Chemical Engineering Journal 145 (2008) 316–327 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Capturing sunlight into a photobioreactor: Ray tracing simulations of the propagation of light from capture to distribution into the reactor Jan-Willem F. Zijffers ∗ , Sina Salim, Marcel Janssen, Johannes Tramper, René H. Wijffels Bioprocess Engineering Group, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands article info Article history: Received 2 June 2008 Received in revised form 14 August 2008 Accepted 16 August 2008 Keywords: Photobioreactor Ray tracing Light guide Fresnel lens Sunlight Microalgae abstract The Green Solar Collector (GSC), a photobioreactor designed for area efficient outdoor cultivation of microalgae uses Fresnel lenses and light guides to focus, transport and distribute direct light into the algae suspension. Calculating the path of rays of light, so-called ray tracing, is used to determine local light intensities inside the photobioreactor based on the focused rays of sunlight. Reflection and refraction of the propagating rays of sunlight from point of focus to refraction into the photobioreactor is calculated. Refraction out of smooth and sandblasted distributor surfaces is simulated. For the sandblasted surface a specific structure is assumed and corresponding reflection and refraction patterns are described by a 2-dimensional modeling approach. Results of the simulations are validated by measurements on real light guide surfaces. The validated model is used to determine the influence of the solar angle on the uniformity and efficiency of light distribution over the light distributor surface. The simulations show that efficient capturing of sunlight and redistribution inside the algal biomass can be achieved in the Green Solar Collector at higher elevation angles of the sun, making the Green Solar Collector suitable for operation at low latitudes with a high level of direct irradiance. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The volumetric biomass productivity of a microalgae culture in a photobioreactor is determined by the light input in the photo- bioreactor and the efficiency of light use for microalgal growth. Light intensity is an important parameter for the photosynthetic efficiency in photobioreactors. Exposure to full sunlight intensi- ties limits the microalgae’s light use efficiency, while prolonged exposure to darkness stops the microalgae’s autotrophic processes. To efficiently cultivate microalgae, the exposure to light has to be carefully regulated and therefore it is important to know local light intensities inside a photobioreactor [1,2]. The Green Solar Collector (GSC) [3] was developed to obtain high area biomass yields by efficiently capturing, transporting and re- distributing available direct sunlight into the microalgae culture. It was designed to supply captured sunlight to the microalgae at reduced intensities. This in combination with a short light path and turbulent mixing is expected to result in high light use efficiency and high volumetric biomass productivity. To be able to run a light efficient cultivation an investigation into distribution of captured sunlight into the reactor compartment is necessary. A uniform dis- ∗ Corresponding author. Tel.: +31 317 483770; fax: +31 317 482237. E-mail address: jan-willem.zijffers@wur.nl (J.W.F. Zijffers). tribution on the light distributor surface is needed to have a reduced and uniform light intensity inside the microalgae culture such that light can be efficiently used by the microalgae. Extracting light uniformly from the lateral surfaces of optical fibers and light guide or distributor like structures has been a prob- lem in previous research [4–6]. The problem was either caused by getting the light into the illumination plate [4] or getting the light out of the distributor over its lateral surface [5,6]. Csögör et al. [7] managed to improve the lateral distribution to a large extend by roughening the surface of the illuminating surface of the distribu- tor. The short distance between the light source and the end of the distributor helped to achieve a more uniform illumination in the work performed by Csögör et al. [7]. Sunlight is captured into the GSC through polymethylmethacry- late (PMMA) Fresnel lenses that are able to rotate over two axes to follow the sun. The lenses can rotate over the light guide and the distance between the lens and light guide is adjustable to maintain the line of focus on top of the light guide as explained by Zijffers et al. [3]. Light focused on top of the light guides refracts into the light guides and internal reflection in the guides directs light into the bioreactor compartment. Internal reflection inside the guides and refraction out of the guides into the algal suspension is calcu- lated based on the specific incident angles of sunlight rays on the interior surface of the light guide. Based on the relation between the light capturing surface of the lens and the light emitting surface of the light guide, the light intensity on the light distributor surface 1385-8947/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2008.08.011