Miguel A Lozano Group of Thermal Engineering and Energy Systems (GITSE) Aragón Institute of Energy Research (I3A) Department of Mechanical Engineering, CPS de Ingenieros Universidad de Zaragoza María de Luna 3, 50018 Zaragoza, Spain
Abstract: The necessity of considering the environment as an additional design factor arises due to increasing environmental conscience worldwide and stricter requirements to reduce the environmental impact of energy systems. Designing such systems very often involves conflicting objectives as eco-friendly technologies are usually more expensive. This paper considers simultaneously economic and environmental criteria in the synthesis of a trigeneration system to be installed in a hospital. Synthesis includes optimal configuration (commercially available equipment) and optimal operation throughout the year. The multiobjective optimization accounts for minimization of total annual cost and CO2 emissions released in the atmosphere, as well as the Eco-indicator 99 (EI-99) in order to broaden environmental considerations in the impact assessment. A Pareto frontier, set of solutions representing optimal trade-offs between the economic and environmental objectives, is obtained from the solution of a Mixed Integer Linear Programming (MILP) model. Two bicriteria problems were solved using the MILP model: (1) annual cost (â¬/yr) versus CO2 emissions (kg CO2/yr) and (2) annual cost (â¬/yr) versus EI-99 Single Score (points/yr). The Pareto solutions consist of optimal configurations that adapt their operational strategy during a specific range in the Pareto frontier. Solutions are compared and it is observed that some configurations are more stable along the Pareto frontier, and that significant reductions in economic cost can be attained if the environmental impact is partially compromised. After the judgment of the solutions obtained and the trade-offs involved, one ultimate configuration is selected, which presents a flexible range of adaptability in the economic/CO2 and economic/EI-99 optimizations.
Abstract: Integration of thermoeconomics and Life Cycle Analysis was carried out within the framework of an Environmental Management Information System. This combined approach identified where environmental loads were generated and tracked environmental loads throughout the system, allowing for a more precise understanding of operational activities. A trigeneration system was modeled, providing electricity, heat, and cooling to a building. The trigeneration system consists of a cogeneration module, auxiliary boiler, absorption chiller and electrical chiller. The trigeneration system model is flexible, as it allows electricity from/to the electric grid to be purchased/sold, and part of the cogenerated heat to be wasted. Umberto software is specifically designed to analyze the distribution of material and energy resources throughout a productive system. The software is based on Petri networks, double-entry bookkeeping and cost accounting, allowing the setup of complex systems and also a combined material, energy and inventory calculation. An assistant was built to include the tracking of emissions through the application of algebra and rules similar to those used in thermoeconomic analysis. It is possible to evaluate the environmental impact in terms of the consumption of natural resources and generation of emissions in the system, from the input of natural resources to the output of the final products. Network parameters were used to calculate the emissions associated with the operation of the system. The issue of allocating environmental loads was introduced and two scenarios for each operational mode were compared: the trigeneration system vs. a conventional energy supply system in which electricity was produced in a representative coal power plant. In this case the trigeneration system operated with significant reduction of the CO2 emitted into the atmosphere.
Abstract: This paper presents a thermoeconomic analysis of a trigeneration system interacting with the economic environment. The aim is to determine the energy and total costs of internal flows and final energy services (electricity, cooling and heat). One of the main difficulties in calculating these costs in trigeneration plants within buildings is the continuous variation of energy supply services. Fuel prices and purchase/sale electricity tariffs can also vary. As a consequence there are different operation conditions that combine the possibilities of purchasing or selling electricity, consuming heat from auxiliary boilers, and wasting the excess of cogenerated heat. A novel cost allocation method valid for all possible operation conditions of the trigeneration system is proposed. The heat produced by cogeneration modules is disaggregated into three fractions: heat that meets the heat demand directly, heat utilized to drive absorption chillers (producing cooling), and heat dissipated to the environment. Cost allocation to all cogeneration co-products is determined by applying the principle of avoided expenditures. The cost allocation proposal is applied to a trigeneration system providing energy services to a hospital with 500 beds located in Zaragoza (Spain), encouraging rational and efficient energy services production and consumption.
Abstract: Environmental information obtained through Life Cycle Analysis techniques has been incorporated into a Mixed Integer Linear Programming (MILP). The solution of the model provides the optimal configuration and operation of an energy supply system to be installed, minimizing the environmental burden associated with production of equipment and consumption of resources. Starting from a superstructure of cogeneration system with additional components highly interconnected, the energy supply system was optimized considering specific demands of a hospital located in Zaragoza, Spain. The objective functions took into account the kilograms of CO2 released and Eco-indicator 99 Single Score. Also considered were price of energy resources, price and amortization possibilities of the equipment and options for selling surplus electricity to the electric grid. The effect of electricity generation conditions on the optimal configuration was examined by varying the source of electricity production in Spain and considering natural gas/electricity mixes from alternate countries. The ratio between local electricity emissions and natural gas emissions (α factor) was found to have the highest impact on the configuration of the system. Therefore the α factor could be considered the strongest influencing factor when deciding the optimal configuration of a system that minimizes environmental loads.
