Automated parametric ray-tracing analysis of fluence rate in a compound parabolic collector (CPC)-based solar Ultraviolet (UV) disinfection reactor

Abstract

Limited access to safe drinking water motivates solar water disinfection (SODIS), but conventional batch methods are constrained by small volumes and long exposure times. Continuous-flow reactors with tracking compound parabolic collectors (CPCs) can substantially increase the Ultraviolet (UV) radiation delivered to water. Their disinfection potential depends on the fluence rate encountered by pathogens along their trajectories in the reactors. Spectral fluence rate is particularly important because solar irradiance and microbial sensitivity vary with wavelength. Total UV irradiance alone cannot represent the radiation relevant to inactivation. Yet the spatial and spectral fluence-rate field inside solar UV reactors has not been systematically characterized across operating conditions and design choices. This study develops an automated parametric Monte Carlo ray-tracing framework for CPCs to resolve that field and quantify its sensitivity to solar conditions, reactor geometry, wavelength, and reflector material. A reference configuration at 317 nm is used to demonstrate model capabilities. Optical gain is defined as the ratio of the mean fluence rate over the entire pipe to the ambient direct normal irradiance at the evaluated wavelength.For the reference case, the mean fluence rate was 1.325 W m−2, corresponding to an optical gain of 7.14. The fluence field was strongly nonuniform in the tube cross section but the mean fluence rate over the tube cross section exhibited a coefficient of variation of less than 1% along the interior length. Under modeled solar tracking conditions, optical gain peaked at solar noon at a valueindependent of site and date, while its diurnal profile depended on hour angle and solar declination. Optical gain varied non-monotonically with wavelength owing to the combined spectral optical properties of the reflector, water, and borosilicate glass. Among the design variables examined, reflector reflectance had the greatest influence on optical gain at the selected wavelength. The resulting fluence fields from this work provide the spatial and spectral inputs needed for subsequent trajectory-based UV dose and microbial-inactivation predictions.