To overcome the limitations of the open pond system in algae cultivation, closed photobioreactors are designed to ensure that algal cells are always grown under optimal con­ditions with high consistency in biomass productivity. Since the conditions in a closed photobioreactor system are strictly controlled, the contamination level in the cultivation me­dium is minimized. This permits the cultivating of single algal strain for a prolonged period, and water sources may be reutilized for subsequent cultivation cycles (Brennan and Owende, 2010; Chisti, 2007). Closed photobioreactors are a more flexible system than the raceway pond because the photobioreactors can be optimized according to the biological and physiological characteristics of the algal strain that is being cultivated (Mata et al., 2010). For example, cultivation pH, temperature, CO2 concentration, mixing intensity, and nutrient level can be manipulated to suit the optimal growing conditions of different algal strains.

These advantages have attracted the interest of many researchers to further improve on the operating conditions of closed photobioreactors for commercial-scale implementations. Depending on the algal strains and cultivation conditions, a closed photobioreactor always offers high biomass productivity, generally in the range of 0.05-3.8 g/L/day (Brennan and Owende, 2010). Several types of closed photobioreactor designs, such as flat plate, tubular, and column, are discussed in Table 12.2. For comparison purposes, the characteristics of a raceway pond are also included in Table 12.2.

Recently, a few LCA studies have been performed to evaluate the overall energy balance for cultivating algal biomass in raceway ponds and airlift tubular closed photobioreactors, as shown in Table 12.3. From the table, we see that the airlift tubular photobioreactor can achieve high biomass productivity compared to the raceway pond, but the energy input to operate the entire system was approximately 350% higher than for the raceway pond. Despite the advan­tages of low contamination and minimum water loss due to evaporation, the airlift tubular photobioreactor consumed a huge amount of electricity to power heavy-duty pumps so that

TABLE 12.2 Various Photobioreactor Designs for Algal Cultivation. (Chisti, 2007; Mata et al., 2010; Sierra et al., 2008; Ugwu et al., 2008; Xu et al., 2009)

sufficient mixing and optimum gas-liquid transfer rate are attained. Cultivating algae using the airlift tubular photobioreactor could easily lead to a negative energy balance in producing algal biofuels if no precautionary steps are taken to reduce the energy input. Furthermore, the energy input does not include the energy used for artificial lights during the nighttime, harvesting and drying of algal biomass, water treatment, lipid extraction, and biodiesel conversion. If these factors are taken into consideration, the overall energy balance for culti­vating algae for biofuel production is expected to be even more negative, as revealed by Stephenson et al. (2010) and Razon and Tan (2011) (Table 12.1). Other photobioreactor de­signs, such as column type and flat plate, are relatively low cost compared to airlift tubular photobioreactors, making them more feasible for commercialization. However, more exten­sive research is required to improve the CO2 transfer and mixing in these photobioreactors with minimum energy input.