The thermo-ecological cost, expressed by means of the cumulative consumption of non-renewable natural resources of exergy, has been applied as a criterion of the selection of technical solutions. This criterion differs from the classical economical one. The general formulation of the objective function of thermo-ecological economy is presented. A detailed formulation of the objective function is demonstrated my means of the optimization of the parameters of a heat exchanger. It indicates the difficulties appearing when determining the influence of the design parameters on the consumption of exergy in fabrication process. Other concepts of the economical application of exergy have been discussed. (C) 2004 Elsevier Ltd. All rights reserved.