Exergy Balance Research Articles

Exergy performance evaluation of a basin-type double-slope solar still equipped with phase-change material and PV/T collector

This study investigated the exergy performance of a basin-type double-slope solar still equipped with phase-change material (PCM) and PV/T collector. PV/T collectors improve fresh water production during the sunshine hours of the day by pre-heating the saline water, and utilization of PCM makes it possible to produce fresh water at night. The governing equations were obtained by writing energy and exergy balances for solar still components. The numerical results in this study were well fitted into the experimental data. System performance was investigated with respect to fresh water production and exergy efficiency for sample winter and summer days in Zahedan, Iran. Results showed that the fresh water productivity increased by 10.6%, and the exergy efficiency improved by 27% and 2% on 6 July and 23 December, respectively, when mass flow rate increased from 0.001 to 0.01 kg/s. By increasing the saline water mass from 20 to 30 kg, the fresh water production was reduced by about 4.8% during the day, but increased by about 7.43% during the night. Exergy efficiency was 45% lower on the winter day than on the summer day. The highest electric power generation values on 6 July and 23 December were 120 and 50 W, respectively.

Experimental study on heat and exergy balance of a dual-fuel combined injection engine with hydrogen and gasoline

In this paper, a dual-fuel engine test rig with gasoline injected in the intake port and gasoline (or hydrogen) injected directly into the cylinder is built up; therefore, two injection models are realized. One is port fuel injection + gasoline direct injection (PFI + GDI), the other is port fuel injection + hydrogen direct injection (PFI + HDI). And the effects of two injection models on heat and exergy balance are investigated experimentally. The results show that, from the perspective of the first law of thermodynamics (heat balance), no matter what the injection mode is, the heat proportion of cooling water is the largest, the exhaust heat ratio and brake power are the second, which two are roughly equivalent, and the uncounted loss is the least. In PFI + GDI mode, the local mixture is too dense due to the increase of mixing ratio, which leads to insufficient combustion and a slight decrease of brake power ratio. However, due to the special characteristics of hydrogen, the increase of direct injection ratio improves the brake power ratio in PFI + HDI mode. Moreover, because of the short quenching distance of hydrogen, the cooling loss rises up with the increase of hydrogen ratio. The engine speed and load also have great impacts on heat distribution, but on account of the different physical and chemical properties between gasoline and hydrogen, resulting in varying degrees of impact and trends. On the basis of the second law of thermodynamics (exergy balance), it is found that no matter what injection mode is, the ratio of exergy destruction is always the highest, accounting for half of the total fuel energy, and the exhaust exergy ratio is lower than the brake power ratio. However, the proportion of exergy contained in cooling water is the smallest, which is quite different from the result of the first law of thermodynamics. The influences of several factors on engine energy balance are analyzed, and the differences and similarities between heat balance and exergy balance are compared. The two analytical methods are interrelated and complementary, and the purpose is to find a reasonable and comprehensive energy balance analysis method for internal combustion engine.

Multiobjective Optimization of a Plate Heat Exchanger in a Waste Heat Recovery Organic Rankine Cycle System for Natural Gas Engines.

A multiobjective optimization of an organic Rankine cycle (ORC) evaporator, operating with toluene as the working fluid, is presented in this paper for waste heat recovery (WHR) from the exhaust gases of a 2 MW Jenbacher JMS 612 GS-N.L. gas internal combustion engine. Indirect evaporation between the exhaust gas and the organic fluid in the parallel plate heat exchanger (ITC2) implied irreversible heat transfer and high investment costs, which were considered as objective functions to be minimized. Energy and exergy balances were applied to the system components, in addition to the phenomenological equations in the ITC2, to calculate global energy indicators, such as the thermal efficiency of the configuration, the heat recovery efficiency, the overall energy conversion efficiency, the absolute increase of engine thermal efficiency, and the reduction of the break-specific fuel consumption of the system, of the system integrated with the gas engine. The results allowed calculation of the plate spacing, plate height, plate width, and chevron angle that minimized the investment cost and entropy generation of the equipment, reaching 22.04 m2 in the heat transfer area, 693.87 kW in the energy transfer by heat recovery from the exhaust gas, and 41.6% in the overall thermal efficiency of the ORC as a bottoming cycle for the engine. This type of result contributes to the inclusion of this technology in the industrial sector as a consequence of the improvement in thermal efficiency and economic viability.

