Driving toward sustainable algal fuels: A harmonization of techno-economic and life cycle assessments

Research efforts have investigated a range of system designs for algae growth and conversion into biofuels. The economic feasibility and environmental impact of these systems are frequently evaluated through techno-economic analyses (TEA) and life-cycle assessment (LCA), which typically determine the levelized cost of fuel production in gallons of gasoline equivalent (MFSP, in $-gge−1), global warming potential (GWP, in g CO2-eq-MJfuel−1), and net energy ratio (NER, unitless). While the outputs from these models seem comparable, the results often conceal the impact of a large number of assumptions inherent to the modeling, limiting direct comparisons. As such, a direct comparison of the modeling results as published can be misleading due to differences in critical assumptions and/or foundational methodology. This work applies harmonization methodology to several sustainability assessment studies found in the literature with the goal of enhancing comparability through implementation of a standard set of assumptions. For the economic evaluation, two harmonization efforts were performed: the first focused on harmonizing productivity, economic assumptions, and cost year for the entire growth to product process; the second compared only downstream fuel conversion technologies by fixing the biomass cost, thereby removing the uncertainty of upstream-growth assumptions. For LCA, harmonization focused on productivity and system boundary. The results of these efforts show a decrease of 43% in the range of reported fuel prices and minimal redcution in the range of LCA results. Both TEA and LCA harmonization efforts then investigated the impact of productivity by simulating a range of biomass productivity values (12.5, 25, and 50 g-m−2 day−1). The work reveals limitations to both the economic and environmental benefits of productivity improvements past approximately 35 g-m−2 day−1. Results highlight the need to redirect research efforts not only to increase productivities, but also to other areas where investments can make a greater impact in terms of economic viability and environmental impact. •Harmonization of algal sustainability assessments over the past decade•Diminishing sustainability returns for productivities above 25–35 g m−2 d−1•Net energy ration (NER) is the most transparent metric for assessment.