Sources Of Exergy Destruction Research Articles

A theoretical study on a novel solar based integrated system for simultaneous production of cooling and heating

An integrated system for simultaneous production of triple-effect cooling and single stage heating is proposed in this paper to harness low grade solar energy. The proposed system combines the heliostat field with a central receiver and the ejector-absorption cycle with the shaft power driven transcritical CO2 cycle. A parametric study based on first and second laws of thermodynamics is carried out to ascertain the effect of varying the exit temperature of duratherm oil, turbine inlet pressure, and evaporators temperature on the energy and exergy output as well as on the energy and exergy efficiencies of the system. The results obtained indicate that major source of exergy destruction is the central receiver where 52.5% of the inlet solar heat exergy is lost followed by the heliostat where 25% of the inlet exergy is destroyed. The energy and exergy efficiencies of the integrated system vary from 32% to 39% and 2.5%–4.0%, respectively, with a rise in the hot oil outlet temperature from 160 °C–180 °C. It is further shown that increase in evaporator temperature of transcritical CO2 cycle from −20 °C to 0 °C increases the energy efficiency from 27.45% to 43.27% and exergy efficiency from 2.51% to 2.97%, respectively. The results clearly show how the variation in the values of hot oil outlet temperature, turbine inlet pressure, and the evaporator temperature of transcritical CO2 cycle strongly influences the attainable performance of the integrated system.

Exergetic study of catalytic steam reforming of bio-ethanol over Pd–Rh/CeO2 with hydrogen purification in a membrane reactor

An exergy analysis of the ethanol steam reforming process using a Pd–Rh catalyst supported on ceria in a Pd–Ag membrane reactor has been performed based on experimental data. Both chemical and physical exergy for chemical species together with thermodynamic loss calculations have been considered to evaluate the exergy efficiency of the process. The system has been studied under different operational conditions in terms of temperature (873–923 K), pressure (4–12 bar), and feed flow rate using a water–ethanol mixture of 50–50% volume (S/C = 1.6). A significant exergy efficiency dependency on the fuel flow rate and pressure has been encountered when considering the heat loss from the reactor. The best results regarding the exergetic and thermal efficiency have been obtained at high flow rates and 8–12 bar, which has been attributed to the higher hydrogen permeation through the membrane. The chemical reactions involved in the reforming process and the heat losses are the main sources of exergy destruction.

Current and theoretical maximum well-to-wheels exergy efficiency of options to power vehicles with natural gas

Lower prices and increased supply of natural gas from hydraulic fracturing could lead to widespread use of natural gas in transportation. There are three primary ways that natural gas could be used in personal vehicles: compressed natural gas (CNG) in a combustion engine, as a source of hydrogen for a fuel cell electric vehicle (FCEV), and to generate electricity for a battery electric vehicle (BEV). In this work, we compare these three paths by analyzing their current and theoretical maximum well-to-wheels (WTW) exergy efficiencies. Each pathway begins with the extraction of natural gas and ends with delivery of work to the vehicle’s wheels. The best current and theoretical maximum well-to-wheels exergy efficiencies for CNG, FCEV, and BEV pathways are found to be 31%/63%, 25%/87% and 44%/84% respectively. The largest exergy destruction for the CNG pathway occurs within the vehicle’s internal combustion engine (ICE) plant, which has a best current efficiency of 35%. For the FCEV pathway the main current sources of exergy destruction are the reforming stage and within the fuel cell engine plant, with best current efficiencies of 69% and 50% respectively. For the BEV pathway, the largest exergetic loss occurs during the conversion from natural gas to electricity at a combined cycle power plant, with a best current efficiency of 59%. While the theoretical maximum succeeds in identifying process steps that limit efficiency, it does not inform how much progress could be made to improve efficiency with what effort.

Exergy destruction and losses on four North Sea offshore platforms: A comparative study of the oil and gas processing plants

The oil and gas processing plants of four North Sea offshore platforms are analysed and compared, based on the exergy analysis method. Sources of exergy destruction and losses are identified and the findings for the different platforms are compared. Different platforms have different working conditions, which implies that some platforms need less heat and power than others. Reservoir properties and composition vary over the lifetime of an oil field, and therefore maintaining a high efficiency of the processing plant is challenging. The results of the analysis show that 27%–57% of the exergy destruction take place in the gas treatment sections, 13%–29% take place in the gas recompression sections and 10%–24% occur in the production manifolds. The exergy losses with flared gas are significant for two of the platforms. The exact potential for energy savings and for enhancing system performances differs across offshore platforms. However, the results indicate that the largest rooms for improvement lie in (i) gas compression systems where large amounts of gas may be compressed and recycled to prevent surge, (ii) production manifolds where well-streams are depressurised and mixed, and (iii) in the installation of flare gas recovery systems.

