Second Law of Thermodynamics Analysis of Transcritical Carbon Dioxide Refrigeration Cycle

In order to identify the locations of irreversible loss within the transcritical carbon dioxide refrigeration cycle with an expansion turbine, a method with respect to the second law of thermodynamics based on exergy analysis model is applied. The effects of heat rejection pressures, outlet temperatures of gas cooler and evaporating temperatures on the exergy loss, exergy efficiency and the coefficient of performance (COP) of the expansion turbine cycle are analyzed. It is found that the great percentages of exergy losses take place in the gas cooler and compressor. Moreover, heat rejection pressures, outlet temperatures of gas cooler and evaporating temperatures have strong influence on the exergy efficiency, COP and the exergy loss of each component. The analysis shows that there exists an optimal heat rejection pressure corresponding to the maximum exergy efficiency and COP, respectively. The results are of significance in providing theoretical basis for optimal design and the control of the transcritical carbon dioxide system with an expansion turbine.

Thermodynamic Analysis and Comparison on Low Temperature CO_2-NH_3 Cascade Refrigeration Cycle

This paper is focused on the cascade refrigeration cycle using natural refrigerant CO 2-NH 3. The properties of refrigerants CO 2 and NH 3 are introduced and analyzed.CO 2 has the advantage in low stage of cascade refrigeration cycle due to its good characteristics and properties. The thermodynamic analysis results of the CO 2-NH 3 cascade refrigeration cycle demonstrates that the cycle has an optimum condensation temperature of low stage and also has an optimum flow rate ratio.By comparing with the R13-R22 and NH 3-NH 3 cascade refrigeration cycles, the mass flow rate ratio of CO 2-NH 3 is larger than those of R13-R22 and NH 3-NH 3, the theoretical COP of CO 2-NH 3 cascade refrigeration cycle is larger than that of the R13-R22 cascade cycle and smaller than that of the NH 3-NH 3 cascade cycle. But the real COP of CO 2-NH 3 cascade cycle will be higher than those of R13-R22 and NH 3-NH 3 because the specific volume of CO 2 at low temperature does not change much and its dynamic viscosity is also small.

Performance comparison and analysis of a combined power and cooling system based on organic Rankine cycle

A novel power and cooling system combined system which coupled organic Rankine cycle(ORC) with vapor compression refrigeration cycle(VCRC) was proposed. R245 fa and butane were selected as the working fluid for the power and refrigeration cycle, respectively. A performance comparison and analysis for the combined system was presented. The results show that dual-pressure ORC-VCRC system can achieve an increase of 7.1% in thermal efficiency and 6.7% in exergy efficiency than that of basic ORC-VCRC. Intermediate pressure is a key parameter to both net power and exergy efficiency of dual-pressure ORC-VCRC system. Combined system can produce maximum net power and exergy efficiency at 0.85 MPa for intermediate pressure and 2.4 MPa for high pressure, respectively. However, superheated temperature at expander inlet has little impact on the two indicators. It can achieve higher overall COP, net power and exergy efficiency at smaller difference between condensation temperature and evaporation temperature of VCRC.

Energy,exergy,and economic analysis of compression-absorption cascade refrigeration cycle using different working fluids

Compression-absorption cascade refrigeration cycle(CACRC)combined with vapor-compression refrigeration and absorption refrigeration cycle attracts great interest due to the less electricity consumption and utilization waste heat.In this work,the performance of the CACRC system was investigated using 16 refrigerants in the vapor compression section and H_(2)O-LiBr in the absorption refrigeration section.Energy,exergy and economic analysis of the CACRC system were carried out and the results were compared.Results show that RE170/H_(2)O-LiBr presents the better coefficient of performance and exergy efficiency amongst all the studied fluids.In addition,the economic optimization,multi-objective optimization,and thermodynamic optimization of the CACRC system based on the RE170/H_(2)O-LiBr working fluid were also carried out.

