Applied Thermal Engineering 24 (2004) 539–548www.elsevier.com/locate/apthermeng

Analysis of power cycle based on cold energy ofliquefied natural gas and low-grade heat source

Wang Qiang *, Li Yanzhong, Wang Jiang

Institute of Refrigeration and Cryogenics Engineering, Xi�an Jiaotong University, Xi�an 710049, Shaanxi, PR China

Received 6 December 2002; accepted 28 September 2003

Abstract

It is important for the sustainable development of economy to exploit new energy. The utilization of low-

grade heat source is confined for its lower utilizing efficiency. Liquefied natural gas (LNG) at 110 K has

plenty of physical cold exergy besides its high quality chemical energy. A power cycle with natural gas

directly expanding based on the cold energy of LNG and a low-grade heat source has been established in

this paper. Some parameters of the cycle have been investigated. The results indicate that the temperature

of low-grade heat source, the condensing temperature of second medium, and the inlet pressure of the

turbine are the key factors affecting the thermal and exergy efficiencies of the power cycle. The thermal and

exergy efficiencies will be improved with the increasing of the temperature of the low-grade heat source, thedecreasing of the condensing temperature and the increasing of the inlet pressure of the turbine. The

thermal efficiency and exergy efficiency of the power cycle will be above 30% and 40%, respectively, after

optimizing the key parameters. In order to further improve the utilizing efficiency of low-grade heat source,

a regenerative power cycle has been put forward and analyzed. The results show that under the same

conditions the thermal and exergy efficiency of the power cycle increase 3–5%.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: LNG; Low-grade heat resource; Cold energy recovery; Regenerative cycle

1. Introduction

It is necessary for sustainable development of social and economy to save energy and protectenvironment. Natural gas is widely used in many areas for its better environmental performance.It is becoming third biggest energy resource following coal and petrol.

* Corresponding author. Fax: +86-29-2668725.

E-mail addresses: jdwq@sina.com (W. Qiang), yzli-epe@mail.xjtu.edu.cn (L. Yanzhong).

1359-4311/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2003.09.010

Nomenclature

p pressure (Pa)t temperature (�C)h specific enthalpy (kJ/kg)s specific entropy (kJ/kgK)ex specific exergy (kJ/kg)c specific heat (kJ/kgK)W work (kW)EX exergy (kW)a mass percent of the steam into the regeneratorm1 mass flow rate of propane (kg/s)m2 mass flow rate of LNG (kg/s)m3 mass flow rate of medium with low-grade heat (kg/s)Q rate of heat flow (kW)DT temperature difference (K)g efficiency

Subscriptsth thermalex exergyef efficient0 ambient stateP1 pump 1P2 pump 2P3 pump 3in inletout outlet

540 W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548

Natural gas is liquefied under cryogenic condition and stored in low temperature of 110 K.Much work should be consumed in the liquefying process. So LNG has plenty of physical coldexergy besides high quality chemical energy. Preliminarily estimated, producing 1 ton of LNGconsumes about 850 kWh of electric energy. When we recover the cold energy for power gen-eration, the power output will reach 240 kWh [1].The physical cold exergy of LNG includes low temperature exergy and pressure exergy [1,2].

Some cycle has been developed to recover the cold energy of LNG [3,4]. In order to achieve higherefficiency, multistage energy recovery system and combined power generation cycle with doublecycle or triple cycle usually were used to recover and utilize the cold energy of LNG [5–7]. Kim,Ondryas and Najjar investigated the effect of cooling the inlet air on the efficiency of the gasturbine power cycle [8–10]. In addition, the cold energy can be recovered by Rankine power cycle.Hisazumi introduced the power generation cycle utilizing waste heat and the cold energy of LNG[11]. The cycle consists of a Rankine cycle using a freon mixture, natural gas Rankine cycle and acombined cycle with gas and steam turbines. The heat source of the Rankine cycle is the latent

W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548 541

heat from the steam turbine�s condenser and the sensible heat of exhaust gas from the heat re-covery boiler.A combined power cycle using refuse incineration and LNG cold energy was proposed by

Miyazaki [12]. The combined cycle consists of an ammonia water Rankine cycle with refuseincinerator and a LNG cold energy cycle. Bisio established a closed-cycle nitrogen turbine torecover the cold energy of LNG [13].In previous studies, the temperature of the heat source of the Rankine cycle is usually higher.

