Total Exergy Loss Research Articles

Energetic and exergetic performance analysis of a solar driven power, desalination and cooling poly-generation system

This paper presents a thermodynamic analysis of a poly-generation system powered by solar thermal energy using parabolic trough collectors. The proposed system consists of an organic Rankine cycle, a multiple effect distillation and an absorption cooling unit. The performance analysis of the solar system is conducted for different configurations: power generation only, cogeneration power and cooling, cogeneration power and desalination, and poly-generation. The effects of turbine inlet temperature and pump inlet temperature on the energetic and exergetic system performance as well as the net power output and total exergy loss of the system are examined. In addition, exergetic parameters, including system total exergy loss, fuel depletion ratio and improvement potential were analyzed. The study reveals that increasing the turbine inlet temperature increases the performance while it reduces the total exergy destruction rate of the system. The result of the study also shows that the two main sources of exergy destruction are the solar thermal collector and desalination unit; with 49.3% of the input exergy (76% of the total exergy loss) destructed in the collector while 9.6% of the inlet exergy (14.9% of the total exergy loss) is destroyed in the desalination system. The overall improvement potential of the system was found to be 64.8%.

Solutions for enhancement of energy and exergy efficiencies in air handling units

In this study, five layouts of using air-to-air heat exchangers (AAHE) were added in the Air Handling Unit (AHU) to diminish the cooling and heating coils energy usage through the energetic and exergetic analysis approach. In the first, second, and third techniques, the main focus is on cooling coil energy usage reduction. In the fourth and fifth layouts, new designs of using two AAHEs (i.e., primary and secondary units) have been incorporated to diminish simultaneously the energy demand of coils. The results showed if the objective function is defined to have the least cooling coil power, the fourth layout, and if the objective function is the lowest heating coil load, the fifth layout is recommended. In all layouts, installation of the AAHE reduced the irreversibility. In the best layout, AHU energy demand diminished by 23.68% which consequently improved the first law efficiency by 31.29%. Similarly, total exergy losses reduced by 26.58% and the second law efficiency improved by 11.79%. Calculations on cost analysis revealed that the least payback time and highest cost saving referred to first and fourth layouts, respectively.

Energy Saving Research on Multi-effect Evaporation Crystallization Process of Bittern Based on MVR and TVR Heat Pump Technology

This work keeps an eye on the energy saving research on evaporation crystallization process of bittern. Based on the thermo sensitivity of solubility of various salts in bittern, the magnesium salts are purified. The conventional evaporation crystallization process used to separate the bittern demands high energy consumption and has low thermodynamic efficiency. Therefore, the multi-effect evaporation (MEE), thermal vapor recompression (TVR) heat pump and mechanical vapor recompression (MVR) heat pump technology were applied to the conventional evaporation crystallization process. The MVR and TVR technology can both make full use of the secondary steam heating materials that will save energy. In addition, Aspen Plus (Version 7.3) was used to simulate the processes of the electrolyte-containing system under the ELECNTRAL thermodynamic model. For the better evaluation of various evaporation crystallization processes, some important evaluation indexes, such as energy consumption, annual total cost (ATC) and exergy loss were chosen as objective functions. Compared with the double-effect evaporation crystallization process coupled with TVR heat pump technology, the results indicated that the double-effect evaporation crystallization process coupled with MVR heat pump technology can save energy consumption and ATC by 80.52% and 15.32% respectively. Furthermore, the MVR heat pump technology takes the lowest effective energy loss, which is a more competitive factor of evaporation crystallization process of bittern.

Time optimal control operation of bulk lyophilisation of materials on trays reduces the exergy losses and increases the efficiency of energy utilisation

Exergy represents a measure of quality of energy and because exergy is destroyed due to process irreversibilities within a system to produce entropy, the exergy expressions and analysis developed in this work could be used to operate the intrinsically unsteady-state lyophilisation processes more efficiently. It is shown that the time optimal control operation of a freeze dryer with respect to heat input to the trays and total pressure in the drying chamber, reduces the duration times of the primary and secondary drying stages by 42.43% and 41.17%, respectively, and this leads to a reduction of 38.19% in the magnitude of the total exergy losses of the lyophilisation system studied in this work. Also, the potential for reducing further the exergy losses when organic co-solvents and water forming non-ideal mixtures of solvents with positive deviations from Raoult's law are used in a lyophilisation system operated under time optimal control, is discussed.

