Highest Exergy Loss Research Articles

Techno-economic comparison of different cycle architectures for high temperature waste heat to power conversion systems using CO2 in supercritical phase

Bottoming thermodynamic systems based on supercritical carbon dioxide as working fluid (sCO2) are a promising technology to tackle the waste heat to power conversion at high temperature levels and that might outperform the conventional power units based on Organic Rankine Cycles. In fact, CO2 is an inexpensive, non-toxic, non-flammable, thermally stable and eco-friendly compound. Moreover, CO2 in its supercritical state shows an extreme increase in density that allows turbomachinery downsizing and a high cycle efficiency due to the reduced work required by the compression stage. In addition, supercritical CO2 permits a better temperature glide matching within the heat source which increases the overall efficiency of waste heat utilization. With the aim of identifying pro and cons of different sCO2 cycle layouts, this paper investigated four design Joule-Brayton configurations at increasing complexity: simple regenerative, with recompression, with reheating and with recompression and reheating. The research methodology is based on 1st and 2nd laws thermodynamic analyses and includes correlations to estimate the investment costs of the equipment. With reference to a high temperature industrial waste heat source, performance, costs and exergy losses in the different cycle layouts are compared. Furthermore, a parametric analysis regarding the effects of the cycle pressure ratio on net power output and back work ratio is carried out.

Exergy analysis: A CO2 removal plant using a-MDEA as the solvent

An exergy analysis was conducted for CO2 removal process from syngas using methyldiethanolamine activated by piperazine (a-MDEA). The process was simulated with Aspen HYSYS and the validation of the results was performed using the field data of Fanavaran Petrochemical Company. The process was divided into five separate sections. In order to determine the exergy loss, exergy balance equation was established around each section. The results showed that the highest exergy loss amongst five sections returns to the flasher (contributes 31.5% of total exergy loss); followed by the absorber column, stripper column, heat recovery section and the pump. In order to decrease the exergy loss in the process, piperazine concentration in a-MDEA solvent was increased which led to reduction in amine solvent flow rate. According to the results, the total exergy loss is decreased from 3.25 MW to 2.63 MW. When the amine flow rate decreased by 27%, the exergy loss, total energy consumption and the total utilities cost decreased by 19%, 18% and 25%, respectively.

Exergy analysis of a shell-and-tube heat exchanger using graphene oxide nanofluids

Nanofluids are the new-generation heat transfer fluids largely used in heat exchangers for thermal energy transport applications. In this paper, graphene oxide was prepared in-house and used for nanofluid development and characterization. The graphene oxide was prepared by oxidizing purified natural flake graphite via the modified Hummers method. The morphology and structure of fabricated graphene oxide were characterized using X-ray diffraction analysis (XRD) and a scanning electron microscope (SEM). The graphene oxide nanofluids were prepared at 0.01 and 0.1wt.% concentrations. The particle size and zeta potential of nanofluids were measured using the dynamic light scattering (DLS) technique. The rheological behaviors of nanofluids were investigated at 25 and 40°C at different shear rates (10–1001/s). Thermal conductivity of nanofluids was measured using the transient hot wire method at 25 and 40°C. Exergy analysis showed that increasing the graphene oxide concentration from 0.01wt.% to 0.1wt.% resulted in 8.7% and 18.9% thermal conductivity enhancement at 25 and 40°C, respectively. Furthermore, the thermal performance of graphene oxide nanoparticles in a shell-and-tube heat exchanger was studied experimentally. The effect of nanofluid concentration, flow rates, temperature inlet and flow regime on the system’s exergy loss was studied experimentally. The results showed that using graphene oxide nanofluids as the hot fluid resulted in less exergy loss in the shell-and-tube heat exchanger under both laminar and turbulent conditions. Comparing the exergy loss of graphene oxide nanofluids to distilled (DI) water showed that DI water caused 22% and 109% higher exergy loss than in laminar conditions.

