Entropy

Editorial

Jump to: Research

5 pages, 181 KiB

Open AccessEditorial

Work Availability and Exergy Analysis

by Pouria Ahmadi and Behnaz Rezaie

Entropy 2018, 20(8), 597; https://doi.org/10.3390/e20080597 - 10 Aug 2018

Cited by 2 | Viewed by 3389

Abstract

n/a Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

Research

Jump to: Editorial

9 pages, 2503 KiB

Open AccessArticle

Exergy Analyses of Onion Drying by Convection: Influence of Dryer Parameters on Performance

by María Castro, Celia Román, Marcelo Echegaray, Germán Mazza and Rosa Rodriguez

Entropy 2018, 20(5), 310; https://doi.org/10.3390/e20050310 - 25 Apr 2018

Cited by 34 | Viewed by 5152

Abstract

This research work is concerned in the exergy analysis of the continuous-convection drying of onion. The influence of temperature and air velocity was studied in terms of exergy parameters. The energy and exergy balances were carried out taking into account the onion drying [...] Read more.

This research work is concerned in the exergy analysis of the continuous-convection drying of onion. The influence of temperature and air velocity was studied in terms of exergy parameters. The energy and exergy balances were carried out taking into account the onion drying chamber. Its behavior was analyzed based on exergy efficiency, exergy loss rate, exergetic improvement potential rate, and sustainability index. The exergy loss rates increase with the temperature and air velocity augmentation. Exergy loss rate is influenced by the drying air temperatures and velocities because the overall heat transfer coefficient varies with these operation conditions. On the other hand, the exergy efficiency increases with the air velocity augmentation. This behavior is due to the energy utilization was improved because the most amount of supplied energy was utilized for the moisture evaporation. However, the exergy efficiency decreases with the temperature augmentation due to the free moisture being lower, then, the moisture begins diffusing from the internal structure to the surface. The exergetic improvement potential rate values show that the exergy efficiency of onion drying process can be ameliorated. The sustainability index of the drying chamber varied from 1.9 to 5.1. To reduce the process environmental impact, the parameters must be modified in order to ameliorate the exergy efficiency of the process. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

17 pages, 1000 KiB

Open AccessArticle

Exergy Analysis and Human Body Thermal Comfort Conditions: Evaluation of Different Body Compositions

by Matheus Martinez Garcia, Rafael Yoshimori Une, Silvio De Oliveira Junior and Carlos Eduardo Keutenedjian Mady

Entropy 2018, 20(4), 265; https://doi.org/10.3390/e20040265 - 10 Apr 2018

Cited by 12 | Viewed by 4949

Abstract

This article focuses on studying the effects of muscle and fat percentages on the exergy behavior of the human body under several environmental conditions. The main objective is to relate the thermal comfort indicators with exergy rates, resulting in a Second Law perspective [...] Read more.

This article focuses on studying the effects of muscle and fat percentages on the exergy behavior of the human body under several environmental conditions. The main objective is to relate the thermal comfort indicators with exergy rates, resulting in a Second Law perspective to evaluate thermal environment. A phenomenological model is proposed of the human body with four layers: core, muscle, fat and skin. The choice of a simplified model is justified by the facility to variate the amount of mass in each tissue without knowing how it spreads around the body. After validated, the model was subjected to a set of environmental conditions and body compositions. The results obtained indicate that the area normalization (Watts per square meter) may be used as a safe generalization for the exergy transfer to environment. Moreover, the destroyed exergy itself is sufficient to evaluate the thermal sensation when the model is submitted to environmental temperatures lower than that considered for the thermal neutrality condition (and, in this text, the thermal comfort) . Nevertheless, for environments with temperatures higher than the calculated for the thermal neutrality, the combination of destroyed exergy and the rate of exergy transferred to the environment should be used to properly evaluate thermal comfort. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

23 pages, 4772 KiB

Open AccessArticle

Engine Load Effects on the Energy and Exergy Performance of a Medium Cycle/Organic Rankine Cycle for Exhaust Waste Heat Recovery

by Peng Liu, Gequn Shu, Hua Tian and Xuan Wang

Entropy 2018, 20(2), 137; https://doi.org/10.3390/e20020137 - 21 Feb 2018

Cited by 15 | Viewed by 5687

Abstract

The Organic Rankine Cycle (ORC) has been proved a promising technique to exploit waste heat from Internal Combustion Engines (ICEs). Waste heat recovery systems have usually been designed based on engine rated working conditions, while engines often operate under part load conditions. Hence, [...] Read more.

