International Journal of Exergy (17 papers in press)
Assessment of the work efficiency with exergy method in aging muscles and healthy and enlarged hearts
by Jale Çatak, Mustafa Özilgen, Ali Bahadir Olcay, Bayram Yılmaz
Abstract: Thermodynamic aspects of skeletal and cardiac muscle work performance are assessed with the data obtained from the literature. Since the second law muscle work efficiency decreases with declining metabolic energy conversion efficiency in the mitochondria, followed by structural failure of the muscles during aging, the thermodynamic aspects of the muscle work aging process were simulated by incorporating the decreasing second law muscle work efficiency with the exercise data obtained with the healthy young adults.
Within limits of the data analyzed here, glucose utilization ability of the cardiac muscle appears to be the most critical factor determining its work performance. The left and the right ventricles of the enlarged heart had the ability of utilizing approximately 3.5 and 2.7 times less glucose, respectively, than their healthy counterparts. The work performance and the entropy generation by the enlarged and the healthy hearts maintained the same ratios. The results presented here for the enlarged heart is case specific and may change drastically for any other unhealthy heart.
Keywords: Aging mitochondria; entropy generation; exergy loss; heart; muscle work performance; second law efficiency.
Investigation on the exergy performance of salt gradient solar ponds with porous mediums
by Hua Wang, Liu Gang Zhang, Yan Yang Mei
Abstract: The exergy performance of solar pond with porous media added in the lower convective zone (LCZ) has been experimentally and theoretically investigated. The experiment is conducted using an independently developed laboratory solar pond. A one dimensional simulation model for temperature and exergy simulation is developed and used to numerical study. The effect of adding porous media in solar pond are investigated under three cases to help distinguish the efficiencies, which are solar pond without adding any porous media (case A), with cinder (case B) and with multi-type porous media of cinder and cobblestones (case C), respectively. The results show the case with multi-type porous media gains the highest LCZ temperature and exergy efficiency, and the case without adding any porous media has the lowest temperature and efficiency; and the maximum transient exergy efficiencies of the three cases are found to be 19.23%, 19.83% and 21.61% respectively. The daily mean exergy efficiency of case C is 2.66% higher than case A, and 1.87% higher than case B. The results indicate that adding porous media, i.e., cinder or the mixture of cinder and cobblestones increases the LCZ temperature of solar pond, and it also improves the exergy performance of solar pond.
Keywords: salt gradient solar pond; porous media; experiment; exergy.
Exergy Analysis of Evacuated Tube Solar Collectors: A Review
by Gaurav Saxena
Abstract: The most recent studies show that different methodologies have been adopted to understand the concept of Exergy in general and exergy efficiency, exergy destruction, exergy losses, etc. in particular. The present paper reviews the concept of evacuated tube solar water heating systems followed by fundamental laws of thermodynamics necessary to provide concept of exergy analysis in detail. Mathematical modelling and experimental data provide the effect of mass flow rate, temperature gradient, inlet temperature, outlet temperature, collector efficiency, etc. on exergy. Finally, the exergy analysis and exergetic efficiencies along with exergy destruction sources for the evacuated tube collectors are presented.
Keywords: solar energy; evacuated tube collector (ETC); exergy analysis; exergy efficiency; solar water heating system.
Effect of pressure drop and longitudinal conduction on exergy destruction in a concentric-tube micro-fin tube heat exchanger
by Binash Imteyaz, Syed Zubair
Abstract: A numerical study is carried out to predict the heat transfer characteristics and various exergy losses in a micro-fin concentric-tube double pipe heat exchanger. The results are compared with the experimental data from the literature and are found to be in a good agreement. The study is extended to include the effect of pressure drop and longitudinal conduction on the exergy losses. Furthermore, Ethylene Glycol-Water as a hot fluid is considered to investigate the effect of higher viscosity on the frictional irreversible losses. It is found that, at lower mass flow rates, fluid friction irreversibility can be ignored for a low viscous fluid; however, there is a substantial exergy loss for a highly viscous fluid. At higher mass flow rates, fluid friction irreversibility dominates over thermal irreversibility for both the fluids investigated in this study.