Exergy efficiency from staple food ingredients to body metabolism: The case of carbohydrates

One of the methods to evaluate the efficiency in the production of foods is using exergy, the share of useful energy, and exergy analysis. In this paper, we propose a link between exergy analysis and nutrition to account for the exergy efficiency (exergy of output per exergy of input) in the metabolism of nutrients from foods in the human body. For this, we analyzed the exergy efficiency of four different chains of carbohydrate-rich products based on semi-industrial preparation processes and we included nutrient bioavailability through the use of several bioavailability indicators, including the glycemic index and protein digestibility. The least exergy efficient chain changed when not only looking at the exergy losses of the food processing chains, but also including the bioavailability and conversion of nutrients into adenosine triphosphate (ATP), the main molecule for energy storage in the body. When including only the processing chain, white bread presented the highest exergy loss, whereas the lowest values pointed to the spaghetti chain when also including metabolism both because of its preprocessing chain and its low bioavailability. In contrast, cooked potatoes were found to be an efficient source of ATP due to both a high metabolic efficiency and low process exergy losses. The carbohydrate bioavailability had a strong influence on the overall exergy efficiency of the studied cases, which shows the importance of including bioavailability aspects in the sustainability assessment of industrial food processing chains.

Novel retrofit designs using a modified coordinate descent methodology for improving energy efficiency of natural gas liquid fractionation process

Tough environmental regulations, intense competition, expensive fossil energy use, and the strong growth predictions of the natural gas market have prompted efforts to retrofit the existing purification processes to reduce their energy requirements. The important goals of retrofit design are to analyze, evaluate and propose suitable technologies to improve the energy efficiency and/or increase the capacity. This paper reports the results of a techno-economic feasibility study to retrofit a natural gas liquid (NGL) fractionation process. A novel hybrid system, side reboiler and heat pump-assisted, thermally-coupled distillation sequence to maximize the energy efficiency, was proposed. Fractional utilization of the area was used as a hydraulic performance indicator to determine if a bottleneck occurs in a retrofit design. A modified coordinate descent methodology was employed to solve the optimization problem. As a result, the modified coordinate descent methodology was successful in finding the optimal proposed sequence structure and the operating variables, which resulted in operating cost savings of 44.55% compared to the representative base case. The short payback period of 14 months and reduced CO2 emissions of up to 42.05% showed that the proposed sequence is an attractive option for retrofitting in industrial implementation. This sequence can be employed for both grass-root and retrofit designs. This study also showed that even the heat pump can reduce the energy requirements significantly, and may have higher exergy loss than the existing conventional distillation columns.

Evaluation of an alternative chlorine production process for energy saving toward sustainability

Exergetic‐based solutions are proposed in this work to reduce the energy footprint of an alternative chlorine production process. By presenting exergy analysis results in the form of visualized exergetic process flowsheets using a solution panel, this study does not only find the low‐exergy‐efficient unit operations, but also proposes ways to improve the exergy efficiency based on the major sources of irreversibility of individual unit operations. Based on the amount of exergy losses, the alternative chlorine production process is divided into three main functional blocks. The total internal and external exergy losses for the entire process are calculated as 14,523.5 and 286.1 kJ/kg, respectively. The electrolysis block shows the highest potential for improvements with the largest exergy loss (responsible for more than 96.0% of total losses; with 14,093.2 kJ/kg of internal losses and 146.5 kJ/kg of external losses). Isothermal mixing, lower reaction temperature, and reduced pressure drop are suggested as avenues for improvement. Exergy losses in the gas treatment block with total exergy losses of 411.7 kJ/kg (equivalent to 2.8% of total losses) are mainly due to the cooling column (241.9 kJ/kg), where modifications in column might be helpful as concentration and temperature gradients along the tower are sources of exergy loss. The highest exergy loss in the feed pretreatment block with the total exergy losses of 158.2 kJ/kg (i.e., 1.1% of total losses) belongs to the mixers (106 kJ/kg) where isothermal and also isobaric mixing is required. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1512–1520, 2016

Potential for exhaust gas energy recovery in a diesel passenger car under European driving cycle

