Erick R. Bandala 1 Instituto Mexicano de Tecnología del Agua, Paseo Cuauhnáhuac 8532, Jiutepec, Morelos 62550, México e-mail: ebandala@tlaloc.imta.mx Claudio Estrada Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, México Comparison of Solar Collection Geometries for Application to Photocatalytic Degradation of Organic Contaminants A comparative study between four different solar collectors was carried out using oxalic acid and the pesticide carbaryl as model contaminants. The comparison was performed by means of a figure-of-merit developed for solar driven Advanced Oxidation Technology systems by the International Union of Pure and Applied Chemistry (IUPAC) for standard- ization purposes. It was found that there is a relationship between the photocatalyst concentration and the overall solar collector performance. Compound parabolic concen- trator was the geometry with the highest turnover rate in the photocatalytic process of oxalic acid, followed by the V trough collector, the parabolic concentrator, and, finally, the tubular collector. When a comparative analysis was carried out using the figure of merit (collector area per order, A CO ), the parabolic trough concentrator (PTC) showed the highest efficiency (lower A CO values) at low photocatalyst loads. The V trough col- lector and the compound parabolic collector showed similar A CO values, which de- creased as the photocatalyst concentration increased. The tubular collector was the worst in all catalyst concentration ranges, with the higher collection surface by the order of oxalic acid. Photocatalytic degradation of the carbamic pesticide was tested using the same experimental arrangement used for oxalic acid. In this case, the use of the figure- of-merit allowed us to observe the same trend as that displayed for oxalic acid, but with slightly higher A CO values. Results of this work demonstrate that a comparison between different reactor geometries for photocatalytic processes is viable using this figure-of- merit approach and that the generated results can be useful in the standardization of a methodology for solar driven processes comparison and provide important data for the scaling up of the process. DOI: 10.1115/1.2390986 1 Introduction Advanced Oxidation Processes AOPs are techniques that in- volve the generation of oxidant species such as the hydroxyl radi- cal HO • . These have emerged as technologies for the oxidation of a wide range of contaminants in water 1. Photocatalysis is a methodology included in the AOPs used in the treatment of nu- merous pollutants. Several studies have been carried out at the laboratory level showing the efficiency of photocatalytic methods in the removal of organic and inorganic contaminants 2,3. In recent years, the use of solar radiation for the application of a photocatalytic process has been increasing as a very attractive cost-effective alternative for this technology. Several laboratory studies and some pilot plant experiments have been performed in order to test the applicability of this technology for waste water depuration 3. A comparison between results obtained is not always easy be- cause, often, experimental parameters are different 4. The prob- lem arises not only for the comparison itself but, most impor- tantly, for the extrapolation of results in a scaling up of processes. The recent approach of this technology to commercial applica- tions has shown the need of solar figures of merit to evaluate and compare between different solar-energy-driven systems 6. Nev- ertheless, despite the fact that some applications of solar photoca- talysis have been developed almost to the point of full-scale com- mercialization 5, generally applicable figures of merit have yet to be established. There are a number of important factors in se- lecting a water-treatment technology, including economics, economy of scale, regulations, effluent quality goals, operation, and robustness. Although all these factors are important, econom- ics is still the most important issue. Comparison work has been conducted between different solar photocatalytic reactor geometries 4,7–13, with an emphasis on the concentrating versus nonconcentrating apparatus. Approaches used for contrast are dissimilar and not always use the same com- parative parameters. For instance, Hilgendorf et al. 7 and Bock- elmann et al. 10 used the photochemical quantum yield, defined by the authors as the ratio between reaction rate and absorbed photon flux, to get equal experimental conditions in different types of reactor and carry out the comparative analysis. Curcó et al. 12, used the total collection area required to achieve the same conversion for each tested photoreactor, using a mathemati- cal model for chemical actinometric characterization. Others 4,8,14,15 have performed the calculation of kinetic constants as function of the radiation absorbed in the photoreactor considering TiO 2 particle density in the slurry for comparative purposes. In all of these cases, approaches including complex mathematical mod- els or the estimation of very sensitive experimental parameters were used. From the engineering point of view, simple models capable of generating useful data for comparative purposes are as desirable as accurate mathematical approaches. In order to follow a standardized procedure, in this work we chose to use a figure of merit proposed by the International Union of Pure and Applied Chemistry IUPAC for the comparative study among Advanced Oxidation Processes for solar-driven sys- tems. This figure of merit is based on a collection area and fit with a phenomenological kinetic order regime for low pollutant con- 1 Corresponding author. Contributed by the Solar Energy Division of ASME for publication in the JOUR- NAL OF SOLAR ENERGY ENGINEERING. Manuscript received June 27, 2005; final manu- script received November 17, 2005. Review conducted by Sixto Malato.