Advanced exergetic evaluation of refrigeration machines using different working fluids

Splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts has many advantages for the detailed analysis of energy conversion systems. Endogenous is the exergy destruction obtained when all other system components are ideal and the component being considered operates with its real efficiency. The difference between total and endogenous exergy destruction is the exogenous exergy destruction caused within the component being considered by the irreversibilities in the remaining components and the structure of the overall system. Unavoidable is the part of exergy destruction within one system component that cannot be eliminated even if the best available technology in the near future would be applied. The avoidable exergy destruction is the difference between total and unavoidable exergy destruction. These concepts enhance an exergy analysis and assist in improving the quality of the conclusions obtained from this analysis. The paper presents the combined application of both concepts to vapor-compression refrigeration machines using different one-component working fluids (R125, R134a, R22 and R717) as well as azeotropic (R500) and zeotropic (R407C) mixtures. The purpose of the paper is not to evaluate these working fluids, some of which cannot be used in future, but to demonstrate the effect of different material properties on the results of advanced exergy analysis.     

Advanced exergetic analysis: Approaches for splitting the exergy destruction into endogenous and exogenous parts

The irreversibilities (exergy destruction) within a component of an energy conversion system can be represented by two parts. The first part depends on the inefficiencies of the considered component while the second part depends on the system structure and the inefficiencies of the other components of the overall system. Thus, the exergy destruction occurring within a component can be split into two parts: (a) endogenous exergy destruction due exclusively to the performance of the component being considered and (b) exogenous exergy destruction caused also by the inefficiencies within the remaining components of the overall system. The paper discusses four different approaches developed by the authors for calculating the endogenous part of exergy destruction as well as the approach based on the structural theory. The advantages, disadvantages and restrictions for applications associated with each approach are presented. It is concluded that all approaches developed by the authors lead to comparable and acceptable results, whereas the structural theory approach should not be used for calculating the endogenous part of exergy destruction because it delivers unacceptable results. Splitting the exergy destruction into endogenous and exogenous parts improves our understanding of the interactions among system components and provides very useful information for improving an exergy conversion system, particularly when this concept is combined with the concept of avoidable and unavoidable exergy destruction.     

Exergy analysis of double effect vapor absorption refrigeration system

Energy and exergy analyses previously performed by the authors for a single effect absorption refrigeration system have been extended to double effect vapor absorption refrigeration system with the expectation of reducing energy supply as well as an interest in the diversification of the motive power employed by HVAC technologies. The total exergy destruction in the system as a percentage of the exergy input from a generator heating water over a range of operating temperatures is examined for a system operating on LiBr–H₂O solution. The exergy destruction in each component, the coefficient of performance (COP) and the exergetic COP of the system are determined. It is shown that exergy destructions occur significantly in generators, absorbers, evaporator2 and heat exchangers while the exergy destructions in condenser1, evaporator1, throttling valves, and expansion valves are relatively smaller within the range of 1–5%. The results further indicate that with an increase in the generator1 temperature the COP and ECOP increase, but there is a significant reduction in total exergy destruction of the system for the same. On the other hand, the COP and ECOP decrease with an increase in the absorber1 temperature while the total exergy destruction of the system increases significantly with a small increase in the absorber1 temperature. The results show that the exergy method can be used as an effective criterion in designing an irreversible double effect absorption refrigeration system and may be a good tool for the determination of the optimum working conditions of such systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Exergy analysis of small DI diesel engine fueled with waste cooking oil biodiesel

ABSTRACT

This study investigates the merits of exergy analysis over energy analysis for small direct injection (DI) diesel engine using the blend of waste cooking oil biodiesel and petroleum diesel. Taguchi’s “L’ 16” orthogonal array has been used for the design of experiment. The engine tested at different engine speeds, load percentages, and blend ratios, using the waste cooking oil biodiesel. Basic performance parameters and fuel input exergy, exergetic efficiency (second law efficiency), exergy associated with heat transfer, exergy associated with the exhaust gas and destruction of exergy are calculated for each blend of waste cooking oil biodiesel and diesel. Results show that the optimum operating conditions for minimum brake-specific fuel consumption (BSFC) and exergy destruction are achieved when engine speed at 1900 rev/min, load percentage is 75%, and the engine is fueled with B40.     