Keywords: micro-fin tube heat exchanger; exergy analysis; longitudinal conduction.
Functional exergy efficiency of an air heat recovery exchanger under varying environmental temperature
by Vytautas Martinaitis, Giedre Streckiene, Darius Biekša, Giedrius Šiupšinskas, Juozas Bielskus
Abstract: The ventilation heat recovery exchanger (HRE) for low energy buildings is becoming one of the important heat transformers in the HVAC systems. The purpose of this paper is to develop the possibilities of exergy analysis for the HRE, when the reference temperature (RT) is variable. The method was prepared through consideration of the exergy flow direction changes, when variable RT was placed below, above and across the operating temperature of working fluids. By using the coenthalpy diagrams, there were obtained three functional exergy efficiencies which had greater sensitivity than universal exergy efficiency. The prepared solution allows the reliable using of the ubiquitous thermodynamic (exergy) analysis for energy chain in the HVAC systems. This methodology may be developed for other heat exchangers with different mass flows, operated in temperatures close to the environment temperature. The results could have a practical application for designing the HVAC systems and implementing their exergy optimum operation controls.
Keywords: HVAC systems; exergy analysis; functional exergy efficiency; ventilation heat recovery exchanger; coenthalpy.
Thermo-ecological performance analysis of a double-reheat Rankine cycle steam turbine system (RCSTS) with open and close feed water heaters
by Guven Gonca
Abstract: This study presents the exergetic and thermo-ecological performance optimization of a Rankine cycle steam turbine system (RCSTS) consisting of three turbines, one open feed water heater (OFWH) and two closed feed water heaters (CFWHs). In this study, different performance characteristics such as effective power, effective power density, exergy efficiency, exergy destruction and ecological coefficient of performance (ECOP) are used to investigate and optimize the system performance. The effects of condenser pressure, OFWH pressure and reheat pressures on the performance characteristics of the system have been examined by considering the irreversibilities and system properties. The results showed that the component pressures considerably affect the system performance. Therefore, they should be optimized to obtain maximum performance characteristics.
Keywords: Steam turbine; Rankine cycle; Thermo-ecological optimization; Power density; ECOP; Exergy.
The mode of interaction of the constituents of a microbial system determines the attainable exergy utilization
by Mustafa Özilgen, Bahar Değerli
Abstract: The typical text book definition of exergy is the maximum work that this system can produce if it is brought to thermal, mechanical and chemical equilibrium with its surroundings via reversible processes. In microbial systems, this ability depends on the interaction, e.g., collaboration, competition or antagonism, between the constituents of the system. Leavening of dough with mixed cultures of microorganisms is a typical example, where the constituents of a system may present different modes of interaction depending on the temperature and their relative amounts. Data pertinent to ten cases of leavening at two different temperatures and six different inoculum ratio of Saccharomyces cerevisiae and Lactobacillus plantarum were proceesed to relate the microbiological and the exergy efficiency optima. The maximum carbon dioxide production, expansion work, and dough volume increase were found to coinside with the case where the system achieved the highest exergy utilization and the highest CO2 production.
Keywords: Exergy efficiency; constituents of the system; ability of resource utilization; sour dough leavening.
Aircraft Fuel System Energy and Exergy Analysis under Hot Day Conditions
by Muhammet Yilanli, Onder Altuntas, Emin Acikkalp, T.Hikmet Karakoc
Abstract: In this paper, energy and exergy analysis of the fuel system is performed to investigate the variation of flow and heat energies of fuel. Thermal balance, formed by many different components of aircraft systems by means of heat management, has been significantly affected by aircraft performance. For that reason, thermal management of the fuel system in the aircraft has a significant indication concerning energy and exergy flow. The results indicate that the rate of energy transfer, which is occurred in the fuel system components throughout flight is respectively 42% for low pressure heat exchanger, 23% for high pressure heat exchanger and 22% for low pressure fuel pump and finally 13% for the high pressure fuel pump. In addition to that; during the flight the exergy destruction rate in the fuel system, in proportion to flight phases, is 51% during climbing phase, 21% during the acceleration phase, 14% during the idle position and 7% during both cruise and descent phase.