The temperature of the gas turbine exhaust or the refuse incineration is about several hundreddegrees. Low-grade heat sources such as solar energy, geothermic energy and industrial waste heatare abundant, but the temperature is usually not high. From the Carnot cycle, we know that theefficiency of a thermal cycle is proportional to the temperature difference between heat source andcold source. The power cycle based on the cold energy of LNG and low-grade heat source en-larges the temperature difference, which can not only efficiently recover and utilize the energyconsumed during the liquefying process of natural gas but also greatly increase the utilizing ef-ficiency of low-grade energy. It is significant for energy saving and environment protection.

2. Power cycle based on cold energy of LNG and low-grade heat source

A combined power cycle with second medium Rankine cycle and natural directly expandingcycle is developed based on cold energy of LNG and the low-grade heat source for recovering thelow temperature exergy and pressure exergy of LNG. The schematic diagram of the power systemis shown in Fig. 1.The combined power cycle includes two parts. The left hand side in Fig. 1 is a second medium

Rankine cycle, which is driven by the temperature difference between the low-grade heat sourceand LNG. It is mainly composed of a turbine, a condenser, a cycle pump, an evaporator and alow-grade heat source. The cycle medium is propane. Low-grade heat source is the hot source andLNG is the cold one of Rankine cycle. Liquid propane is pumped into the evaporator 7 by thecycle pump P1 and gasifies with the heating of low-grade heat source. The high-pressure propane

1P2

2 3

4

57

6

a

b

c

def

g h

i

j

P1

88 8

9

Fig. 1. Schematic diagram of power cycle based on cold energy of LNG and low-grade heat source: (1) LNG reservoir,

(2) condenser, (3) heat exchanger, (4) turbine, (5) heat exchanger, (6) turbine, (7) evaporator, (8) low-grade heat source

and (9) natural gas consumer.

Table 1

Parameters of LNG cold energy recovering cycle

Second medium power cycle Natural gas directly expanding cycle

Medium Propane Medium Natural gas

Efficiency of turbine 0.8 Efficiency of turbine 0.8

Efficiency of pump 0.7 Efficiency of pump 0.7

Evaporator outlet temperature (�C) 60–90 LNG tank pressure (MPa) 0.1

Condenser outlet temperature (�C) )40 to )80 Natural gas supplying pressure (MPa) 0.4

Turbine 5 inlet pressure (MPa) 1.6–2.0

542 W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548

steam expands to the condenser pressure in the turbine 6. LNG flowing adversely in the condenser2 releases the cold energy to condense the propane steam to saturated state.The right hand side in Fig. 1 is an open style cycle with natural gas directly expanding. LNG

with normal pressure is pumped into the condenser 2 by the pump P2, where it gasifies and releasescold energy. Natural gas is further heated in the heat exchanger 3 to increase the temperature andpressure. After it expands in the turbine 4, where power is generated, its pressure decreases to thegas-supplying pressure. The natural gas from the turbine 4 is heated continuously in the heatexchanger 5 by the low-grade heat source, and then it goes into the gas-supplying system. Theparameters of the power cycle are listed in Table 1. The ambient temperature is t0 ¼ 20 �C, and theambient pressure is p0 ¼ 0:1 MPa.

