1,035 research outputs found
Sensitivity analysis applied to the multi-objective optimization of a MCFC hybrid plant
No full textIn this paper, the multi-objective optimization of a molten carbonate fuel cell (MCFC) based hybrid plant fueled with landfill gas is performed. System operation is significantly affected by off-design conditions. These are due to variations methane concentration occurring as the landfill depletes, performance degradations of the components, particularly the fuel cell, and ambient conditions. For these reasons, the objective functions are defined considering the plant lifetime. Some of the parameters affecting the results, as the voltage degradation, the cost of fuel cell, the methane concentration and the unit cost of landfill gas can be only estimated or forecasted and their actual values are uncertain. Therefore, the optimization is performed considering a sensitivity analysis in order to estimate the effects of possible variations on the Pareto front. The following free design variables are considered: pressure and temperature operation of the MCFC, turbine inlet temperature, fuel mass flow rate. In addition, the optimal configuration of the heat exchanger network is selected for each set of the design variabl
Entropy generation analysisfor the design optimizationof solid oxide fuel cells
No full textPurpose - The aim of this paper is to investigate performance improvements of a monolithic solid oxide fuel cell geometry through an entropy generation analysis. Design/methodology/approach - The analysis of entropy generation rates makes it possible to identify the phenomena that cause the main irreversibilities in the fuel cell, to understand their causes and to propose changes in the design and operation of the system. The various contributions to entropy generation are analyzed separately in order to identify which geometrical parameters should be considered as the independent variables in the optimization procedure. The local entropy generation rates are obtained through 3D numerical calculations, which account for the heat, mass, momentum, species and current transport. The system is then optimized in order to minimize the overall entropy generation and increase efficiency. Findings - In the optimized geometry, the power density is increased by about 10 per cent compared to typical designs. In addition, a 20 per cent reduction in the fuel cell volume can be achieved with less than a 1 per cent reduction in the power density with respect to the optimal design. Research limitations/implications - The physical model is based on a simple composition of the reactants, which also implies that no chemical reactions (water gas shift, methane steam reforming, etc.) take place in the fuel cell. Nevertheless, the entire procedure could be applied in the case of different gas compositions. Practical implications - Entropy generation analysis allows one to identify the geometrical parameters that are expected to play important roles in the optimization process and thus to reduce the free independent variables that have to be considered. This information may also be used for design improvement purposes. Originality/value - In this paper, entropy generation analysis is used for a multi-physics problem that involves various irreversible terms, with the double use of this physical quantity: as a guide to select the most relevant design geometrical quantities to be modified and as objective function to be minimized in the optimization proces
Thermoeconomic approach for the analysis of control system of energy plants
No full textIn this paper a thermoeconomic approach is applied to the dynamic model of a Power System in order to investigate the effects of the control system on the primary energy consumption and on the economic costs of the product. To achieve this objective, various control strategies are compared when variations of the operation condition, due to some internal or external causes, are produced. These variations cause the intervention of the control system, which rearranges the operating condition in order to have the controlled quantities within acceptable ranges. Generally the plant efficiency changes, depending on the selected strategy. A microturbine is considered as the case study. The analysis here proposed allows one to quantify the effect of the control on the performance variation of the components. The approach associates an exergetic cost and a thermoeconomic cost to the control system operation, which expresses the additional resource (primary energy and economic resources) consumptions that may be associated to the control. The impact on the initial and final steady states as well as the transient evolution are considered. This can be usefully applied to improve energy system operation acting on the control system, both in the off-design steady states and transient operations. In the particular application considered in this paper, reductions of about 8% in fuel consumption and 5% in the total costs are achieved. Concerning transient operation, it is shown that the control system can produce large variation in the operation cost
Thermoeconomic approach for the analysis of low temperature district heating systems
No full textIn this paper a thermoeconomic analysis of district heating systems is performed. The analysis aims at comparing possible options to supply heat to the users, using low temperature networks. Thermoeconomic analysis consists a powerful tool to perform such analysis as it allows one to evaluate the possible options in terms of primary energy cost or economic costs. In the first case, the use of exergy as the quantity that is transported along the network makes it possible to properly consider the various qualities of energy that are used to supply heat to the network and to distribute it to the users. In the case of economic cost, the various cost contributions are considered: investment cost, cost of heat supplied to the network, pumping cost. A different cost can be calculated for the various users depending on their position and characteristics of the heating devices. This is a useful information in order to compare possible options for supply them hea
Thermoeconomics as a tool for the design and analysis of energy savings initiatives in buildings connected to district heating networks