Abstract: The development of trigeneration systems is especially important in the buildings sector, where the thermal loads are imposed by the needs of heating, domestic hot water, and cooling. A strong seasonal character is indicated, since the demands depend totally on local climatic conditions and vary considerably throughout the year. Geographic locations were chosen so as to represent the climatic variety in Spain: Canary Islands, Mediterranean Coast, Atlantic Coast, and different locations in the interior of the Iberian Peninsula. The solution of a mixed integer linear programming model (MILP) that incorporated local economic/environmental conditions determined the optimal configuration of the different energy supply plants as well as the optimal operation modes throughout an entire representative year. From an economic point of view, the optimal configuration for all localities included cogeneration modules. From an environmental point of view, the optimal solution was strongly dependent on the origin of the electricity supplied by the grid.
Abstract: The energy requirements for most residential and commercial buildings show both annual or seasonal, and daily variations. In addition, depending on the electrical rate schedule and location, there can be time-of-use variation in the prices a customer pays for electricity. Such facts make it harder to select (a) the best-fit equipment for the host building and (b) the best operating strategy for the selected equipment. This article proposes a mixed integer programming (MILP) model to help decide the optimal make up of a prospect cogeneration system as applied to a residential complex located in Zaragoza, Spain. The model considers the possibility of whether to use or not a set of proposed alternative technologies within a previously defined superstructure. Such superstructure is defined through binary variables and takes into account the optimal operation of all feasible combinations of technologies throughout a typical meteorological year. The model's objective function is to minimize the total annual cost, which includes the cost of invested capital, subject to technical, economic and legal constraints.
Abstract: Many countries in Europe promote cogeneration as a way to meet energy needs in residential and commercial buildings. They do this to save primary energy and reduce CO2 emissions. This article presents an energy and economic analysis approach for cogeneration plants hosted by such buildings. The plants use gas-fired internal combustion engines as prime movers. Technical criteria to characterize annual operation for cogeneration systems with seasonally and daily variable heat demand are defined. The focus is on determining the total engine size or output by considering different operational strategies. The methodology is illustrated by applying it to a cogeneration plant that meets domestic hot water and heating demand in a residential complex in Spain. The resulting graphical analysis allows one to compare various operational strategies.
Abstract: The sugar cane industry represents one of the most important economic sectors in Brazil. It produces sugar and ethanol for the internal and external markets. Also, thermal and electric energy are produced for the own factory consumption, using sugar cane bagasse as fuel in cogeneration plants. Almost all the sugar cane factories in Brazil are self-sufficient in terms of energy supply and in the last few years some of them have been selling their surplus for the grid. The introduction of steam power plants operating at higher pressure and temperature levels or even Biomass Gasification Systems operating in combined cycles are new alternatives for increasing the efficiency of these systems. The purpose of this paper is to analyze different options of cogeneration systems in sugar cane factories in order to evaluate the possibilities of increasing electricity generation. The analysis of the power plant is performed together with the steam demand reduction of sugar production process once the two systems are interlinked.
Abstract: Sugar and ethanol production from sugar cane in Brazil is one of the most competitive sectors of the national economy. The sugar production is done basically by several steps: extraction, juice clarification, evaporation, syrup treatment, sugar boiling, crystallization and centrifugation where the crystal sugar and the molasses are obtained. This paper presents a thermoeconomic optimization of thermal energy consumption in a sugar production process looking for the minimum investments and operation costs. Data from Brazilian sugar factories are used to define the process parameters. The methodology proposed is used to evaluate the cost of the steam consumed by the factory and the optimal design of the evaporation system as well as the juice and syrup heaters network.
Abstract: The conceptual significance of thermoeconomic costs is analyzed in this paper. Marginal and average costs are acurately defined and the relationship between both costs is studied. Marginal and average costs can be unified in a single body of doctrine which is the same for the cost accounting and optimization theories. The chain rule of derivation is applied as a general method for calculating average and marginal costs according to the model describing the behavior of the system. The more accurate the physical model of the system is, the greater physical significance will have the costs. The chain rule also provides the factors that generate costs and allow to know the detailed process of cost formation in a plant. Finally the factors characterizing the thermoeconomic significance of the costs are analyzed. Only when the costs have a well known meaning can be used to solve real and practical problems. Criteria to define a good productive structure, needed in the thermoeconomic analysis or optimization of a plant, are also provided.
