Exergy Balance Research Articles

A new solar share evaluation method of solar aided power generation (SAPG) system by tracing exergy flows and allocating exergy destruction

Solar thermal and coal are two major energy sources in solar aided power generation (SAPG) systems. Reasonably distinguishing the solar shares of the total electricity output is a problem that must be solved. This study modeled the energy balance, exergy balance and solar exergy balance of the key subsystems in an SAPG system. Meanwhile, this study also designed a new solar share evaluation method by tracing solar exergy flows and allocating exergy destruction. How and how much solar exergy was transferred from the sun to electricity could be theoretically estimated by analyzing the distributions of solar share and solar exergy destruction. The proposed method considered energy level differences between solar energy and coal without assuming plant efficiency or solar shares in each energy stream to be constant.The proposed method could not only be used to calculate solar-generated electricity on the point of exergy transfer and destruction, but also be further used for excavating the potential of solar share expansion.

Thermodynamic analysis and performance optimization of the supercritical carbon dioxide Brayton cycle combined with the Kalina cycle for waste heat recovery from a marine low-speed diesel engine

With the continuous rise in world oil prices and increasing environmental awareness, how to improve ship energy efficiency and reduce ship pollution emissions has become a common concern of the shipping industry. Waste heat recovery technology is an effective method to improve the fuel economy of ships and help the future ships to meet the increasingly stringent Energy Efficiency Design Index of the International Maritime Organization. Under the thermodynamic analysis results of the 8S90ME-C10.2 low-speed marine diesel engine, this paper proposed a waste heat recovery scheme that combined the supercritical carbon dioxide Brayton cycle power generation system with the Kalina cycle power generation system. According to the energy and exergy balances of the combined cycle system, a MATLAB program based on the REFPROP database was established. With the application of control variate method, the influence of the key operating parameters including the main compressor inlet temperature, the turbine inlet temperature, the main compressor outlet pressure, the expander inlet pressure, and the ammonia solution mass concentration on the system performance was thoroughly analyzed. Moreover, the multi-objective optimization matching between the diesel engine and the combined power generation system was carried out from the viewpoints of the thermodynamic performance and economic performance and the impact of the system on the fuel economy and the Energy Efficiency Design Index of the ship was calculated. The results showed that the combined power generation system was used to recycle the waste heat of diesel engine exhaust gas and bypass exhaust gas to generate electricity, which reduced the annual fuel consumption and the Energy Efficiency Design Index to 16.62% and 15.01%, respectively. Finally, this study provides a reference for researchers to study the combined use of supercritical carbon dioxide Brayton cycle and Kalina cycle to recycle the waste heat of the marine diesel engine.

Improving performance of AHU using exhaust air potential by applying exergy analysis

In this study, the exergy analysis of an AHU equipped with a heat and exergy recovery unit was investigated. The equations obtained from energy and exergy balance were solved based on a program developed in engineering equation solver. Through the air-to-air heat exchanger, the energy is transferred from the fresh air to the exhaust one but the exergy is transferred from the exhaust air to the fresh one. Therefore, the cooling coil power consumption and irreversibility were reduced. The efficacy of installing an air-to-air heat exchanger is dependent on the temperature and relative humidity of the ambient. Based on the results, at the lowest ambient temperature and relative humidity, the power consumption is reduced by 10.8%, while in the highest ambient temperature and relative humidity, this figure was 33%. Under ambient conditions with low temperatures and high relative humidity, installation of heat recovery unit reduced the irreversibility by 5.18%, while in the highest temperature and lowest relative humidity this figure was 12.8%.

