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The exergy analyses performed in this study are based on three thermodynamic performance parameters namely exergy destruction, exergy efficiency and thermodynamic improvement potentials. After mathematical analysis with Aspen Plus software, the results showed that 64% and 44% of the total exergy content of the input resources into microalgal methyl ester (MME) and jatropha methyl ester (JME) production plants were destroyed respectively for 1 ton of biodiesel produced. This implies that only 36% and 56% (for MME and JME production plants respectively) useful energy in the products is available to do work. The highest and lowest exergy destructions were recorded in the oil extraction units (38% and 39% of the total exergy destroyed for MME and JME plants respectively) and transesterification units (5% and 2% of total exergy destroyed for MME and JME plants respectively) respectively for 1 ton biodiesel produced. Since sustainable biodiesel production depends on cultivation of feedstock, oil extraction and transesterification processes, exergy analysis which is carried out on only the transesterification unit cannot justify the thermodynamic feasibility of the whole biodiesel production plant unless a complete thermodynamic assessment has been done for the whole plant. Thus, according to this study which considers all the biodiesel production processes, MME and JME production plants are not thermodynamically feasible.

Biodiesel production growth rate keeps increasing every year with the hope of replacing fossil based liquid fuel which is near exhaustion. Energy efficiency methods coupled with environmental and economic sustainabilities would render biodiesel production attractive. Algal cultivation and oil extraction processes are found to be energy intensive hence exergy destructive. This study presents the thermodynamic feasibility of microalgal biodiesel production plant via exergy analysis. Mathematical modelling with Aspen Plus shows that the centrifuge for algae–medium separation recorded the highest exergy loss of 925 MJ representing 15% of the total exergy loss. 64% of the total exergy input into the whole plant is destroyed to attain the products (biodiesel and glycerin).