exergy analysis of liquefied natural gas cold energy recovering cycles

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2005; 29:65–78Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/er.1040

Exergy analysis of liquefied natural gas cold energyrecovering cycles

Wang Qiangn,y, Li Yanzhongz and Chen Xi

Institute of Refrigeration and Cryogenic Engineering, Xi’an Jiaotong University, Xi’an 710049, China

SUMMARY

Liquefied natural gas (LNG) is known as ‘green fuel’ used in power plant, automobile and so forth due toits higher energy density and environmentally friendly advantages. LNG, besides its high quality chemicalexergy, has plenty of physical exergy such as cold exergy and pressure exergy, which could be utilizedfurther. Analysis of physical exergy and its affected factors has been conducted. Based on the analysis,several cycles used for recovering and applying the physical exergy of LNG, such as combined power cycle,gas turbine power generation cycle and automobile air-conditioning system have been proposed. Theparameters affecting the performance of the cycles are discussed. The recovery and utilization of physicalexergy of LNG are the important measures to save energy and protect the environment. Copyright # 2005John Wiley & Sons, Ltd.

KEY WORDS: exergy; liquefied natural gas; combined power cycle; gas turbine; vehicle air-conditioning

1. INTRODUCTION

It is strongly brought forward with the demand of energy saving and environment protection fora sustainable development of the society and economy. Natural gas is widely used in many areasas a ‘green’ fuel. Natural gas is usually liquefied to become liquid, known as liquefied natural gas(LNG) in order to store and transport conveniently. LNG industry has thus developed rapidlyin recent years.

LNG is usually stored at low temperature about 110K at normal pressure and has plenty ofphysical exergy besides its high quality chemical energy for the work consumed in the liquefyingprocess. Preliminarily estimated, producing one ton of LNG consumes about 850 kWh ofelectric energy. The power output may reach up to 240 kWh when we recover the cold energy forpower generation (Hongtan et al., 1999).

Received 12 March 2003Accepted 27 October 2003Copyright # 2005 John Wiley & Sons, Ltd.

yE-mail: [email protected]

nCorrespondence to: W. Qiang, Institute of Refrigeration and Cryogenic Engineering, Xi’an Jiaotong University,Xi’an 710049, China.

zE-mail: [email protected]

Contract/grant sponsor: University Skeleton Teacher FoundationContract/grant sponsor: Doctoral Foundation

A combined power cycle, which is based on the physical exergy of LNG and low-grade heatsources such as solar energy, geothermic energy or industrial waste heat, works well withsatisfied efficiency. The physical exergy of LNG is recovered and utilized efficiently and theutilizing efficiency of low-grade energy can be improved. Hisazumi et al. (1998) and Miyazakiand Kang (2000) proposed a combined power cycle for recovering cold energy of LNG andindicated that the efficiency of a combined power cycle is obviously higher than that of simpleRankine cycle, i.e. a gas expanding cycle. It is known that the efficiency of a gas turbine is rathersensitive to the ambient temperature, which is taken as the exhaust heat sink. That is why thepower output and efficiency of a gas turbine in high temperature seasons are always lower thanin the standard condition (Kim and Ro, 2000). In order to improve the performance of a gasturbine, external refrigeration methods such as evaporation cooling and absorption cooling areusually adopted to decrease the inlet air temperature. These methods have to consume extraenergy, however, the cooling process can easily be conducted in a LNG gas turbine plant.Ondryas et al. (1991) and Yousef et al. (1996) used the cold energy recovered from LNG to coolthe inlet air of gas turbine and improved the performance of the cycle.