3. Analysis on energy of power cycle

3.1. Analysis of thermal efficiency

The thermal analysis of each facility in the power cycle shown in Fig. 1 can be done withfollowing equations:Turbine 6: W6 ¼ m1ðhb � hcÞTurbine 4: W4 ¼ m2ðhh � hiÞPump 1: WP1 ¼ m1ðha � hdÞPump 2: WP2 ¼ m2ðhf � heÞThe heat provided by low-grade heat source:

Qin ¼X

cm3DT

Therefore, the thermal efficiency of the cycle is:

gth ¼ðW6 � WP1Þ þ ðW4 � WP2Þ

Qin

3.2. Analysis of exergy efficiency

The exergy balance formula is

EXin ¼ EXef þ EXloss

W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548 543

The exergy analysis of each facility of the power cycle shown in Fig. 1 is done with followingequations:Exergy parameter of the cycle:

ex ¼ ðh� h0Þ � T0ðs� s0Þ

The effective exergy:

EXef ¼ ðEX6 � EXP1Þ þ ðEX4 � EXP2Þ

where,

EX6 ¼ m1ðexb � excÞ

EXP1 ¼ m1ðexa � exdÞ

EX4 ¼ m2ðexh � exiÞ

EXP2 ¼ m2ðexf � exeÞ

The input exergy of the power cycle is:

EXin ¼X

m3ðexin � exoutÞ þ m2exLNG

Exergy efficiency of the cycle:

gex ¼EXef

EXin

4. Results and discussions

In order to investigate the effect of the parameters on the thermal efficiency and exergy effi-ciency of the power cycle, analysis has been conducted under the conditions such as, different low-grade heat source temperature, different condensing temperature and different inlet pressure of theturbine 4. The results are illustrated from Figs. 2–4.

60 65t (°C)

70 75 80 8510

20

30

40

50

60

η

(%)

ηexη th

Fig. 2. The effect of the low-grade heat source temperature on the thermal and exergy efficiency.

1.6 1.7 1.8 1.9 2.010

20

30

40

50

60

η (%

)

ηexηth

P (MPa)

Fig. 4. The effect of the inlet pressure of the turbine 4 on the thermal and exergy efficiency.

-80 -70 -60 -50 -4020

30

40

50

60

η (%

)

ηexηth

t (°C)

Fig. 3. The effect of condensing temperature on the thermal and exergy efficiency.

544 W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548

The effect of different low-grade heat source temperature on the thermal efficiency and exergyefficiency is shown in Fig. 2, where, the inlet pressure of the turbine 4 is 1.8 MPa, the condensingtemperature is )60 �C and the rest conditions are listed in Table 1.It is seen that both the thermal efficiency and exergy efficiency of the power cycle are increasing

with the temperature of the low-grade heat source goes higher. The changing tendency of thecurves is similar with each other. This is because the temperature of heat source in Rankine cycleincreases with the increasing of the low-grade heat source temperature. The thermal efficiency andexergy efficiency of the cycle will be improved. In addition, Fig. 2 also shows that even thetemperature of the low-grade heat source is lower, the thermal efficiency and exergy efficiency canalso reach about 30% and 40%, respectively.Fig. 3 shows the effect of different condensing temperature on the thermal efficiency and exergy

efficiency, where, the inlet pressure of the turbine 4 is 1.7 MPa and the temperature of the low-grade heat source is 80 �C, and the rest conditions are listed in Table 1. We can see from Fig. 3that, under natural gas supplying pressure, the thermal efficiency and exergy efficiency increasewith the decreasing of the condensing temperature. This is because the lower condensing tem-perature results in the lower cold source temperature of Rankine cycle, and leads to a higherthermal efficiency of the power cycle. The outlet pressure of the turbine 6 decreases for thecondensing temperature decreases, which will directly result in the increasing of the effective ex-ergy EXef of the cycle. The exergy efficiency will also be improved.

W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548 545

The effect of the inlet pressure of the turbine 4 on the thermal efficiency and exergy efficiency isshown in Fig. 4, where, the temperature of the low-grade heat source is 75 �C and the condensingtemperature is )50 �C, the rest conditions are listed in Table 1. Fig. 4 shows that under natural gassupplying pressure, the thermal efficiency and exergy efficiency will increase with the increasing ofthe inlet pressure of the turbine 4. The increasing extent of the efficiency decreases with the in-creasing of the inlet pressure, because the increasing of the inlet pressure is propitious to powergenerating of the turbine 4. Under natural gas supplying pressure (0.4 MPa), the increasing extentof pressure ratio decreases with an increasing of the inlet pressure, so the increasing extent of thethermal efficiency and exergy efficiency also decreases with the increasing of the inlet pressure.The power cycle can sufficiently recover and utilize the physical cold exergy of LNG and ef-

