۱- Introduction

۲- Microalgae, light and, photosynthesis

۳- Light and microalgae growth

۴- Light distribution systems

۵- Techno-economical and policy analysis

۶- Conclusion

References

Abstract

Raceway open ponds are preferred cultivation system for mass algal commodity production. For operational reasons, large-scale raceway ponds must be operated at a depth greater than 20 cm meaning that algal cultures are normally light limited as light cannot penetrate into the depth below 5 cm. For the efficient distribution of light into the culture, different light delivery systems such as temporal and spatial have been proposed. If the proper mixing created, the flashing light effect can be created and that would result in a significant increase in biomass productivity. However, to date, this method has not been achieved in outdoor raceway open ponds. On the other hand, spatial light dilution systems are found to be more effective and economical that temporal light dilution systems. Among spatial dilution systems, luminescent solar concentrator (LSC) panels have a potential to be commercialized for mass microalgae production. Luminescent solar concentrators combine spectrum shifting properties with spatial dilution to channel the light into the culture where it is needed. There is also the possibility of electricity production as well as higher algal biomass production when using LSC panels in open ponds or PBRs. Additionally, compared to other proposed methods, the lower capital cost can be expected when using LSCs in algal cultivation systems as there is no need to use a solar tracking system to track the sun. In this review article, the effects of photolimitation, photosaturation and, photoinhibition in concentrated microalgal cultures, as well as the impact of applying different light distribution systems on the biomass productivity and photosynthetic efficiency as a result of having more uniform distribution of light into the culture, have been outlined.

Introduction

Since 1965, microalgae have been grown commercially in various fields such as high value products (e.g., β-carotene and astaxanthin), human and animal nutrition, pharmacy and cosmetics [1–۳]. Further, microalgae have the potential to be commercialized for commodity products such as biofuel and food [4,5], as well as a tool for carbon dioxide bioremediation [6,7]. There are two main proposed microalgae cultivation systems, raceway open ponds and closed photobioreactors. To date, paddle wheel driven raceway ponds are found to be the most cost-effective cultivation systems, especially for large scale mass cultivation of commodity products [8]. Achieving higher yields per illuminated surface area and culture volume as well as shorter specific growth rates are primary goals in microalgal cultivation [9]. Large scale open ponds must be operated in depth of 20–۳۰ cm, however, there is more availability of light into the depth of shallower ponds [10]. Solar energy plays a significant role in the growth and productivity of microalgae [11]. In any cultivation system, culture productivity depends heavily on capturing light energy efficiently while the growth of microalgae is usually saturated at an irradiance of around 200 μmol m−۲ s −۱ , which is about 1/ 10 of the maximum irradiance of a summer day [1,12]. The main aim of any algal grower is to achieve maximum yield of targeted product at the shortest doubling time resulting in the highest productivity [13]. Considering that one would have to operate the culture at specific depth [14] and biomass concentrations are normally set at the highest achievable yield [15], there is a very limited control on light availability to the cell in open ponds.