Thermodynamics of premixed combustion in a heat recirculating micro combustor

A thermodynamic model has been developed to evaluate exergy transfer and its destruction in the process of premixed combustion in a heat recirculating micro combustor. Exergy destruction caused by process irreversibilities is characterized by entropy generation in the process. The entropy transport equation along with the solution of temperature and species concentration fields in the wake of flame sheet assumptions have been used to determine the different components of entropy generation. The role of thermal conductivity and thickness of combustor wall, and Peclet number on transfer and destruction rate of exergy is depicted in the process of flame stabilization via heat recirculation. The entropy generations due to gas phase heat conduction and chemical reaction are identified as the major sources of exergy destruction. The total irreversibility in pre-flame region is confined only within a small distance upstream of the flame. It has been observed that the local volumetric entropy generation is higher near the axis than that near the combustor wall. The second law efficiency is almost invariant with heat loss from the combustor, Peclet number, and thermal conductivity and thickness of combustor wall.

Exergy analysis of parabolic trough solar collectors integrated with combined steam and organic Rankine cycles

In this paper, detailed exergy analysis of selected thermal power systems driven by parabolic trough solar collectors (PTSCs) is presented. The power is produced using either a steam Rankine cycle (SRC) or a combined cycle, in which the SRC is the topping cycle and an organic Rankine cycle (ORC) is the bottoming cycle. Seven refrigerants for the ORC were examined: R134a, R152a, R290, R407c, R600, R600a, and ammonia. Key exergetic parameters were examined: exergetic efficiency, exergy destruction rate, fuel depletion ratio, irreversibility ratio, and improvement potential. For all the cases considered it was revealed that as the solar irradiation increases, the exergetic efficiency increases. Among the combined cycles examined, the R134a combined cycle demonstrates the best exergetic performance with a maximum exergetic efficiency of 26% followed by the R152a combined cycle with an exergetic efficiency of 25%. Alternatively, the R600a combined cycle has the lowest exergetic efficiency, 20–21%. This study reveals that the main source of exergy destruction is the solar collector where more than 50% of inlet exergy is destructed, or in other words more than 70% of the total destructed exergy. In addition, more than 13% of the inlet exergy is destructed in the evaporator which is equivalent to around 19% of the destructed exergy. Finally, this study reveals that there is an exergetic improvement potential of 75% in the systems considered.

Simulation and exergetic evaluation of CO2 capture in a solid-oxide fuel-cell combined-cycle power plant

The incorporation of fuel cells into power plants can enhance the operational efficiency and facilitate the separation and capture of emissions. In this paper a fuel-cell unit, consisting of solid-oxide fuel-cell stacks, a pre-reformer, and an afterburner is incorporated into a combined-cycle power plant with CO2 capture. The thermodynamic performance of the plant is examined using an exergetic analysis and it is compared with a conventional combined-cycle power plant (reference plant) without CO2 capture, as well as with other plants with CO2 capture.The inefficiencies of the chemical reactions taking place in the fuel-cell unit are found to be the main source of exergy destruction among the plant components. However, the additional power generated in the fuel-cell stacks and the afterburner enhances the overall efficiency and compensates for the energy needed for the capture and compression of the carbon dioxide. When compared with the reference plant and with alternative capture technologies, the solid-oxide fuel-cell plant with CO2 capture operates more efficiently and appears to be a thermodynamically promising approach for carbon capture.

Investigating the effect of several thermodynamic parameters on exergy destruction in components of a tri-generation cycle

Multiple energy generating cycles such as tri-generation cycles, which produce heat and cold in addition to power through burning of a primary fuel, have increasingly been used in recent decades. On the other hand, advanced exergy analysis of thermodynamic systems by splitting exergy destruction into endogenous and exogenous parts identifies internal irreversibilities of each of the components and the effect of these irreversibilities on the performance of other components. Therefore, main sources of exergy destruction in cycles can be highlighted and useful recommendations in order to improve the performance of thermodynamic cycles can be presented. In the present work, a tri-generation cycle with 100 MW power production, 70 MW heat and 9 MW cooling capacity is considered. For this tri-generation cycle, effects of various thermodynamic parameters on the amount of endogenous and exogenous exergy destructions, exergy loss and the amount of fuel consumption, are investigated. The results indicate that, increasing compressor pressure ratio, pre-heater outlet temperature and excess air leads to better combustion and lower exergy loss and fuel consumption. Increasing the mass flow rate of steam generator, while keeping the cycle outlet temperature constant and considering cooling capacity variable, lead to increase the first- and second-law efficiencies of the cycle.