Performance assessment of a geothermally heated building

This study deals with an exergetic performance evaluation of a geothermally heated building. This building used in the analysis has a volume of 1147.03 m³ and a net floor area of 95.59 m², while indoor and exterior air temperatures are 20 and 0 °C, respectively. The geothermal heating system used for the heat production was constructed in the Ozkilcik heating center, Izmir, Turkey. Thermal water has a pressure of 6.8 bar, a temperature of 122 °C and a mass flow rate of 54.73 kg/s, while it is reinjected at 3.2 bar and 72 °C. The system investigated feeds three regions. Among these, the Ozkanlar region has supply/return pressure and temperature values of 4.6/3 bar and 80/60 °C, respectively. Energy and exergy flows are studied to quantify and illustrate exergy destructions in the overall system. Total exergy input rate to the system is found to be 9.92 kW and the largest exergy destruction rate occurs in the primary energy transformation at 3.85 kW.     

Exergetic analysis of the refrigeration system in ethylene and propylene production process

The exergetic analysis is a tool that has been used successfully in many studies aiming a more rational energy use reducing the cost of the processes. With this analysis it is possible to perform an evaluation of the overall process, locating and quantifying the degradation of exergy. In this context, the present work aimed the exergetic analysis of the refrigeration cycles in ethylene and propylene production process, calculating the loss of exergy, in order to propose changes in the operational variables of the cycles used, trying to reduce the rate of destroyed exergy in the process. The commercial simulator Hysys^© (version 3.2) was used to obtain thermodynamic properties of the process streams and to perform mass and energy balances. The application of new operational conditions in these cycles resulted in a reduction of about 13% of the losses of exergy for the refrigeration system of the process.     

分析方法在热电联产中的评价作用

本文通过对热电联产机组进行分析,建立了热电联产机组的分析数学模型,引入了折合的概念,同时对几种常用的评价准则进行了比较。该模型克服了其他方法的局限性,物理意义明确,较其他方法更为合理。     

Parametric analysis and optimization for a combined power and refrigeration cycle

A combined power and refrigeration cycle is proposed, which combines the Rankine cycle and the absorption refrigeration cycle. This combined cycle uses a binary ammonia–water mixture as the working fluid and produces both power output and refrigeration output simultaneously with only one heat source. A parametric analysis is conducted to evaluate the effects of thermodynamic parameters on the performance of the combined cycle. It is shown that heat source temperature, environment temperature, refrigeration temperature, turbine inlet pressure, turbine inlet temperature, and basic solution ammonia concentration have significant effects on the net power output, refrigeration output and exergy efficiency of the combined cycle. A parameter optimization is achieved by means of genetic algorithm to reach the maximum exergy efficiency. The optimized exergy efficiency is 43.06% under the given condition.     

过冷式小型冰蓄冷系统(火用)分析

通过对过冷式小型冰蓄冷系统火用分析模型的建立,采用[火用]分析的方法,揭示了小型过冷式冰蓄冷系统的能量转换的薄弱环节以及过冷度对各部件[火用]损失系数和系统火用效率的影响。分析了减少火用损失的途径,节约能源。并为系统的改进和优化提供有力的理论参考。     

基于_分析的发电厂改造方法研究

根据分析理论,把锅炉、汽轮机及发电厂热力系统作为一个整体,通过对发电厂各环节进行分析找到具体损失环节并对具体位置提出改造方案,是发电厂改造的一种新方法。以某超超临界机组为例,运用分析理论找到损失环节,找到二次风温及给水温度对锅炉系统及电厂热力系统影响的关系,通过增加一级高压加热器提高给水温度,同时在保证锅炉排烟温度不变的前提下适当地提高二次风温,分析在给水温度及二次风温的双重作用下,锅炉乃至整个机组性能的变化。结果表明,当给水温度由299.5℃升高至322℃时,二次风温由327.8℃升高至360℃,锅炉系统传热损失由3 443 kJ/kg降低至3 254 kJ/kg,燃烧环节损失由6 204 kJ/kg降低至6 158 kJ/kg,锅炉效率由54.15%升高至54.45%,机组目的效率由42%升高至46.7%。     

