Minimum Exergy Destruction Research Articles

Optimization design of helical micro fin tubes based on exergy destruction minimization principle

• Increased heat exchange area and the secondary flow are the primary sources of heat transfer enhancement. • The local exergy destruction rates are related to the equivalent heat transfer coefficient and the unit pressure drop. • Optimal design tends to have a larger number of starts and a lower micro fin height. • The exergy destruction minimization principle is proved to be still applicable in the case of the periodic model and fully developed turbulence. The helical micro fin tubes (HFT) are commonly used in various double pipe heat exchangers because of the excellent processing and anti-fouling performance. It is of great significance to further improve the overall efficiency of the HFT so as to diminish energy consumption. In this work, the heat transfer and flow characteristics of the HFT are studied by numerical simulation. The results show that the heat transfer enhancement factors of the HFT are the secondary flow generated near the wall and the increase of the heat exchange area. In addition, the effects of the geometrical parameters on thermal–hydraulic performance are studied at Re = 36,636. It is found that the micro fin height ( e ), the helical angle ( φ ), and the number of starts ( Ns ) have a significant impact on the overall performance, and there is a strong mutual coupling between them. According to the parametric analysis, the HFT with a low micro fin height and a large number of starts is considered to be a better geometrical type. Finally, in order to select (or design) the HFT quickly under the specific working conditions, based on the exergy destruction minimization principle, the geometrical parameters are optimized by using the artificial neural network and genetic algorithm. An optimal solution ( e = 0.23 mm, φ = 36.1°, and Ns = 66) is selected from the Pareto front by the TOPSIS method. The results indicate that the optimal solution has a sensible balance between the exergy destruction caused by heat transfer and fluid flow. Besides, it has a better thermal–hydraulic performance as well ( PEC = 1.73). This work fills the gap of heat transfer and the geometrical optimization study of HFT based on the second law of thermodynamics and provides strong evidence that the exergy destruction minimization principle is still applicable in the case of the periodic model and fully developed turbulence. We hope that it will be contributed to the structural design of the HFT.

Optimization of the exergy efficiency, exergy destruction, and engine noise index in an engine with two direct injectors using NSGA-II and artificial neural network

Direct Dual Fuel Stratification (DDFS) strategy is a novel Low Temperature Combustion (LTC) strategy that has comparable thermal efficiency to the Reactivity Controlled Compression Ignition (RCCI) strategy, while it offers more control over the combustion process and the rate of heat release. The DDFS strategy uses two direct injectors for the low- and high-reactivity fuels (gasoline and diesel) to benefit from the RCCI concept. In this study, the injection strategy of the injectors of a gasoline/diesel DDFS engine was optimized from the thermodynamic perspective to maximize exergy efficiency and minimize exergy destruction and an engine noise index. An artificial neural network was developed with 576 samples from a CFD code to predict the DDFS mode behavior, and the non-dominated sorting genetic algorithm (NSGA-II) was used to obtain the Pareto Front and the optimal solutions. Compared to the base case, the exergy efficiency of the optimal cases increased by up to 2%, exergy destruction and Peak Pressure Rise Rate (PPRR) reduced by about 2.3%, and 2 bar/deg, respectively, in the optimal solutions. NOX and soot emissions were reduced by 40% and 35%, respectively, in the best-case scenarios.

Evaluation of The Use R450A as an Alternative to R134a in Low and Medium Temperature Heat Pump Systems: 4-E (Energy, Exergy, Environmental and Enviro-Economic) Analysis

This investigation presents a theoretical evaluation of the use R450A replace to R134a in the low and medium temperature heat pump systems. The energy, exergy, environmental and enviro-economic analyzes of heat pumps have been performed for two refrigerants (R134a and its alternative R450A). The thermodynamic and environmental analyses have been made for two heat source temperatures (10 oC and 20 oC) and six heat sink temperatures (from 30 oC to 55 oC). It has been observed that R450A's mass flow rate is lower than R134a. Because the density of R450A in the heat pump suction line is lower than R134a. Although the heating capacity of R134a is more than R450A, the reason why R134a has a similar COP value with R450A is that R450A's compressor energy consumption is lower than R134a. Exergy destruction occurred mostly in the compressor under all operating conditions for both refrigerants. At low heat sink temperatures, minimum exergy destruction occurs in the expansion valve, as the minimum exergy destruction occurs in the evaporator at high heat sink temperatures. The exergy results show that R450A behaves similarly to R134a. In conclusion, as alternative refrigerant R450A can be used in heat pumps replace to R134a according to energy, exergy, environmental and environmental-economic analysis results.