Keywords: energy analysis; exergy analysis; thermal management; aircraft fuel system.
Analysis of Effect of Construction Parameters on Energetic and Exergetic Efficiency of Induction Air Heaters
by Umit UNVER, Alper KELESOGLU, Ahmet YUKSEL, Halil Murat UNVER, Fikret YUKSEL
Abstract: The use of induction heating systems in different areas is increasing every day and the energy efficiency of induction heaters became an important issue. Thus, this study is concerned with the energy and exergy efficiency analysis of a novel air heater that operates with induction principle. The aim of this paper is investigation of the energy and the exergy efficiency augmentation of the new K-1 induction air heater prototype. Energy and exergy performance of the K-1 prototype of induction air heater was analysed and compared with the previous K-0 prototype. The analyses were performed with experimental study and with CFD simulation. The improvement of the flow geometry leaded to reduce the natural convection and the radiation heat losses 61.5 W and 115.5 W respectively. The new design boosted the energy efficiency up to 22.98%. The exergy efficiency of the K-1 was calculated to be around 11.5 % max. The CFD simulation results showed that energy efficiency increases linearly with the inlet velocity increment. A 1 m/s increase in the inlet velocity yields 3.1% and 0.06% augmentation in the energy and the exergy efficiency respectively. On the other hand, the inlet temperature increase negatively affects the energy and the actual exergy efficiencies. It was concluded that the induction air heaters would be more efficient and competitive with the appropriate flow field enhancements.
The improvement of the flow geometry leaded to reduce the natural convection and the radiation heat losses 61.5 W and 115.5 W respectively. The new design boosted the energy efficiency up to 22.98%. The exergy efficiency of the K-1 was calculated to be around 11.5 % max. The CFD simulation results showed that energy efficiency increases linearly with the inlet velocity increment. A 1 m/s increase in the inlet velocity yields 3.1% and 0.06% augmentation in the energy and the exergy efficiency respectively. On the other hand, the inlet temperature increase negatively affects the energy and the actual exergy efficiencies. It was concluded that the induction air heaters would be more efficient and competitive with the appropriate flow field enhancements.
Keywords: air heating; induction air heater; energy analysis; exergy analysis; thermal efficiency; exergetic efficiency.
Energy and Exergy Analysis of Advanced Absorption Power Cycles using Salt-Water Mixtures as Working Fluids
by Hasan Ozcan, Salem Yosaf
Abstract: In advanced absorption power cycles (AAPCs), a jet ejector is installed at the absorber inlet and serves for two main functions; it assists pressure recovery, and improves the mixing between the weak solution and the vapor coming from the turbine. These effects enhance the absorption of the vapor into the solution resulting in a better performing power cycle. The influence of the jet ejector on the energy and exergy efficiencies of an AAPC is evaluated, and the thermodynamic efficiencies of the AAPC are compared to those of conventional absorption power cycle (APCS) using three different working fluids, namely ammonia-water solution (NH3-H2O) , lithium bromide-water solution (LiBr-H2O), and lithium chloride-water solution (LiCl-H2O). Five cases are studied that represent the improvement in the AAPCS efficiencies as results of jet ejector integration in the cycle. Some parametric studies are performed comparatively by taking into account both power cycle configurations. Results of parametric studies depict that LiCl-H2O exhibits the highest energy and exergy efficiencies used in AAPC.
Keywords: Absorption power cycle; advanced absorption power cycle; Jet ejector; LiBr-water; NH3-water; LiCl-water; energy; exergy.