This work addresses the potential for waste energy recovery from exhaust gases in a diesel passenger car mounted in a chassis dynamometer. The New European Driving Cycle was followed, while recording relevant operating variables. Tests were performed under three temperature conditions, and exergy analysis was included to find the potential of exhaust gases to produce useful work at six points in the exhaust system. Results include mean temperature at each point, as well as the energy quality index, which was lower than 33%, meaning that less than one-third of the energy of exhaust gases can be converted into useful work in a recovery system. In general, the highest exergy losses were found in the muffler. Although the greatest recovery potential corresponds to the highest temperature of gases, environmental regulations for vehicles restrict waste energy recovery to be performed downstream after-treatment devices, which, in the present work, was the outlet of the diesel particle filter. Temperature of gases at this location varied in the range 115–320°C, and potential fuel saving varied between 8% and 19% for the complete driving cycle.

Exergy analysis as a scoping tool for cleaner production of chemicals: a case study of an ethylene production process

High energy consumption is one of the main challenges of the chemical industry. The energy footprint of most processes can, however, diminish through the solutions presented in the current paper, leading to cleaner ways of producing chemicals. Ethylene production process is selected as a case study to demonstrate the approach as it is one of the most energy consuming processes in the chemical industry. The study involves an exergetic diagnosis and does not only find the low-exergy-efficient unit operations, but also proposes tools to improve these units based on their key sources of irreversibility. For the ethylene production process, this is conducted by first splitting the flowsheet into four main functional blocks (namely cracking, compression, refrigeration, and separation and purification) according to their exergy losses. This results in identifying the cracking block as the most inefficient block with more than 45% of total exergy losses and thus the first block to be improved so that overall losses reduce (examples of which include increasing the number of furnace tubes while reducing their lengths). Although the compression block is found to have the lowest contribution to internal exergy losses, the inefficient unit operations such as the water cooler (with an exergy loss of 214 kJ/kg) can still be improved through solutions such as system isolation. The refrigeration block is also shown to have the second highest exergy losses with its ethylene and propylene compressors being the main contributors. Solutions are again provided to improve the block performance with specific focus on intercooler design improvement and system isolation. Finally, exergy losses in the purification and separation block are identified to be mainly due to demethanator, deethanator, and ethylene column where modifications in column design might be helpful as concentration and temperature gradients along the towers are the main sources of exergy losses. The approach used in the current study can also be applied to other chemical processes and the findings suggest that even for a well-developed process technology, there is still opportunity for thermodynamically justifiable energy efficiency improvements. Therefore, it is important for process developers to continuously revisit existing processes, in order to ensure lessons learned in one area can be applied to another one. Using a panel of solutions, which has been constructed from a number of previous case studies helps to make this approach more systematic and user-friendly.

Evaluation and optimization of the alkaline water electrolysis ohmic polarization: Exergy study

A model is created in order to investigate the effect of different material parameters on the ohmic overpotential of the alkaline water electrolysis for hydrogen production, which influences the exergy efficiency of the water electrolysis. In this research, it was demonstrated that the electrode material parameters and electrolyte conditions of the water electrolysis components such as electrode and membrane resistivity, distance between the electrodes, oxygen and hydrogen bubbles that covers the electrode surface, electrolyte concentration, electrolyte ionic conductivity, and temperature have various effects on the ohmic overpotential, which consequently affect the exergy efficiency of the alkaline water electrolysis. The results of our model has illustrated that the highest exergy loss is due to hydrogen bubbles followed by the electrolyte ionic resistance and oxygen bubbles resistance, respectively. The model has also provided a strong direction for how to optimize the exergy efficiency by reducing the ohmic overpotential, which is affected by various material parameters and operating conditions.

