Exergy Balance Research Articles

Energy, exergy and economic analysis of a hybrid spray-assisted low-temperature desalination/thermal vapor compression system

Integrating thermal desalination systems with vapor compression is an effective way to improve the energy efficiency. This paper investigates a spray-assisted low-temperature desalination system that is integrated with a thermal vapor compression system (SLTD-TVC). A detailed thermodynamic model is judiciously developed based on the principles of heat and mass transfer, heat balance, mass balance, and exergy balance. Applying the model, the energy efficiency of the combined SLTD-TVC process is first evaluated. The production ratio of the combined system is found to be 10–35% higher than that of the conventional SLTD process. Accordingly, an exergy analysis is conducted to quantify the sources of irreversibility within the system. The steam jet ejector is found to be the major source of thermodynamic irreversibility, accounting for more than 40% of the exergy destruction. The overall system efficiency is improved at a lower motive steam pressure, a higher number of operating stages and a medium cooling water flowrate. Finally an economic analysis is carried out, which reveals that the changes of both initial plant cost and operation cost are marginal after the integration of the thermal vapor compression system.

Exergoeconomic optimization of liquefying cycle for noble gas argon

Today, gas condensate has an important application in industries. One of the high consumption liquid gases is Argon which is used different methods for its liquefaction. However, the costs of this procedure is very high.In this paper, the best cycle from thermodynamic and economic point of view was achieved by thermoeconomic analysis. For this purpose, first, some applicable cycles were investigated and then, liquefying cycle was chosen. In order to optimize cycle, thermoeconomic equations including exergy balance, entropy and energy balance were applied for each component,. Performance coefficient values for dual pressure and pre-heated Linde Hampson cycles are achieved respectively by 2 and 1.8 times multipling by simple cycle. Also, the best performance coefficient of each compressor pressure ratio were evaluated in low ratios, comparing to other pressure ratios. On the other hand, it was illustrated that very low and very high pressure ratios are far from optimum circumstances and just for compressors, the best efficiency can be achieved in relatively low pressure ratios.

Performance analysis and multi-objective optimization of an organic Rankine cycle with binary zeotropic working fluid employing modified artificial bee colony algorithm

From a thermal point of view, zeotropic mixtures are likely to be more efficient than azeotropic fluids in low-temperature power cycles for reduction in exergy destruction occurring during heat absorption/rejection processes due to their suitable boiling characteristics. In this study, comprehensive energetic and exergetic analyses are mathematically performed for an organic Rankine cycle (ORC) system employing a potential binary zeotropic working fluid, namely R717/water. For this purpose, initially mass, energy, and exergy balance equations are derived. With regard to the similarity in molar mass of R717 (17.03 g mol−1) and water (18.01 g mol−1), there is no need to alter the size of the ORC components such as turbine and pump. In order to achieve the optimal thermal and exergy efficiencies of the ORC system, modified version a powerful and relatively new optimization algorithm called artificial bee colony (ABC) is used taking into account different effective constraints. The main motivation behind using ABC lies on its robustness, reliability, and convergence rate speed in dealing with complicated constrained multi-objective problems. Convergence rates of the algorithm for optimal calculation of the efficiencies are presented. Subsequently, due to the importance of exergy concept in ORC systems, exergy destructions occurring in the components are computed. Finally, the impacts of pressure, temperature, mass fraction, and mass flow rate on the ORC thermal and exergy efficiencies are discussed.

Integrated power generation cycle (Kalina cycle) with auxiliary heater and PCM energy storage

Kalian cycle is a modified Rankine cycle for power generation, that its working fluid is a mixture of ammonia-water instead of pure water and can be driven in higher temperatures comparing to the Rankine cycle. In this study a hybrid Kalina power cycle and solar thermal flat plate collector using phase change material (PCM) as thermal energy storage material is introduced and analyzed. Bandar Aabas (a city in south of Iran), because of its sunny days and use of the Persian Gulf sea water for cooling in the condenser has been selected. The amount of solar fraction in year is 83.8%. This integrated process requires 53.85 GWh heat load during the year that 45.13 GWh of energy per year in the system, is supplied by the sun during the hours of 7–19 and released energy from the PCM during the night from 19.01 to 6.59. Rest of the needed heat load for the integrated system of 8.72 GWh per year is provided with auxiliary heater. By using energy and exergy balance equations, all components were analyzed. According to the results, thermal efficiency of the Kalina cycle is 6.12%. Results of the exergy analyses show 23.21% of the total destroyed exergy in the cycle, occurred in one of the heat exchangers as the main component wasting exergy. Also it was found that most the exergy efficiency is related to the one of the heat exchangers and flash separator with 99.24% and 99.11%, respectively.

