ECOS 2021, the 34th conference of the series, will be held in Giardini Naxos, Taormina, Sicily (Italy) from june 27 through july 2, 2021. The purpose of the conference is for participants to present new research results and exchange views on theoretical and applied engineering topics related to performance, economics, and environmental impact of energy conversion systems. We cordially welcome you to Giardini Naxos, and expect that you will enjoy a rewarding and fruitful week at the conference while also getting a taste of the exceptional cultural and environmental scenario of eastern Sicily.