The carbon footprint and non-renewable energy demand of algae-derived biodiesel Pooya Azadi a , George Brownbridge a , Sebastian Mosbach a , Andrew Smallbone b , Amit Bhave b , Oliver Inderwildi c,d , Markus Kraft a,⇑ a Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom b cmcl innovations, Sheraton House, Castle Park, Cambridge CB3 0AX, United Kingdom c World Economic Forum, 91-93 route de la Capite, Geneva, CH-1223 Cologny, Switzerland d Smith School of Enterprise and the Environment, University of Oxford, Oxford OX1 2BQ, United Kingdom highlights  Global sensitivity analysis is performed to determine the environmental impact of algal biodiesel.  GHG emission of algal biodiesel ranges from 40 to 125 g e-CO 2 /MJ.  Biodiesel from dried algae may prove sustainable if a low carbon solution e.g. solar drying is used. article info Article history: Received 18 July 2013 Received in revised form 2 September 2013 Accepted 14 September 2013 Available online 10 October 2013 Keywords: Algae Biodiesel Life cycle assessment Carbon footprint Uncertainty analysis abstract We determine the environmental impact of different biodiesel production strategies from algae feedstock in terms of greenhouse gas (GHG) emissions and non-renewable energy consumption, we then bench- mark the results against those of conventional and synthetic diesel obtained from fossil resources. The algae cultivation in open pond raceways and the transesterification process for the conversion of algae oil into biodiesel constitute the common elements among all considered scenarios. Anaerobic digestion and hydrothermal gasification are considered for the conversion of the residues from the wet oil extrac- tion route; while integrated gasification–heat and power generation and gasification–Fischer–Tropsch processes are considered for the conversion of the residues from the dry oil extraction route. The GHG emissions per unit energy of the biodiesel are calculated as follows: 41 g e-CO 2 /MJ b for hydrothermal gas- ification, 86 g e-CO 2 /MJ b for anaerobic digestion, 109 g e-CO 2 /MJ b for gasification–power generation, and 124 g e-CO 2 /MJ b for gasification–Fischer–Tropsch. As expected, non-renewable energy consumptions are closely correlated to the GHG values. Also, using the High Dimensional Model Representation (HDMR) method, a global sensitivity analysis over the entire space of input parameters is performed to rank them with respect to their influence on key sustainability metrics. Considering reasonable ranges over which each parameter can vary, the most influential input parameters for the wet extraction route include extractor energy demand and methane yield generated from anaerobic digestion or hydrothermal gasi- fication of the oil extracted-algae. The dominant process input parameters for the dry extraction route include algae oil content, dryer energy demand, and algae annual productivity. The results imply that algal biodiesel production from a dried feedstock may only prove sustainable if a low carbon solution such as solar drying is implemented to help reducing the water content of the feedstock. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Despite tremendous improvements in the cultivation and processing of microalgae feedstock, the overall impact of algal bio- fuels on the environment upon widespread use has remained a highly controversial issue. The underlying elements that prevent a general analysis of such impacts are primarily attributed to the following: (i) wide ranges of reported algae oil content and annual productivity, (ii) lack of a proven technology for the extraction of oil from wet algae feedstocks or for solar drying of dilute algae slurries, (iii) lack of information about the conversion of oil-ex- tracted algae in biological and thermochemical processes alike, (iv) lack of practical information regarding the extent to which nutrients can be supplied from wastewater or from a recycle stream within the biorefinery, and (v) the inherent differences 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.09.027 ⇑ Corresponding author. Tel.: +44 (0)1223 762784. E-mail address: mk306@cam.ac.uk (M. Kraft). Applied Energy 113 (2014) 1632–1644 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy