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OCR for page 64
Potential Applications of Concentrated Solar Photons APPENDIX D Intensity Influence The photocatalytic process at a TiO2 particle can be schematically represented in a simplified reaction scheme as: The actual reaction mechanism is more complicated and can involve a number of intermediates, such as hydroxyl radicals species adsorbed on the TiO2 surface, radicals, and radical ions. A summary of literature and Solar Energy Research Institute data  for the effect of intensity [irradiance (photons/area-time)] on the photocatalyzed rate of oxidation indicates that at or above I sun near-ultraviolet equivalence, the reaction rate varies as the square root of intensity. This square root dependence can be rationalized by the increased importance of processes like e-h+ recombination [Eq. (4)] and the reaction of photogenerated e- and h+ with intermediates. A collector also brings a reflectance loss. The impact of these two phenomena on the achievable rate is now estimated (see Figure D-1). A. Photon Rate Arriving at Catalyst i. Plate: (direct + diffuse)(area) = (Id + 0.25Id)(1.0) = 1.25 Id (photons/time) ii. Concentrator: (direct)(collector x-section) + (diffuse)(tube x-section) (0.8 Id)(1.0) + (0.25Id)(0.1) (reflectance = 0.8) = 0.825Id
OCR for page 65
Potential Applications of Concentrated Solar Photons FIGURE D-1 Illustration of calculation for configurations and concentrator. B. Rate Loss Let PR = photon rate RA = reactor area Since intensity I = PR/RA and rate per unit area varies as the square root of I, Rate = (rate per area)(area)
OCR for page 66
Potential Applications of Concentrated Solar Photons REFERENCE 1. Turchi, C., and Ollis, D.F. J. Catal. 119, 483 (1989).
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