Exergy and energy analysis of an AC steel electric arc furnace under actual conditions

The energy and exergy analyses of an existing steel Electric Arc Furnace (EAF) are performed to estimate the potential for increasing the furnace efficiency. To obtain realistic results, the effect of air infiltration into the furnace was taken into account. The results of the analyses revealed that the energy and exergy efficiencies of the furnace are low and should be increased. The main sources of energy waste are stack gases followed by heat transfer to the cooling water, while the main sources of exergy destruction are combustion and heat transfer. Hot stack gases contain 18.3% and 12.2% of the total input energy and exergy, respectively. Increasing the air infiltration reduces the energy and exergy efficiencies of the EAF. By using the energy of flue gas to preheat the sponge iron, the electrical energy consumption of the furnace can be reduced by 89 GJ, dictating a 21.4% reduction in electrical energy consumption and a 13.6% increase in steel production.

Pathway for Advanced Architectures of Oxy-pulverized Coal Power Plants: Minimization of the Global System Exergy Losses

The objective of this paper is to assess the improvement potential as well as heat integration opportunities of the oxy- pulverized coal power plant by means of the mapping exergy destruction sources on a conventional architecture as a first step towards an integrated, highly efficient system with optimized operating conditions and advanced architecture. On the basis of a first generation oxy-fired power plant, composed by conventional cryogenic ASU and CPU, a standard pulverized-coal boiler with supercritical steam cycle, the whole system is modeled at elementary equipment level. Operating conditions and current state-of-the-art design are considered in order to set a base-case for the identification of the exergy destruction.The exergy analysis reveals the location and the magnitude of the losses. Main losses occur in the boiler, steam generation, turbines, distillation unit, compression steps of the ASU and CPU, gas quality control system, and in the regenerative heater. According to this assessment, a novel architecture is investigated. Compression heat integration, bypass regenerative heater with improved heat exchange, preheating of the oxygen flow with the bypass flow surplus heat and reheat of the cold depolluted flue gas in a regenerative heater are implemented. Important reduction in exergy destruction is reported and the exergy efficiency of the integrated power plant increases from 36.4% to 39.6% (considering CO2 as a product), which corresponds to an overall exergy destruction diminution of 16%. The resulting net plant energy efficiency is 36.1%LHV, which is a 3.5%-pts increase compared to the base-case oxy-fired power plant, reducing the energy penalty down to 7.9%-pts when compared to the reference air-fired unit without CCS.

Thermofluidics and energetics of a manifold microchannel heat sink for electronics with recovered hot water as working fluid

A detailed thermo-hydrodynamic analysis of a hot water cooled manifold microchannel heat sink for electronic chip cooling is presented. The hot water cooling enables efficient recovery of heat dissipated by the even hotter chip by using hot water recovered from a secondary application. Contrary to usual expectation of laminar flow in electronic cooling, high flow rate and high fluid temperatures result in turbulent flow conditions in the inlet and outlet manifolds of the heat sink with predominantly laminar flow conditions in microchannels. To simulate these complex flow conditions, a three dimensional (3D) conjugate heat transfer model with turbulent flow is developed. Microchannel heat transfer structure is modeled as porous medium with permeability parameters extracted from a 3D model for a single microchannel. The energetic performance of the heat sink is analyzed in terms of 2nd law efficiency and sources of exergy destruction are identified by detailed local entropy generation analysis at low and high Reynolds number conditions of 2400 and 11200 respectively. This analysis shows that entropy generation due to heat transfer dominates the net entropy generation in the heat sink for both conditions. Although entropy generation due to viscous dissipation increases significantly with increased Reynolds number, it still contributes less than a third to the total entropy generated at high Reynolds numbers. Use of hot water reduces the heat transfer component of entropy generation significantly, thus leading to higher 2nd law efficiency.

Energy and exergy analyses of a biomass trigeneration system using an organic Rankine cycle

In this study, energy and exergy analyses of a biomass trigeneration system using an organic Rankine cycle (ORC) are presented. Four cases are considered for analysis: electrical-power, cooling-cogeneration, heating-cogeneration and trigeneration cases. The results obtained reveal that the best performance of the trigeneration system considered can be obtained with the lowest ORC evaporator pinch temperature considered, Tpp = 20 K, and the lowest ORC minimum temperature, T9 = 345 K. In addition, this study reveals that there is a significant improvement when trigeneration is used as compared to only electrical power production. This study demonstrates that the fuel utilization efficiency increases, in average, from 12% for electrical power to 88% for trigeneration. Moreover, the maximum exergy efficiency of the ORC is 13% and, when trigeneration is used, it increases to 28%. Furthermore, this study reveals that the electrical to cooling ratio can be controlled through changing the ORC evaporator pinch point temperature and/or the pump inlet temperature. In addition, the study reveals that the biomass burner and the ORC evaporator are the main two sources of exergy destruction. The biomass burner contributes to 55% of the total destructed exergy whereas the ORC evaporator contributes to 38% of the total destructed exergy.

