Ideal solar absorber

Figure 2 Spectral absorptance of an ideal solar absorber. Transition from total to zero absorptance is at 2 (im wavelength.

Figure 4 Spectral reflectance of the ideal solar absorber and of silver.

The application of UV absorbers, i.e. compounds absorbing the harmful solar radiation, represents an effective solution of the problem (Rabek, 1990). The absorbed radiation is deactivated by intramolecular radiative and radiationless processes. The ideal UV absorber is expected to absorb all terrestrial UV-A and UV-B radiation but no radiation having wavelengths higher than 400 nm. Different classes of commercialized UV absorbers fulfil requirements on effective plastics protection. A group of UV absorbers acting by excited state intramolecular proton transfer (ESIPT) mechanism (Pospfsil and Nespurek, 1997) includes phenolic derivatives of benzophenone (37), various benzotriazoles, such as 38 or 39, and 1,3,5-triazine 40. Non-phenolic UV absorbers are represented by oxamide 41 and a-cyanoacrylate 42. 

Fig. 4.9. Energy diagram of the ideal solar cell structure in Fig. 4.5 absorber between an n- and a p-membrane [6]. Electrons can be exchanged through the n-membrane, while holes are blocked by a barrier in the valence band. Holes can only be exchanged through the p-membrane, while electrons are blocked by a barrier in the conduction band...

Hanna and Nozik (2006) have used the detailed balance model to calculate the power conversion efficiency of single-gap and two-gap tandem solar conversion devices which employ QD absorbers capable of MEG after photon absorption. The detailed balance model has been used previously (Shockley and Queisser, 1961 Werner et al, 1994 Spirkl and Ries, 1995 Brendel et al, 1996 Wiirfel, 1997 de Vos and Desoete, 1998 Landsberg and Badescu, 2002) to calculate the limiting efficiency of ideal solar... 

A further reduction may be achieved by assuming radiation to pass in sequence through n ideal solar cells of decreasing band gaps so that longer wavelengths are absorbed later. At the same time each cell emits radiation to keep in a steady state and this is absorbed by the two neighbouring cells. This radiation becomes important as n -> >, since for many cells the photons... 

An ideal solar-thermal surface would absorb all solar radiation up to a certain cutoff wavelength, Xc- At all wavelengths greater than Xc, the emittance would be zero. The ideal cutoff wavelength varies slightly depending on the temperature and, thus, the shape of the emittance curve. However, a value usually taken for the cutoff is 2 pm. The spectral absorptance of the ideal solar-thermal surface with a 2-pm cutoff would look like that of Fig. 2, where absorptance (a) is 1 for X < 2 pm and emittance (e) is 0 for X > 2 pm. 

The spectral selectivity of a solar absorber can be judged by comparing its solar absorptance and thermal emittance with the ideal values. Because of the overlap of the solar and thermal spectra, as seen in Fig. 1, the solar absorptance of the ideal surface is limited to a maximum of about 0.99 and the total emittance (600 K) is limited to a minimum of about 0.01. 

The conversion efficiency is probably the best single indicator of the potential usefulness of a selective solar absorber. The conversion efficiency is dependent on both the solar absorptance and the thermal emittance. For the ideal surface mentioned before, operating at 600 K, CE = 0.98. Only a few absorbers have been developed that give a CE(600 K) greater than 0.3 without resorting to solar concentration. Many of the known selective surfaces are unable to give a CE(600 K) greater than 0. 

The spectral reflectance curve of a surface can readily be compared with the ideal curve to give a visual indication of spectral selectivity. Figure 4 shows the the reflectance of the ideal solar photothermal absorber with the transition at 2 pm. The reflectance curve for silver is also included. Silver reaches a maximum reflectance of about 0.985 in the long-wavelength region, thus placing a practical limit on longwave reflectance of a solar absorber. 

Fig. 3.18 Schematic outline and ideal band diagram of an extremely thin absorber solar cell. The n-Ti02 crystallites are clustered together to form a relatively open, network-like morphology, accommodating a thin layer of CdTe absorber, with p-ZnTe at the back contact. (Reprinted from [270], Copyright 2009, with permission from Elsevier)...

The optimal sensitizer for the dye-sensitized solar cell should be panchromatic, i.e., it should absorb visible light of all colors. Ideally, all photons at wavelengths shorter than a threshold of about 920 nm (see Section 9.16.1.1) should be harvested and converted to electric current.1,2 In addition, the sensitizer should fulfill several other demanding conditions ... 

However, unlike photosynthesis in green plants, the titanium oxide photocatalyst in itself does not allow the use of visible light and can make use of only 3-4% of solar beams that reach the earth. Therefore, to address such enormous tasks, photocatalytic systems which are able to operate effectively and efficiently not only under UV but also under the most environmentally ideal energy source, sunlight, must be established. To this end, it is vital to design and develop unique titanium oxide photocatalysts which can absorb and operate with high efficiency under solar and/or visible light irradiation.3 4 ... 

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