A Review on Technologies for the Use of CO2 as a Working Fluid in Refrigeration and Power Cycles

The use of carbon dioxide as a working fluid has been the subject of extensive studies in recent years, particularly in the field of refrigeration where it is at the heart of research to replace CFC and HCFC. Its thermodynamic properties make it a fluid of choice in the efficient use of energy at low and medium temperatures in engine cycles. However, the performance of transcritical CO2 cycles weakens under high temperature and pressure conditions, especially in refrigeration systems;On the other hand, this disadvantage becomes rather interesting in engine cycles where CO2 can be used as an alternative to the organic working fluid in small and medium-sized electrical systems for low quality or waste heat sources. In order to improve the performance of systems operating with CO2 in the field of refrigeration and electricity production, research has made it possible to develop several concepts, of which this article deals with a review of the state of the art, followed by analyzes in-depth and critical of the various developments to the most recent modifications in these fields. Detailed discussions on the performance and technical characteristics of the different evolutions are also highlighted as well as the factors affecting the overall performance of the systems studied. Finally, perspectives on the future development of the use of CO2 in these different cycles are presented.

Exergy analysis of R1234ze（Z） as high temperature heat pump working fluid with multi-stage compression

In this paper, the simulation approach and exergy analysis of multi-stage compression high tempera- ture heat pump （HTHP） systems with R1234ze（Z） working fluid are conducted. Both the single-stage and multi-stage compression cycles are analyzed to compare the system performance with 120℃ pressurized hot water supply based upon waste heat recovery. The exergy destruction ratios of each component for different stage compression systems are compared. The results show that the exergy loss ratios of the compressor are bigger than that of the evaporator and the condenser for the single-stage compres- sion system. The multi-stage compression system has better energy and exergy etticiencies with the increase of compression stage number. Compared with the single- stage compression system, the coefficient of performance （COP） improvements of the two-stage and three-stage compression system are 9.1% and 14.6%, respectively. When the waste heat source temperature is 60℃, the exergy efficiencies increase about 6.9% and 11.8% for the two-stage and three-stage compression system respec- tively.

Working fluids of a low-temperature geothermally-powered Rankine cycle for combined power and heat generation system

A novel combined power and heat generation system was investigated in this study. This system consists of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger and a commercial R134a-based heat pump subsystem. The advantages of the novel combined power and heat generation system are free of using additional cooling water circling system for the power generation subsystem as well as maximizing the use of thermal energy in the low-temperature geothermal source. The main purpose is to identify suitable working fluids (wet, isentropic and dry flu-ids) which may yield high PPR (the ratio of power produced by the power generation subsystem to power consumed by the heat pump subsystem) value and QQR (the ratio of heat supplied to the user to heat produced by the geothermal source) value. Parameters under investigation were evaporating temperature, PPR value and QQR value. Results indicate that there exits an optimum evaporating temperature to maximize the PPR value and minimize the QQR value at the same time for individual fluid. And dry fluids show higher PPR values but lower QQR values. NH3 and R152a outstand among wet fluids. R134a out-stands among isentropic fluids. R236ea, R245ca, R245fa, R600 and R600a outstand among dry fluids. R236ea shows the highest PPR value among the recommended fluids.

Selection of organic Rankine cycle working fluids in the low-temperature waste heat utilization

In the current study, simulations based on the engineering equation solver （EES） software are performed to determine the suitable working fluid for the simple organic Rankine cYcle system in different temperature ranges. Under the condition of various temperatures and a constant thermal power of the flue gas, the influence of different organic working fluids on the efficiency of the subcritical organic Rankine cycle power generation system is studied, and its efficiency and other parameters are compared with those of the regenerator system. It is shown that the efficiency of the subcritical organic Rankine cycle system is the best when the parameters of the working fluid in the expander inlet are in the saturation state. And for the organic Rankine cycle, the R245fa is better than other working fluids and the efficiency of the system reaches up to 10.2% when the flammability, the toxicity, the ozone depletion, the greenhouse effect and other factors of the working fluids are considered. The R60 l a working fluid can be used for the high-temperature heat source, however, because of its high flammability, new working fluid should be investigated. Under the same condition, the efficiency of the organic Rankine cycle power generation system with an internal heat exchanger is higher than that of the simple system without the internal heat exchanger, but the efficiency is related to the properties of the working fluid and the temperature of the heat source.