Design and analysis of dual mixed refrigerant processes for high-ethane content natural gas liquefaction

Recovery and purification of ethane has a significant impact on economic benefit improvement of the high-ethane content natural gas.However,current LNG-NGL integrated processes mainly focus on conventional natural gas,which are not applicable to natural gas with high ethane content.To fill this gap,three dual mixed refrigerant processes are proposed for simulation study of high-ethane content natural gas liquefaction.The proposed processes are optimized by a combination method of sequence optimization and genetic algorithm.Comparatively analysis is conducted to evaluate the three processes from the energetic and exergetic points of view.The results show that the power consumption of Process 3 which compressing natural gas after distillation is the lowest.For safety or other considerations,some common compositions of the mixed refrigerant may need to be removed under certain circumstances.Considering this,case studies of mixed refrigerant involving six composition combinations are carried out to investigate the effects of refrigerant selection on the process performance.

基于SolidWorks Flow Simulation的热电制冷箱装配体安装架构的仿真优化

为了提高热电制冷箱的制冷性能,对市面上常见的热电(半导体)制冷箱进行分析,并对制冷箱的制冷装配体与箱壁之间的架构方式进行探究。采用仿真工具SolidWorks Flow Simulation对制冷箱的制冷装配体与箱壁之间的架构方式进行建模仿真。分析了重力、风扇方向、开槽尺寸、槽内所加支架等参数对半导体制冷箱制冷性能的影响规律,结果表明:制冷箱在开槽尺寸一定时,重力方向处于箱体尺寸最长边,风扇进气方向为吸气,槽内加对称支架,并对支架的4个侧面做尽可能多的圆形镂空处理,且圆越大越好,此时热电制冷箱的制冷性能最优。

Performance analysis of cogeneration systems based on micro gas turbine(MGT),organic Rankine cycle and ejector refrigeration cycle

In this paper,the operation perfonnance of three novel kinds of cogeneration systems under design and off-design condition was investigated.The systems are MGT(micro gas turbine)+ORC(organic Rankine cycle)for electricity demand,MGT+ERC(ejector refrigeration cycle)for electricity and cooling demand,and MGT+ORC+ERC for electricity and cooling demand.The effect of 5 different working fluids on cogeneration systems was studied.The results show that under the design condition,when using R600 in the bottoming cycle,the MGT+ORC system has the lowest total output of 117.1 kW with a thermal efficiency of 0.334,and the MGT+ERC system has the largest total output of 142.6 kW with a thermal efficiency of 0.408.For the MGT+ORC+ERC system,the total output is between the other two systems,which is 129.3 kW with a thermal efficiency of 0.370.For the effect of different working fluids,R123 is the most suitable working fluid for MGT+ORC with the maximum electricity output power and R600 is the most suitable working fluid for MGT+ERC with the maximum cooling capacity,while both R600 and R123 can make MGT+ORC+ERC achieve a good comprehensive performance of refrigeration and electricity.The thermal efficiency of three cogeneration systems can be effectively improved under oredesign condition because the bottoming cycle can compensate for the power decrease of MGT.The results obtained in this paper can provide a reference for the design and operation of the cogeneration system for distributed energy systems(DES).

Performance Analysis and Evaluation of a Supercritical CO2 Rankine Cycle Coupled with an Absorption Refrigeration Cycle

The utilization of sensible waste heat such as flue gas and industrial surplus heat is essential for energy saving. Supercritical CO2 power generation cycle is a promising way to be used in this field. In this paper, a new supercritical CO2 Rankine cycle coupled with an absorption refrigeration cycle is proposed, which consists of a reheating supercritical CO2 cycle, a mixed-effect Li Br-H2O absorption refrigeration cycle and solar subsystem including evacuated-tube collector and a hot water storage tank. The system has four variants according to the presence or absence of solar subsystem and net cooling energy output. The thermodynamic model of the proposed system was established and its performance was evaluated. The proposed system is able to realize cascade utilization of flue gas waste heat and efficient conversion of solar energy. It has much higher thermodynamic efficiency than the reference system(i.e., the conventional supercritical CO2 Brayton cycle). Taking combined power and cooling system driven by flue gas waste heat and solar energy as an example, its thermal efficiency and exergy efficiency are 20.37% and 54.18% respectively, compared with the 14.74% and 35.96% of the reference system. Energy Utilization Diagrams(EUD) are implemented to investigate the irreversible losses and variation of the exergy destruction in the energy conversion process. Parametric analysis in two key parameters is conducted to provide guidance for the system optimal design.

