A systematic methodology for the synthesis of unit process chains using Life Cycle Assessment and Industrial Ecology Principles

The integration of environmental impacts in the conceptual process design procedure can be achieved by combining Life Cycle Assessment (LCA) tools with flowsheeting software and process integration techniques. However, current LCI databases assume a given unit process chain for each product or waste, and do not allow for the systematic synthesis of the process chain minimizing the environmental impacts of raw materials or auxiliary flows, accounting systematically for the different alternatives and recyclings. This paper presents a methodology for systematically designing process units LCI database for the optimal design of integrated systems with a life cycle perspective.

Méthodologie de la synthèse des systèmes énergétiques industriels

Raffaele Bolliger

This work presents a synthesis method that leads to the preliminary design of industrial energy systems. Such systems are composed of several technologies that transform, through a set of physical unit operations, raw materials and energy into products and energy services. The purpose of the preliminary synthesis is to define which technologies form the system, to calculate their technical characteristics, their operating conditions and their interactions, based on performance indicators such as for example the cost of the system or its environmental impact. This area is studied since a long time and many synthesis approaches are available. However, these methods have significant limitations, particularly in their ability to face the study of more and more complex systems. The main goal of the method presented in this work is to enable the study of large systems and promote the use of the experience gained from previous studies in the form of models and methodological approaches. The synthesis method of industrial energy systems is based on the overall system optimization using a model that is obtained by assembling several modules, representing each a set of physical unit operations and whose equations are formulated with the black box technique, and an integration model that can represent the possible interactions within the system. Optimization is performed using a multi-objective evolutionary algorithm, whose objective functions are defined on the basis of calculation of several performance indicators based on the model of the system. To simplify the resolution of this complex system, the optimization problem is decomposed into a master problem, responsible for calculating the characteristics of the units and their operating conditions, and an optimization slave subproblem, which selects the units being part of the system and their interconnections. To ensure its robustness, the slave subproblem is formulated as a mixed-integer linear optimization problem. The slave subproblem is formulated by using process integration techniques, which are extended in this work to allow the synthesis of multiple heat and mass transfer networks. The synthesis problem can thus be defined using an explicit superstructure or by one generated automatically, either implicit or explicit. This work also introduces a technique that greatly reduces the number of degrees of freedom of the integration model. Instead of separately optimizing each ΔTmin/2 associated with heat streams, a formula is applied to calculate their value from a reference case. The ΔTmin/2 optimization is thus reduced to the optimization of a single decision variable related to the reference case, regardless of the size of the problem. The proposed method uses a set of heterogeneous elements, including flowsheeting software for modeling physical unit operations, mathematical programming tools for the formulation of the integration problem, methods for calculating performance indicators and calculation tools such as the optimization algorithm. This work introduces new tools developed to systematically apply the proposed methodology and to automate recurring operations of data transfer and the call of the various used software. In particular, a syntax description is defined as an abstraction layer to describe and to structure the exchanged information. A computing platform has been created to support the application of the method and to ensure the data transfer between its components. Two case studies are presented to illustrate the various aspects of the synthesis method. A first case, involving the synthesis of two combined cycles, has been chosen to illustrate the different application stages of the method and to show the potential reuse of certain modules. Through the integration techniques, it has been possible to identify potential heat recovery that can increase the performance of one of the cycles beyond what had been expected by experts using conventional simulation techniques. The second case study is about the treatment of waste generated by an industrial site active in the field of fine chemicals. Waste treatment can recover different materials and energy services useful for process units, thereby reducing the quantities purchased in the market. The model of multi-network integration can easily solve the complexity of the problem of waste management in developing strategies for allocating waste to the various treatments available for different objective functions related to operating costs and environmental impact.

EPFL2010

Integration of Life Cycle Assessment in the conceptual design of renewable energy conversion systems

Léda Gerber

Life Cycle Assessment (LCA) is a well-established methodology that has been extensively used for the environmental evaluation of emerging technologies in the field of renewable energy conversion systems. In order to identify effective solutions for impact mitigation already at the conceptual process design stage, it is necessary to take into account the environmental aspects in the process design procedure, in consistency with economic and thermodynamic criteria. This thesis presents a systematic methodology for the integration of LCA in the conceptual design of renewable energy systems, using process design, process integration and multi-objective optimization techniques. The developed methodology is illustrated by three application case studies. The first one concerns the evaluation of different candidate technologies for the combined production of Synthetic Natural Gas and electricity from lignocellulosic biomass by thermochemical conversion. The second case study treats of the environomic optimal configurations of Enhanced Geothermal Systems for combined heat and power production, considering different potential conversion technologies. The third case study deals with the design and the synthesis of an urban energy system, accounting for the different energy services to be supplied, the different potential resources, the candidate conversion technologies, and the waste to be treated. In this last case study, the overall supply chains from resources or wastes to services are synthesized, accounting for the industrial ecology possibilities. A first major outcome of these application case studies is that accounting for the process integration in the evaluation of the environmental impacts leads to an important impact reduction. Secondly, the developed methodology allows for calculating the trade-offs between the environmental criteria and the thermo-economic ones. Therefore, the optimal promising configurations of an emerging technology or of a system considering its environmental performances can be identified. Finally, it is demonstrated that integrating the environmental impacts in the process design procedure leads to a different decision-making than if only thermo-economic criteria are considered.

EPFL2012

Environomic design of vehicle energy systems for optimal mobility service

Zlatina Kirilova Dimitrova, François Maréchal

The main design criteria for the modern sustainable development of vehicle powertrains are the high energy efficiency of the conversion system, the competitive cost and the lowest possible environmental impacts. An innovative decision making methodology, using multi-objective optimization technics is currently under development. The idea is to obtain a population of possible design solutions corresponding to the most efficient energy system definition. These solutions meet technical, economic and environmental optimality. This article applies the methodology on an electric vehicle, in order to define the powertrain configuration of the vehicle, to estimate the cost of the equipment and to show the environmental impacts of the technical choices of the powertrain configurations in a life cycle perspective. A physical model of the electric vehicle is made and coupled with a cost model for the vehicle and LCA (Life Cycle Assessment) technics are used for the environmental assessment. After multi-objective optimizations with thermo-economic and environmental objectives, the solutions obtained from the Pareto frontiers curve are analyzed. Conclusions about the environomic design of the vehicle for optimal mobility service are made. The greenhouse gas emissions are calculated from a well-to-wheel perspective for different countries of use of the electric vehicle, according to their respective electricity production mixes.