Entropy generation analysisfor the design optimizationof solid oxide fuel cells

Purpose - 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 low temperature district heating systems

In 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

District 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 approach for the analysis of control system of energy plants

In 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

Embodied exergy-based analysis of a municipal solid waste treatment system with uncertainty inclusion

The development of an integrated solid waste management (ISWM) system is still a matter of interest in many countries. One of the main challenges is to efficiently allocate the material streams in order to save energy and recover materials. The aim of this paper is to use the embodied exergy criteria to evaluate the distribution of the material streams into a solid waste (SW) treatment system composed by: a mechanical biological treatment (MBT) plant for refuse derived fuel (RDF) production and a paper recycling plant for cardboard production. Two scenarios are compared, based on the inlet mass flow to the MBT plant and the cardboard production. Stochastic tools based on Monte Carlo simulation are adopted for generating simulation scenarios, in order to account for the uncertainty that occurs in external (e.g., waste composition) and internal (e.g., equipment energy consumption) parameter

Compact physical model for simulation of thermal networks

Optimal design and management of DH networks require numerical models for simulating the physical behavior of the network in various operating conditions. DH network models usually rely on the physical description of the fluid-dynamic and thermal behaviours. The use of physical models can represent a limitation in various cases: a) when extended networks are considered (several thousands of nodes); b) when multiple simulations are required in real-time; c) when multi-energy networks are optimized. In these cases, compact models are preferable

Optimization of the Thermal Load Profile in District Heating Networks through "Virtual Storage" at Building Level

Abstract Thermal storage is of extreme importance in modern district heating networks in order to increase the share of waste heat and heat produced through renewable sources and cogeneration. Nevertheless, installation of large storage volumes is not always feasible, especially in dense urban areas. A possible option consists in virtual storage, which is obtained through variation of the thermal request profiles of some of the connected buildings with the goal of producing an effect similar to that obtained using storage. To perform such approach there are three crucial elements: 1) an advanced ICT solution able provide real time information about the thermal request of the buildings and the thermodynamic conditions at the thermal substations; 2) a detailed thermo fluid-dynamic model of the district heating network able to simulate the temperature evolution along the network as the function of time; 3) a compact model of the buildings in the district able to check the acceptability of the internal temperatures following the modified strategies. The model produces changes in the start-up time of the buildings connected with the network as well as possible pauses during the day. These changes in the request profiles usually involve a slightly larger heat load. Nevertheless, peak shaving is accompanied by a reduction in heat generation of boilers and an increase in the thermal production of efficient systems, such as cogeneration units. This results in a significant reduction in the primary energy consumption. An application to the Turin district heating network, which is the largest network in Italy, is presented. In particular, a subnetwork connecting the main transport network to about 100 buildings located in the central area of the town is considered. The analysis if performed in selected days where the optimization was conducted the day before on the basis of weather forecasts and then applied to the network. Despite the changes in the request profiles could be applied only to a limited number of buildings, the analysis show that the peak request can be reduced. Simulations performed considering the application of changes to a larger number of buildings show that reduction in the primary energy consumptions of the order of 5% can be obtained

Entropy generation analysis of wildfire propagation

Entropy generation is commonly applied to describe the evolution of irreversible processes, such as heat transfer and turbulence. These are both dominating phenomena in fire propagation. In this paper, entropy generation analysis is applied to a grassland fire event, with the aim of finding possible links between entropy generation and propagation directions. The ultimate goal of such analysis consists in helping one to overcome possible limitations of the models usually applied to the prediction of wildfire propagation. These models are based on the application of the superimposition of the effects due to wind and slope, which has proven to fail in various cases. The analysis presented here shows that entropy generation allows a detailed analysis of the landscape propagation of a fire and can be thus applied to its quantitative descriptio

Automatic fouling detection in district heating substations: Methodology and tests

Abstract Diagnosis of anomalies in heat exchangers of district heating substations is an essential point to assure high comfort level in buildings, as well as to exploit energy sources efficiently. The aim of this paper is to propose a methodology for automatically detecting fouling in the heat exchangers located in the substations of a district heating system. The methodology is tailored for large district heating networks, where a large number of buildings should be examined with reasonable availability of data. Fouling is analysed using only the data collected by the meters installed in the substations: the mass flow rate on the primary side and the temperatures on both sides of the heat exchanger. Evaluation is difficult due to the rawness of the data gathered and the variable operating conditions, which are adjusted on the basis of the external temperatures and set-points. The software created to implement the proposed methodology receives rough data as the input and it is able to manage data gap and lack of data. Furthermore, it provides a graphical output, which can be used for assisting the operators who manage the network and plan the cleaning schedules. The software has been tested considering space heating substations in six distribution networks of the Turin district heating system, for a total amount of 325 heat exchangers. A regular application of the approach and the cleaning of the heat exchangers presenting fouling is expected to lead to an average annual decrease of about 1.6% of the primary energy consumption in the entire network