Exergy Balance Research Articles

A novel combined cooling-heating and power (CCHP) system integrated organic Rankine cycle for waste heat recovery of bottom slag in coal-fired plants

The extensive use of organic Rankine cycle (ORC) in waste heat recovery motivates the employment of ORC technique to recover the waste heat in coal-fired plants. A novel combined cooling-heating and power (CCHP) system integrated Organic Rankine Cycle (CCHP-ORC) for recovering the waste heat of bottom slag in coal-fired plant is first proposed in this paper. A MATLAB procedure with REFPROP database is developed for the proposed system based on the mass, energy and exergy balances of each component. With the application of Analytic Hierarchy Process (AHP) method, R1234ze(E) and heptane/R601a are adopted as the optimal working fluids for the Sing Fluid CCHP-ORC and Dual Fluid CCHP-ORC systems, respectively. Parametric studies are conducted to investigate the effect of superheat degree of turbine inlet temperature, chilled water mass flow rate and condenser temperature on the coefficient of performance, thermal efficiency, heat exergy, cooling exergy, total exergy production and exergy production rate. The results show that the superheat degree of turbine inlet temperature benefits the enhancement of the exergy production rate. The enhancement of chilled water mass flow rate leads to an increase in coefficient of performance and refrigerating capacity while the cooling exergy, total exergy production and exergy production rate decrease. Except for exergy production rate in Dual Fluid CCHP-ORC system, the rise of condenser temperature leads to a decrease of performance parameters. As the condenser temperature rises from 25 °C to 40 °C, the thermal efficiency of Cycle2 declines 19.4% and 18.3% respectively in the Sing Fluid CCHP-ORC system and Dual Fluid CCHP-ORC system.

Computer-Aided Exergy Sensibility Analysis of Nitrobenzene Production through Benzene Nitration Using an Acid Mixture

Nitrobenzene is widely produced via benzene nitration to be applied in several industries such as pharmaceutical, textile, and agricultural. In this work, an exergy sensibility analysis was performed with the aim of identifying possible opportunities of process improvements. The irreversibilities, exergy of wastes, and efficiency were calculated per stage through exergy balance. The simulation software Aspen plus V10.1 provided the physical exergies of process streams while chemical exergies were found in literature. A sensibility analysis was also carried out in order to study the effect of efficiency of some stages (polyfunctional reaction and cooling 1) on global exergy efficiency. This analysis reveals that nitrobenzene production from benzene is an efficient process from an exergy viewpoint (88%). The total irreversibilities, total exergy of wastes, and exergy of utilities-inlet were calculated in 41,647,341.85 MJ/h, 5,537,487.3 MJ/h, and 18,137,363.71 MJ/h, respectively. The results obtained from sensibility analysis suggested that heat flow of heat exchangers HE-201, HE-303, and HE-301 could provide energy requirements for heating.

Comprehensive exergoeconomic analysis of a municipal solid waste digestion plant equipped with a biogas genset

A comprehensive exergoeconomic performance analysis of a municipal solid waste digestion plant integrated with a biogas genset was conducted throughout this study in order to highlight its bottlenecks for further improvements. Exergoeconomic performance parameters of each component of the plant were determined by solving exergy and cost balance equations based on Specific Exergy Costing (SPECO) approach. The analysis was conducted to reveal the cost structure of the plant based on actual operating information and economic data. The exergy unitary cost of two main products of the plant, i.e., bioelectricity and biofertilizer were determined at 26.27 and 2.27 USD/GJ, respectively. The genset showed the highest overall cost rate (101.27 USD/h) followed by digester (68.41 USD/h). Furthermore, the net bioelectricity amounted to 67.81% of the overall cost rate of the products, while this value was 32.19% for both liquid and dewatered digestates. According to the results obtained, efforts should mainly focus on enhancing the efficiency of the genset in order to boost the overall performance of the system exergoeconomically. In addition, minimizing the investment-related cost of the digester could also substantially enhance the exergoeconomic performance of the plant.