The Organic Rankine Cycle (ORC) has been proved a promising technique to exploit waste heat from Internal Combustion Engines (ICEs). Waste heat recovery systems have usually been designed based on engine rated working conditions, while engines often operate under part load conditions. Hence, it is quite important to analyze the off-design performance of ORC systems under different engine loads. This paper presents an off-design Medium Cycle/Organic Rankine Cycle (MC/ORC) system model by interconnecting the component models, which allows the prediction of system off-design behavior. The sliding pressure control method is applied to balance the variation of system parameters and evaporating pressure is chosen as the operational variable. The effect of operational variable and engine load on system performance is analyzed from the aspects of energy and exergy. The results show that with the drop of engine load, the MC/ORC system can always effectively recover waste heat, whereas the maximum net power output, thermal efficiency and exergy efficiency decrease linearly. Considering the contributions of components to total exergy destruction, the proportions of the gas-oil exchanger and turbine increase, while the proportions of the evaporator and condenser decrease with the drop of engine load. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

22 pages, 3212 KiB

Open AccessArticle

Exergoeconomic Assessment of Solar Absorption and Absorption–Compression Hybrid Refrigeration in Building Cooling

by Yue Jing, Zeyu Li, Liming Liu and Shengzi Lu

Entropy 2018, 20(2), 130; https://doi.org/10.3390/e20020130 - 17 Feb 2018

Cited by 20 | Viewed by 4285

Abstract

The paper mainly deals with the match of solar refrigeration, i.e., solar/natural gas-driven absorption chiller (SNGDAC), solar vapor compression–absorption integrated refrigeration system with parallel configuration (SVCAIRSPC), and solar absorption-subcooled compression hybrid cooling system (SASCHCS), and building cooling based on the exergoeconomics. Three types [...] Read more.

The paper mainly deals with the match of solar refrigeration, i.e., solar/natural gas-driven absorption chiller (SNGDAC), solar vapor compression–absorption integrated refrigeration system with parallel configuration (SVCAIRSPC), and solar absorption-subcooled compression hybrid cooling system (SASCHCS), and building cooling based on the exergoeconomics. Three types of building cooling are considered: Type 1 is the single-story building, type 2 includes the two-story and three-story buildings, and type 3 is the multi-story buildings. Besides this, two Chinese cities, Guangzhou and Turpan, are taken into account as well. The product cost flow rate is employed as the primary decision variable. The result exhibits that SNGDAC is considered as a suitable solution for type 1 buildings in Turpan, owing to its negligible natural gas consumption and lowest product cost flow rate. SVCAIRSPC is more applicable for type 2 buildings in Turpan because of its higher actual cooling capacity of absorption subsystem and lower fuel and product cost flow rate. Additionally, SASCHCS shows the most extensive cost-effectiveness, namely, its exergy destruction and product cost flow rate are both the lowest when used in all types of buildings in Guangzhou or type 3 buildings in Turpan. This paper is helpful to promote the application of solar cooling. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

16 pages, 926 KiB

Open AccessArticle

Exergy Analysis of the Musculoskeletal System Efficiency during Aerobic and Anaerobic Activities

by Gabriel Marques Spanghero, Cyro Albuquerque, Tiago Lazzaretti Fernandes, Arnaldo José Hernandez and Carlos Eduardo Keutenedjian Mady

Entropy 2018, 20(2), 119; https://doi.org/10.3390/e20020119 - 11 Feb 2018

Cited by 11 | Viewed by 6134

Abstract

The first and second laws of thermodynamics were applied to the human body in order to evaluate the quality of the energy conversion during muscle activity. Such an implementation represents an important issue in the exergy analysis of the body, because there is [...] Read more.