No full textDistrict Heating (DH) is a rational way to supply heat to buildings in urban areas. This is expected to play an important role in future energy scenarios, mainly because of the possibility to recover waste heat and to integrate renewable energy sources. Even if DH is a well known technology, there are open problems to face. Some of these problems are related to tendencies to reduce design temperatures, the improvement of control strategies, connection of new users to existing networks, implementation of energy savings initiatives and the access of multiple heat producers to the same network. This paper aims to show that exergy is an appropriate quantity for the analysis of DH systems and thermoeconomics can be profitably used to improve their design and operation. Three possible applications of thermoeconomic theories are presented: variation of supply temperature along the heating season, opportunities to connect new users, effects of energy savings initiatives in buildings connected with the network
Thermoeconomic Diagnosis of anUrban District Heating System based onCogenerative Steam and Gas Turbines
No full textThe energy system diagnosis is a experimental technique applied for the detection and the location of possible anomalies. These information are obtained by comparing the values assumed by opportune variables in a operation condition with the corresponding reference values. In this thesis a procedure based on the use of thermodynamic variables, elaborated using the thermoeconomic methods, is proposed. The main originality, in comparison with other thermoeconomic diagnosis procedures, consists on the evaluation of the regulation system effects on the plant working condition; these effects can be then isolated to achieve the purpose of the malfunction location. The usefulness of such an operation is shown by applying the procedure to some cases of anomalies, obtained using a mathematical model of two thermal power plants, located in Moncalieri (Turin). Thermoeconomics is an engineering discipline born in sixties, consisting in the contemporary use of thermodynamic principles and economic concepts. It allows to associate a cost to all the productive processes taking place inside a system and in particular their products. This cost can be measured in monetary units and eventually in pure thermodynamic units. The key of the thermoeconomic analysis procedures consists on a productive model of the system, called productive structure, generally different from its physical model. Every energy transformation is represented and quantified in terms of supplied product, i.e. the useful effect obtained, resources required and possible losses. Such quantities in modern thermoeconomics are expressed using exergy fluxes. The system is first divided in some subsystems or components, in each one a significant energy transformation takes place. The grade of detail depends on the available information and on the aim of the analysis. The thermodynamic variables (mass and energy flows, temperatures, pressures etc.) required to characterize the fluxes entering and exiting the component must be known. In this way a more detailed analysis furnish more information, but it requires the measures of a larger number of physical variables. The ratio between every resource used by a component and its product is called unit exergy consumption. The values assumed by the whole of the unit exergy consumptions completely describes the thermoeconomic model of the system. This means that the description of the system based on its productive model is a simplification of the physical model. The information are summarized in the thermoeconomic parameters, which substitute the thermodynamic variables measured in correspondence of the fluxes crossing the boundaries of the control volumes. The thermoeconomic diagnosis is made by studying the temporal variation of the unit exergy consumptions, while, the methodologies usually applied in the energy systems, the variation of a whole of different quantities is analysed. The thermoeconomic diagnosis allows the use of the same procedure for all the anomalies, so it is a general methodology. On the contrary the other methodologies use a different procedure, depending on the kind of anomaly wants to be detected; the available data must be chosen and organized so that they could furnish the required information. Nevertheless the thermoeconomic diagnosis only allows to detect anomalies having sensible repercussions on the thermodynamic behaviour of the system. Moreover some information are lost when the physical structure is substitute with the productive structure, which could make the procedure unable to locate some kind of malfunctions. These considerations suggest the contemporary use of the thermoeconomic diagnosis together with other methodologies, as they are often complementary. In particular the other techniques are normally devised to prevent the anomalies which can cause, if not repaired, failures. On the contrary the aim of the thermoeconomic diagnosis consists on the detection and the location of the anomalies causing the reduction of the system efficiency. Moreover it also allows to evaluate the costs associated to the variation of the working condition, which is more significant than the simple variation of the efficiency of a single process. The same efficiency variation can in fact involves a different fuel impact depending on where it takes place. This consideration is known as principle of non equivalence of the irreversibilities. The procedures of thermoeconomic diagnosis consist on the determination of the values assumed by the unit exergy consumptions in a operation condition and a reference condition, on the calculation of opportune evaluation indices based on these quantities and on their comparison. The two states must be characterized by the boundary conditions: the environment must be characterized by the same temperature, pressure and humidity, the plant production must be the same in quality and quantity (the same electric power and, in case of thermal production, the exiting flow must be characterized by the same energy flow, temperature, pressure and thermodynamic quality) and finally the fuel quality must be the same. Due to these constraints, the reference condition is usually determined by means of a simulator. The correct anomaly location is only possible in the cases where its effect is largely concentrated in the component where it has taken place. This does not happens always. The first effect of an anomaly is the reduction of the efficiency of the component where it has occurred (intrinsic malfunction). If the component resource has been maintained constant, the anomaly causes the reduction of its product. As this product is generally resource of other components, their production is affected too and in particular it decreases. This effect is not negative, but can have a negative consequence: the efficiency of the components generally depends on the working condition, so the variation of their resources involve a variation of their efficiency too. A malfunction, called induced malfunction, takes so place in the other components, although any anomalies have