Exergy Research Articles

Effects of combustion products composition models on the exergetic analysis of spark-ignition engine fuelled with ethanol

Effects of combustion products composition models on the exergetic analysis of spark-ignition engine fuelled with ethanol

Exergy Profit Evaluation of Municipal Solid Waste Processing

This article evaluates the processing of Municipal Solid Waste (MSW) and its contribution to sustainability from the viewpoint of its net exergy balance. The evaluation follows the recent framework for estimation of Exergy Footprint and Exergy Profit for a given Process System. The method evaluates the Exergy Assets and Liabilities, estimating the Exergy Profit as the balance of these two. The concept of avoided exergy by substituting the useful chemical to its primary production, similar to the concept of avoided emission is also proposed in this work. The obtained estimate is the specific contribution of the considered MSW processing system. For estimating the overall impacts, the obtained Exergy Profit has to be added to the contributions of the potential upstream and downstream processes. The results show the potential value that can be obtained and sustainability contribution to be achieved by the extraction of useful chemicals from waste and waste-to-energy conversion. The chemical extraction option shows a marginally higher Exergy Profit compared with the typical MSW incineration practice, indicating that, as long as there is a sufficient market demand for the chemical products, this route can be more beneficial. An important outcome is that the product substitution can lead to three-fold increase in the Exergy Profit and the related emission savings.

Exergy Analysis of a Natural Gas-Fired Gas Turbine Combined Cycle Power Plant with Post-Combustion Carbon Capture

Post-combustion carbon capture (PCC) plays an important role in reducing the greenhouse gas emissions. In the present study, an exergy analysis is conducted to assess the exergy destruction and exergetic efficiency of a natural gas-fired combined cycle gas turbine (CCGT) system coupled with the PCC unit. The overall exergetic performance of the system is compared against the baseline CCGT by using realistic data. The working temperature and composition of the exhaust flue gas are two critical parameters that have significant impact on the performance of the absorption liquid and equipment operation. Results show that the highest exergy destruction occurs in the combustion chamber and condenser are 15.69 MW and 11.78 MW, occupying more than 45 % of the exergy destruction of the overall system for the conventional CCGT system. For the CCGT system with PCC unit, the exergy destruction of absorber is relatively high with an exergetic efficiency of 56.18 %. The highest exergetic efficiency is found in the units of combustion chamber and heat recovery steam generator (HRSG), which are 90.23 % and 87.01 %, while the condenser has the lowest efficiency. Identification of the low efficiency component presents an opportunity for improvement of the system.

EXERGY AND ENERGY ANALYSES OF CORN DRYING PROCESS: AN EXPERIMENTAL STUDY IN A LABORATORY-SCALE DRYER

EXERGY AND ENERGY ANALYSES OF CORN DRYING PROCESS: AN EXPERIMENTAL STUDY IN A LABORATORY-SCALE DRYER

A New Approach for Exergoeconomics Evaluation by Considering Uncertainty with Monte Carlo Method

The exergoeconomics analysis combines thermodynamic assessments based on exergy analysis with economic concepts. this article suggests a new method for exergoeconomics analysis and evaluation of energy systems by considering uncertainty in economic parameters. As the first step, the future values of economic parameters that influence the operating cost of the energy system are forecasted by the Monte Carlo Method. Then, as a novel approach, principles of exergoeconomics analysis method are coupled with the Monte Carlo Method for exergoeconomics evaluation of energy systems. Also, three new parameters, i.e. Risk Factor (RF), Risk Factor Sensitivity (RFS), and Product Cost Sensitivity (PCS), are proposed. Two different approaches are considered in the evaluation process to improve the system: a) decreasing the total cost of products and b) reducing the risk of the cost of products. Also, the proposed method is applied to the CGAM system as a benchmark. Eventually, the results of the first and second approaches show that the total cost of products can be reduced 4.1% (from 22.270 $/GJ to 21.358 $/GJ) and also the risk of the cost of the products can be reduced 5.8% (from 25.8% to 24.3%).

