Light/dark cycle of microalgae cells in raceway ponds: Effects of paddlewheel rotational speeds and baffles installation

Highlights

• The light/dark cycle for microalgae cells in raceway ponds was simulated.

• Light/dark cycles decreased with the increase of paddle rotational speed.

• Baffles can increase the light time and the ratio of light time to light/dark cycle.

Abstract

The aim of this work was to study the light/dark (L/D) cycle in raceway ponds (RWPs) by the computational fluid dynamics (CFD) method via determining the hydrodynamics of culture media and cell trajectories. The effects of paddlewheel rotational speed and flow-deflector baffles installation on the L/D cycle were analyzed. The results indicated that, the L/D cycles of microalgae cells decreased with the increase of the paddlewheel rotational speeds, when the paddlewheel rotational speeds ranged from 5 to 12 rpm. In addition, the installation of the flow-deflector baffles in RWPs can greatly increase the light time and the ratio of light time to L/D cycle for microalgae cells. The study provided an effective method to characterize the L/D cycles in RWPs, and may have important implications for designing the effective large-scale microalgae culture system.

Introduction

Microalgae, as the largest photosynthetic microorganisms in the world, are able to utilize sunlight, carbon dioxide, water and inorganic salts to produce high-value bioproducts, synthesize potential biofuels and mitigate carbon dioxide. Due to their extremely fast growth rate, low nutrient requirement and ability to accumulate high lipid carbohydrate content, microalgae are not only suitable to produce valuable bioproducts, but also been considered as one of the most promising renewable energy sources (Carvalho et al., 2011). Large-scale microalgae cultivation is actually a complex process, which affects by multiple factors, including photosynthetic mechanisms of microalgae cells, supply of light and gases, hydrodynamics, temperature, nutrient concentration and pH of culture medium, etc. (Carvalho et al., 2011, Singh and Singh, 2015). Among these factors, it is acknowledged that light availability has been considered as one of the greatest challenges for mass microalgae cultivation (Abu-Ghosh et al., 2016, Apel and Weuster-Botz, 2015, Carvalho et al., 2011). To date, previous studies indicated that light with excessive intensity may lead to photo-inhibition, and light with low intensity will cause growth-limiting, and only light with a proper intensity, duration and wavelength is suitable for the microalgae cultivation. Recently, some researchers have found that the light/dark (L/D) cycle, in which the microalgae cells alternated between the light zones and dark zones with a proper frequency, can promote the microalgae growth than continuous light (Amini Khoeyi et al., 2012, Li et al., 2014). Therefore, understanding the L/D cycle characteristics of microalgae cells has important implications for designing the effective large-scale microalgae culture systems, and thus has fascinated various scientific communities.

To date, the characteristics of L/D cycle for the microalgae cells have been studied by two methods generally. One is to regulate the light time and dark time by the artificial light (Jacob-Lopes et al., 2009, Matthijs et al., 1996, Park and Lee, 2000). The other one is to determine the L/D cycle by setting the volume ratio of light-to-dark and controlling the flow rate of suspension through the light and dark zone (Grobbelaar, 1991, Liao et al., 2014, Merchuk et al., 1998). However, due to the complexity of L/D cycle (depending on the cell trajectory of microalgae cells, hydrodynamics and light field in culture media), both of these two methods cannot provide more information on the L/D cycles of each cells (Moberg et al., 2012, Nedbal et al., 1996). Recently, based on the computational fluid dynamics (CFD) analysis, some researchers have developed a new approach to determine the L/D cycle by identifying the cell trajectory via combining Lagrangian particle tracking method and flow field simulation. Using this method, the cell trajectories and L/D characteristics of microalgae cells in photobioreactors (PBRs) such as helical PBRs (Perner-Nochta and Posten, 2007), flat-plate PBRs (Zhang et al., 2013) and tubular PBRs (Krujatz et al., 2015, Moberg et al., 2012, Zhang et al., 2012, Zhao et al., 2015) have been investigated. These successes confirmed that the L/D cycles in microalgae cultivation systems can be determined by simulating light field and flow field using the CFD method. However, due to the difficulty in simulating flow field driven by paddlewheel (Prussi et al., 2014), the L/D cycles for microalgae cells in raceway ponds (RWPs), which are considered as a promising large-scale method to cultivate microalgae cells, were seldom known.

To address this gap, in this paper, based on Lagrangian particle tracking method, the L/D cycles of microalgae cells in RWPs were studied by identifying the cell trajectories and hydrodynamic characteristics via a commercial CFD software, i.e., ANSYS CFX. Moreover, the effects of paddlewheel rotational speed and flow-deflector baffles installation on L/D cycles of microalgae cells were also investigated. This study may have important implications in designing the effective large-scale RWPs by optimizing the microalgae cells’ L/D cycles via CFD method.