fectively improve the utilizing efficiency of the low-grade energy, such as, solar energy, geothermicenergy and industrial waste heat. It provides a good method for utilizing low grade-heat source.From the viewpoint of the second law, the cycle efficiency is lower for the relatively low-tem-perature of the heat source. If the temperature of the propane were raised during the heat-additionprocess in Rankine cycle, the efficiency of the cycle above would more nearly approach that of theCarnot cycle [14]. A practical method is by the use of a regeneration process [15,16]. The re-generation power cycle is shown in Fig. 5.A regenerator 10 is added in this improved power cycle. Liquid propane pumped into the

evaporator 7 evaporates for the heat-addition process. Part of the propane steam which enters theturbine 6, is extracted from the turbine, which is an intermediate state in the turbine expansionprocess. The extracted stream is directed into the regenerator 10. The portion of the proponesteam which is not extracted expands completely to the condensation pressure, and then it iscondensed to the saturated liquid in the condenser 2. The pump P1 increases the pressure of theliquid from the condenser 2 isentropically to the same pressure as that of the extracted steam. Thecompressed liquid enters the regenerator 10 and mixes directly with the flow stream extractedfrom the turbine 6. The regenerator 10 is benefit to increasing the inlet temperature of the turbine6 and decreasing the heat exchanging temperature difference in the evaporator. The irreversibleloss will decrease.

1P2

2 3

4

57

6

a

b

c

d

ef

g h

i

j

P3

P1k

m

n

88 8

10

9

Fig. 5. Schematic diagram of regenerative cycle based on cold energy of LNG and low-grade heat source: (1) LNG

reservoir, (2) condenser, (3) heat exchanger, (4) turbine, (5) heat exchanger, (6) turbine, (7) evaporator, (8) low-grade

heat source, (9) natural gas consumer and (10) regenerator.

546 W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548

The thermal and exergy analysis of the regenerative cycle are as follows:

ha ¼ ahmð1� aÞhk

a ¼ ha � hkhm � hk

Turbine 6: W6 ¼ m1½hb � ahm � ð1� aÞhc�Pump 1: WP1 ¼ m1ð1� aÞðhk � hdÞPump 2: WP2 ¼ m2ðhf � heÞPump 3: WP3 ¼ m1ðhn � haÞ

Thermal efficiency of the system:

gth ¼ðW6 � WP1 � WP3Þ þ ðW4 � WP2Þ

Qin

EXef ¼ ðEX6 � EXP1 � EXP3Þ þ ðEX4 � EXP2Þ

The thermal and exergy efficiency of the power cycle with or without regenerator are shown in

Figs. 6 and 7, respectively. From Figs. 6 and 7 we can see that, the regenerator is important for

60 65 70 75 80 8510

20

30

40

50

60

η th(%

)

with regenerator without regenerator

t (°C)

Fig. 6. The thermal efficiency of the power cycle with and without regenerator.

60 65 70 75 80 8510

20

30

40

50

60

η ex(%

)

with regenerator without regenerator

t (°C)

Fig. 7. The exergy efficiency of the power cycle with and without regenerator.

W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548 547

low-grade heat source to improve the thermal and exergy efficiency of the power cycle. When thetemperature of the low-grade heat source is below 70 �C, the efficiency will increase 3–5% byintroducing the regeneration process.According to this idea, a multistage regenerative cycle can be designed to improve the efficiency

of the combined power cycle in practice.

5. Conclusions

1. A combined power cycle based on the cold energy of LNG and low-grade heat source has beenestablished in this paper. It can not only effectively recover the low temperature exergy andpressure exergy of LNG, but also utilize low-grade energy. Recovering the cold energy ofLNG and utilizing low-grade energy are significant to energy saving and environment protec-tion.