Exergy loss based allocation method for hybrid renewable-fossil power plants applied to an integrated solar combined cycle

This paper presents a novel Exergy Loss based (EL) allocation method for the electricity produced in hybrid renewable-fossil power plants. The rationale behind this approach is that the electricity allocated to the fossil and renewable resources are obtained by subtracting from the respective source input exergies, the corresponding exergy losses, that are identified by dividing the plant into three parts; namely: the renewable, fossil and hybrid sections. The advantage of this approach is that the allocation is based only on the performance of the power plant given by its internal exergy balances, and hence the results are independent from any external arbitrary assumptions on the reference conversion efficiencies of the two resources, as it is typical of classical methods. We show that the allocations obtained using the proposed EL approach, applied to an existing integrated solar combined cycle, are consistent and comparable with the allocations obtained using the Separate Production Reference (SPR) method, as long as reasonable efficiency values of the reference scenario are selected.

Thermodynamic analysis of an oxy-hydrogen combustor supported solar and wind energy-based sustainable polygeneration system for remote locations

In this study, a solar and wind energy-based system integrated with H2O2 combustor is developed to produce fresh water from sea-water desalination, electricity, cooling, hydrogen, and oxygen as well as to provide food drying and domestic water heating. The main components of the proposed system contains concentrated solar power (CSP), wind turbine, Rankine cycle, multi stage flash (MSF) desalination unit, water electrolyzer, a refrigeration unit, a food drying system, oxy-hydrogen combustor, domestic water heater, as well as hydrogen and oxygen storage units. Furthermore, for continuous operation of the system during night time and in cloudy weather conditions, a thermal energy storage (TES) unit and oxy-hydrogen combustion unit are integrated to the system. Based on energy and exergy balances, performance assessment of the proposed system is conducted. Moreover, effects of various parameters such as solar irradiation, wind speed and ambient temperature on some of the outputs of the system are investigated. The results illustrate that the proposed system fulfills most of the remote community requirements in an efficient, environmentally benign and uninterrupted way. The obtained results for the reference case show that with installation of parabolic trough concentrators (PTCs) on an area of 111,728 m2, the plant produces net electrical power of approximately 11.4 MW, approximately 828 m3/day of freshwater, about 36 kg/s of hot air for food drying, about 31 kg/s of heated domestic water, approximately 920 kg/day of H2 and about 2.26 MW of cooling. The overall energy efficiency of the system is found to be 50%, while the exergy efficiency of the system is 34%. In addition, the energy and exergy efficiencies of single generation in which there is only electrical power output are approximately 15% and 16%, respectively.

A framework for waste heat energy recovery within data centre

Data Centres are emerging as a large industrial sector, consuming three percent of the global electricity supply while contributing for two percent of total greenhouse gas emissions, with as much as half ultimately wasted as heat. Consequently, research has focused on technologies for harvesting this waste heat energy. A framework and its accompanying decision support tool is presented which identifies the compatibility of waste heat source(s) and sink(s), exergy balance and temporal availability, along with economic and environmental benefits of available heat exchanger technologies to propose a streamlined and optimised heat recovery strategy. Substantial improvement in data centre energy efficiency together with reduction in payback time for heat recovery has been demonstrated in the included case study.

Thermodynamic analyses of the solar thermal energy driven Rankine cycle with CO2-H2O binary mixture as a working fluid

The solar energy driven Rankine cycle with CO2-H2O binary mixture as a working fluid driven by solar energy is proposed to increase the pressure ratio and specific work in this work. As compared with CO2 working fluid, the condensing temperature increases with the increment of water fraction, and the working fluid with lower pressure is condensed into liquid at ambient temperature. As compared with supercritical CO2 (S-CO2) cycles, the pressure ratio of the proposed cycle is improved by reducing the outlet pressure of turbine, and the consumption work of compression is reduced by compressing the liquid working fluid. As a result, the specific work of the proposed cycle is higher than that of S-CO2 cycle The characteristics of working fluid and parameters optimization are performed with different pressures and compositions of working fluid, and the energy and exergy balances at the design point are conducted to analyze the performances of the system. Theoretical results show that there is an optimal value of cycle efficiency when the mole fraction of water is increased. The solar-to-electric efficiency on the nominal condition reaches to 29.36%, which is higher than existing solar tower systems. Research findings introduce a promising approach for the efficient utilization of the abundant solar resources.

