Cell efficiencies

EH( is the enthalpy of formation and is also considered to be the calorific value of fuel. The value of EH( is positive when energy is required for the process whereas its negative value represents release of energy. The maximum possible efficiency is defined as  

The efficiency of a fuel cell is determined by considering the standard free energy of the formation of water 

The difference between the two heats of formation is 44.01 kJ/mol and gives the molar enthalpy of vaporisation of water (latent heat). The efficiency of an ideal fuel cell operation at standard conditions is (H ) = 0.83 or 83% for the free energy of 237.3 kJ/mol. 

The efficiency depends upon the temperature and pressure, etc., of the system, which is discussed later on in this chapter. The efficiency of a system can also be expressed in terms of the maximum EMF of a cell. Due to the losses in the fuel cells, the efficiency of systems decreases and actual cell voltage (Vactuai) decreases as compared to the ideal cell voltage (Videai)- 

For illustration, the efficiency of a hydrogen fuel cell operating at 25°C and after pressure is given by 

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Cell efficiency is improved by inhibiting or regulating the synthesis of unneeded enzymes, so there are two classes of enzymes—those that are constitutive and always produced and those that are inducible, i.e., synthesized when needed in response to an inducer, usually the initial substrate in a pathway. Enzymes that are induced in one organism may be constitutive in another. 

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 ... 

When the fuel gas is not pure hydrogen and air is used instead of pure oxygen, additional adjustment to the calciJated cell potential becomes necessary. Since the reactants in the two gas streams practically become depleted between the inlet and exit of the fuel cell, the cell potential is decreased by a term representing the log mean fugac-ities, and the operating cell efficiency becomes ... 

ANIMAL CELLS EFFICIENTLY CAPTURE THE ENERGY RELEASED DURING CATABOLISM... 

In the 1880s there was great interest in trying to develop storage cells efficient enough and with... 

The great energy consumption, limited recources of traditional fuels and environmental problems have lead to intensive research on the conversion of solar energy during the last fifteen years. Conversion into electrical energy has been realized in technical devices consisting of pn-junction photovoltaic cells. Efficiencies of up to 20 % have been obtained with single crystal devices and around 9 % with polycrystalline or amorphous layers. 

FIG. 62. Normalized solar cell efficiency as a function of illumination time for different power densities as obtained by continuous illumination of 1000-W/m" AM 1.5 light. The initial efficiencies of the four cells were 9%, 109f. 9c. and 69c for 28-. 42-. 57-. and 113-mW/cm power density, respectively. 

Using the tri-iodide/iodide redox couple and the sensitizers (22) and (56), several groups have reported up to 8-10% solar cell efficiency where the potential mismatch between the sensitizer and the redox couple is around 0.5 V vs. SCE. If one develops a suitable redox couple that decreases the potential difference between the sensitizer and the redox couple, then the cell efficiency could increase by 30%, i.e., from the present value of 10% up to 13%. Towards this goal, Oskam et al. have employed pseudohalogens in place of the triiodide/iodide redox couples, where the equilibrium potential is 0.43 V more positive than that of the iodide/iodide redox couple.17 Yamada and co-workers have used cobalt tris-phenanthroline complexes as electron relays (based on the CoII/m couple) in dye-sensitized solar cells.95... 

Enhanced photovoltaic cell efficiency was achieved via incorporation of highly electron-deficient oxadiazole moieties on side chains of poly(phenylene vinylenejs and poly(fluorene)s <2006SM949>. The synthesis of terminal... 

A nanostructured Ti02/CdS/CuSCN solar cell has been fabricated, where the CdS layer was grown by the basic SILAR technique. The cell efficiency... 

As potential vectors for gene therapy, adenoviruses display a number of both advantages and disadvantages (Table 14.3), and they have been used in over 300 gene therapy trials to date. Their major advantage relates to their ability to infect non-dividing cells efficiently and the usually... 

A wide range of operating conditions and design philosophies affect mercury cell efficiency. For example, the fundamental distinction between a resaturation and a waste brine process influences the temperature and brine strength profile along the length of the cell and hence the overall efficiency. Another important factor is the quality of the brine. Impurities in the brine can cause base-plate deposits, which tend to reduce the anode/cathode gap. This gradual reduction in gap requires either manual or automatic adjustment and, eventually, the cell must be taken off-line and the thick mercury removed. 

For both low temperature electrolysers, the biggest gain in efficiency is to be expected from an improvement in Balance of Plant components, taking into account the big gap between cell efficiency (80-90%) and system efficiency (50-60%). In the case of SPE electrolysers, catalytic research should therefore be directed to making the catalysts more tolerant to contaminants. For alkaline electrolysers, in addition to this, more active electrodes could lower capital costs. 

P.T. Landsberg, H. Nussbaumer, G. Willeke, Band-band impact ionization and solar cell efficiency, J. Appl. Phys. 7A (1993) 1451-1452. 

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