Определение термодинамической эффективности реакторов частичного окисления энергохимических установок

Актуальность исследования обуславливается необходимостью повышения энергоэффективности в области использования углеводородов для комбинированной выработки химической продукции и энергоносителей с применением технологий газификации. Это даст возможность снижать издержки производства и расширять возможности газохимии. Цель исследования: определение термодинамической эффективности реакторов частичного окисления топлива, входящих в состав энергохимических установок комбинированной выработкой энергоносителей и синтез-газа. Основное внимание уделено влиянию режимных параметров на численное значение эксергетического КПД. Объект: реактор частичного окисления с набором вспомогательного оборудования, обеспечивающий получение синтез-газа. При этом учитывается системный фактор, т. е. параметры энергосистемы, в рамках которой планируется сооружение подобного типа энергохимической установки. Методы: проведение численного эксперимента с привлечением традиционных подходов к составлению эксергетического баланса теплоэнергетических установок. Также использованы разработанные ранее авторами математические модели для расчета параметров синтез-газа, образующегося в процессе переработки углеводородного сырья в проточных автотермических некаталитических реакторах. Результаты. Полученные результаты позволяют выбирать наиболее целесообразные режимные параметры работы реактора частичного окисления, входящего в состав энергохимических установок с газификацией нестабильного газового конденсата. Так, максимальные значения эксергетического КПД достигаются при использовании атмосферного воздуха без обогащения кислородом как для случая с подачей водяного пара (ŋ ex =78-83 %), так и для случая без подачи пара (ŋ ex =82-88 %). При этом некоторые варианты тепловых схем энергохимических установок с газификацией топлива в термодинамическом плане могут считаться равноэффективными, а решение по количеству подаваемого водяного пара в зону реакции должно соответствовать требованиям к синтез-газу. Выбор уровня рабочего давления в реакторе требует дополнительных исследований, поскольку очевидность применения реактора атмосферного типа на данном этапе исследований не выявлена.

Performance analysis of a solar chimney power plant design aided with reflectors

A new design of a solar chimney power plant (SCPP) with enhanced incident solar radiation, realised using reflectors, was developed and validated. A detailed thermodynamic analysis was performed to access the performance of the new SCPP design, and its performance was compared with a conventional SCPP. All the formulated energy and exergy balance equations were solved simultaneously using engineering equation solver software. Study reveals that the increase in incident solar radiation using reflectors enhances the efficiency and power output by 22.61% and 133%, respectively. The improvement in power output is due to the higher mass flow rate. Furthermore, a laboratory scale working model of an SCPP aided with reflectors was built to determine how well the empirical results agree with the results obtained using the analytical model developed in the present study. The model developed predicts the empirical values of temperature with an accuracy of 5%. The proposed model was evaluated in Dhahran, Saudi Arabia to be used as an illustrative example. For solar irradiation data of 2017, the average energetic efficiency and the power output for the SCPP design aided with reflectors are 0.641% and 230 kW, respectively. Moreover, the power output, energy efficiency, variation of the temperature of the floor and air, variation of the mass flow rate and inlet velocity of the turbine, and the density variation of air in the collector for each month of the year are reported. Also, the results of the comparative study with a conventional SCPP are presented.

