Exergy Loss Rate Research Articles

Performance assessment and optimization of industrial pasta drying

SUMMARY Drying is a high-energy-intensive operation and an important step in the pasta production. In this study, exergy analysis of a four-step drying system in a farfalle pasta production line using actual operational data obtained from a plant located in Izmir, Turkey, was performed. Exergy loss rates, evaporation rates, exergy efficiencies, and improvement in potential rates for each dryer section were determined in this drying system. The exergy efficiency values varied between 0.25% and 5.27% from the predrying to the final drying section. The exergy efficiency value for the entire drying system was calculated to be 2.96%, and the highest exergetic improvement in potential rate was 165.54 kW for the first dryer section. Copyright © 2013 John Wiley & Sons, Ltd.

Exergoeconomic analysis of a central heating system from the generation stage to the building envelope

This paper deals with exergoeconomic analysis of a dormitory heated by a conventional boiler in a central heating system. The energy and exergy flows are examined from the generation stage to the envelope of the building. The energy and exergy flows between the stages are obtained using a predesign tool for an optimized building design. Exergetic and exergoeconomic analyses of some heating equipment are comprehensively evaluated. A conventional boiler and a water heater in the central heating system are considered in the analysis. Based on the analysis results obtained, the total exergy input rate is calculated to be 1455kW while the largest exergy loss rate is found to be 987.6kW. Exergetic efficiencies of the conventional boiler and the water heater are also obtained to be 15.6% and 20.4%, respectively.

A comparative study on energetic, exergetic and environmental performance assessments of novel M-Cycle based air coolers for buildings

In this study, three various novel air coolers based on M-Cycle are evaluated using energy and exergy analyses based efficiency assessments along with environmental impact and sustainability parameters. The M-Cycle systems are considered to cool a building room air while their inlet air parameters are same, but outlet cooled air parameters are different. Systems I and III draw electricity directly taken from an electric grid in the building while System II, which is stand alone system, produces and draws electricity from its solar PV panels. In the energy analysis, wet bulb effectiveness, cooling capacity, Coefficient of Performance (energetic COP) and Primary Energy Ratio ( PER) are found. In the exergy analysis, exergy input and output rates, exergy loss rate, exergy destruction rate, Exergetic Coefficient of Performance ( COP ex ), Primary Exergy Ratio ( PE xR ) and exergy efficiency are obtained for six different dead state temperatures changing between 10 °C and 35 °C. Also, sustainability assessments of the systems are obtained using sustainability index ( SI) tool for these various dead state temperatures. Finally, environmental assessments of the systems are calculated from their greenhouse gas (GHG) emissions (gCO 2/kW h) due to their electricity consumptions. Maximum exergy efficiencies and sustainability assessments are found to be 35.13% and 1.5415 for System III and 34.94% and 1.5372 for System II, respectively. GHG emissions of the systems are calculated to be 2119.68 gCO 2/day, 153.6 gCO 2/day and 3840 gCO 2/day for Systems I, II and III respectively. So, System II becomes a good choose to prevent the global warming and to attain sustainable future.

Heating load, COP, exergy loss rate, exergy output rate and ecological analyses and optimisations for irreversible universal steady flow variable temperature heat reservoir heat pump cycle model

AbstractThe performance of an irreversible universal steady flow heat pump cycle model with variable temperature heat reservoirs is analysed and optimised by using finite time thermodynamics. The universal heat pump cycle consists of two heat absorbing branches, two heat releasing branches and two irreversible adiabatic branches, with the losses of heat resistance and internal irreversibility. Expressions of heating load, coefficient of performance (COP), exergy output rate, exergy loss rate and ecological function of Brayton, Otto, Diesel, Atkinson, Dual, Miller and Carnot heat pump cycles are derived. There exists an optimal heat conductance distribution and an optimal thermal capacity rate matching between the working fluid and heat reservoir which leads to maximum heating load, maximum exergy output rate and maximum ecological function respectively. Performance comparisons among heating load, COP, exergy output rate, exergy loss rate and ecological function objectives are carried out. The effects of t...