Sustainable production of hydrocarbon fields guided by full-cycle exergy analysis

To mitigate the negative impacts of hydrocarbon fuels on climate change complementary decision tools should be considered when selecting or evaluating the performance of a certain production scheme. The exergy analysis can give valuable information on the management of the oil and gas reservoirs. It can also be used to calculate the CO2 footprint of the different oil recovery mechanisms. We contend that the concept of exergy recovery factor can be used as a powerful sustainability indicator in the production of the hydrocarbon fields. The exergy-zero recovery factor is determined by considering exergy balance of full cycle of hydrocarbon-production systems and defines boundaries beyond which production of hydrocarbons is no longer sustainable. An example of the exergy analysis is presented in the paper.

Energy and Exergy Analyses of Angra 2 Nuclear Power Plant

Nuclear Power Plants (NPPs) based on Pressurized Water Reactors (PWRs) technology are considered an alternative to fossil fuels plants due to their reliability with low operational cost and low CO2 emissions. An example of PWR plant is Angra 2 built in Brazil. This NPP has a nominal electric power output of 1300 MW and made it possible for the country save its water resources during electricity generation from hydraulic plants, and improved Brazilian knowledge and technology in nuclear research area. Despite all these benefits, PWR plants generally have a relatively low thermal efficiency combined with a large amount of irreversibility generation or exergy destruction in their components, reducing their capacity to produce work. Because of that, it is important to assess such systems to understand how each component impacts on system efficiency. Based on that, the aim of this work is to evaluate Angra 2 by performing energy and exergy analyses to quantify the thermodynamic performance of this PWR plant and its components. The methodology consists in the development of a mathematical model in EES (Engineering Equation Solver) software based on thermodynamic states in addition to energy and exergy balance equations. According to the results, Angra 2 has energy efficiency of 36.18% and exergy efficiency of 49.24%. Reactor core is the most inefficient device in the NPP; it has exergy efficiency of 67.16% and is responsible for 63.88% of all exergy destroyed in Angra 2.

Evaluaation the energy potential of compressed air for determining efficiencty of air-accumulating electric

The thermodynamics analysis of processes flowing into the storages of compressed air energy are considered in the paper. The exergy balance has been propose as the instrument for study. The main indices have been estimate: the compressed air potential, efficiencies for all elements of the scheme and air-accumulating electric station (AAES) as a whole. The study procedure demonstrated on the example of adiabatic AAES. The relation of final temperatures, specific exergy and compression work per 1000 m3 of atmospheric air to compression ratio are determined. On the base, it can concluded that adiabatic AAES has not high efficiency, but it has some advantages: simple scheme, low investment, and combined well with renewable energy sources, for instance, wind-driven plants. To raise the efficiency of an adiabatic AAES, it should be transform to diabatic type. For that it is required modify the scheme and work regime of its elements, and combined it in complex with gas turbine plants.

Exergy analysis of a 1.2 MW waste heat power generation project

AbstractAccording to systematic features, analysis method based on exergy balance is established. Basic indicators in the system, the subsystem, and facilities are put forward in this paper. By using this method to analyze the generation system of megawatt‐scale in one chemical enterprise, it is found that the objective exergy efficiency of the system is 35.67%, and exergy loss of organic Rankine cycle (ORC) is the highest. The thermal efficiency of the total system is 9.61%. For the condenser, the thermal efficiency is 91.18%, and the exergy efficiency is only 23.44%. The objective exergy efficiency of the evaporator is 74.04%. The influence coefficient of exergy loss of condenser is higher than that of pump and expander, but input exergy of the condenser is lower than that of the expander. It is revealed that ORC subsystem is the part which needs to be focused on, and the condenser is the most important component of ORC subsystem which should be optimized firstly.