Investigation of the effect of compression ratio on the energetic and exergetic performance of a CI engine operating with safflower oil methyl ester

This study examines the energy and exergy analyses for a single-cylinder, four-stroke, direct-injection, compression-ignition (CI) engine when it was run on the safflower (Carthamus tinctorius L.) oil methyl ester (SOME) and traditional diesel (as reference fuel) at various engine loading conditions (from 25% to full load in 25% steps) at a fixed speed of 1500 rpm. In addition, the compression ratio (CR) was changed between 12:1 and 18:1 in order to monitor its effect on the engine characteristics. The experimental outcomes obtained from the above-stated analyses showed that the tested engine spent more alternating fuel because of its lower energy content than that of diesel fuel in an effort to ensure the output power to be the same for the test fuels. Furthermore, the results exhibited that the energetic and exergetic efficiency enhanced with the increase of load and CR. In this contest, at the CR of 18:1 and full load, the maximum energy efficiency values for SOME and diesel fuel were found to be 28.67% and 29.78%, respectively meanwhile the related exergetic efficiency values were calculated to be 26.41% and 27.94%, respectively. The minimum exergy destruction figures were revealed at the CR of 18:1 and a load of 100% for tested fuels with the findings of 56.59% for SOME and 55.22% for diesel fuel. In conclusion, the results of the conducted analyses for the tested diesel engine powered by SOME appeared to be fairly close to those of diesel fuel at various loads and CRs. SOME will eventually be classified as an alternate fuel to petroleum-based diesel fuel.

Constructal design of a circular micro-channel Stirling regenerator based on exergy destruction minimization

Regenerator is a key component in Stirling engines, and its characteristics have a significant influence on engine performance. Herein, subjected to the global constraints of the fixed total volume and porosity, constructal design of a circular micro-channel regenerator was conducted to achieve its optimal structures. The total exergy destruction rate in the irreversible regenerative process, which includes three types of individual exergy destruction contributions caused by the heat transfer with a finite temperature difference, flow resistance, and axial heat conduction of the solid matrix, was adopted as the optimization objective. The optimal channel diameter was obtained, and the effects of the regenerator inner radius, total volume of regenerators, regenerator porosity, engine speed, piston phase angle, hot part temperature, charge pressure, and thermal conductivity ratio due to the segmentation of regenerators on the optimization results were analyzed. The results show that the total exergy destruction rate offers a minimum at a relatively low channel diameter of around 0.5 mm under the given parameter values. All the parameters studied have an obvious effect on the minimum total exergy destruction rate, and the regenerator inner radius, total volume and porosity of regenerators, and engine speed also have an evident influence on the optimal structure of the regenerator. These indicate that the regenerator performance can be further improved by selecting appropriate global geometric parameter values of regenerators and operating parameter values of engines. The findings of this study can provide some guidelines for the optimal design of micro-channel regenerators and the performance improvement of Stirling engines.

Optimal equipment arrangement of a total site for cogeneration of thermal and electrical energy by using exergoeconomic approach

The optimal equipment arrangement for a system for cogeneration of thermal and electrical energy for a total site is determined with exergoeconomic analysis. A steam power plant can operate concurrently in a dual-use mode producing steam and electricity. It is important to know the potential for cogeneration before designing and optimizing a central utility system in order to establish on site fuel demand targets. In the current study, two case studies with different backpressure steam turbines arrangement are considered and, if extra power generation is needed, designs with a system of condensing and gas turbines are examined. The minimum exergy destruction rate, maximum exergy efficiency, minimum total capital cost rate and minimum exergy destruction cost rate for these arrangements are investigated with exergoeconomic analysis. Finally, selected backpressure steam turbines arrangement in conjunction with the gas turbine system are introduced for finding the optimum arrangement of the cogeneration system in the total site with an exergoeconomic approach.

Is exergy destruction minimization the same thing as energy efficiency maximization?