Time optimal control operation of bulk lyophilisation of materials on trays reduces the exergy losses and increases the efficiency of energy utilisation

Exergy represents a measure of quality of energy and because exergy is destroyed due to process irreversibilities within a system to produce entropy, the exergy expressions and analysis developed in this work could be used to operate the intrinsically unsteady-state lyophilisation processes more efficiently. It is shown that the time optimal control operation of a freeze dryer with respect to heat input to the trays and total pressure in the drying chamber, reduces the duration times of the primary and secondary drying stages by 42.43% and 41.17%, respectively, and this leads to a reduction of 38.19% in the magnitude of the total exergy losses of the lyophilisation system studied in this work. Also, the potential for reducing further the exergy losses when organic co-solvents and water forming non-ideal mixtures of solvents with positive deviations from Raoult's law are used in a lyophilisation system operated under time optimal control, is discussed.

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.

Exergy Analysis and Process Optimization with Variable Environment Temperature

In its usual definition, exergy cancels out at the ambient temperature which is thus taken both as a constant and as a reference. When the fluctuations of the ambient temperature, obviously real, are considered, the temperature where exergy cancels out can be equated, either to the current ambient temperature (thus variable), or to a constant reference temperature. Thermodynamic consequences of both approaches are mathematically derived. Only the second approach insures that minimizing the exergy loss maximizes performance in terms of energy. Moreover, it extends the notion of reversibility to the presence of an ideal heat storage. When the heat storage is real (non-ideal), the total exergy loss includes a component specifically related to the heat exchanges with variable ambient air. The design of the heat storage can then be incorporated into an optimization procedure for the whole process. That second approach with a constant reference is exemplified in the case study of heat pumping for heating a building in wintertime. The results show that the so-obtained total exergy loss is the lost mechanical energy, a property that is not verified when exergy analysis is conducted following the first approach.

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.

Analysis on the Performance of Steam Absorption Chiller at Various Operating Conditions

Steam Absorption Chiller (SAC) is the main component of Cogeneration Plant which provides chilled water by utilizing the steam as system input for the cooling purpose. In this paper, the performance of a double effect lithium bromide-water steam absorption chiller was analyzed by conducting the energy and exergy analysis. An excel spreadsheet program was developed to calculate the exergy destructions within the systems and total exergy loss of the system in guiding future improvement of the plant. Various operating conditions like steam inlet temperature, chilled water inlet temperature and load factor were analyzed in detail. The result showed that the highest exergy destruction occurred at the high temperature generator followed by the absorber. Increasing the load factor, steam and chilled water inlet temperature will increase the system performance.

Performance study on a photovoltaic thermal (PV/T) stepped solar still with a bottom channel

This study presents a novel type of stepped PV/T solar still in which a bottom channel is designed under the base of the still, aiming to enhance heat transfer. In order to assess the performance of the desalination system, a theoretical model is established, and validated by the existing experimental data, thus can be used for energy and exergy analysis. The effects of bottom channel structure on thermal performance (temperatures of components, heat transfer coefficients, thermal efficiency), exergy performance (total exergy loss of each component, exergy efficiency) and freshwater productivity are investigated. The comparative results show that, when the depth of bottom channel H is 0.01 m, the average heat transfer rate from absorber plate to saline water is improved by 44%, thereby the average temperature of saline water is raised by 16.4% and the daily freshwater productivity is improved by 51.7%. Moreover, the average thermal efficiency and exergy efficiency are increased by 17% and 3% respectively. However, the freshwater productivity could be decreased due to the increase of H. Compared with H = 0.01 m, the daily fresh water production is decreased by 17% when H = 0.03 m. The results of the exergy analysis indicate that the optimum depth of bottom channel is around 0.01 m.