LNG Vehicle (LNGV) makes up many disadvantages of compressed natural gas vehicle(CNGV) for the higher energy density and lower storage pressure. It can run several folds oflong distance after one charge than CNGV. The cold energy of LNG can be recovered andapplied for cold storage in LNGV, by which a traditional CFC refrigeration system could bereplaced (Kleffmann, 1996; Ihlenburg and Kesten, 1998). The LNGV air conditioner is a newidea, which cannot only save energy and decrease mechanical noise pollution, but also avoid theozone layer depletion and greenhouse effect due to the leak of CFCs. As a fresh idea andpreliminary investigation, an analysis and discussion for the cold exergy recovery for airconditioner in LNGV are completed in this paper for its further research and development.

2. ANALYSIS ON PHYSICAL EXERGY OF LNG

The physical exergy of LNG exists and is distinguished due to the temperature difference andpressure difference between LNG and the environment states such as atmosphere, seawater, etc.(Bisio, 1995). Exergy analysis is a usual way to indicate the quality of energy and its convertiblefraction of the variety of energy quantitatively. It is known that a maximum work output can beobtained between a cold source and a heat source through Carnot cycle. The practical workoutput may be changed according to the variation of heat source temperature and therefrigerant states. The quantity of usable energy during a thermodynamic process is shown inFigure 1, in which the temperature of refrigerant increases during the process absorbing heatfrom external source. If the heat absorbed in an infinitesimal process in the temperature rangebetween T1 and T2 is dq; the maximum usable work dwmax in the process will be expressed as

dwmax ¼ dq� dqT

T0ð1Þ

The maximum usable energy wmax in the thermodynamic process between T1 and T2 istherefore

wmax ¼Z T2

T1

dwmax ¼Z T2

T1

1�T

T0

� �dq ð2Þ

Copyright # 2005 John Wiley & Sons, Ltd. Int. J. Energy Res. 2005; 29:65–78

W. QIANG, L. YANZHONG AND C. XI66

Now we consider the energy equivalence and exergy quantity in LNG including its phasechange process happening during the temperature increasing. LNG has the primary state (Ts, ps)at its storage state and goes to the equilibrium state (T0, p0) at atmosphere eventually. Theconservation equation of energy for the change is as follows:

dq ¼ dhþ dwmax ð3Þ

The work output is

dwmax ¼ dq� dh ¼ T0 ds� dh ð4Þ

The maximum usable work wmax from the process is the integral of Equation (4) from state sto state 0, which is namely the physical exergy of LNG written as

ex ¼ ðh� h0Þ þ T0ðs0 � sÞ ¼ �Z T0

Ts

cp dT þ T0

Z T0

Ts

cpdT

T� T0

Z p0

ps

@v

@T

� �p

dp ð5Þ

where the first two items are the function of temperature, and the last item is the function ofpressure. Equation (5) shows that the physical exergy of LNG is composed of two parts, whichare sensible cold exergy exc;s and pressure exergy exp; and written as

exph ¼ exc;s þ exp ð6Þ

The former is aroused by the non-equilibrium heat between the system and environment, andthe latter is aroused by the non-equilibrium pressure exergy between system and atmosphere.

Natural gas has a main component of methane. The physical characteristics such as specificheat changes hardly from 110 to 300K at a certain pressure and can be assumed as a constantfor a simple analysis. The sensible cold exergy of LNG is shown as

exc;s ¼ �cpðT0 � TsÞ þ cpT0

Z T0

Ts

dT

T¼ �cpðT0 � TsÞ þ cpT0 ln

T0

Tsð7Þ

S1 S2 S

T q

T0

ds

T1

T2

δ

Figure 1. Exergy indication in T2S diagram during temperature variation.