Diagnosis of an alternative ammonia process technology to reduce exergy losses

Ammonia production through more efficient technologies can be achieved using exergy analysis. Ammonia production is one of the most important but also one of most energy consuming processes in the chemical industry. Based on a panel of solutions previously developed, this study helps to identify potential areas of improvement using an exergy analysis that covers all aspects of conventional ammonia synthesis and separation. The total internal and external exergy losses are calculated as 3,152 and 6,364kJ/kg, respectively. The process is then divided into five main functional blocks based on their exergy losses. The reforming block contains the largest exergy loss (3,098kJ/kg) and thus the largest potential for improvement including preheating cold feed through an economizer, developing technology towards isobaric mixing, and pressure drop reduction in the secondary reformer as the main contributors to the irreversibility (1,302kJ/kg) in this block. The second largest exergy loss resides in the ammonia synthesis block (3,075kJ/kg) where solutions such as reduced temperature rise across the compressor, proper compressor isolation, reducing undesired components such as argon in the reactor feed, and using lower temperatures for reactor outlet streams, are proposed to decrease the exergy losses. Throttling process in the syngas separator is the key contributing mechanism for the irreversibility (1,635kJ/kg exergy losses) in the gas upgrading block. The exergy losses in the residual ammonia removal block (833kJ/kg exergy losses) are mainly due to the stripper and the absorber column where a modified column design might be helpful. The highest exergy loss in the preheating block belongs to the compressors (518kJ/kg exergy losses) where a lower inlet temperature and better system isolation could help to reduce losses.

Hidrojen Üretimi Amaçlı Entegre Güneş Enerjisi Temelli Kimyasal Reaktör Sisteminin Termodinamik Analizi

The biggest advantage of the renewable energy based systems is that these energy systems are environmentally friendly, since they emit very few pollutants. The solar parabolic trough collector systems generate thermal energy by using solar radiation. These renewable energy systems are the most deployed type of the solar concentrating collectors. Especially, they are very suitable for middle-temperature solar power system applications. Storing of the solar energy is not a suitable way due to the irreversibility production associated with the heat transfer. Instead of that, solar energy should be used to produce hydrogen energy using a solar reactor system. By using the parabolic concentrating collectors, hydrogen may be produced by applying water-gas shift reaction with H 2 O and CO which emitted to atmosphere by any reaction under 475 K. Produced hydrogen can be used in energy generation systems or chemical industries while carbon dioxide can be used in green houses or carbon industry. The water-gas shift reactions have the benefit of generating long term storable energy carriers from the solar radiation. This conversion also allows transportation of solar radiation from the sunbelt to remote population centers. This paper gives a second law analysis based on an exergy concept for the simple solar cylindrical parabolic reactor for a better evaluation. The scientific approach of detailed energy and exergy analyses of the solar cylindrical parabolic reactor and dispersion of the exergy losses are presented in this study too. Exergy analysis of the solar energy conversion processes helps to investigate the optimum system that covers the imposed thermal and economic limitations. In this paper, it is given that the highest exergy losses take place at the solar collector and receiver sub-system. The outcomes of theoretical analysis should be used for analyzing the system components and irreversibilities of the solar cylindrical parabolic reactor.

Exergy Analysis of Energy Consumption for Central Air Conditioning System

The energy consumption of several central air conditioning systems in summer was researched by the method of exergy analysis. Combined with actual example,the exergy loss of all the equipments and the exergy efficiency of three systems were calculated. The results show that the exergy efficiency of three systems is very low. Relatively speaking, the exergy efficiency of primary return air conditioning system with supplying air in dew point is highest. The equipment of highest exergy loss is air-conditioned room, while the exergy loss of surface air cooler is smallest. Based on this, several improvement measures were proposed to reduce exergy loss and improve exergy efficiency.

Exergy analysis for fuel reduction strategies in crude distillation unit

Inefficient furnaces and heat exchangers contribute to the depletion of fossil fuel problem due to higher fuel demand and higher carbon emission. The method of exergy analysis is applied to the furnace and crude preheat train (CPT) in a crude distillation unit (CDU) to determine performance benchmark of the system. This paper presents exergy analysis and strategies to reduce exergy loss through process modification. The highest exergy loss was found to be located at the inlet furnace. The proposed options for fuel reduction strategies are reduction of heat loss from furnace stack and overall cleaning schedule of CPT. The feasibility and economic analysis for both options are investigated. From the results, overall cleaning schedule of CPT contributes to the highest energy saving of 5.6%. However, reduction of heat loss from furnace stack is the highest cost saving by about 6.4%.