This paper discusses whether the exergy destruction minimization or energy efficiency maximization comes first in resolving the climate emergency problem and provides sustainable solution options regarding the 2nd Law of thermodynamics. It has been shown that low-temperature district energy systems with renewable energy sources and waste heat are effective in minimizing exergy destructions, while energy efficiency has a secondary impact. The research has been based on the Rational Exergy Management Model. The corresponding rational exergy management efficiency was directly related to nearly-avoidable CO2 emissions responsibility with a global magnitude of around 80% of direct emissions in the built environment. One conclusion deduced from such an unrecognized magnitude so far is that nearly-avoidable CO2 emissions may not be ignored anymore to develop new strategies for sustainable decarbonization, while the 1st Law measures have limited remaining capabilities. New equations were developed to show the impact of exergy destructions on total CO2 emissions. Sample results show that a 30 percent-point decrease of exergy destructions comparing to the supplied exergy in thermo-mechanical systems has the potential of reducing total CO2 emissions by 35%. The paper argues that current exergy destruction is around 0.8 of the supply exergy, as an industry average, which gives ample room for improvement using the 2nd Law, while the 1st Law efficiency is already higher, and there is less room available for improvements concerning CO2 emissions. The paper shows that the 1st Law efficiency may be increased by about 0.15 points, which gives a window of opportunity about a 25 percent-point decrease in emissions. The second main conclusion is that nowadays, new decarbonization strategies are needed based on the 2nd Law, which will positively impact when coupled with the current 1st Law measures towards meeting the Paris agreement.

Optimum power generation assessment in an H-Darrieus vertical axis wind turbine via Exergy Destruction Minimization

In recent years, Vertical Axis Wind Turbines have emerged as a viable complement to the ever-expanding worldwide wind power generation. Compared to conventional wind turbines, they are independent of wind direction, emit less noise, and require a smaller installation area, making them attractive for urban and remote implementations. In this work, a Second Law of Thermodynamics approach is utilized to deepen the understanding of the fluid dynamics phenomena that reduce the turbines' power generation. The turbulent unsteady fluid flow around the turbine is computed using a two-dimensional Finite Volume approach via the commercial software ANSYS Fluent, which is validated using experimental data available from the literature. The Exergy Destruction and the Reversible Work grow in direct proportion to the flow speed, although their growth rate depends on the turbine operating regime: attached flow or dynamic stall. The latter explains how the Second Law Efficiency maximizes at an intermediate Tip Speed Ratio, which coincides with a more efficient power generation.

Thermodynamic optimization of a rack-level water-cooling infrastructure in high load data centers based on the principle of minimum exergy destruction

Exergy model and the least exergy loss principle are introduced to analyze the exergy loss distribution in heat transfer and air/water flow process, for a combined server/cabinet cooling network. Based on the optimized case suggested by the exergy model, a multi-stage water cooling scheme is designed, with two-stage heat exchanger in server cabinet, and double-chiller in the water loop. Both thermal and energy performance of such cooling infrastructure has been tested in a data center in China, with the measured performance in good agreement with the designed values, and the exergy loss model has been validated.

Energy and exergy analyses of fabricated solar drying system with smooth and rough surfaces at different conditions: A case study

AbstractFor this experimental work, the solar dryer system has been fabricated. Various experiments have been performed to evaluate the performance of the fabricated dryer system. Energy and exergy concepts have been used for the assessment. And comparative analyses have also been done with smooth and rough surfaces: Grade 150 and Grade 300. This experimental work has been concluded as the maximum collector (i.e., 74°C) and cabinet temperatures (i.e., 66°C) are attained at 13:00 p.m. with Grade 300 surface. Maximum moisture loss (i.e., 35 g) and percentage moisture loss (i.e., 7.53%) are recorded at 13:00 p.m. again with a rough surface. Minimum exergy destruction rate (i.e., 0.294 W) but minimum exergy efficiency (i.e., 20.82%) are found at 17:00 p.m. with a black painted surface, which is not an acceptable condition. Maximum energy efficiency (i.e., 35.98%) and heat removal factor (i.e., 0.56) are obtained at 13:00 p.m. with a rough surface. The best performance from the fabricated solar system is received between 12:00 and 14:00 p.m. This study recommends rough surfaces and 12:00–13:00 p.m. timings for the solar drying system as the performance of the system is better in terms of energy–exergy.

