An optical and energy absorption analysis of the solar compound parabolic collector photoreactor (CPCP): The impact of the radiation distribution on its optimization

This work presents a systematic and quantitative study of the optical and photon absorption performance of the solar compound parabolic collector photoreactor (CPCP) by a rigorous radiative Monte Carlo model. The parameters of the system, such as the properties of the receiver wall (refractive index and wall thickness), the collector reflectivity, the solar irradiation conditions and the optical thickness were varied to represent different experimental conditions and to investigate their impact on the radiation absorption of the CPCP. The results showed that the role of the receiver wall should be described accurately to determine an optimal CPCP design and to model its energy absorption field (the exclusion of the wall effect causes errors up to 12%). Finally, a new method to optimize solar CPCP was proposed based on an Energy Absorption Distribution Index (EADI). This index accounts for the cost of increasing the total rate of energy absorption (TREA) on the radiation distribution in the photoreactor volume and therefore is a more comprehensive approach than the extended method followed in the literature, which determines the optimum photocatalyst concentration only from the maximization of the TREA. An optimum apparent optical thickness interval was derived from these two approaches, which is tau(app,opt) = (2.20-3.58) +/- 0.20 for all the solar irradiance conditions studied (diffuse or direct radiation at different incidence angles). Therefore, to operate CPCPs efficiently, the tau(app,opt) should be at least 2.20 +/- 0.20 and a further increase will depend on the intrinsic kinetics and hydrodynamics of the process, but should not exceed tapp = 3.58 +/- 0.20.