Thermodynamic analysis of hybrid cooling system integrated with waste heat reusing and peak load shifting for data center

A hybrid cooling system aimed at enhancing energy, exergy and economic efficiency of data center is proposed. The proposed system combines a high cooling efficiency spray cooling system, a waste heat reusing absorption chiller and a peak load shifting chilled water storage. This combination is implemented by replacing the condenser of spray cooling system with the generator of absorption chiller, and storing cooling capacity of absorption chiller during off-peak time to supplement the cooling load during on-peak time. A thermodynamic model is developed based on energy and exergy balance. Parametric sensitivity analysis is performed to investigate the effects of spray cooling inlet temperature, chilled water temperature, cooling water temperature and ambient temperature on the energy saving efficiency, exergy efficiency and operation cost. Comparison study and economic analysis are conducted based on Dawning 5000A supercomputer in Shanghai, China. Sensitivity analysis results show that the effect of spray cooling inlet temperature is the largest, while the effects of chilled water temperature and cooling water temperature are the smallest. At appropriate condition, the absorption chiller and chilled water storage could totally eliminated the cooling load of air conditioner during on-peak time. The energy saving efficiency, exergy efficiency and operation cost saving could be up to 51%, 17%, and 71% with the cost payback period as low as 286 days.

Node Temperature of the Coupled High-Low Energy Grade Flus Gas Waste Heat Recovery System

Coupled high-low energy grade flus gas waste heat recovery systems (CWHRS) have been applied in power plants to improve unit efficiency. In this study, to evaluate the rationality of waste heat recovery, the energy-grade balance coefficient (EBC) of the CWHRS was derived using the theory of heat balance, exergy balance and energy grade balance. The inlet flue gas temperature (IFT) of the low-temperature economizer was defined as the node temperature of the CWHRS. The optimal node temperature (ONT) was optimal when the absolute value of the EBC was the smallest. The exergy efficiency and EBC of the system installed on a supercritical 600 MW unit were calculated and the result shows that the ONT of the system was about 115 °C, the ONT decreased from about 135 °C to about 113 °C when the IFT increased from 335 °C to 380 °C and the ONT decreased from about 144 °C to about 113 °C when the inlet air temperature increased from −10 °C to 35 °C. The node temperature is recommended as an adjusting parameter of CWHRS to ensure the effect of waste heat recovery.

Hydrogen production with a photovoltaic thermal system enhanced by phase change materials, Shiraz, Iran case study

Whereas Photovoltaic Thermal Systems (PVT), Phase Change Materials (PCM) and Proton Exchange Membrane (PEM) electrolyzer have been thoroughly studied individually, the effects of their combination need to be more investigated. The current study proposed a new PVT system integrated with PCM and PEM electrolyzer to produce hydrogen in a hydrogen fuel filling station. Based on the energy and exergy balance equations, a mathematical model is developed to analyse the effects of different types of PV and PCM sets on the thermal and electrical performances. Variations in the temperature of system components, generated electricity, hydrogen production as well as the energy/exergy amounts and efficiencies with time are presented for different effective parameters. Based on the obtained results, we found that PV type is one of the most dominant parameters of the system. PCM utilization improves the electrical, thermal energies and exergy efficiencies. The highest daily amount of produced hydrogen is obtained for 16th August 2018 with mono-crystalline semitransparent PV and 120 kg of RT35 PCM type (88.71 gr/day). While the hydrogen production for the same PVT system without PCM is 5.32% less than the case with PCM. Moreover, the maximum diurnal energy efficiency is obtained 35.04% for mono-crystalline semitransparent PV and RT35 PCM during the summer, while the maximum daily exergy efficiency of 15.17% is achieved for the integration of mono-crystalline semitransparent PV and RT28 PCM type in the winter.