The first and second laws of thermodynamics were applied to the human body in order to evaluate the quality of the energy conversion during muscle activity. Such an implementation represents an important issue in the exergy analysis of the body, because there is a difficulty in the literature in evaluating the performed power in some activities. Hence, to have the performed work as an input in the exergy model, two types of exercises were evaluated: weight lifting and aerobic exercise on a stationary bicycle. To this aim, we performed a study of the aerobic and anaerobic reactions in the muscle cells, aiming at predicting the metabolic efficiency and muscle efficiency during exercises. Physiological data such as oxygen consumption, carbon dioxide production, skin and internal temperatures and performed power were measured. Results indicated that the exergy efficiency was around 4% in the weight lifting, whereas it could reach values as high as 30% for aerobic exercises. It has been shown that the stationary bicycle is a more adequate test for first correlations between exergy and performance indices. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

22 pages, 1998 KiB

Open AccessArticle

Comparative Evaluation of Integrated Waste Heat Utilization Systems for Coal-Fired Power Plants Based on In-Depth Boiler-Turbine Integration and Organic Rankine Cycle

by Shengwei Huang, Chengzhou Li, Tianyu Tan, Peng Fu, Ligang Wang and Yongping Yang

Entropy 2018, 20(2), 89; https://doi.org/10.3390/e20020089 - 29 Jan 2018

Cited by 13 | Viewed by 7021

Abstract

To maximize the system-level heat integration, three retrofit concepts of waste heat recovery via organic Rankine cycle (ORC), in-depth boiler-turbine integration, and coupling of both are proposed, analyzed and comprehensively compared in terms of thermodynamic and economic performances. For thermodynamic analysis, exergy analysis [...] Read more.

To maximize the system-level heat integration, three retrofit concepts of waste heat recovery via organic Rankine cycle (ORC), in-depth boiler-turbine integration, and coupling of both are proposed, analyzed and comprehensively compared in terms of thermodynamic and economic performances. For thermodynamic analysis, exergy analysis is employed with grand composite curves illustrated to identify how the systems are fundamentally and quantitatively improved, and to highlight key processes for system improvement. For economic analysis, annual revenue and investment payback period are calculated based on the estimation of capital investment of each component to identify the economic feasibility and competitiveness of each retrofit concept proposed. The results show that the in-depth boiler-turbine integration achieves a better temperature match of heat flows involved for different fluids and multi-stage air preheating, thus a significant improvement of power output (23.99 MW), which is much larger than that of the system with only ORC (6.49 MW). This is mainly due to the limitation of the ultra-low temperature (from 135 to 75 °C) heat available from the flue gas for ORC. The thermodynamic improvement is mostly contributed by the reduction of exergy destruction within the boiler subsystem, which is eventually converted to mechanical power; while the exergy destruction within the turbine system is almost not changed for the three concepts. The selection of ORC working fluids is performed to maximize the power output. Due to the low-grade heat source, the cycle with R11 offers the largest additional net power generation but is not significantly better than the other preselected working fluids. Economically, the in-depth boiler-turbine integration is the most economic completive solution with a payback period of only 0.78 year. The ORC concept is less attractive for a sole application due to a long payback time (2.26 years). However, by coupling both concepts, a net power output of 26.51 MW and a payback time of almost one year are achieved, which may promote the large-scale production and deployment of ORC with a cost reduction and competitiveness enhancement. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

18 pages, 1520 KiB

Open AccessArticle

Exergetic Analysis, Optimization and Comparison of LNG Cold Exergy Recovery Systems for Transportation

by Paweł Dorosz, Paweł Wojcieszak and Ziemowit Malecha

Entropy 2018, 20(1), 59; https://doi.org/10.3390/e20010059 - 13 Jan 2018

Cited by 33 | Viewed by 7759

Abstract

LNG (Liquefied Natural Gas) shares in the global energy market is steadily increasing. One possible application of LNG is as a fuel for transportation. Stricter air pollution regulations and emission controls have made the natural gas a promising alternative to liquid petroleum fuels, [...] Read more.

