The paper shows how a thermal system design can be optimized using energy, economy and environment as separate objectives. Comparisons with a single-objective thermo-economic optimization and a two-objective energetic and economic optimization are also discussed. The test case plant of the CGAM problem is taken as an example of application for the three-objective approach. An environmental impact objective function is defined and expressed in cost terms by weighting carbon dioxide and nitrogen oxide emissions according to their unit damage costs. An evolutionary algorithm is used to find the surface of optimal solutions in the space defined by the three objective functions. (C) 2003 Elsevier Ltd. All rights reserved.