Comparison of single and double stage regenerative organic rankine cycle for medium grade heat source through energy and exergy estimation

Regenerative organic Rankine cycle (RORC) can be used to improve organic Rankine cycle (ORC) performance. This paper presents a comparison of a single (SSRORC) and double stage regenerative organic Rankine cycle (DSRORC) using a medium grade heat source. Performance for each system is estimated using the law of thermodynamics I and II through energy and exergy balance. Solar thermal is used as the heat source using therminol 55 as a working fluid, and R141b is used as the organic working fluid. The initial data for the analysis are heat source with 200°C of temperature, and 100 L/min of volume flow rate. Analysis begins by calculating energy input to determine organic working fluid mass flow rate, and continued by calculating energy loss, turbine power and pump power consumption to determine net power output and thermal efficiency. Exergy analysis begins by calculating exergy input to determine exergy efficiency. Exergy loss, exergy destruction at the turbine, pump and feed heater is calculated to complete the calculation. Energy estimation result shows that DSRORC determines better net power output and thermal efficiency for 7.9% than SSRORC, as well as exergy estimation, DSRORC determines higher exergy efficiency for 7.69%. ©2019. CBIORE-IJRED. All rights reserved

Exergic analysis of the installation for controlled dehydration of fine particulate products

An installation is designed for dehydration of fine-particulate bulk products in the active hydrodynamic mode. A thermal system for heat-mass transfer during rapeseed dehydration in the active hydrodynamic mode is assessed for thermodynamic efficiency. Diagrams of thermal and exergy balance for heat-mass transfer processes are presented and heat exchange and exergy analysis are performed. Exergy loss by rapeseed dehumidification are calculated The research results show that the greatest loss of exergy occurs due to irreversibility of dehydration processes during the phase transformation, as well as due to the pressure drop in the device.

Unsteady-state exergy analysis for heat conduction of homogeneous solids under periodic boundary conditions

Thermal exergy analysis is typically performed under a steady-state assumption. This assumption is reasonable when the temperature difference between the environment and the system of interest is very large; however, when the temperature difference is small, the exergetic behavior of the system becomes highly sensitive to the environmental temperature. Additionally, when the system has a large thermal capacity, the storage effect cannot be ignored; as such, the steady-state assumption is not suitable. Under such conditions, unsteady-state exergy analysis should be performed to improve understanding of the system behavior. In this study, we conducted numerical unsteady-state exergy analyses for heat conduction based on the complete forms of energy, entropy, and exergy equations. The system of interest was a one-dimensional homogeneous solid with a thermal diffusivity of 0.5 × 10−6 J/(m3∙K). We analyzed two cases by assigning two different time-varying temperature boundary conditions using a 24 h periodic sinusoidal function with a constant amplitude of 10 °C but different midlines of 20 °C and 10 °C. This study was focused on the complex exergetic behavior inside the solid based on the concept of exergy balance, how the exergy flows in and out of a subsystem, and how it is stored and consumed. For intuitive interpretation of the unsteady-state results, a zonal classification method was proposed. This method was used to interpret the state of exergy flow (warm/cool) and its direction (inflow/outflow) that varies depending on the temperature relationships. Additionally, the zonal classification method was utilized to interpret the state (warm/cool) and temporal increase/decrease of the exergy storage rate, which uniquely appears in unsteady-state analyses. The application of the developed method to various transient thermal problems will lead to more complete understanding of and greater insight into various thermodynamic phenomena.

Exergy return on exergy investment analysis of natural-polymer (Guar-Arabic gum) enhanced oil recovery process

