==Publication:**All accepted (Registered & Presented) papers of REEE 2021 will be collected in the E3S conference proceedings (eISSN: 2267-1242), which is indexed by *Scopus*, *Ei Compendex*, Google Scholar, CAS, DOAJ, EBSCO, ProQuest, etc. **Special Issue-Energies(ISSN 1996-1073)Prize for the best full paper will be the publication of 1 (one) paper in Special Issue in Energies (ISSN 1996-1073; CODEN: ENERGA).--Impact Factor: 2.702 (2019) ;5-Year Impact Factor: 2.822 (2019) ==History:REEE 2018 proceedings are published online (E3S Web of Conferences,Volume 80,2019),indexed by Ei Compendex & Scopus already. REEE 2019 proceedings are published online (E3S Web of Conferences,Volume 122,2019),indexed by Ei Compendex & Scopus already. REEE 2020 proceedings are published online (E3S Web of Conferences,Volume 191,2020),indexed by Ei Compendex & Scopus already. ==Committee:Conference Chair:Prof. Elmar Bollin, Offenburg University of Applied Sciences, Germany Program Chairs:Prof. Muhammad Shakeel Virk, University of Troms?, Norway;Prof. Carlos Felgueiras, Polytechcnic of Porto, Portugal. Local Chair:Prof. Nidia Caetano, Polytechcnic of Porto, Portugal Guest editors:Prof. Jorge Loredo, University of Oviedo, Spain;Prof. Jin Su Jeong,Universidad de Extremadura, Spain.