EXERGY ANALYSIS OF LIQUEFIED NATURAL GAS 67

Considering the vaporization heat of LNG is r, and then the latent cold exergy is

exc;l ¼T0

Ts� 1

� �r ð8Þ

So the cold exergy of LNG is

exc ¼ exc;l þ exc;s ¼T0

Ts� 1

� �r� cpðT0 � TsÞ þ cpT0 ln

T0

Tsð9Þ

From the state equation of a practical gas, we have

pv ¼ zRT ð10Þ

And after differential treatment it is

@v

@T

� �p

¼zR

pð11Þ

The pressure exergy exp is

exp ¼ �zRT0

Z p0

ps

dp

p¼ zRT0 ln

ps

p0ð12Þ

The whole physical exergy of LNG is

exph ¼ exc þ exp ¼T0

Ts� 1

� �r� cpðT0 � TsÞ þ cpT0 ln

T0

Ts

� �þ zRT0 ln

ps

p0ð13Þ

Equation (13) expresses a usable energy from LNG combining those from lower temperatureand higher pressure differing from ambient temperature and atmosphere pressure.

Liu et al. (1999) indicated that the main factors affecting the characteristics of the cold exergyand pressure exergy of LNG are the ambient temperature T0 and the system pressure ps.

3. THE COLD ENERGY OF LNG RECOVERING CYCLES

3.1. Combined power cycle based on low-grade energy and LNG

From the exergy analysis of LNG above, we can establish a combined power cycle, which iscomposed of a second refrigerant Rankine cycle based on the cold exergy of LNG and thenatural gas directly expanding cycle based on the pressure exergy of LNG.

To the low-grade energy, it is difficult to improve the efficiency of second refrigerant Rankinecycle by increasing the inlet pressure of the turbine for the limitation of the heat sourcetemperature. A regenerative cycle is practical to improve the inlet pressure of the turbine. Theschematic diagram of the combined power cycle is shown in Figure 2.

The combined cycle in Figure 2 consists of two parts, in which the left part is a secondrefrigerant Rankine cycle driven by the temperature difference between low-grade energy andLNG. The cycle is mainly composed of a turbine, a condenser, a cycle pump, an evaporator, aregenerator and the low-grade heat source. The cycle medium is propane (R290), which ispumped into the evaporator 7 by cycle pump P3 and gasifies with the heating of the low-gradeenergy. The propane steam goes into the turbine 6 to expand and generate power. Part of thehigh-pressure propane steam in the turbine 6, which does not expand completely yet, is extracted



from the turbine and is introduced into the regenerator. The left portion of the propone steamcontinuously expands to the condensation pressure, and then condenses into saturated liquid inthe condenser 2. The pump P1 increases the pressure of the liquid isentropically from thecondenser 2 to the same pressure as that of the extracted steam. The pressurized liquid enters theregenerator 10 and mixes directly with the flow stream extracted from the turbine 6. Theregenerator increases the inlet temperature and pressure of turbine 6, and the cycle efficiency ofthe cycle can be improved (Kenneth, 1989).

The right part of Figure 2 is a natural gas directly expanding open style power cycle. LNGstored at normal pressure is pumped into the condenser 2 by P2; where it gasifies and releasescold energy. The natural gas is further heated in the heat exchanger 3 and reaches highertemperature and pressure. After it expands in the turbine 4, where the power is generated, itspressure decreases to gas-supplying pressure. The natural gas outlet of the turbine 4 iscontinuously heated in the exchanger 5 by low-grade energy and finally goes into gas-supplyingsystem.

Some important parameters for the combined cycle are listed in Table I. The exergy efficiencyof the combined power cycle with different condenser outlet temperature is shown in Figure 3, inwhich the ambient temperature and pressure are 293K and 0.1MPa, respectively, andPh=1.7MPa, Tb=353K.

From Figure 3, we can see that the exergy efficiency increases with the decreasing of thecondenser outlet temperature. It is because the lower condenser outlet temperature results inlower outlet pressure of the turbine 6, and also results in greater temperature difference betweenthe heat source and the cold source in the Rankine cycle. This will directly result in theincreasing of the exergy output and hence the exergy efficiency of the cycle is improved.

