Unlike open raceway ponds, where risk of contamination is distressing, single spe­cie culture may be grown in closed photo-bioreactors (CPBs) without any contami­nation risk. Huge quantities of algal biomass have been produced using CPBs for fuel production previously using different types of CPBs, including tubular, col­umn, and flat-panel (Fig. 18.3) (Sanchez Miron et al. 1999; Pulz 2001; Carvalho et al. 2006).

A tubular photo-bioreactor comprises of an array of straight transparent (plastic or glass) tubes. This tubular array also called as a solar collector captures the sun­light. The solar collector tubes are generally <0.1 m in diameter. Tube diameter is limited because light does not penetrate too deeply in the dense culture broth that is essential to ensure higher biomass productivities in the CPB. Microalgal broth is circulated from a reservoir to the solar collector and back to the reservoir making it a continuous operation. Capturing of sunlight may be enhanced through the orienta­tion of the collector facing sun pathway and through the use of transparent pipes or flat panels as solar collectors. Alternatives of tubular photo-bioreactors exist in con­sidering flaws related to tubular photo-bioreactors (Carvalho et al. 2006; Pulz 2001)

but these alternatives are not commonly used for mass culture. Technically, achiev- ability of tubular photo-bioreactors is maximum by artificial illumination (Pulz 2001) but costly than natural source of light (sun).

The CPBs require cooling during day-light hours; temperature control at night is also useful, in a way. For example, the loss of biomass due to respiration at night may be reduced by lowering the temperature at night. Outdoor tubular photo­bioreactors are effectively and inexpensively cooled using heat-exchangers. This may be achieved by placing a heat-exchange coil in the degassing column. Evaporative cooling by water sprayed on tubes (Terry and Raymond 1985) is also used and has demonstrated successfully in dry climates. Large tubular photo­bioreactors have also been sited within temperature controlled green-houses (Pulz 2001), but doing so is not worthwhile for economical production of biodiesel.