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

Abstract

Liquefied natural gas (LNG), an increasingly widely applied clean fuel, releases a large number of cold energy in its regasification process. In the present paper, the existing power generation cycles utilizing LNG cold energy are introduced and summarized. The direction of cycle improvement can be divided into the key factors affecting basic power generation cycles and the structural enhancement of cycles utilizing LNG cold energy. The former includes the effects of LNG-side parameters, working fluids, and inlet and outlet thermodynamic parameters of equipment, while the latter is based on Rankine cycle, Brayton cycle, Kalina cycle and their compound cycles. In the present paper, the diversities of cryogenic power generation cycles utilizing LNG cold energy are discussed and analyzed. It is pointed out that further researches should focus on the selection and component matching of organic mixed working fluids and the combination of process simulation and experimental investigation, etc.

Keywords

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

Introduction

Liquefied natural gas (LNG) is the final product of natural gas (NG) in the processes of de-acidification, de-hydration and liquefaction, which comes to the ultimate state of liquid mixture with a low temperature of –162°C [ 1]. The main composition of LNG consists of methane (90%), ethane (0.1%–5%), nitrogen (0.5%–1%) and a small amount of hydrocarbon of C3–C5. Compared with the conventional energies, such as coal and oil, the absence of sulfur in LNG composition decreases the amount of environmentally hazardous gases produced after regasification and combustion. Besides, the greenhouse gases generated during NG burning are only 1/2 and 2/3 of an equal amount of coal and oil, respectively. Considering the severe environmental issue, it is reasonable to believe that LNG with low emission problems will be the dominating energy in the future in preference to coal and oil.

In practical applications, only after regasification can LNG be used. However, the traditional ways of regasification using seawater or air vaporizer will lead to a huge loss of the cold energy of LNG. Therefore, the establishment of cryogenic power generation cycles utilizing LNG cold energy becomes an important way to recovery cold energy. Due to the high quality of electricity, turbine coupled to an electrical generator is usually adopted to achieve electricity output in cryogenic power generation cycles.

The present paper focuses on the power generation cycles taking advantages of LNG cold energy, classifying the existing studies into the key factors affecting basic power generation cycles and the structure enhancement, which provides a profile of alternative power generation cycles using LNG cold energy.

Basic power generation cycles utilizing LNG cold energy and the effects of key factors

Basic LNG power generation cycles

Basic LNG power generation cycles utilizing cold energy mainly include direct expansion of LNG, Rankine cycle using intermediate cooling medium, and the combination of aforementioned cycles. These three kinds of power generation cycles have been in practical use with relatively mature technology. In Osaka Gas Company in Japan, for example, as early as 1979 and 1982, the Rankine cycle using propane as the working fluid and the combined cycle have been in use and achieved an output power of 1450 kW and 6000 kW, respectively [ 2].

LNG direct expansion is an open cycle, as shown in Fig. 1, and converts the pressure exergy of LNG into power output, which exploits LNG exergy in a relatively direct but inefficient way. In this case, LNG from the storage tank is pumped up and vaporized into NG with a high temperature and pressure. NG is fed into the turbine and is expanded to output electric energy by a generator linked to a rotating shaft. The turbine exhaust is heated by a heat source, where seawater or residual heat is always employed, and finally is transported to user’s receiving terminal as demanded. In spite of principle-simplicity and cost-saving, the inefficiency of direct expansion has restricted it to the assisting of other basic cycles in application.

In Rankine cycle, intermediate cooling medium (working fluid with low boiling temperature is usually employed) goes through four stages of condensation, compression, vaporization and expansion to generate power, using LNG as its heat sink and low quality heat (including solar energy, air, water, industrial waste heat, etc.) as its heat source, as presented in Fig. 2. Compared to direct expansion, the Rankine cycle primarily utilizes cryogenic exergy of LNG instead of pressure exergy and achieves higher efficiency in performance due to the reduced turbine back pressure. However, the large temperature difference between the single working fluid and LNG in the process of heat exchange in the condenser always causes a great loss of exergy, which increases the irreversibility of the whole cycle.