Thermodynamic design of a cascade refrigeration system of liquefied natural gas by applying mixed integer non-linear programming

Liquefied natural gas(LNG) is the most economical way of transporting natural gas(NG) over long distances. Liquefaction of NG using vapor compression refrigeration system requires high operating and capital cost. Due to lack of systematic design methods for multistage refrigeration cycles, conventional approaches to determine optimal cycle are largely trial-and-error. In this paper a novel mixed integer non-linear programming(MINLP)model is introduced to select optimal synthesis of refrigeration systems to reduce both operating and capital costs of an LNG plant. Better conceptual understanding of design improvement is illustrated on composite curve(CC) and exergetic grand composite curve(EGCC) of pinch analysis diagrams. In this method a superstructure representation of complex refrigeration system is developed to select and optimize key decision variables in refrigeration cycles(i.e. partition temperature, compression configuration, refrigeration features, refrigerant flow rate and economic trade-off). Based on this method a program(LNG-Pro) is developed which integrates VBA,Refprop and Excel MINLP Solver to automate the methodology. Design procedure is applied on a sample LNG plant to illustrate advantages of using this method which shows a 3.3% reduction in total shaft work consumption.

基于双级节流制冷循环的天然气液化工艺模拟与优化

液化天然气以其安全可靠、汽化潜热高、环境污染小等特点,在天然气贸易中占据重要位置,但天然气液化工艺系统功耗大以及液化成本较高等问题仍然存在。为此,基于2个双级节流制冷循环的天然气液化工艺系统,采用Aspen HYSYS软件,以比功耗、㶲效率、液化率和总成本作为评价指标,分析了关键参数对系统性能的影响;然后对2个系统进行了单目标和多目标优化,确定了系统的最优运行工况;最后对2个系统的最优性能进行对比,并采用LINMAP与TOPSIS方法对Pareto前沿进行决策,在最优工况下对系统设备进行了㶲分析。研究结果表明:①节流阀V1、V3、V4的节流压力增大,或乙烯质量流量增大,均会使系统比功耗先减后增;单目标优化结果显示系统B性能优于系统A;②采用LINMAP与TOPSIS方法对Pareto前沿的决策结果有所差别,基于LINMAP方法的决策结果表明,系统B的总成本优于系统A,系统A的比功耗和㶲效率更优,综合性能更好;③基于TOPSIS方法的决策结果表明,系统A的比功耗优于系统B,系统B的㶲效率和总成本更优,综合性能更好;④㶲分析结果显示,节流阀及换热器的㶲损较大。结论认为,开展双级节流制冷循环天然气液化工艺研究,对其进行参数分析和优化,能够为天然气液化工艺设计创新提供借鉴,将有助于天然气液化行业的高质量发展。

基于太阳能吸附制冷的载/蓄冷充填降温系统性能分析

含冰粒的载/蓄冷充填降温是解决深井热害问题的有效方式,但其制冰能耗大、系统运行费用较高。为此,研发了基于太阳能吸附制冷的矿井载/蓄冷充填降温系统(Mine Cold Load/Storage Backfill Cooling System Based on Solar Adsorption Refrigeration),该系统由地面太阳能集热子系统、吸附制冷子系统和地下输冰子系统组成。通过建立子系统的数学模型和地面系统的TRNSYS模型,分别分析甲醇解吸量、吸附制冷量和吸附制冰量在不同太阳能辐射强度、不同季节和不同地区影响下的变化规律,进而得出太阳辐射强度和太阳辐射连续性是造成制冰量差异的主要原因。选取太阳辐射强度和太阳连续性较优的淮南、南宁两区域进行系统制冰能效分析,与传统蒸汽压缩式制冷系统相比,该系统的吸附制冷子系统平均节能效率达到64.71%。研究结果反映出,太阳能吸附制冷与载/蓄冷充填降温相结合的新型矿井降温系统的研发,对于高效解决矿井热害问题有所裨益。

Finite time thermodynamic analysis and optimization of water cooled multi-spilit heat pipe system(MSHPS)

This study focuses on the heat transfer characteristics of the evaporation terminal,the cool distribute unit(CDU)and refrigerant flow distribution of a water cooled multi-spilit heat pipe system(MSHPS)used in data center.The finite time thermodynamic analysis,the exergy method and the software SIMULINK was employed to build the simulation model of the combined system.The results show that the IT servers should concentrate on arranging at the location below 1.3 m.The CDU has a heat transfer of about 74 J in a period of 6 s.And the optimum flow rate of the CDU is 0.82 kg/s.The flow distribution characteristic of a CDU which connect 2 heat pipe evaporator terminals of 6 kW was calculated,and the working fluid is R22.Then the free cooling time,part time free cooling and energy saving potential in major cities of China were analysised.The energy saving potential is from 61%to 25%.The results are of great significance for the operational control and practical application of a MSHPS and other pipe-net systems.

