Luminescent solar concentrators to increase microalgal biomass productivity

Light is the main limiting factor of any mass microalgal cultivation resulting in relatively low biomass productivity in raceway ponds. Microalgal cells in open ponds are normally photoinhibited on the surface and photolimited at the depth of the cultures where there is total darkness. Delivering light to the microalgal cells at the depth of cultures in large scale raceway ponds can increase biomass productivity. Luminescent solar concentrators (LSCs) can potentially be an economical light-diffusing system to be used in algal biotechnology. The main advantage of luminescent solar concentrators is that a solar tracking system is not needed. This results in less cost compared to other diffusing systems. Luminescent particles such as organic dyes or quantum dots (QDs) are the main constituents of LSCs. Luminescent particles absorb photons when light hits the surface of LSCs and the absorbed light is reflected internally and emitted from the edges at a longer wavelength. To the best of my knowledge, to date, there have been no attempts in using LSCs as a light guide for the growth of microalgae in any open system. Thus, the main aim of this study was to evaluate the effect of LSCs as a light guide to deliver light to the depth of microalgal cultures in raceway ponds to increase both biomass and high-value productivities. To assess the viability and efficacy of the LSCs system in an algal raceway pond, it is first necessary to select the most suitable microalgae species for this purpose. Three species, Arthrospira platensis (MUR 129), Scenedesmus sp. (MUR 268) and Chlorella sp. (MUR 269). were chosen for a laboratory experiment to investigate the effect of red and blue LSCs on the productivity of cultures. Arthrospira platensis showed up to 9% higher productivity when red LSCs were used compared to control and blue LSCs. The biomass productivity of Scenedesmus sp. cultures under red LSCs was also 30% and 4.5% higher compared to that in control and blue LSCs. The growth rate of Chlorella sp. cultures did not improve under red and blue LSCs. Furthermore, Scenedesmus sp. culture resulted in 30% higher cell density in cultures with red LSCs compared to that in control. Thus, Arthrospira platensis and Scenedesmus sp. were chosen as the most suitable species for further outdoor investigations using micro raceway ponds. In the next stage, Arthrospira platensis and Scenedesmus sp., were grown using red and blue LSCs and compared with control cultures with no LSCs using micro raceway ponds (0.1 m2) with the final culture volume of 21.5 L. The LSCs were installed on the edge of raceway ponds to have 200 mm of a panel inside the raceway pond and 100 mm of the panel out of the pond facing the sun to collect visible and diffuse light from sunlight, downgrade and, transfer it to the depth of A. platensis cultures. The bottom part of LSCs inside the A. platensis culture was also laser-cut to have enough surface area to increase the irradiance. Arthrospira platensis cultures when grown with red LSCs, reached a significantly higher biomass yield (1.77 ± 0.014 g L−1) compared to control (1.53 ± 0.002 g L−1) and blue LSCs (1.59 ± 0.056 g L−1). The biomass productivity of 57 ± 3.2 mg L−1 d−1 (12.2 g m−2 d−1) was obtained when Arthrospira cultures in raceway ponds were equipped with red LSCs. This was 24% and 26% higher than the biomass productivity of Arthrospira cultures when grown in raceway ponds with blue LSCs and control. There was no significant difference between the productivity of Arthrospira cultures with blue LSCs and control. Furthermore, the maximum phycocyanin productivity in Arthrospira cultures with red LSCs was 8.49 ± 0.9 mg L−1 d−1, which was 14% and 44% higher than that in cultures with blue LSCs and control cultures. In addition, the phycocyanin content of A. platensis was 136 mg L−1 (77 mg gbiomass−1) and 141 mg L−1 (89 mg gbiomass−1) under red and blue LSCs, respectively. The results of showed that red LSCs can significantly increase Arthrospira’s growth and productivity. Based on the outcome of this study, only red LSCs were applied to outdoor Scenedesmus sp. cultures in the next experiment. When grown with red LSCs, Scenedesmus sp. cultures reached a higher cell density compared to the control. Furthermore, the maximum specific growth rate (µ) of Scenedesmus sp. cultures with red LSCs was 16% higher than control with no LSCs. The biomass productivity of 43.6 ± 1.3 mg L-1 d-1 (9.4 g m-2 d-1) was obtained for Scenedesmus sp. cultures equipped with red LSCs which was 18.5% higher than that for Scenedesmus sp. cultures when grown in raceway ponds with no LSCs. Further, the protein content of Scenedesmus sp. under red LSCs was 436 mg gbiomass-1 (43.6%) which was 17.5% higher than that in control. The lipid content of Scenedesmus cultures under red LSCs (133 mg gbiomass-1) was also 10% higher compared to control with no LSCs. However, the carbohydrate content of Scenedesmus sp. cultures with red LSCs and control was not significantly different. The results of all indoor and outdoor experiments showed that using red LSCs on Arthrospira platensis and Scenedesmus sp. cultures was promising. More light availability to microalgal cells into the depth of the cultures is the most likely reason for having higher productivity in cultures with red LSCs. From the energy perspective, the results showed that the total amount of photosynthetic active radiation (PAR) available for A. platensis and Scenedesmus sp. cells at the depth of each pond emitting from four red LSCs is 34 µmol photons s−1. In other words, using red LSCs in each outdoor raceway pond bring about 34 µmol photons s−1 more light to the depth of A. platensis and Scenedesmus sp. cultures. This means injecting 34 µmol photons s−1 deep into the A. platensis and Scenedesmus sp. cultures where it would otherwise be in full darkness. This helps move the light from the photosaturated surface to the depth of the microalgal cultures. Moreover, based on the mixing rate, the thickness of the LSCs and surfaces of each red LSC, A. platensis and Scenedesmus sp. cells received brief bursts of light when they pass an edge and a surface of LSCs. For instance, considering PAR emitting from an edge of a red LSC (110 Wm−2/506 µmol photons m−2 s−1), A. platensis and Scenedesmus sp. cells received around 506 µmol photons m−2s−1 in 27 ms from each edge and 276 µmol photons m−2 s−1 in 218 ms when they pass each surface of a red LSC. In other words, it can be said that A. platensis and Scenedesmus sp. cells with red LSCs received brief bursts of light with different intensities for durations less than a second inside the cultures while there was total darkness for the cultures without LSCs. Finally, the costs of biomass and phycocyanin production using luminescent solar concentrators as a light delivering system on an industrial scale raceway pond cultivation of Arthrospira was assessed. The results showed that using red luminescent solar concentrators would result in a biomass and phycocyanin production costs of AU$3.16 and AU$ 125 per kg, respectively, which are 14% and 35% lower than the corresponding costs in a conventional raceway pond with no LSCs. The biomass and phycocyanin production costs of Arthrospira cultivation in conventional raceway ponds (with no LSCs) were AU$3.67 and AU$ 187 per kg, respectively. These results showed that using LSCs for growing Arthrospira can significantly lower the cost of biomass and phycocyanin production if the same size production facility is used. In conclusion, this study clearly showed that using LSCs in a raceway open ponds can be a promising method to increase the biomass productivity of a microalgal culture while reducing the production costs of biomass and the desired high-value product