Extremely small energy requirement by poliovirus to proliferate itself is the key to an outbreak of an epidemic

Poliovirus is an RNA virus which proliferate itself in the intestines. Thermodynamic assessment of the proliferation process has been assessed after calculating the energy and exergy of the RNA with the group contribution method. Mass, energy and exergy balances are performed to calculate the energy and exergy exploited from the host cell as 4.65 × 10−19 and 3.35 × 10−17 kJ, respectively. In a typical epidemic in 1988, 350,000 cases were reported, implying that the total of 1.627 × 10−9 kJ of energy 1.174 × 10−7 kJ of exergy was exploited from them during the epidemic. Although the energy and exergy efficiency of virus generation in a host cell were very small, e.g., 1.376 × 10−6 and 9.78 × 10−5, respectively; energy and exergy employed in this process was very small implying that extremely small energy and exergy requirement by poliovirus to proliferate itself is the key to an outbreak of an epidemic.

Extremely small energy requirement by poliovirus to proliferate itself is the key to an outbreak of an epidemic

Poliovirus is an RNA virus which proliferate itself in the intestines. Thermodynamic assessment of the proliferation process has been assessed after calculating the energy and exergy of the RNA with the group contribution method. Mass, energy and exergy balances are performed to calculate the energy and exergy exploited from the host cell as 4.65 × 10−19 and 3.35 × 10−17 kJ, respectively. In a typical epidemic in 1988, 350,000 cases were reported, implying that the total of 1.627 × 10−9 kJ of energy 1.174 × 10−7 kJ of exergy was exploited from them during the epidemic. Although the energy and exergy efficiency of virus generation in a host cell were very small, e.g., 1.376 × 10−6 and 9.78 × 10−5, respectively; energy and exergy employed in this process was very small implying that extremely small energy and exergy requirement by poliovirus to proliferate itself is the key to an outbreak of an epidemic.

Thermal performance of a gas turbine based on an exergy analysis

This work concerns the calculation and the analysis of the thermal performance of the components ofan MS 7001 type gas turbine with a nominal power of 87 MW using the concept of exergy. The exergy balance is used in addition to the energy balance to estimate the irreversibility of the air compressor, the combustion chamber and the turbine. The exergy analysis is carried out by applying the equilibrium equations obtained from the general definitions of the irreversibility of the thermodynamic processes and the data provided by the manufacturer. The results show that the exergy destruction of the gas turbine depends on the variation of the thermodynamic parameters: ambient temperature, compression ratio, air–fuel ratio. The combustion chamber has the highest exergy destruction estimated at 36.34 MW. The air compressor has an exergy efficiency of 84.19% that of the combustion chamber is 75.91% whilethat of the turbine expansion is 92.58%. The total exergy destruction of the gas turbine is 53.51 MW and itsefficiency is 32.44%. Improving the performance of the gas turbine requires decreasing the temperatureof the intake air.

Energy and exergy analysis of compact automotive air conditioning (AAC) system

Automotive Air conditioning (AAC) is a unit that uses a high measure of energy from a car total engine power. In equatorial climate country such as Malaysia, high usage of AAC is inevitable due to hot, humid and rainy weather throughout the year. An understanding about the energy and exergy losses is essential to find the potential improvement to maximise the efficiency in an AAC system. The main objective of this study is to study the performance of energy and the exergy of a compact automotive air conditioning system. This cycle uses R134a and PAG lubricant as the working fluid. The different ranges of initial refrigerant charge and compressor speed have been tested on the AAC to evaluate the effect of different major thermodynamic parameters in performance. A theoretical model is developed to work out the thermodynamic parameters such as coefficient of performance, exergy destruction ratio, component efficiency defect as well as the dimensionless exergy balance for the AAC system components. The results of this study have shown that most of the energy has been destructed in evaporator part. In order to maximize the efficiency and performance of AAC system, further optimization needs to be done in order to improve the evaporator component.