利用ORC-VCR回收压缩热的预冷式CAES系统性能分析

常规非绝热压缩空气储能(D-CAES)系统的储能过程通常采用四级以上的压缩机组以减少空气压缩功的消耗,导致产生大量的低品位压缩热直接排放到环境中,能源浪费严重。针对这一问题,本工作提出了一种采用有机朗肯循环-蒸汽压缩制冷(ORC-VCR)回收压缩热的预冷式CAES系统(ORC-VCR-CAES),通过回收空气压缩阶段压缩机产生的压缩热来对压缩机入口空气进行预冷,可以进一步降低空气压缩功的消耗,提高系统的循环效率。对ORC-VCR-CAES耦合系统进行了热力学分析和经济性分析。结果表明,不同ORC-VCR循环工质对系统性能的影响较大,采用R152a作为循环工质的ORC-VCR-CAES系统综合性能最佳。其系统循环效率可达64.15%,比常规D-CAES系统提高了5.94%;在考虑外部废热能量输入时,ORC-VCR-CAES系统(火用)效率为51.90%,比常规D-CAES系统提高了4.81%。通过压缩热的回收有效减少了冷却器的(火用)损失,但压缩机组的(火用)损失仍然较大,是系统进一步优化的关键部件;经济性分析表明,当峰谷电价为1.26元和0.30元时,ORC-VCR-CAES系统的项目净现值相比于常规D-CAES系统可增加12.48%,且峰谷电价差越小,ORC-VCRCAES相比于常规D-CAES系统的项目净现值增加百分比越高。

六类十六种制冷剂在制冷循环中的循环特性分析

研究了制冷剂在标准式制冷循环中的循环特性,具体理论分析了无机化合物类、HCs、CFC、HFCs、HCFC、HFOs六类等十六种制冷剂(R600a、R245fa、R1234yf等)单一冷凝温度变化时对系统的单位制冷量、COP、压比和压缩排气温度的影响规律。结果表明,在蒸发温度-10℃、冷凝温度30℃~50℃的工况下,HCs类制冷剂、HFOs类制冷剂和HFCs类中的R245fa制冷剂在制冷循环中具有良好的循环性能,为将来选择合适的制冷剂有重要的研究价值。

Research of refrigeration system for a new type of constant temperature hydraulic tank

Different from the traditional hydraulic oil cooling method,a new type of constant temperature oil tank cooling system based on semiconductor refrigeration technology is designed. This paper studies the principle of semiconductor refrigeration and establishes a heat transfer model. Semiconductor cooler on piping refrigeration is simulated,and influence of the parameters on the outlet temperature,such as pipe pressure difference of inlet and outlet,pipe length,pipe radius,are gotten,and then hydraulic tank semiconductor refrigeration system is proposed. The semiconductor refrigeration system can control temperature at 37 ± 1°C.

The mass and heat transfer process through the door seal of refrigeration

As one of the main reasons causing leakage heat load in a refrigerator,mass and heat transfer through refrigerator door seal is of great importance to be studied.In this paper,a model is presented for numerical simulation of mass and heat transfer process through refrigerator door seal,and an experiment apparatus is designed and set up as well for comparison.A two-dimensional model and tracer gas method are used in simulation and experiment,respectively.It can be found that the relative deviations of air infiltration rate between the simulated results and experimental results were less than 1%,and the temperature difference errors at two special points of the door seal were less than 2.03℃.In conclusion,the simulated results are in good agreement with the experimental results.This paper initially sets up a model that can accurately simulate the heat and mass transfer through the refrigerator door seal,and the model can be used in refrigerator door seal optimization research in the follow-up study.

