**12. Conclusion**

The information on energy recovery helps to establish an overall energy balance. The improvement of energy recovery through optimizing configuration, operation, microbiology and materials will make AFCs more attractive. On the other hand, adopting proper strategies to reduce the energy requirement of operation may compensate for low energy recovery. Incorporating other energy producing processes such as biogas production, algal biomass harvesting, biohydrogen etc., will increase the energy independency. Further, modifying the process for desalination, nutrient recovery and production of valuable chemicals will also

AFC has attractive properties that ensure further development and applications of this technology. It can be easily combined with green roofs to create electricity where photosynthetic and electrochemical reactions are carried out by a continuously growing population of microorganisms in living solar cells. This makes the system capable of self-repair, giving long lifetime and low maintenance. Moreover, using these reproducing organisms living in solar cells does not require any special catalysts that in solar cells are costly and toxic. Therefore, it can be used in natural surroundings with no risk of pollution. AFC also has organic material as intermediate energy carriers between the photosynthetic and the electrochemical portions of the cell which help them in generating electricity at night [48]. Closed loop AFC systems can preserve nutrients for the organisms which enable long-term, low-maintenance power production. Integrated AFC will add value to other applications such as food, agriculture, biomass for bio-energy production etc. [49, 50]. Similarly, it can be coupled with wastewater and surface water treatment to supply extra organic matter for energy production and in turns providing treated water [51].

Algae fuel cells are not without limitations. They need high cost infrastructure and energy for harvesting and growth. Another problem associated with microbial fuel cells is the pH membrane gradient which reduces cell voltage and power output. This problem is caused by acid production at the anode, alkaline production at the cathode and the nonspecific proton exchange through the membrane. The high cost of membrane commonly used in laboratories as a proton-permeable membrane would also limit the applications [52]. In addition, the slow rate of oxygen reduction at cathode electrodes is also a major limiting factor for power generation. Need for improved engineering on downstream algae biofuel processing from AFC for sustainable energy production is another challenge. It includes effective strategies for nutrient circulation and light exposure in designing photo-bioreactors that are reasonably cheap for large-scale deployment in low-cost systems. The secondary challenge related to this is the extraction of crude algae oil which is mostly addressed from the engineering side. The extraction technologies which are successfully demonstrated are relatively expensive. On the other hand, challenges associated with the management of algae bio-oil conversion to usable liquid

fuels need improved catalysts similar to petroleum crude.

maximize the benefits of AFC.

98 Microalgal Biotechnology

**11. Challenges**

**10. Application and adaptability**

AFC is a developing technology with a huge potential to capture solar energy and convert it to electricity. Similarly, the regenerated biomass during the process can be converted into secondary biofuels like solid biomass, bioethanol, biogas, etc. which is an added advantage. This technology also remediates wastewater, removes heavy metals, dye decolorizes, etc. Even though various studies have focused on increasing the performance parameters, physical and catalytic parameter variations, improvement of power generation, cost effective electrode materials, selection of bioactive organisms and finding out an alternative membrane to give cost effective solution need to be addressed. In near future, algae will become a sustainable technology and development in this research area. The possibility of using bioengineering, molecular biology, biotechnology and electrical engineering together to improve the efficiency of AFC is not a farfetched idea. Some studies like life cycle analysis based on commercial-scale, increasing power density, optimization technological methods on AFC configuration need special attention and investigation.