LNG (Liquefied Natural Gas) shares in the global energy market is steadily increasing. One possible application of LNG is as a fuel for transportation. Stricter air pollution regulations and emission controls have made the natural gas a promising alternative to liquid petroleum fuels, especially in the case of heavy transport. However, in most LNG-fueled vehicles, the physical exergy of LNG is destroyed in the regasification process. This paper investigates possible LNG exergy recovery systems for transportation. The analyses focus on “cold energy” recovery systems as the enthalpy of LNG, which may be used as cooling power in air conditioning or refrigeration. Moreover, four exergy recovery systems that use LNG as a low temperature heat sink to produce electric power are analyzed. This includes single-stage and two-stage direct expansion systems, an ORC (Organic Rankine Cycle) system, and a combined system (ORC + direct expansion). The optimization of the above-mentioned LNG power cycles and exergy analyses are also discussed, with the identification of exergy loss in all components. The analyzed systems achieved exergetic efficiencies in the range of

20 %

to

36 %

, which corresponds to a net work in the range of 214 to 380 kJ/kg

_{L N G}

. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

3324 KiB

Open AccessArticle

Exergy Analysis of a Pilot Parabolic Solar Dish-Stirling System

by Ehsan Gholamalizadeh and Jae Dong Chung

Entropy 2017, 19(10), 509; https://doi.org/10.3390/e19100509 - 21 Sep 2017

Cited by 30 | Viewed by 6532

Abstract

Energy and exergy analyses were carried out for a pilot parabolic solar dish-Stirling System. The system was set up at a site at Kerman City, located in a sunny desert area of Iran. Variations in energy and exergy efficiency were considered during the [...] Read more.

Energy and exergy analyses were carried out for a pilot parabolic solar dish-Stirling System. The system was set up at a site at Kerman City, located in a sunny desert area of Iran. Variations in energy and exergy efficiency were considered during the daytime hours of the average day of each month in a year. A maximum collector energy efficiency and total energy efficiency of 54% and 12.2%, respectively, were predicted in July, while during the period between November and February the efficiency values were extremely low. The maximum collector exergy efficiency was 41.5% in July, while the maximum total exergy efficiency reached 13.2%. The values of energy losses as a percentage of the total losses of the main parts of the system were also reported. Results showed that the major energy and exergy losses occurred in the receiver. The second biggest portion of energy losses occurred in the Stirling engine, while the portion of exergy loss in the concentrator was higher compared to the Stirling engine. Finally, the performance of the Kerman pilot was compared to that of the EuroDish project. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

595 KiB

Open AccessArticle

Energy, Exergy and Economic Evaluation Comparison of Small-Scale Single and Dual Pressure Organic Rankine Cycles Integrated with Low-Grade Heat Sources

by Armando Fontalvo, Jose Solano, Cristian Pedraza, Antonio Bula, Arturo Gonzalez Quiroga and Ricardo Vasquez Padilla

Entropy 2017, 19(10), 476; https://doi.org/10.3390/e19100476 - 21 Sep 2017

Cited by 36 | Viewed by 6843

Abstract

Low-grade heat sources such as solar thermal, geothermal, exhaust gases and industrial waste heat are suitable alternatives for power generation which can be exploited by means of small-scale Organic Rankine Cycle (ORC). This paper combines thermodynamic optimization and economic analysis to assess the [...] Read more.

Low-grade heat sources such as solar thermal, geothermal, exhaust gases and industrial waste heat are suitable alternatives for power generation which can be exploited by means of small-scale Organic Rankine Cycle (ORC). This paper combines thermodynamic optimization and economic analysis to assess the performance of single and dual pressure ORC operating with different organic fluids and targeting small-scale applications. Maximum power output is lower than 45 KW while the temperature of the heat source varies in the range 100–200 °C. The studied working fluids, namely R1234yf, R1234ze(E) and R1234ze(Z), are selected based on environmental, safety and thermal performance criteria. Levelized Cost of Electricity (LCOE) and Specific Investment Cost (SIC) for two operation conditions are presented: maximum power output and maximum thermal efficiency. Results showed that R1234ze(Z) achieves the highest net power output (up to 44 kW) when net power output is optimized. Regenerative ORC achieves the highest performance when thermal efficiency is optimized (up to 18%). Simple ORC is the most cost-effective among the studied cycle configurations, requiring a selling price of energy of 0.3 USD/kWh to obtain a payback period of 8 years. According to SIC results, the working fluid R1234ze(Z) exhibits great potential for simple ORC when compared to conventional R245fa. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Graphical abstract

