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Engineered surface scatterers in edge-lit slab waveguides to improve light delivery in algae cultivation

Syed Saad Ahsan, Brandon Pereyra, Erica E Jung, and David Erickson

Open Access Open Access

Abstract

Most existing photobioreactors do a poor job of distributing light uniformly due to shading effects. One method by which this could be improved is through the use of internal wave-guiding structures incorporating engineered light scattering schemes. By varying the density of these scatterers, one can control the spatial distribution of light inside the reactor enabling better uniformity of illumination. Here, we compare a number of light scattering schemes and evaluate their ability to enhance biomass accumulation. We demonstrate a design for a gradient distribution of surface scatterers with uniform lateral scattering intensity that is superior for algal biomass accumulation, resulting in a 40% increase in the growth rate.

© 2014 Optical Society of America

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Figures (5)
Fig. 1
Fig. 1 Scatterers on slabwaveguides for algal cultivation. (a) Algae can also be excited via evanescent waves where growth is confined closer to the surface of the waveguide; (b) Uniform distribution of scatterers results in non-uniform illumination across the length of the reactor; (c) Spatially varying the distribution of pillars results in more uniform illumination along the reactor; (d) SEMs of the pillars at different densities, from left to right is variance down the length of the reactor.

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Fig. 2
Fig. 2 Characterizing angular scattering from surface scatterers. (a) Results from the 2D FEM simulation environment; (b) periodic positions of the pillars scatter the laser light in predictable manner creating interference patterns; (c) Angular scattering profiles vary with respect to the side angle of incidence of the laser; (d) angular scattering profiles also seem to vary depending on the length along the waveguide when seen through pinholes at different locations from front (1cm from front edge) and back (3.5cm from front edge)

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Fig. 3
Fig. 3 Characterizing longitudinal scattering illumination in shallow dye channels. (a) Schematic of shallow channel dye experiments; (b) the surface coverage along the length of the reactor of the posts required for uniform scattering; (c) the scattering along the length of the shallow dye channel when sample has uniform surface coverage of posts of 25%; (d) the scattering along the length of the shallow dye channel when sample has gradient surface coverage of pillars as in (b).

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Fig. 4
Fig. 4 The surface coverage of photobioreactors with different scattering schemes over the course of three days. (a) evanescent excitation; (b) uniform density of posts at 50% coverage; (c) chemically etched waveguides; (d) gradient density of pillars

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Fig. 5
Fig. 5 (a) The total surface coverage as a function of the length after the first day for the different scattering schemes;(b) the total surface coverage for different scattering schemes over the course of the three days; (c) the scattering intensity across the width of a gradient pillar sample in shallow channel dye experiments; (d) a fluorescent image of the bacteria under the uniform density of posts at 50%. Notice that algal growth occurs only in between pillars and seems to be spatially confined.

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Equations (6)

Equations on this page are rendered with MathJax. Learn more.

S ( x ) = ∫ d θ A ( θ ) e − k ( θ ) x ≈ A e − k int x
k( sc )= k i sc s c i
K( x )= 1 ( 1/ k 0 −x )
k 0 = k max ( 1+L* k max )
SC( x )= k i s c i *( 1/ k 0 −x )
P( t )=K* P 0 e rt / ( K+ P 0 ( e rt −1 ) )
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