The energy-saving mechanism of coal-fired power plant with S–CO2 cycle compared to steam-Rankine cycle

S–CO2 (Supercritical-CO2) coal-fired power plant is a promising technology for efficient and clean utilization of coal for power generation. The comparative study between the S–CO2 coal-fired power plant and the power plant with steam Rankine cycle from aspects of energy and exergy balances is conducted. The conversion and transfer of the energy and exergy in the power plants are revealed. With the main gas parameters of 32MPa/620 °C and double-reheat process, the power generation efficiencies of the S–CO2 coal-fired power plant and the power plant with steam Rankine cycle are 49.06% and 48.12%, respectively. The corresponding exergy efficiencies are 48.02% and 47.10%, respectively. The energy-saving mechanism of the S–CO2 coal-fired power plant is revealed: the smaller boiler efficiency and larger exergy efficiency of the boiler system in the S–CO2 coal-fired power plant make the energy level of the energy being transferred to the S–CO2 cycle is higher than that of the energy being transported to the Rankine cycle. The CO2 absorbs the high-level energy and produces more mechanical power through the S–CO2 cycle to obtain higher power efficiency.

Thermodynamic analysis of an Energy-Water-Food (Ewf) nexus driven polygeneration system applied to coastal communities

Continued rise in global human population, per capita consumption, urbanization and migration towards coastal cities present challenges in fulfilling the energy, water and food demands of coastal communities in sustainable manner. In this regard, as a solution to the problem, a new multigeneration system is proposed to address some of the most common and vital needs of such communities. The system developed is based on principles of sustainability and decentralisation and is driven by renewable energy sources including sun and biomass. It provides electricity, fresh water, hot water for domestic use, HVAC for space air-conditioning and food storage, in addition to hot air for food drying. In the proposed hybrid system, biomass energy is integrated with solar energy in a complimentary manner as a means to maximise outputs and enhance system resilience against weather conditions and day/night cycles. Designing for resilience enables a type of operation that fulfils parallel demands in a continuous stable and flexible operation which can be optimised depending on the requirements. The main sub-systems used in the proposed multigeneration system consist of a Biomass combustor, Concentrated Solar Power (CSP), a Rankine Cycle, a desalination unit and an Absorption Cooling System (ACS). A comprehensive integrated thermodynamic model of the entire system is developed by application of energy, mass, entropy and exergy balance equations. Moreover, effects of various inputs and environmental variables on the outputs and performance has also been studied. Results reveal that the proposed system is capable of fulfilling some of the coastal community’s essential requirements in an efficient and ecologically benign manner. The energy and exergy efficiencies of the proposed system are 55% and 18%, respectively. The outputs of the system include 1687 m3/day of produced fresh water, ~4 MW of cooling, ~13 MW of electricity, ~73 kg/s of hot air for food drying, and ~41 kg/s of hot water for domestic use. Furthermore, the highest amount of exergy destruction is observed in biomass combustion unit and the solar PTCs.

Human body exergy consumption in the thermal environment of shelter gymnasium in winter

This study attempts to evaluate the thermal stress in the winter shelter gymnasium in terms of human body exergy balance analysis. The human body's exergy consumption rate was quantitatively analysed, revealing that the total human exergy consumption differs depending on the difference in heat insulation performance of the evacuation gymnasium. Integrated exergy consumption after 30 days is 15.7 MJ/m2 for an 'existing outer skin with no heating', 14.4 MJ/m2 for an 'existing outer skin with radiant heating', and 12.5 MJ/m2 for a 'modified outer shell with no heating' It decreased to 9.6 MJ/m2 in the 'rehabilitating envelope with radiant heating'. In the case of existing hulls, accumulation of thermal stress was found to be greater than in the case of renovation hulls.

