A combined power cycle utilizing low-temperature waste heat and LNG cold energy

Energy Conversion and Management 50 (2009) 567–575

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A combined power cycle utilizing low-temperature waste heat and LNG cold energy Xiaojun Shi, Defu Che * State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

a r t i c l e

i n f o

Article history: Received 25 August 2007 Received in revised form 1 April 2008 Accepted 28 October 2008 Available online 9 December 2008 Keywords: Power generation Low-grade waste heat Ammonia–water mixture LNG Thermodynamic analysis

a b s t r a c t This paper has proposed a combined power system, in which low-temperature waste heat can be efficiently recovered and cold energy of liquefied natural gas (LNG) can be fully utilized as well. This system consists of an ammonia–water mixture Rankine cycle and an LNG power generation cycle, and it is modelled by considering mass, energy and species balances for every component and thermodynamic analyses are conducted. The results show that the proposed combined cycle has good performance, with net electrical efficiency and exergy efficiency of 33% and 48%, respectively, for a typical operating condition. The power output is equal to 1.25 MWh per kg of ammonia–water mixture. About 0.2 MW of electrical power for operating sea water pumps can be saved. Parametric analyses are performed for the proposed combined cycle to evaluate the effects of key factors on the performance of the proposed combined cycle through simulation calculations. Results show that a maximum net electrical efficiency can be obtained as the inlet pressure of ammonia turbine increases and the peak value increases as the ammonia mass fraction increases. Exergy efficiency goes up with the increased ammonia turbine inlet pressure. With the ammonia mass fraction increases, the net electrical efficiency increases, whereas exergy efficiency decreases. For increasing LNG turbine inlet pressure or heat source temperature, there is also a peak of net electrical efficiency and exergy efficiency. With the increase of LNG gas turbine outlet pressure, exergy efficiency increases while net electrical efficiency drops. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The consumption of fossil fuels continues to increase to satisfy the demand for energy and electricity in the world. This results in serious energy shortage and environmental pollution. In order to save energy and protect the environment, greater attention than ever has been paid to the utilization of low-grade waste heat to generate power in recent years. Much work has been carried out on using organic Rankine cycle to recover low-temperature waste heat [1–6]. It is found that less energy in the low-temperature waste heat can be converted to power by organic Rankine cycle because boiling essentially takes place at constant pressure and temperature in the evaporator. Kalina [7] developed a new power cycle, which utilizes ammonia– water mixture as working fluid. Kalina cycle shows higher exergy efficiency than organic Rankine cycle due to better temperature profile matching between heat source and working fluid [8–9]. But it operates with high temperature waste heat, such as the heat in the flue gas from gas turbine. Furthermore the lower pressure ratio across turbine limits its efficiency improving. A novel ammnia-water binary mixture thermodynamic cycle capable of producing power and refrigeration has been proposed by Goswami [10]. This cycle can be driven by low-temperature heat source and has * Corresponding author. Tel.: +86 29 82665185; fax: +86 29 82668703. E-mail address: [email protected] (D. Che). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.10.015

been thoroughly studied [11,12]. Although the cycle exhibits higher thermal efficiency at typical working conditions, the higher temperature of exit heat source fluid lowers heat source utilization efficiency. Liquefied natural gas (LNG) is produced by cryogenic refrigeration of natural gas after removing the acid and water. Producing one ton of LNG consumes about 850 kWh of electric energy [13]. At receiving terminal, LNG, which is approximately at atmospheric pressure and at a temperature of around 160 °C, has to be regasified and fed to a distribution system at the ambient temperature and at a suitably elevated pressure. Typically sea water is used as the heat source to vaporize LNG. This process not only consumes a large amount of power for driving the sea water pump but also wastes plenty of physical cold energy. With the increasing demand for cleaner fuels, LNG is now playing an even significant role as energy resource. It is estimated that the amount of LNG imported to China will be 20 million tons by 2010. Therefore, the utilization of the cold energy generated during LNG vaporization becomes more and more important. Bisio et al. [14] proposed a closed-cycle nitrogen turbine to recover the cold energy of LNG. Some power generation cycles utilizing low-grade heat source and the cold energy of LNG have been put forward by Hisazumi et al. [15], Wang et al. [16]. In previous studies, ammonia–water mixture that is suitable for sensible source was not used as working fluid. Then the electricity efficiency of power cycle is far from

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X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575

Nomenclature cp ex E_ in h hs _ m P Q_ r Rg s T _ W x z

specific heat at constant pressure (kJ kg1 K1) specific exergy (kJ kg1) exergy input (kW) specific enthalpy (kJ kg1) isentropic specific enthalpy (kJ kg1) mass flow rate (kg s1) pressure absolute (MPa) heat transfer rate (kW) latent heat of vaporization (kJ kg1) gas constant (kJ kmol1 K1) specific entropy (kJ kg1 K1) temperature (K) work (kW) mass fraction of ammonia compress factor

