SGEM SGEM 2020. 20th International Multidisciplinary Scientific Geoconference. Volume 20. Energy and Clean Technologies : conference proceedings. Albena, Bulgaria, 18 - 24 August, 2020 Issue 4.1 Renewable Energy Sources and Clean Technologies, Recycling, Air Pollution and Climate Change. -- 1. vyd. -- Sofia : STEF 92 Technology, 2020. -- 560 s. -- ISBN : 978-619-7603-09-5. -- ISSN : 1314-2704. -- 10.5993/sgem2020/4.1.