SGEM SGEM 2021. 21th International Multidisciplinary Scientific Geoconference. Volume 21. Energy and Clean Technologies : conference proceedings of selected papers. 16-22 August, 2021 - Albena, Bulgaria Issue 4.1 Renewable Energy Sources and Clean Technologies, Recycling, Air Pollution and Climate Change. -- 1. vyd. -- Sofia : STEF 92 Technology, 2021. -- ISBN : 978-619-7603-26-2. -- 978-619-7603-27-9 (DVD). -- ISSN : 1314-2704. -- 10.5593/sgem2021/4.1.