Heat Transfer Performance of Microgroove Back Plate Heat Pipes with Working Fluid and Heating Power

Micro heat pipes(MHP) cooling is one of the most efficient solutions to radiate heat for high heat flux electronic components in data centers. It is necessary to improve heat transfer performance of microgroove back plate heat pipes. This paper discusses about influence on thermal resistance through experiments and numerical simulation with different working fluids, filling ratio and heat power. Thermal resistance of the CO2 filled heat pipe is 14.8% lower than the acetone filled heat pipe. In the meantime, at the best filling ratio of 40%, the CO2 filled heat pipe has the optimal heat transfer behavior with the smallest thermal resistance of 0.123 K/W. The thermal resistance continues to decline but the magnitude of decreases is going to be minor. In addition, this paper illustrates methods about how to enhance heat pipe performance from working fluids, filling ratio and heat power, which provides a theoretical basis for practical applications.

Energetic Analysis and Working Fluids Selection for a New Power and Refrigeration Combined Ecological System

The main purpose of this study is to analyze the performance of a new system that combines organic Rankine Cycle(ORC) and vapor compression refrigeration cycle(VCRC) for refrigeration and cogeneration. This system uses low-temperature heat sources such as solar energy, geothermal, industrial waste heat and biomass. The novelty of the proposed system manifests itself essentially in: the development of new ORC-VCRC combination architecture, lowering the ORC condensing temperature, the possibility of refrigeration production by the ORC upstream of the pumping phase, preheating of ORC using VCRC fluid and new configurations based on the integration of heat recovery systems to improve the overall system performance. The first part of this study presents the energetic analysis for the basic system using different working fluids and investigation of the operating parameters effect on the system performance(The system performance is described by the ORC thermal efficiency, the VCRC coefficient of performance and the system overall efficiency). Ten working fluids have been selected in order to provide the most adequate candidates for the proposed system. The results showed that the heating temperature and the cooling temperature have a significant effect on the system performance. The choice of fluid was also mentioned;the obtained results confirmed that the best combination for the basic system is R236fa-acetone. Four system configurations are developed and analyzed in the second part of the study. Also in the same part of the study, we will compare these configurations in terms of the performance rate retained. In the last part, we will make a comparison of this new system with another system.

MY WORK IN THE FIELD OF FLUID MECHANICS

I graduated from the National Peiyang University (now called Tianjin University) in 1950, majoring in hydraulic engineering. Starting from 1952, my teaching work was basically in mechanics.My first academic probe was in cybernetics, resulting in the publication of the first Chinese paper concerning optimal control. After 1963, I worked on the theory of hydrodynamic stability. My explorative thrust is at the eigenvalues of the Orr Sommerfeld Equation,a non-self adjoint problem in

CFD-Based Optimization of a Diesel Engine Waste Heat Recycle System

A dedicated heat exchanger model is introduced for the optimization of heavy-duty diesel engines.The model is a prerequisite for the execution of CFD simulations,which are used to improve waste heat recovery in these systems.Several optimization methods coupled with different types of working fluids are compared in terms of exergy efficiency and heat exchanger complicity.The three considered optimization methods all lead to significant improvements in the R245fa and R1233zd systems with a comparatively low evaporation temperature.The optimal R245fa system has the highest efficiency increase(77.49%).The cyclopentane system displays the highest efficiency among the optimized ORC(Organic Rankine Cycle)systems,yet achieved by using a much heavier evaporator HEC(Heat Exchanging Core).In contrast,the 96.84%efficiency increase for the optimized R1233zd is achieved with only 68.96%evaporator weight.

Performance Analysis and Multi-Objective Optimization of Two Organic Rankine Cycles with Different Fluids for Low Grade Waste Heat Recovery

The basic organic Rankine cycle(BORC)and ORC with an internal heat exchanger(IHORC)are studied with different working fluids under a given heat source condition to analyse the thermodynamic performances and net power output.The results demonstrate that the external exergy efficiency of IHORC is lower than that of BORC while the internal exergy efficiency is on the opposite with the same overall exergy efficiencies.A multi-objective optimization model with inlet pressure and temperature of expander as independent parameters and exergy and heat recovery efficiencies as objective functions is solved by NSGA-II(the second non-dominated sorting genetic algorithm).The Pareto optimal solutions are obtained by the optimization models.By calculation with the optimum conditions,it is determined that R236ea has the best comprehensive performance with exergy efficiencies being 40.69%and 41.38%,and heat recovery efficiencies being 83.2%and 75.6%in IHORC and BORC,respectively.The evaporators occupy the maximum exergy destruction,which can be reduced by decreasing pinch point temperatures and increasing evaporation pressures.

