energies Review Small-Scale Biodiesel Production Plants—An Overview Maria Gabriela De Paola, Ivan Mazza, Rosy Paletta, Catia Giovanna Lopresto and Vincenza Calabrò *   Citation: De Paola, M.G.; Mazza, I.; Paletta, R.; Lopresto, C.G.; Calabrò, V. Small-Scale Biodiesel Production Plants—An Overview. Energies 2021, 14, 1901. https://doi.org/10.3390/ en14071901 Academic Editor: João Fernando Pereira Gomes Received: 22 February 2021 Accepted: 26 March 2021 Published: 30 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Informatics, Modeling, Electronics and System Engineering (DIMES), University of Calabria, Via P. Bucci, Cubo 39 C, 87036 Rende, CS, Italy; mariagabriela.depaola@unical.it (M.G.D.P.); mazzaivan432@gmail.com (I.M.); pltrsy93p61h919i@studenti.unical.it (R.P.); catiagiovanna.lopresto@unical.it (C.G.L.) * Correspondence: vincenza.calabro@unical.it; Tel.: +39-0984-496703 Abstract: Small-scale plants that produce biodiesel have many social, economic and environmental advantages. Indeed, small plants significantly contribute to renewable energy production and rural development. Communities can use/reuse local raw materials and manage independently processes to obtain biofuels by essential, simple, flexible and cheap tools for self-supply. The review and understanding of recent plants of small biodiesel production is essential to identify limitations and critical units for improvement of the current process. Biodiesel production consists of four main stages, that are pre-treatment of oils, reaction, separation of products and biodiesel purification. Among lots of possibilities, waste cooking oils were chosen as cheap and green sources to produce biodiesel by base-catalyzed transesterification in a batch reactor. In this paper an overview on small-scale production plants is presented with the aim to put in evidence process, materials, control systems, energy consumption and economic parameters useful for the project and design of such scale of plants. Final considerations related to the use of biodiesel such as renewable energy storage (RES) in small communities are discussed too. Keywords: small-scale plants; biodiesel; waste cooking oils; in-situ analysis; nanogrid; renewable energy storage (RES) 1. Introduction During last years, research is focusing on design and implementation of small plants to produce biodiesel from waste cooking oils (WCO), in order to achieve the double aim of making people independent in biofuel production and taking advantage of wastes destined to expensive disposals. Biodiesel is a renewable, biodegradable and green energy source, which can be used as a fuel in the transport sector and for heating, without the need to make modifications to engines or boilers, in partial or total replacement of the diesel fuel. As a biofuel, it guarantees the reduction of polluting emissions, as it does not contain sulfur and aromatic compounds, and it contributes to the reduction of particulate matter emitted [1]. Biodiesel may be effectively used in pure form or blended with fossil diesel. European motor manufacturers undertook successful tests on blends with different diesel compositions (5–10%, 25–30%, 100% pure) [2]. Minor modifications (seals, piping) are required when 100% biodiesel is used, but no change is necessary in the distribution system, therefore avoiding expensive infrastructure changes. Biodiesel is also used as an efficient heating oil. The biodiesel use decreases global warming impacts, reduces emissions, increases energy independence and has positive impact on agriculture. The use of 1 kg of biodiesel leads to the reduction of 3 kg of CO 2 [3]. Consequently, CO 2 emissions are significantly lower by 65–90% than with conventional diesel use, as well as particulate emissions and other harmful emissions. Moreover, biodiesel has a high lubricity and fast biodegradability. European countries promoted and regulated the use of biodiesel by specific legislation and publication of strict guidelines in compliance with European Committee for Stan- dardization (CEN). The European fuel standard EN 14214 was developed in co-operation Energies 2021, 14, 1901. https://doi.org/10.3390/en14071901 https://www.mdpi.com/journal/energies