The Thermo-Dielectric Behavior of Biological Multi-Layer Structures In Dielectric Fields

The article propose an original bioengineering protocol (BPE) for thermo-dielectric behaviour prediction and an reverse bioengineering protocol (RBEP) for optimal tailored-end response based on controlled dielectric field distribution and optimal exergy balance in complex biological multi-layers structures (MLS).The prediction of MLS thermo-dielectric behaviour were proposed to be realise with an three bio-markers kit (water, sodium chloride, vitamin B12). The investigation of relationship between the polarization level and temperature profile and between the subsequently conduction effect and the reological parameters characterize the direct modeling of MLS dielectric response. The temperature profile and Laplacian stress pressure profile in MLS define the thermo-dielectric response correlated with designed dielectric antenna biomarkers chart. The temperature profile (the time-temperature history) was obtained by thermal transfer iteration using the Fourier law, as first order thermal kinetics model of heat distribution in radial slabs. The finite time and gradient assumptions were used in Fourier model for assessing the uniform volumetric heating rates. Controlled activation level procedures with screening of exergy distribution in dielectric field bio-captors was originally described in the current reverse bioengineering protocol for maximal thermal equivalent conversion of electromagnetic field energy. The MLS tailored-made response reached regulating the specific thermo-dielectric impact with MLS dielectric bio-captors (nano-particles with specific Curie point: iron/iron oxide, iron yttrium/iron yttrium oxide, zinc /zinc oxide) define the optimal reverse bioengineering protocol. An exergy optimization model was genuine proposed for efficient energy distribution in biological MLS (transfer, conversion and loose coefficient).

Heat transfer analysis of energy and exergy improvement in water-tube boiler in steam generation process

Steam generation is an essential process in steel production. Saturated steam is used in the cold-rolling mills of sample steel company to raise the acid pool temperature. The studied water-tube boiler had a rated saturated steam generation capacity of 47 tons per hour. This study used energy and exergy balance equations for each of the components in the cycle to calculate exergy efficiency and irreversibility rate. Results from the exergy analysis along with the exergy dissipation rate of the cycle were obtained. Based on the literature results, it was recommended to increase the input air temperature by means of a preheater. First, the exergy dissipation in the boiler was reduced by 68 kJ kg−1 with the installation of an economizer. The most important cause of exergy destruction was then presented, and some recommendations were provided on how to increase the temperature of the makeup water in order to increase the boiler efficiency and exergy.

The relation between thermal comfort and human-body exergy consumption in a temperate climate zone

Human body exergy balance calculation method gives minimum human body exergy consumption rates at thermal neutrality (TSV = 0) providing more information on human thermal responses than other methods. The literature is lacking the verification of this method in various climatic zones. The aim of this study is to investigate the relationship between thermal comfort and human body exergy consumption in a temperate climate zone. A small office building in Izmir Institute of Technology campus, Izmir/Turkey, was chosen as a case building and equipped with measurement devices. The occupant was subjected to a survey via a mobile application to obtain his Thermal Sensation Votes. Objective data were collected via sensors and used for predicting occupant thermal comfort and for exergy balance calculations. Under given conditions, the results show that Thermal Sensation Votes are generally zero at a Ti range of 21–23 °C and, are mostly lower than Predicted Mean Votes in summer while the opposite is observed in winter. Predicted Mean Votes at minimum Human Body Exergy Consumption rates were on slightly warm side while Thermal Sensation Votes are zero. It means that for given case, the HBexC rate calculation gave a better prediction of the environmental parameters for the best thermal comfort.

Detailed study of key boundary parameters influence on a turbocharged diesel engine based on thermodynamic analysis