Exergy and Energy Analysis of Combined Cycle systems with Different Bottoming Cycle Configurations

SUMMARY The paper deals with thermodynamic analysis of cooled gas turbine-based gas-steam combined cycle with single, dual, or triple pressure bottoming cycle configuration. The cooled gas turbine analyzed here uses air as blade coolant. Component-wise non-dimensionalized exergy destruction of the bottoming cycle has been quantified with the objective to identify the major sources of exergy destruction. The mass of steam generated in different configurations of heat recovery steam generator (HRSG) depends upon the number of steam pressure drums, desired pressure level, and steam temperature. For the selected set of operating parameters, maximum steam has been observed to be generated in the case of triple pressure HRSG = 19 kg/kg and minimum in single pressure HRSG = 17.25 kg/kg. Plant-efficiency and plant-specific works are both highest for triple-pressure bottoming cycle combined cycle. Non-dimensionalized exergy destruction in HRSG is least at 0.9% for B3P, whereas 1.23% for B2P, and highest at 3.2% for B1P illustrating that process irreversibility is least in the case of B3P and highest in B1P. Copyright © 2012 John Wiley & Sons, Ltd.

Design and Exergy Analysis of a Thermal Power Plant Using Different Types of Turkish Lignite

Normal 0 21 false false false TR X-NONE X-NONE The purpose of this study is to design a 240 MW el thermal power plant (TPP) to be operated with ten different types of Turkish lignite and fulfill an exergy analysis including the determination and comparison of the performance for each type of lignite. Additionally an examination of the effect of the ambient temperature on the power plant’s exergy efficiency and calculation of the CO 2 emissions of each lignite type is carried out. Exergy destruction of each component is investigated by using conservation of mass, conservation of energy and exergy destruction in an open system at steady state. Sivas Kangal lignite is selected for designing. N et energy and exergy efficiencies of the plant are calculated as 37.16 and 34.84 % respectively. The analysis of the case study revealed that the boiler is the major source of exergy destruction with 299.10 MW and 83.29 % of the total exergy destruction of the overall plant . Another point is that, fuel consumption and CO 2 emissions of the TPP increase with rising ambient temperature. It should be noted that, comparison between the performances and emission data of the power plants can provide useful data for the future coal-fired thermal power plants investments in Turkey.

Exergy analysis of gas turbine – solid oxide fuel cell-based combined cycle power plant

Fuel cell technology is one of the promising systems for cleaner and competitive alternate power generation systems. When the high temperature fuel cells like solid oxide fuel cell and molten carbon fuel cell are integrated with the gas turbines, the total thermal efficiency of the combined cycle can be obtained greater than 60%. In the present work, thermodynamic analysis of SOFC-GT combined system (3MW) has been carried out for the fuel methane to evaluate the energy efficiency, exergy efficiency and exergy destruction of each component and compared with other fuels like coal gas and ethanol. The effect of compression ratio, turbine inlet temperature and ambient temperature of air on the performance of the system has been analysed. The outcome of the system modelling reveals that SOFC and combustion chamber are the main sources of exergy destruction. At the optimum compression ratio 9 and at the turbine inlet temperature 1,250 K, the total thermal efficiency and the exergy efficiency are found to be 63.3% and 60.85% respectively.

Investigation of effect of variation of cycle parameters on thermodynamic performance of gas-steam combined cycle

The paper deals with second law thermodynamic analysis of a basic gas turbine based gas-steam combined cycle. The article investigates the effect of variation of cycle parameters on rational efficiency and component-wise non-dimensionalised exergy destruction of the plant. Component-wise inefficiencies of the combined cycle have been quantified with the objective to pin-point the major sources of exergy destruction. The parameter that affects cycle performance most is the TIT (turbine inlet temperature). TIT should be kept on the higher side, because at lower values, the exergy destruction is higher. The summation of total exergy destruction of all components in percentage terms is lower (44.88%) at TIT of 1800 K & rp,c = 23, as compared to that at TIT = 1700 K. The sum total of rational efficiency of gas turbine and steam turbine is found to be higher (54.91%) at TIT = 1800 K & rp,c = 23, as compared to that at TIT = 1700 K. Compressor pressure ratio also affects the exergy performance. The sum total of exergy destruction of all components of the combined cycle plant is lower (44.17%) at higher value of compressor pressure ratio (23)& TIT = 1700 K, as compared to that at compressor pressure ratio (16). Also exergy destruction is minimized with the adoption of multi-pressure-reheat steam generator configuration.