Performance analysis of solar driven combined recompression main compressor intercooling supercritical CO_(2) cycle and organic Rankine cycle using low GWP fluids

Current study deals with performance evaluation of the solar power tower driven recompression with main com-pressor intercooling(RMCIC)supercritical CO_(2)cycle incorporating the parallel double evaporator organic Rank-ine cycle(PDORC)as bottoming cycle using low global warming potential fluids to reduce the global warming and ozone depletion.Using the PDORC instead of the basic organic Rankine cycle,waste heat from the intercooler and cycle exhaust were recovered simultaneously to enhance performance of the standalone RMCIC cycle.Exergy,thermal efficiency,efficiency improvement and waste recovery ratio were considered as performance parameters.A computer program was made in engineering equation solver to simulate the model.It was concluded that by the incorporation of the PDORC thermal efficiency was improved by 7-8%at reference conditions.Maximum combined cycle’s thermal and exergy efficiency were found 54.42%and 80.39%respectively of 0.95 kW/m^(2)of solar irradiation based on R1243zf working fluid.Among the results it was also found that maximum waste heat was recovered by the R1243zf about 54.22%at 0.95 effectiveness of low temperature recuperator.

Performance evaluation of absorption thermal energy storage/transmission using ionic liquid absorbents

Efficient thermal energy storage and transmission are considered as two of the most significant challenges for decarbonisation in thermal energy utilization.The liquid-gas absorption thermal energy storage/transmission sys-tem is promising approach to tackle these challenges,owing to the long-term stability,flexibility in heat/cooling output,and liquid medium.At present,the shortcomings of conventional absorption working fluids have trig-gered considerable interest in searching for novel working pairs,such as ionic liquids(ILs).However,it is still unknown whether ILs can work effectively in thermal energy transmission with long distance.In this study,the absorption thermal energy storage/transmission systems using IL absorbents are theoretically investigated.mod-eling frameworks for working pairs screening and performance evaluation are proposed.Results show that the IL-based working pairs present better or comparable performance than conventional working pairs(including H_(2) O/Salts and NH 3/Salts).Among the investigated IL-based working pairs,H_(2) O/[EMIM][EtSO 4]presents high-est COP(around 0.62)and exergy efficiency(around 0.32),and is relatively close to H_(2) O/LiBr.As for energy storage density,H_(2) O/[EMIM][Ac]performs better than H_(2) O/LiBr,presenting 137.4 kWh/m 3 with a desorption temperature of 115°C.The present work provides a straightforward screening of IL absorbents for thermal energy storage and transmission purposes.

Optimization of Low-Temperature Exhaust Gas Waste Heat Fueled Organic Rankine Cycle

Low temperature exhaust gases carrying large amount of waste heat are released by steel-making process and many other industries, Organic Rankine Cycles （ORCs） are proven to be the most promising technology to re- cover the low-temperature waste heat, thereby to get more financial benefits for these industries. The exergy analysis of ORC units driven by low-temperature exhaust gas waste heat and charged with dry and isentropic fluid was per- formed, and an intuitive approach with simple impressions was developed to calculate the performances of the ORC unit. Parameter optimization was conducted with turbine inlet temperature simplified as the variable and exergy effi- ciency or power output as the objective function by means of Penalty Function and Golden Section Searching algo- rithm based on the formulation of the optimization problem. The power generated by the optimized ORC unit can be nearly as twice as that generated by a non-optimized ORC unit. In addition, cycle parametric analysis was performed to examine the effects of thermodynamic parameters on the cycle performances such as thermal efficiency and exergy efficiency. It is proven that performance of ORC unit is mainly affected by the thermodynamic property of working fluid, the waste heat temperature, the pinch point temperature of the evaporator, the specific heat capacity of the heat carrier and the turbine inlet temperature under a given environment temperature.

Experimental Study on the Heating Effect of a Wind-Energy Stirring Heater

In this paper,an experiment system of wind-energy stirring heater has been designed and built.Its heating performance under the rotation speed of 300 r/min has been studied through changing stirring rotor’s layer number or using different working fluids.With the help of CFD numerical simulation method,we studied the influence factors including temperature rise,total heat,heating power of each experimental group,and analyzed why these factors have such an effect.The results show:increasing the layer number of the stirring rotors can increase the motion intensity of working fluid and improve the heating effect;the quantity of effective working fluid in the rotor area can be increased significantly with the increasing of liquid level;the working liquid with high density,low viscosity,low specific heat capacity will be the ideal one.

Review on Applications of Zeotropic Mixtures

Compared with the pure fluids,the zeotropic mixtures can balance the requirements of environmental protection,heat source matching and system safety,and exhibit excellent thermodynamic performance.However,compared to the widespread applications of pure fluids,zeotropic mixtures are rarely exploited in thermodynamic cycles,and there is a lack of targeted summary on refrigeration systems,organic Rankine cycle systems and combined power and refrigeration systems.In the recent years,zeotropic mixtures are developing at an unprecedented pace,while the working fluids components are inevitably explored in the process.In this paper,the research progress of zeotropic mixtures in the field of refrigeration systems,organic Rankine cycle systems and combined power and refrigeration systems are reviewed.Based on the review of zeotropic working mixtures,the reasonable predictions can be proposed.In the future,environmental problems will still be one of the most important concerned issues.Therefore,the zeotropic mixtures consisting of natural hydrocarbons and carbon dioxide,which are environmentally friendly,have great potential for development.Furthermore,zeotropic mixtures of natural working fluids can improve comprehensive energy efficiency of combined systems and will play an important role in future carbon emission reduction technologies.