Optimal design for cryogenic structured packing column using particle swarm optimization algorithm

Large scale cryogenic air separation is the most efficient and cost-effective approach to produce high-purity air products by present. Structured packing columns (SPC) are widely focused and applied due to their characteristics of high efficiency and energy saving in the cryogenic distillation process. The optimal design of the SPC is to reduce the energy consumption and initial investment, while it is a highly nonlinear and multivariable problem. The coexistence of real variables and integer variables, such as the flow rates and the positions of materials at the inlets/outlets, makes the optimization become a typical mixed integer nonlinear programming (MINLP) problem. The purpose of this paper is to study the optimal design method for the cryogenic SPC using the particle swarm optimization (PSO) algorithm. A modified PSO for handling the MINLP problem (MI-PSO) is proposed. A multi-objective optimal design for the SPC in cryogenic air separation plant with the capacity of 17,000 Nm3/h is investigated as a instance. By MI-PSO algorithm, the total exergy loss theoretically reduces 36.3% and the main condenser heat load decreases 5.4% after optimization, which can provide test prediction for the cryogenic distillation experiment.

Global minimization of total exergy loss of multicomponent distillation configurations

AbstractThe operating cost of a multicomponent distillation system comprises two major aspects: the overall heat duty requirement and the temperature levels at which the heat duties are generated and rejected. The second aspect, often measured by the thermodynamic efficiency of the distillation system, can be quantified by its total exergy loss. In this article, we introduce a global optimization framework for determining the minimum total exergy loss required to distill any ideal or near‐ideal multicomponent mixture using a sequence of columns. Desired configurations identified by this new framework tend to use milder‐temperature reboilers and condensers and are thus attractive for applications such as heat pump assisted distillation. Through a case study of shale gas separations, we demonstrate the effectiveness of this framework and present various useful physical insights for designing energy efficient distillation systems.

Analysis of exergy losses in laminar premixed flames of methane/hydrogen blends

Combustion is the primary source for exergy loss in power systems such as combustion engines. To elucidate the exergy loss behaviors in combustion and explore the principle for efficiency improvement, the second-law thermodynamic analysis was conducted to analyze the energy conversion characteristics in laminar premixed flames of methane/hydrogen binary fuels. The sources causing exergy losses in laminar premixed flames included five parts, namely heat conduction, mass diffusion, viscous dissipation, chemical reactions and incomplete combustion, respectively. The calculations were conducted at both atmospheric and elevated pressures, with the equivalence ratio varying from 0.6 to 1.5 and the hydrogen blending ratio increasing from 0% to 70%. The results indicated that the total exergy loss firstly increased and then decreased with increased equivalence ratio, and reached the minimum value at the equivalence ratio of 0.9. This was primarily due to the trade-off relation between the decreased exergy loss from entropy generation and the increased exergy loss from incomplete combustion, as equivalence ratio increased. As the hydrogen blending ratio increased from 0% to 70%, the total exergy loss decreased by 2%. Specifically, the exergy loss from heat conduction decreased, primarily due to the decreased flame thickness. Moreover, the reactions with H2, H and H2O as reactants were inhibited, leading to decreased the exergy loss from chemical reactions. As pressure increased from 1 atm to 5 atm, the total exergy loss decreased by 1%, because the exergy losses induced by heat conduction and chemical reactions decreased as the flame thickness was reduced. The exergy loss from incomplete combustion also decreased, because elevated pressure inhibited dissociations and decreased the mole fractions of incomplete combustion products.

Design and optimization of natural gas liquefaction process using brazed plate heat exchangers based on the modified single mixed refrigerant process

The multi-stream plate-fin heat exchangers (PFHEs) are widely used as the core heat transfer equipment in small-scale natural gas liquefaction processes. However, the PFHEs have disadvantages such as the strict pre-treatment standard and the complicated engineering and transportation. The brazed plate heat exchangers (BPHEs) have been suggested to replace the PFHEs to overcome such disadvantages. In this paper, the methodology for transforming PFHE processes into BPHE processes was introduced and a novel liquefaction process using BPHEs was proposed based on the modified single mixed refrigerant process. The key parameters of the new process were optimized by using the genetic algorithm, and the optimal operating conditions were compared with the base case. The results showed that the specific energy consumption (SEC) of the optimized case was 10.3% lower than that of the base case, while it was slightly higher than the SEC of the original PFHE process. Additionally, the exergy losses of the major equipment in the process were analyzed. The exergy losses were reduced by 7.6%, 19.5%, 29.5% and 15.3% respectively for the compressors, the heat exchangers, the Joule-Thompson valves and the water coolers in the optimized case. The total exergy loss was lower by 14.7% after the optimization.