Energy and Exergy Analyses of a Refrigerant Pump Integrated Dual-Ejector Refrigeration (DER) System

The aims of the present study are to design and perform the analysis of a dual-ejector refrigeration system (DER). The system is constructed by adding a second ejector and a refrigeration pump to the classical single-ejector refrigeration system (SER). In the theoretical analysis, two different refrigerants are employed, R134a and R600, and the cooling coefficient of performance (COP), exergy destruction and exergy efficiency are selected as the performance indices. The performance indices of the DER system are investigated under the variation of the evaporation, and condensing temperatures and results are compared with the SER system operating at the same conditions. In the given conditions, the maximum cooling COP and exergy efficiency are achieved with the DER system by 7.52 and 38.8%, respectively. In the DER system, the minimum exergy destruction occurs with R134a by 9.3 kJ/kg at 10 °C evaporation and 40 °C condensing temperatures. Moreover, 5.3% increments in the cooling COP and exergy efficiency are achieved with R600 when the condenser temperature is 55 °C and the evaporator temperature is 5 °C. The results also showed that the improvements achieved in the cooling COP and exergy efficiency with the DER system are greater at high condensing and low evaporation temperatures.

Energy and exergy study of Shatt Al-Basra gas turbine power plant

Energy and exergy analyses have been carried out on General Electric (GE) gas turbine unit in Shatt Al-Basra power plant located in Basra-Iraq. The analysis is based on both full-load and part-load actual operating data during the year 2017. An obvious drop off for plant performance characteristics is observed during the hot season. Energy and exergy analyses show that maximum thermal and exergy efficiencies were in February. Minimum exergy destruction in the unit is predicted in March. Improvement of the unit performance is recommended by installing intake air cooling as well as utilizing the discharged heat power that contains considerable work potential.

A comprehensive analysis for second law attributes of spiral heat exchanger operating with nanofluid using two-phase mixture model: Exergy destruction minimization attitude

The present article focuses on the second law attributes of a counter-flow spiral heat exchanger working with an Al2O3–H2O nanofluid with employing the two-phase mixture model. To improve the cogency of the simulations, the turbulence modeling is performed using four-equation transition Shear Stress Transport (SST) model. The simulations are conducted for different nanoparticle volume fractions and nanofluid flow rates. It is shown that by dispersing further nanoparticles in the common fluid, the total entropy generation of the hot nanofluid significantly diminishes, whereas the cold water and the heat exchanger body exhibit higher thermal entropy generation. The overall exergy destruction in the heat exchanger significantly decreases by the increase of the volume fraction, while it tends to increase by the flow rate increment. For instance, an about 9.2% reduction in the overall exergy destruction is observed as the volume fraction increases from 0.01 to 0.04. All the conditions exhibit great second law efficiency so that the minimum second law efficiency is larger than 0.84, and increases with the raise of either the volume fraction or flow rate.

Conjugate heat transfer enhancement in the mini-channel heat sink by realizing the optimized flow pattern

Mini-channel heat sinks are popular in cooling high heat flux devices due to the high convective heat transfer performance associated with moderate pressure drop penalty. It is of great significance to further improve the thermal hydraulic performance in the mini-channel heat sink so as to adapt it to higher heat flux conditions. In this paper, a new method of realizing the optimized flow field to enhance the thermal hydraulic performance was carried out successfully in the mini-channel heat sink. The chosen mini-channel heat sink was 30 mm × 30 mm in substrate size with Reynolds number ranging from 100 to 1100. Through conjugate heat transfer optimization based on exergy destruction minimization principle, the optimized flow pattern was characterized by three pairs of longitudinal swirls flow. Subsequently, the inclined parallelepiped ribs were proposed to realize the optimized flow pattern in the mini-channel heat sink. The heat transfer enhancement mechanism was investigated by analyzing velocity distributions, temperature distributions, and heat convection intensity. Besides, the total thermal resistance was decreased and the exergy destruction minimization principle was verified. As a result, the variation ranges of Nu/Nu0, f/f0, efficiency evaluation criterion (EEC), and overall performance criterion (R3) were 1.35–5.92, 1.27–8.75, 0.68–1.12, and 1.31–4.22, respectively. The maximum average heat flux could achieve 3.2 × 106 W/m2 within a temperature difference of 60 K between substrate and fluid. The configured parameters pitch ratio (PR) and width ratio (WR) were recommended as 1 and 0.2, respectively. This work is conducive to the structural design of mini-channel heat sinks.

