Page 91 - Advances in bioenergy (2016)

on biofuel production, e.g., producing liquid biofuel and using the residual biomass to generate
methane by anaerobic conversion 13,39,48,49 or to be subjected to thermochemical conversion
(e.g., Ref 42). Another option is to generate a variety of partially nonenergetic microalgal
coproducts, using a biorefinery (e.g., Refs 23–26, 50, and 51). Still another option is the
coproduction of biodiesel, glycerol, and algal cake (which may serve as feed) (e.g., Ref 27).

Estimating realistic future biomass and lipid yields from open ponds is hard. On one hand,
−2
there are efforts to increase biomass and lipid yields (m ) of algal cultivation area by
selection, by genetic engineering including reconfiguration of the photosynthetic system and
metabolic engineering, and by synthetic biology. 11, 52-58 It has been proposed to increase
biomass yields by genetic engineering for truncated antennae, and to increase biomass and
lipid yield by metabolic engineering. Demonstrations of the intended increases in yield under
conditions used for commercial production have however been absent so far. Modalities of
cultivation, which may increase lipid yields such as additives and N-limitation are also
studied. 59-61

On the other hand, in practice, commercial production tends to generate lower yields than

achievable in well controlled experimental settings. 2,61 The differences in yields can
apparently be large when production is in open ponds. For the cultivation of Spirulina,
2
Vonshak and Richmond, e.g., found the yield from commercial production to be a factor 5 to 6
lower than from well controlled small-scale production. A major problem of growing
microalgae in open systems such as ponds is the vulnerability of autotrophic microalgal
cultures to contamination with competing microorganisms, grazers (consumers of algae), and
infective agents, including viruses and bacteria. 37, 61-64 In practice, strategies to keep out

competing microorganisms and grazers in open systems have led to commercial growing of
autotrophic algae suitable for lipid-based liquid biofuel production under extreme conditions
(such as a high pH or a high NaCl concentration). 61,63 Such extreme conditions are not
conducive to high biomass yields. And in such systems, production may still be lowered
61
because of, e.g., heavy rainfall (leading to less extreme conditions) and infections. So far,
there has apparently been no practical demonstration of successful open systems continuously
producing autotrophic microalgae for biofuels, while not employing extreme conditions. 61,63
Moreover, expansion of algal cultivation may well lead to an increase of infection pressure,
64
negatively affecting future yields. All in all, future lipid yields in commercial open ponds,
which—as pointed out above—would seem preferable from the perspective of cost and scale,
would seem as yet highly uncertain.

AN ENERGETIC CRITERION FOR ENERGY SOURCES

To make energetic sense, energy sources should show a good energetic return on energy
investment. 65-68 This return, energy return on investment, abbreviated as EROI, is the ratio of
energy delivered to the life cycle input of (man-made) energy. Current major transport energy
sources have EROIs of more than 5 (e.g., Refs 27, 66, and 67). This holds both for mineral oil-
based fuels such as gasoline and diesel and for electricity. 27,66,67 An EROI of more than 5 also