New Sorption Cycles for Heat and/or Cold Production Adapted for Long Distance Heat Transmission

The analysis of the combinations of dipoles, linked by a gas transfer between an endothermic element and an exothermal element, through an exothermal physical-chemical process in thermal contact with an other endothermic process, is the basis of a new process for the transmission at long distance of heat/cold production. The yield of such cycles is identified through the values of the unused exergy, the untapped exergy and the exergy which is produced in excess by the process.

Numerical Simulation of Thermal Energy Storage in Cylindrical Cells

This paper presents the results of a -numerical transient model for phase change in a storage cell filled with a phase change material (PCM). Phase change occurs under the presence of natural convection. The PCM is encapsulated in a cylindrical energy storage cell. Two cases of PCM melting are analyzed, (1) the surface temperature of the bottom half of the cylindrical cell is kept at a constant temperature, which is higher than the melting temperature of the PCM, and (2) a fluid flows under the cell with an inlet temperature that is higher than the melting point of the PCM. The results show the evolution of the solid-liquid interface, isotherms and flow lines during the melting process.

The Helical Turbine and Its Applications for Hydropower Without Dams

The objective of this paper is to introduce an environmentally friendly Helical Turbine that has been developed to operate in free or ultra low-head water currents without dams. The turbine is a cross flow unidirectional rotation machine that makes it particularly valuable for ocean applications, such as reversible tidal streams in ocean bays, estuaries and canals, streams in open ocean, underwater currents generated by wave fluctuations etc.

A Simplified Thermodynamic Analysis of a LiBr-H2O Vertical Tube Absorber

One of the most important components of an absorption air-conditioning/heat pump system is the absorber, where the refrigerant water vapour is absorbed into the liquid solution. While absorption systems have been in use for several years, the complex transport phenomena occurring in the absorber are not fully elucidated yet. Thus, an attempt is made to model the absorption process of water vapour in aqueous solutions of lithium bromide considering a falling-film, vertical-tube absorber. The proposed analysis is based on the formulation of four differential equations describing the spatial variation (parallel to the tube-axis) of solution mass, temperature, mass fraction and coolant temperature. The system of ordinary differential equations is numerically solved using a non-stiff numerical method. Thermophysical properties and especially, heat and mass transfer coefficients are calculated using widely-accepted and reliable relationships, which are extracted from the literature using recently published information on wavy-laminar flows. In the present study, the questionable assumption of treating the water vapour as an ideal gas is heavily modified utilizing. Consequently, the hypothesis of saturated water vapour at the steam-solution interaction surface is revised by introducing an energy difference between the superheated steam and the liquid water within the binary solution. The last correction encouraged us to compare theoretical results for solution temperature, mass fraction and mass flow rate, which were obtained using both assumptions. It was proved that the initial treatment causes an underestimation of the absorbed steam mass and correspondingly, an underestimation of solution temperature and mass fraction at the mass exchange interface. An attempt is made also to identify the effect of mass transfer coefficient on the effectiveness of the absorption process and on the energy differences between the superheated steam and the liquid water either as pure substance or as component of the binary mixture. It was shown that the increase of mass transfer coefficient leads to an increase of steam mass transfer rate and to a corresponding decrease of solution temperature slope at the entrance of a tube. Correspondingly, the increase of mass transfer coefficient results in an increase of heat of absorption and heat of dilution at the same variation range of the solution mass fraction.

Gibbs Systems Dynamics: A Simple But Powerful Tool for Process Analysis, Design and Optimization

Dynamic process modeling by the mean of Equivalent Gibbs systems is described here. It allows to model a large number of processes and only requires standard engineering knowledge. This method is issued from thermodynamics of irreversible processes, initiated by I. Prigogine, but applied here to process engineering. First, an Equivalent Gibbs System (EGS) is defined for each component involved in the process. In such system, mass, energy and entropy are linked through Gibbs equation and entropy production can easily be expressed according to fluxes and their related forces. Assuming linear phenomenological laws, phenomenological coefficients can be calculated from common engineering correlations, or evaluated from technical data if available. As an example, a conventional vapor compression chiller is simulated. Three control modes are analyzed on an exergy basis: on/off control with constant or floating condensing pressure, PID control with variable compressor speed.

Impact on Overall Efficiency of Component Efficiency Increases for an Existing Thermal Electrical Generating Station

Most electrical generating utilities are striving to improve the efficiencies of their existing thermal electric generating stations, many of which are old. Exergy methods have been shown to provide meaningful insights that can assist in increasing the efficiency of conventional coal-to-electricity technologies. Here, exergy analysis is used to assess measures for improving the efficiencies of coal-fired electrical generating stations. This scope of the study is limited to minor practical improvements, which can be undertaken with limited effort and cost and are not overly complex. The plant considered is the coal-fired Nanticoke Generating Station (GS) in Ontario, Canada. The findings suggest that the results of exergy analyses should be used, along with other pertinent information, to guide efficiency improvement efforts for thermal generating stations. Also, efficiency improvement efforts should focus on plant components responsible for the largest exergy losses: the steam generator (where large losses occur from combustion heat transfer across large temperature differences), the turbines, the electrical generator and the transformer. Possible improvements in these areas should be assessed in conjunction with other criteria, and other components should be considered where economically beneficial improvements can be identified.