The effect of inlet pressure of the turbine 4 on the exergy efficiency in natural gas directlyexpanding cycle is shown in Figure 4. The ambient temperature T0=293K, the ambientpressure P0=0.1MPa, Tb=343K, Td=223K. Figure 4 shows that under natural gas supplyingpressure, the exergy efficiency will increase with the increasing of inlet pressure of the turbine 4,but the increasing gradient diminishes. It is known that the power output of turbine 4 isproportional to the inlet pressure and hence the exergy efficiency increases. With the increasingof the inlet pressure, the increasing gradient of pressure ratio between the inlet and outletdecreases, which results in a smaller increasing gradient of the exergy efficiency.

1

2 3

57

6

10 P1

P3

P2

88 8

9

n

m

a

b

c

kd

g

f e

jh

i

4

Figure 2. The schematic diagram of combined power cycle for recovering physical exergy of LNG basedon low-grade energy (1: LNG tank; 2: condenser; 3: heat exchanger; 4: turbine; 5: heat exchanger;6: turbine; 7: evaporator; 8: low-grade heat source; 9: natural gas supplying system; 10: regenerator).



3.2. Gas turbine power generation cycle

The cold energy of LNG can be used to cool the inlet air temperature of gas turbine to improvethe efficiency of the cycle. The saturation temperature of LNG is about 110K at standardpressure. In order to avoid the icing in gas compressor for very low inlet air temperature andeffectively recover and utilize the cold energy of LNG, a gas turbine power generation cycle withRankine cycle based on cold exergy of LNG has been proposed. The schematic diagram of thepower generation cycle is shown in Figure 5.

The combined cycle comprises of the gas turbine power generation cycle and the Rankinecycle. The part on the left side of Figure 5 is the Rankine cycle. The second refrigerant Rankine

190 200 210 220 230

20

30

40

50

60

T(K)

η ex

(%)

Figure 3. The effect of the condenser outlet temperature on the exergy efficiency.

Table I. The parameters of combined power cycle.

Component parameters Medium

Cycle-one Propane

Efficiency of turbine 0.8Efficiency of pump 0.7Evaporator outlet temperature (K) 333–363Heat transfer temperature difference in the evaporator (K) 5Condenser outlet temperature (K) 233–193

Cycle-two Natural gas

Efficiency of the turbine 0.8Efficiency of the pump 0.7LNG tank pressure (MPa) 0.12Natural gas supplying pressure (MPa) 0.4Turbine 5 inlet pressure (MPa) 1.6–2.0



cycle is driven by temperature difference between LNG and the exhaust of the gas turbine. Itmainly includes a turbine, a condenser, a recycle pump, an evaporator, etc. The refrigerant ofthe cycle is R23. The exhaust of the gas turbine is the high temperature heat source and LNG isused as low temperature heat source. The liquid refrigerant from the condenser is pumped intothe evaporator by P2; where it absorbs heat and gasifies into high-pressure state. After the steamexpands for power output in the turbine 9, the temperature and pressure decrease to thesaturated state in the condenser 2.

The right part of Figure 5 is the gas turbine power generation cycle. LNG stored at standardpressure is pumped by P1 and gasifies after releasing cold energy in condenser 2. The natural gaswith lower temperature is used to cool the inlet air from the filter in the heat exchanger 3.The cooled air mixing with natural gas is pumped into the combustor 6 by the compressor 5.The high temperature gas from the combustion enters the gas turbine 7 for power output. The

i

2

ml

P2

P1

9

8g

waste gas

u

n

air

1h

j

k

3

ab

d

c 5

6e

f

7

Figure 5. The gas turbine power generation cycle based on cold energy of LNG(1: LNG tank; 2: condenser; 3: inlet air cooler; 4: filler; 5: air compressor; 6: combustor;

7: gas turbine; 8: evaporator; 9: turbine).

1.6 1.7 1.8 1.9 2.010

20

30

40

50

60

P(MPa)

η ex (%

)

Figure 4. The effect of inlet pressure of the turbine 4 on the exergy efficiency.



exhaust from the gas turbine eventually flows into the evaporator 8 as the heat source of secondrefrigerant Rankine cycle.