In present study, the test platform and computational fluid dynamics simulation model of a turbocharged diesel engine are employed to explore energy balance, exergy balance, exergy terms variation, exergy destruction formation causes and spatial distribution in wider adjustment ranges of intake air temperature, coolant temperature and exhaust gas recirculation rate at different working conditions in detail. The results indicate that first, adjusting intake air temperature from 30 °C to 70 °C, exergy efficiency, exergy destruction and exhaust energy are negatively correlated with intake air temperature, while the change trends of exhaust exergy and heat transfer terms are opposite. Secondly, enhancing coolant temperature from 50 °C to 90 °C, heat transfer terms decreases significantly and effective work increases, while exhaust terms and exergy destruction remain basically unchanged. Third, exhaust gas recirculation rate increases from 5% to 30%, heat transfer exergy decreases, while effective work increases first and then decreases, in-cylinder exergy destruction decreases first and then increases. Fourth, there are several characteristics of in-cylinder exergy destruction at different key boundary parameters: (a) exergy destruction chiefly occurs in combustion process, (b) high exergy destruction rate primarily focuses in richer mixture region, (c) exergy destruction rate is negatively correlated with in-cylinder local temperature in the same heat release rate region. Finally, root reasons for the influences of key boundary parameters on exergy destruction are in-cylinder local equivalent ratio and temperature. Adjusting of intake air temperature, coolant temperature and exhaust gas recirculation rate reasonably to enhance temperature and promote diluted mixing of oil-air appropriately can effectively suppress exergy destruction and optimize efficiency.

Exergoeconomic analysis of a solar assisted tea drying system

In this study, an integrated system which consists of a batch type tea dryer and a PV/T unit is developed and analyzed through the exergoeconomic approach. The EXCEM method based on mass, energy, exergy, and cost balances is performed to find out the exergoeconomic performance of the drying system. The parametric studies are used to see the effect of changing properties utilized in the system on performance. The results of the present study show that the capital cost and the capital productivity of the system are $5953 and 1.54, respectively. The energetic and exergetic loss ratios are calculated as 76.45 MJ/$ and 72.63 MJ/$, respectively. The exergy efficiency and exergy destruction for the whole system are found to be 74% and 201.6 GJ, respectively.

Novel Affordable, Reliable and Efficient Technologies to Help Addressing the Water-Energy-Food Nexus

Improving the food system sustainability and security is becoming an urgent global challenge. In this regard, one of the most effective routes is the shift of the human diet toward healthier and more sustainable consumption, involving in particular the prevalence of plant-based raw food materials. Controlled hydrodynamic cavitation (HC) technologies could help considerably in this transition. HC techniques are gaining increased scientific interest, and are quickly spreading across a wide range of technical fields, recently showing surprising performances with biological raw materials related to the food, agricultural and forestry sectors and resources. HC processes enjoy recognized advantages in the acceleration of the processing steps of plant-based food, the extraction of valuable bioactive compounds, the reduction and the valorization of waste streams, as well as the superior efficiency in resource use, energy consumption, process yield, and exergy balance than competing processes. Thus, HC is very promising candidate to help addressing the water-energy-food nexus, and, ultimately, sustainability. Findings obtained from direct experimental trials and recent literature concerning the applications of HC to food processing, provide a strong basis for novel investigation aimed at standardization, starting from the identification of the most suitable devices and the optimal processing parameters, eventually oriented to further spreading of HC applications.

Quantitative Analysis of Carbon Emissions in Precision Turning Processes and Industrial Case Study

With growing concerns regarding the global environment, the industrial sector has played a significant role; that is, it is responsible for consuming large amount of energy and resources, while simultaneously producing wastes and carbon dioxide. A quantitative calculation of carbon emission in the turning process is presented in this paper. A generic carbon emission system boundary based on exergy balance is proposed first to avoid blurred system boundaries or the omission of elements. Then, a carbon emission model (iERWC) is formed by converting energy consumption (E), resource depletion (R) and waste generation (W) to equivalent carbon emissions (C) based on information flow (i), which effectively solves the problem of quantifying the impact of the machining process on the environment. Finally, the model is verified by experiments, and a simulation analysis is carried out. Additionally, the influence rule of processing parameters on carbon emissions is analyzed, and the cutting parameter that produces the lowest carbon emission is given.

