In recent decades, utilization of exergy has spread to the fields of physics and engineering as a useful decision making tool. The ratio of energy to exergy in a substance can be considered as an indicator for the improvement of the energy efficiency of large scale energy systems. In this study, an exergy analysis of a 300 MW lignite thermoelectric power plant, located in Ptolemais Greece, is carried out. In order to promote a more efficient solution, the existing lignite thermoelectric power plant is compared with three different cogeneration (CHP (combined heat and power)) systems, employing the Rankine cycle, designed to use the same amount of fuel as the existing one. The performed exergy analysis reveals that the utilization of the proposed CHP systems may result to a significant increase of the energy efficiency, compared to the performance of the existing power plant.