Exergy analysis is being used to investigate different design schemes of a power plant in Crete, Greece in two parts. The following cases are analyzed: 1) installation of a steam turbine unit that will utilize heavy fuel oil as fuel, 2) installation of a gas turbine unit that will utilize diesel oil, 3) installation of a 95 MW diesel power plant that uses heavy fuel oil, and 4) installation of a cogeneration unit that will consist of a 70 MW gas turbine and a 25 MW steam turbine. In particular, the conversion of the 25 MW unit (unit No. 5) of the power plant of Linoperamata to a 95 MW cogeneration unit is studied. In this article (Part I) the analysis of the existing unit is performed. All necessary data are obtained from the existing power plants (steam, gas turbine and diesel fuel) located at Linoperamata. Exergy analysis is used for the evaluation of the proposed solutions.