The parameters of total gas turbine power generation cycle are shown in Table II, in whichthe ambient temperature T0 and pressure P0 are 293K and 0.1MPa, respectively.

The effects of the condenser outlet temperatures on the power output and exergy efficiency areshown in Figure 6, where the inlet temperature of the air compressor varies from 93 to 323K. Itis shown that both the power output and exergy efficiency decrease gradually with the increasingof the outlet temperature of the condenser, which represents the increasing of the cold sourcetemperature of the Rankine cycle. It can also be found from the figure that, with suitable outlettemperature of the condenser (from 213 to 173K), the exergy efficiency of the cycle remains upto about 50%, which indicates that the cycle makes effectively use of the total energy of LNGand the effect of energy saving is evidently achieved.

The effects of the inlet air temperature on the power output and exergy efficiency have beenshown in Figure 7, where the outlet temperature of the condenser is about 193K. Figure 7 shows

Table II. The parameters of the gas turbine power generation cycle.

Second refrigerant Rankine cycle withR23 Component parameters

Gas turbine power generationcycle with natural gas Component parameters

Efficiency of turbine 0.80 Efficiency of gas turbine 0.88Efficiency of pump 0.70 Efficiency of pump 0.70Temperature of waste (K) 293 Efficiency of air compressor 0.90Outlet temperature of condenser (K) 213–173 Pressure ratio of air compressor 15

Pressure of LNG storage tank (MPa) 0.12Inlet temperature of gas turbine (K) 1573Exhaust temperature of gas turbine (K) 823Inlet temperature of gas compressor (K) 278–313

170 180 190 200 210

30

40

50

60

T(K)

Ex

(MJ/

t.LN

G)

ηex

Exef

5

6

7

8

η ex (%

)

Figure 6. The effects of outlet temperatures of the condenser on power output and exergy efficiency.



that the inlet air temperature of the gas turbine has a significant effect on the power output ofthe gas turbine cycle. With the decreasing of the inlet air temperature of the gas turbine, thepower output evidently improves. When the environment temperature is above 303K, the poweroutput increases about 10% in average with every 10K reducing step of the inlet airtemperature. The cycle power output decreases about 25% while the air temperature increasesfrom 278 to 303K. Due to the increasing of environment temperature, the air density decreasesand in turn the air mass flow rate will decrease, the power output will decreases as well. Exergyefficiency changes greatly with different inlet air temperature. In the same way, the exergyefficiency of the gas turbine cycle decreases about 4% while the environment temperatureincreases from 278 to 303K. It is necessary to cool the inlet air temperature in the areas withhigher annual average air temperature.

3.3. Automobile air-conditioning system for recovering the cold energy of LNG

The cold exergy of LNG can also be recovered and utilized in auto air-conditioning system.Wang et al. (2002) developed an air-conditioning system to recover the cold energy of LNG.Usually the pressure of automobile LNG storage system is just above atmosphere (over0.1MPa). According to the physical exergy analysis of LNG above, the pressure exergy of LNGtakes only a little fraction of the physical exergy comparing with its cold exergy. This means thatthe recovery of the cold energy of LNG for air-conditioning system should mainly be from thecold exergy.

It is known that the storage temperature of LNG is about 110K and the air temperaturefor air conditioning is normally required at about 283K. If the cold natural gas is directlyused to cool inlet air, the steam and CO2 in the air would be frozen and block the channelof heat exchanger. In this paper, a multi-stage heat exchange of cold energy recoveringcycle is proposed, which includes a self-heating heat exchanger to avoid freezing.A cold accumulation cycle with a green refrigerant of glycol solution is also developed todiminish the heat transfer temperature difference in low temperature condition. At the same

ηex

Exef

30

275 280 285 290 295 300 305 310 315

40

50

60

T(K)

Ex

(MJ/

t.LN

G)

5

6

7

8

η ex (%

)

Figure 7. The effects of inlet air temperature on power output and exergy efficiency.



time, the cold accumulation cycle can effectively adjust the requirement of cold capacityto satisfy the variety of cooling load of air-conditioning system. The schematic diagram ofauto air-conditioning system for recovering the cold energy of LNG is shown in Figure 8.The air-conditioning system consists of both a cold energy recovering cycle and a coldaccumulating cycle.