Exergy analysis of Joule–Thomson cryogenic refrigeration cycle with an ejector

In this paper, exergy method is applied to analyze the ejector expansion Joule–Thomson (EJT) cryogenic refrigeration cycle. The exergy destruction rate in each component of the EJT cycle is evaluated in detail. The effect of some main parameters on the exergy destruction and exergetic efficiency of the cycle is also investigated. The most significant exergy destruction rates in the cycle are in the compressor and ejector. The ejector pressure ratio and compressor isothermal efficiency have a significant effect on the exergetic efficiency of the EJT cycle. The exergy analysis results show the EJT cycle has an obvious increase in the exergetic efficiency compared to the basic Joule–Thomson refrigeration cycle. A significant advantage from the use of the ejector is that the total exergy destruction of the EJT cycle can be reduced due to much more decreasing of the exergy destruction rates in the compressor and expansion valve. The exergy analysis also reconfirms that applying an ejector is a very important approach to improve the performance of the Joule–Thomson cryogenic refrigeration cycle.     

Quantifying global exergy resources

Exergy is used as a common currency to assess and compare the reservoirs of theoretically extractable work we call energy resources. Resources consist of matter or energy with properties different from the predominant conditions in the environment. These differences can be classified as physical, chemical, or nuclear exergy. This paper identifies the primary exergy reservoirs that supply exergy to the biosphere and quantifies the intensive and extensive exergy of their derivative secondary reservoirs, or resources. The interconnecting accumulations and flows among these reservoirs are illustrated to show the path of exergy through the terrestrial system from input to its eventual natural or anthropogenic destruction. The results are intended to assist in evaluation of current resource utilization, help guide fundamental research to enable promising new energy technologies, and provide a basis for comparing the resource potential of future energy options that is independent of technology and cost.     

Conventional and enhanced exergy analysis of a hydrogen liquefaction system

In the present work, conventional and enhanced exergy analyses were applied to the cryogenic liquefaction process of hydrogen gas. The hydrogen liquefaction unit consists of a multi-stage compressor, booster compressor-turbine pair, and heat exchanger block. Convectional exergy analysis cannot identify parts of exergy inefficiencies. In addition, by convectional exergy analysis, it cannot determine inevitable exergy losses that occur due to technological limits. For this reason, enhanced exergy analysis should be applied to the system. The exergy destruction affecting the exergy efficiency of the hydrogen liquefaction unit was investigated in detail. This study suggests an enhanced exergy analysis of a cryogenic liquefaction system. According to the results of the convectional exergy analysis, exergy efficiency of the whole liquefaction process are 32.22%. Also, the highest and lowest endogenous exergy destruction among whole components is calculated as 9563 kW and 92.83 kW in the turbine and CM-1, respectively. With these calculated results, the potential for improvement in the turbine in the liquefaction system was found to be high.     

非共沸混合工质压缩制冷循环的不可避免(火用)损失

以非共沸混合工质替代 CFCS是比较有效的替代方案。通过对给定节点温差下的蒸发器和冷凝器内的温度匹配分析 ,提出利用调节非共沸混合工质的配比来优化蒸发器和冷凝器内的温度匹配 ,并可计算出循环的实际不可避免灯用损失 ,从而提出采用非共沸混合工质的蒸气压缩制冷循环的实际不可避免灯用损失的计算方法 ,并提出利用最佳配比和实际不可避免的灯用损失的计算 ,对各种非共沸混合工质对进行筛选 ,以进一步减少循环可避免的灯用损失 ,为优化蒸气压缩制冷循环 ,提高循环的性能奠定基础。     

Definitions and nomenclature in exergy analysis and exergoeconomics

This paper presents the definitions of some terms used in exergy analysis and exergy costing, discusses options for the symbols to be used for exergy and some exergoeconomic variables, and presents the nomenclature for the remaining terms.     

Exergy analysis of multistage cascade refrigeration cycle used for natural gas liquefaction

This paper provides an exergy analysis of the multistage cascade refrigeration cycle used for natural gas liquefaction. The equations of exergy destruction and exergetic efficiency for the main cycle components such as evaporators, condensers, compressors, and expansion valves are developed. The relations for the total exergy destruction in the cycle and the cycle exergetic efficiency are obtained. Also, an expression for the minimum work requirement for the liquefaction of natural gas is developed. It is shown that the minimum work depends only on the properties of the incoming and outgoing natural gas, and it increases with decreasing liquefaction temperature. The minimum work for a typical natural gas inlet and exit state is determined to be 456.8 kJ kg^?1 of liquefied natural gas (LNG), which corresponds to a coefficient of performance (COP) of 1.8. Using a typical actual work input value; the exergetic efficiency of the multistage cascade refrigeration cycle is determined to be 38.5% indicating a great potential for improvements. Copyright © 2002 John Wiley & Sons, Ltd.     

