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

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.

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

Daniel Favrat, Hongtao Li, François Maréchal

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.

2007

Swiss Motor. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Fonctionnement avec mélange stoechiométrique (l=1) et pauvre (l>>1).

Daniel Favrat, Geoffrey Basil Leyland, Roger Röthlisberger

The Swiss limit for NOx exhaust gas emissions from decentralised electrical power and heat cogeneration units using stationary I.C. engines operating on natural gas, despite a recent increase from 80 to 250 mg/mN3 (5% O2), remains the most severe in force in the world. The only technology currently able to meet this limit - the use of a stoichiometric mixture and a three-way catalyst - results in a relatively low global engine efficiency and suffers from a limited reliability of the exhaust gas treatment system. In this context, the Laboratory for Energy Systems of the EPFL has undertaken to develop new technologies (mixture, ignition,...) on the basis of a converted diesel engine for natural gas operation and to evaluate their reliability as well as their potential to reduce exhaust gas emissions, particularly NOx. The objective of this project is to perform an experimental study of the influence of the operating mode, either a stoichiometric mixture (l = 1) and a 3-way catalyst for exhaust gas treatment or a lean mixture (l >> 1) without catalyst, on the performance and emissions of a Liebherr G 926 engine, in order to establish a basis of comparison for future evaluation of new technologies. The realisation of this project required the construction of a new test bed especially developed for the study of gas engines. In the case of the stoichiometric operating mode, the results show that at an MBT spark timing of 20 °CABTDC, a mechanical power output of 106.3 ± 0.5 kW at 1502 ± 5 rpm and a global efficiency of 35.9 +1,7/-0.6% is achieved. A series of tests with different sparkplugs showed only a weak influence on the engine behaviour. The use of a 3-way catalyst for exhaust gas treatment reduces the NOX and CO emissions comfortably below the limits prescribed by the Swiss Federal Clean Air Act. In the case of the naturally aspirated engine, lean operation with an air to fuel ratio over 1.6 keeps the NOX emissions under the prescribed limit without a catalyst. However, the power output falls below 69 kW and the global efficiency below 35%. By turbocharging and intercooling the intake air and gas mixture the power output was increased from this low level to 150 kW (pme = 12 bar). As with stoichiometric operation, changing the spark plugs has only a weak influence on the engine behaviour. However, an increase of the ignition coil discharge time led to a perceptible reduction of the cycle-by-cycle variability. The results show that the best method of reducing the amount of energy dissipated by the exhaust manifold is to insulate the manifold, rather than water cool it. The insulation preserves more energy for the turbocharger and a high temperature for a possible catalytic exhaust gas after treatment. The reduction of the dead volume located between the cylinder liner, the cylinder head and the cylinder gasket led to a 40 % decrease of the THC emissions. Further, the use of a new piston, generating a higher level of turbulence during the combustion, raised the lean limit and thus reduced NOX emissions below 150 mg/mN3, 5% O2, while still keeping the global efficiency over 37%; these results are valid for a intake mixture temperatures of both 50 and 80 °C, however in the second case at a somewhat lower power output than 150 kW. Despite these improvements, and because of an extremely low CO raw emissions limit, the use of a oxidation catalyst is needed to fulfil the requirements of the Swiss Federal Clean Air Act. In order to reduce the raw CO emissions and to increase the engine global efficiency, this project will continue with a third part aiming to study the potential of an ignition system based on a prechamber.

1998

Multi-objective 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 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 CO2 separation options has been coupled within a multi-objective evolutionary algorithm to characterize the economic and environmental performances of such complex systems within various contexts. The method has been applied to a case of power generation in Germany. The optimum options for system integration under different boundary conditions are revealed by the Pareto Optimal Frontiers. Results show the influence of the configuration and technical parameters on the electrical efficiencies of the Pareto optimal plants and their sub-systems. The results provide information on the relationship between power generation cost and CO2 emissions, and allow sensitivity analyses of important economical parameters like natural gas and electricity prices. Such a tool is of interest for power generation technology suppliers, for utility owners or for project investors, and for policy makers in the context of CO2 mitigation schemes including emission trading.

Elsevier2006

Swiss Motor. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Fonctionnement avec mélange stoechiométrique (l=1) et pauvre (l>>1).

Daniel Favrat, Geoffrey Basil Leyland, Roger Röthlisberger

1998