Effect of Boiler Feedwater Inlet Locations on the Water Circulation Characteristics in a Firetube Boiler With the Non-Symmetrically Arranged Tube Passes 3 and 4

Since the furnace section of a boiler is the primary heat transfer surface for the production of vapor, the overall water circulation patterns in the boiler will be significantly influenced by the circulation patterns near the furnace area. Boiler water circulation characteristics for a newly designed 4-pass firetube boiler with the non-symmetric arrangement of tube passes 3 and 4 were investigated in the previous work [1], in which the attraction forces between 28 different temperature nodes on the furnace wall were evaluated to predict the characteristics of water circulation near the boiler furnace. It was found that various non-symmetric water circulation patterns would occur for different firing conditions. As a consequence, in this paper, the analysis methods developed in the authors’ previous work are explicitly employed to predict and improve the water circulation in a firetube boiler when 6 different boiler feedwater inlet locations (3 on the right-hand side and 3 on the left-hand side of the boiler vessel shell) are used. Each side has 3 different feedwater inlet locations below the centerline of the boiler pressure vessel along its length. Investigation of the analysis results reveals that non-symmetric water circulation patterns are not unavoidable, but improvements in the water circulation and potentially the heat transfer rate can be achieved when the boiler feed water inlet is located near the front head of the boiler pressure vessel.

Using Experimental Data for Predicting Performance and Emissions of a Real Gas Turbine Plant

Precise performance evaluation at design and off-design operations is needed for a correct management of power plants. This need is particularly strong in gas turbine power plants which can quickly react to load variations and are very sensitive to ambient conditions. The paper aims at presenting a simple tool to determine the values of the thermodynamic quantities in each point of the plant and the overall plant performances of a real gas turbine plant. Starting from experimental data, a zero-dimensional model is developed which properly considers the effect of ambient conditions and water injection for pollutant abatement at different load settings under the action of the control system. An emission model taken from the literature is also included, after tuning on experimental data, to predict carbon monoxide and nitrogen oxide pollution.

Thermoeconomic Diagnosis: Zooming Strategy Applied to Highly Complex Energy Systems: Part 1 — Detection and Localization of Anomalies

This paper presents a summary of our most recent advances in Thermoeconomic Diagnosis, developed during the last three years [1–3], and how they can be integrated in a zooming strategy oriented towards the operational diagnosis of complex systems. In fact, this paper can be considered a continuation of the work presented at the International Conference ECOS’99 [4–6] in which the concepts of malfunction (intrinsic and induced) and dysfunction [7] were analyzed in detail. These concepts greatly facilitate and simplify the analysis, the understanding and the quantification of how the presence of an anomaly, or malfunction, affects the behavior of the other plant devices and of the whole system. However, what remains unresolved is the so-called inverse problem of diagnosing [3], i.e. given two states of the plant (actual and reference operating conditions), find the causes of deviation of the actual conditions with respect to the reference conditions. The present paper tackles this problem and describes significant advances in addressing how to locate the actual causes of malfunctions, based on the application of procedures for filtering induced effects that hide the real causes of degradation. In this paper a progressive zooming thermoeconomic diagnosis procedure, which allows one to concentrate the analysis in an ever more specific zone is described and applied to a combined cycle. In an accompanying paper (part 2 [8]) the accuracy of the diagnosis results is discussed, depending on choice of the thermoeconomic model.

Reliable Performance Evaluation of Single Cells Through an Effective Experimental Test Rig and Measure Chain

Fuel cells are known to be efficient and environmental friendly electricity generation devices. Great expectations are put on their contribution for future ultra-clean energy production. Nevertheless, the requests from deregulated energy market prompt fast commercialization of systems that are not yet fully optimized. Low efficiencies of first generation commercial fuel cell plants could result in failure when satisfying end users’ requirements thus creating an obstacle for subsequent market penetration. In this context, the availability of reliable data on fuel cells, necessary for their correct integration in full energy systems for plant optimization and feasibility assessment constitutes a priority. On the other hand, while measuring fuel cells performance is a difficult task nevertheless within reach for most research departments; the challenge for the scientific community is to reliably assess performance dependence on all the most relevant input parameters. As a result, most of the experimental data find on literature on fuel cells performances refer to voltage measures at increasing currents for fixed gas compositions and flow rates. In this work an experimental facility has been set up, test rigs have been designed and constructed both for fuel cells and reforming section testing; the main aim was to allow great operational flexibility. Great attention has been paid on test procedures and on input parameterisation as well on reliable advanced control systems. Dependence on the most relevant input parameters, i.e. current density, operating temperature, fuel and oxidant utilization factor, fuel humidification and dilution has been deeply analysed. Performances have been analysed both in terms of output voltage and efficiency and in terms of time degradation and expected total lifetime. The contribution of the work done consist in defining adimensional parameters which, thanks to their direct relation with the theoretical equations which govern a fuel cell, can greatly improve performance evaluation capability of experimental tests. Moreover those parameters can represent a way to standardize test procedures and constitute a means for comparing and exchanging results in a easier and effective way. A second contribution consist in designing and developing a unique control system that can improve test reliability thanks to the feature that allows to change single parameters while keeping the others constant and greatly enhance the number of experimental points that can be obtained in a test.