1408 KiB

Open AccessArticle

Exergy Analysis of a Parallel-Plate Active Magnetic Regenerator with Nanofluids

by Ibai Mugica, Steven Roy, Sébastien Poncet, Jonathan Bouchard and Hakim Nesreddine

Entropy 2017, 19(9), 464; https://doi.org/10.3390/e19090464 - 2 Sep 2017

Cited by 17 | Viewed by 5204

Abstract

This paper analyzes the energetic and exergy performance of an active magnetic regenerative refrigerator using water-based Al₂O₃ nanofluids as heat transfer fluids. A 1D numerical model has been extensively used to quantify the exergy performance of a system composed of [...] Read more.

This paper analyzes the energetic and exergy performance of an active magnetic regenerative refrigerator using water-based Al₂O₃ nanofluids as heat transfer fluids. A 1D numerical model has been extensively used to quantify the exergy performance of a system composed of a parallel-plate regenerator, magnetic source, pump, heat exchangers and control valves. Al₂O₃-water based nanofluids are tested thanks to CoolProp library, accounting for temperature-dependent properties, and appropriate correlations. The results are discussed in terms of the coefficient of performance, the exergy efficiency, and the cooling power as a function of the nanoparticle volume fraction and blowing time for a given geometrical configuration. It is shown that while the heat transfer between the fluid and solid is enhanced, it is accompanied by smaller temperature gradients within the fluid and larger pressure drops when increasing the nanoparticle concentration. It leads in all configurations to lower performance compared to the base case with pure liquid water. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

1463 KiB

Open AccessArticle

An Improved System for Utilizing Low-Temperature Waste Heat of Flue Gas from Coal-Fired Power Plants

by Shengwei Huang, Chengzhou Li, Tianyu Tan, Peng Fu, Gang Xu and Yongping Yang

Entropy 2017, 19(8), 423; https://doi.org/10.3390/e19080423 - 19 Aug 2017

Cited by 22 | Viewed by 7348

Abstract

In this paper, an improved system to efficiently utilize the low-temperature waste heat from the flue gas of coal-fired power plants is proposed based on heat cascade theory. The essence of the proposed system is that the waste heat of exhausted flue gas [...] Read more.

In this paper, an improved system to efficiently utilize the low-temperature waste heat from the flue gas of coal-fired power plants is proposed based on heat cascade theory. The essence of the proposed system is that the waste heat of exhausted flue gas is not only used to preheat air for assisting coal combustion as usual but also to heat up feedwater and for low-pressure steam extraction. Air preheating is performed by both the exhaust flue gas in the boiler island and the low-pressure steam extraction in the turbine island; thereby part of the flue gas heat originally exchanged in the air preheater can be saved and introduced to heat the feedwater and the high-temperature condensed water. Consequently, part of the high-pressure steam is saved for further expansion in the steam turbine, which results in additional net power output. Based on the design data of a typical 1000 MW ultra-supercritical coal-fired power plant in China, an in-depth analysis of the energy-saving characteristics of the improved waste heat utilization system (WHUS) and the conventional WHUS is conducted. When the improved WHUS is adopted in a typical 1000 MW unit, net power output increases by 19.51 MW, exergy efficiency improves to 45.46%, and net annual revenue reaches USD 4.741 million while for the conventional WHUS, these performance parameters are 5.83 MW, 44.80% and USD 1.244 million, respectively. The research described in this paper provides a feasible energy-saving option for coal-fired power plants. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

1049 KiB

Open AccessArticle

Thermal and Exergetic Analysis of the Goswami Cycle Integrated with Mid-Grade Heat Sources

by Gokmen Demirkaya, Ricardo Vasquez Padilla, Armando Fontalvo, Maree Lake and Yee Yan Lim

Entropy 2017, 19(8), 416; https://doi.org/10.3390/e19080416 - 17 Aug 2017

Cited by 37 | Viewed by 6794

Abstract

This paper presents a theoretical investigation of a combined Power and Cooling Cycle that employs an Ammonia-Water mixture. The cycle combines a Rankine and an absorption refrigeration cycle. The Goswami cycle can be used in a wide range of applications including recovering waste [...] Read more.

