By accessing geothermal resources it is possible to use stored heat from the deep subsurface for direct heat provision, to transfer the heat to a higher or lower (“cold”) temperature level using appropriate techniques, or to convert it into electricity. For conversion into electricity, typically, power plant cycles applying the Clausius-Rankine or a modified Clausius-Rankine process are used. Although the use of geothermal heat for production or storage of energy is based on the same thermodynamic processes as in conventional energy engineering, most importantly, effects of geological conditions have to be considered and adjusted design algorithms as well as optimization strategies have to be developed. The competence cluster “Process and Plant Technologies” is concerned with investigations on energy and process technological aspects of subsurface use as part of a sustainable energy supply. Our research applies to the fields of energy and process engineering, materials selection as well as test and pilot plant engineering which also includes related economic and ecological aspects.