Energy and exergy assessments of a perlite expansion furnace in a plaster plant

In this study, energy and exergy assessments of a perlite expansion furnace in a plaster factory are performed. Chemical properties of the perlite and its application areas are described. Energy and exergy relations according to the First and Second Law of Thermodynamics are derived. Energy and exergy efficiencies, losses and exergy destructions are calculated based on the measurements obtained from the system. Evaluations of environmental impact, energy management and economic are finally generalized according to the obtained results. Energy and exergy efficiencies of the furnace are determined to be 66% and 26%, respectively. Reference temperature effects on exergy efficiency, destruction and sustainability index are also presented. The results showed that the furnace has not been well designed in terms of thermal aspects due to high energy and exergy losses and the manufacturer producing the perlite expanded should establish an energy management structure.

Critical analysis of a biogas powered absorption system for climate change mitigation

The importance of biogas as a renewable alternative is being studied because of an increase in the cost of conventional fuels. The present article suggests a numerical study of a biogas powered NH3-H2O absorption refrigeration system where biogas is used to heat the water which serves as an energy input to generator of an absorption system. A computational model has been developed for the analysis which involves the determina- tion of effect of generator temperature on various perfor- mance parameters, i.e., exergy losses in the different components, COPcooling, COPheating and the exergy effi- ciency. The results indicate that COPcooling and COPheating lies in the range of 0.159-0.33 and 1.16-1.33, respectively, whereas exergetic efficiency lies in the range of 0.29-0.80 for the same variation in generator temperature ranging from 50 to 70 � C. The highest exergy loss is found in the generator while the lowest is found in the condenser and it is also found that with an increase in the evaporator as well as absorber and condenser temperature, the COP increases and decreases, respectively. The effect of ambient tem- perature on exergy loss in the different components is also studied. Exergy analysis is an excellent tool to pin point the losses in the system due to irreversibility which are the basis for the further improvement in the system components.

Biocatalytic conversion of lignocellulose to platform chemicals

Naturally occurring lignocellulose can be used as a renewable resource for the sustainable production of platform chemicals that can in turn be converted to valuable fine chemicals, polymers, and fuels. The biocatalytic conversion of lignocellulose is a very promising approach due to its high selectivity, mild conditions, and low exergy loss. However, such biocatalytic processes are still seldom applied at the industrial scale since the single conversion steps (pretreatment, hydrolysis, and fermentation) may exhibit low conversion rates, low efficiencies, or high costs. The biocatalytic conversion of lignocellulose to platform chemicals is reviewed in this work. Structures and production rates of lignocellulose are described, and platform chemicals that may be produced from lignocellulose are summarized. Biocatalytic conversion of lignocellulose is distinguished from conventional non-selective approaches. All essential conversion steps used in biocatalytic approaches (pretreatment, hydrolysis, and fermentation) are reviewed in detail. Finally, potential interactions between these conversion steps are highlighted and the advantages as well as disadvantages of integrated process configurations are elucidated. In conclusion, a comprehensive understanding of the biocatalytic conversion of lignocellulose is provided in this review.

Energy and exergy analysis of hydrogen-oriented coal gasification with CO2 capture

Coal gasification is seen as an attractive way for hydrogen production. The main advantage of this approach is the low fuel price compared to reforming of natural gas. On the other hand, considering the EU climate policy, the emission of CO2 has to be reduced. The CO2 capture from the raw syngas should be thus considered. The main purpose of this work is to propose implementation of a fully “zero-emission” plant by introduction of biomass into the gasifier. The structure of the plant has been proposed and modeled. It is composed basically of gasifier, water gas shift reactors, acid gas removal units, pressure-swing adsorption process for hydrogen separation, combined cycle for power production and essential auxiliary components. The analyzed installation has been evaluated from the energy and exergy points of view by calculation of energy utilization factor and relative exergy losses occurring in main components of the system. Such an approach enables determination of the contribution of each component to the total system irreversibility. The results indicate that the highest exergy loss takes place in the gasifier. The overall exergy efficiency of the coal-to-hydrogen system is equal to ca. 57%, while the energy utilization factor is close to 65%.