Global warming, environmental and sustainability aspects of a geothermal energy based biodigester integrated SOFC system

In this study, global warming, environmental and sustainability aspects of a geothermal energy based biodigester integrated SOFC system are parametrically analyzed. In this regard, a system is designed, consisting of three main subsystems such as Solid Oxide Fuel Cell, Anaerobic Digester, and a Heat Recovery Steam Generator. In order to investigate the global warming, environmental and sustainability aspects of the system, the energy and exergy analyses are performed, and the following indicators are taken into consideration, which are i) unit CO2 emission, ii) environmental effect factor, iii) waste exergy ratio, iv) exergy destruction ratio, v) exergy recovery ratio, vi) exergetic sustainability index. Accordingly, the maximum exergetic sustainability index and exergy efficiency of the integrated system are calculated to be 0.486 and 0.367, respectively, in case the SOFC inlet temperature is equal to 633.3 °C while electric current density is 5500 A/m2. On the other hand, the minimum exergy destruction ratio and the minimum environmental effect factor are obtained to be 0.74 and 2.33 while SOFC inlet temperature is 633.3 °C and SOFC current density is 8000 A/m2. The minimum unit CO2 emission of the whole system is determined to be 368.4 kg/MWh at 5500 A/m2 of SOFC current density and 727 °C of SOFC inlet temperature while determined as 258.3 kg/MWh at 8000 A/m2 of SOFC current density and 680 °C of SOFC inlet temperature. Thus, it can be said that such a system may be applied for reducing the CO2 based global warming effects and improving the environmental sustainability.

Parametric analysis of combined power and freshwater producing system for natural gas engine heat recovery

The present paper focuses on exergoeconomic optimization of a proposed system with the goal of cogenerating freshwater and power, simultaneously. The cogeneration system includes Internal Combustion Engine, Organic Rankine Cycle, and Reverse Osmosis desalination unit. The thermodynamic model of the cogeneration system is investigated from energy, exergy, and exergoeconomic viewpoint. In addition, the variation of some key parameters including the engine speed, turbine inlet pressure, the number of the active vessels, freshwater mass flow rate, and the unit cost of freshwater to obtain optimum working condition is analyzed. The results show that the optimum value of the unit cost of power generation and freshwater occurs when the engine speed is 4000 rpm. Based on the optimization, the optimal condition does not occurred at the speed that exergy destruction becomes minimum. Furthermore the optimum value of the turbine inlet pressure was obtained respect to the maximum exergy efficiency and minimum unit cost of fresh water. The result showed that the optimum value of the turbine inlet pressure is 2690 kPa. There is an optimum value for the number of the active vessels that is achieved when 3 pressure vessels exist in the stage. In addition, the multi-objective optimization of the system based on the Pareto method revealed that at the optimum working condition, the exergy efficiency and freshwater mass flow rate are 30.7% and 2.926 kg/s, respectively. As a final result, it can be stated that the optimal values are not achieved in the minimum exergy destruction.

Improved Exergy Evaluation of Ammonia-Water Absorption Refrigeration System Using Inverse Method

Abstract In this study, the compatibility of exergy destruction minimization (EDM) as the main objective is checked by plotting coefficient of performance (COP), exergy coefficient of performance (ECOP), and overall exergy destruction rate by simultaneously varying input operating temperatures for a 28 TR cooling load absorption system. The component-wise variation in exergy destruction is also considered and it is found that the maxima of COP and ECOP, and the minima of overall exergy destruction lies on a common point, and when the variation of operating temperatures is further extended, the exergy destruction in one of the component becomes negative, which marks the upper bound of the present analysis. At highest valid generator temperature (155 °C), the minimum possible overall exergy destruction rate is 53.50 kW and maximum COP is 0.523. Through inverse optimization (IO) using dragonfly algorithm (DA), the same overall exergy destruction rate is achieved for a wide range of generator temperatures much below than 155 °C, and as low as 127.34 °C. The above variation is explained in terms of flow ratio, mass flowrate of steam, and mass flowrate of cooling water.

