Thermoeconomic and environomic modeling and optimization of energy systems.

The increasing concern for the environmental impact of human activities stimulates the development of new methods for the analysis and design of industrial processes including energy systems. It is generally admitted that the considerations of environmental and other life cycle factors as early as possible in the design can contribute to a reduction of overall costs, favoring a cost effective evaluation of trade-offs between internal process improvements and post-treatments of effluents. For energy systems with significant time dependence of the major input and output parameters additional important questions for the designer are where and when to invest, in particular when staged or deferred cost investments are worth considering. Additionally advanced design methods have to be able to deal with availability (reliability) considerations, which are known to be of high relevance for energy systems. Modern energy systems are not only made of integrated technologies but they cannot be considered as fixed as they, like living bodies, tend to adapt to the decision making environment, which at present is mainly based on economic factors and regulatory issues. Ideally assessments at all levels (company, national, international) should be made in a coherent framework where all technology options can freely compete particularly when a major departure from present day economics can be envisaged, at least for sensitivity analyses. This paper intends to review a new approach meeting the above requirements and called environomic modeling and optimization, which has been extensively developed at LENI-EPFL with successful demonstration to the following applications : Ž Combined cycle power plant with or without cogeneration and with or without advanced features like CO2 separation, gas recirculation, oxygen enrichment, etc. Ž District heating network with centralized and/or decentralized heat pumps and cogeneration units, Ž Waste incineration plants with cogeneration and with or without toping cycle Results of the optimization of the above applications using genetic algorithms and including sensitivity analyses, in particular to the internalization of some pollution costs, will be presented. Further applications to other energy systems currently under study will be mentioned.

Sustainable Development Based Methodology for the Optimized Integrated Design and Operation of Energy Conversion Processes and Networks.

Daniel Favrat

A design methodology allowing engineers to consider simultaneously and from the earliest phases of design some of the key factors of sustainable development (pollution, life cycle data) has been developed and successfully demonstrated on two examples of integrated energy systems. The principal idea is to take diverse factors related to thermodynamics, economics and the environment and integrate them in a way that the most optimal syntheses and designs (and operation) can be found. The solution space resulting from such a complete formulation, which can include tens of independant variables, is highly non linear and includes locally continuous but generally non contiguous subspaces. Solution procedures based on specially adapted genetic algorithms, have been implemented and shown to allow the user to tracks the most effective solution paths in a robust way. Pollution penalty factors accounting for both emissions and immission levels have been proposed and applied in dedicated sensitivity analyses. Using a simplified quasi-stationary formulation application examples include the predesign of combined cycle power plant with or without cogeneration and with or without CO2 separation as well as the predesign of district heating systems considering the use of both centralized and/or decentralized heat pumps as well as cogeneration units. Both exergetic and economic optimum solutions are compared. As FN did not finance the entire proposal, additional contributions from FOGA and Alliance for Global Sustainability allowed the completion of this project. Although the present results open exciting prospects and are already being extended in the framework of four different international initiatives, further work is still required. It includes, among others, a further refinement of the link between entropy and/or exergy and pollution, an extension to a broader range of pollutants including those having synergistic effects, a better account for the time dependancy and uncertainties, the coupling with top down approaches including neural net based simplified submodels and parallelisation and an extension of the models of technologies considered to other integrated systems such as direct electrochemical conversion or renewables based devices (new proposal in preparation).

1999

An Environomic Approach for the Life Cycle Modeling and Optimization of a District Heating Network Based on Centralized and Decentralized Heat Pumps (Part I: Methodology).

Vinicio Curti, Daniel Favrat

Although heat pump based district heating is often an obvious solution, adapting the delivery temperature to the most exigent users is detrimental to overall system performance. This pitfall can be avoided with a centralized plant of heat pumps, cogeneration and an auxiliary furnace, supplemented by decentralized heat pumps. However, the problem of mixed energy production and delivery which this poses is complex and presents for the daunting if not impossible task of adequately, much less optimally, determining the best system for the job. An environomic methodology for aiding in this task is proposed. It is used to model the thermodynamic, economic, and environmental characteristics of such a system in order that its final configuration and corresponding component designs can be optimized. Our approach is an attempt to respond at the synthesis, design and operational level of an energy system to the concept of sustainablility. The optimization of the model developed results in a highly nonlinear, non- contiguous solution space effectively searched using a genetic algorithm. The system’s environmental characteristics are introduced into the model through pollution damage cost terms and pollution penalty functions adapted to the system’s changing emissions and to local and global pollutant conditions. Results are shown for various district heating user distributions and fuel and electricity prices. In this first of a series of two articles, our methodology is described and the details of the environomic model for a district heating network based on centralized and decentralized heat pumps presented. In the accompanying article [1], a complete set of results for the optimal synthesis, design and operation of the network is given and discussed.

1998

Sustainable Development Based Methodology for the Optimized Integrated Design and Operation of Energy Conversion Processes and Networks.

Daniel Favrat

1999