*1.2.4. Hydrogen*

Microalgae can directly produce hydrogen from sunlight and water, only in the complete absence of oxygen. Hydrogen is a promising future energy source because it does not emit greenhouse gases and releases water as a by-product [18]. There are limitations existing regarding the large-scale production of hydrogen as fuel. At present, hydrogen is produced by stream reformation, photofermentation [19] and photolysis of water mediated by photosynthetic algae [20]. Purple non-sulfur bacteria derive hydrogen from diverse substrates, while green sulfur bacteria get hydrogen gas from hydrogen sulfide (H<sup>2</sup> S). Other microalgae can make hydrogen directly from sunlight and water, although only in the complete absence of oxygen.

were explored. The initiation of research on algae as a source of renewable energy began by virtue of the energy crisis in the 1970s. The cultivation of algae requires few relatively simple conditions: light, water, carbon source, micro- and macronutrients and optimum temperature. Over the years, different culture systems have been developed keeping in mind the optimum conditions for microalgal growth, although it is a challenging task. The cultivation system for the growth of algae is an important requirement to aid in enhanced production of biofuels which includes open air ponds and closed controlled systems. The development of profitable algae-based fuel generation technology is yet in transition state wherein the final

Biofuels from Microalgae

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The cultivation of microalgae is a significant factor leading to enhanced biofuel production. The choice of cultivation system has to be emphasized because the phycoremediation efficiency and the yield of biofuels and other value-added products would largely depend on it. Broadly, the cultivation systems meant for microalgae are either open systems or closed systems. Hybrid systems, which are a combination of an open system and a closed system,

The open microalgal pond systems are commonly used for cultivation of microalgae as they have good opportunity to utilize the atmospheric carbon dioxide readily available in the atmosphere. There are several configurations of microalgae cultivation systems for biomass production and enhanced phycoremediation of industrial, domestic and agricultural wastewaters. The most commonly used systems for research and industrial microalgal cultivation

For open systems, location is an important criterion keeping in mind, the sufficient sunlight availability and the requirement of the algae to be cultivated. The open ponds can be natural or artificial in nature and usually include natural lagoons, circular ponds, tanks and raceway ponds. Cultivation of *Chlorella* sp. was traditionally done in circular ponds, which are usually made up of concrete. They are also equipped with rotating arm to ensure mixing of the culture and prevention of sedimentation of algal biomass. Generally, the raceway ponds comprise race track or oval channel made up of concrete, and they are meant to circulate nutrients and

The closed systems (photobioreactors (PBRs)) have well-controlled growth conditions. Generally, these reactors are designed to increase the light accessibility. They also allow

configuration is still to be explored and demonstrated at the industrial scale [29].

can be used to achieve high biomass productivity with high nutrient removal [28].

*1.3.1. Open microalgal systems*

are as follows:

• The raceway pond

• The circular pond tank

• The shallow big pond

*1.3.2. Closed algal systems*

carbon dioxide regularly to the algal cultures [30].

• The closed pond

Exploring new organisms for hydrogen production, optimization of growth conditions and use of biotechnological techniques can open new doors in making hydrogen a viable fuel for future [21, 22].
