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Current and future dynamics, of environmental legislations and trading market, of the fuel and electricity prices due to the market liberalization or resource shortages, and of the innovations of power/cogeneration technologies, impose great challenges on optimal design and operation evaluation of integrated energy systems. Developing new design tools for power and cogeneration technologies is therefore a key concern guiding the work presented in this thesis, which becomes the central problem that has been studied in this work. Based on 'Environomic Modeling', a decisive criteria decomposition and multi-objective optimization approach has been further developed based upon the previous work in the Laboratory for Industrial Energy Systems of the Swiss Federal Institute Technology of Lausanne. It has been implemented to evaluate the optimal design of integrated energy systems for power and/or cogeneration in the CO2 abatement context, or for solving the power load dispatching problems. By taking the specific investment cost versus the CO2 emission rate as the two objectives, the developed approach has been implemented to 'Typify' the environomic performance of natural gas combined cycle plants with market available gas turbines for both power and/or cogeneration applications, in the context of CO2 abatement. The dynamic exogenous parameters that impose difficulties in project-based evaluation, such as the fuel price, annual operating hours and interest rate, have been successfully treated through simple post optimization analysis from the derived 'Typification Map'. The developed methodology has also been used to study the potential of Clean Development Mechanism (CDM) defined in the Kyoto Protocol for the analyzed natural gas combined cycle (NGCC) projects in China, for both power alone and cogeneration applications. It is shown that in the latter case, a break-even Certified Emission Reduction (CER) price of 14.5 US$/tonCO2 to 16.5 US$/tonCO2 will be required to make NGCC plants able to compete with coal plants. When a NGCC plant is used for cogeneration, it is also shown that a reduced break-even CER price can be expected due to the profits from heat selling. Simultaneous optimization of CO2 emission rate or the annual total CO2 emissions, against the economic criteria of the project, e.g. cost of electricity or heating specific cost with sensitivity analyses for dealing with the dynamics of fuel and electricity prices have been applied to two project-based case studies. Those have even more complex superstructures including options of electricity importation or exportation. For the case of advanced natural gas combined cycle (NGCC) plants with a possible CO2 capture option, the obtained results provide information on the relationship between power generation cost and CO2 emission performance. This approach is intended for power/cogeneration technology suppliers, for utility owners or project investors, and for policy makers in the context of CO2 mitigation schemes including emission trading. One of the interesting results shows that a break-even value as high as 69 US$/ton has been identified for the CO2 tax or price of CO2 allowance that can bring a 400MW NGCC plant with CO2 capture into practice, due to the relatively high investment cost of CO2 capturing. For the case of an integrated heating plant, it is shown that the heating fuel specific consumption can be dramatically reduced down to 19.8 - 21.4 [kg coal.equ./GJ] with a significant reduction of associated emissions at a heating specific total cost of respectively 3.1 and 6.6 [US$/GJ]. These range performances are achieved whether power exportation is allowed or not, by implementing heat pump and/or cogeneration technologies. The 'Environomic modeling and multi-objective optimization' methodology has also been implemented for load dispatching for a given power generation system composed with power generation units associated with different fuel consumption, emission and cost performances. The obtained daily CO2 emissions versus daily power generation costs Pareto Optimal Frontiers (POFs) give the operator an increased flexibility and structured knowledge to take the decisions on the best power dispatching solution under different environmental regulation circumstances.