Evaluation of Natural Gas Combined Cycle Plants in China through Environomic Modeling and Multi-objective Optimization

Due to the requirements of lower capital investment, higher operational flexibility, and the pressures from environmental legislations on local pollutants and global greenhouse gases (GHGs) emissions, power generation market is in favour of natural gas based electricity production in many regions, which is expected to be tripled between now and 2030. This trend will be particularly marked in developing countries such as China, where electricity demand is expected to rise most rapidly, while their local environment has been or is being severely damaged by heavily using low efficiency high polluted pulverized coal plants. Such a fuel switching can both enhance the local environment protection, and reduce the COz emissions, however, at a higher and more dynamic power generation cost. In this paper, the Pareto Optimal solutions regarding to two objectives of levelized cost of electricity and the associated COz emissions in relation to the design configurations and technical process parameters are obtained and presented for a natural gas combined cycle project in China, based on the developed 'environomic' models and a multiobjective optimizer. The obtained Pareto Optimal Frontiers tend to supply a flexible tool for handling the dynamics of important economic/market and environmental factors, so that the most proper design under a given circumstance can be easily identified through post-optimization simulation.Based upon which, the Clean Development Mechanism potentials of NGCC project in China are also discussed.

Environomic modeling and multi-objective optimisation of integrated energy systems for power and cogeneration

Hongtao Li

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.

EPFL2006

Small scale SOFC systems

Nordahl Autissier

The increasing cost of carbon based fuels pushes the electricity generation market towards more efficient electricity generation technologies. In this context, fuel cells offer the opportunity to increase efficiency and lower CO2 and NOx emissions, in particular with co-generation units. The complexity, as well as the number and different combinations of components involved with a SOFC stack make their analysis impossible without dedicated tools. The simultaneous consideration of economic and thermodynamic considerations is a key challenge for engineers and decision makers. The present work contributes to the development of design tools for fuel cells and energy systems. A detailed model of fuel cell stack and a model of a complete system were developed. Based on a commercial CFD code, additional routines were implemented to model the electro-chemical behavior of a fuel cell as well as the reforming reaction schemes. The model is able to predict cell behavior under various fuel feeds. The detailed 3-D model of stack repeat unit allowed to identify weaknesses of a first stack geometry. With the combination of experimental observations and modeling activities, specific problems of the existing concept were identified and a new geometry designed, together with the corresponding stack concept. A new stack design is presented with successful experiments up to 1 kWe. Besides the stack, a co-generation fuel cell system is composed of multiple components such as heat exchangers, reformer, recycle loops... The number, the type and size of those components influence the system costs but also its efficiencies (thermal and electrical). A multi-objective thermo-economic optimization methodology was defined, allowing to optimize at the same time the configuration and operating parameters of a system. This method was applied to two sizes of co-generators : a 1 kW unit and a 30 kW unit. HTceramix SA (Yverdon, Switzerland) is developing part of a co-generation unit. The work indicates the potential improvement of their prototype. The 1 kW unit was optimized in a first phase on thermal and electrical efficiencies, and in a second phase on electrical efficiency and installed costs. Those two results indicate the influence of the objective function selection on the results. The high operating temperature of Solid Oxide Fuel Cells (SOFC) offers good opportunity for coupling with a gas turbine. A simple concept of pressurized SOFC-µGT was studied and optimized, for an net electrical output of 30 kW. The simple layout described in this work allows present technical feasibility of the concept. Moreover, the costs of electricity, lower than 0.20$/kWh, makes it economically viable solutions. The methodology and the tools developed allow the engineer to explore the solution space through optimal configurations, in order to select those fitting with its priorities and technical constraints.

EPFL2008

Multi-Criteria Optimization of an Advanced Combined Cycle Power Plant including CO2 Separation Options

Meinrad Bürer, Daniel Favrat, Hongtao Li, François Maréchal

This paper illustrates a methodology developed in order to facilitate the analysis of complex systems characterized by a large number of technical, economical and environmental parameters. Thermo-economic modeling of a natural gas combined cycle including monoethanolamine absorption CO2 separation option has been integrated within a multi-objective optimizer based on a genetic algorithm in order to characterize the economic and environmental potential of such complex systems within various contexts. A natural gas combined cycle project in a district of Germany is given as a case study. The results show the influences of the configuration and technical parameter changes on the evolution of electrical efficiency of the combined cycle plant as well as on those of its sub-systems, such as gas turbine cycle and steam cycle. The optimum integrations of such a complex system under different situations are revealed by the Pareto Optimal Frontier obtained through the multi-objective optimization process, which provides information on the relationship between power generation cost and CO2 emission performances. Such information is of direct relevance for policy makers to define coherent emission tax levels, or for utility owners or project investors to choose the appropriate emission levels to be reached by the new plant, or for power generation technology suppliers to identify the market potential of their products as well as the most appropriate design for a given power unit, under given policies and economic contexts.

2004