The combined cycle is the integration of direct expansion and Rankine cycle, as illustrated in Fig. 3. The combined cycle can use the cryogenic and pressure exergy simultaneously and the connection between the two sub-cycles is realized through the condenser where the working fluid obtains the cold energy released by LNG. The power output is generated from both the working fluid turbine and the NG turbine. As far as the efficiency is concerned, the combined cycle is the most commonly used one among cryogenic power generation cycles of LNG in practical power plant.

Effects of key parameters of basic LNG power generation cycles

The effects of key parameters on the cycle performance chiefly revolve around basic Rankine cycle and the combined cycle for their extensive application. In this section, LNG-side parameters, circulatory working fluids and input and output factors of crucial equipment are discussed.

Effect of LNG-side parameters

Liu and Guo [ 3] have determined the impacts of LNG temperature, pressure and concentration of methane in LNG on LNG exergy. It is reported that the cryogenic exergy, the pressure exergy and the total exergy of LNG increase with the rising ambient temperature at a specified composition of methane. When the pressure of LNG rises up, the pressure exergy increases while the cryogenic exergy decreases, both of which make the total exergy first reduce then level off. The reason for the decline of cryogenic exergy is that the rise of the LNG pressure makes itself a higher bubble-point, which accordingly decreases the temperature difference between the LNG and the ambient. In the meantime, the state of LNG gets much closer to the critical section where there is a lower latent heat of vaporization. In addition, the cryogenic exergy, the pressure exergy and the total exergy of LNG increase as the mole fraction of methane rises at a specified ambient temperature and pressure.

Bai [ 4] has adjusted the pressure and temperature of LNG and the concentration of methane in LNG respectively in the proposed Rankine cycle using propane as the working fluid, with molar flow change of propane to match the adjustment. The power output of the cycle is influenced by the above LNG-side parameters and it is further analyzed that the rise of the methane composition in LNG at a certain pressure will decrease the bubble-point of LNG and increase the temperature difference between LNG and the ambient, which clearly makes a higher cryogenic exergy. Besides, the consequent decline of molar mass causes an augmented pressure exergy, which improves the overall ability of power generation.

The temperature difference and pressure difference between LNG and reference environment mutually contribute to a considerable LNG cold exergy. The research results above show that the temperature and pressure of LNG directly influence LNG exergy and the performance of cycle while the concentration change of the dominating composition of methane mediately alters the temperature and pressure difference and affects LNG exergy as a result, by changing its own physical attributes, embracing bubble-point and molar mass, etc. Thus a proper type of cycle should be chosen to utilize LNG exergy effectively according to the condition of LNG. The concentration of methane in LNG has determined the ability of cycle power output within limit. As for the practical applications, the upper bound of methane concentration interval can be regarded as an ideal working condition and the lower bound as a severe condition, so as to simulate practical engineering in a comprehensive way.

Working fluid

1) Single working fluid

The principles of the selection of the single working fluid in cryogenic Rankine cycle and the combined cycle are presented as follows: friendly to environment, large heat of vaporization, good chemical and thermal stability, high thermal conductivity, small kinetic viscosity, nearly vertical liquid saturation line, non-toxic, easy to produce and low cost, etc [ 5]. Moreover, the triple point of the working fluid should be lower than the lowest temperature of system operation, which ensures that the solidification and the jam of the working fluid will not happen at any place in the circulation [ 4].

Lu et al. [ 6] have selected different working fluids to compare their performance in sub-critical Rankine cycle with LNG as its heat sink and seawater as its heat source. The regularity appears similar when employing four working fluids of R152a (CH₃CHF₂), R290 (C₃H₈), R600 (C₄H₁₀) and R134a (CH₂FCF₃) in the investigated cycle, i.e., the total amount of electricity generated from circulation first increases and then decreases with the evaporation temperature rising, which confirms that there exists an optimal evaporation temperature where the power output gets the maximum. Meanwhile, R290 is proved to be with the comprehensively optimum properties in contrast to other working fluids in enthalpy drop within equal entropy, saturation pressure and other attributes. Corresponding to a heat source temperature of 20°C, the best evaporation temperature of R290 maintains at 11.08°C.

Liu et al. [ 7] have established the dependencies between LNG utilized temperature and unit power output in LNG-seawater Rankine cycle. It is revealed that at a specified LNG vaporization pressure, an optimal LNG utilized temperature exists which makes the unit power output maximum. In view of the existence of temperature difference between cold and hot fluids in the heat transfer process happened in condenser, the optimal condensing temperature of working fluid is supposed to be deduced according to the optimal LNG utilized temperature to obtain the max output.

Consequently, the optimum condensing and vaporizing temperature ought to be taken into account as part of the principles when selecting the single working fluid in Rankine cycle and the combined cycle.

Zhang et al. [ 8] have screened and constructed the refrigerant fluids in LNG-seawater power system, adopting a novel group contribution method (GCM). Vital attribute values, in which critical point, thermal conductivity, enthalpy of vaporization and heat capacity are included, can be calculated based on GCM in the selection of working fluids. According to GCM calculation model, CHF₃ is recommended to be a prior choice to the conventional refrigerant fluids embracing R22 (CHClF₂), R134a (CH₂FCF₃) and R410a with the evaluating indicators of LNG exergetic efficiency and thermal efficiency. High density of CHF₃ decreases the total flow rate in circulation and economizes the material cost; its lower boiling point makes trans critical cycle possible, which improves the overall cycle performance; a larger heat capacity reduces the flow rate of working fluid at a required amount of heat transmission, shrinks the size of the pipelines, and decreases the pump power input.

2) Mixed working fluids

Although Rankine cycle and the combined cycle employing propane as working fluid are the most feasible ways to recovery LNG cold exergy, isothermal phase change process of single medium in condenser makes a great LNG exergy loss which confines the cycle performance to a relatively low level. Considering the fact that the condensing temperature of mixed working fluid matches well with LNG vaporization temperature, due to the temperature-variation phase change of mixed media, the single working fluid is supposed to be substituted by the mixed one to enhance the overall cycle performance.

The mixture of R22 and R142b (CH₃CClF₂) has been proved to perform better than any one of two pure refrigerated fluids in LNG cold energy power system, according to the study conducted by Kim et al. [ 9]. However, the gradually severe environment issue constricts the use of Freon, and organic mixed fluids turn into commonly employed media.

Zhu et al. [ 10] have compared the performance of propane and ternary mixed fluids consisting of propane, ethylene and isobutane at a mole fraction ratio of 0.39:0.16:0.45 in LNG-air Rankine cycle. The investigation shows that the LTMD (logarithmic mean temperature difference) of ternary mixed fluids gets much smaller than the propane and the utility efficiency of cold energy is increased by 41.04%.

Wang [ 5] has proposed an isentropic fluid, which is a mixture of propane and isobutane at a mole fraction ratio of 0.7:0.3, in the LNG-waste gas Rankine cycle. The isentropic mixture is able to maintain the turbine exhaust at a lower pressure and temperature, and consequently improves the efficiency of the turbine. It is reported that the cycle employing the isentropic mixture performs with the highest thermal efficiency when altering the mole fraction ratio of propane and isobutane under the similar working condition. However, adding isobutane to pure propane to constitute the mixture will elevate the allowable minimum condensing temperature and narrow working fluid operational temperature interval of the cycle.

Chen [ 11] has adopted the mixture of propane and butane as working fluid in Rankine cycle using LNG as its heat sink. The study states the variation trend of the exergetic efficiency of vaporizer and condenser along with the change of mixing ratio and mixture flow rate. When the flow rate of mixture is fixed, the variation tendency of the exergetic efficiency of the vaporizer along with the change of the mixing ratio is complex while the exergetic efficiency of the condenser declines as the concentration of propane in mixture rises up. Additionally, there exists an optimal mixture flow rate where the exergetic efficiency of the vaporizer reaches the peak while the exergetic efficiency of the condenser declines with the flow rate of the mixture rising.

Besides, plenty of researches have been devoted to Rankine cycle and combined cycle employing ammonia-water mixture as its working fluid. Miyazaki et al. [ 12] have built a combined cycle with exhaust gas as heat source and LNG as heat sink to separately analyze the performance of ammonia-water and water vapor. It is presented that the exergetic efficiency and thermal efficiency of the former is 1.53 and 1.43 times of the latter, respectively. Gao and Wang [ 13] have proposed a novel LNG power generation cycle employing ammonia-water as the working medium in the basis of conventional thermodynamic cycle and LNG cold energy utilization. The thermal efficiency of this novel cycle rises up by 14.5% and the exergetic efficiency reaches 53.6%. It is also concluded in the report that the critical reasons for efficiency improvement are the decrease of average exothermic temperature and proper matching between the vaporization and the condensation. Wang et al. [ 14] have adopted a heat recovery vapor generator (HRGV), where vaporization is classified into three different regions including a sub-cooled region, an evaporation region, and a super-heated region, to replace the conventional vaporizer in Rankine cycle using ammonia-water as the working medium. An optimal components ratio of ammonia and water is discovered at an ammonia mass fraction of 0.7 to obtain the peak of the net work output in the discussed cycle.

Inlet and outlet thermodynamic parameters of equipment

Research on the inlet and outlet key parameters of equipment provides available directions for cycle optimization, which is conducted by changing key parameters to obtain the variation regularity of cycle power output, thermal and exergetic efficiency and other valuable indicators. The inspected parameters include but are not limited to the turbine inlet temperature and pressure, as well as the vaporizer outlet temperature, etc.

In LNG-seawater combined cycle, the increase of the turbine inlet temperature of the working fluid and NG will markedly improve the overall power output of cycle, as reported by Bai [ 4]. Rao et al. [ 15] have suggested LNG-residual heat Rankine cycle using ethane as its working fluid and presented that the systematic thermal efficiency and total power output increase with the pressure ratio ascending between the vaporizer outlet pressure and the condenser outlet pressure of the working fluid. Xue et al. [ 16] have proposed a two-stage LNG-exhaust gas Rankine cycle. It is pointed out that the cost per net power output increases with the rising turbine inlet pressure and the mass flow of the working fluid in each stage, which indicates an enhancement in cycle economic performance.

Structural enhancement

A few studies [ 17, 18] of the structural enhancement are conducted toward LNG direct expansion. Most structural enhancement types in LNG cold energy power cycles are built on Rankine cycle and Brayton cycle. Meanwhile, Kalina cycle which characteristically employs absorber and separator are proposed to make use of the property of non-azeotropic mixtures. Compound cycles with higher structural complicacy are established by integrating Rankine cycle, Brayton cycle or Kalina cycle to make better cycle performance available.

Structural enhancement based on Rankine cycle

The working fluids employed in Rankine cycle vary from most widely used propane to ethane, ammonia-water, organic refrigerating mixture and CO₂, etc. The heat sources are also extended from seawater to industrial waste heat, exhaust gas, geothermic heat, and solar energy, etc.

Kim and Kim [ 19] have conducted a modified combined cycle with regeneration, using a low-level heat of 200°C and ammonia-water as working fluid at an ammonia mass fraction of 60%, as presented in Fig. 4. In this ammonia-water cycle, the liquid ammonia-water, which is first pumped up by the working fluid pump, is preheated in the regenerator by the hot stream of the exhaust mixture from the turbine. The process utilizes the heat of turbine exhaust to raise the inlet temperature of turbine and to improve the work output afterwards. Huang et al. [ 20] have proposed a trans critical regenerative Rankine cycle, adopting CO₂ as the working medium and LNG and geothermic heat as the heat sink and heat source, respectively. Figure 5 depicts the schematic diagram of the system. The regenerative Rankine part is accomplished by the working fluid of CO₂. The heat source consisting of water vapor and CO₂ is condensed by the working fluid of CO₂ and LNG successively then fed to the separator where liquid water outflows from the bottom and CO₂ from the top. The CO₂ separated from water transfers heat to LNG and finally turns into the state of liquid for recovery. In this system, the cold exergy of LNG, on the one hand, drives the working fluid to export power through turbine, and on the other, provides cooling capacity of CO₂ liquefaction to control carbon emission.

Poly-stage inclination is another important approach of Rankine cycle to make structures enhanced, mainly based on the theory of cascade utilization of LNG cold energy according to LNG regasification curve.

