Energy Conversion and Management

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1 Energy Conversion and Management 50 (2009) Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: 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 , China article info abstract 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 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.: ; fax: address: dfche@mail.xjtu.edu.cn (D. Che). 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 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 /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.enconman

2 568 X. Shi, D. Che / Energy Conversion and Management 50 (2009) Nomenclature c p specific heat at constant pressure (kj kg 1 K 1 ) e x specific exergy (kj kg 1 ) _E in exergy input (kw) h specific enthalpy (kj kg 1 ) h s isentropic specific enthalpy (kj kg 1 ) _m mass flow rate (kg s 1 ) P pressure absolute (MPa) _Q heat transfer rate (kw) r latent heat of vaporization (kj kg 1 ) R g gas constant (kj kmol 1 K 1 ) s specific entropy (kj kg 1 K 1 ) T temperature (K) _W work (kw) x mass fraction of ammonia z compress factor Abbreviations HX heat exchanger LNG liquefied natural gas Greek symbols g 1 net electrical efficiency of combined cycle g 2 exergy efficiency of combined cycle waste heat recovery efficiency g wh g EG g P g T 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 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. 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: Low-temperature waste heat A 3 B C HX1 Separator T V City gas 11 Mixer 17 HX2 16 LP 20 HX3 2 FP 7 14 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 CP T2 12 T1 LNG tank 6 5 Condenser 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.

3 X. Shi, D. Che / Energy Conversion and Management 50 (2009) 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. 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% Feed pump 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) 167 ( ) Heat source flow rate 46 Generator efficiency 0.99 Inlet pressure of ammonia turbine (MPa) 3.0 ( ) Inlet pressure of LNG turbine (MPa) 3.0 (2 4.35) Natural gas supplying pressure (MPa) 0.3 ( ) Ammonia mass fraction of basic solution 0.5 ( ) Isentropic efficiency of turbine 0.9 Isentropic efficiency of pump 0.8 Ambient temperature ( C) 15 Ambient pressure (MPa) 0.1 Effectiveness of heat exchanger 0.85 Pressure loss of heat exchanger 0.05 _W FP ¼ _m A ðh 2 h 1 Þ where h 2 ¼ h 1 þðh s2 h 1 Þ=g P 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: _Q HX 3 ¼ _m A ðh 3 h 2 Þ¼ _m W ðh 18 h 19 Þ ð3þ 3.3. Separator 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. T 3 ¼ T 8 ¼ T 4 ð4þ The total mass balance is given as _m A ¼ _m B þ _m C 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 _W T1 ¼ _m B ðh 4 h 5 Þ ð6þ h 5 ¼ h 4 ðh 4 h s5 Þ=g T Net power output of ammonia turbine cycle is defined as follows: _W AC ¼ W _ T1 g EG W _ FP W _ CP 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 _Q condenser ¼ _m B ðh 5 h 6 Þ¼ _m LNG ðh 13 h 12 Þ ð9þ The mass flow rate of vaporized LNG is equal to _ Q condenser =r LNG, where r LNG is the LNG latent heat of vaporization at the pressure of LNG pump outlet Condensate pump The saturated ammonia liquid leaving the condenser is compressed to mixer pressure. The pump power consumption is calculated as follows: ð1þ ð2þ ð5þ ð7þ

