future science group 387 ISSN 1759-7269 10.4155/BFS.12.39 © 2012 Future Science Ltd Raceway ponds mixed by a paddlewheel (Figure 1) have been used since the 1950s to culture microalgae for vari- ous purposes [1–6] . The ponds are typically 0.25–0.30 m deep, shaped like a race track and may approach 0.5 ha or more in area. A high surface-to-volume ratio of the raceways is intended to provide a large area for absorp- tion of the sunlight required by algae to grow. Raceways are the most successful method for growing large quantities of algae in commercial operations. Algae cultures require mixing to keep the cells sus- pended, prevent thermal stratification of the raceway, improve absorption of CO 2 for photosynthesis, achieve a uniform distribution of the nutrients and ensure that no algal cells remain in the deeper dark regions for extended continuous periods [7,8] . The cost of operating a raceway pond, the energy demand for the mixing [9] and the algae biomass pro- ductivity achieved depend on the geometric configura- tion of the raceway. A good design should minimize the energy consumption for mixing [10,11] , prevent the biomass from settling and eliminate the dead zones, which adversely affect mixing, biomass productivity and energy consumption. The ponds are mixed con- tinuously, generally for the entire duration of the algal culture. With increased interest in producing algae for biofuels [6,12] , attention to minimizing the energy requirements for algae cultivation is important in order to improve net energy recovery in the biomass [10,11,13] . This study focuses on the use of computational fuid dynamics (CFD) to simulate the flow of various com- mon configurations in raceway ponds to identify an optimal configuration that minimizes the occurrence of dead zones and is energy efficient to operate. The use of raceway ponds for algae culture is discussed in detail elsewhere [14] . CFD studies of other types of algae culture systems have been reviewed by Bitog et al. [15] . Methodology The starting point was the simulation of the fluid flow pattern in a typical raceway pond with two bends (Figure 1) , as ponds with more than two bends have a higher energy consumption and are, therefore, not Design of raceway ponds for producing microalgae Kawisra Sompech 1 , Yusuf Chisti 2 & Thongchai Srinophakun* 3 Background: A raceway pond for producing algal fuels must be designed to minimize the energy required for mixing, otherwise the net energy recovery in the biofuel will be low. Results & discussion: Computational fuid dynamics modeling was used to characterize the energy demands for mixing full-scale raceways of various confgurations. The locations of the dead zones and the conditions required for eliminating them were identifed. The existing geometric confgurations of the raceways were compared, to identify the best confguration. Conclusion: An inexpensive raceway confguration with a minimum of three semicircular defector bafes and a modifed end of the central divider was found to be the most energy efcient, while also being able to completely eliminate the dead zones. RESEARCH ARTICLE 1 School of Chemical Engineering Practice, Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand 2 School of Engineering, Massey University, Private Bag 11 222, Palmerston North, New Zealand 3 Center of Excellence for Petroleum, Petrochemicals & Advanced Materials; Center for Advanced Studies in Industrial Technology, Kasetsart University, Bangkok 10900, Thailand *Author for correspondence: E-mail: fengtcs@ku.ac.th Biofuels (2012) 3(4), 387–397 For reprint orders, please contact reprints@future-science.com