Techno-economic analysis of autotrophic microalgae for fuel production Ryan Davis ⇑ , Andy Aden, Philip T. Pienkos National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401, United States article info Article history: Received 18 February 2011 Received in revised form 6 April 2011 Accepted 6 April 2011 Available online 17 May 2011 Keywords: Algae Autotrophic Pond Photobioreactor Techno-economic Green diesel abstract It is well-established that microalgal-derived biofuels have the potential to make a significant contribu- tion to the US fuel market, due to several unique characteristics inherent to algae. Namely, autotrophic microalgae are capable of achieving very high efficiencies in converting solar energy into biomass and oil relative to terrestrial oilseed crops, while at the same time exhibiting great flexibility in the quality of land and water required for algal cultivation. These characteristics allow for the possibility to produce appreciable amounts of algal biofuels relative to today’s petroleum fuel market, while greatly mitigating ‘‘food-versus-fuel’’ concerns. However, there is a wide lack of public agreement on the near-term eco- nomic viability of algal biofuels, due to uncertainties and speculation on process scale-up associated with the nascent stage of the algal biofuel industry. The present study aims to establish baseline economics for two microalgae pathways, by performing a comprehensive analysis using a set of assumptions for what can plausibly be achieved within a five-year timeframe. Specific pathways include autotrophic production via both open pond and closed tubular pho- tobioreactor (PBR) systems. The production scales were set at 10 million gallons per year of raw algal oil, subsequently upgraded to a ‘‘green diesel’’ blend stock via hydrotreating. Rigorous mass balances were performed using Aspen Plus simulation software, and associated costs were evaluated on a unit-level basis. Upon completing the base case scenarios, the cost of lipid production to achieve a 10% return was determined to be $8.52/gal for open ponds and $18.10/gal for PBRs. Hydrotreating to produce a diesel blend stock added onto this marginally, bringing the totals to $9.84/gal and $20.53/gal of diesel, for the respective cases. These costs have potential for significant improvement in the future if better microalgal strains can be identified that would be capable of sustaining high growth rates at high lipid content. Given that it is difficult to maximize both of these parameters simultaneously, it was determined that the near-term research should focus on maximizing lipid content as it offers more substantial cost reduc- tion potential relative to an improved algae growth rate. Additional economic sensitivity studies were established to identify other important cost drivers, and a resource assessment comparison was made to evaluate parameters such as water and CO 2 requirements. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Microalgae exhibit great versatility as an energy source, as algal cells can be used either directly as a biomass feedstock in many biochemical (e.g., fermentation) or thermochemical (e.g., gasifica- tion, pyrolysis, and liquefaction) conversion pathways, or can be exploited specifically for their relatively high oil content [1]. In the latter case, algal oil can be extracted and upgraded into infra- structure-compatible fuels with a high energy density, while a variety of co-products can simultaneously be produced from other algal constituents. Algal growth is divided into two main classes, autotrophic and heterotrophic. Autotrophic algae utilize CO 2 as the carbon source, while heterotrophic algae utilize sugars derived from other biomass sources. Each category has its own advantages; for example, autotrophic growth directly consumes CO 2 which on a large scale would typically be supplied from an upstream power plant or other emissions source, thereby providing for an effective carbon capture and recycle opportunity. While the same carbon re- cycle indirectly takes place in the heterotrophic pathway (CO 2 is utilized for growing the biomass sugar source), it does so less effi- ciently as it relies on terrestrial biomass growth which exhibits lower photosynthetic efficiency relative to autotrophic algae [2]. However, the heterotrophic pathway utilizes currently available and proven fermentation technology for algal cultivation, and typ- ically exhibits higher cellular lipid content [1,3]. The autotrophic pathway has generally received more public attention, as attribut- able to the large number of companies dedicated either in whole or in part to pursuing autotrophic algal biofuels, relative to a smaller number of entities in the heterotrophic field. As such, there is a much larger amount of public data on autotrophic growth 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.04.018 ⇑ Corresponding author. Tel.: +1 303 384 7879. E-mail address: ryan.davis@nrel.gov (R. Davis). Applied Energy 88 (2011) 3524–3531 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy