REVIEWARTICLE

Feier XUE, Yu CHEN, Yonglin JU

A review of cryogenic power generation cycles with liquefiednatural gas cold energy utilization

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2016

Abstract Liquefied natural gas (LNG), an increasinglywidely applied clean fuel, releases a large number of coldenergy in its regasification process. In the present paper,the existing power generation cycles utilizing LNG coldenergy are introduced and summarized. The direction ofcycle improvement can be divided into the key factorsaffecting basic power generation cycles and the structuralenhancement of cycles utilizing LNG cold energy. Theformer includes the effects of LNG-side parameters,working fluids, and inlet and outlet thermodynamicparameters of equipment, while the latter is based onRankine cycle, Brayton cycle, Kalina cycle and theircompound cycles. In the present paper, the diversities ofcryogenic power generation cycles utilizing LNG coldenergy are discussed and analyzed. It is pointed out thatfurther researches should focus on the selection andcomponent matching of organic mixed working fluidsand the combination of process simulation and experi-mental investigation, etc.

Keywords liquefied natural gas (LNG) cold energy,power generation cycle, Rankine cycle, compound cycle

1 Introduction

Liquefied natural gas (LNG) is the final product of naturalgas (NG) in the processes of de-acidification, de-hydrationand liquefaction, which comes to the ultimate state ofliquid mixture with a low temperature of –162°C [1]. Themain composition of LNG consists of methane (90%),

ethane (0.1%–5%), nitrogen (0.5%–1%) and a smallamount of hydrocarbon of C3–C5. Compared with theconventional energies, such as coal and oil, the absence ofsulfur in LNG composition decreases the amount ofenvironmentally hazardous gases produced after regasifi-cation and combustion. Besides, the greenhouse gasesgenerated during NG burning are only 1/2 and 2/3 of anequal amount of coal and oil, respectively. Considering thesevere environmental issue, it is reasonable to believe thatLNG with low emission problems will be the dominatingenergy in the future in preference to coal and oil.In practical applications, only after regasification can

LNG be used. However, the traditional ways of regasifica-tion using seawater or air vaporizer will lead to a huge lossof the cold energy of LNG. Therefore, the establishment ofcryogenic power generation cycles utilizing LNG coldenergy becomes an important way to recovery cold energy.Due to the high quality of electricity, turbine coupled to anelectrical generator is usually adopted to achieve electricityoutput in cryogenic power generation cycles.The present paper focuses on the power generation

cycles taking advantages of LNG cold energy, classifyingthe existing studies into the key factors affecting basicpower generation cycles and the structure enhancement,which provides a profile of alternative power generationcycles using LNG cold energy.

2 Basic power generation cycles utilizingLNG cold energy and the effects of keyfactors

2.1 Basic LNG power generation cycles

Basic LNG power generation cycles utilizing cold energymainly include direct expansion of LNG, Rankine cycleusing intermediate cooling medium, and the combinationof aforementioned cycles. These three kinds of powergeneration cycles have been in practical use with relativelymature technology. In Osaka Gas Company in Japan, forexample, as early as 1979 and 1982, the Rankine cycle

Received October 23, 2015; accepted December 20, 2015

Feier XUE, Yonglin JU (✉)Institute of Refrigeration and Cryogenics, Shanghai Jiao TongUniversity, Shanghai 200240, ChinaE-mail: yju@sjtu.edu.cn

Yu CHENCollege of Mechanical Engineering, Shanghai University of EngineeringScience, Shanghai 201620, China

Front. EnergyDOI 10.1007/s11708-016-0397-7

using propane as the working fluid and the combined cyclehave been in use and achieved an output power of 1450kW and 6000 kW, respectively [2].LNG direct expansion is an open cycle, as shown in

Fig. 1, and converts the pressure exergy of LNG into poweroutput, which exploits LNG exergy in a relatively directbut inefficient way. In this case, LNG from the storage tankis pumped up and vaporized into NG with a hightemperature and pressure. NG is fed into the turbine andis expanded to output electric energy by a generator linkedto a rotating shaft. The turbine exhaust is heated by a heatsource, where seawater or residual heat is alwaysemployed, and finally is transported to user’s receivingterminal as demanded. In spite of principle-simplicity andcost-saving, the inefficiency of direct expansion hasrestricted it to the assisting of other basic cycles inapplication.In Rankine cycle, intermediate cooling medium (work-

ing fluid with low boiling temperature is usuallyemployed) goes through four stages of condensation,compression, vaporization and expansion to generatepower, using LNG as its heat sink and low quality heat(including solar energy, air, water, industrial waste heat,etc.) as its heat source, as presented in Fig. 2. Compared todirect expansion, the Rankine cycle primarily utilizescryogenic exergy of LNG instead of pressure exergy andachieves higher efficiency in performance due to thereduced turbine back pressure. However, the largetemperature difference between the single working fluidand LNG in the process of heat exchange in the condenseralways causes a great loss of exergy, which increases theirreversibility of the whole cycle.

The combined cycle is the integration of directexpansion and Rankine cycle, as illustrated in Fig. 3.The combined cycle can use the cryogenic and pressureexergy simultaneously and the connection between the twosub-cycles is realized through the condenser where theworking fluid obtains the cold energy released by LNG.The power output is generated from both the working fluidturbine and the NG turbine. As far as the efficiency is

concerned, the combined cycle is the most commonly usedone among cryogenic power generation cycles of LNG inpractical power plant.

2.2 Effects of key parameters of basic LNG powergeneration cycles

The effects of key parameters on the cycle performancechiefly revolve around basic Rankine cycle and thecombined cycle for their extensive application. In thissection, LNG-side parameters, circulatory working fluidsand input and output factors of crucial equipment arediscussed.

2.2.1 Effect of LNG-side parameters

Liu and Guo [3] have determined the impacts of LNGtemperature, pressure and concentration of methane inLNG on LNG exergy. It is reported that the cryogenicexergy, the pressure exergy and the total exergy of LNGincrease with the rising ambient temperature at a specifiedcomposition of methane. When the pressure of LNG risesup, the pressure exergy increases while the cryogenicexergy decreases, both of which make the total exergy firstreduce then level off. The reason for the decline ofcryogenic exergy is that the rise of the LNG pressuremakes itself a higher bubble-point, which accordinglydecreases the temperature difference between the LNG andthe ambient. In the meantime, the state of LNG gets muchcloser to the critical section where there is a lower latentheat of vaporization. In addition, the cryogenic exergy, thepressure exergy and the total exergy of LNG increase as the

Fig. 1 Schematic diagram of direct expansion of LNG

Fig. 2 Schematic diagram of Rankin cycle

Fig. 3 Schematic diagram of Rankine cycle with direct expan-sion of LNG

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mole fraction of methane rises at a specified ambienttemperature and pressure.Bai [4] has adjusted the pressure and temperature of

LNG and the concentration of methane in LNG respec-tively in the proposed Rankine cycle using propane as theworking fluid, with molar flow change of propane to matchthe adjustment. The power output of the cycle is influencedby the above LNG-side parameters and it is furtheranalyzed that the rise of the methane composition in LNGat a certain pressure will decrease the bubble-point of LNGand increase the temperature difference between LNG andthe ambient, which clearly makes a higher cryogenicexergy. Besides, the consequent decline of molar masscauses an augmented pressure exergy, which improves theoverall ability of power generation.The temperature difference and pressure difference

between LNG and reference environment mutually con-tribute to a considerable LNG cold exergy. The researchresults above show that the temperature and pressure ofLNG directly influence LNG exergy and the performanceof cycle while the concentration change of the dominatingcomposition of methane mediately alters the temperatureand pressure difference and affects LNG exergy as a result,by changing its own physical attributes, embracing bubble-point and molar mass, etc. Thus a proper type of cycleshould be chosen to utilize LNG exergy effectivelyaccording to the condition of LNG. The concentration ofmethane in LNG has determined the ability of cycle poweroutput within limit. As for the practical applications, theupper bound of methane concentration interval can beregarded as an ideal working condition and the lowerbound as a severe condition, so as to simulate practicalengineering in a comprehensive way.

2.2.2 Working fluid

1) Single working fluidThe principles of the selection of the single working

fluid in cryogenic Rankine cycle and the combined cycleare presented as follows: friendly to environment, largeheat of vaporization, good chemical and thermal stability,high thermal conductivity, small kinetic viscosity, nearlyvertical liquid saturation line, non-toxic, easy to produceand low cost, etc [5]. Moreover, the triple point of theworking fluid should be lower than the lowest temperatureof system operation, which ensures that the solidificationand the jam of the working fluid will not happen at anyplace in the circulation [4].Lu et al. [6] have selected different working fluids

to compare their performance in sub-critical Rankine cyclewith LNG as its heat sink and seawater as its heat source.The regularity appears similar when employing fourworking fluids of R152a (CH3CHF2), R290 (C3H8),R600 (C4H10) and R134a (CH2FCF3) in the investigatedcycle, i.e., the total amount of electricity generatedfrom circulation first increases and then decreases with

the evaporation temperature rising, which confirms thatthere exists an optimal evaporation temperature wherethe power output gets the maximum. Meanwhile, R290 isproved to be with the comprehensively optimum propertiesin contrast to other working fluids in enthalpy drop withinequal entropy, saturation pressure and other attributes.Corresponding to a heat source temperature of 20°C,the best evaporation temperature of R290 maintains at11.08°C.Liu et al. [7] have established the dependencies between

LNG utilized temperature and unit power output in LNG-seawater Rankine cycle. It is revealed that at a specifiedLNG vaporization pressure, an optimal LNG utilizedtemperature exists which makes the unit power outputmaximum. In view of the existence of temperaturedifference between cold and hot fluids in the heat transferprocess happened in condenser, the optimal condensingtemperature of working fluid is supposed to be deducedaccording to the optimal LNG utilized temperature toobtain the max output.Consequently, the optimum condensing and vaporizing

temperature ought to be taken into account as part of theprinciples when selecting the single working fluid inRankine cycle and the combined cycle.Zhang et al. [8] have screened and constructed the

refrigerant fluids in LNG-seawater power system, adoptinga novel group contribution method (GCM). Vital attributevalues, in which critical point, thermal conductivity,enthalpy of vaporization and heat capacity are included,can be calculated based on GCM in the selection ofworking fluids. According to GCM calculation model,CHF3 is recommended to be a prior choice to theconventional refrigerant fluids embracing R22 (CHClF2),R134a (CH2FCF3) and R410a with the evaluatingindicators of LNG exergetic efficiency and thermalefficiency. High density of CHF3 decreases the total flowrate in circulation and economizes the material cost; itslower boiling point makes trans critical cycle possible,which improves the overall cycle performance; a largerheat capacity reduces the flow rate of working fluid at arequired amount of heat transmission, shrinks the size ofthe pipelines, and decreases the pump power input.2) Mixed working fluidsAlthough Rankine cycle and the combined cycle

employing propane as working fluid are the most feasibleways to recovery LNG cold exergy, isothermal phasechange process of single medium in condenser makes agreat LNG exergy loss which confines the cycle perfor-mance to a relatively low level. Considering the fact thatthe condensing temperature of mixed working fluidmatches well with LNG vaporization temperature, due tothe temperature-variation phase change of mixed media,the single working fluid is supposed to be substituted bythe mixed one to enhance the overall cycle performance.The mixture of R22 and R142b (CH3CClF2) has been

proved to perform better than any one of two pure

Feier XUE et al. Cryogenic power generation cycles utilizing cold energy of liquefied natural gas 3

refrigerated fluids in LNG cold energy power system,according to the study conducted by Kim et al. [9].However, the gradually severe environment issue con-stricts the use of Freon, and organic mixed fluids turn intocommonly employed media.Zhu et al. [10] have compared the performance of

propane and ternary mixed fluids consisting of propane,ethylene and isobutane at a mole fraction ratio of0.39:0.16:0.45 in LNG-air Rankine cycle. The investiga-tion shows that the LTMD (logarithmic mean temperaturedifference) of ternary mixed fluids gets much smaller thanthe propane and the utility efficiency of cold energy isincreased by 41.04%.Wang [5] has proposed an isentropic fluid, which is a

mixture of propane and isobutane at a mole fraction ratio of0.7:0.3, in the LNG-waste gas Rankine cycle. Theisentropic mixture is able to maintain the turbine exhaustat a lower pressure and temperature, and consequentlyimproves the efficiency of the turbine. It is reported that thecycle employing the isentropic mixture performs with thehighest thermal efficiency when altering the mole fractionratio of propane and isobutane under the similar workingcondition. However, adding isobutane to pure propane toconstitute the mixture will elevate the allowable minimumcondensing temperature and narrow working fluid opera-tional temperature interval of the cycle.Chen [11] has adopted the mixture of propane and

butane as working fluid in Rankine cycle using LNG as itsheat sink. The study states the variation trend of theexergetic efficiency of vaporizer and condenser along withthe change of mixing ratio and mixture flow rate. When theflow rate of mixture is fixed, the variation tendency of theexergetic efficiency of the vaporizer along with the changeof the mixing ratio is complex while the exergeticefficiency of the condenser declines as the concentrationof propane in mixture rises up. Additionally, there exists anoptimal mixture flow rate where the exergetic efficiency ofthe vaporizer reaches the peak while the exergeticefficiency of the condenser declines with the flow rate ofthe mixture rising.Besides, plenty of researches have been devoted to

Rankine cycle and combined cycle employing ammonia-water mixture as its working fluid. Miyazaki et al. [12]have built a combined cycle with exhaust gas as heatsource and LNG as heat sink to separately analyze theperformance of ammonia-water and water vapor. It ispresented that the exergetic efficiency and thermalefficiency of the former is 1.53 and 1.43 times of thelatter, respectively. Gao and Wang [13] have proposed anovel LNG power generation cycle employing ammonia-water as the working medium in the basis of conventionalthermodynamic cycle and LNG cold energy utilization.The thermal efficiency of this novel cycle rises up by14.5% and the exergetic efficiency reaches 53.6%. It is alsoconcluded in the report that the critical reasons forefficiency improvement are the decrease of average

exothermic temperature and proper matching between thevaporization and the condensation. Wang et al. [14] haveadopted a heat recovery vapor generator (HRGV), wherevaporization is classified into three different regionsincluding a sub-cooled region, an evaporation region,and a super-heated region, to replace the conventionalvaporizer in Rankine cycle using ammonia-water as theworking medium. An optimal components ratio ofammonia and water is discovered at an ammonia massfraction of 0.7 to obtain the peak of the net work output inthe discussed cycle.

2.2.3 Inlet and outlet thermodynamic parameters of equip-ment

Research on the inlet and outlet key parameters ofequipment provides available directions for cycle optimi-zation, which is conducted by changing key parameters toobtain the variation regularity of cycle power output,thermal and exergetic efficiency and other valuableindicators. The inspected parameters include but are notlimited to the turbine inlet temperature and pressure, aswell as the vaporizer outlet temperature, etc.In LNG-seawater combined cycle, the increase of the

turbine inlet temperature of the working fluid and NG willmarkedly improve the overall power output of cycle, asreported by Bai [4]. Rao et al. [15] have suggested LNG-residual heat Rankine cycle using ethane as its workingfluid and presented that the systematic thermal efficiencyand total power output increase with the pressure ratioascending between the vaporizer outlet pressure and thecondenser outlet pressure of the working fluid. Xue et al.[16] have proposed a two-stage LNG-exhaust gas Rankinecycle. It is pointed out that the cost per net power outputincreases with the rising turbine inlet pressure and the massflow of the working fluid in each stage, which indicates anenhancement in cycle economic performance.

3 Structural enhancement

A few studies [17,18] of the structural enhancement areconducted toward LNG direct expansion. Most structuralenhancement types in LNG cold energy power cycles arebuilt on Rankine cycle and Brayton cycle. Meanwhile,Kalina cycle which characteristically employs absorberand separator are proposed to make use of the property ofnon-azeotropic mixtures. Compound cycles with higherstructural complicacy are established by integratingRankine cycle, Brayton cycle or Kalina cycle to makebetter cycle performance available.

3.1 Structural enhancement based on Rankine cycle

The working fluids employed in Rankine cycle vary frommost widely used propane to ethane, ammonia-water,

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organic refrigerating mixture and CO2, etc. The heatsources are also extended from seawater to industrial wasteheat, exhaust gas, geothermic heat, and solar energy, etc.Kim and Kim [19] have conducted a modified combined

cycle with regeneration, using a low-level heat of 200°Cand ammonia-water as working fluid at an ammonia massfraction of 60%, as presented in Fig. 4. In this ammonia-water cycle, the liquid ammonia-water, which is firstpumped up by the working fluid pump, is preheated in theregenerator by the hot stream of the exhaust mixture fromthe turbine. The process utilizes the heat of turbine exhaustto raise the inlet temperature of turbine and to improve thework output afterwards. Huang et al. [20] have proposed atrans critical regenerative Rankine cycle, adopting CO2 asthe working medium and LNG and geothermic heat as theheat sink and heat source, respectively. Figure 5 depicts theschematic diagram of the system. The regenerativeRankine part is accomplished by the working fluid ofCO2. The heat source consisting of water vapor and CO2 iscondensed by the working fluid of CO2 and LNGsuccessively then fed to the separator where liquid wateroutflows from the bottom and CO2 from the top. The CO2

separated from water transfers heat to LNG and finallyturns into the state of liquid for recovery. In this system, thecold exergy of LNG, on the one hand, drives the workingfluid to export power through turbine, and on the other,provides cooling capacity of CO2 liquefaction to controlcarbon emission.

Poly-stage inclination is another important approach ofRankine cycle to make structures enhanced, mainly basedon the theory of cascade utilization of LNG cold energyaccording to LNG regasification curve.Yang [21] has established a LNG-seawater two-stage

Rankine cycle of horizontal focusing on 7 MPa LNG in thecondition of super-critical regasification. As demonstratedin Fig. 6, the loops from left to right in turn are the first andsecond stage of the whole system and ethane andpropylene are employed respectively. In this system, theworking fluids in the two stages obtain LNG cold energysuccessively and individually drive their own Rankine sub-cycle. Compared with the Rankine cycle employing only

propylene, the proposed two-stage Rankine system has animprovement in electrical power output by 36.47%.In view of the great exergetic loss in the condensers of

the horizontal cycle, Yang [21] has established a three-stage vertical cycle to reduce the systematic exergetic loss,as illustrated in Fig. 7. The loops from the bottom to the topare the first, second and third stage, using propylene,ethylene and ethylene as working fluid, respectively. Bothof the outlet gases from turbine in the top and the centralstage are divided into two streams, one of which is used toassist the LNG regasification through HX2 and HX1.Besides, the top stage takes seawater as its heat sourcewhile the central and bottom stage respectively utilizesexothermic heat from the turbine exhausted in the previousstage. The transformation from horizontal type to verticaltype implements the segmented utilization of LNG coldenergy, which improves the performance of the wholesystem.Considering the fact that the condenser outlet tempera-

ture of LNG is still low and the corresponding part of LNGcold energy is not utilized yet, Choi et al. [22] haveestablished a triple cascade Rankine cycle, as displayed inFig. 8. In Cd1, LNG acts as the cold fluid and the outworking fluid as the hot fluid; in Cd2, the central workingfluid works as the hot fluid while LNG and outer mediumprovide cold capacity; in Cd3, only inner working fluidserves as hot fluid while the other three fluids absorb the

Fig. 4 Schematic diagram of the regenerative Rankine cyclewith direct expansion of LNG [19]

Fig. 5 Schematic diagram of CO2 regenerative Rankine cycle[20]

Fig. 6 Schematic diagram of two-stage Rankine cycle ofhorizontal [21]

Feier XUE et al. Cryogenic power generation cycles utilizing cold energy of liquefied natural gas 5

heat released. Compared to the basic Rankine cycle, thethermal efficiency of the proposed one is significantlyincreased at the expense of higher systematic structuralcomplexity and instability due to adopting the multi-streamheat exchangers.A brief summary of main structural improvements of

Rankine cycles [4,5,12,14,15,21,23–28] is presented inTable 1.

3.2 Structural enhancement based on Brayton cycle

Brayton cycle, which makes use of LNG cold energy toreduce the compressor inlet gas temperature, significantlyreduces the power consumption of the compressor underthe condition of a specified compression ratio andimproves the net work output of the circulation; thetemperature curves of LNG regasification and working gascooling match better than those in the Rankine cycleemploying single working medium, which can effectivelyimprove the cycle thermodynamics performance. Thetypical nitrogen Brayton cycle combining with LNG direct

expansion is exhibited in Fig. 9. On one side, thecompressed LNG passes cooling capacity to nitrogen,goes through heat exchanging with heat source and is fedto NG turbine to export power. On the other side, nitrogenobtaining cooling capacity enters the compressor at a lowertemperature, and after heated by heat source it enters theturbine for power output in a state of high temperature andhigh pressure and then returns to the LNG-nitrogen heatexchanger.Agazzani et al. [29] have proposed an improved Brayton

cycle employing helium as the working fluid and heat ofcombustion as the heat source. A regenerator has beeninstalled between the helium compressor and HX2 toincrease the helium turbine inlet temperature, as presentedin Fig. 10. Morosuk and Tsatsaronis [30] have built up anovel Brayton cycle, as shown in Fig. 11. After two-stagecompression with intermediate cooling, air assists fuelburning in the combustion chamber combustion. Thencombustion gas with a high temperature enters the gasturbine for power output and the exhaust provides the heatfor helium Brayton cycle. LNG completes direct expansionthrough the NG turbine after vaporizing in HX1 and HX2.Dispenza et al. [31] have established another structure-enhanced Brayton cycle, using open NG combustionexhaust gas from two-stage expansion as the heat source.The overall power output of the proposed circulationcomes from both the expansion of helium and NG, inwhich way the power-output ability of the whole cycle isimproved. Other researchers also put forward cyclessimilar to Dispenza’s structure [32,33].Considering the fact that most of the turbine power

output is consumed by compressor driving, which limitsthe net work output of the whole system, a cascade ofcompression and expansion becomes one of theapproaches to modify Brayton cycle. Tomków andCholewiński [34] has proposed a novel Brayton cyclewith two-stage compression and two-stage expansion, asshown in Fig. 12. Both HX1 and HX2 are multi-streamheat exchangers, which provide intermediate heating fortwo-stage expansion and intermediate cooling for two-stage compression, respectively. In addition, the complexMGT (mirror gas turbine) cycle is recommended byKaneko et al. [35]. In this proposed cycle, LNG is firstemployed for intercooling of four-stage air compressionthen fed to the NG turbine to export electrical power in gasstate. NG out of the NG turbine burns with the compressedair in the combustion chamber then goes into the gasturbine. In spite of the superior performance compared tobasic types, the structural complexity of MGT restricts itsscope of practical application.

3.3 Structural enhancement based on Kalina cycle

The advantage of Kalina cycle which employs the non-azeotropic mixtures (ammonia-water is used mostly) as theworking medium mainly lies in two aspects: the variable

Fig. 7 Schematic diagram of three-stage Rankine cycle ofvertical [21]

Fig. 8 Schematic diagram of three-stage cascade Rankine cycle[22]

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temperature evaporation of the working fluid reduces theirreversibility of endothermic process and the smalltemperature change in the condenser of basic working

medium, which contains less solute, alleviates irreversi-bility of condensation process.In recent researches, the working media used in Kalina

Table 1 A brief summary of main structures of enhanced Rankine cycles

Type enhanced Researchers Structuraldescription

Working fluid Heat source Cycle characteristics

RC a) Rao et al. [15] RC Ethane Residualheat

When the temperature of heat source is below 260°C and LNG vaporizingpressure is greater than a certain value, auxiliary heat source should beapplied for LNG regasification in case of insufficient heat transfer in the

condenser.

Wang et al. [14] RC Ammonia-water

Residualheat

Cycle uses HRVGb) instead of general vaporizer to enhance turbine inlettemperature of ammonia steam.

Thecombinedcycle

Miyazaki et al.[12]

RC+DEC c) Ammonia-water

Exhaustgas

Temperature glide occurred in heat transfer process in the condenser betweenammonia-water and LNG effectively reduces heat loss.

Bai [4] RC+DEC Ethane Residualheat

Working medium compressed by the pump exchanges heat with cold waterand waste heat successively; cooling capacity obtained by cold water can beused for air conditioning system or compressor cooling, etc. Two-stage heat

exchanging improves the turbine inlet temperature.

Cao and Lu [23] RC+DEC Propane Seawater Working medium propane vaporized by seawater is shunt for two strands,one of which drives Rankine cycle and the other directly transfers heat toLNG out of the condenser; both strands get remixed through mixer and fed

into working medium pump.

Wang et al. [24] Trans criticalRC+DEC

CO2 Geothermicheat

After absorbing the heat from geothermic water, CO2 is fed into the turbineto export electrical power; LNG heated by CO2 and the ambient, in gas state

is also sent into the NG turbine to do work.

Sun et al. [25] RC+DEC Propylene Solarenergy

Water heated by solar energy and assistant electric heater provides heatneeded for the system. Propylene completes Rankine cycle as working fluid.

Installingregenerator

Sun et al. [26] RegenerativeRC+DEC

Mixture(methane:ethylene:propanemol%=

0.3:0.4:0.3)

Residualheat

MSCHE (multi-stream cryogenic heat exchanger) is used to substituteconventional vaporizer in the cycle. In MSCHE, LNG through the firstchannel releases a large amount of cold; boosted mixed working mediumsent into the second channel gains cold energy then completes heat transferto refrigerant and external heat source respectively and expands to outputpower; high-temperature gas from medium turbine outlet goes into the thirdchannel as a hot fluid to release heat. In addition, cold energy obtained by therefrigerant from mixed medium will be supplied to the air conditioning

system.

Wang [5] RegenerativeRC+DEC

Mixture(propane andisobutane)

Exhaustgas

Working medium is vaporized by regenerator and exhaust gas heater insuccession, which effectively realizes utilization of exhaust gas waste heat

and reduces condenser loads.

Poly-stage RC Yang [21] 3-stage RCof horizontal

Ethane/Ethane/propylene

Seawater Considering three-stage recovery of LNG cold energy, LNG successivelyacts as cold fluid for left ethane RC, central ethane RC and right propyleneRC. Three loops from left to right operate individually and in each loop two-stage expansion is achieved by working medium with intermediate heat

absorption from seawater.

Cao et al. [27] 2-stage RCof horizontal

+DEC

CO2/R245fa(CF3CH2CHF2)

Residualheat

LNG serves as the heat sink for super-critical CO2 Rankine cycle and R245faRankine cycle while residual heat provides requisite heat in the opposite

order.

Zhu et al. [28] 2-stage RCof vertical

Ethylene/propane

Exhaustgas

Lower ethylene Rankine cycle and upper propane Rankine cycle are linkedthrough ethylene-propane heat exchanger where ethylene is vaporized andpropane is condensed. Additionally, a strand of stream from propane turbineis elicited to heat LNG out of ethylene condenser and fed to propane pump in

liquid state.