Yang [ 21] has established a LNG-seawater two-stage Rankine cycle of horizontal focusing on 7 MPa LNG in the condition of super-critical regasification. As demonstrated in Fig. 6, the loops from left to right in turn are the first and second stage of the whole system and ethane and propylene are employed respectively. In this system, the working fluids in the two stages obtain LNG cold energy successively and individually drive their own Rankine sub-cycle. Compared with the Rankine cycle employing only propylene, the proposed two-stage Rankine system has an improvement in electrical power output by 36.47%.

In view of the great exergetic loss in the condensers of the horizontal cycle, Yang [ 21] has established a three-stage vertical cycle to reduce the systematic exergetic loss, as illustrated in Fig. 7. The loops from the bottom to the top are the first, second and third stage, using propylene, ethylene and ethylene as working fluid, respectively. Both of the outlet gases from turbine in the top and the central stage are divided into two streams, one of which is used to assist the LNG regasification through HX2 and HX1. Besides, the top stage takes seawater as its heat source while the central and bottom stage respectively utilizes exothermic heat from the turbine exhausted in the previous stage. The transformation from horizontal type to vertical type implements the segmented utilization of LNG cold energy, which improves the performance of the whole system.

Considering the fact that the condenser outlet temperature of LNG is still low and the corresponding part of LNG cold energy is not utilized yet, Choi et al. [ 22] have established a triple cascade Rankine cycle, as displayed in Fig. 8. In Cd1, LNG acts as the cold fluid and the out working fluid as the hot fluid; in Cd2, the central working fluid works as the hot fluid while LNG and outer medium provide cold capacity; in Cd3, only inner working fluid serves as hot fluid while the other three fluids absorb the heat released. Compared to the basic Rankine cycle, the thermal efficiency of the proposed one is significantly increased at the expense of higher systematic structural complexity and instability due to adopting the multi-stream heat exchangers.

A brief summary of main structural improvements of Rankine cycles [ 4, 5, 12, 14, 15, 21, 23– 28] is presented in Table 1.

Structural enhancement based on Brayton cycle

Brayton cycle, which makes use of LNG cold energy to reduce the compressor inlet gas temperature, significantly reduces the power consumption of the compressor under the condition of a specified compression ratio and improves the net work output of the circulation; the temperature curves of LNG regasification and working gas cooling match better than those in the Rankine cycle employing single working medium, which can effectively improve the cycle thermodynamics performance. The typical nitrogen Brayton cycle combining with LNG direct expansion is exhibited in Fig. 9. On one side, the compressed LNG passes cooling capacity to nitrogen, goes through heat exchanging with heat source and is fed to NG turbine to export power. On the other side, nitrogen obtaining cooling capacity enters the compressor at a lower temperature, and after heated by heat source it enters the turbine for power output in a state of high temperature and high pressure and then returns to the LNG-nitrogen heat exchanger.

Agazzani et al. [ 29] have proposed an improved Brayton cycle employing helium as the working fluid and heat of combustion as the heat source. A regenerator has been installed between the helium compressor and HX2 to increase the helium turbine inlet temperature, as presented in Fig. 10. Morosuk and Tsatsaronis [ 30] have built up a novel Brayton cycle, as shown in Fig. 11. After two-stage compression with intermediate cooling, air assists fuel burning in the combustion chamber combustion. Then combustion gas with a high temperature enters the gas turbine for power output and the exhaust provides the heat for helium Brayton cycle. LNG completes direct expansion through the NG turbine after vaporizing in HX1 and HX2. Dispenza et al. [ 31] have established another structure-enhanced Brayton cycle, using open NG combustion exhaust gas from two-stage expansion as the heat source. The overall power output of the proposed circulation comes from both the expansion of helium and NG, in which way the power-output ability of the whole cycle is improved. Other researchers also put forward cycles similar to Dispenza’s structure [ 32, 33].