EXERGY ANALYSIS OF A CO2 BOIL-OFF GAS RE-LIQUEFACTION SYSTEM

EXERGY ANALYSIS OF A CO2 BOIL-OFF GAS RE-LIQUEFACTION SYSTEM

Study on Energy Saving of Epichlorohydrin Unit Based on Entransy Theory

The mathematical models for calculating the energy utilization efficiency of heat exchanger networks (HENs) based on entransy analysis method was built. The energy utilization efficiency of HENs for epichlorohydrin unit were calculated by exergy analysis and entransy analysis. Aiming at the maximum energy recovery, the entransy analysis of epichlorohydrin unit’s HENs was done. By taking three different ?Tmin, 10 K, 15 K and 20 K, the results show that the exergy efficiencies are 61.01 %, 59.28 % and 57.27 %. The entransy transfer efficiency is 42.81 %, 42.13 % and 41.00 %. Using the entransy analysis method, utilities were saved by 19.41 %, 18.01 % and 15.70 %. Using the exergy analysis method, utilities were saved by 16.59 %, 14.86 % and 12.02 %.The results of different methods were analysed and compared. Exergy is more complicated than entransy in the calculation process. The exergy analysis method can not complete HENs Synthesis. But the entransy analysis method not only can complete HENs synthesis, but also can calculate the energy utilization efficiency of HENs. The entransy analysis method is more suitable for analyzing the energy utilization of epichlorohydrin unit.

Optimisation and Analysis Based on Bi-Level Reduced-Order Model for Coupled Heat and Mass Transfer Processes Under Uncertainty

This work proposes an integrated framework to build fast and high-fidelity bi-level reduced-order models (ROMs) for heat and mass transfer processes systems. In the proposed framework, the bi-level ROMs including the ROM for output variables (yROM) and the ROM for state variables (zROM) corresponding to its input variables of the systems are developed by integrating principal component analysis (PCA) with artificial neural networks (ANN), which can closely approximate the results of computational fluid dynamics (CFD). In particular, the CPU time is significantly reduced with the fast bi-level ROMs. A case study is used to demonstrate the benefits of using our proposed bi-level ROMs methodology for simulation and optimisation for heat and mass transfer processes systems, where a bi-level ROMs for the closed wet cooling towers (CWCTs) is built with the help of exergy analysis. The design points served for CFD simulations are randomly sampled by stochastic reduced-order modelling (SROM). The developed yROM is used for stochastic optimisation of exergy efficiency ratio and then the corresponding optimal exergy flux fields are then reconstructed and analysed based on zROM.

Performance Comparison of Absorption Heat Pump and Refrigeration in Low Temperature Waste Heat Recovery

Absorption heat pump and absorption refrigeration cycle are widely used in industries as two of the most important low-temperature waste heat recovery technologies. In order to compare their energy and economic performances, this paper establishes the models of one effect LiBr –H2O absorption heat pump and one effect LiBr -H2O absorption refrigeration cycle in the process simulation software Aspen PLUS. The two systems are simulated for waste heat source under temperature from 95 (C to 125 (C saturated water to output 130 (C saturated steam (absorption heat pump), or 7 and 10 °C cold water (absorption refrigeration). The energy and economic performances of the two technologies are compared and analyzed through three parameters, coefficient of performance, exergy efficiency, and operation cost per unit exergy output. The results show that the performance of heat pump is better than that of refrigeration cycle under each of these evaluation indexes. The work in this paper aims to provide some theoretical support and technology selection suggestion for low temperature waste heat recovery.

Multi-component Fluid Cycles in Liquid Air Energy Storage

With the penetration of renewable energy sources into the energy market, an emerging problem is the energy stability during energy supply and delivery. Energy storage technologies have the ability to overcome the intermittent nature of energy sources (such as wind and solar energy) and respond to unexpected situations like power failure. Liquid air energy storage (LAES) achieves good round-trip efficiencies by using hot and cold energy storages for heat transfer between the charging and discharging operations. In this work, the LAES technology has been studied and multi-component fluid cycles (MCFCs) are considered for the first time to replace the previously suggested single-component fluids in the cold thermal energy storage cycles to increase the performance of the cold cycles. Two case studies related to single MCFC and dual MCFC are simulated, optimized and compared. The optimal composition for the multi-component fluids is determined by using a particle swarm optimization (PSO) method. The objective function is to maximize the round-trip efficiency of the process. The results indicate that the LAES process with dual MCFC is superior to the case with single MCFC in terms of liquid yield (95.1 % vs. 87.1 %), exergy efficiency (86.1 % vs. 82.4 % for charging and 85.5 % vs. 78.5 % for discharging) and round-trip efficiency (61.8 % vs. 53.9 %).

