**9. Energy analysis**

pigments respectively for light energy absorption [31, 32]. The chlorophyll pigment adsorbs wavelength between 650 and 750 nm in the red region while carotenoids pigment absorbs wavelength between 450 and 500 nm in the blue region. This mechanism of transferring excitation energy by both chlorophyll and carotenoids results in higher efficiency of photosynthesis over a wide range of wavelengths [32]. However, the absorption of wavelength by the pigments

During photosynthesis, light energy absorbed by chlorophyll induces the transfer of electrons and hydrogen ions from water to an acceptor called NADP+ where they are temporarily stored. The light reactions use solar power to reduce NADP+ to NADPH by adding a pair of

Photosynthesis rate can be increased by proper light source and light intensities leading to higher cell growth and generation of electrons. As a result, higher bioelectricities might be observed with an optimized light source installed in photo microbial fuel cells. However, only a few studies focusing on the influence of specific light supply or intensities upon power generation and cell growth of photosynthetic microorganisms have been carried out. Xing et al. [35] found that the exposure of AFC to incandescent light increased power densities by 8–10% for glucose fed reactors and 34% for acetate fed reactors when compared to the reactors operated under dark condition. But, Fu et al. [36] obtained a higher power density and open-circuit voltage when AFC was operated under dark condition by using *Spirulina platensis* as biocatalyst. Yeh et al. [37] had investigated the effect of the type and light intensity of artificial light sources on the cell growth of microalgae *Chlorella vulgaris*. They found that fluorescent light source was effective in indoor cultivation of these microalgae with an overall productivity of 0.029 g dry cell weight L−1d−1 and it was obtained by using light source having a light intensity of 9 W m−2. Similarly, *S. platensis* and *H. pluvialis* cultivated under red LED light condition showed better growth profile [35, 38, 39]. On the other hand, *Nannochloropsis sp.* showed a maximum specific growth rate of 0.64, 0.51, 0.54 and 0.58 d−1 when exposed to blue, red, green and white light respectively [40].

The most important component of AFC is a membrane which acts as a physical separator and ion selective in passing protons. Moreover, it also hinders the passage of other materials and prevents the crossover of oxygen from the cathode to the anode. Microbes grow on the surface of the membrane causing membrane fouling when AFC is operated for a long term. Membrane fouling occurs when organic foulants such as extracellular polymeric substances aggregate on the surface of the membrane. The negatively charged sulfonate groups in the membrane are prone to this type of fouling especially at low pH [41]. This bond eventually

Oxygen reduction reaction occurs on the exposed area of catalyst and its framework present in three-phase boundaries. Over potential of this is efficiently reduced by commonly used

contributes to the formation of a strong biofouling layer on the membrane.

electrons along with a proton from which electrical power may be generated [34].

depends entirely on the type and history of microalgae [33].

**8. Influence of fouling**

**8.1. Membrane fouling**

96 Microalgal Biotechnology

**8.2. Biofouling**

Economic success of AFC is directly related to power generation, algal biomass production in combination with other application in a fully biotic cell. Even though there is enormous progress in the research in this area, there are still difficulties in practical applications. The overall power output of the system decreases with the increase in the dimension of AFC. This is mainly due to poor mixing and deprived configuration of electrodes. Laboratory scale reactors having a capacity less than 50 mL relatively generate high power densities greater than 500 Wm−3 whereas configurations having larger than 2 L normally produce a power density less than 30 Wm−3. The energy data of AFC are generally expressed in normalized energy recovery expressed in kilowatt hours per cubic meter based on the volume of reactor. Simple anode substrate produces more electricity than complex substrate due to easy degradation pathways. For instance, acetate produce much higher power densities than glucose (<0.03), sucrose (−< 0.01) and wastewaters (<0.01) which are complex. Similarly, average normalized energy recovery with acetate, glucose and wastewater are 0.25, 0.18 and 0.04 kWh m−3 respectively [43].

A good separation between the electrodes is necessary to prevent interaction between oxygen diffusion, anolyte, catholyte and other materials. This is facilitated by a solid electrolyte or an oxygen gradient. The commonly used solid electrolytes include cation exchange membrane, anion exchange membrane, proton exchange membrane and other materials like textiles, woven fabrics, eggshell, papers, glass wool, etc. [44]. These materials greatly affect energy recovery, performance and capital cost of AFC. Some of the researches show that ion exchange membranes have a lower normalized energy recovery 0.14 ± 0.40 kWh m−3 when compared to the membrane-less system which has 0.23 ± 0.46 kWh m−3 (p < 0.05) [43].

Stacking AFC in parallel or series configuration helps to achieve preferred voltage and current output [45]. This shows some encouraging results for the technical feasibility of operating multiple AFCs. It is proven that a stack consisting of 40 identical 20 mL units can achieve an open-circuit voltage of 13.03 V [46]. Similarly, by shuffling the parallel and serial electric connections in a stack an external power management system can extract a power of ~200 mW which can drive a 60-W DC motor [47].

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 maximize the benefits of AFC.

**12. Conclusion**

**Author details**

Pandian Sivakumar1

Coimbatore, India

**References**

India

Radhakrishnan Kannan4

Ragunathan Balasubramanian<sup>1</sup>

Amritapuri Campus, Kollam, India

Sciences. 2010;**34**:289-292

Hydrogen Energy. 2009;**34**:7555-7560

configuration need special attention and investigation.

\*, Karuppasamy Ilango<sup>2</sup>

\*Address all correspondence to: sivakumar.p@spt.pdpu.ac.in

3 Centre for Biotechnology, AC Tech, Anna University, Chennai, India

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

, Nagarajan Praveena3

, Arumugamurthy Sakthisaravanan4 and

1 School of Petroleum Technology, Pandit Deendayal Petroleum University, Gandhinagar,

2 Department of Electrical and Electronics Engineering, Amrita Vishwa Vidyapeetham,

4 Department of Petrochemical Engineering, RVS College of Engineering and Technology,

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