Evaluation of Cogeneration Plant with Steam and Electricity Production based on Thermoeconomic and Exergoenvironmental Analyses
by Eduardo Jose Cidade Cavalcanti
Abstract: A cogenerative system which produces 83.6 MW of steam and 119 MW of electricity was evaluated. The exergoeconomic and exergo environmental approach were carried out in order to calculate the cost rate and environmental impact per exergy unit of both products. The Specific Exergy Costing (SPECO) approach was used in balance. The specific cost rate of steam and electricity are 66.6 $/GJ and 62.8 $/GJ, respectively. And the specific environmental impact rate of steam and electricity are 15545 mPt/GJ and 14350 mPt/GJ, respectively. The results reveal that the combustor chamber has the most exergy destruction and higher cost rate of exergy destruction in both analyses. The higher total cost rate and environmental impact were in the gas turbine and the superheater, respectively. The lower exergoeconomic factor and exergoenvironmental factor was in the combustor. In addition, the effect of air environmental temperature at products (electricity and steam) was evaluated. The outcomes were the increase of air temperature, reduce the electrical power and exergetic efficiency. The increase of air temperature, increase the specific cost rate of electricity and steam. And the increase of air temperature, increase the specific environmental impact rate of electricity and steam.
Keywords: Cogeneration; exergoeconomy; steam; exergoenvironmental analyses; eco-indicator 99.
Energy-Exergy-Economic (E3) Analysis of Stand-Alone Solar Thermal Cogeneration Power Plant
by K.S. Reddy
Abstract: Concentrating solar power (CSP) based solar cogeneration plants are more energy efficient compared to single purpose power plants. In this paper, energy, exergy and economic analysis have been carried out for the stand-alone solar thermal co-generation power plant (SASTCPP), which is based on Rankine cycle. Both direct and indirect steam generation is adopted for the comparative E3 analysis. To achieve high energy efficiency, back pressure steam turbine with regenerative feed water heating is incorporated in power block. Reference cogeneration power plant has a capacity of 50 MWth. Levelized cost of electricity (LCOE) associated to SASTCPP is calculated with three different cost allocation approach direct, equivalent fuel consumption, and exergy approach. The observed exergy efficiency (33 to 35%) is comparatively less than energy efficiency (62 to 70 %) and the maximum exergy destruction takes place within the solar field. Minimum LCOE calculated as Indian rupees (INR) 5.46 with direct steam generation.
Keywords: concentrating solar power; energy-exergy-economic analysis; levelized cost of electricity; solar thermal cogeneration power plant; direct steam generation.
Thermodynamic optimization of rectangular and elliptical microchannels with nanofluids
by Bouthayna Khlifi, Brahim Kalech, Mourad Bouterra, Afif Cafsi
Abstract: In this paper, the thermodynamic optimization of laminar flow in rectangular and elliptical microchannels subjected to low or high heat flux is analytically studied. Two mathematical models are developed, by incorporating the aspect ratio term, to evaluate entropy generation numbers. Two relations for calculating the conductivity and the viscosity of CuO-water nanofluids, based on experimental data from literature, are proposed. An entropy generation analysis is proposed to optimize the geometry of the microchannel (aspect ratio, shape) and the working fluid (type of nanofluid, nanoparticles concentration, thermophysical properties) for each heat flux condition. It has been revealed that at low heat flux, using water as working fluid flowing inside microchannels with aspect ratio ε=1 is more efficient from the standpoint of thermodynamic criteria. However, at high heat flux, it is more advisable to use microchannels with the smallest possible aspect ratio containing CuO-water nanofluids with higher conductivity and higher nanoparticles concentration. For the two cases of heat flux values, the rectangular microchannel reaches the minimum value of total entropy generation number compared to the elliptical microchannel.
Keywords: Thermodynamic performance; entropy generation; irreversibilities; rectangular microchannel; elliptical microchannel; aspect ratio; nanofluids; thermophysical properties ; Nusselt number; Poiseuille number.