Optimal performance for a steady-flow endoreversible refrigerator model including five typical cycles

The optimal performance of a universal endoreversible steady-flow refrigerator cycle consisting of a constant thermal-capacity heating branch, a constant thermal-capacity cooling branch and two adiabatic branches with heat transfer loss was analysed using finite-time thermodynamics. The analytical formulae about cooling load, coefficient of performance, exergy loss rate, exergy output rate and ecological criterion of the endoreversible steady-flow refrigerator cycle are derived. Moreover, analysis and optimisation of the model were carried out in order to investigate the effect of the cycle process on the performance of the cycles using numerical examples. The results obtained include the performance characteristics of endoreversible steady-flow Carnot, Diesel, Otto, Atkinson and Brayton refrigeration cycles. The results can provide some theoretical guidance for the design of practical refrigerators.

Ecological performance of an endoreversible Carnot refrigerator with complex heat transfer law

The optimal ecological objective of an endoreversible Carnot refrigerator based on a new generalised heat transfer law , including generalised convective heat transfer law and generalised radiative heat transfer law, is derived. The ecological objective function representing the best compromise between the exergy output rate and exergy loss rate (entropy production rate) of the refrigerator. The results include the optimal ecological performance of the endoreversible Carnot refrigerator with various heat transfer laws, which obtained in many literatures.

Exergetic Analysis of Textile Convective Drying with Stenters by Subsystem Models: Part 1—Exergetic Modeling and Evaluation

This study, which consists of two parts, deals with the exergy analyses and assessments of the direct gas–heated (DGHS) and hot oil–heated (HOHS) stenters. In the first part, a new model for the exergetic analysis of the convective drying of textiles at stenters was presented and the variations of exergetic parameters for each chamber of the stenter were examined. It was emphasized that the exergy efficiency of the last chambers decreased drastically due to the lower evaporation rate of the falling rate period of drying. Additionally, the subsystems of the chambers were analyzed. For this purpose, the detailed control volume models were conducted for the stenters. It was determined that the combustion chamber and mixing unit of the DGHS and the hot oil boiler of the HOHS led to higher exergy destruction rates. Furthermore, total exergy destruction and loss rates of the HOHS were higher compared to those with the DGHS. The exergy efficiency values of each chamber of the DGHS were calculated to be 10.9, 14.9, 15.3, 12.2, 9.8, and 5.3%, respectively.

Thermodynamic analysis of a building using exergy analysis method

This study deals with exergetic assessment of an educational building heated by a conventional boiler in a heating center. The heating system is examined from the generation stage to the envelope of the building. In general the heat loss calculations are made using both energy and exergy analysis methods. The energy and exergy flows between the stages are obtained using a pre-design tool for an optimized building design. Energy and exergy losses are obtained to evaluate the performance of the system. A conventional boiler in the heating center and a fan coil unit in a room are considered in the analysis. Total exergy input rate is calculated to be 694.5 kW, while the largest exergy loss rate is obtained to be 333 kW. Exergetic efficiencies of the conventional boiler and the fan coil unit are also found to be 13.4% and 37.6%, respectively.

Performance and thermo-economic assessments of geothermal district heating system: A case study in Afyon, Turkey

In this study energy, exergy and exergoeconomic analysis of the Afyon geothermal district heating system (AGDHS) in Afyon, Turkey is performed through thermodynamic performances and thermo-economic assessments. In the analysis, actual system data are used to assess the district heating system performance, energy and exergy efficiencies, exergy losses and loss cost rates. Energy and exergy losses throughout the AGDHS are quantified and illustrated in the flow diagram. The energy and exergy efficiencies of the overall system are found to be 37.59% and 47.54%, respectively. The largest exergy loss occurs in the heat exchangers with 14.59% and then in the reinjection wells with 14.09%. Besides, thermo-economic evaluations of the AGDHS are given in table. Energy and exergy loss rates for the AGDHS are estimated to be 5.36 kW/$ and 0.2 kW/$, respectively.

Exergy analysis of ice storage air-conditioning system with heat pipe during charging period

The ice storage air-conditioning system with heat pipe was presented, and analytical model relating to the exergy loss amount and loss rate was developed in each component of the ice storage air-conditioning system. Exergy analysis results of the ice storage air-conditioning system with heat pipe were compared with those of the ice storage air-conditioning system with ice-on-coil. The energy saving and exergy loss reduction possibilities were also analyzed and discussed. The results indicated that the exergy efficiency of the ice storage air-conditioning system with heat pipe is 9.55% higher than that of the ice storage air-conditioning system with ice-on-coil. The exergy loss in the condenser can be reduced by reducing thermal resistance and the temperature difference of heat transfer or utilizing exergy of heat rejected by the condenser. The exergy loss in the expansion valve is reduced through increasing supercooling temperature of the refrigerant before entering into the expansion valve or decreasing pressure difference between the condensing pressure and the evaporating pressure.