Advanced exergy analysis of heat exchanger network in a complex natural gas refinery

In this paper, the advanced exergy analysis was used to determine the avoidable and unavoidable exergy destructions of the heat exchanger networks (HENs) in a complex natural gas refinery in the South Pars gas field, focusing only on the improvement of the avoidable part. Exergy balances were evaluated for the HENs and the equations of advanced exergy destruction and exergetic efficiency for the HENs were developed. The total exergetic efficiency of the HEN in the plant was determined to be 62.8% that could be increased up to 84.2%, suggesting a high potential for improvement. Also, it was shown that the HEN1 had the most severe condition in the plant with the highest inefficiencies among the other networks. The avoidable and inevitable irreversibilities were calculated for all the heat exchangers running in the plant network. The advanced exergy analysis revealed that the exergy destruction had two major contributors. First, about 59% of the total irreversibility of the system was avoidable and could be eliminated by the well-known optimization techniques. Secondly, only 18 most inefficient heat exchangers (17% in numbers) contributed to more than 61% of the total exergy destruction in the plant network. Hence, there was a high potential for improvement of the operational cost in such a huge consuming energy system.

Performance assessment and optimization of a humidification dehumidification (HDH) system driven by absorption-compression heat pump cycle

Feasibility investigation of a humidification dehumidification (HDH) desalination system driven by an absorption-compression heat pump cycle (ACHPC) is carried out in this paper. The proposed hybrid desalination system uses both heating capacity of the discharged brine as a waste heat for the ACHPC and mechanical power of the ACHPC for the HDH system. The system's performance is investigated under different optimal design modes, using genetic algorithm (GA) as the most robust tool for optimization. A steady-state mathematical model based on the mass, energy, exergy, and exergoeconomic balance equations for components of the proposed system is developed and the predicted results are validated using the available experimental and numerical data. The results indicated that the proposed hybrid system can produce the maximum freshwater of 0.647 kg/s with the gain output ratio (GOR) and exergy-based GOR (EGOR) of 9.02 and 3.04%, respectively. In this optimal mode, the cost of produced freshwater and specific work consumption (SW) are estimated 7.13 $/l and 262.9, respectively. Furthermore, a comprehensive parametric study is conducted leading to the determination of the proposed system's behavior due to any variation in the key thermodynamic parameters. It is found that the GOR and EGOR can be maximized with the desalination top temperature, desalination mass flowrate ratio, and compression ratio, while the cost of freshwater can be minimized with the absorber terminal temperature difference and compression ratio. At last, multi-variable regression analysis is employed to correlate the main performance criteria of the system with the key thermodynamic parameters.

Improvement of a Biorefinery from Palm using Exergetic Sensibility Analysis

Background: Vegetable oils have received a great attention to reduce depending on fossil fuels because of its biodegradable, low toxicity and being derived from renewable resources. Palm oil is the most important vegetable oil and it is transformed commercially into numbers of product. Nevertheless, great amount of energy have been wasted in oil production when residues such as palm rachis and palm cake are not used to obtain value-added products. Objectives: In this work, sensibility analysis was performed to palm-based biorefinery in order to evaluate the effect of process variations (stream leaving hydrogen separation stage and considering sludge as products instead of wastes) and exergy efficiency of gasifier on global exergy efficiency. Methods/Analysis: Physical and chemical exergies of process stream were quantified using a robust commercial simulation software. An exergy balance was performed to determine total exergy entering and leaving the system. Findings: The global exergy efficiency was 37.90% for original process, however, when sludge were considered as products, global exergy efficiency was 37.93%, showing a non-significant increase. In addition, exergy efficiency of gasifier did not affect considerably global exergy efficiency and the total exergy of wastes was reduced when stream of selexol leaving separation stage was assumed as product. Novelty/Improvement: These results indicated that exergy efficiency of palm oil; palm kernel oil and hydrogen production could be improved through adding commercial value to selexol wastes. Keywords: Exergy, Efficiency, Hydrogen, Palm, Vegetable oil

Design of biodiesel and bioethanol production process from microalgae biomass using exergy analysis methodology

Microalgae are a promising source of energy and its principal advantage that does not compete with the feeding supply. Besides, microalgae have other benefits, such as the use of infertile land for its cultivation and seawater and flue gas as nutrients source. Exergy analysis is a useful tool for measure the quantity and quality of the energy sources and studies the sustainability process. This methodology requires analysing material and energy flows of each stage of the production process. In this study, exergy analysis was applied to one scenario taking a production capacity of 100 t/y of biodiesel from microalgae biomass. Chlorella sp was used as reference species. This strain has been widely studied, and their characteristics are known. The process designed have five stages: cultivation, harvesting, drying, oil transformation, and residual biomass transformation. The process was simulated using specialized software. The mass, energy and exergy balances were performed for each stream of the process. Finally, the exergy analysis was developed, and it was found that the residual biomass transformation stage with 32 % value present the lowest exergy efficiency of the process, followed by oil extraction stage with an efficiency of 52 %. The overall exergy efficiency of the process was 89 %, which indicates that designed process is adequate and have potential from the energy point of view.