Thermodynamic assessment of the integrated gasification-power plant operating in the sawmill industry: An energy and exergy analysis

Biomass thermochemical conversion into heat and electricity is a promising technological alternative for the management of the biomass residues from the sawmill process. In this study, an energetic and exergetic analysis for syngas production from biomass gasification has been performed, including its potential use for heat and power generation. A stoichiometric model of biomass gasification with air as gasifying agent was accomplished to evaluate the syngas production and the potential energy recovery from pinewood chips residues. From the thermodynamic analysis of the biomass residues gasification process and syngas production, it was observed that a cold-gas and hot-gas efficiencies close to 74.5% and 84.6% could be achieved by considering an ER ratio of 0.34, respectively; while energy losses represented 15.3% of the total energy input to the gasifier. Furthermore, an exergy balance of the integrated gasification-power plant (IGPP) was considered. Biomass gasification and power generation processes showed a higher contribution to the total destroyed exergy; reaching values of 42.4% and 45.5% of the total destroyed exergy, respectively. According to energy balance, the IGPP and heat recovery from exhaust gases could supply 52.6% of electricity and 38.9% of thermal energy requirements for the sawmill process.

Performance enhancement of a humidification–dehumidification desalination system

The purpose of the present work is to investigate humidification–dehumidification desalination system and to explore the effect of pertinent parameters on the overall performance of the process taking in account the irreversibilities and energy losses. The system has been inspected using first and second laws of thermodynamics, and an optimization of the performance along with design development has been performed based on mathematical calculation and modeling for the fundamental equations associated with mass, energy, exergy and salinity balance incorporating the effects of irreversibilities and thermal losses which in turn helps in establishing an efficient desalination system by reducing these losses. The results show a good improvement compared to previous studies. The model target is to increase heat exchange in humidifier and dehumidifier compartment as well as augmenting pure water capacity and lessening energy consumption. Results expose that the inlet water temperature and flow rate represent the main factors affecting the system performance. It is found that the heater has the main part of exergy losses. Increasing the temperature of the water in the dehumidifier outlet allows minimizing the exergy losses in the dehumidifier.

Comparison of Exergy Losses for Reformers Involved in Hydrogen and Synthesis Gas Production

AbstractA user‐coded, customized methodology for exergy balance and analysis of chemical and thermal processes is developed with the Aspen HYSYS process simulator, to assess and explain exergy loss in steam methane reforming. Exergy analysis is presented for four configurations of primary and secondary reformers to produce synthesis gas: (1) an autothermal reformer (ATR) alone, (2) a top‐fired reformer (TFR) alone, (3) ATR‐TFR configured in parallel, and (4) ATR‐TFR configured in series. The same states and feed conditions, i.e., mass flow rate, temperature, pressure, and feed gas composition, are applied to the four reformer configurations considered, with the single ATR showing the lowest exergy loss of about 0.43 W kg−1 of dry productivity.

Exergy and Energy Analysis of Coal Gasification in Supercritical Water with External Recycle System

Abstract Supercritical water gasification (SCWG) is a novel and clean technology for lignite translating into hydrogen-rich gas. Previous experimental researches show that the use of external recycle system of liquid residual can improve the energy efficiency, but there is not a theoretical model to figure out the component of which exergy lost most and to provide guidance for further optimization of the existing system. In this paper, the thermodynamic model of liquid residual external recycle system was established, based on which energy and exergy balance of the system was evaluated and the exergy efficiency of the main equipment was calculated. Moreover, the influence of recycle flow ratio (0–37.5 %), gasification temperature (550 °C–650 °C), gasification pressure (23–25 MPa) and slurry concentration (2.73–4.15 %) on the exergy and energy efficiency were analyzed. The results showed that the exergy destruction rate of reactor was the highest, which reached 5.52 kW. Both energy and exergy efficiency increased as recycle flow ratio, gasification temperature and pressure increased. The energy and exergy efficiency of the system reached 70.26 % and 56.86 % respectively at the condition of recycle flow ratio of 30 %, gasification temperature of 650 °C, pressure of 25 MPa and slurry concentration of about 2.93 %.