Abbreviations HX heat exchanger LNG liquefied natural gas Greek symbols net electrical efficiency of combined cycle exergy efficiency of combined cycle waste heat recovery efficiency

g1 g2 gwh

maximization. The research on utilizing the cold energy of LNG to enhance the efficiency of the power cycle with ammonia– water mixture as working fluid is quite limited. Although Miyazaki [17] established a combined power cycle using refuse incineration and LNG cold energy, which operates at high temperatures, ammonia–water cycle using LNG vaporization as low-temperature thermal sink has not been studied in the lowgrade energy source utilization area. In this paper, a novel combined power system is proposed to convert more energy in the low-temperature waste heat to power and utilize the cold energy of LNG to the utmost extent. The proposed combined cycle provides power output as well as vaporization LNG with power generation as the primary goal and contributes both to saving of energy and to environmental protection.

gEG gP gT

generator efficiency isentropic pump efficiency isentropic turbine efficiency

Subscripts 1–20 state points in Fig. 1 A basic solution of ammonia–water AC ammonia–water power cycle B ammonia vapor FP feed pump C weak solution of ammonia–water cc combined power cycle CG city gas CP condensate pump i inlet condition or composition e exit condition LC LNG power cycle LP LNG pump P pump T1 ammonia turbine T2 LNG turbine W hot water 0 reference state

Low-temperature waste heat

4

B

Separator

A

18

3

T1

14 C 8

HX3

T2

HX1

5 15

2 9

13

FP

19 1

City gas 17

12

TV

Condenser

10 11

Mixer HX2 20

LP

16 7

LNG tank 6

CP

2. The proposed combined power system description This proposed combined system consists of the Rankine cycle with ammonia–water mixture as the working fluid and the LNG power generation cycle. The schematic diagram of the system is shown in Fig. 1. In the Rankine cycle, low-temperature waste heat and cold energy of LNG are used as heat source and as thermal sink, respectively. Low-temperature waste energy can be recovered from the exhaust of industrial processes and power plants. The LNG cycle with natural gas directly expanding utilizes the latent heat of the spent ammonia vapor from the ammonia turbine and the sensible heat of the weak solution of ammonia–water returning to mixer as heat sources for power generation. In order to thoroughly recover both the low-temperature waste heat and the heat exchanged within the system components, the heat exchanges are cascaded according to the temperature of both hot and cold streams.The regasified LNG is heated continuously up to the ambient temperature and delivered to the gas supplying system. The main advantages of the proposed combined system in comparison to other power cycles utilizing low or mid temperature sources are as follows:

Fig. 1. Schematic diagram of the proposed combined power cycle. A, basic solution of ammonia–water; B, ammonia vapor; C, weak solution of ammonia–water; CP, condensate pump; FP, feed pump; HX, heat exchanger; LP, LNG pump; T1, ammonia turbine; T2, LNG turbine; TV, throttle valve.

 Ammonia–water mixture is used as working fluid to recover low-temperature waste heat because multi-component working fluid is suitable for sensible heat source. The boiling temperature of the ammonia–water mixture increases during the boiling process, so that a better thermal matching between the heat source and working fluid is obtained and exergy destruction is decreased in the heat exchanger HX3.  Low-temperature heat can be efficiently converted to electrical energy. Power is generated by the ammonia–water mixture Rankine cycle as long as the temperature of the ammonia–water mixture is higher than bubble point, such that partial boiling produces ammonia vapor in the heat exchanger HX3. But for Kalina cycle, there will be no power output unless all liquid ammonia–water mixture is vaporized. Therefore, Kalina cycle is not suitable to recover low-temperature waste heat.

X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575

 The proposed combined system can not only efficiently recover low-temperature waste heat but also fully utilize the cold energy of LNG. Because ammonia is a working fluid with low boiling point and the cold energy generated during the LNG vaporization is used to condense the ammonia turbine exhaust, the ammonia vapor can expand to a much lower temperature. Compared to other conventional power cycles with ammonia– water as working fluid, the pressure ratio of ammonia turbine is increased greatly. In the condenser, both the ammonia vapor and the LNG go through a low constant pressure phase change process, so that a better thermal matching is obtained and the exergy destruction is reduced.  The proposed combined cycle vaporizes natural gas as useful products while it mainly produces electrical energy. Therefore, a great amount of electrical power for driving sea water pump can be saved because sea water is no longer required as the heat source to vaporize the LNG.