3.3.1. Cold energy recovering cycle. In this cycle, LNG is stored in a low temperaturestorage tank with vacuum shield. It stands in the saturation state at the pressure of0.12MPa. When the valve of the storage tank open, the saturated cold natural gas goes intodouble tube heat exchanger under the system pressure, where it releases partial cold energy tothe backward flowing gas and increases its temperature. The warmed natural gas continuouslygoes into a plate-fin heat exchanger and absorbs the heat from glycol solution flowingconversely. The natural gas out of the plate-fin gets higher temperature further and flows backinto a coil heat exchanger, which is dipped inside of the LNG storage tank, to gasify the LNGand be cooled down again after obtaining the latent cold energy. The phase-changing processhappens only inside the LNG storage tank to void the freezing problem of glycol solution inheat exchangers. The latent cold energy is transmitted to natural gas from coil to plate-fin heatexchanger once and again. After several cycles of natural gas between the coil and the plate-finheat exchanger for picking up the cold energy from LNG storage tank, the cold energy istransferred from LNG to glycol solution. The natural gas eventually flows through the doubletube and plate-fin heat exchanger to the supplying system of engine. Furthermore, it releases therest of cold energy to the cooling water in motor system and finally flows into inter-combustionengine.

cold accumulating reservoir

pump air heat exchanger

filter

plate-fin heat exchanger

double tube heatexchanger

coil heat exchanger

LNG storage tankT

P charging tankof LNG

P

cryogenicvalveP

Figure 8. The schematic diagram of an auto air-conditioning system using the cold energy of LNG.



3.3.2. Cold accumulating air-conditioning cycle. The glycol solution is selected as cool-carryingmedium due to its low freezing point, high specific heat and harmless to environment. It takes inthe cold energy of LNG in plate-fin heat exchanger, stores it in the solution reservoir andreleases it to the air in air-conditioning system. When the air-conditioning cycle operates, theglycol solution from cold accumulating reservoir is pumped into air heat exchanger, and thecold is transferred from glycol solution to the returning air of the carriage. The cold air is mixedwith fresh air in a certain proportion and sent into the carriage by a fan.

The cold accumulating reservoir can adjust the cooling load of air-conditioning. The coolingload is generally larger at starting and may become lower while automobile runs at high speed.In order to quickly decrease the outlet air temperature of the air heat exchanger in the starting, aby-pass tube is designed in cold accumulating cycle, by which, the glycol solution flowing out ofthe plate-fin heat exchanger directly flows into the air heat exchanger instead of flowing into thecold accumulating reservoir. When auto stops or runs at a low speed, the cold energyaccumulated in the reservoir will release and meet the demand of cooling load.

As the first step of experiment, a simulative investigation with liquefied nitrogen (LN2),instead of LNG, has been conducted to verify the feasibility of the automobile air-conditioningcycle. An analysis of the simulation experiment shows it is practicable and capable ofeffectiveness. The experimental measurements of the LN2 system are shown in Figures 9 and 10.

