To commercialize algal biofuels, the first challenge is the mass production of algal biomass with minimal energy input and in a cost-effective manner. Phototrophic cultivation appears to be the preferred method to cultivate algae because sunlight is abundantly available at no cost. Apart from that, phototrophic algae are able to capture CO2 from flue gases and could potentially act as a superior carbon sink, offering an added advantage to this cultivation method. However, this method has its limitations, especially in temperate countries where suitable sunlight intensity is not always available throughout the year (Lam and Lee, 2012). The open pond system and the closed photobioreactor are among the cultivation systems that are suitable for growing phototrophic algae. An ideal cultivation system should meet the fol­lowing requirements: (1) has a large effective illumination area, (2) utilizes optimal gas-liquid transfer, (3) is simple to operate, (4) maintains a low contamination level, (5) has low capital and operating costs, and (6) utilizes a minimal amount of land (Xu et al., 2009). Unfortunately, this ideal cultivation system is yet to be realized, even with intensive research and field trials.

The following section details the basic design of the open and closed photobioreactors, including their advantages, limitations, and factors to consider before attempting to scale up both cultivation systems.