Microalgal Cultivation Reactor Systems - Biotechnological Applications of Microalgae - page 72

lie in enhancing productivity sufficiently that it outweighs the additional cost of closed
reactors. Another alternative is to attempt to design PBRs that are cheap to build in terms
of construction materials, as well as efficient in terms of light distribution, mixing, gas
sparging, etc, which makes them cheap to operate by lowering energy requirements.
A major but rarely recognized concern, particularly for energy products such as
biofuels, is the energy balance of the production system. For a process to be econom-
ically viable and sustainable, the energy generated when the product is used must
be greater than that involved in its manufacture. The energy inputs in microalgal
reactors are particularly focused on the mixing and gas pressurization, as well as the
embodied energy in reactor materials; therefore, open systems have a more favorable
energy balance than closed systems (Richardson, 2011).
5.5 CONCLUSION
In the production of algal energy products, the aim is the biological conversion of
sunlight to a more convenient, portable, storable, and accessible form of fuel. In
the case of biodiesel production, this entails the production of algal lipids. Lipid
productivity is dependent on both biomass productivity and lipid content (Griffiths
and Harrison, 2009), which is determined by both the species used and the culture
conditions provided by the reactor.
Most large-scale commercial algal production systems to date have been for food,
feed, neutraceutical, or fine chemical production. As biofuel is a bulk commodity prod-
uct, production must be on a grand scale, and costs must be extremely low. Sterility,
particularly microbial contamination, is perhaps less of a concern for energy produc-
tion than it would be for a product such as a neutraceutical or fine chemical for human
consumption. A particular consideration with an energy product is that the energy
balance must be positive; that is, the energy recovered from the product must exceed
the energy input required for production. LCA (life cycle assessment) studies to date
suggest that biofuel production in closed reactors is unable to achieve a net energy ratio
(energy out/energy into process) of above one (Lardon et al., 2009; Richardson, 2011).
It is generally considered that closed PBRs alone will be incapable of cost- effectively
producing microalgal biomass on the scale required for biofuels (Greenwell et al.,
2010). While productivities will inevitably be lower in open raceways, it is envisaged
that open systems, due to their lower cost, simplicity of operation, and ability to scale
to large volumes, will form the basis of microalgal production for biofuels (Sheehan
et al., 1998). The lipids necessary for biodiesel production are often produced under
nutrient stress conditions. Therefore, it is likely that a two-phase system using closed
reactors to generate contamination-free inoculum with a high biomass concentration
for a second product-generating stage in open systems could be advantageous.
REFERENCES
Acién Fernández, F.G., Fernández Sevilla, J.M., Sánchez Pérez, J.A., Molina Grima,
E., and Chisti, Y. (2001). Airlift-driven external-loop tubular photobioreactors for
outdoor production of microalgae: Assessment of design and performance. Chemical
Engineering Science , 56: 2721-2732.