Conversion of solar energy

In principle, electrochemical photovoltaic cells (also known as regenerative solar cells) can be constructed very simply. They consist of a semiconductor electrode, an electrolyte containing a redox system and a counter electrode, as illustrated in the lower part of Fig. 2.30. The energy scheme for a cell with an -type semiconductor and a redox system with a standard potential close to the valence band is shown in the upper part of Fig. 2.30. On illumination of such a cell, holes are created and transferred to the reduced species of the redox couple. The electrons reach the ohmic contact on the backside of the semiconductor, traverse the external circuit, do useful work, and reach the counter electrode, at which they reduce the oxidised component 

Besides the photocnrrent, the photovoltage of a photovoltaic cell is of importance. It is obtained for i = 0, i.e., U = U. We have then 

The maximum conversion efficiency of a photovoltaic cell is defined as 

In many cases, instead of a complete i-V curve, the photocurrent iph is plotted versus the photovoltage Vph as a power plot. The latter is obtained by connecting the semiconductor and the counter electrode through a load resistauce (as iu the lower part of Fig. 2.30) that is varied from iufiuity to zero. 

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Gratzel M (2007) Photovoltaic and photoelectrochemical conversion of solar energy. Phil Trans R Soc A 365 993-1005... 

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. 

Despite the potential impact of novel photosynthetic routes based on these developments, the most ambitious application remains in the conversion of solar energy into electricity. Dvorak et al. showed that photocurrent as well as photopotential response can be developed across liquid-liquid junctions during photoinduced ET reactions [157,158]. The first analysis of the output power of a porphyrin-sensitized water-DCE interface has been recently reported [87]. Characteristic photocurrent-photovoltage curves for this junction connected in series to an external load are displayed in Fig. 22. It should be mentioned that negligible photoresponses are observed when only the platinum counterelectrodes are illuminated. Considering irradiation AM 1, solar energy conversions from 0.01 to 0.1% have been estimated, with fill factors around 0.4. The low conversion... 

In experiments investigating conversion of solar energy to chemical energy, systems of semiconductor suspensions have also been used. These systems have several advantages and disadvantages ... 

De Maria, G. et al., Thermochemical conversion of solar energy by steam reforming of methane, Energy, 11, 805, 1986. 

A.W. Adamson, University of Southern California You mentioned a thermodynamic limitation to the efficiency of conversion of solar energy to useful work. This is an interesting point, and I would like to hear more about it. I can see an entropic effect in terms of the following hypothetical cell for the system A = A, where A is the excited state of species A ... 

Phototendering, dye-induced, 9 388-389 Photothermal conversion, of solar energy, 23 10... 

Two conclusions can be made at this stage of the discussion. Firstly, when surface area constraints are important, direct conversion of solar energy wins out over biomass. Estimates of an increase in biomass yields using bio-engineering approaches amount to not more than a factor of 2 [7]. 

However, there are many challenges to exploiting direct conversion of solar energy into electricity and/or chemical fuels. For a comprehensive review of the basic research challenges in the photovoltaic conversion of solar energy see Refs. [2, 3, 9]. 

For capture and conversion of solar energy, semiconductors possess several key advantages, including relatively high absorption coefficients and the possibility to separate the photogenerated carriers at homo and heterojunctions. The energy... 

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

Ru(CN)jNO reactions with OH , SH and SOj" resemble those of the nitroprusside ion, with attack at the coordinated nitrosyl to give analogous transients and similar second-order rate constants. Ruthenium(II) complexes of the general type Ru(N2), Nj = biden-tate hgands, are important reactants. The relative inertness of Ru(NH3) + and Ru(diimine)f+ towards substitution makes these complexes definite, although weak, outer-sphere reductants (Tables 5.4, 5.5, 5.6 and 5.1). Ruthenium(ll) complexes of the general type Ru(diimine)f +, and particularly the complex Ru(bpy)j+, have unique excited state properties. They can be used as photosensitizers in the photochemical conversion of solar energy. Scheme 8.1 ... 

The abundance of incident solar energy, particularly in large desert regions with few interruptions due to cloud cover, lends to the appeal of solar heat engines. Electrical power produced via thermal conversion of solar energy by means of a conventional Rankine cycle is technically achievable. 

Nakato Y, Tsumura A, Tsubomura H (1982) Efficient photoelectrochemical conversion of solar energy with n-type silicon semiconductor electrodes surface doped with IIIA elements. Chem Lett 1071-1074... 

A model proposed for photochemical conversion of solar energy 11,14) is shown in Fig. 3. The system is made of a photoreaction couple, two kinds of electron mediator, and reduction as well as oxidation catalysts. It is designed to share the necessary functions among the various compounds because it would be difficult for one single compound to bear all the functions. A single component carrying out the total conversion would of course be the best system. 

Fig. 3. A model system for photochemical conversion of solar energy. R2 Reducing agent, R, Oxidizing agent C2 Oxidation catalyst Cj Reduction catalyst T2, T, Electron mediators P2-Pj = P Photoreaction center...

Photochemical conversion of solar energy in artificial systems (as in natural photosynthesis) is essentially based on electron-transfer between an excited photosensitizer (A ) and an appropriate electron acceptor (B)... 

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