In this paper, we introduce a new thermodynamic concept of greenizing energy systems and propose a new greenization factor which is defined as the amount of environmental impact reduction by the system greenized divided by the reference environmental impact for the original (reference) case. A greenization factor of 1 indicates that the system is fully greenized case in which its environmental impact is zero or minimal. The greenization options of energy systems are studied extensively for an actual coal-fired power plant in Ontario. Multiple case studies are presented under various greenization criteria. The greenization factors and sustainability indexes for each option are determined and presented comparatively. The results can be extended to other energy systems, processes and applications (e.g., fossil fuel based power plants and transportation vehicles) for assessment purposes. (C) 2011 Elsevier Ltd. All rights reserved.