One of the decisions to be taken in the cultivation of microalgae is regarding the use of open or closed photobioreactors. Closed photobioreactors of the vertical tubular, helical tubular, and flat panel type are considered to have high photosynthetic efficiency and degree of control. Closed reactors have some advantages and disadvantages over open ones.

1.1.1 Closed Photobioreactors

Due to the high productivity achieved in cultures carried out in closed photobioreactors, much attention has been paid to these systems. The configurations tested on a laboratory or pilot scale include vertical reactors, flat plate, annular, plastic bags, green wall panel (GWP), and various forms of tubular reactors, stirred mechanically or by airlifting.

Closed photobioreactors are highly efficient at biofixation of CO2, mainly due to better homogeneity of the medium and mass transfer. However, these reactors are limited by the excess O2 produced (Ho et al., 2011). The costs of these reactors are generally high (Table 1.1). Contamination can be controlled in sterile systems; however, this causes an in­crease in production costs (Amaro et al., 2011). The scale-up of open photobioreactors gener­ally occurs by increasing the diameter of the tube, but the cells do not receive sufficient light for growth (Ugwu et al., 2008).