occurred in them. A second consequence of the variation of the working condition consists on the variation of some control parameters. In particular the total production of the plant has varied and some set-points can have varied. The working condition originated as direct effect of the anomaly is unacceptable, so the control system intervenes to operate a regulation in order to restore the setting values of these parameters. The intervention modifies the natural effects of the anomaly, so other malfunctions and dysfunctions are induced. The location of the intrinsic malfunctions becomes more difficult once the regulation system has intervened. The thermoeconomic diagnosis procedure here proposed is based on the determination of the working condition that would have taken place if the regulation system did not intervene. This condition is fictitious, as the constraints imposed by the control system are not complied, so it must be mathematically calculated. If the anomaly is sufficiently little, the effect of the regulation parameters on the unit exergy consumptions can be calculated using a Taylor's development. The independent variables are represented by the characteristic variables of the regulation system, i.e. a set of variables which completely individuated its positioning. In this way an artificial working condition can be built, where the effects of the regulation system are not present but the effects of the anomaly are. This condition is here called free condition. The diagnosis is made by comparison of the values assumed by the unit exergy consumptions in free and reference conditions. The thermoeconomic diagnosis procedures proposed in literature are based on the comparison between operation and reference conditions. In this comparison the contribution of the regulation system is hidden and sometimes makes impossible the correct location of the anomalies, as shown in the proposed applications. The proposed procedure is here applied to two energy systems: a steam turbine and a gas turbine plants, both able to also provide thermal power to an urban district heating network. A mathematical model of the plants, described in the first chapter, has been built in order to simulate their behaviour. Some anomalies have been simulated by varying the values of the characteristic parameters of the components, like efficiencies, heat transfer coefficients and pressure drops. The model also takes into account the regulation system. In particular its characteristic parameters in the gas turbine plant are the fuel mass flow, the opening grades of the inlet guided vanes and of the by-pass valve and the water mass flow passing through the recuperator. The regulation parameters of the steam turbine are the fuel mass flow, the opening grade of the throttles and the mass flow of the steam extraction for the cogeneration. The effect of these variables on the productive structure fluxes has been differently evaluated for the two plants: an analytical calculation, using the mathematical model of the plant, is proposed for the gas turbine plant, while a numerical calculation, using some working conditions, is proposed for the steam turbine plant. The analytical development has been expressed in form of a constrained optimization problem, mathematically described using a Lagrangian function. Such expression is particular significant as the Lagrange multipliers coincide with the marginal costs associated to every variable. In this way a cost can be associated to the regulation parameters. The procedure is applied to some cases of single and multiple malfunctions. In all the cases it allows to locate where the anomalies have taken place. The procedure is particularly helpful in the application to the gas turbine plant, where the effects induced by the regulation system are sometimes larger than the intrinsic malfunction, so that the correct location is impossible using the ordinary thermoeconomic procedures. On the contrary, in the steam power plant the effect of the malfunctions are mainly intrinsic, so that the correct location is in most of the cases possible using both the procedures. A further develop of the diagnosis technique consists on the erasure of the contribution of the effects induced by the specific components behaviour, i.e due to the efficiency variations caused by the variation of the resources. To take into account this contribution the system can be split into its components, each one considered separately. The knowledge of different working conditions, corresponding to as many regulations, allows to build a linear thermoeconomic model of the components. The each product can be calculated as resources vary. This dependence is acceptable only if the difference between the fluxes in free and reference conditions is sufficiently low. The unit exergy consumptions of every component in a condition characterized by the same resources as in free condition can be calculated. In this condition any anomaly is present in the system, as it is built starting from the reference state. A difference between the unit exergy consumptions respect to the reference values is due to the behaviour of the components. The induced malfunctions caused by the dependence of the efficiencies on the quality and amount of resources can be so erased. The more desegregate is the productive structure and the better works this technique. The use of structures defined by splitting exergy into its components is recommended. The procedure has been applied to some gas turbine operation conditions, where single malfunctions and a triple malfunction have been simulated. In all cases it has allowed to find at the same time how many were the intrinsic effects and where they had occurred. This is an important improvement in the application to the real systems, as the number of malfunctioning components is a priori unknown. The procedure is described in the forth chapter, while the applications to the power plants is shown in chapters 5 and 6. In this last chapter an application obtained using measured data relative to the steam power plant is proposed. These results do not constitute a demonstration of the absolute validity of the methodology for the energy system diagnosis. Nevertheless an important result has been obtained: a correct thermoeconomic diagnosis is impossible without considering the regulation system. It is not a finish line, but the starting point for future studies in this field. In particular, when if more than one anomaly are present in the system, the proposed diagnosis procedure does not allow to correctly predict the technical energy saving obtained by completely removing each one. In fact, this information requires the use of a mathematical model of the system. A second aspect of the thermoeconomic