Exergoeconomic analysis of an underground air tunnel system for greenhouse cooling system

This paper investigates some exergoeconomic parameters for an underground air tunnel system based upon some operating conditions. The ratio of exergy loss rate to capital cost ( R ex) changes between 0.052 and 0.552. The total exergy losses values are obtained to be from 0.26 kW to 2.50 kW for the system. The daily average maximum cooling coefficient of performances (COP) values for the system are also obtained to be 11.96 for experimental period, while the total average COP is found to be 5.89. The overall exergy efficiency value for the system on a product/fuel basis is found to be 56.9%.

Second-law performance of heat exchangers for waste heat recovery

Exergy change rate in an ideal gas flow or an incompressible flow can be divided into a thermal exergy change rate and a mechanical exergy loss rate. The mechanical exergy loss rates in the two flows were generalized using a pressure-drop factor. For heat exchangers using in waste heat recovery, the consumed mechanical exergy is usually more valuable than the recovered thermal exergy. A weighing factor was proposed to modify the pressure-drop factor. An exergy recovery index (ηII) was defined and it was expressed as a function of effectiveness (ɛ), ratio of modified heat capacity rates (C∗), hot stream-to-dead-state temperature ratio, cold stream-to-dead-state temperature ratio and modified overall pressure-drop factor. This ηII–ɛ relation can be used to find the ηII value of a heat exchanger with any flow arrangement. The ηII−Ntu and ηII−Ntuh relations of cross-flow heat exchanger with both fluids unmixed were established respectively. The former provides a minimum Ntu design principle and the latter provides a minimum Ntuh design principle. A numerical example showed that, at a fixed heat capacity rate of the hot stream, the heat exchanger size yielded by the minimum Ntuh principle is smaller than that yielded by the minimum Ntu principle.

Ecological optimization of an irreversible quantum Carnot heat engine with spin-1/2 systems

A model of a quantum heat engine with heat resistance, internal irreversibility and heat leakage and many non-interacting spin-1/2 systems is established in this paper. The quantum heat engine cycle is composed of two isothermal processes and two irreversible adiabatic processes and is referred to as a spin quantum Carnot heat engine. Based on the quantum master equation and the semi-group approach, equations of some important performance parameters, such as power output, efficiency, entropy generation rate and ecological function (a criterion representing the optimal compromise between exergy output rate and exergy loss rate), for the irreversible spin quantum Carnot heat engine are derived. The optimal ecological performance of the heat engine in the classical limit is analyzed with numerical examples. The effects of internal irreversibility and heat leakage on ecological performance are discussed in detail.

Ecological optimization of an irreversible harmonic oscillators Carnot heat engine

A model of an irreversible quantum Carnot heat engine with heat resistance, internal irreversibility and heat leakage and many non-interacting harmonic oscillators is established in this paper. Based on the quantum master equation and semi-group approach, equations of some important performance parameters, such as power output, efficiency, exergy loss rate and ecological function for the irreversible quantum Carnot heat engine are derived. The optimal ecological performance of the heat engine in the classical limit is analyzed with numerical examples. Effects of internal irreversibility and heat leakage on the ecological performance are discussed. A performance comparison of the quantum heat engine under maximum ecological function and maximum power conditions is also performed.

Energy and exergy analysis of novel solar bi-ejector refrigeration system with injector

Energy and exergy balances were done on a novel solar bi-ejector refrigeration system with R123, whose circulation pump is replaced by an injector. The analysis result of the novel system was compared with that of the original one. The effect of operation condition on system energy efficiency, exergy efficiency and exergy loss was analyzed, and the dynamic performance of a designed solar bi-ejector refrigeration system was also studied. The comparative results indicate that under the same operating condition, the novel system and the original system have equal energy efficiency, exergy efficiency and exergy loss, and the only difference between them is the exergy losses of the generators and the added injector. The other conclusions mainly include: the solar collector has the largest exergy loss rate of over 90% and for the bi-ejector refrigeration subcycle, the ejector has the largest exergy loss rate of about 5%; the total exergy loss changes inversely proportional to the evaporation temperature and positively proportional to the condensation temperature; when the other parameters are fixed, there exists an optimum generation temperature, at which the overall energy and exergy efficiencies are both the maximum and the total exergy loss is the minimum. The study points out the direction for optimizing the novel solar bi-ejector refrigeration system. Copyright © 2009 John Wiley & Sons, Ltd.