This paper presents a theoretical investigation of a combined Power and Cooling Cycle that employs an Ammonia-Water mixture. The cycle combines a Rankine and an absorption refrigeration cycle. The Goswami cycle can be used in a wide range of applications including recovering waste heat as a bottoming cycle or generating power from non-conventional sources like solar radiation or geothermal energy. A thermodynamic study of power and cooling co-generation is presented for heat source temperatures between 100 to 350 °C. A comprehensive analysis of the effect of several operation and configuration parameters, including the number of turbine stages and different superheating configurations, on the power output and the thermal and exergy efficiencies was conducted. Results showed the Goswami cycle can operate at an effective exergy efficiency of 60–80% with thermal efficiencies between 25 to 31%. The investigation also showed that multiple stage turbines had a better performance than single stage turbines when heat source temperatures remain above 200 °C in terms of power, thermal and exergy efficiencies. However, the effect of turbine stages is almost the same when heat source temperatures were below 175 °C. For multiple turbine stages, the use of partial superheating with Single or Double Reheat stream showed a better performance in terms of efficiency. It also showed an increase in exergy destruction when heat source temperature was increased. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

7078 KiB

Open AccessArticle

Multi-Objective Optimization for Solid Amine CO₂ Removal Assembly in Manned Spacecraft

by Rong A, Liping Pang, Meng Liu and Dongsheng Yang

Entropy 2017, 19(7), 348; https://doi.org/10.3390/e19070348 - 10 Jul 2017

Cited by 6 | Viewed by 5115

Abstract

Carbon Dioxide Removal Assembly (CDRA) is one of the most important systems in the Environmental Control and Life Support System (ECLSS) for a manned spacecraft. With the development of adsorbent and CDRA technology, solid amine is increasingly paid attention due to its obvious [...] Read more.

Carbon Dioxide Removal Assembly (CDRA) is one of the most important systems in the Environmental Control and Life Support System (ECLSS) for a manned spacecraft. With the development of adsorbent and CDRA technology, solid amine is increasingly paid attention due to its obvious advantages. However, a manned spacecraft is launched far from the Earth, and its resources and energy are restricted seriously. These limitations increase the design difficulty of solid amine CDRA. The purpose of this paper is to seek optimal design parameters for the solid amine CDRA. Based on a preliminary structure of solid amine CDRA, its heat and mass transfer models are built to reflect some features of the special solid amine adsorbent, Polyethylenepolyamine adsorbent. A multi-objective optimization for the design of solid amine CDRA is discussed further in this paper. In this study, the cabin CO₂ concentration, system power consumption and entropy production are chosen as the optimization objectives. The optimization variables consist of adsorption cycle time, solid amine loading mass, adsorption bed length, power consumption and system entropy production. The Improved Non-dominated Sorting Genetic Algorithm (NSGA-II) is used to solve this multi-objective optimization and to obtain optimal solution set. A design example of solid amine CDRA in a manned space station is used to show the optimal procedure. The optimal combinations of design parameters can be located on the Pareto Optimal Front (POF). Finally, Design 971 is selected as the best combination of design parameters. The optimal results indicate that the multi-objective optimization plays a significant role in the design of solid amine CDRA. The final optimal design parameters for the solid amine CDRA can guarantee the cabin CO₂ concentration within the specified range, and also satisfy the requirements of lightweight and minimum energy consumption. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

3288 KiB

Open AccessArticle

Exergy Dynamics of Systems in Thermal or Concentration Non-Equilibrium

by Enrico Sciubba and Federico Zullo

Entropy 2017, 19(6), 263; https://doi.org/10.3390/e19060263 - 8 Jun 2017

Cited by 9 | Viewed by 4843

Abstract

The paper addresses the problem of the existence and quantification of the exergy of non-equilibrium systems. Assuming that both energy and exergy are a priori concepts, the Gibbs “available energy” A is calculated for arbitrary temperature or concentration distributions across the body, with [...] Read more.

