Page 97 - Advances in bioenergy (2016)

to be strongly dependent on the assumptions that microalgal lipid yields per hectare are high
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(often assuming biomass yields in the order of 80–90 mg year and/or lipid contents of 25–
50%), and/or assuming that there are low or no energy inputs in water supply and/or that inputs
of nutrients, CO , or heat are available from wastes, with no environmental burden allocated
2
on these inputs.
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Assumed yields of 70–90 mg algal dry weight ha year are much higher than actual
microalgal yields from current commercial ponds (e.g., Refs 22, 27, 61, 92, and references
therein). Whether these high yields combined with very high lipid contents can be achieved in
the future is uncertain. The assumption that very high biomass yields can be combined with
very high lipid contents does not take sufficient account of the increase in photon demand,
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which is associated with high lipid contents. The proposal to achieve much higher algal
biomass yields than currently achieved by genetic engineering for truncated antennae is also
problematic. Algae with truncated antennae are less competitive than their counterparts with
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full antennae. This may well have a negative impact on their yields in open ponds where
contamination by airborne full-antenna organisms may occur. 58

Relatively good energetic outcomes for liquid microalgal lipid-based biofuels have, as pointed
out above, been achieved in available LCAs when wastes (e.g., waste heat, waste water,
including nutrients present therein) are used as inputs and the environmental burden of these
inputs is valued at 0 (e.g., Refs 68, 88, 93, and 102). The assumption that the environmental
burden of ‘wastes’ can be valued at 0 merits further consideration. In line with the tenets of

industrial ecology, 109 wastes are increasingly viewed as resources. This also holds for waste
water. The nutrients present therein are increasingly considered for recovery to be applied in
food production. 110 Moreover, when microalgal biofuels are to become major suppliers of
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energy limitations to the availability of CO , water and nutrient wastes may emerge. In view
2
thereof, the absence of any allocation of environmental burdens to the use of ‘wastes’ as inputs
in microalgal production is debatable, as such inputs might also be considered coproducts. If
the latter assumption is subscribed to, the environmental burden of the production process
should partly be allocated to the inputs in the production of microalgae, which are currently
considered to be ‘wastes’. This would negatively affect the estimated EROI of microalgal
biofuels.

To the extent that aspects downstream from the production of microalgae in ponds are
concerned, the assumed use of algal biofuel residues to generate energy and to provide for
nutrient inputs leads to relatively low life cycle energy inputs. So does the assumption that
energy inputs in processing of algal cultures to biodiesel can be lowered much. The
substitution of dry technologies for the isolation and processing of algae by wet technologies
has been assumed to be beneficial to expected energy efficiency in producing microalgal
biofuels (e.g., Ref 93). Examples of such wet technologies are treatments with supercritical
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methanol, ethanol, 112 or CO plus solvent and the in situ transesterification with very high
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amounts of methanol. 114 Still, even when wet technologies are applied, the algae Chlorella,
Spirulina and Dunaliella, which are grown at a density typically around approximately 0.1–