1 Introduction
The exponential increase in the world population and rapid industrialization has
resulted in continuous rise of the global energy demand. This has led to the
depletion of fossil energy reserves, climate change and environmental pollution. To
address these problems, we are compelled to find sustainable, renewable and
carbon-neutral energy sources. In this regard, hydrogen is considered as a clean and
efficient energy carrier. It has the highest energy density (142 kJ/g) among any
other known fuels (Kumar et al. 2013), and on combustion it only produces water
vapour. In addition, hydrogen can easily get converted into electricity in a fuel cell
without any pollution (Batista et al. 2014) and may be used directly as a transportation fuel in an internal combustion engine (Das et al. 2014).
Different conversion technologies can be used for hydrogen production, but till
today, it is produced through conventional technologies which include reforming
processes, gasification and water splitting. Among the conventional processes,
steam reforming of methane is widely used thermo-chemical technology and
contributes 48% of the global hydrogen demand. About 30% of world hydrogen is
produced by the reforming of oil/naphtha and 18% from the coal gasification (Das
et al. 2014). Water electrolysis is another efficient method which produces hydrogen of very high purity and accounts for 3.9% of hydrogen production (Das et al.
2014), but this technology is challenged by the high cost of electricity. Biomass
(crop residues, animal wastes, waste paper, municipal solid wastes, etc) gasification
is also employed for hydrogen production, but it has the drawback of low thermal
efficiency (Holladay et al. 2009). To overcome the various socioeconomic and
environmental limitations associated with the currently existing industrial processes
of hydrogen production, research focusing the biological hydrogen production
technology has received substantial importance. This technology is not only
environmentally benign but also requires less energy input as it can be carried out
under ambient operating conditions (Das and Veziroglu 2001). Production of
hydrogen through biological pathways is primarily controlled by the domain of
bacteria and algae. In recent days, microalgae are considered valuable and
tremendously potential source for the sustainable generation of biohydrogen.
Interest in microalgae for hydrogen production has been ensued due to the fact that
they can carry out the production of hydrogen through the process of photosynthesis utilizing most abundant natural resources, sunlight and water. Evidence of
microalgal hydrogen production through biophotolysis of water was firstly put on
record by Gaffron and Rubin in 1942. They studied the hydrogen metabolism in a
unicellular green microalga, Scenedesmus obliquus, and reported the hydrogen
production by this microorganism in the presence of light energy under anaerobic
condition after an adaptive dark phase (Gaffron and Rubin 1942). Although photobiological hydrogen production by algae has been studied for several years, in
recent decades considerable advances in this field have been made (Torzillo et al.
2015; Marquez-Reyes et al. 2015). Apart from this, over the past few years, dark
fermentation utilizing microalgal substrate for biohydrogen production has also
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H. Singh and D. Das
The exponential increase in the world population and rapid industrialization has
resulted in continuous rise of the global energy demand. This has led to the
depletion of fossil energy reserves, climate change and environmental pollution. To
address these problems, we are compelled to find sustainable, renewable and
carbon-neutral energy sources. In this regard, hydrogen is considered as a clean and
efficient energy carrier. It has the highest energy density (142 kJ/g) among any
other known fuels (Kumar et al. 2013), and on combustion it only produces water
vapour. In addition, hydrogen can easily get converted into electricity in a fuel cell
without any pollution (Batista et al. 2014) and may be used directly as a transportation fuel in an internal combustion engine (Das et al. 2014).
Different conversion technologies can be used for hydrogen production, but till
today, it is produced through conventional technologies which include reforming
processes, gasification and water splitting. Among the conventional processes,
steam reforming of methane is widely used thermo-chemical technology and
contributes 48% of the global hydrogen demand. About 30% of world hydrogen is
produced by the reforming of oil/naphtha and 18% from the coal gasification (Das
et al. 2014). Water electrolysis is another efficient method which produces hydrogen of very high purity and accounts for 3.9% of hydrogen production (Das et al.
2014), but this technology is challenged by the high cost of electricity. Biomass
(crop residues, animal wastes, waste paper, municipal solid wastes, etc) gasification
is also employed for hydrogen production, but it has the drawback of low thermal
efficiency (Holladay et al. 2009). To overcome the various socioeconomic and
environmental limitations associated with the currently existing industrial processes
of hydrogen production, research focusing the biological hydrogen production
technology has received substantial importance. This technology is not only
environmentally benign but also requires less energy input as it can be carried out
under ambient operating conditions (Das and Veziroglu 2001). Production of
hydrogen through biological pathways is primarily controlled by the domain of
bacteria and algae. In recent days, microalgae are considered valuable and
tremendously potential source for the sustainable generation of biohydrogen.
Interest in microalgae for hydrogen production has been ensued due to the fact that
they can carry out the production of hydrogen through the process of photosynthesis utilizing most abundant natural resources, sunlight and water. Evidence of
microalgal hydrogen production through biophotolysis of water was firstly put on
record by Gaffron and Rubin in 1942. They studied the hydrogen metabolism in a
unicellular green microalga, Scenedesmus obliquus, and reported the hydrogen
production by this microorganism in the presence of light energy under anaerobic
condition after an adaptive dark phase (Gaffron and Rubin 1942). Although photobiological hydrogen production by algae has been studied for several years, in
recent decades considerable advances in this field have been made (Torzillo et al.
2015; Marquez-Reyes et al. 2015). Apart from this, over the past few years, dark
fermentation utilizing microalgal substrate for biohydrogen production has also
202
H. Singh and D. Das