Yang [21] 2-stageregenerativeRC of vertical

Propylene /ethylene

Seawater Ethylene, the working fluid of the upper Rankine cycle, is divided into threeshares after expansion, one of which is directly used for LNG regasification,and two remaining shares supply needed heat for two-stage expansion in thelower propylene Rankine cycle, which makes up the systematic defect of

insufficient work output.

Notes: a) RC—Rankine cycle; b) HRGV— heat recovery vapor generator; c) DEC— direct expansion cycle

Feier XUE et al. Cryogenic power generation cycles utilizing cold energy of liquefied natural gas 7

cycle have been extended from ammonia-water toethylene-propane mixture and trafluoromethane-propanemixture, etc. Structural enhancement based on Kalinacycle includes installing regenerator, two-stage expansionor combination with LNG direct expansion, etc. The mainstructures of Kalina cycles are summarized in Table 2[4,36–39].Figure 13 is a schematic diagram of a novel Kalina cycle

combined with LNG direct expansion, adopting ammonia-water mixture as the working medium [36]. After

absorbing heat from the heat source, ammonia-watermixture with a high temperature is fed into the separatorwhich separates the mixture into rich ammonia vapor andweak ammonia-water solution. The rich ammonia vapordrives the medium turbine and the exhaust is condensed byLNG and boosted into the absorber, where the vapor isremixed with weak ammonia-water solution, which hascompleted heat exchanging with the LNG in theregenerator and the throttling. The NG turbine exhaustobtains heat from the remixed solution and is transported touser’s terminal while the remixed solution cooled by theLNG cold energy enters the working fluid booster andreturns to exchange heat with the external heat source. Inthis loop, the rich ammonia vapor possesses a great abilityto export power and the weak ammonia-water solutionwith a high temperature can provide the heat needed forLNG direct expansion, both of which increase the poweroutput from the cycle.

3.4 Compound cycles

The Rankine cycle, Brayton cycle, Kalina cycle and LNGdirect expansion are frequently integrated to formcompound cycles which have a superior cycle performancewith relatively complex structures. Some researchers havealso established novel compound cycles combining withthe gas turbine. The main structures of compound cyclesare presented in Table 3 [2,34,40–44]. The compoundcycles raised by Zhang and Lior [42] and Tomków andcholewiński [34] are elaborated as follows, respectively.Zhang and Lior [42] have set up a near-zero CO2

emission thermal cycle combining the CO2 super-criticalRankine cycle with the CO2 Brayton cycle. As shown inFig. 14, NG turbine exhaust burns with the product of airseparator unit (ASU) in the combustion chamber and thecombustion gas expands in the gas turbine and providesheat for CO2 vaporization in three-channel and commonheat exchangers successively. Then the mixture out of thecommon heat exchanger is fed to the separator whichseparates water out of the bottom and CO2 out of the top.

Fig. 9 Schematic diagram of Brayton cycle with direct expan-sion of LNG

Fig. 10 Schematic diagram of the regenerative Brayton cyclewith direct expansion of LNG [29]

Fig. 11 Schematic diagram of improved Brayton cycle withdirect expansion of LNG [30]

Fig. 12 Schematic diagram of two-Stage Brayton cycle [34]

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The cooled CO2 goes through a two-stage compressionand returns to the combustion chamber. One stream of theCO2 from the low-level compressor completes the super-critical Rankine cycle. In the present compound cycle,LNG cold energy is used for cooling the inlet gases of thecompressors and the CO2 condensation, while the

systematic power output is generated by the Rankinecycle and the gas turbine. One part of the CO2 produced byNG combustion acts as the working fluid of the Rankinecirculation and the other part is condensed into liquid forstorage instead of direct discharge to the environment,which makes the whole cycle close to zero-CO2 emission.The integration of Rankine cycle and Kalina cycle is

established by Tomków and Cholewiński [34], asillustrated in Fig. 15. The reason for the employment ofkrypton-ethane mixture as working medium in the Kalinapart is that the boiling point of krypton differs far awayfrom that of ethane, which makes it easy to separate onefrom the other; besides, it is harder for the inert gas kryptonto react with other components. Representative devices ofKalina cycle are the separator and the absorber. The formerseparates the krypton-ethane mixture into rich krypton gaswhich accomplishes two-stage expansion for power outputand weak krypton solution which heats the mixture pumpoutlet liquid krypton-ethane solution. The latter remixesrich and weak krypton solution back to krypton-ethanemixture. The three-channel condenser realizes the connec-tion between the propane Rankine cycles and the Kalinacycles, where LNG works as the cold fluid and propaneand rich krypton turbine exhaust serve as the hot fluids.