Analysis of the thermodynamic performance limits of the organic Rankine cycle in low and medium temperature heat source applications

In this paper,an exploration of the practical thermodynamic performance limits of the organic Rankine cycle(ORC)under working fluid and cycle parameter restrictions is presented.These performance limits are more realistic benchmarks for the thermodynamic cycle than the efficiency of the Carnot cycle.Subcritical ORC configuration with four typical case studies that are related to temperature ranging from 373.15 to 673.15 K is taken into account.The ORC is defined by its cycle parameters and working fluid characteristic properties.The cycle parameters involve evaporation temperature(T_(eva)),condensation temperature(T_(con))and superheat degree(ΔT_(sup)),while the working fluids are represented by the characteristic properties including critical temperature(T_(c)),critical pressure(p_(c)),acentric factor(ω),and molar ideal gas isobaric heat capacity based on the principle of corresponding states.Subsequently,Pareto optimum solutions for obtained hypothetical working fluids and cycle parameters are achieved using multi-objective optimization method with the consideration of both thermal efficiency(η_(th))and volumetric power output(VPO).Finally,sensitivity analysis of the working fluid characteristic properties is conducted,and the second law of thermodynamics analysis,especially the applicability of entropy generation minimization,is performed.The results show that the current commonly used working fluids are widely scattered below the Pareto front that represents the tradeoff betweenη_(th) and VPO for obtained hypothetical fluids.T_(eva) and T_(con) are the most dominant cycle parameters,while T_(c) and ωtend to be the most dominant characteristic property parameters.The entropy generation minimization does not give the same optimal results.

Construction and Optimization of Liquefied Natural Gas Regasification Cold Energy Comprehensive Utilization System on Floating Storage Regasification Unit

In this paper, the efficient utilization of liquefied natural gas(LNG) vaporization cold energy in offshore liquefied natural gas floating storage regasification unit(FSRU) is studied. On the basis of considering different boil-off gas(BOG) practical treatment processes, a cascade comprehensive utilization scheme of cold energy of LNG based on the longitudinal three-stage organic Rankine cycle power generation and the low-grade cold energy used to frozen seawater desalination was proposed. Through the comparative analysis of the effects of the pure working fluid and eight mixed working fluids on the performance of the new system, the combination scheme of system mixed working fluid with the highest exergy efficiency of the system was determined. Then, the genetic algorithm was used to optimize the parameters of the new system. After optimization, the net output power of the LNG cold energy comprehensive utilization system proposed in this paper was 5186 kW, and the exergy efficiency is 30.6%. Considering the power generation and freshwater revenue, the annual economic benefit of the system operating is 18.71 million CNY.

Low-temperature compression-assisted absorption thermal energy storage using ionic liquids

Thermal energy storage technologies play a significant role in building energy efficiency by balancing the mis-match between renewable energy supply and building energy demand.The absorption thermal energy storage(ATES)stands out due to its high energy storage density(ESD),high coefficient of performance(COP),low charg-ing temperature and wider application flexibility.A hybrid compression-assisted ATES(CATES)using ionic liquid(IL)-based working fluids is investigated to address the problems of the existing ATES cycle.Models for mixture property and cycle performance are established with verified accuracies.Four ILs([DMIM][DMP],[EMIM][Ac],[EMIM][DEP],and[EMIM][EtSO_(4)])are compared with H_(2)O/LiBr.Results show that the CATES effectively avoid the crystallization,decreases the circulation ratio,lowers the charging temperature,and improves the COP/ESD.H_(2)O/[DMIM][DMP]has the highest COP and performs better than H_(2)O/LiBr with generation temperatures above 86℃,while H_(2)O/[EMIM][EtSO_(4)]shows the highest COP with generation temperatures below 75℃.Among the H_(2)O/IL mixtures,H_(2)O/[EMIM][Ac]shows the highest ESD with generation temperatures above 86℃,otherwise H_(2)O/[EMIM][EtSO_(4)]shows the highest.The optimal compression ratio is 1.6-2.8 for H_(2)O/[DMIM][DMP]under the generation temperatures of 90-70℃with the maximum COP of 0.758-0.727.The ESD increases significantly with the compression ratio.