Variable refrigerant flow cooling assessment in humid environment using different refrigerants

Variable Refrigerant Flow (VRF) is a cooling system developed to anticipate and minimize operating and maintenance costs. VRF allows personalized control and maximizes flexibility to accommodate changing tenants in high rise and compound buildings in hot and humid environments. Although many studies have previously modeled VRF systems from an energy perspective, minor attempts have been made to analyze the exergy performance of this technology. The aim of this paper is to present an exergy/energy analysis of VRF technology to address the effect of refrigerant flow as well as the cooling-air flow rates on the electric energy saving in the hot and humid zones. This analysis will be then used to investigate its performance in humid areas, particularly the Gulf region. VRF is an advanced air conditioning system that is developed to manage load variability by controlling the compressor speed and the expansion valve opening. It is proposed that the system be implemented at Masdar City buildings, located in Abu Dhabi.In this study, VRF units from different manufacturers were modeled and compared using engineering equation solver (EES) and IPSEpro software. The models, which were mainly developed on EES were repeated for validation on similar models that have been developed using the IPSEpro software, and the results were in agreement within 6% uncertainty. Parametric studies were done after modeling the system on EES, where both the high and low pressures in the cycle were varied to obtain the corresponding COP and second law efficiency operating range. It was noticeable that COP and second law efficiency are significantly affected by the evaporator and condenser temperatures and pressures. In addition, as significant concern has been raised due to the impact of refrigerants on global warming, different refrigerants were considered and the results showed that refrigerant R-410a would be the second most efficient refrigerant, after ammonia, for such systems. Finally, an effectiveness NTU evaporator model was implemented to minimize the overall power consumption of the VRF system under various zone loads. It was found that, under a typical set of zone loads, the optimal refrigerant evaporating temperature is 11 °C, when the sum of indoor fan power and outdoor unit power is minimized using the exergy balance. These results helped decision makers to install the VRF system for the first time in that hot and humid area. The preliminary results for May and June sowed an average energy saving of 32% with variable evaporators and condenser speed and variable compressor frequency.

Improving performance of an inverted absorber multi-effect solar still by applying exergy analysis

In this paper, the energetic and exergetic analyses of an inverted absorber multi-effect solar still has been studied. A new criterion was defined to determine the efficiency of the first law of thermodynamics for multi-effect solar stills. Energy and exergy balance equations have been developed for water basins, condensing surfaces and absorber plate to evaluate the irreversibility through the various components. Heat and modified mass transfer coefficients along with accurate properties of humid air were used to solve the governing system of differential equations. Results affirmed that the exergy loss through the absorber plate, water basins and condensing covers in a certain basin decrease as the number of effects increase. However, the increase in basins number from one to ten leads to increase in total irreversibility through the water basins by 337%. Calculations on energy balance equations showed that with increase in effect from one to ten, heat transfer to ambient decreased by 74.8%, which in turn increased the first law efficiency by 174.6%. It is found that with increase in effect from one to ten, the total yield, irreversibility and overall second efficiency increased by 407.3%, 10.4% and 471%, respectively, but global second efficiency decreased by 20.6%.

