It is considered that the electric, thermal and cooling loads of a building complex are covered by a tri-generation system consisting of a gas engine with heat recovery, an absorption chiller driven by thermal energy, electrically driven compression chillers and two thermal storage tanks (one with hot and one with cold water). Supplementary electricity is supplied by the local network. The behavior of such a system during transients is characterized primarily by the transient performance of the storage tanks and the absorption chiller (start-up and shut-down). The objective of the work reported here is the operation optimization of the system under load-varying conditions taking into consideration the transient behavior of the three aforementioned components. The simulation of the dynamic behavior of the absorption chiller, in particular, is based on the Gompertz function (sigmoid curve). Minimization of the total cost for covering the energy needs of the building is selected as the objective function, while the operating point of each component is to be determined by optimization under specified constraints. The optimal values of the control variables are determined using a multi–stage control vector parameterization method and optimization software based on Sequential Quadratic Programming. The solution of the optimization problem is accompanied by a sensitivity analysis. Suggestions are written for continuation and improvement of the work.