Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Solar irradiation assisted synthesis of biodiesel from waste cooking oil using calcium oxide derived from chicken eggshell Rupam Bharti a , Abhishek Guldhe a , Dipesh Kumar b , Bhaskar Singh a, ⁎ a Department of Environmental Sciences, Central University of Jharkhand, Ranchi 835205, India b Department of Chemical Engineering, Indian Institute of Technology, Guwahati 781039, India GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Solar irradiance Biodiesel Catalyst Transesterification Waste cooking oil Sustainability ABSTRACT The conventional approaches in transesterification have several techno-economic and sustainability challenges. To this end, the utilization of waste resources and renewably sourced energy has tremendous appeal. The present study reports the findings on the transesterification of waste cooking oil (WCO) catalyzed by CaO (sourced from waste chicken eggshells) using solar irradiation based thermal energy. A co-solvent (ethyl acetate) was used to enhance the miscibility of transesterification reactants. The process variables (alcohol to oil molar ratio, catalyst concentration, reaction time, and reaction temperature) were optimized using response surface methodology based Box-Behnken design. During the experiments, the intensity of solar radiation varied, and the reactor temperature ranged between 40 and 46 °C. At the predicted optima, a conversion of 90.14% was attained, which was in reasonable agreement with the predicted response (93.64%). A gradual loss of catalytic activity was observed over consecutive batch transesterification runs, and at the end of a 3rd run, 73.13% conversion of WCO was attained. The solar radiation available at the ground level during the experimental period was ≈ 5.15 kW m −2 d −1 , while the conventional approach (hot-plate based heating) led to an energy input of 2.5 kWh per run at the optimized reaction condition. With process intensification strategies, solar irradiation is poised to https://doi.org/10.1016/j.fuel.2020.117778 Received 15 January 2020; Received in revised form 18 March 2020; Accepted 2 April 2020 ⁎ Corresponding author. E-mail address: bhaskar.singh@cuj.ac.in (B. Singh). Fuel 273 (2020) 117778 0016-2361/ © 2020 Elsevier Ltd. All rights reserved. T