It has been estimated that 17% of the recovered hydrocarbon exergy in oil fields [1] is spent on fluid handling and recovery costs. Therefore, improving the efficiency of oil production can give an some contribution to more efficient energy usage and therefore minimizing to some extent the carbon footprint. By way of example we present in this paper a work-flow, which can serve as a template for computing the fluid handling and recovery costs for natural polymer (Guar-Arabic Gum) flooding. The main contributors to the exergy investment in an Exergy Return on Exergy Investment analysis (ERoEI) are, the fluid circulation costs, the steel costs of the tubing and casing and to some degree the drilling costs. The main contributor to the exergy gain is the exergy of the produced oil. The fluid circulation costs represent the largest exergy investment and usually approximately accounts for 80% of the exergy used for the recovery of oil. For quantifying the circulation costs, the paper uses a 1-D displacement model of polymer flooding of oil to compare the enhanced oil recovery (EOR) history for three scenarios, i.e., (1) water injection, (2) natural-polymer water injection and (3) natural-polymer slug injection. The advantage of a 1-D model is that it allows multiple comparisons of many scenario's avoiding time consuming simulations but this goes at the expense of ignoring 3-D effects. The 1-D model can be extended to a 2-D or 3-D model, which makes it possible to include the improvement of vertical and areal sweep-efficiency. A numerical solution of the EOR model is obtained with COMSOL. We analyze the exergy balance of viscosified water, e.g., with natural-polymer. A comparison as to the displacement efficiency is made between the three scenarios, viz., water, Guar-Arabic gum, and slug injection. The viscosity behavior of Guar-Arabic gum is obtained from laboratory data. It is argued that an ERoEI analysis, which is used on its own or complementary to an economic analysis, can be used to show the advantage of using Guar-Arabic gum slugs with respect to permanent polymer-injection to enhance the oil recovery. Moreover, the analysis shows that at the end of the project, the concept of exergy-zero recovery time or zero-time marks, for each scenario the termination point, i.e., when the circulation exergy costs (exergy investment) become equal to the recovery exergy (exergy return), and thus recovery should be abandoned. For the conditions considered a single polymer injection displacement leads to optimal results.

Energy and exergy analysis of cold and power production from the geothermal reservoir of Torre Alfina

Geothermal power plants can provide clean and renewable energy and can be proposed as integrated units for simultaneous production of cooling and power. This paper presents a cascade arrangement of an organic Rankine cycle (ORC) and a water/lithium bromide (LiBr) absorption chiller (ABS). Starting from a literature reference layout which is taken as benchmark, some improvements are proposed at system level.To assess the performance of the system, a thermodynamic model is developed in EES and the energy and exergy balance is calculated. The proposed system is re-evaluated with reference to resource conditions corresponding to a planned power plant in central Italy, Torre Alfina (TA). A sensitivity analysis is performed in order to investigate the operating range of the plant and the possibility of adapting its design to the requirements of the customers. Under optimized conditions, the TA Case (targeted on a 5 MW power output) showed an energy utilization factor (EUF) of 46.2% and an exergy efficiency of 27.7%, neglecting the brine reinjection loss. The highest exergy destructions occur in the ORC economizer (8.6%), in the ABS generator (6.3%) and absorber (5.5%). The good resource conditions in TA case drive the design optimization to production of power rather than cold.

Exergy analysis and performance evaluation of a newly developed integrated energy system for quenchable generation

This paper presents an innovative use of waste heat recovered from a cement slag for multigeneration purposes, including power, ammonia, heat, hot water and oxygen production. A novel approach of ammonia production is employed in this study. The proposed system consists of four major subsystems; copper-chlorine (Cu-Cl) cycle, cryogenic Air Separation Unit (ASU), ammonia synthesis reactor and two steam Rankine cycles. The Aspen plus 9.0 version is employed for modeling and simulation of the proposed system. A comparative study is also conducted considering the different CuCl cycle based integrated systems for multigenerational purpose with numerous energy sources. The parametric studies are carried out by varying parameters, namely flow rate, compressor and turbine discharge pressures and ammonia reactor conversion rate. The exergy analysis is comprehensively conducted for each of the system components through exergy balance equations and exergy efficiencies. The overall exergy efficiency of the designed system is found to be 36.1%. Further results are also presented and discussed.