_ FP ¼ m _ A ðh2  h1 Þ W

569

ð1Þ

where

h2 ¼ h1 þ ðhs2  h1 Þ=gP

ð2Þ

3.2. Heat exchanger HX3 The basic solution of ammonia–water coming out of the feed pump is heated in the HX3 by low-temperature waste heat and partial boiling produces ammonia vapor. Saturated hot water is selected as the low-temperature heat source. It is assumed that the temperature of the basic solution at the HX3 outlet is 17 K lower than that of the hot water. The heat transfer in the HX3 is calculated as follows:

_ A ðh3  h2 Þ ¼ m _ W ðh18  h19 Þ Q_ HX 3 ¼ m

ð3Þ

3.3. Separator 3. Analysis To determine the performance of the proposed combined system, the steady-state component models are used. Every component is modelled in consideration of mass, energy and species balances. In this study, the saturated hot water, which is produced in an improved LNG fuelled combined cycle power plant with waste heat recovery system established by Shi and Che [18], is selected as a typical waste heat for the proposed combined system analysis. Main parameters of the proposed combined cycle for the calculations are listed in Table 1. The values within the parentheses represent the variable range for parametric analysis. The thermodynamic properties of working fluid are calculated by REFPROP 7.1 [19]. The following assumptions are made for the proposed system analysis:  The flow is steady and the state of the working fluid at each specific location within the system does not change with the time.  All components are well insulated.  The LNG is assumed to be pure methane.  Pressure drop and heat loss in pipe lines are neglected.  The ammonia–water mixture is saturated liquid at ambient temperature in the mixer.  To avoid moisture erosion at the turbine outlet, the quality of ammonia vapor turbine exhaust is maintained higher than 90%.

In the separator, the basic solution is separated to ammonia and a weak solution. The ammonia is in the vapor-phase, while the weak solution is in the liquid-phase. The temperatures of the ammonia vapour and weak solution leaving the separator are assumed to be equal to that of the basic ammonia–water mixture from the HX3.

T3 ¼ T8 ¼ T4

ð4Þ

The total mass balance is given as

_ Bþm _C _A¼m m

ð5Þ

3.4. Ammonia vapour turbine The ammonia vapor passes through ammonia turbine to produce work. After expansion in the turbine, it drops to the lowest possible temperature to generate more power output and to reduce the exergy loss in the condensation process. The turbine back pressure is determined by an iterative procedure under the condition that the turbine exhausts quality of 90%. Gross power output of ammonia turbine is

_ T1 ¼ m _ B ðh4  h5 Þ W

ð6Þ

h5 ¼ h4  ðh4  hs5 Þ=gT

ð7Þ

Net power output of ammonia turbine cycle is defined as follows: 3.1. Feed pump

_ AC ¼ W _ T1 g  W _ FP  W _ CP W EG

The ammonia–water mixture leaves the mixer as saturated liquid at ambient temperature and is pumped to a high pressure. The required pump work rate is given by: Table 1 Main parameters for the calculations. Parameters

Value

Heat source temperature (°C) Heat source flow rate (kg/s) Generator efficiency Inlet pressure of ammonia turbine (MPa) Inlet pressure of LNG turbine (MPa) Natural gas supplying pressure (MPa) Ammonia mass fraction of basic solution Isentropic efficiency of turbine Isentropic efficiency of pump Ambient temperature (°C) Ambient pressure (MPa) Effectiveness of heat exchanger Pressure loss of heat exchanger

167 (157–197) 46 0.99 3.0 (1.8–3.2) 3.0 (2–4.35) 0.3 (0.3–0.8) 0.5 (0.48–0.52) 0.9 0.8 15 0.1 0.85 0.05

ð8Þ

3.5. Condenser Utilizing the cold energy generated during the LNG vaporization, the ammonia turbine exhaust is condensed into a state of saturated liquid at very low-temperature. At the same time, LNG is converted into saturated gas. The condenser heat duty is

_ B ðh5  h6 Þ ¼ m _ LNG ðh13  h12 Þ Q_ condenser ¼ m

ð9Þ

The mass flow rate of vaporized LNG is equal to Q_ condenser =rLNG , where rLNG is the LNG latent heat of vaporization at the pressure of LNG pump outlet. 3.6. Condensate pump The saturated ammonia liquid leaving the condenser is compressed to mixer pressure. The pump power consumption is calculated as follows:

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_ CP ¼ m _ B ðh7  h6 Þ W

ð10Þ

where

h7 ¼ h6 þ ðhs7  h6 Þ=gP

ð11Þ

_ T2 ¼ m _ LNG ðh14  h15 Þ W

ð18Þ

h15 ¼ h14  ðh14  hs15 Þ=gT

ð19Þ

Net power output of LNG turbine cycle is defined as follows:

_ LC ¼ W _ T2 g  W _ LP W EG

3.7. Throttle valve

ð20Þ

The poor liquid mixture rejects heat to natural gas in the HX1 (weak solution/natural gas heat exchanger) and throttle into the mixer. Throttle valve is used to lower pressure of the weak solution from the separator pressure to the mixer pressure. The throttling process is assumed adiabatic. Mass balance is

3.12. Heat exchanger HX2 (hot water/natural gas heat exchanger)

_9 _ 10 ¼ m m

_ LNG ðh17  h16 Þ _ W ðh19  h20 Þ ¼ m m

ð12Þ

Energy balance is

h10 ¼ h9

ð13Þ

Natural gas from the mixer passes through the HX2 and heated continuously up to ambient temperature by the hot water leaving the HX3. Finally, it is delivered to the gas supplying system. Energy balance