Theoretical analysis of LiBr/H2O absorption refrigeration systems

A computational model is developed for the parametric investigation of single-effect and series flow double-effect LiBr/H2O absorption refrigeration systems. The effects of generator, absorber, condenser, evaporator and dead state temperatures are examined on the performance of these systems. The parameters computed are coefficient of performance (COP), exergy destruction rates, thermal exergy loss rates, irreversibility and exergetic efficiency. The results indicate that COP and exergetic efficiency of both the systems increase with increase in the generator temperature. There exist different optimum values of generator temperature for maximum COP and maximum exergetic efficiency. The optimum generator temperature is lower corresponding to maximum exergetic efficiency as compared to optimum generator temperature corresponding to maximum COP. The effect of increase in absorber, condenser and evaporator temperatures is to decrease the exergetic efficiency of both the systems. The irreversibility is highest in absorber in both systems. Copyright © 2009 John Wiley & Sons, Ltd.

Effect of a complex generalised heat transfer law on the ecological performance of an endoreversible Carnot heat pump

SYNOPSIS The effect of a complex generalised heat transfer law q ∝ (ΔTn)m on the ecological performance of an endoreversible Carnot heat pump is derived by taking as the objective function an ecological optimisation criterion representing the best compromise between the exergy output rate and exergy loss rate (entropy production rate) of the heat pump. The results obtained include the optimal ecological performance of an endoreversible heat pump with various heat transfer laws, as obtained in existing literature.

Analysis of Some Exergoeconomic Parameters of Small Wind Turbine System

This paper investigates some exergoeconomic parameters (energy and exergy loss ratios) for a 1.5 kW wind turbine system, linking capital costs and thermodynamic losses, based on some operating conditions. The results show that while the ratio of energy loss rate to capital cost (Ren) changes between 0.007 and 0.813 at different wind speeds, the ratio of exergy loss rate to capital cost (Rex) changes between 0.006 and 0.411. In addition, the maximum Ren and Rex values are obtained at a wind speed of 12 m/s. Furthermore, a parametric study is undertaken to investigate how varying wind speed will affect the exergoeconomic parameters of the wind turbine system and to develop a correlation between the ratio of thermodynamic loss rate to capital cost and wind speed for practical applications of wind energy systems.

Exergy analysis of a vapour-compression refrigerating system using R410A as refrigerant

In this study, the first and second laws of thermodynamics are employed to analyse the performance of an experimental Vapour-Compression Refrigerating Machine (VCRM) using R410A. The exergy method is used to evaluate the irreversibility of each component when some working parameters are varied. The dimensionless exergy loss rate of each component is analysed, emphasising their contribution to the total exergy loss rate.

Exergy-based ecological optimisation for an endoreversible air heat pump cycle

SYNOPSIS On the basis of exergetic analysis, ecological performance analysis and optimisation of an endoreversible simple air heat pump cycle with constant-temperature heat reservoirs is carried out in this paper. An exergy-based ecological optimisation criterion, which consists of maximising a function representing the best compromise between the exergy output rate and exergy loss rate (entropy generation rate and environment temperature product) of the heat pump cycle, is taken as the objective function. The analytical formulae concerning the relationship between the exergy-based ecological function and pressure ratio is derived with heat resistance losses in the hot- and cold-side heat exchangers. The effects of the effectivenesses of the heat exchangers, the ratio of hot-side heat reservoir temperature to ambient temperature and the heat reservoir temperature ratio on the exergy-based ecological function are analysed. The cycle performance optimisations are performed by identifying the optimum pressure ratio and the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for a fixed total heat exchanger inventory, respectively. The expressions of the maximum ecological function are derived. The influences of some design parameters, including heat capacity rate of the working fluid and heat exchanger inventory on the optimal performance of the endoreversible air heat pump are provided by numerical examples. The results show that exergy-based ecological optimisation is an important and effective criterion for the evaluation of air heat pumps.