The paper addresses the problem of the existence and quantification of the exergy of non-equilibrium systems. Assuming that both energy and exergy are a priori concepts, the Gibbs “available energy” A is calculated for arbitrary temperature or concentration distributions across the body, with an accuracy that depends only on the information one has of the initial distribution. It is shown that A exponentially relaxes to its equilibrium value, and it is then demonstrated that its value is different from that of the non-equilibrium exergy, the difference depending on the imposed boundary conditions on the system and thus the two quantities are shown to be incommensurable. It is finally argued that all iso-energetic non-equilibrium states can be ranked in terms of their non-equilibrium exergy content, and that each point of the Gibbs plane corresponds therefore to a set of possible initial distributions, each one with its own exergy-decay history. The non-equilibrium exergy is always larger than its equilibrium counterpart and constitutes the “real” total exergy content of the system, i.e., the real maximum work extractable from the initial system. A systematic application of this paradigm may be beneficial for meaningful future applications in the fields of engineering and natural science. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

4171 KiB

Open AccessArticle

The Exergy Loss Distribution and the Heat Transfer Capability in Subcritical Organic Rankine Cycle

by Chao He, Youzhou Jiao, Chaochao Tian, Zhenfeng Wang and Zhiping Zhang

Entropy 2017, 19(6), 256; https://doi.org/10.3390/e19060256 - 3 Jun 2017

Cited by 8 | Viewed by 4365

Abstract

Taking net power output as the optimization objective, the exergy loss distribution of the subcritical Organic Rankine Cycle (ORC) system by using R245fa as the working fluid was calculated under the optimal conditions. The influences of heat source temperature, the evaporator pinch point [...] Read more.

Taking net power output as the optimization objective, the exergy loss distribution of the subcritical Organic Rankine Cycle (ORC) system by using R245fa as the working fluid was calculated under the optimal conditions. The influences of heat source temperature, the evaporator pinch point temperature difference, the expander isentropic efficiency and the cooling water temperature rise on the exergy loss distribution of subcritical ORC system are comprehensively discussed. It is found that there exists a critical value of expander isentropic efficiency and cooling water temperature rise, respectively, under certain conditions. The magnitude of critical value will affect the relative distribution of exergy loss in the expander, the evaporator and the condenser. The research results will help to better understand the characteristics of the exergy loss distribution in an ORC system. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

2922 KiB

Open AccessArticle

Second Law Analysis of a Mobile Air Conditioning System with Internal Heat Exchanger Using Low GWP Refrigerants

by Vicente Pérez-García, Juan M. Belman-Flores, José L. Rodríguez-Muñoz, Víctor. H. Rangel-Hernández and Armando Gallegos-Muñoz

Entropy 2017, 19(4), 175; https://doi.org/10.3390/e19040175 - 19 Apr 2017

Cited by 19 | Viewed by 6507

Abstract

This paper investigates the results of a Second Law analysis applied to a mobile air conditioning system (MACs) integrated with an internal heat exchanger (IHX) by considering R152a, R1234yf and R1234ze as low global warming potential (GWP) refrigerants and establishing R134a as baseline. [...] Read more.

This paper investigates the results of a Second Law analysis applied to a mobile air conditioning system (MACs) integrated with an internal heat exchanger (IHX) by considering R152a, R1234yf and R1234ze as low global warming potential (GWP) refrigerants and establishing R134a as baseline. System simulation is performed considering the maximum value of entropy generated in the IHX. The maximum entropy production occurs at an effectiveness of 66% for both R152a and R134a, whereas for the cases of R1234yf and R1234ze occurs at 55%. Sub-cooling and superheating effects are evaluated for each one of the cases. It is also found that the sub-cooling effect shows the greatest impact on the cycle efficiency. The results also show the influence of isentropic efficiency on relative exergy destruction, resulting that the most affected components are the compressor and the condenser for all of the refrigerants studied herein. It is also found that the most efficient operation of the system resulted to be when using the R1234ze refrigerant. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