ð21Þ

3.13. Efficiency Power output of the proposed combined system is

3.8. Mixer

_ CC ¼ W _ AC þ W _ LC W

The liquid ammonia at the condensate pump outlet condition mixes with the weak solution from the throttle valve in the mixer to regenerate the basic of ammonia–water. Then, the basic solution leaves the mixer as saturated liquid at ambient temperature to complete the Rankine loop. The natural gas from the LNG turbine is used as the cooling fluid for the mixer. The conversation of energy in the mixer gives:

The corresponding net electrical efficiency is defined as

_ B h7 þ m _ C h10 ¼ m _ A h1 þ m _ LNG ðh16  h15 Þ m

ð14Þ

3.9. LNG pump

ð22Þ

_ W

CC g1 ¼ _ mW ðh18  h19 Þ

ð23Þ

Electrical energy and thermal energy are at different grade. Therefore, in order to have deeper insight into the thermodynamic performance, exergy efficiency is defined as the exergy output divided by the exergy input to the system. The exergy input is taken as the exergy change of the heat source. The exergy output is the exergy of the net work and the exergy of the city gas.

g2 ¼ ðW_ CC þ m_ LNG ex17 Þ=E_ in

The LNG at a low temperature of 162 °C is removed from the storage tank and pumped to the required pressure. The required pump work rate is given by:

_ LP ¼ m _ LNG ðh12  h11 Þ W

ð15Þ

where

_ LNG ex11 _ w ðex18  ex19 Þ þ m E_ in m

h12 ¼ h11 þ ðhs12  h11 Þ=gP

ð25Þ

The specific exergy is defined as follows in the calculations:

For LNG : ex ¼

where

ð24Þ

  T0 T0 p  1 r  C p ðT 0  TÞ þ C p T 0 ln þ zRg T 0 ln p0 T T

ð16Þ ð26Þ

3.10. Heat exchanger HX1 (weak solution/natural gas heat exchanger)

For water : ex ¼ ðh  h0 Þ  T 0 ðS  S0 Þ

ð27Þ

The regasified LNG at saturate state is further heated in the HX1 by recovering the heat from the weak solution from the separator. The heat balance for the HX1 is

The reference conditions are taken as T0 = 288.15 K and P0 = 0.101325 MPa. Waste heat recovery efficiency is defined as

_ LNG ðh14  h13 Þ _ C ðh9  h8 Þ ¼ m m

gwh ¼ ðh18  h20 Þ=ðh18  hwh;288K Þ

ð17Þ

ð28Þ

3.11. LNG turbine

3.14. Validation

The superheated natural gas expands through LNG turbine to the gas supplying pressure to generate power. Gross power output of LNG turbine is

All component models are individually validated by comparison with previously published and validated models [17,20]. Table 2 compares the results of this work with the simulation results from

Table 2 Comparison of the component models of this work with the simulation results from reference [21]. Component

Main assumption

Simulation results Ref. [21]

This work

Pump Heat exchanger Turbine Throttle valve

Ti = 40.4 °C, Pi = 150 kPa, Pe = 1484 kPa, g1 = 0.75, m = 1kg/s, x = 0.3 Ti = 37.5 °C, Pi = 1400 kPa, Te = 9.2 °C, m = 0.229 kg/s, x = 0.979 Ti = 450 °C, Pi = 5100 kPa, Pe = 19.1 kPa, g1 = 0.75 , m = 0.771 kg/s, x = 0.098 Ti = 9.2 °C, Pi = 1358 kPa, Pe = 165 kPa, x = 0.098

_ = 2.2 kw W Q_ = 59.4 kw _ = 726.9 kw W

_ = 2.09 kw W Q_ = 55.2 kw _ = 757.8 kw W

Te = 22.7 °C

Te = 21.3 °C

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X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575

reference [21] for the same operating conditions. The maximum deviation is within 5%, which may be caused by using different thermodynamic properties calculation models of ammonia–water mixtures. The results of this work are in good agreement with the results from reference [21] on condition that the thermodynamic properties of working fluid are calculated with the same thermal property method. 4. Results and discussion 4.1. Results of the energy analysis Based on the above mentioned modules, a computer program is developed to simulate the proposed combined system. Table 3 summarizes the thermodynamic properties for each state of the system with the ammonia mass fraction of basic solution of 0.5, the heat source temperature of 167 °C, the natural gas supplying pressure of 0.3 MPa, the LNG turbine inlet pressure of 3 MPa and the ammonia turbine inlet pressure of 3 MPa. The performance of the proposed combined system under the typical operating condition is presented in Table 4. This system can generate a power of 8.3 MW and the net electrical efficiency is estimated as 33.28%. At the same time, about 58.9 t h1 of LNG can be heated up to 35.8 °C, about 0.2 MW of electric power for operating sea water pumps can be saved due to eliminating about 2356 t h1 of sea water as the heat source to vaporize 58.9 t h1 of LNG. The temperature of hot water leaving the system drops to 35.5 °C at this typical condition. Therefore, the waste heat recovery efficiency of the system can reach about 86.57% according to Eq. (28). The exergy efficiency of the proposed combined cycle is equal to 48.87% based on the Eq. (24) and higher than the net electrical efficiency. 4.2. Effect of ammonia turbine inlet pressure In the calculations, the heat source temperature is set to be 167 °C, the natural gas supplying pressure is taken as 0.3 MPa and the LNG turbine inlet pressure is taken as 3 MPa. Other thermal conditions are still the input data shown in Table 1. When the ammonia mass fraction of basic solution x is 0.48, 0.50 and 0.52 respectively, evaluation is made to clarify the effect of the ammonia turbine inlet pressure P4 varying from 1.8 to 3.2 MPa on net electrical efficiency g1 and exergy efficiency g2. The calcu-