离心式CO_(2)逆布雷顿制冷系统的[火用]分析研究

离心式逆布雷顿循环(CRBC)是一种利用离心力势能-压力能转换的制冷循环,工质在旋转管内的离心和向心流动实现高效压缩和膨胀过程,为提升气体制冷循环的效率提供了潜在可能性。以CO_(2)为制冷工质,水为冷却介质,建立了离心式逆布雷顿制冷系统的热力学模型,并进行[火用]分析,研究系统的[火用]效率及[火用]损分布情况,为系统的评估及改进提供理论依据。研究结果表明:在相同送风温度下,加热器进口温度存在优化可能,系统最大[火用]损率出现在离心等温压缩、绝热压缩和向心绝热膨胀流动过程,分别约为20%,空气冷却过程和离心绝热压缩流动过程的[火用]损率较小且与加热器进口温度呈相反的变化趋势。离心式逆布雷顿循环的[火用]效高达19.2%,远高于CO_(2)逆布雷顿循环的8.1%、开式空气逆布雷顿循环的4.9%和闭式空气逆布雷顿循环的2.3%。

蒸发吸收热交换式−吸收式制冷循环性能分析

为了优化吸收式制冷循环性能,对一种蒸发吸收热交换式-吸收式制冷循环进行了热力学分析。选择高压冷凝温度T8和蒸发吸收热交换器出口温差ΔTEAX为自变量,研究了ΔTEAX与中压节流温度T12的对应关系,分析了T8对ΔTEAX取值范围的影响;考察了循环中溶液放气范围的变化,分析了ΔTEAX对制冷剂流量、循环制冷量和循环利用效率的影响。结果表明:当高温发生器出口温度和ΔTEAX一定时,存在最佳T8使循环利用效率取极大值;最佳T8随ΔTEAX增加而逐渐增大,对应的循环利用效率极大值逐渐减小。

具有喷射制冷的SOFC/TRCC电冷联供系统4E分析

提出了一种通过跨临界CO_(2)循环(TRCC)和喷射制冷循环(ERC)回收固体氧化物燃料电池/燃气轮机(SOFC/GT)余热的新型电冷联供系统.在工程求解器(EES)软件环境下建立了系统的数学模型,从能量、㶲、环境和经济(4E)的角度对系统进行研究,分析了关键参数变化对系统性能的影响.结果表明:联供系统在设计工况下净输出功率为272.874 kW,可向用户提供15.785 kW冷负荷,系统总成本为0.288 8元/(kW·h);系统总发电效率、总㶲效率和能源综合利用效率分别为68.12%、66.60%和72.06%;增加燃料利用率或提高SOFC入口温度和增大TRCC透平入口压力都可提高联供系统的㶲效率,当空燃比达到10.88时,系统㶲效率达到最大值66.94%;增大燃料利用率和SOFC入口温度或降低TRCC透平入口压力可降低系统总成本,在空燃比为10.57时达到最小值0.280 8元/(kW·h).

吸收式制冷系统应用于高温热害隧道降温的理论分析研究

为解决高地温隧道施工过程中出现的热害问题,利用隧道施工过程中出现的高温涌水作为吸收式制冷系统的热源,驱动溴化锂吸收式制冷机组工作产生冷冻水,通过空冷器释放冷量,作为隧道施工降温辅助措施。提出一个新的热害资源利用方法——利用高温涌水驱动吸收式制冷循环并服务于自身隧道降温,该方法适用于施工过程中遭遇高水温的隧道。基于能量守恒和质量守恒原理对循环的各个部件建立热力学模型,借助EES数值模拟平台对单效溴化锂吸收式制冷循环和2级溴化锂吸收式制冷循环的运行性能进行计算和分析。结果表明:1)当蒸发温度为5℃时,可以利用60℃以上的隧道涌水作为热源驱动吸收式制冷系统。2)提高蒸发温度、降低冷凝温度,系统能够在更低的热源温度下启动,从而能够利用温度更低的隧道涌水作为热源驱动吸收式制冷系统。3)当蒸发温度提高至15℃时,可以利用50℃以上的隧道涌水作为热源驱动吸收式制冷系统,从理论上分析了隧道高温涌水作为热源驱动溴化锂吸收式制冷系统的可行性。4)根据隧道涌水温度能够匹配对应的吸收式制冷系统,当隧道涌水温度在50~63℃时,可以采用2级溴化锂吸收式制冷系统;当隧道涌水温度在63℃以上时,可以采用单效溴化锂吸收式制冷系统。