Light guide systems enhance microalgae production efficiency in outdoor high rate ponds

Microalgae provide a powerful solar driven biotechnology platform for the production of the increasing quantity of food, fuel, high value products, fine chemicals and clean water required to supply our expanding global population. Light capture fuels all photosynthetically driven microalgae processes and consequently, maximizing light capture efficiency at minimal cost is central to process design. Raceway-style and paddle wheel operated open ponds, also known as high rate ponds (HRP₀), are widely used, easy to operate and offer low-cost evaporative cooling compared to energy intensive closed systems. Here, the concept of delivering light from the surface into greater depths of the algae solution was tested at pilot-scale, by fitting a standard HRP₀ with light transmitting guides designed to allow the capture of sunlight between ~10:00 am and ~3:00 pm to improve light distribution throughout the depth of the solution (HRP+). This prototype (HRP+) was analyzed to compare light distribution and biomass production efficiency against two control systems in two separate experiments such as: 1) a standard High Rate Pond (HRP₀), and 2) a standard HRP₀ fitted with no light transmitting dummy guides (HRP-) to maintain comparable fluid dynamics system characteristics. Under semi-continuous cultivation, HRP+ showed a significantly improved light distribution within the culture compared to the HRP- when operated at moderate to high densities. An increased proportion of the culture in the HRP+ was kept within the desired optimal photosynthetic zones while dark zones (unfit for photosynthesis and causing biomass loss) were drastically reduced compared to the HRP-. The HRP+ system exhibited a 3.9-fold higher maximum areal productivity of 49.2 ± 8.3 g·m⁻²·d⁻¹ compared to HRP- (March–April 2018). Overall, this study establishes proof-of-concept of the functionality of light guides to improve light distribution and productivities in microalgae cultivation systems compared to the control systems run at the same incident light levels.