Table 4 Calculation results for the proposed combined cycle. _ T1 (kW) W

_ T2 (kW) W

_ CC (kW) W

g1 (%)

g2 (%)

gwh (%)

_ CG m (kg/s)

TCG (°C)

3450.7

5147.3

8334.6

33.28

48.87

86.57

16.36

35.8

lated results are shown in Fig. 2. In order to illustrate the impact of the ammonia turbine inlet pressure P4 and the ammonia mass fraction of basic solution x on efficiency, parametric analysis is performed for the ammonia vapor flow rate, pressure ratio of ammonia turbine, LNG turbine flow rate, LNG turbine inlet specific enthalpy, ammonia turbine work output and LNG turbine work output variations. The results are demonstrated in Fig. 3. It can be seen that net electrical efficiency goes up first to a maximum and then decreases with increased ammonia turbine inlet pressure. The maximum net electrical efficiency increases as the ammonia mass fraction increases. It is known that the enthalpy drop across the turbine decreases as the pressure ratio decreases. As shown in Fig. 3, the pressure ratio of ammonia turbine decreases with increase ammonia turbine inlet pressure under the condition of the quality of ammonia vapor turbine exhaust maintained about 90%. In addition, increasing ammonia turbine inlet pressure leads to less ammonia vaporized in the HX3. Therefore, the lowered ammonia vapor flow rate and pressure ratio result in a lower ammonia turbine work output. LNG flow rate decreases with decreased ammonia vapour flow rate because LNG is vaporized through absorbing the latent heat of spent ammonia vapor from the ammonia turbine. When the ammonia vapor flow rate decreases, there is more weak solution flowing through the HX1 to heat the natural gas, so natural gas specific enthalpy at LNG turbine inlet increases. Although LNG flow rate decreases with increased ammonia turbine inlet pressure, the LNG turbine inlet specific enthalpy increases rapidly as the pressure goes up, such that the LNG turbine work output increases. The variation of the LNG turbine work output first has a more significant effect on the net electrical efficiency than that of ammonia turbine work output, while it is reversed as the ammonia turbine inlet pressure increases. Compared to net electrical efficiency, exergy efficiency continuously increases as the pressure increases. It is can be explained that exergy input decreased with the increase of the pressure due to the reduced LNG flow rate.

Table 3 Thermodynamic parameters of the working fluids in the proposed combined cycle power plant. State No.

Fluid

TK

P kPa

h kJ/kg

s kJ/(kg K)

ex kJ/kg

_ kg/s m

x

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Basic solution Basic solution Basic solution Ammonia Ammonia Ammonia Ammonia Weak solution Weak solution Weak solution LNG LNG Natural gas Natural gas Natural gas Natural gas Natural gas Water Water Water

288.15 288.59 423.15 423.15 219.61 218.79 218.82 423.15 309.64 310.14 111.67 113.06 178.81 383.62 238.99 293.61 308.9 440.15 311.57 308.65

0.22385 3.15789 3 3 0.033 0.03135 0.23563 3 2.85 0.2356 0.1013 3.3241 3.1579 3 0.3 0.2850 0.285 0.8400 0.7980 0.7581

14.404 10.002 1037.5 1903.4 1387.61 99.425 99.786 584.13 54.954 54.954 0 9.518 536.18 1092.903 778.3413 897.995 932.382 706.02 161.650 149.418

0.73308 0.73613 3.5656 6.0848 6.3459 0.48015 0.48048 2.2851 0.83413 0.84371 0 0.01688 3.3013 5.4713 5.6228 6.0998 6.2139 2.012 0.55102 0.5117

52.359 55.883 288.072 498.83 136.09 309.82 310.09 117.46 6.399 3.642 1013.7 1018.3 598.55 530.01 171.81 154.01 155.5 127.860 4.472 3.570

23.9055 23.9055 23.9055 6.6901 6.6901 6.6901 6.6901 17.2154 17.2154 17.2154 16.3636 16.3636 16.3636 16.3636 16.3636 16.3636 16.3636 46 46 46

0.5 0.5 0.5 1 1 1 1 0.30569 0.30569 0.30569 – – – – – – – – – –

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X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575

X=0.48 X=0.50 X=0.52

0.52

X=0.48 X=0.50 X=0.52

0.50 0.330

Exergy efficiency

Net electrical efficiency

0.335

0.325

0.320

0.48

0.46

0.315

0.44

1.6

2.0

2.4

2.8

3.2

1.6

Ammonia turbine inlet pressure (MPa)

2.0

2.4

2.8

3.2

Ammonia turbine inlet pressure (MPa)

Pressure ratio of ammonia turbine

Fig. 2. Effect of ammonia turbine inlet pressure on the efficiency.

