The search for alternative fuels for the transport industry has revived the interest in biofuels from sustainable cultivation and feedstock which is not in competition with food or animal feed [1]. While biofuels remain a viable alternative to fossil fuels, a full replacement of our 90 % hydrocarbon-based transport industry is unlikely in the near to mid-term future [2–8]. With the emphasis shifting towards the development of advanced biofuels, microalgae has proven to be a promising feedstock to overcome land use and food security issues while growing in waste or seawater [7, 9–11]. The oil-rich algae biomass, with its superior production yields [12–14] has attracted considerable attention as potential domestic and renewable substitute for imported fossil fuels. Given microalgae’s high production yields, the required global land mass necessary to satisfy global fossil fuel consumption could be considerably reduced. For algae-derived biodiesel with a yield of 850 GJ/ha/year, to replace the total production of 748 million tons of petroleum-derived diesel in 2009, a land mass of around 57.3 million ha would be required. This land size approximated to an area somewhat smaller than Texas [7]. The inherent potential advantage of biodiesel production from algae is lower lifecycle Greenhouse Gases (GHG) emissions, as algae biomass converts atmospheric CO2 through photosynthesis into bio-plant material which is eventually released back to the atmosphere via micro-organisms when used as a fuel, via engine tail pipe emissions [10, 15, 16]. Fossil fuel combustion releases additional carbon which took million of years to be removed from the atmosphere [17]. Moreover, compared to cultivation requirements for other advanced biofuel sources microalgae growth mainly requires solar radiation, carbon dioxide, water and nutrients in the form of inorganic salts [18]. Given an existing base of 50,000 species of known microalgae, only a fraction is appropriate for biodiesel production, due to algae strains’ varying lipid content and productivity levels [10, 19]. On average algal lipid content varies between 20 and 50 % by weight of dry biomass, although some strains can under certain optimally induced conditions accumulate as much as 90 % oil yield ratios [18, 20, 21].

Microalgae is cultivated in either open raceway ponds or closed photobioreactors (PBR), each of which has been designed in a variety of operating configurations [10]. Both farming approaches are still held back by substantial technical and economic hurdles. Although photobioreactors benefit from higher productivity rates, lower evaporative water losses or likelihood of culture collapse, the farming approach is still challenged by its significantly larger capital investment and operating cost base [9, 22]. While open pond algae cultivation is operated on a 60–100 % lower cost level, the technology is held back from its large-scale commercial breakthrough by problems of poor light utilization efficiency, lower volumetric productivity and higher risk of culture contamination or collapse [10, 19, 22, 23]. However, since this industry is not mature, there is ample space for optimization and we expect the future rise in oil prices to add to this effect.

The main stages of the algae biodiesel production process consist of algae farming, biomass harvesting, oil extraction, transesterification of algae oil, fuel distribution and combustion, see Fig. 1.

Fig. 1
figure 1

Algae to biodiesel carbon cycle [7]

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As part of our LCA study, microalgae batches are cultivated in open pond farming installations, harvested and dried in subsequent stages to generate 75 tonnes/ha/year of dry algae biomass. The extracted 30 % share of algae lipids (22.5 tonnes/ha/year) is further transesterified to yield 850 GJ/ha/year of biodiesel. 89 GJ/ha/year of glycerol is generated as a by-product and is exported as animal feed. The remaining 70 % residual algae biomass (689 GJ/ha/year) is used in various co-product utilization methods to offset the production process’ substantial energy requirements. When we consider all energy inputs of the biodiesel production cycle as 100 % fossil fuel sourced, the production process is 2.5 times as energy intensive as conventional diesel from the United States and nearly equivalent to the high fuel-cycle energy use of oil shale diesel, see Fig. 2. The major disadvantage inherent in biodiesel production from microalgae is driven by the high energy input in the form of heat and electricity.

Fig. 2
figure 2

Benchmarking the life cycle energy requirements of fossil fuels against algae biodiesel [7]

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Biodiesel from algae biomass can only realize its inherent environmental advantages of GHG emissions reduction, once every step of the production chain is fully optimized and decarbonized. This will entail the sourcing of all direct energy input in the form of heat and electricity, as well as indirect requirements for transport and building materials, from low-carbon energy sources. Moreover, the offset of carbon-intensive fertilizer requirements through the recycling of wastewater will be critical. Recent studies suggest that without the recycling of harvest water, the algae-to-biodiesel water footprint is as high as 3,726 kg water/kg biodiesel [11]. Further improvements in the production of cycle’s carbon footprint can be achieved through the commercialization of new oil extraction technologies. By eliminating the need to dry algae biomass to a 90 % solid content [13] required for the subsequent oil extraction in vegetable oil mills, considerable energy savings could be achieved.

As a priority, countries will need to defossilize primary energy sources used by their electricity grids, as only then can the transport sector move towards low GHG emissions. In the case of Saudi Arabia which operates on a 100 % fossil-based electricity and heat grid, the inherent environmental advantages of producing algae biodiesel would be heavily overshadowed by the nation’s carbon intensive energy and power sector. In contrary, Brazil and France, which essentially operate on a defossilized electricity grid, have the potential for biodiesel from algae to be a viable alternative to conventional diesel [7]. Ultimately with the transport fuel industry remaining to be fuelled by hydrocarbons for the foreseeable future, biofuels will need to contribute as well as complement our current fuel mix as part of a larger mission to diversify our global transport system within the mid-to long-term future.