Table 2 Summary of main structures of Kalina cycles

Researchers Structural description Working fluids Heat source Utilization efficiency of available energy/%

Liu and Guo [37] KC Mixture (ethylene: propanemol% = 0.60：0.40)

Seawater 25.3

Liu and Guo [38] KC Mixture (trafluoromethane:propane mol% = 0.73：0.27)

Seawater 23.5

Bai [4] KCa) + DECb) Mixture (trafluoromethane:propane mol% = 0.73：0.27)

Industrial wasteheat

24.0

Shi and Che [39] KC+ DEC Mixture (ammonia: watermass% = 0.50：0.50)

Industrial wasteheat

33.28

Wang et al. [36] KC+ DEC Mixture (ammonia: watermass% = 0.52：0.48)

Industrial wasteheat

39.33

Notes: a) KC—Kalina cycle; b) DEC—direct expansion cycle

Fig. 13 Schematic diagram of Kalina cycle with direct expan-sion of LNG [36]

Table 3 Summary of main structures of compound cycles

Researchers Structural description Working fluids Heat source Utilization efficiency ofavailable energy/%

Xia et al. [40] BCa) + regenerative RCb)

+ DECc)Nitrogen/ammonia-water Industrial waste heat 51.04

Lu and Wang [41] RC+ DEC+ gas turbine Exhaust gas/ammonia-water Heat of combustion —

Zhang and Lior [42] Super-critical RC+ BC CO2/CO2 Heat of combustion 50

Shi et al. [43] RC+ DEC+ gas turbine Exhaust gas/water vapor Heat of combustion 59.24

Najjar [44] RC+ 2-stage DEC+ gasturbine

Propane/exhaust gas Seawater-heat of combustion 35.84

Hisazumi et al. [2] 2-stage RC+ gas turbine+ 2-stage DEC

Freon/water vapor/exhaust gas Heat of combustion 56

Tomków and Cholewiński [34] RC+ KCd) Propane/mixture of krypton and ethane Seawater 18.8

Notes: a) BC—Brayton cycle; b) RC—Rankine cycle; c) DEC—direct expansion cycle; d) KC—Kalina cycle

Feier XUE et al. Cryogenic power generation cycles utilizing cold energy of liquefied natural gas 9

4 Remarks and conclusions

The present paper approximately classified studies of thepower generation cycle utilizing LNG cold energy into keyfactors affecting basic power generation cycles andstructural enhancement. Thanks to the existing maturetechnology in basic loops, studies of key factors caneffectively provide valuable guidance for deeper optimiza-tion of the practical projects. The great diversity ofstructural enhancement provides a new way for furthermodification of power generation cycle using LNG coldenergy. Several aspects worthy of deeper development inthe future mainly include:

1) Component analysis of organic mixed working fluidemployed in Rankine cycle and the combine cycleAt present, employing organic mixed working medium

in Rankine cycle and the combined cycle has become animportant and feasible approach for cycle optimization.However, most existing researches are simply conducted ata fixed mixture component ratio, which does not make adetailed and systematical analysis of the selection ofmixture components and determination of its ratio. Theoriginal intention of using mixture medium is to find abetter matching of temperature curve between the workingmedium and LNG, so that the mixed working mediumcomponents will closely relate to LNG components to

Fig. 14 Schematic diagram of integration of Rankine cycle and Brayton cycle [42]

Fig. 15 Schematic diagram of integration of Kalina cycle and Rankine cycle [34]

10 Front. Energy

some degree. It will be of great significance to define thecriteria of choosing mixture components and the compo-nent ratio according to different sources of LNG.2) Combination of cycle simulation and experimental

investigationCurrent structural improvement mainly depends on the

process simulation by software, which makes it worthnoticing that devices in simulation process are always setto operate under an ideal condition and will consequentlybring a larger deviation to evaluate the feasibility of theproposed cycle. It is difficult to establish experimental set-up in LNG power generation system, but it is of greatimportance to transform from plentiful theoretical studiesto practical appliance. Therefore, the combination of cyclesimulation and experimental investigation will pose greatchallenges but it is a right step toward the right directionfor LNG cold energy power generation system in thefuture.3) Feasible and economical analysesFor those theoretically mature cycles of LNG cold

energy power generation, feasible and economical ana-lyses can be supplemented for further practical implement,including type selection of vital devices, pipeline establish-ment of circulation, cost evaluation of equipment, calcula-tion of annual power generation benefit and life ofreturning investment, etc.

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