7.6

mB (kg/s)

7.2

6.8

6.4

6.0

X=0.48 X=0.50 X=0.52

160 140 120 100 80 1300

18

1200

17

1100

16 15

h14 (kJ/kg)

mL N G (kg/s)

19

180

X=0.48 X=0.50 X=0.52

1000

X=0.48 X=0.50 X=0.52

900

4200

5400

4000

5200

3800

5000

3600 3400 3200 3000

X=0.48 X=0.50 X=0.52

WT 2 (KW)

WT 1 (KW)

800

2.0 2.2 2.4 2.6 2.8 3.0 3.2

Ammonia turbine inlet pressure (MPa)

X=0.48 X=0.50 X=0.52

4800 4600 4400 4200 2.0 2.2 2.4 2.6 2.8 3.0 3.2

Ammonia turbine inlet pressure (MPa)

Fig. 3. Ammonia vapour flow rate, pressure ratio of ammonia turbine, LNG turbine flow rate, LNG turbine inlet specific enthalpy, ammonia turbine work output and LNG turbine work output vs. ammonia turbine inlet pressure.

If the ammonia mass fraction becomes high, ammonia turbine work output increases due to more ammonia vapour expanding through the ammonia turbine for given ammonia turbine inlet pressure, whereas LNG turbine work output is decreased due to the reduced natural gas specific enthalpy at the LNG turbine inlet.

The work output variation of ammonia turbine is weaker than that of the LNG turbine, so total work output of the system increases with the increase of ammonia mass fraction at a given ammonia turbine inlet pressure. However the rise in ammonia mass fraction causes an increase of the flow rate of LNG regasified in the con-

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X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575

denser and consequently a rise in the exergy input. Furthermore, the increased amplitude exergy input is larger than that of the total work output of the system. Therefore, the net electrical efficiency increases as the ammonia mass fraction increases, but exergy efficiency decreases as shown in Fig. 2.

pressure ratio across the LNG turbine increases while the natural gas specific enthalpy at the LNG turbine inlet continuously decreases with increased the LNG inlet pressure. The variation of the exergy efficiency with LNG turbine inlet pressure is similar to that of net electrical efficiency. However the maximum point of exergy efficiency does not coincide with the maximum LNG turbine work output. This is mainly because increasing the LNG turbine inlet pressure results in a reduction the LNG latent heat of vaporization at the corresponding pressure, and then LNG flow rate increases rapidly, leading to a higher exergy input.

4.3. Effect of LNG turbine inlet pressure The heat source temperature is 167 °C, the inlet pressure of ammonia turbine is 3 MPa, the natural gas supplying pressure is taken as 0.3 MPa and the ammonia mass fraction of basic solution is set to be 0.5, other thermal conditions summarized in Table 1 are kept unchanged. The performance of the proposed combined system is evaluated for LNG turbine inlet pressures from 2 to 4.35 MPa. The calculated results are presented in Fig. 4 and 5. From the results, net electrical efficiency increases first as the pressure goes up. Then due to the reduced LNG turbine work output at a pressure of about 4 MPa, net electrical efficiency goes down at the corresponding pressure. The LNG turbine work output curve can be explained in Fig. 5, which shows the specific enthalpy drop of the LNG turbine peaks with the pressure. The reason is that

4.4. Effect of low-temperature heat source temperature It should be pointed out that there are three cases of the system with the increase of the heat source temperature. If the temperature of the basic solution at the HX3 outlet is lower than that of the bubble point, no ammonia will be vaporized to generate power in the ammonia turbine and then all basic solution from the separator will be used to heat LNG which is expanded through the LNG turbine. However, it will be reversed if the temperature of the basic solution at the HX3 outlet is higher than that of the dew point, i.e. basic solution will be completely vaporized by waste heat in the HX3 and then no regasified LNG drive the LNG turbine. What interests us is the third case of partial vaporization, the temperature of basic solution is between the bubble and dew point at the HX3 pressure, partial vaporized ammonia is sent to ammonia turbine and weak solution from the separator heat the LNG to drive LNG turbine. Both the inlet pressure of ammonia turbine and LNG turbine are set to be 3 MPa, keeping other thermal conditions unchanged as summarized in Table 1. The performance of the proposed combined system is calculated for different heat source temperatures at the natural gas supplying pressure of 0.3 MPa and the ammonia mass fraction of basic solution of 0.5. The calculated results are presented in Fig. 6 and 7. Fig. 6 shows that the net electrical efficiency increases to a maximum point and then decreases with increased the heat source temperature. The variation of exergy efficiency is almost in same trend as that of the net electrical efficiency. As the heat source temperature approaches the bubble point temperature of the basic ammonia–water mixture, ammonia turbine work output approaches zero due to less ammonia vapour vaporized in the HX3. When heat source temperature increases, more ammonia vapour