Considering the fact that most of the turbine power output is consumed by compressor driving, which limits the net work output of the whole system, a cascade of compression and expansion becomes one of the approaches to modify Brayton cycle. Tomków and Cholewiński [ 34] has proposed a novel Brayton cycle with two-stage compression and two-stage expansion, as shown in Fig. 12. Both HX1 and HX2 are multi-stream heat exchangers, which provide intermediate heating for two-stage expansion and intermediate cooling for two-stage compression, respectively. In addition, the complex MGT (mirror gas turbine) cycle is recommended by Kaneko et al. [ 35]. In this proposed cycle, LNG is first employed for intercooling of four-stage air compression then fed to the NG turbine to export electrical power in gas state. NG out of the NG turbine burns with the compressed air in the combustion chamber then goes into the gas turbine. In spite of the superior performance compared to basic types, the structural complexity of MGT restricts its scope of practical application.

Structural enhancement based on Kalina cycle

The advantage of Kalina cycle which employs the non-azeotropic mixtures (ammonia-water is used mostly) as the working medium mainly lies in two aspects: the variable temperature evaporation of the working fluid reduces the irreversibility of endothermic process and the small temperature change in the condenser of basic working medium, which contains less solute, alleviates irreversibility of condensation process.

In recent researches, the working media used in Kalina cycle have been extended from ammonia-water to ethylene-propane mixture and trafluoromethane-propane mixture, etc. Structural enhancement based on Kalina cycle includes installing regenerator, two-stage expansion or combination with LNG direct expansion, etc. The main structures of Kalina cycles are summarized in Table 2 [ 4, 36– 39].

Figure 13 is a schematic diagram of a novel Kalina cycle combined with LNG direct expansion, adopting ammonia-water mixture as the working medium [ 36]. After absorbing heat from the heat source, ammonia-water mixture with a high temperature is fed into the separator which separates the mixture into rich ammonia vapor and weak ammonia-water solution. The rich ammonia vapor drives the medium turbine and the exhaust is condensed by LNG and boosted into the absorber, where the vapor is remixed with weak ammonia-water solution, which has completed heat exchanging with the LNG in the regenerator and the throttling. The NG turbine exhaust obtains heat from the remixed solution and is transported to user’s terminal while the remixed solution cooled by the LNG cold energy enters the working fluid booster and returns to exchange heat with the external heat source. In this loop, the rich ammonia vapor possesses a great ability to export power and the weak ammonia-water solution with a high temperature can provide the heat needed for LNG direct expansion, both of which increase the power output from the cycle.

Compound cycles

The Rankine cycle, Brayton cycle, Kalina cycle and LNG direct expansion are frequently integrated to form compound cycles which have a superior cycle performance with relatively complex structures. Some researchers have also established novel compound cycles combining with the gas turbine. The main structures of compound cycles are presented in Table 3 [ 2, 34, 40– 44]. The compound cycles raised by Zhang and Lior [ 42] and Tomków and cholewiński [ 34] are elaborated as follows, respectively.

Zhang and Lior [ 42] have set up a near-zero CO₂ emission thermal cycle combining the CO₂ super-critical Rankine cycle with the CO₂ Brayton cycle. As shown in Fig. 14, NG turbine exhaust burns with the product of air separator unit (ASU) in the combustion chamber and the combustion gas expands in the gas turbine and provides heat for CO₂ vaporization in three-channel and common heat exchangers successively. Then the mixture out of the common heat exchanger is fed to the separator which separates water out of the bottom and CO₂ out of the top. The cooled CO₂ goes through a two-stage compression and returns to the combustion chamber. One stream of the CO₂ from the low-level compressor completes the super-critical Rankine cycle. In the present compound cycle, LNG cold energy is used for cooling the inlet gases of the compressors and the CO₂ condensation, while the systematic power output is generated by the Rankine cycle and the gas turbine. One part of the CO₂ produced by NG combustion acts as the working fluid of the Rankine circulation and the other part is condensed into liquid for storage instead of direct discharge to the environment, which makes the whole cycle close to zero-CO₂ emission.

