An iterative procedure is suggested to evaluate and improve the energy system design. The procedure considers the information deriving from complementary evaluation approaches, each applied within its appropriate time-space window of interest: (a) Process-related, local-scale methods (Energy, Exergy and Thermoeconomic analyses); (b) Environmental assessment methods (Impact Assessment, Emergy Synthesis); (c) Economic methods (Micro- and Macro-Economic and Externality Evaluations). Process-related methods are applied first, in order to provide localscale performance indicators able to suggest optimization procedures from a user-side point of view. Environmental evaluation approaches are then used to judge the overall environmental quality of the design, in the largest regional and biosphere scales. Finally, micro- and macro-economic evaluation approaches are applied in order to ascertain the soundness of the proposed solution as far as the economic return on the investment as well as global benefits to the Society are concerned. New choices for the design configuration and parameters may be suggested by implementing the iterative procedure. A cogeneration system working in a town of Northern Italy is used as a case study: starting from the present configuration of the plant, modifications are suggested and evaluated, thus identifying the way for improving the performance under various viewpoints. A proposal for plant transformation from Steam Cycle to Combined Cycle is suggested, capable of increasing the plant electric power from 136 to 332 MWe, increasing the energy efficiency from 60 to 63%, increasing the exergy efficiency from 37 to 49%, and decreasing the overall demand for environmental support (transformity) from 1.84 X 10(5) to 1.27 X 10(5) seJ/J. The paper points out several benefits and bottlenecks of the existing plant and of the proposed solutions. © 2004 Elsevier Ltd. All rights reserved.