0.50 0.48 0.46 0.44

net electrical efficiency exergy efficiency

0.40 0.38 0.36 0.34 0.32 0.30 2.0

2.5

3.0

3.5

4.0

4.5

LNG turbine inlet pressure (MPa) Fig. 4. Effect of LNG turbine inlet pressure on the efficiency.

1100

h1 4 (kJ/kg)

mL N G (kg/s)

20

18

1000

16

900

320 5400 300

WT 2 (KW)

specific enthalpy drop of LNG turbine (kJ/kg)

Efficiency

0.42

280

5100 4800 4500

260 2

3

4

LNG turbine inlet pressure (MPa)

2

3

4

LNG turbine inlet pressure (MPa)

Fig. 5. LNG flow rate, LNG turbine inlet specific enthalpy, specific enthalpy drop of LNG turbine and LNG turbine work output vs. LNG turbine inlet pressure.

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X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575 0.50 0.50

0.45 0.45

Efficiency

Efficiency

net electrical efficiency exergy efficiency

0.40

0.35

net electrical efficiency exergy efficiency

0.40

0.35

0.30

0.30

0.25

0.25 420

430

440

450

0.3

460

0.4

0.5

0.6

0.7

0.8

LNG turbine outlet pressure (MPa)

Heat source temperature (K)

Fig. 8. Effect of LNG turbine outlet pressure on the efficiency.

Fig. 6. Effect of heat source temperature on the efficiency.

expands through the ammonia turbine, such that ammonia turbine work output continuously goes up. The variation of LNG flow rate is almost the same trend as that of ammonia flow rate because more LNG is vaporized in the condenser with the increase of ammonia vapour flow rate. Natural gas specific enthalpy at the LNG turbine increases first due to the increasing of heat source temperature and then decreases because less weak solution flow through the HX1 to heat the natural gas. Therefore the peak value of LNG turbine work output exists as the heat source temperature increases, which results in the curves of the net electrical efficiency and exergy efficiency.

5500

75 70

5000

4.5. Effect of LNG turbine outlet pressure

60 4500

WT 2 (KW)

City gas temperature

65

55 50 45

3500

40

LNG turbine outlet pressure is decided by the natural gas supplying pressure. The performance of the proposed combined system is calculated as the LNG turbine outlet pressure increases from 0.3 to 0.8 MPa. The effect of LNG turbine outlet pressure are presented in Fig. 8 and 9 at the ammonia turbine inlet pressure of 3 MPa, LNG turbine inlet pressure of 3 MPa and the ammonia mass fraction of basic solution of 0.5.

4000

35 3000 0.3

0.4

0.5

0.6

0.7

0.8

LNG turbine outlet pressure (MPa)

0.3

0.4

0.5

0.6

0.7

0.8

LNG turbine outlet pressure (MPa)

Fig. 9. City gas temperature and LNG turbine work output vs. heat source temperature.

20

8

mL N G (kg/s)

mB (kg/s)

18

6

16 14 12

4 1150 4500

1050

4000

W (KW)

h14 (kJ/kg)

5000 1100

1000 950

3500 3000

ammonia turbine LNG turbine

2500

900

2000 420

430

440

450

460

heat source temperature (K)

420

430

440

450

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Heat source temperature (K)

Fig. 7. Ammonia vapor flow rate, LNG flow rate, LNG turbine inlet specific enthalpy, ammonia turbine work output and LNG turbine work output vs. heat source temperature.

X. Shi, D. Che / Energy Conversion and Management 50 (2009) 567–575

According to the calculated results, the net electrical efficiency decreases as the LNG turbine outlet pressure increases. This is because lowered pressure ratio across the LNG turbine results in a lower LNG turbine work output. However, the exergy efficiency increases with the pressure. This can be explained as follows: for increasing the LNG outlet pressure, although LNG turbine work output decreases, city gas exergy increases because city gas temperature increases with the pressure. Furthermore, the change of the city gas exergy is larger than that of the LNG turbine work output. Then exergy output increases with the pressure. The variation of LNG turbine outlet pressure has no effect on the performance of ammonia turbine cycle.