Effects of dimethyl carbonate and 2-ethylhexyl nitrate on energy distribution, combustion and emissions in a diesel engine under different load conditions

Dimethyl carbonate (DMC)/diesel blended fuels are receiving increasing attention due to the advantages of DMC (oxygen content 53.3%) that improved the fuel-rich zone of diesel engines. However, the low cetane number (CN) of DMC/diesel dual-fuel at low loads resulted in a delayed combustion progress. Since the combustion phase had a significant effect on thermal efficiency, exergetic efficiency, energetic and exergetic losses; 2-ethylhexyl nitrate (EHN) was added to the mixed fuel as a CN improver, so that the combustion phase closed to that of diesel to ensure proper combustion characteristics of the DMC/diesel dual-fuel. In this study, a four-cylinder turbocharged diesel engine was used to study performance and emissions. Based on experiments, combustion characteristics including energy balance and exergy balance were focused, in which the EHN was added to the DMC/diesel mixture at different concentrations. The five test fuels included diesel (D100), and a mixture of 20% DMC with 80% diesel (DMC20). In addition, EHN was added to DMC20 at a ratio of 0.5%, 1%, and 2%. The results showed that EHN could shorten the ignition delay time of DMC and reduced maximum pressure rise rate (MPRR). The use of EHN improved brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC), under the same load, using 0.5% and 2% EHN, BTE increased by 0.2% and 1%, relative to DMC20, respectively. The study found that the energy efficiency and exergy efficiency were higher after adding EHN, which made up for the burning problem of DMC. Moreover, the irreversible loss reduced, and the available work and available energy increased. Thermodynamic analysis showed that DMC had better combustion performance than diesel after adding 2% EHN. In this study, the method of EHN/DMC/diesel blended fuel combining with 25% EGR was proposed and confirmed that the trade-off relationships of NOX and soot have been improved noteworthy.

The effect of temperature and amount of water in co-injection of diesel-water on exergy and irreversibility rate of VGT diesel engine

The water introduction to combustion chamber can leave two-sided effect on diesel engine performance, thus its role on combustion must be delineated. Accordingly, three levels of water amount in 14.21, 21.31, and 28.41 mg/cycle are adopted to be injected from an upper angled nozzle hole, while fixed diesel amount of 31.3 mg/cycle is injected at 3 °CA BTDC from lower angled nozzle. The water temperature is also taken into account in two levels of 27 °C and 60 °C. By high water injection (HWI) at 60 °C, the rate of pressure rise increases by 17.1% compared to the base case (no water injection) due to steam pressure in the cylinder. The results indicate that WI scheme is effective for fuel consumption, NOx, and soot reduction. In terms of exergy balance by input fuel exergy, it is concluded that the more heat loss exergy is transferring to work exergy in low and medium water injection, while the fuel burn exergy increases drastically. Altogether, the case of diesel injection with high water amount at 27 °C water temperature, i.e. “diesel-HWI-27 °C” is recommended from exergetic viewpoint since it represents the highest total exergy (838.31 J) and the lowest irreversibility (132.98 J).

Energetic-exergetic analysis of an air handling unit to reduce energy consumption by a novel creative idea

PurposeThis study aims to simulate the flow and heat transfer through an air handling unit to reduce its energy consumption by a novel creative idea of using an air-to-air heat exchanger.Design/methodology/approachTo do this, both first and second laws of thermodynamics energy and exergy balance equations were solved numerically by an appropriate developed computer code.FindingsUsing the air-to-air heat exchanger in dry conditions decreases the cooling coil load by 0.9 per cent, whereas the reduction for humid conditions is 27 per cent. Similarly, using air-to-air heat exchanger leads to an increase in the first law of efficiency in dry and humid conditions by 0.9 per cent and 36.8 per cent, respectively.Originality/valueThe second law of efficiency increases by 1.55 per cent and 2.77 per cent in dry and humid conditions, respectively. In other words, the effect of using an air-to-air heat exchanger in humid conditions is more than that in dry conditions.