The Application of Transparent Glass Sponge for Improvement of Light Distribution in Photobioreactors

Abstract

A new concept for improving light dilution and light distribution in photobioreactors by applying transparent, openpored sponges was realized during cultivation experiments. A manufacturing process based on the polymer replica technique was established. A polyurethane template is impregnated with a nanoscaled SiO 2 powder suspension (solids loading of 60 wt% at around pH 10) and dried. The green body is prepared by burning-out the polymer at 800°C. Subsequently the sintering of the remaining SiO 2 structure to transparent cellular bodies is carried out at 1330°C. Many different factors influence the process. The slurry stabilization (viscosity, zeta-potential, pH) is important to generate a stable and self-supporting SiO 2 shell around the template and achieve appropriate green bodies. The sintering temperature determines the transition of amorphous (transparent) into crystalline silica. The 8 ppi sponge has a porosity of 0.9, the specific surface area of 568 m -1 means an increase of the (inner) surface to volume ratio of the reactor. Porous glass has a very high surface to volume ratio (about 0.26 m 2 /g of glass at 10 µm pore size). Because of multiple and complex reflection of light into the porous glass, the algae can obtain light energy at any location of the total volume. First cultivation experiments in special designed Miniplate reactors show an increase in growth rate (about +25%) at low cell densities. The photo conversion efficiency was enlarged from 4.9% for the empty reactor to 5.6% and 5.9% for the 8 and 15 ppi sponge filled reactor. The effect and the improved efficiency of biomass build-up per applied light will be more apparent at high cell densities. Therefore concentration of the standard Tris-Phosphate (TP) medium (2.5 fold) and feeding are necessary.

Integration in microalgal bioprocess development: Design of efficient, sustainable, and economic processes

“…For the required light intensity, two cultivation strategies have been developed in the recent years. The first approaches the achievement of high growth rates at low light intensities as in case of high cell densities or light dilution . In the second strategy the saturation effects of high light intensities are considered.…”

Hydrogen production with the microalga Chlamydomonas reinhardtii grown in a compact tubular photobioreactor immersed in a scattering light nanoparticle suspension

“…While preparing this manuscript, another interesting application involving scattering nanoparticles to improve light distribution was proposed [27]. As a scattering matrix, sponges coated with scattering SiO 2 were used, directly immersed in the cultures.…”

“…Between the two layers is an intermediate zone where light becomes increasingly limited [17]. To attenuate the effects of this physical constraint that actually acts as a strong drawback in algal biotechnology, different techniques have been proposed so far, such as fast mixing in order to induce rapid light/dark cycles within the culture depth [11,18e20], ultra-dense thin-layer cultures flowing on tilted smooth or corrugated surfaces (cascades) [21e23], high intensity tangent light dilution/scattering devices [24e26], and application of scattering materials in the culture suspension [27]. All these strategies aim at improving light conversion efficiency by "diluting" the incident light over a larger culture surface and to reduce, as much as possible, the amount of time the cells spend in the dark i.e., deep layers of the culture and dark parts of the reactors.…”

Photobioreactors with internal illumination – A survey and comparison

“…Another way to internally illuminate PBRs via light guides was presented by Jacobi et al (2012) [33]. To guide the light from the reactor surface deeper into the culture they used transparent sponges made of glass.…”