Biopower and biofertilizer production from organic municipal solid waste: An exergoenvironmental analysis

In this study, the environmental performance of a genset-coupled anaerobic digestion plant is analyzed at component-level using an exergoenvironmental method. The plant digests organic municipal solid waste (MSW) while producing two main products, i.e., biopower and biofertilizer. A comprehensive exergoenvironmental modeling of the plant is conducted using actual operating data in order to highlight the main units consuming exergy and causing environmental burdens. The exergoenvironmental indicators of all units of the system are computed by integrating exergy and environmental impact balances. The unitary exegetic environmental impact of biopower and biofertilizer are determined at 11.10 and 0.36 mPts/GJ, respectively. This means that the biofertilizer generation causes less environmental burden over the biopower due to the ease of its production. The highest total environmental impact rate (37.05 mPts/h) is caused by the genset followed far behind by the digester (8.56 mPts/h). Although the genset has the highest operation-related environmental impact rate (36.97 mPts/h), the highest component-related environmental impact rate (7.87 mPts/h) is associated with the digester. Therefore, the exergoenvironmental performance of the plant can be boosted by minimizing the rate of exergy dissipation of the genset while mitigating the environmental impacts related to the development and construction of the digester.

Ексергетичні втрати в повітронагрівачі теплоутилізаційної системи котельної установки

Однією з причин зниження ефективності теплоутилізаційних систем та їх окремих елементів є втрати ексергетичної потужності. Такі втрати пов'язані з гідродинамічним опором при русі теплоносіїв, з незворотними процесами при теплообміні між теплоносіями, з процесами теплопровідності. Зниження втрат ексергетичної потужності дає змогу підвищити ефективність теплоутилізаційних систем. Це визначає актуальність робіт, присвячених вирішенню зазначеної проблеми. Для розрахунку втрат ексергетичної потужності в теплоутилізаційних системах та їх окремих елементах розроблено комплексну методику, яка поєднує ексергетичні методи з методами, побудованими на розрахунку дисипаторів ексергії. Розроблена методика дає змогу розділити втрати ексергетичної потужності згідно з причинами та зонами їх локалізації і виявити умови, за яких ці втрати будуть мінімальними. Основні етапи методики включають розробку математичної моделі досліджуваних процесів на основі рівняння ексергії, рівнянь балансів ентропії і ексергії, рівняння нерозривності, рівняння для внутрішньої енергії. У межах розробленої математичної моделі отримано диференціальні рівняння ентропії та ексергії і формули для розрахунку дисипаторів ексергії, що характеризують гідродинамічні втрати і втрати ексергетичної потужності внаслідок нерівноважного теплообміну між теплоносіями. Визначено значення дисипаторів ексергії для пластинчастого повітронагрівача теплоутилізаційної системи котельної установки за різних режимів роботи котла. Встановлено внесок кожного виду втрат у сумарні втрати ексергетичної потужності у повітронагрівачі і визначено область максимальних втрат цієї потужності.

An analysis method for exergy efficiency of wind turbines based on experiment and simulation

ABSTRACTThis paper discusses the result of the exergy efficiency analysis of direct-drive wind turbines by obtaining the exergy loss and destruction characteristics as well as the law of influence of every original factor on the dynamic change in the exergy efficiency. After proposing an exergy balance equation for wind power systems, a mathematical model of exergy analysis is constructed. Based on this model, the exergy utilization rate was improved by considering unavoidable exergy destruction. At first, the exergy loss characteristics were obtained using particle image velocimetry technology and the exergy destruction characteristics were obtained using simulation of finite volume method. Then, by combining the result of experiment and simulation, it was found that the exergy loss rate reached the maximum value at a tip speed ratio of 4.5–5.0. The exergy destruction rate was weakened between 10 and 15 degrees. The maximum value of exergy efficiency rate was 0.38, corresponding to exergy loss and destruction rate of 0.58 and 0.06. Finally, five factors were analyzed to determine the exergy efficiency influence weights. These were as follows: slot type = 0.30813, air gap length = 0.28471, yaw angle = 0.13637, the tip speed ratio = 0.04116, and matching characteristic = 0.25250.