Organisms live at far-from-equilibrium with their surroundings while maintaining homeostasis, importing exergy and exporting entropy

Organisms live at far-from-equilibrium with their surroundings while maintaining homeostasis, importing exergy and exporting entropy

Compressor shaft work targeting using new numerical exergy problem table algorithm (ex-pta) in sub-ambient processes

One of the major challenges faced by energy intensive industries is an excessive greenhouse gas emission to the atmosphere due to high consumption of fossil fuels throughout the production processes. Due to high electricity usage in intensive industries, it is strongly advisable to all the engineers and other stakeholders to reduce the amount of energy needed for production, thus reducing carbon footprints. This is because the power generation industry emits approximately 37 % of total carbon dioxide emission to the atmosphere. Process improvement via Process Integration enhances the process energy efficiency, which contributes to the minimisation of fossil fuels and electricity consumption. Sub-ambient processes with refrigeration system are one of the highly energy intensive processes, which consumes a vast amount of electrical energy to run the compressors for fulfilling the process cooling demand. It is crucial to optimise the process-utility heat transfer in the system including the placement of compressors and expanders to minimise the exergy losses and shaft work consumption. This paper presents a combined energy analysis technique of Pinch Analysis and Exergy Analysis to determine exergy losses and compressor shaft work targeting in sub-ambient processes via a novel numerical approach known as the Exergy Problem Table Algorithm (Ex-PTA). The validity of the novel approach is demonstrated by using an illustrated case study. Based on the new numerical method, the total exergy loss in the process is 4.42 kW which is equivalent to 7.4 kW potential savings of compressor shaft work. In the economic analysis, the wasted potential shaft work costs a loss of approximately 19,926 MYR/y which should be avoided to minimise process operating cost.

Development of a New Graphical Tool for Calculation of Exergy Losses in Sub-Ambient Processes

This paper presents new graphical tools, which can quickly identify all thermal exergy losses occurring in process design particularly in condensers/evaporators of low-temperature processes. Although the Omega -H diagram is a powerful tool for getting insights about exergy loss in a heat exchanger network (HEN), the calculation of the enclosed area is not straightforward due to non-linearity of the curves. The Omega Composite Curves (OCCs) and Omega Grand Composite Curve (OGCC) developed in this research are new graphical tools that can be applied to any process, including sub-ambient processes. These curves are linear, and all enclosed areas have a rectangular shape. So, all thermal exergy losses can readily be calculated and also necessary modifications to enhance the efficiency of refrigeration systems, either in new design or retrofit study can graphically be suggested. To demonstrate the capability of the new graphical tools, two ammonia refrigeration cycles (one single-stage and one three-stage) have been designed to fulfil the cooling demand of a sub-ambient process and achieve minimum shaft work requirements. The exergy loss associated with each unit in the refrigeration cycle is directly calculated from the OGCC diagram. This is a convenient tool for calculation of exergy losses and can be used to compute the enclosed areas between the OGCC and the condenser/evaporator horizontal lines. The enclosed areas show that exergy loss for 3-stage compression is much less than that in the 1-stage cycle. Also, for the throttling valve, the exergy loss in the 3-stage refrigeration is considerably less than that in the 1-stage cycle. The new linear curves can easily be plotted and implemented to show the share of inefficiencies occurring in different unit operations.

Exergoeconomic Performance Assessment of a Sulfuric Acid Production Unit Using the Specific Exergy Costing Method

In this work, an industrial sulfuric acid production unit was evaluated using exergetic and exergoeconomic analysis to determine possible improvements in its efficiency and cost effectiveness. The exergoeconomic evaluation was performed by Specific exergy costing method (SPECO). The case study process is the production of sulfuric acid from liquid sulfur by a double absorption contact. The exergy efficiency, exergy losses and exergoeconomic performance of each component in the overall plant were calculated and analyzed. The exergetic analysis, on the studied process, identifies the location, magnitude, and sources of thermodynamic inefficiencies. The obtained results indicated that the largest exergy destruction took place in the combustion stage, representing a 67 % of the total exergy losses. The exergy efficiency of the unit was around 44.7 %. Through the exergoeconomic analysis, the selling price of the produced H2SO4 as well as the total investment cost were estimated and the exergoeconomic factor of each component was quantified. The highest exergoeconomic factor, of 86 %, was observed in the recovery boiler, meaning that reducing the equipment cost of the studied process is of high importance to reduce the final product of the overall system. On the other hand, the combustion oven showed the lowest exergoeconomic factor, of 1.22 %, implying that improving its thermodynamic efficiency would be compulsory for that component.

Economic, Exergy, Environmental (3E) Analysis of Methanol Production from Shale Gas

Compared with direct utilisation as fuel, the downstream processing of shale gas shows great potential in economic competitiveness, in which methanol is a particularly important product. This study simulates a technical route for producing methanol from shale gas, which is evaluated from the points of economic, exergy and environment. The economic target is estimated by purchased cost curves, while exergy analysis demonstrates the distribution of exergy destruction. A hybrid LCI (Life cycle inventory) model is established to assess pollutant emissions by sources. The results show that methane reforming features the highest capital cost, while methanol synthesis requires the highest operating cost. The reaction systems account for 73.3 % of total exergy destruction. Electricity consumption dominates the environmental impacts of the whole process.