Novel pinch point method based exergetic optimization of subcritical organic Rankine cycle for waste heat recovery
by Jahar Sarkar
Abstract: New pinch point design methodology for subcritical organic Rankine cycle is proposed to predict pinch point locations in both evaporator and condenser, and optimize working fluid mass flow rate to get maximum heat recovery and exergetic efficiency for given heat source and heat sink conditions. Selected working fluids have been compared based on various performance parameters for various heat source conditions. The present method seems to be better than previous pinch point design methods as it optimize the cycle by considering both source and sink. At optimum operation, ammonia is best in terms of lower mass flow rate and turbine size, toluene is best in terms of lowest heat exchanger size and cost, whereas, R-1233zd and isopentane are best in terms of higher power output and heat recovery, exergy and entrancy efficiencies. Contour plots are presented as well to select optimum design parameters for available heat source and sink.
Keywords: Waste heat recovery; Organic Rankine Cycle; Pinch point temperature difference; Exergetic efficiency; Irreversibility; Entrancy efficiency.
Special Issue on: IEEES-9 Exergetic Dimensions of Energy Systems
Energy and exergy analyses of a solar air heater with wire mesh-covered absorber plate
by Atilla G. Devecioğlu, Vedat Oruç, Zafer Tuncer
Abstract: Thermodynamic analysis on a novel design of solar air heater having absorber plate covered with copper wire mesh was investigated in this study. The newly-designed collector was tested for various air flow rates and tilt angles of collector, and then the results were compared for the case without wire mesh. The experimental study was carried out for mass flow rates of 0.030 and 0.055 kg/s as well as collector tilt angles of 25
Keywords: Solar air heater; wire mesh; exergy; thermal efficiency.
Comparative Energy and Exergy Studies of Combined CO2 Brayton-Organic Rankine Cycle Integrated with Solar Tower Plant
by Abdullah AlZahrani, Ibrahim Dincer
Abstract: The present study investigates the performance of a combined cycle power plant used as a power block for solar tower (central receiver) system. The combined power cycle employs a supercritical carbon dioxide (S-CO2) Brayton cycle as a topping cycle and an Organic Rankine Cycle (ORC) as bottoming cycle. A solar concentrating system consists of a central receiver, and a heliostat field provides thermal power to the combined cycle. To evaluate the system performance, a thermodynamic analysis is conducted through energy and exergy approaches for each subsystem and hence evaluation of its overall energy and exergy efficiencies. Furthermore, the energy and exergy efficiencies of the integrated system are parametrically studied under different operating conditions. The features of using CO2 as a working fluid in a Brayton cycle are elaborated in the context of high temperature solar tower technologies. Moreover, the opportunity of recovering the topping cycle low-grade heat is investigated, and an n-butane based ORC is, in this regard, proposed as a bottoming cycle. The integrated solar plant with a combined cycle and a solar tower is modeled, and the conceptual feasibility of the integration of the S-CO2 Brayton cycle with ORC is illustrated.
Keywords: Solar tower; Carbon dioxide; Brayton cycle; ORC; Energy and Exergy analysis; integrated power system.
Exergo-economic Analysis of Parabolic Trough Integrated Cogeneration Power Plant
by K.S. Reddy
Abstract: Cogeneration plant produces heat and electricity simultaneously, and have a great potential towards best use of primary energy resources as compared to other plants, which produces heat and electricity separately. Parabolic trough integrated cogeneration power plant (PTICPP) is one of the attractive alternatives to utilizing solar energy in an efficient manner. The economically viable PTICPP could bring an opportunity for its utilization at large scale. Therefore, estimation of levelised cost of electricity (LCE) is done by two different rational cost apportionment method namely; lost-kilowatt and exergy method. To achieve the objective of this paper, a reference PTICPP having a capacity of 20 MWe and 61.94 MWth is designed, and comprehensive thermal performance of reference plant is analyzed at the variable amount of irradiation. In Indian climatic condition, the minimum estimated LCE are 0.127 $/kWh and 0.114 $/kWh respectively by lost-kilowatt and exergy method.
Keywords: Parabolic trough integrated cogeneration power plant; direct steam generation; exergo-economics; levelized cost of electricity.