Energy-efficient extractive pressure-swing distillation for separating binary minimum azeotropic mixture dimethyl carbonate and ethanol

An energy-efficient extractive pressure-swing distillation process is proposed for separating binary minimum azeotropic mixture ethanol and dimethyl carbonate. It can be observed that the minimum amount flow rate of entrainer will be decreased when the operating pressure of extractive distillation column is increased via the thermodynamic feasibility insights (i.e., residue curve map and isovolatility lines). Therefore, an extractive pressure-swing distillation process with 4 bar for the extractive distillation column is designed. Process variables of the proposed design are optimized by combining the sensitivity analysis and sequence quadratic program approaches with minimum total annual cost as objective function. Total annual cost, CO2 emissions, and exergy loss of the optimized extractive pressure-swing distillation with 4 bar for the extractive distillation column is reduced by 44.09%, 44.16% and 41.54%, respectively when compared with the existing process with 1 bar for the extractive distillation column, which mainly attributing the flow rate of entrainer decreasing from 200.020 kmol/h to 44.963 kmol/h. Furthermore, the extractive pressure-swing distillation with heat integration is studied to further reduce energy cost because the enough heat transfer temperature difference could be provided by increasing operation pressure.

Investigation of energy-saving azeotropic dividing wall column to achieve cleaner production via heat exchanger network and heat pump technique

A thermally coupled azeotropic dividing wall column (ADWC) configuration is explored for the separation of industrial wastewater to recycle the organic solvent tert-butanol. Heat pump technology is used to the ADWC configuration to improve the released heat duty quality of the condenser achieving the energy-saving. A gas preheater before the compressor is installed in the heat pump assisted ADWC configuration in increasing the temperature of the inlet vapour stream of the compressor that achieves effectively reducing the power and compression ratio of the compressor. To fully utilize a large amount of superheat energy produced in heat pump system indicated by the temperature-enthalpy and Grand Composite Curve diagrams, a green and sustainable Heat Integrated ADWC (HI-ADWC) separation configuration is proposed by the combined use of heat exchange network and heat pump implementations. Three indexes involving total annual cost, CO2 emissions, and exergy loss are introduced to evaluate the economic, environmental and thermodynamic performances. The results illustrate that the TAC of the proposed green and sustainable HI-ADWC configuration is significantly reduced by 32.91% with a ten-year payback period compared to that of the existing configuration. CO2 emissions are reduced by 86.43% and exergy loss of the HI-ADWC configuration by 36.72%. The proposed method for the green and sustainable HI-ADWC configuration could be widely extended to other industrial processes reduce energy consumption and related CO2 emissions.

Exergy and Exergoeconomic Analysis of a Combined Cooling, Heating, and Power System Based on Solar Thermal Biomass Gasification

The purpose of this paper is to improve the utilization of renewable energy by exergy and exergoeconomic analysis of the novel combined cooling, heating, and power (CCHP) system, which is based on solar thermal biomass gasification. The source of heat to assist biomass and steam gasification is the solar heat collected by a dish collector, and the product gas being fuel that drives the internal combustion engine to generate electricity and then to produce chilled/hot water by a waste heat unitization system. The analysis and calculation of the exergy loss and exergy efficiency of each component reveal the irreversibility in the heating and cooling conditions. Then, the exergoeconomic costs of multi-products such as electricity, chilled water, heating water, and domestic hot water are calculated by using the cost allocation method based on energy level. The influencing factors of the unit exergy cost of products are evaluated by sensitivity analysis, such as initial investment cost, biomass cost, service life, interest rate, and operating time coefficient. The results reveal that the internal combustion engine takes up 49.2% of the total exergy loss, and the most effective method of products cost allocation is the exergoeconomic method based on energy level and conforms to the principle of high energy level with high cost.