Exergy Analysis of a Process Converting Power and Biomass to a Liquid Fuel

A power and biomass to liquid process (PBtL) via Fischer-Tropsch synthesis is exergetically analysed. Synthesis gas is generated through an entrained flow gasifier and extra hydrogen is produced through high temperature steam electrolysis in a solid oxide electrolysis cell (SOEC). The gasification section of the process has the highest exergy destruction rate, followed by the Fischer-Tropsch and SOEC sections. To improve the process, the focus should be on the design and operation of these sections. The plant has an overall exergetic efficiency of 69 %, evaluated as the ratio of the exergy of the products to the exergy of the biomass and electricity supplied to the process.

Organic Rankine Cycle with Pure and Mixed Working Fluids for LNG Cold Energy Recovery

Liquefied Natural Gas (LNG) contains a large amount of cold exergy, which is normally wasted during the regasification process. Organic Rankine Cycles (ORCs), which have been widely employed for low-temperature waste heat utilization above ambient temperature, can also exploit LNG cold energy efficiently. Due to the low regasification temperature of LNG, the ORC for LNG cold energy recovery is quite different from the conventional ORCs recovering low-temperature waste heat. In this study, we investigate the working fluids for such an ORC recovering LNG cold energy. Both pure and mixed working fluids are studied and compared. According to open literature, mixed working fluids perform better than pure working fluids because the mixed working fluid can achieve a better match with the heat source. Therefore, the main motivation for mixed working fluids is to obtain gliding temperature for a better match with sensible heat sources. However, a few studies show that pure working fluid outperform mixed working fluids when the evaporating condition is close to the critical point. In addition, if the evaporation pressure is greater than the critical pressure, pure working fluids can also match well with a sensible heat source. Therefore, it is too general to claim that mixed working fluids will perform better than pure working fluids. For cases where the mixed working fluids perform better than pure working fluids, the optimal composition of the working fluid has to be determined. In this study, the performance of ORCs with pure and mixed working fluids are investigated for different types of ORCs. The optimal working fluid and composition for an ORC recovering LNG cold energy can be determined based on a simulation-based framework. The results show that mixed working fluids do not show significant improvement in net power output of the integrated system.

Optimization of an Absorption-Based Biogas Upgrading and Liquefaction Process

The present work proposes a methodology for optimization of a liquefied biomethane (LBM) production plant. The LBM production plant comprises amine-based absorption upgrading followed by a single expander refrigeration cycle. The processes were modeled using Aspen HYSYS® and optimized through a Sequential Quadratic Programming algorithm. Any changes in the operating conditions of the upgrading process will affect the cooling demand in the liquefaction, while the opposite is not true. Based on this, a sequential optimization approach starting with the upgrading process is proposed. In order to accommodate the connection between the processes, different objective functions were formulated for the sequential optimization approach. The results from the sequential approach were compared with an overall optimization approach, where the entire LBM plant was optimized simultaneously. The results indicate that the same solution was obtained both for the sequential approach and the simultaneous approach. For the sequential approach, however, the best result was observed when the interaction between the upgrading and liquefaction processes was accounted for by considering the effect of the upgrading process on the exergy requirement in the liquefaction process.

Exergy Analysis of Vapour Compression Refrigeration System Using R507A, R134a, R114, R22 and R717

Exergy Analysis of Vapour Compression Refrigeration System Using R507A, R134a, R114, R22 and R717

Thermodynamic Model of Geothermal Resources for Low-medium Temperatures Energy Conversion Process Optimization

Environmental and economic concerns are motivating manufacturers and public entities towards the use of renewable energy, which is continuously increasing its market penetration. Among the possible renewable energy resources, geothermal is particularly attractive compared to others such as solar and wind, mainly because of its continuity and dispatchability. Most studies in geothermal energy conversion systems model the resource as pure water or steam, while in reservoir simulations it is common practice to apply advanced geochemical modelling to estimate the long-term productivity. The presence of CO2 and saline equilibria may determine power plant optimisation conditions that differ from the ones which assume the resource as pure water or steam. The thermodynamic properties of the geothermal resource with high carbon dioxide (CO2) contents (1 to 8% in mass) within the 298 – 473 K temperature and 15 bar pressure are examined. The model applied focuses on the Equations of State (EoS) to be used in the calculation of the physical/thermodynamic properties of a mixture. The obtained results are validated through the comparison of different commercial software (UNISIM®, EES®, REFPROP®, TREND®). As an example, the Torre Alfina geothermal resource data (Middle Italy) are considered and the effects on the performance of a binary (ORC) geothermal power plant producing are examined in terms of energy and exergy efficiency. A sub-critical Iso-butane and a supercritical R134a power cycle are compared, and the advantages of the supercritical solution are demonstrated.