4 570 X. Shi, D. Che / Energy Conversion and Management 50 (2009) _W CP ¼ _m B ðh 7 h 6 Þ ð10þ _W T2 ¼ _m LNG ðh 14 h 15 Þ ð18þ where h 7 ¼ h 6 þðh s7 h 6 Þ=g P ð11þ h 15 ¼ h 14 ðh 14 h s15 Þ=g T Net power output of LNG turbine cycle is defined as follows: _W LC ¼ W _ T2 g EG W _ LP ð19þ 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 _m 10 ¼ _m 9 ð12þ Energy balance is h 10 ¼ h Mixer ð13þ 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: _m B h 7 þ _m C h 10 ¼ _m A h 1 þ _m LNG ðh 16 h 15 Þ ð14þ 3.9. LNG pump 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: _W LP ¼ _m LNG ðh 12 h 11 Þ where h 12 ¼ h 11 þðh s12 h 11 Þ=g P ð15þ ð16þ Heat exchanger HX2 (hot water/natural gas heat exchanger) 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 _m W ðh 19 h 20 Þ¼ _m LNG ðh 17 h 16 Þ ð21þ Efficiency Power output of the proposed combined system is _W CC ¼ _ W AC þ _ W LC The corresponding net electrical efficiency is defined as _W g CC 1 ¼ _m W ðh 18 h 19 Þ ð22þ 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. g 2 ¼ð W _ CC þ _m LNG e x17 Þ= E _ in ð24þ where _E in _m w ðe x18 e x19 Þþ _m LNG e x11 The specific exergy is defined as follows in the calculations: For LNG : e x ¼ T 0 T 1 r C p ðt 0 TÞþC p T 0 ln T 0 T þ zr gt 0 ln p p 0 ð25þ ð26þ Heat exchanger HX1 (weak solution/natural gas heat exchanger) For water : e x ¼ðh h 0 Þ T 0 ðs S 0 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 _m C ðh 9 h 8 Þ¼ _m LNG ðh 14 h 13 Þ ð17þ The reference conditions are taken as T 0 = K and P 0 = MPa. Waste heat recovery efficiency is defined as g wh ¼ðh 18 h 20 Þ=ðh 18 h wh;288k Þ ð28þ LNG turbine The superheated natural gas expands through LNG turbine to the gas supplying pressure to generate power. Gross power output of LNG turbine is Validation 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 T i = 40.4 C, P i = 150 kpa, P e = 1484 kpa, g 1 = 0.75, m = 1kg/s, x = 0.3 _W = 2.2 kw _W = 2.09 kw Heat exchanger T i = 37.5 C, P i = 1400 kpa, T e = 9.2 C, m = kg/s, x = Q _ = 59.4 kw Q _ = 55.2 kw Turbine T i = 450 C, P i = 5100 kpa, P e = 19.1 kpa, g 1 = 0.75, m = kg/s, x = _W = kw _W = kw Throttle valve T i = 9.2 C, P i = 1358 kpa, P e = 165 kpa, x = T e = 22.7 C T e = 21.3 C

5 X. Shi, D. Che / Energy Conversion and Management 50 (2009) 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 h 1 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 h 1 of sea water as the heat source to vaporize 58.9 t h 1 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 Effect of ammonia turbine inlet pressure Table 4 Calculation results for the proposed combined cycle. _W T1 (kw) _W T2 (kw) _W CC (kw) g 1 (%) g 2 (%) g wh (%) _m CG T CG ( C) 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 P 4 varying from 1.8 to 3.2 MPa on net electrical efficiency g 1 and exergy efficiency g 2. The calculated results are shown in Fig. 2. In order to illustrate the impact of the ammonia turbine inlet pressure P 4 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 T K P kpa h kj/kg s kj/(kg K) e x kj/kg _m kg/s x 1 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

6 572 X. Shi, D. Che / Energy Conversion and Management 50 (2009) Net electrical efficiency Exergy efficiency Ammonia turbine inlet pressure (MPa) Ammonia turbine inlet pressure (MPa) Fig. 2. Effect of ammonia turbine inlet pressure on the efficiency. m B m L N G Pressure ratio of ammonia turbine h 14 (kj/kg) W T 1 (KW) W T 2 (KW) Ammonia turbine inlet pressure (MPa) 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-

7 X. Shi, D. Che / Energy Conversion and Management 50 (2009) 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 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 Efficiency net electrical efficiency exergy efficiency LNG turbine inlet pressure (MPa) Fig. 4. Effect of LNG turbine inlet pressure on the efficiency. 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 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 m L N G h 1 4 (kj/kg) specific enthalpy drop of LNG turbine (kj/kg) W T 2 (KW) LNG turbine inlet pressure (MPa) 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.

8 574 X. Shi, D. Che / Energy Conversion and Management 50 (2009) Efficiency net electrical efficiency exergy efficiency Efficiency net electrical efficiency exergy efficiency Heat source temperature (K) LNG turbine outlet pressure (MPa) Fig. 6. Effect of heat source temperature on the efficiency. Fig. 8. Effect of LNG turbine outlet pressure 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 Effect of LNG turbine outlet pressure 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. City gas temperature W T 2 (KW) LNG turbine outlet pressure (MPa) LNG turbine outlet pressure (MPa) Fig. 9. City gas temperature and LNG turbine work output vs. heat source temperature m B 6 m L N G h 14 (kj/kg) W (KW) heat source temperature (K) ammonia turbine LNG turbine 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.

9 X. Shi, D. Che / Energy Conversion and Management 50 (2009) 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 kg s 1 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. 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