direct combustion with oxygen, that is ... 

Another notable electrocatalytic reaction where surface oxides take part is the electrooxidation of hydrazine (N2H4). The use of hydrazine as a fuel in the direct hydrazine fuel cell has attracted research interest because the hydrazine/02 fuel cell exhibits a high open circuit potential of 1.61 V (as opposed to 1.23 V in H2/O2 fuel cell) and its theoretical energy-conversion efficiency (AG/AH) is 100 % (as opposed to 83 % in H2/O2 fuel cell) [85]. The electrooxidation of hydrazine occurs via a 4-electron oxidatirm process yielding molecular N2 [86] ... 

The theoretical solar conversion efficiency of a regenerative photovoltaic cell with a semiconductor photoelectrode therefore depends on the model used to describe the thermodynamic and kinetic energy losses. The CE values, which consider all the mentioned losses can generally only be estimated the full line in Fig. 5.65 represents such an approximation. Unfortunately, the materials possessing nearly the optimum absorption properties (Si, InP, and GaAs) are handicapped by their photocorrosion sensitivity and high price. 

In the discussions by many authors of the energy conversion efficiency of semiconductor photoelectrochemical systems, it has been tacitly assumed that the maximum theoretical photovoltages produced is the difference between E (in units of eV) and E(0x/R). The best conversion efficiency should then be obtained with a redox couple whose standard redox potential is as low as possible, with a reasonable margin x, say 0.3 V, above E (Fig. 11). From this it follows that the maximum photovoltage obtainable is equal to the band gap, Eg, in an eV unit, minus a small margin x plus A. 

The theoretical quantum efficiency of such a system would be rather low, how ever, if one considers that photosynthetic starch accumulation requires a minimum of 48 quanta/mol glucose and fermentation of the starch derived glucose presently only yields 2 mol of H2 per mol glucose. The overall quantum efficiency of the system would be 2 H2/48 absorbed quanta, or a maximum of 2% on an incident solar quantum basis. The energy conversion efficiencies would be even lower. 

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