Green heating system: characteristics and illustration with multi-criteria optimization of an integrated energy system

The characteristics of the 'Green Heating System', synonymous with 'Environmentally Friendly Heating System', is academically defined from the total energy systems' point of view, using the concept of 'reversible mode of heating' under the current and future technical, economic and environmental protection context. The exergy-based Specific Consumption Analysis Approach is used to quantitatively evaluate the influence of subsystems' exergy efficiencies on the overall performance of a heating system. Through a case study in the city of Beijing, it is shown that heating fuel specific consumption and the associated emissions can be dramatically reduced as a result of the implementation of a reversible mode of heating system. A multi-criteria optimization process based on a new evolutionary multi-objective algorithm is undertaken to investigate the trade-off between cost and environmental performances associated with such a system.

Defining Optimal Configurations of Geothermal Systems Using Process Design and Process Integration Techniques

Léda Gerber, François Maréchal

The use of geothermal resources for the polygeneration of energy services has recently gained interest and is expected to know an important development in the future. Major research questions concern the increase of the efficiency in the usage of geothermal resources, as well as the increase of their economical profitability and the minimization of the generated life-cycle environmental impacts. This can be achieved by applying process design and process integration techniques to the overall geothermal system, formed by the subsystems of resources harvesting, energy conversion systems and demand. This paper presents a systematic methodology for the optimal design of geothermal systems configurations. In a first step, the different components of the system superstructure are separately modeled using a flowsheeting software. The superstructure includes not only the potential conversion technologies, but as well the different types of potential resources and the demand profiles for the district heating and cooling. It covers a wide pannel of conventional resources and technologies like deep and shallow aquifers, heat pumps, flash systems or organic Rankine cycles, as well as emerging resources and technologies, like Enhanced Geothermal Systems, Kalina cycles, supercritical cycles or organic Rankine cycles using fluid mixtures. Then, resources, technologies and demand profiles models are integrated together using process integration techniques. The resolution of the mixed integer linear programming problem extracts the configuration of the geothermal system from the superstructure. The state variables of the final configuration are then used to define the size of the equipment in the system. Based on these results, the economic, energetic and environmental performances of the integrated system are finally calculated. Performance indicators considered include energy and exergy efficiency, investment costs, operating costs and profitability. Life cycle impact assessment is as well performed by fully integrating the life cycle inventory in the process design framework. To account for the seasonal variation of the demand in energy services, a multi-period optimization model of the integrated system is proposed. The calculation sequence is implemented in a multi-objective optimization framework to calculate the optimal system configurations with their associated technologies, resources and process operating conditions, choosing the objective functions in the different considered performance indicators. This results into the thermo-economic Pareto Frontier that includes the set of the most attractive configurations of the system. The methodology is illustrated by an application to a case study of a city of Switzerland. The results are discussed, as well as their practical implications in terms of technological orientations to be favored in the development of geothermal energy.

Aidic Servizi Srl, Via Giuseppe Colombo 81/A, Milano, Mi 20133, Italy2011

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

Decentralized production in urban systems, the vision of the Institute of Energy Sciences of EPFL

François Maréchal

In 1998, the Board of the Swiss Federal Institute of Technology established the challenging vision of the 2000 W society. The goal is to reach, for Switzerland by the year 2050, an energy intensity target of 2000 Wyear/year/cap. Referring to a present intensity of 5000 W year/year/cap, this means an expected factor 4 reduction of the intensity. For the Swiss Federal Institute of Technology, the challenge is to contribute to this target by its Research and Development activities. In the Ecole Polytechnique Fédérale de Lausanne (EPFL), the Institute of Energy Sciences (ISE) of the School of Engineering (STI) is a group of research laboratories that has committed to contribute to this target in the field of efficient conversion and distribution of energy. Decentralised production of electricity in urban areas is one of the multidisciplinary headlight projects that has been selected to reach the 2000 W target. Decentralised production is considered as a way of producing the energy services required by a community in an integrated approach allowing for polygeneration of heating, cooling, refrigeration and electricity services. This new systemic approach requires a new vision where several networks that have to be operated simultaneously. Electricity has to be distributed through microgrids connected with the major grids. In this field, the laboratories from electrical engineering are developing new tools for the design and the operation of such grids as well as new equipments that guarantee the safe operation of the system and its power quality. The design of district heating networks and their integration with the energy services requirements is a complex task since the demands vary with the seasons, the hours of the day and are by nature stochastic due to the behavior of the inhabitants and the weather conditions. Furthermore, the profitability and the conversion efficiency of such systems mainly rely on the appropriate system design, i.e. the size of the piping and the equipments, as well as on its optimal management. Applying techniques such as thermo-economic modeling, process integration , exergy analysis and multi-objective optimisation, the Laboratory for Industrial Energy Systems (LENI) develops computer aided design methods for helping the different stakeholders (energy services companies, technology developers, final users, policy makers) in defining the most appropriate investments both from the side of the infrastructure and the energy conversion equipments and also from the optimal operation management side in order to place the technologies in the energy market context. The Laboratories of the Energy Sciences Institute also work on the development of new equipments for advanced energy conversion, such as high speed turbocompressors for heat pumping, solid oxide fuel cells (SOFC) and cogeneration concepts combining SOFC and gas turbines. Within the Institute, this advanced R&D effort is performed in a multidisciplinary approach combining experimental work with multi-scale multi-physics computer aided design tools like CFD and optimisation.