中温地热能驱动的跨临界有机朗肯-蒸气压缩制冷系统的火用分析

建立中温地热能驱动跨临界有机朗肯−蒸气压缩制冷系统的火用分析热力学模型,采用R143a作为系统循环工质,探讨膨胀机入口压力、地热流体进口温度、冷凝温度、蒸发温度对火用效率的影响规律,分析系统各个部件的火用损失。计算结果表明：合理的膨胀机入口压力应该小于1.8倍临界压力;存在最佳的地热流体进口温度使得系统的火用效率最大;降低冷凝温度和提高蒸发温度都可以提高?效率,但需要增加换热器等效换热面积作为代价;冷凝器、发生器、膨胀机、节流阀、压缩机、蒸发器、工质泵的火用损失依次降低;随着地热流体进口温度升高,冷凝器及发生器的火用损失所占的比例增大,其它部件的火用损失对应的比例则降低。本文可以为跨临界有机朗肯−蒸气压缩制冷系统的设计提供依据。     

Comprehensive exergy analysis of a ground-source heat pump system for both building heating and cooling modes

This paper presents a comprehensive exergy analysis of three circuits and whole system of a ground-source heat pump (GSHP) for both building heating and cooling modes. The purpose is to search out the key potential energy saving components. The analytical formulae of exergy loss, exergy efficiency, exergy loss ratio, exergy loss coefficient and thermodynamic perfect degree are derived, respectively. The results show that these exergy indexes should be used integratively, and in the whole system the location of maximum exergy loss ratio is the compressor, while the location of minimum exergy efficiency and thermodynamic perfect degree is the ground heat exchanger, so that the compressor and the ground heat exchanger should be primarily improved. The results also indicate that the exergy loss of a GSHP system for building heating mode is bigger than that of cooling mode, and the exergy efficiency of a whole GSHP system is obviously lower than those of its components for both building heating and cooling modes. Therefore, a comprehensive exergy analysis of a GSHP should be paid more attention to. The results may provide guidelines for the design and optimization of GSHP systems.     

Evolution of the decrease in mineral exergy throughout the 20th century. The case of copper in the US

A mineral deposit is a natural resource whose exergy can be calculated from a defined reference environment (RE). This RE can be compared to a thermodynamically dead planet, where all materials have reacted, dispersed and mixed. Like any substance, a mine is characterized by its quantity, chemical composition and concentration (ore grade). The mine’s exergy measures the minimum (reversible) energy to extract and concentrate the materials from the RE to the conditions in the mine. And the mine’s exergy replacement cost accounts for the actual exergy required to accomplish this, with available technologies. The exergy assessment of the natural resource wealth of the Earth defined from a RE is named as exergoecology. The aim of this paper is to prove the usefulness of these two indicators for assessing the degradation of mineral deposits over history. As an example, the exergy decrease of US copper mines due to copper extraction throughout the 20th century has been determined. The results indicate that the exergy decrease was 65.4 Mtoe, while the exergy replacement cost 889.9 Mtoe. During the past century, the US extracted the equivalent of 2.5 and 1.2 times of its current national exergy reserves and base reserve of copper, respectively.     

三级复叠式制冷系统低温级制冷剂的应用分析

为研究三级复叠制冷系统中低温循环制冷剂替代的可行性方案,采用R1150/R170/R717、R50/R170/R717和R14/R170/R717三种工质组合,对三级复叠式制冷系统的高低温循环压缩机的排气温度、压缩机输入功率、COP、热力学完善度、系统的■效率、■损失以及系统中各个部件■损失所占比例随蒸发温度的变化进行热力学分析。研究结果表明:不同蒸发温度下均存在最佳中间循环冷凝温度,使COP值最大。蒸发温度由-100.0℃升高到-80.0℃时,R1150/R170/R717的■损失最小,COP、热力学完善度和■效率最大。R1150/R170/R717的COP由0.60增大到0.82。R1150/R170/R717的COP比R14/R170/R717的COP高3.47%～4.49%。主要的■损失部件是冷凝器,冷凝器的■损失所占比例随蒸发温度的升高而升高。推荐在三级复叠式制冷系统中采用R1150/R170/R717制冷剂组合方案,研究结果为三级复叠式制冷系统工质组的选择提供理论依据。