The Energy Return on Investment for Algal Biocrude: Results for a Research Production Facility

Abstract

This study is an experimental determination of the energy return on investment (EROI) for algal biocrude production at a research facility at the University of Texas at Austin (UT). During the period of this assessment, algae were grown at several cultivation scales and processed using centrifugation for harvesting, electromechanical cell lysing, and lipid separation in an enhanced coalescence membrane. The separated algal lipids represent a biocrude product that could be refined into fuel. To determine the EROI, a second order analysis was conducted, which includes direct and indirect energy flows, but does not consider capital energy expenses. At the time that the data in this study was collected, the research program was focused on improving biomass and lipid productivity. As a result, some higher efficiency processing steps were replaced by lower efficiency ones to permit other experiments. Although the production process evaluated here was energy negative, the majority of the energy consumption resulted from non-optimized growth conditions. Therefore, the experimental results do not represent an expected typical case EROI for algal fuels, but rather outline the important parameters to consider in such an analysis. The results are the first known experimental energy balance for an integrated algal biocrude production facility. A Reduced Case is presented that speculates the energy use for a similar system in commercial-scale production. In addition, an analytical model that is populated with data that have been reported in the literature is presented. For the experiments, the Reduced Case, and Literature Model, the estimated EROI was 1.3 × 10−3, 0.13, and 0.57, respectively (refining energy requirements are not included in the experimental or Reduced Case EROI value). These results were dominated by growth inputs (96.59%, 94.15%, and 76.32% of the total energy requirement, respectively). For the experiments and Literature Model, lipid separation was the most energy intensive processing step (2.47% and 10.06%, respectively), followed by harvesting, refining, and then electromechanical cell lysing.