Evaluación energética y exergética en un motor de combustión interna ciclo Otto de 1.6L

El presente trabajo tuvo como objetivo: Evaluar el comportamiento de un motor de combustión interna mediante el balance energético y exergético para la valorización de su desempeño, considerando como caso de estudio a un motor de vehículo Mazda 3 – 1.6; el cálculo energético se desarrolló en ruta previamente caracterizada a velocidad constate de 50 km/h y 90 km/h. Tras una investigación de tipo experimental, con base en observación y medición, además de métodos de inducción, deducción y síntesis, se determinó, dentro de las principales conclusiones, que el análisis del balance energético y exergético aporta a generar ideas para la optimización y un uso más eficiente de hidrocarburos; se determinó, además, que el motor estudiado presenta un rendimiento energético promedio de 27.57 % para una velocidad de 50 km/h, mientras que el flujo de exergía total del sistema es 22 %, no existiendo diferencia significativa con los resultados de eficiencia a 90 km/h.

Energy and exergy analysis of hydraulic free-piston engines

A hydraulic free-piston engine is an unconventional reciprocating piston internal combustion engine in which the piston assembly motion is determined by in-cylinder gas pressure and load force. Fuel combustion energy is directly converted into hydraulic energy. These affect the work process of cylinder and efficiency of energy conversion. In order to study the energy utilization efficiency and to explore the recovery potential of waste heat energy of hydraulic free-piston engine, in this paper, the energy distribution and waste heat energy characteristics of hydraulic free-piston engine have been studied by combining energy and exergy analysis. The thermal efficiency was analyzed by the first law of thermodynamics, and exergy balance was analyzed by the second law. The effect of the characteristic parameters on the thermal and exergy efficiency was studied through the simulation analysis comparing the energy utilization of hydraulic free-piston engine and conventional engines. The results show that control of the injection timing parameter is effective for optimizing efficiency because the cycle characteristic parameters can be controlled by changing the injection timing. The experimental results show that the thermal efficiency is 40.8% and the exergy efficiency is 46.3%. The simulation result show that the thermal efficiency of hydraulic free-piston engine is 38.0% and the conventional diesel engine is 33.0%.

Performance, emission, energy and exergy analyses of gasoline fumigated DI diesel engine

In this paper, the effect of gasoline fumigation on the performance, emission, energy and exergy balance of a direct injection diesel engine is experimentally investigated. Gasoline fumigation is induced at three different ratios based on energy. In the present study, computer-controlled injection using LabVIEW software is attempted for gasoline fumigation and common rail is used to inject at the same pressure always. The characteristics of the engine are studied with and without fumigation. Results reveal that gasoline fumigation increases the energy and exergy efficiency to about 5% at medium and high loads. For all the operating points, the percentage of energy and exergy transfer through the exhaust gases decreases by an average of 2.6% using gasoline fumigation. Fumigation decreases diesel fuel consumption by 33.53%, CO2 emissions by 5.8%, NOx by 10.4% and smoke by 13–17% during full load at 30% of gasoline fumigation. Also, fumigation increases brake thermal efficiency by 3.78%, CO emissions from 0.2 to 0.96% by volume and UBHC emissions from 51 to 205 ppm during full load at 30% of gasoline fumigation. Results show that the gasoline fumigation replaces diesel and reduces both nitrogen oxides and smoke.