Greenhouse gas emission and exergy assessments of an integrated organic Rankine cycle with a biomass combustor for combined cooling, heating and power production

Abstract In this study, greenhouse gas emission and exergy assessments of an integrated organic Rankine cycle (ORC) with a biomass combustor for combined cooling, heating, and power production as a trigeneration system are conducted. This trigeneration system consists of a biomass combustor, an ORC, a single-effect absorption chiller, and a heat exchanger. Four special cases are considered in this comprehensive study, namely, electrical power, cooling-cogeneration, heating-cogeneration, and trigeneration cases. Various exergetic and environmental output performance parameters, namely, exergy efficiency, exergy destruction rate, and greenhouse gas emissions, are examined under varying ORC evaporator pinch point temperature, pump inlet temperature, and turbine inlet pressure. This study shows that using trigeneration considerably increases both energy and exergy efficiencies and decreases the greenhouse gas emissions as compared to the electrical power case. This study reveals that the heating-cogeneration and trigeneration cases are less sensitive to the considered temperature and pressure variations as compared with the electrical power and cooling-cogeneration cases. In addition, the results show that when the trigeneration case is used, the exergy efficiency increases significantly to 27% as compared with the exergy efficiency of the electrical power case, which is around 11%. It is also found that the main two sources of exergy destruction are the biomass combustor and ORC evaporator. Moreover, this study shows that the emissions of CO2 in kg/MWh are significantly high for the electrical power case while for the trigeneration case, the emissions per MWh of trigeneration drop significantly to relatively low level. Specifically, the emissions drop to around one seventh per MWh produced when trigeneration is used as compared with only electrical power production case.

Exergy analysis of a passive direct methanol fuel cell

An analytical, one-dimensional, steady state model is employed to solve for overpotentials at the catalyst layers along with the liquid water and methanol distributions at the anode, and oxygen transport at the cathode. An iterative method is utilized to calculate the cell temperature at each cell current density. A comprehensive exergy analysis considering all possible species inside the cell during normal operation is presented. The contributions of different types of irreversibilities including overpotentials at the anode and cathode, methanol crossover, contact resistance, and proton conductivity of the membrane are investigated. Of all losses, overpotentials in conjunction with the methanol crossover are considered as the major exergy destruction sources inside the cell during the normal operation. While the exergy losses due to electrochemical reactions are more significant at higher current densities, exergy destruction by methanol crossover at the cathode plays more important role at lower currents. It is also found that the first-law efficiency of a passive direct methanol fuel cell increases as the methanol solution in the tank increases in concentration from 1 M to 3 M. However, this is not the case with the second-law efficiency which is always decreasing as the concentration of the methanol solution in the tank increases.

Entropy Transport Equation in Large Eddy Simulation for Exergy Analysis of Turbulent Combustion Systems

The transport equation of entropy is introduced in large eddy simulation to perform exergy analysis of turbulent combustion systems. The sources of exergy destruction can be evaluated by analyzing entropy generation terms, which appear in unclosed forms in this equation. The closure is based on the filtered density function (FDF) methodology. The primary advantage of FDF is that chemical reaction and its entropy generation effects appear in closed forms. This methodology involves a stochastic model, which is being developed to account for the subgrid scale transport of entropy.

English

Fuel cells offer a more efficient and fuel flexible technology that produces heat and power with lower emissions. It is observed that by integrating fuel cells with the gas turbines, more than 70%thermal efficiency can be achieved from the combined cycle. This proves to be a better exergetic performance when compared to the conventional gas turbine plants. A Solid Oxide Fuel Cell-Gas Turbine combined cycle power plant is modeled and parametric exergy analysis is done to evaluate the energy efficiency, exergy efficiency and the exergy destruction of each component in the system. The effect of pressure ratio, turbine inlet temperature and ambient temperature on the performance of the system is investigated by using diesel and gasoline as fuels. The outcome of the modeled system reveals that SOFC and combustion chamber are the main sources of exergy destruction. At optimum pressure ratio, the total thermal efficiency using gasoline and diesel is found to be 71.36% and 70.72% while the exergy efficiency is found to be 66.53% and 66.76% respectively.