The temperature variation of the supplying air and the glycol solution in steady state is shownin Figure 9. It is seen that the glycol solution temperature from inlet to outlet in the air heatexchanger increases steadily. The temperature of supplying air keeps in steady state, whichmeans the cold recovering from LN2 in good effects. Figure 10 shows the case that theautomobile stops running and LNG stops providing cold energy. The cooling load of the airconditioning cycle is offered by the cold capacity stored in the cold accumulating reservoir. Thetemperatures of the glycol solution and the supplying air with the time elapsed are measured inthe simulative experiment. It is shown that the variation of the supplying air temperatureincreases gradually after the auto stops. The temperature increases about 2K after 10min,

0 300 600 900 1200 1500250

260

270

280

290

t(s)

supplying air temperature

the outlet glycol temperature

the inlet glycol temperature

T(K

)

Figure 9. The temperature variation of the supplying air and glycol solution in steady state.



which commendably satisfies the requirement of air conditioning system. In addition, the coldaccumulation avoids the great fluctuation of the system temperature.

4. CONCLUSIONS

1. Liquefied natural gas (LNG) used as the ‘green’ fuel has much physical exergy besideshigh quality chemical exergy. The physical exergy is composed of cold exergy andpressure exergy. Cold exergy includes two parts, sensible cold exergy and latent coldexergy. Recovery and utilization physical exergy of LNG cannot only save much energy,but also protect the environment.

2. A combined power cycle with regenerative cycle based on the cold energy of LNG andlow-grade heat source has been proposed in this paper. It effectively utilizes low-gradeenergy and recovers the cold exergy and pressure exergy of LNG. The exergy efficiencyof the combined power cycle increases with the decreasing of the condenser outlettemperature and the increasing of the turbine inlet pressure. The exergy efficiency of thecombined power cycle reaches about 40–50%.

3. The gas turbine power generation cycle based on the cold energy of LNG sufficientlyutilizes the chemical exergy of LNG and effectively recovers and utilizes the cold exergyof LNG. The power output and exergy efficiency of the cycle decrease with the increasingof the outlet temperature of the condenser. The cold energy of LNG can also be used tocool the inlet air of gas turbine at warm seasons, which leads to more output of power.10K of decrease of the inlet air temperature will lead to an average improvement of 10%for the power output and 2% for the cycle efficiency at high ambient temperature. Theoverall exergy efficiency of the cycle is as high as 50%.

4. An automobile air-conditioning system using the cold energy of LNG is developed inthis paper for recovering the cold exergy of LNG. The simulative experiment of LN2

0 120 240 360 480 600 720 840 960 1080270

275

280

285

290supplying air temperature

glycol solution temperature

T (

K)

t (s)

Figure 10. The temperature variation of the glycol solution and the supplying air.



system has been conducted and shown an expected refrigerating effect. It realizes the heatexchange with a significant temperature difference under low temperature conditions andsuccessfully completes the transition of cold energy from cryogenic cycle to refrigeratingcycle. The glycol cold accumulating cycle can automatically adjust the cooling capacityaccording to the cooling load at different conditions. The fresh idea of automobile air-conditioning system for recovering the cold energy of LNG has important significance tosave energy and diminish the ozone layer depletion caused by the leak of CFCs.

NOMENCLATURE

c =specific heat (kJ kg�1K�1)ex =specific exergy (kJ kg�1)Ex =exergy (kJ)h =specific enthalpy (kJ kg�1)P =pressure (Pa)q =heat (kJ kg�1)r =latent heat (kJ kg�1)R =gas constant (kJ kg�1K�1)s =specific entropy (kJ kg�1K�1)t =time (s)T =temperature (K)v =specific volume (m�3 kg�1)w =specific work (kJ kg�1)z =compress factor (dimensionless)

Greek letter

Z =efficiency (dimensionless)

Subscripts

c =coldex =exergyl =latent heatmax =maximum0 =ambient statep =isobaric stateph =physicals =saturated state, sensible heat

ACKNOWLEDGEMENTS

This work was supported by the University Skeleton Teacher Foundation of the Ministry of Education,People’s Republic of China, and the Doctoral Foundation of Xi’an Jiaotong University.



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exergy analysis of liquefied natural gas cold energy recovering cycles

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