Exergetic performance of eco friendly absorption heat transformer for seawater desalination

Vapour Absorption Heat Transformers (VAHT) have shown good potential for upgrading waste heat to useful temperature levels. VAHT can be effectively coupled with Multi Effect Desalination (MED) systems for water purification. In this study an exergetic analysis is carried out for VAHT integrated with MED. A simulation model has been developed to evaluate the exergetic efficiency, exergy losses in the components and exergy balance of the system. The effects of operating parameters such as heat source temperature, sink temperature, effectiveness of solution heat exchanger are evaluated on the performance of the system. Exergy efficiency varies from 11.14% to 35.35% depending upon the temperatures of heat source and sink. It is found that the highest exergy loss is in the condenser of VAHT, followed by second effect of MED.

Framework for analysis of solar energy systems in the built environment from an exergy perspective

Exergy analysis is a more powerful tool than mere energy analysis for showing the improvement potential of energy systems. Direct use of solar radiation instead of degrading other high quality energy resources found in nature is advantageous. Yet, due to physical inconsistencies present in the exergy analysis framework for assessing direct-solar systems commonly found in literature, high exergy losses arise in the conversion process of solar radiation in direct-solar systems. However, these losses are disregarded in indirect-solar systems. In this paper, contradictions and physical inconsistencies which result from including the conversion of solar radiation only for direct-solar systems are shown. An evaluation framework physically coherent for systems making direct and indirect use of solar radiation is derived and its physical correctness is thoroughly discussed. Results from case studies using the proposed framework are presented and compared with the conventional approach, enabling their direct comparison and better understanding of the benefits and correctness of the proposed method. The new method allows recognizing clearly the suitability of direct-solar systems, being appropriate for highlighting more sustainable energy supply systems. Although this paper focuses on building systems, the framework might be used for exergy analysis of direct-solar systems in the context of other energy uses.

Influence of blast-furnace process thermal parameters on energy and exergy characteristics and exergy losses

The technology ‘blast furnace—converter’ dominates at present in the production of steel all over the world. Therefore, the blast-furnace process is continuously being improved, among others, by raising the thermal parameters, such as temperature and oxygen-enrichment of the blast, as well as the addition of auxiliary fuels. The changes in the consumption of coke go together with changes in the consumption of blast, the production of top-gas and its consumption in Cowper stoves, as well as the production of electricity in the recovery turbogenerator utilizing the waste exergy of the top-gas due to raised pressure. Related to a unit of pig iron production, these values are called energy (exergy) characteristics of the blast-furnace plant. They serve as a quantity measure of the thermal improvement of the blast-furnace process. This paper presents an algorithm of the process exergy analysis of simulative investigations of the influence of increased thermal parameters on the thermodynamic perfection of the process and the blast-furnace plant. This algorithm bases on the theoretical empirical balance method of the ‘input–output’ type. By means of this algorithm the influence of increased thermal parameters of the process not only on the saving of coke but also of the blast can be determined, as well as of the production and composition of top-gas, the consumption of top-gas in the Cowper stoves and the production of electric energy in the recovery turbine. The blast-furnace process displays a high exergy efficiency, whereas the process of compressing and preheating the blast is characterized by rather high exergy losses due to the application of the combustion process. It has been shown that the internal exergy losses in the blast furnace are comparable with the exergy losses in the processes of compressing and preheating of the blast. Calculations were carried out for a modern Polish blast furnace with a volume of 3200 m3. Copyright © 2005 John Wiley & Sons, Ltd.