5. Conclusion This paper has proposed a combined power system that can efficiently recover low-temperature waste heat and fully utilize the cold energy of LNG as well. The proposed combined system consists of the Rankine cycle with ammonia–water mixture as the working fluid to recover low-temperature waste heat and the LNG power generation cycle. It produces power and provides certain amount of regasified LNG. The steady-state component models are developed in consideration of mass, energy and species balances, and validated by comparison with previously published models and simulation results of other investigators. Based on the validated models, this system has been simulated for a typical operating condition. The results show that the net electrical efficiency and the exergy efficiency of the proposed combined cycle are 33.28% and 48.87%, respectively. The waste heat recovery efficiency of the proposed combined cycle reaches 86.57%. About 16.36 kg s1 of natural gas, whose conditions are 35.8 °C, 0.3 MPa, can be delivered to the natural gas supplying system and no less than 0.2 MW of electrical power for operating sea water pumps can be saved. To improve the net electrical efficiency and exergy efficiency of the system, a parametric analysis has been performed to investigate the effects of ammonia mass fraction of basic solution, heat source temperature, natural gas supplying pressure, as well as the inlet pressure of ammonia turbine and LNG turbine. It is found that a maximum net electrical efficiency can be obtained as the inlet pressure of ammonia turbine increases and the maximum value increases as the ammonia mass fraction increases. Exergy efficiency goes up with the increased ammonia turbine inlet pressure. As the ammonia mass fraction increases at given ammonia turbine inlet pressure, the net electrical efficiency increases, whereas the exergy efficiency decreases. There is also a peak of net electrical efficiency and exergy efficiency with increased LNG turbine inlet pressure or heat source temperature. As the LNG gas turbine outlet pressure increases, exergy efficiency increases while net electrical efficiency drops. The parametric analysis results imply that there is

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a improvement potential for the system by applying optimization techniques. In order to further evaluate the advantages of the proposed combined system and compare it with other power cycle recovering low-temperature waste heat and determine if this system is profitable, a more detailed design and a techno-economic study based on the results of thermodynamic analyses are expected. References [1] Sternlicht Beno. Waste energy recovery: an excellent investment opportunity. Energy Conversion and Management 1982;22:361–73. [2] Hung TC, Shai TY, Wang SK. A review of organic Rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 1997;22(7):661–7. [3] Hung Tzu-Chen. Waste heat recovery of organic Rankine cycle using dry fluids. Energy Conversion and Management 2001;42:539–53. [4] Liu BT, Chien KH, Wang CC. Effect of working fluids on organic Rankine cycle for waste heat recovery. Energy 2004;29:1207–17. [5] Wei D, Lu X, Lu Z, Gu J. Performance analysis and optimization of organic Rankine cycle (ORC) for waste heat recovery. Energy Conversion and Management 2007;48:1113–9. [6] Madhawa Hettiarachchi HD et al. Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources. Energy 2007:01. [7] Kalina IA. Combined cycle system with novel bottoming cycle. ASME Journal of Engineering for Gas Turbine and Power 1984;106:737–42. [8] Marston CH. Parametric analysis of the Kalina cycle. ASME Journal of Engineering for Gas Turbine and Power 1990;112:107–16. [9] Nag PK, Gupta AVSSKS. Exergy analysis of the Kalina cycle. Applied Thermal Engineering 1998;18:427–39. [10] Goswami DY, Xu F. Analysis of a new thermodynamic cycle for combined power and cooling using low and medium temperature solar collector. ASME Journal of Solar Energy Engineering 1999;121:91–7. [11] Tamm G, Goswami DY, Lu S, Hasan AA. Theoretical and experimental investigation of an ammonia–water power and refrigeration thermodynamic cycle. Solar Energy 2004;76:217–28. [12] Martin C, Goswami DY. Effectiveness of cooling production with a combined power and cooling thermodynamic cycle. Applied Thermal Engineering 2006;26:576–82. [13] Hongtan Liu, Lixin You. Characteristics and applications of the cold heat exergy of liquefied natural gas. Energy Conversion & Management. 1999;40: 1515–25. [14] Bisio G, Tagliafico L. On the recovery of LNG physical exergy by means of a simple cycle or a complex system. Eexergy, an international journal. 2002;2:34–50. [15] Hisazumi Y, Yamasaki Y, Sugiyama S. Proposal for a high efficiency LNG power generation system utilizing waste heat from the combined cycle. Applied Energy 1998;60:169–82. [16] Wang Q, Li YZ, Wang J. Analysis of power cycle based on cold energy of liquefied natural gas and low-grade heat source. Applied Thermal Engineering 2004;24:539–48. [17] Miyazaki T. A combined power cycle using refuse incineration and LNG cold energy. Energy 2000;25:639–55. [18] Shi X, Che D. Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system. International journal of energy research. 2007;31:975–98. [19] REFPROP Version 7.1, NIST Standard Reference Database 23, the Us Secretary of Commerce, America; 2003. [20] Donghong Wei, Xuesheng Lu, et al. Performance analysis and optimization of organic Rankine cycle (ORC) for waste heat recovery. Energ Convers Manage 2007;48:1113–9. [21] Zhang N, Lior N. Methodology for thermal design of novel combined refrigeration/power binary fluid systems. International Journal of Refrigeration 2007;30:1072–85.