analysis here studied in deep is the effect of the choice of the productive structure on the results. The definition of fuels and products is not universally accepted, although many studies and applications have allowed to achieve a certain agreement. Some grade of freedom are so available for the analyst. The choice of the productive structure has a sensible impact on the cost calculation, in particular when some losses occur in the system, i.e. some fluxes characterized by a non zero exergy exit the system without being provided (and sold) to the users. These fluxes are not products, as they do not have any usefulness, so they can not exit the system in the productive model. The components of the system must be charged for them. Different criteria allow to make this operation. A different productive structure, an so a different cost accounting, corresponds to each criterion. In the third chapter some criteria are described and applied to the Moncalieri plants. A particular emphasis is given to the choice of the productive models for the gas turbine plant. The diagnosis procedure is not sensitive to the choice of the productive structure: all the examined cases give information in coherent to indicate the components responsible for the malfunctions. Moreover a detailed structure, obtained splitting exergy into mechanical and thermal (and if necessary chemical) components to define fuels and products, also allows to obtain a more detailed information. In particular, if the gas turbine plant is considered, a more detailed structure allows to individuate the causes of pure mechanical or thermal malfunctions. On the contrary if other kinds of malfunctions occur, the location becomes more difficult, as the effects are split on terms of the unit exergy consumption matrix. Nevertheless the information does not contradict the one given by a simpler structure, so the contemporary use of both of them is suggested. The last contribution of this thesis is the evaluation of the exergy cost to be associated to the regulation system intervention. This quantity is obtained considering the fuel consumption and the total product in operation and free conditions. The unit cost is defined as the ratio between the variation of the resources and the corresponding variation of the products. This parameter allows to evaluate the incidence of internal constraints, like set-points, on the plant efficiency. If the plant does not present any anomaly this parameter is equal to the marginal cost calculated in reference condition, otherwise it assumes a different value. An higher value means that the regulation system intervention causes an increase in the cost of the products, while a lower value causes a cost decrease. negative values are associated the contemporary decrease (or increase) of the plant efficiency and the total production. From the malfunction analysis point of view, a value of the unit cost of the regulation higher than the unit cost of the plant products means that the regulation system induces malfunctions in the system. In that case the use of the proposed procedure is particularly suitable, as it allows to eliminate those malfunctions from the syste
Design improvement of circular molten carbonate fuel cell stack through CFD Analysis
No full textMolten carbonate fuel cell (MCFC) is a promising technology for distributed power generation. The core of an MCFC power generation unit is the stack, where various fuel cells are connected together in series and parallel in order to obtain the desired voltage and power. Stack geometry and configuration are major engineering topics, as inhomogeneous temperature or mass fractions cause inefficient performances of the fuel cells, as efficiency and power smaller than the expected and shorter lifetime. A detailed model is a useful tool to improve stack performances, through design improvements. In this paper, a 3D model of a stack composed of 15 circular MCFC, considering heat, mass and current transfer as well as chemical and electrochemical reactions is presented. The model validation is conducted using some preliminary experimental data obtained for an MCFC stack developed in the Fabbricazioni Nucleari laboratories. These results are examined in order to improve the stack configuration. It is shown that power density may be increased of about 20% through double side feeding. In addition, the average temperature gradients in the axial direction are reduced of more than 70%. Significant reductions in the temperature gradients, especially in transversal direction, can be achieved by adjusting the mass flow rate of cathodic gas supplied to the various cell
Numerical Analysis of a medium scale latent energy storage unit for district heating systems
No full textThe present paper describes the application of computational fluid-dynamics (CFD) to the design and characterization of a medium scale energy storage unit for district heating systems. The shell-and-tube LHTES unit contains a technical grade paraffin (RT100) as phase change material (PCM) and uses water as heat transfer fluid (HTF). The system has been designed to transfer heat from the district to the building heating networks. After an initial description of the LHTES unit and a wide literature overview on the subject, the paper discusses the need for thermal enhancement to improve the thermal conductivity of the PCM. A solution based on a paraffin-graphite composite with a 15% graphite volume fraction has been found to be well performing in this particular application. Several operating scenarios characterized by heat requests ranging between 130 kWand 400 kWhave been explored and the main outputs presented as function of Re and St numbers. The timewise variations of other significant quantities such as liquid fraction, sensible and latent energy content, HFT outlet temperature and heat fluxes have been also presented and discussed. A final discussion on the possible system configurations shows that in comparison to traditional water storage systems for district heating, LHTES systems provide, depending on the chose alternative, higher energy storage densitie
Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement
No full textThe present paper describes the analysis of the melting process in a single vertical shell-and-tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid-dynamic (CFD) model that takes into account of the phase-change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase-change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase-change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano-enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano-enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detecte
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