2. In order to increase the thermal efficiency and exergy efficiency of the power cycle, some keyparameters have been analyzed. The results show that the temperature of the low-grade heatsource and the condensing temperature of second medium greatly affect the thermal efficiencyand exergy efficiency of the power cycle. When the temperature of low-grade heat sourceincreases or the condensing temperature decreases, both the thermal and exergy efficiency will in-crease. The inlet pressure of the turbine in natural gas directly expanding cycle is another factoraffecting the efficiency of the power cycle. Under gas-supplying pressure, the thermal efficiencyand exergy efficiency will increase with the increasing of the inlet pressure of the turbine 4. Thethermal efficiency and exergy efficiency of the power cycle are about 30% and 40%, respectively,when the key parameters are optimized.

3. A regeneration process internal to the combined cycle is proposed in this paper based on thecold energy of LNG and low-grade heat source to improve the performance of the power cycle.The regenerator plays an important role in utilizing low-grade energy. When the temperature ofthe low-grade heat source is below 70 �C, the thermal efficiency and exergy efficiency of thepower cycle with regenerator are 3–5% higher than the power cycle without regenerator.

Acknowledgements

This work is supported by the University Skeleton Teacher Foundation of Ministry of Edu-cation of PRC and the Doctoral Foundation of Xi�an Jiaotong University.

References

[1] H. Liu, L. You, Characteristics and applications of the cold heat exergy of liquefied natural gas, Energy

Conversion and Management 40 (1999) 1515–1525.

[2] G. Bisio, Thermodynamic analysis of the use of pressure exergy of natural gas, Energy 20 (2) (1995) 161–167.

[3] C.W. Kim, S.D. Chang, S.T. Ro, Analysis of the power cycle utilizing the cold energy of LNG, International

Journal of Energy Research 19 (1995) 741–749.

[4] T. Miyazaki, A. Akisawa, T. Kashiwagi, LNG cold energy cycle, in: Proceedings of the JSRAE, 1998, pp. 157–160.

548 W. Qiang et al. / Applied Thermal Engineering 24 (2004) 539–548

[5] A. Akiwasa, T. Miyazaki, A. Yuzawa, T. Kashiwagi, Multistage energy recovery system using LNG cold energy,

in: Proceedings of the Japanese Energy Resources Conference, 1998, pp. 41–44.

[6] Y.S.H. Najjar, Efficient use of energy by utilizing gas turbine combined cycle, Applied Thermal Engineering 21

(2001) 407–438.

[7] I.O. Marerro, A.M. Lefsaker, et al., Second law analysis and optimization of a combined triple power cycle, Energy

Conversion and Management 43 (2002) 557–573.

[8] T.S. Kim, S.T. Ro, Power augmentation of combined cycle power plants using cold energy of liquefied natural gas,

Energy 25 (2000) 841–856.

[9] I.S. Ondryas, D.A. Wilson, M. Kawamoto, et al., Options in gas turbine power augmentation using inlet air

chilling, Transactions on ASME Journal of Engineering for Gas Turbines and Power 113 (1991) 203–211.

[10] Y.S.H. Najjar, Enhancement of performance of gas turbine engines by inlet air-cooling and cogeneration cycle,

Applied Thermal Engineering 16 (2) (1996) 173–185.

[11] Y. Hisazumi, Y. Yamasaki, et al., Proposal for a high efficiency LNG power generation system utilizing waste heat

from the combined cycle, Applied Energy 60 (1998) 169–182.

[12] T. Miyazaki, Y.T. Kang, A combined power cycle using refuses incineration and LNG cold energy, Energy 25

(2000) 639–655.

[13] G. Bisio, L. Tagliafico, On the recovery of LNG physical exergy by means of a simple cycle or a complex system,

Exergy 2 (2002) 34–50.

[14] K. Wark, Thermodynamics, McGraw-Hill Book Company, New York, 1989.

[15] W. Cheng, A cryogenics power generation cycle for recovering cold energy of LNG, Journal of China University of

Science and Technology 29 (6) (1999) 671–676.

[16] F.J. Wang, J.S. Chiou, Performance improvement for a simple cycle gas turbine GENSET––a retrofitting example,

Applied Thermal Engineering 22 (2) (1996) 1105–1115.