Energy conversion, efficiency

Finally, we shall discuss the theoretical efficiencies which can be reached with semiconductor-based electrochemical devices for the conversion of solar light into redox energy. The limitations are very much the same as for solid state semiconductor devices and the derivation of the theoretical conversion efficiencies can be done in a fully analogous way. 

The denominator and the numerator of the above equation are delineated in Fig. IV.20. One sees that the energy of the band gap controls the available conversion efficiency critically. For very small 

Assuming that the losses by entropy production, band bending and overvoltages in the redox reactions amount to a value of 0.6 eV one obtains from the efficiency equation above together with the solar spectrum the curve of Fig. IV.21 for the conversion efficiency in dependence on the band gap. 

Solar energy conversion efficiencies of regenera-tive cells and storage cells. 

Clearly, a storage cell can only be operated with a semiconductor having a wide enough band gap in order to provide the necessary photovoltage. Charging of the system will need a sufficient illumination intensity. Therefore, the efficiencies given in Fig. IV.21 are probably too optimistic for normal conditions. 

---

Lasers act as sources and sometimes as amplifiers of coherent k—uv radiation. Excitation in lasers is provided by external particle or photon pump sources. The high energy densities requked to create inverted populations often involve plasma formation. Certain plasmas, eg, cadmium, are produced by small electric discharges, which act as laser sources and amplifiers (77). Efforts that were dkected to the improvement of the energy conversion efficiencies at longer wavelengths and the demonstration of an x-ray laser in plasma media were successful (78). 

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 ... 

Typical polarization curves for alkaline fuel cells are shown in Fig, 27-63, It is apparent that the all aline fuel cell can operate at about 0,9 and 5()() rnA/cnr current density. This corresponds to an energy conversion efficiency of about 60 percent IIII, The space shuttle orbiter powder module consists of three separate units, each measuring 0,35 by 0,38 by I rn (14 by 15 by 40 in), weighing 119 kg (262 lb), and generating 15 kW of powder. The powder density is about 100 W/L and the specific powder, 100 W/kg,... 

For the criterion of fuel to electrical energy conversion efficiency, the diesel and gas turbine are natural choices for prime fuels such as oil and gas. 

The excellent photosensitivity and relatively high energy conversion efficiencies obtained from the bulk hetcrojunction materials arc promising. The monochromatic power efficiencies for conjugated polymer photovoltaic devices are around... 

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

Singh P, Singh R, Gale R, Rajeshwar K, DuBow J (1980) Surface charge and specific ion adsorption effects in photoelectrochemical devices. J Appl Phys 51 6286-6291 Bard AJ, Bocarsly AB, Pan ERF, Walton EG, Wrighton MS (1980) The concept of Fermi level pinning at semiconductor/liquid junctions. Consequences for energy conversion efficiency and selection of useful solution redox couples in solar devices. J Am Chem Soc 102 3671-3677... 

The band edges are flattened when the anode is illuminated, the Fermi level rises, and the electrode potential shifts in the negative direction. As a result, a potential difference which amounts to about 0.6 to 0.8 V develops between the semiconductor and metal electrode. When the external circuit is closed over some load R, the electrons produced by illumination in the conduction band of the semiconductor electrode will flow through the external circuit to the metal electrode, where they are consumed in the cathodic reaction. Holes from the valence band of the semiconductor electrode at the same time are directly absorbed by the anodic reaction. Therefore, a steady electrical current arises in the system, and the energy of this current can be utilized in the external circuit. In such devices, the solar-to-electrical energy conversion efficiency is as high as 5 to 10%. Unfortunately, their operating life is restricted by the low corrosion resistance of semiconductor electrodes. 

Transportation is expected to be an especially important apphcation of fuel cells, since their uniquely high energy conversion efficiency may result in a substantial decrease in... 

By using a multichamber system [129], exchange of residual gases between successive depositions will be strongly decreased, and very sharp interfaces can be made. Furthermore, the use of a load-lock system ensures high quality of the background vacuum, and thus low levels of contaminants in the bulk layers. Multichamber reactor systems have been used for the fabrication of solar cells, and considerable improvements in energy conversion efficiency have been achieved [130, 131]. 

To calculate the optical to chemical energy conversion efficiency, Halmann95 used the following equation ... 

Furthermore, unique reactions would be expected to occur because both oxidation and reduction sites exist close to each other (on the same particle). On the other hand, reverse reactions of the desired ones easily occur, resulting in low energy conversion efficiency. 

The photoelectrochemical reduction of C02 at illuminated p-type semiconductor electrodes is also effective for C02 reduction to highly reduced products. The combination of photocathodes with catalysts for C02 reduction leads to a marked decrease in the apparent overpotential. At present, however, light to chemical energy conversion efficiencies are still very low, and negative in some cases. 

---