Syngas production from oil sludge gasification and its potential use in power generation systems: An energy and exergy analysis

Thermo-chemical conversion of oil sludge (OS) is a promising technological alternative for management petroleum waste from crude oil refinery. In this study, an energetic and exergetic analysis has been approached, taking into account the exergy destruction by irreversibilities associated with syngas production and its potential use to power generation. A simulation kinetic model with air/steam mixtures by using Aspen-Hysys 8.6® software was accomplished to evaluate the effect of gasifying agents on OS gasification parameters. It was noted a decreasing trend of produced syngas LHV when equivalence ratio (ER) and steam to oil sludge ratio (SOS) are increased, reaching values between 5 and 10 MJ/Nm3 for ER > 0.30 ratios and SOS between 0.5 and 1.5, while syngas yield can range from 2.0 to 5.0 Nm3 H2/kg OS with hydrogen content close to 25% (mol.%) at the same conditions. Furthermore, when an integrated gasification power generation system and exergy balance were taken into account, exergy destruction was increased almost three times with respect to syngas production from OS gasification process, resulting in 64.4% of total exergy input of integrated plant. Considering the thermodynamic analysis, OS gasification process could become an added value technological alternative for oily waste treatment from the crude oil refining.

Efficiency of the air heater in a heat recovery system at different thermophysical parameters and operational modes of the boiler

We have examined, for the plate heater included in a heat recovery system of the boiler plant, the influence of its thermophysical parameters on the losses of exergetic power under different operational modes of the heating boiler. A procedure for the calculation of losses in a given heat recovery unit is based on an integrated approach that combines exergetic methods with the methods of thermodynamics of irreversible processes. A mathematical model includes a differential equation of the exergy balance and an equation of thermal conductivity for an air heater under boundary conditions of the third kind. The differential equation of exergy balance has been solved jointly with the equation of thermal conductivity. The result of solving them is the obtained estimation dependences for determining the losses of exergetic power associated with the processes of heat transfer. We have calculated losses of exergetic power in the examined heat recovery units at a change in the coefficient of thermal conductivity of the plate, in the heat transfer coefficient from flue gases and an operational mode of the boiler. The derived dependences on a thermal conductivity coefficient for the considered operating modes of the boiler have two distinct sections, along the first of which there is a relatively small increase in the losses of exergetic power while reducing the coefficient of thermal conductivity, along the second ‒ the loss of exergetic power in a heat recovery unit increase relatively sharply. For the considered sequence of regimes of the boiler a transition from its maximum heat output to the minimal one is accompanied by a decrease in the losses of exergetic power. A similar character is also demonstrated by the dependence on a thermal conductivity coefficient of the relative contribution of losses of exergetic power in the heat transfer processes to their totals in a heat recovery unit. In this case, there are minor differences in the relative contribution of these losses under different operational modes of the boiler. The heat transfer coefficient from the side of flue gases within a framework of a single operating mode of the boiler affects less significantly, compared to the thermal conductivity coefficient of the material of a heat exchange surface, the losses of exergetic power in the heat transfer processes. We have established regions of change in the thermal conductivity coefficient, as well as operational modes of the boiler, in the range of which the losses of exergetic power in a heat recovery unit are minimal

Multi-objective optimization of double effect series and parallel flow water–lithium chloride and water–lithium bromide absorption refrigeration systems

In this study, multi-objective optimization of the series and parallel flow type double effect absorption refrigeration systems is presented. Genetic algorithm is used to find the optimal solutions and the Pareto-optimal fronts. First, a thermodynamic model is developed to simulate the double effect series and parallel flow configurations with water–lithium chloride and water–lithium bromide solution pairs. Energy and exergy balances are applied to evaluate coefficient of performance, exergy efficiency and irreversible losses in the system components separately for the two working fluid pairs. Next, optimization is done considering coefficient of performance, exergy efficiency and the total system irreversibility rate as objective functions. The low and high pressure generator temperatures are taken as decision variables in the series configuration while for the parallel system; additionally the distribution ratio is also taken as a decision parameter. The optimization is done for four different cases of fixed evaporator, absorber and condenser temperatures. For each case, the optimal decision parameter values are determined for both the water–lithium chloride and water–lithium bromide operated double effect series and parallel configurations. The optimized coefficients of performance were found slightly lower for water–lithium chloride. Contrary to this, the optimal exergy efficiencies were more and the total system irreversibility were less for water–lithium chloride. Further, the optimal performances of the double effect water–lithium chloride systems were obtained comparatively at lower generator temperatures than those of the water–lithium bromide systems.