**Author details**

perfect mixing to permit the light to be within an optimum value for cell growth and to improve gas exchange. Since photobioreactors solve many problems of the open cultivation, researchers have focused on designing photobioreactors for large microalgal biomass production [30]. There is a wide variation in the design of the photobioreactor depending upon their geometry and construction. Photobioreactors can be built as bags tanks, and towers. Photobioreactors can be plates or tubular and made up of plastic or glass. Tubular photobioreactors seem to be the most suitable. Bubble columns and airlift photobioreactors can also be considered since they produce a relatively high concentration of microalgal biomass product [31]. An auxiliary tank is used to separate the oxygen produced from the photosynthesis. This is important considering that excessive oxygen can negatively affect the microalgae growth [32]. Despite the advancements in the design of photobioreactors for enhancing biofuel productivity in algae cultivation, bottlenecks are yet to be addressed efficiently considering the cost economics of biofuels and their productivity.

The hybrid systems are cost-effective and can be used for large algae cultivation [33]. Hybrid systems overcome the limitations of open systems and the high initial and operating cost associated with closed systems. In the hybrid system, the microalgae are initially cultured in closed and controlled photobioreactor system and then shifted to open system in order to enhance the biomass yield [34]. This system offers promising options for algal cultivation

Alternate energy sources are to be explored because nonrenewable energy sources are getting depleted, and environmental pollution is increasing globally. The economic viability of microalgal biofuel production should be significantly enhanced by a high-value coproduct strategy, which would, conceptually, involve sequentially the cultivation of microalgae in a

algal biomass, thermal processing and utilizing residual biomass for extraction of high-value products. The synergy approach toward biofuel production can make the technology more

Enhanced production of these biofuels will help conserve our natural resources and save our environment. Algae technology has enormous potential not only for algae-based biofuels but also for food, feed, renewable chemicals and many other products that are critical for a more sustainable society. Major technological challenges like effective design of photobioreactors, innovative upstream processing and downstream processing ought to be addressed before

commercialization of microalgae as a source of biofuels for a better future.

mitigation), extracting bioreactive products from harvested

*1.3.3. Hybrid algal systems*

246 Advances in Biofuels and Bioenergy

toward biofuel production.

microalgal farming facility (CO<sup>2</sup>

viable and economically more feasible [35].

**2. Conclusion**

Archana Tiwari<sup>1</sup> \* and Thomas Kiran<sup>2</sup>