Experimental exergy and entransy analyses on designed and fabricated crossflow heat exchanger

AbstractA crossflow heat exchanger (CFHEx) is designed and fabricated in a workshop. For designing this heat exchanger (HEx), the number of passes, frontal areas, HEx volumes, heat transfer areas, free‐flow areas, ratios of minimum free‐flow area to frontal area, densities, mass flow rates of flowing fluids, maximum/minimum heat capacities, heat capacity ratio, outlet temperatures of hot/cold fluids, average temperatures, mass velocities, Reynolds numbers, and convective heat transfer coefficients are evaluated by considering Colburn/friction factors. After fabrication of the HEx, effectiveness, exergy destruction, entransy dissipation, entransy dissipation‐based thermal resistance, entransy dissipation number, and entransy effectiveness for hot/cold fluids sides are found at different flow rates and inlet temperatures of fluids. By experimental results, optimum operating conditions are found, which gives maximum effectiveness and entransy effectiveness but minimum rates of exergy destruction, entransy dissipation, entransy dissipation‐based thermal resistance, and entransy dissipation number for the fabricated CFHEx. This study is concluded as follows: minimum exergy destruction and entransy dissipation rates (ie, 3.061 kJ/s·K and 1125.44 kJ·K/s, respectively) are found during experiment 2. Maximum entransy effectiveness of hot/cold fluids (ie, 0.689/0.21) is achieved in experiment 1. Moderate values of entransy dissipation number (ie, 4.689), entransy dissipation‐based thermal resistance (ie, 0.04 s·K/J), exergy destruction (ie, 3.845 kJ/s·K), and entransy dissipation (ie, 1374.04 kJ·K/s) rates are found during experiment 1. Maximum effectiveness (ie, 0.4) for the fabricated HEx is also obtained through experiment 1. After comparative analyses, it is found that experiment 1 provides optimum results, which shows the best performance of the fabricated HEx.

Optimization of a New Design of Molten Salt-to-CO2 Heat Exchanger Using Exergy Destruction Minimization.

One of the ways to make cost-competitive electricity, from concentrated solar thermal energy, is increasing the thermoelectric conversion efficiency. To achieve this objective, the most promising scheme is a molten salt central receiver, coupled to a supercritical carbon dioxide cycle. A key element to be developed in this scheme is the molten salt-to-CO2 heat exchanger. This paper presents a heat exchanger design that avoids the molten salt plugging and the mechanical stress due to the high pressure of the CO2, while improving the heat transfer of the supercritical phase, due to its compactness with a high heat transfer area. This design is based on a honeycomb-like configuration, in which a thermal unit consists of a circular channel for the molten salt surrounded by six smaller trapezoidal ducts for the CO2. Further, an optimization based on the exergy destruction minimization has been accomplished, obtained the best working conditions of this heat exchanger: a temperature approach of 50 °C between both streams and a CO2 pressure drop of 2.7 bar.

Thermodynamic analysis of gasoline-fueled electronic fuel injection digital three-spark ignition (EFI-DTSI) engine

Numerous researchers have devised various strategies to improve the combustion efficiency of the internal combustion engine. Despite continuous improvement during past decades, there is still scope for further development in engine performance. This paper analyzes the energy and exergy distribution of a single-cylinder 199.5 cc electronic fuel injected three-spark ignited high-speed petrol engine. The engine is operated at 25%, 50%, 75%, and 100% throttle positions for different speeds of 4000–10,000 rpm with an increment of 2000 rpm. From the detailed heat balance analysis, the best results obtained are: maximum brake thermal efficiency of 34.9% corresponding to 6000 rpm at 50% throttle opening, minimum heat carried away by exhaust gas of 18.5% at 4000 rpm and 25% throttle position, minimum heat carried by cooling water as 13% for 10,000 rpm and 25% throttle, and the minimum unaccounted energy loss of 26.6% under the condition 8000 rpm and 75% throttle position. However, the best results of exergy analysis are: second law efficiency of 51.93% corresponding to 4000 rpm and 25% throttle, maximum exergy transfer for useful work as 33.7% concerning 6000 rpm and 50% throttle, minimum exergy transfer for exhaust gas as 15.7% for 6000 rpm and 25% throttle, minimum exergy transfer associated with coolant of 4.6% at 4000 rpm and 25% throttle, and minimum exergy destruction of 39.4% corresponding to 8000 rpm and 50% throttle, respectively.