Energy analysis of two-phase secondary refrigeration in steady-state operation, part 2: Exergy analysis and effects of phase change kinetics

A great deal of attention is paid to secondary refrigeration as a means of reducing excessively high emissions of refrigerants (most of which have a potent greenhouse effect) due to leaks in large cooling units. Among the environmentally friendly fluids that can be used in secondary circuits for transporting and storing cold, hydrate slurries offer the advantage of significant latent heats of fusion associated with good fluidity. Research programs have focused attention on hydrate systems, including CO2, TBPB (tetra-n-butyl-phosphonium-bromide), and mixed CO2-TBPB hydrates. In addition to feasibility concerns, energy efficiency is also a crucial concern requiring an objective analysis of the improvements likely to result from these new materials. A numerical model of secondary refrigeration system was built for slurries ranging from ice-slurry to TBPB-CO2 mixed hydrate slurries. The circuit was designed for maximizing performance under various constraints relating to power rate, heat transfer areas, and flow ability. The effects of phase change kinetics on thermal exchanges were introduced in the model and in the exergy balance. The results, analyzed in terms of exergy losses, demonstrate the couplings through which kinetics influences global performance.

Studies of the unavoidable exergy loss rate and analysis of influence parameters for pipeline transportation process

According to the exergy balance relationship among the various items in the crude oil transportation process system, the exergy balance equation is established and each exergy loss is calculated. In a sense, the essence of exergy analysis method relies on the calculation and the analysis of system exergy loss. The unavoidable exergy loss is defined based on the required minimum exergy loss from the technology standpoint during the pipeline conveying process. In order to truly reflect the degree of effective energy utilization, this article puts forward the unavoidable exergy loss rate as evaluation index, which is based on the exergy analysis criterion consisted by exergy loss coefficient and exergy loss rate. Taking an oil pipeline as an example, the changes of the unavoidable exergy loss are studied under different designed parameters. And the unavoidable thermal exergy loss is the dominant position. Change situations of the unavoidable exergy loss rate are explored. Through analysis, the trend and the regularity of the unavoidable total exergy loss rate are identical with the unavoidable thermal exergy loss rate. Then the orthogonal experiment method is used to compare the different influence degrees to the pipeline unavoidable exergy loss rate. The results are as follows: diameter, insulation thickness and buried depth, which can provide the reference for the energy-saving transportation of crude oil pipeline.

Multi-generation Renewable Energy System for Dairy Farms: Exergy Analysis

Exergy analyses of multi-generational micro gas turbine systems are reported for a grid-independent dairy farm in Ontario, Canada, with an aim of being environmentally benign. Onsite anaerobic digesters utilize farm waste to produce carbon neutral biogas for combustion in the micro gas turbine modules. A range of micro gas turbines coupled with absorption refrigeration units and an organic Rankine cycle are driven by the recovered waste heat to meet the cooling and electrical needs of farm sizes between 250 and 6000 cows. Exergy balances are applied to each component as well the overall system configuration, and exergy efficiencies are obtained. Small farm sizes with one absorption cooling unit are found to be more exergy efficient than large farm sizes with more than one absorption cooling unit, but the difference is less than 0.5%. Most of exergy destruction within the micro gas turbine module occurs in the combustion chamber, which contributes 79% of the exergy destruction of the system. Farm sizes between 250 and 6000 cows are observed to be capable of having their cooling and electricity needs met with micro gas turbines ranging in capacity from 100 to 1000 kW output, while exhibiting similar exergy efficiencies.

Industrial waste heat recovery: A systematic approach

Globally one third of energy consumption is attributable to the industrial sector, with up to fifty percent ultimately wasted as heat. Unlike material waste that is clearly visible, waste heat (WHE) can be difficult to identify and evaluate both in terms of quantity and quality. Hence by being able to understand the availability of waste heat energy, and the ability to recover, there is an opportunity to reduce industrial energy costs and associated environmental impacts. A waste heat energy recovery framework is developed to provide manufacturers with a four step methodology in assessing production activities in facilities, analysing the compatibility of waste heat source(s) and sink(s) in terms of exergy balance and temporal availability, selecting appropriate heat recovery technologies and decision support based on economic benefits. The economic opportunity for industrial energy recovery is demonstrated in an industrial case study. The applicability of the framework for wider industrial application is discussed.

