Klaus Vajen is director of the Institute of Thermal Energy Engineering at University of Kassel (DE). He holds a PhD in applied physics, and his scientific work focuses mainly on experimental and numerical investigations of (solar) thermal energy components and systems, where he published more than 300 scientific papers.

He founded and coordinates the master programme "Renewable Energies and Energy Efficiency (re2)" at University of Kassel as well as the European graduate school for PhD-students "Solar Energy Network (SolNet)". He is furthermore chair of the council "RE Teaching" of more than 130 professors and lecturers in German speaking countries.

Klaus Vajen is president of ISES since 2020. He was chair of the Solar World Congress 2011 and the EuroSun 2022 in Kassel.