Multi-dimensional life cycle assessment of decentralised energy storage systems

The intermittent nature of renewable energy sources like solar and wind energy stimulates the use of centralised and decentralised energy storage systems. The sustainability of lead acid, lithium-ion and concentration gradient flow batteries, compressed air and pumped hydro energy storage (PHES) systems is investigated by conducting a multi-dimensional life cycle assessment. The environmental, economic and exergetic sustainability are assessed by calculating ReCiPe 2016 indicators, the present worth ratio and the Total Cumulative Exergy Loss, respectively. The multi-dimensional sustainability assessment did not lead to one preferred system. The PHES causes the lowest damage to human health, ecosystem diversity and resource availability and results in the lowest global warming potential. The concentration gradient flow battery system named BBS is preferred from an economic viewpoint, while the PHES is second-best. The lithium-ion battery system causes the lowest exergy losses, followed by the PHES. It is recommended to pay attention to the exergetic sustainability of technological systems as exergy losses are independent of environmental models, weighting factors, market prices, subsidies etc. More research into the specifications of the energy storage systems is needed to be able to draw firm conclusions with regard to which system is preferred.

Thermodynamics and LCA analysis of biomass supercritical water gasification system using external recycle of liquid residual

Biomass supercritical water gasification is clean and renewable, which can convert biomass into hydrogen rich gas. Previous studies indicated that external recycle of liquid residual could improve gas yield and gasification efficiency. However, the influence mechanism of external recycle on energy and exergy efficiency is complicated and theoretical model for system optimization is insufficient. Thermodynamic model of external recycle of liquid residual was built in this paper. Exergy efficiency of the main components and exergy loss distribution were specified and the result showed that exergy loss of reactor and preheater accounted for 26.06% and 35.88% of the total exergy loss, which were the main exergy loss sources. Effective ways to reduce exergy loss of components with large exergy loss and to improve energy and exergy efficiency of the system were proposed. Moreover, life cycle assessment of biomass gasification process was carried out. The results indicated that the increase of gasification temperature, pressure and external recycle flow rate of liquid residual and decrease of biomass concentration could improve energy and exergy efficiency of the system. Energy and exergy efficiency reached 63.67% and 48.29% respectively at the condition of gasification temperature of 560 °C, pressure of 25 MPa, recycle flow ratio of 32.43%, biomass concentration of 2.78%. Besides, the increase of gasification temperature and decrease of biomass slurry concentration and pressure could decrease GWP.

Numerical study on exergy losses of iso-octane constant-volume combustion with water addition

A numerical study on exergy losses in the iso-octane auto-ignition processes with water addition was conducted in an adiabatic constant-volume system. Detailed chemical mechanism of iso-octane was used in the chemical kinetic simulation, under variable initial temperatures, equivalence ratios and water to fuel ratios. The cooling, dilution, thermal and chemical effects of water addition on the exergy losses in four reaction stages, defined as the fuel-series reaction stage, the fuel fragment reaction stage, the H2O2 loop reaction stage and the H2-O2 reaction stage were analyzed, respectively. The cooling effect was the most influential factor, which increased the exergy losses from the first to third reaction stages, but the increase magnitude was reduced at higher initial temperatures. The dilution effect increased the exergy losses in all the four reaction stages. The thermal effect also increased the exergy losses of the four reaction stages due to the decrease of temperature. However, its influence could be almost negligible compared to the other three effects. The chemical effect decreased the exergy losses in the fuel fragment reaction stage at both low and high initial temperatures. On the other hand, the exergy losses caused by the incomplete combustion products and combustion intermediates were decreased by the cooling, dilution and thermal effects while increased by the chemical effect of water addition. In general, the four effects on the exergy losses from incomplete combustion products and combustion intermediates were greater than those on the exergy losses from chemical reactions, therefore the total exergy losses in the iso-octane auto-ignition processes exhibited a declination trend with water addition, e.g. a decrease by 5% at a higher initial temperature.