1892 KiB

Open AccessArticle

Use of Exergy Analysis to Quantify the Effect of Lithium Bromide Concentration in an Absorption Chiller

by Andrew Lake, Behanz Rezaie and Steven Beyerlein

Entropy 2017, 19(4), 156; https://doi.org/10.3390/e19040156 - 1 Apr 2017

Cited by 20 | Viewed by 7210

Abstract

Absorption chillers present opportunities to utilize sustainable fuels in the production of chilled water. An assessment of the steam driven absorption chiller at the University of Idaho, was performed to quantify the current exergy destruction rates. Measurements of external processes and flows were [...] Read more.

Absorption chillers present opportunities to utilize sustainable fuels in the production of chilled water. An assessment of the steam driven absorption chiller at the University of Idaho, was performed to quantify the current exergy destruction rates. Measurements of external processes and flows were used to create a mathematical model. Using engineering equation solver to analyze and identify the major sources of exergy destruction within the chiller. It was determined that the absorber, generator and condenser are the largest contribution to the exergy destruction at 30%, 31% and 28% of the respectively. The exergetic efficiency is found to be 16% with a Coefficient of performance (COP) of 0.65. Impacts of weak solution concentration of lithium bromide on the exergy destruction rates were evaluated using parametric studies. The studies reveled an optimum concentration that could be obtained by increasing the weak solution concentration from 56% to 58.8% a net decrease in 0.4% of the exergy destruction caused by the absorption chiller can be obtained. The 2.8% increase in lithium-bromide concentration decreases the exergy destruction primarily within the absorber with a decrease of 5.1%. This increase in concentration is shown to also decrease the maximum cooling capacity by 3% and increase the exergy destruction of the generator by 4.9%. The study also shows that the increase in concentration will change the internal temperatures by 3 to 7 °C. Conversely, reducing the weak solution concentration results is also shown to increase the exergetic destruction rates while also potentially increasing the cooling capacity. Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

3432 KiB

Open AccessArticle

Numerical Study of the Magnetic Field Effects on the Heat Transfer and Entropy Generation Aspects of a Power Law Fluid over an Axisymmetric Stretching Plate Structure

by Payam Hooshmand, Hamed Rajabzadeh Gatabi, Navid Bagheri, Isma’il Pirzadeh, Ashkan Hesabi, Mohammad Yaghoub Abdollahzadeh Jamalabadi and Majid Oveisi

Entropy 2017, 19(3), 94; https://doi.org/10.3390/e19030094 - 1 Mar 2017

Cited by 7 | Viewed by 5041

Abstract

Numerical investigation of the effects of magnetic field strength, thermal radiation, Joule heating, and viscous heating on a forced convective flow of a non-Newtonian, incompressible power law fluid in an axisymmetric stretching sheet with variable temperature wall is accomplished. The power law shear [...] Read more.

Numerical investigation of the effects of magnetic field strength, thermal radiation, Joule heating, and viscous heating on a forced convective flow of a non-Newtonian, incompressible power law fluid in an axisymmetric stretching sheet with variable temperature wall is accomplished. The power law shear thinning viscosity-shear rate model for the anisotropic solutions and the Rosseland approximation for the thermal radiation through a highly absorbing medium are considered. The temperature dependent heat sources, Joule heating, and viscous heating are considered as the source terms in the energy balance. The non-dimensional boundary layer equations are solved numerically in terms of similarity variable. A parameter study on the Nusselt number, viscous components of entropy generation, and thermal components of entropy generation in fluid is performed as a function of thermal radiation parameter (0 to 2), Brinkman number (0 to 10), Prandtl number (0 to 10), Hartmann number (0 to 1), power law index (0 to 1), and heat source coefficient (0 to 0.1). Full article

(This article belongs to the Special Issue Work Availability and Exergy Analysis)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Work Availability and Exergy Analysis

Share This Special Issue

Special Issue Editors

Special Issue Information

Keywords

Benefits of Publishing in a Special Issue

Published Papers (19 papers)

Editorial

Research

Further Information

Guidelines

MDPI Initiatives

Follow MDPI