Exergoeconomic analysis and optimization of a novel hybrid cogeneration system: High-temperature proton exchange membrane fuel cell/Kalina cycle, driven by solar energy

Abstract Designing energy conversion systems with high efficiencies and low pollutant emission is essential for sustainable development. A new cogeneration system including a high-temperature proton exchange membrane fuel cell, integrated with a solar methanol steam reformer, and a Kalina cycle is proposed to produce electricity and heat. Using energy, exergy and cost balance, the proposed system is analyzed from the viewpoints of exergy, economy, and environmental impact. A Parametric study is performed and shows that a higher fuel cell temperature is in favor of the total product unit cost and carbon dioxide mass specific emission. Also, the exergy efficiency is maximized, and the total product unit cost as well as the carbon dioxide mass specific emission are minimized at some specific values of anode recirculation ratio. Optimization results show that the average daily exergy efficiency can increase by up to 29.3% and the total product unit cost as well as the carbon dioxide mass specific emission can decrease by up to 17.72% and 16.3%, respectively compared to the corresponding values under the base conditions. It is concluded that combining a Kalina cycle with a high-temperature proton exchange membrane fuel cell along with utilizing solar energy for reforming process yields an efficient energy conversion system with low emission.

Biomass Gasification Integrated with Chemical Looping System for Hydrogen and Power. Coproduction Process – Thermodynamic and Techno‐Economic Assessment

AbstractThree biomass gasification‐based hydrogen and power coproduction processes are modeled with Aspen Plus. Case 1 is the conventional biomass gasification coupled with a shift reactor, cases 2 and 3 involve integration of biomass gasification with iron‐based and calcium‐based chemical looping systems. The effects of important process parameters on the performance indicators such as hydrogen yield and efficiencies are evaluated by sensitivity analyses. These parameters include gasification temperature, molar ratios of steam to biomass in the gasifier, Fe2O3 to syngas in the fuel reactor, Fe/FeO to steam in the steam reactor, CaO to CO, and steam to CO in the carbonator. The energy and exergy balance distributions for the above three cases are comprehensively discussed and compared. Furthermore, techno‐economic assessments are performed to evaluate the three cases in terms of capital cost, operating cost, and leveled cost of energy.

Two objective optimization for a new molten carbonate fuel cell based power producing system

A new molten carbonate fuel cell based combined cycle is proposed and analysed in detail from the viewpoints of exergoeconomics. The system utilizes methane as a fuel and makes use of liquid natural gas cooling effect as a heat sink. The system performance is simulated using the conservations of mass and energy as well as the exergy and cost balances. By considering different organic fluids for each stage of the cascade organic Rankine cycle, it is observed that the best performance is achieved with using toluene and n-pentane for the top and bottom organic Rankine cycles, respectively. It is observed that the highest and second highest exergy destructions occur in the after burner and fuel cell, respectively. Parametric studies are performed to specify the decision parameters prior to optimization process. Performance optimization is accomplished in order to minimize the sum of product cost (SUPC), case A; and to maximize the exergy efficiency, case C. The results show that the highest exergy efficiency is obtained as 67.96% and the lowest produced power unit cost is calculated as $32.09/GJ. Performing a multi objective optimization for the system resulted in an exergy efficiency of 66.56% and a SUPC of $33.57/GJ, case B.

Evaluation of Large-Scale Production of Chitosan Microbeads Modified with Nanoparticles Based on Exergy Analysis

Novel technologies for bio-adsorbent production are being evaluated on the lab-scale in order to find the most adequate processing alternative under technical parameters. However, the poor energy efficiency of promising technologies can be a drawback for large-scale production of these bio-adsorbents. In this work, exergy analysis was used as a computer-aided tool to evaluate from the energy point of view, the behavior of three bio-adsorbent production topologies at large scale for obtaining chitosan microbeads modified with magnetic and photocatalytic nanoparticles. The routes were modeled using an industrial process simulation software, based on experimental results and information reported in literature. Mass, energy and exergy balances were performed for each alternative, physical and chemical exergies of streams and chemical species were calculated according to the thermodynamic properties of biomass components and operating conditions of stages. Exergy efficiencies, total process irreversibilities, energy consumption, and exergy destruction were calculated for all routes. Route 2 presents the highest process irreversibilities and route 3 has the highest exergy of utilities. Exergy efficiencies were similar for all simulated cases, which did not allow to choose the best alternative under energy viewpoint. Exergy sinks for each topology were detected. As values of exergy efficiency were under 3%, it was shown that there are process improvement opportunities in product drying stages and washing water recovery for the three routes.