Energy and Productivity Yield Assessment of a Traditional Furnace for Noncentrifugal Brown Sugar (Panela) Production

Noncentrifugal brown sugar (called panela in Colombia) is a natural sweetener obtained from the extraction, purification, and concentration of sugarcane juices. In this work, energy and productivity yield of a traditional furnace for panela production were evaluated, considering five performance indices. Experimental productions were developed in a pilot plant facility, analyzing furnace gas emissions of furnace and bagasse properties. Mass, energy, and exergy balances were performed. The following indices were obtained from the experimental runs: energy efficiency 12.726 ± 1.091%, exergy efficiency 9.013 ± 0.710%, energy losses through chimney 72.293 ± 11.507%, yield 0.144 ± 0.021 kgpanela/kgbagasse, productivity 7.450 ± 0.520 kgpanela/h, and bagasse consumption 1.258 ± 0.139 kgbagasse consumed/kgbagasse produced. It was found that these outcomes were strongly influenced by excess air and gas circulation velocity through the furnace, which affects the combustion rate and heat transfer between the gases and the juices. Finally, it was concluded that the traditional scheme is inefficient and requires various critical operational adjustments, such as combustion chamber, chimney draft control, and heat exchangers design.

Exergy Analysis and Optimization of Sintering Process

Iron ore sintering is an important and large energy‐consumption segment in the iron and steel industry. It is thus essential to save energy in the sintering process considering both quality and quantity. This paper takes exergy loss minimization as an optimization objective, and a sintering optimization model is established based on material balance and exergy balance. Optimized results are obtained by solving constrained multidimensional nonlinear programming problems. Two types of sintering processes, in which solid fuels consist of coke and coke/coal powder, are calculated and analyzed. The effects of fuel ratio, fuel preheat temperature, and sinter basicity on exergy losses are also studied. The results show that the exergy losses obtained from optimizations for two types of solid fuels decrease by 5.0% and 8.1%, respectively. Exergy loss has a negative correlation with the coal ratio and fuel preheat temperature and a positive correlation with the coke ratio and sinter basicity. Exergy efficiencies increase from 29.7% to 35.0% and 35.5%, for each optimization. The research in this paper is highly significant to exploring of the sintering process, guiding steel production, and reducing energy consumption.

Investigation of a combined cycle power plant coupled with a parabolic trough solar field and high temperature energy storage system

This study was aimed at exergetically investigating a combined cycle power plant coupled with a parabolic trough solar field and high temperature energy storage system. Thermodynamic modeling of the combined cycle was conducted by ASPEN HYSYS simulation software, while the modeling of solar and energy storage section was carried out using MATLAB software. The main goal of the present research work was to reveal the reasons and sources of the thermodynamic losses occurring in the power plant based on the thermodynamic outcomes obtained from modeling. After writing the exergy balance equations for all the subunits of the power plant, their exergetic performance parameters were measured individually. Moreover, the effect of different influential parameters such as gas turbine inlet temperature and outlet pressure, and the R113 working fluid mass flow rate on the exergy and electrical efficiencies of the whole system was also investigated and scrutinized. The exergetic value of the net electric power was found to be 93,580 kW. The overall exergy destruction of the system stood at 216,710 kW. The contribution of solar and energy storage combination, as the main subunit wasting exergy, to the overall exergy destruction of the system was determined at 43%. Moreover, combustion chamber subunit stood in the next rank in terms of exergy destruction, contributing 15.7% of the overall exergy destruction of the power plant. The overall exergy efficiency and electrical efficiency of the power cycle was determined at 38.2% and 47%, respectively.

A study on combustion characteristics under enriched-oxygen condition by exergy analysis

The object of this paper is to investigate the effect of operating parameters on Oxygen-enriched combustion (OEC) by exergy balance analysis during methane combustion process. In this work, the specific exergy value of fuel, feed gas and products were calculated on the basis of the second law of thermodynamics in different reaction conditions, so that availability destructions and exergy efficiencies of OEC process can be obtained based on exergy balance, and the influence of initial fuel temperature, oxygen concentration, exhaust gas recirculation ratio and equivalence ratio were addressed. It is observed that during methane OEC process, the exergy efficiency is mainly affected by products temperature, in the case of products temperature lower than 1870K, exergy efficiency of combustion increase with products temperature, and opposite changes was appeared when the products temperature higher than 1870K. Meanwhile, appropriate control of equivalence ratio can also be used to improve exergy efficiency by suppressing the dissociation of product.