The integration of Rankine cycle and Kalina cycle is established by Tomków and cholewinński [ 34], as illustrated in Fig. 15. The reason for the employment of krypton-ethane mixture as working medium in the Kalina part is that the boiling point of krypton differs far away from that of ethane, which makes it easy to separate one from the other; besides, it is harder for the inert gas krypton to react with other components. Representative devices of Kalina cycle are the separator and the absorber. The former separates the krypton-ethane mixture into rich krypton gas which accomplishes two-stage expansion for power output and weak krypton solution which heats the mixture pump outlet liquid krypton-ethane solution. The latter remixes rich and weak krypton solution back to krypton-ethane mixture. The three-channel condenser realizes the connection between the propane Rankine cycles and the Kalina cycles, where LNG works as the cold fluid and propane and rich krypton turbine exhaust serve as the hot fluids.

Remarks and conclusions

The present paper approximately classified studies of the power generation cycle utilizing LNG cold energy into key factors affecting basic power generation cycles and structural enhancement. Thanks to the existing mature technology in basic loops, studies of key factors can effectively provide valuable guidance for deeper optimization of the practical projects. The great diversity of structural enhancement provides a new way for further modification of power generation cycle using LNG cold energy. Several aspects worthy of deeper development in the future mainly include:

1) Component analysis of organic mixed working fluid employed in Rankine cycle and the combine cycle

At present, employing organic mixed working medium in Rankine cycle and the combined cycle has become an important and feasible approach for cycle optimization. However, most existing researches are simply conducted at a fixed mixture component ratio, which does not make a detailed and systematical analysis of the selection of mixture components and determination of its ratio. The original intention of using mixture medium is to find a better matching of temperature curve between the working medium and LNG, so that the mixed working medium components will closely relate to LNG components to some degree. It will be of great significance to define the criteria of choosing mixture components and the component ratio according to different sources of LNG.

2) Combination of cycle simulation and experimental investigation

Current structural improvement mainly depends on the process simulation by software, which makes it worth noticing that devices in simulation process are always set to operate under an ideal condition and will consequently bring a larger deviation to evaluate the feasibility of the proposed cycle. It is difficult to establish experimental set-up in LNG power generation system, but it is of great importance to transform from plentiful theoretical studies to practical appliance. Therefore, the combination of cycle simulation and experimental investigation will pose great challenges but it is a right step toward the right direction for LNG cold energy power generation system in the future.

3) Feasible and economical analyses

For those theoretically mature cycles of LNG cold energy power generation, feasible and economical analyses can be supplemented for further practical implement, including type selection of vital devices, pipeline establishment of circulation, cost evaluation of equipment, calculation of annual power generation benefit and life of returning investment, etc.

References

[1] Chen L Q, Xu P L, Sun L, Dong W H, Ma K. Present situation analysis of LNG cold energy power generation technology. Natural Gas and Oil, 2013, 31(6): 39–44

[2] 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(3): 169–182

[3] Liu Y N, Guo K H. Efficiency of power generation by LNG cold energy. In: Proceedings of Power and Energy Engineering Conference (APPEEC), 2010 Asia-Pacific. IEEE, 2010, 1–4

[4] Bai F F. Integrated and cost-effective design utilizes LNG cryogenic energy for power generation. Dissertation for the Master Degree. Guangzhou, China: South China University of Technology, 2011

[5] Wang T. The theoretical study of working fluid for Rankine cycle utilizing cold energy of LNG. Dissertation for the Master Degree. Shanghai: Shanghai Jiao Tong University, 2011

[6] Lu Y W, Yang H C, Lv P F, Liu G L, Ma C F, Wu Y T. Parametric analysis and working fluid selection of power generation system based on LNG cold energy. Journal of Beijing University of Technology, 2011, 37(12): 1874–1879

[7] Liu Y N, Guo K H. Efficiency of power generation by LNG cold energy. Cryogenics and Superconductivity, 2010, 38(2): 13–17

[8] Zhang L, Gao W, Yu L M, Zhang X R, Yu S C. LNG cold energy optimization of working fluid power systems. Chemistry & Industry, 2015, 33(1): 25–29

[9] Kim C W, Chang S D, Ro S T. Analysis of the power cycle utilizing the cold energy of LNG. International Journal of Energy Research, 1995, 19(9): 741–749

[10] Zhu H M, Sun H, Shu D.Simulation of Rankine power cycle to recovery the cooling nature gas. Cryogenics and Superconductivity, 2010 (12): 12–14

[11] Chen M. Development and optimization of power generation air-conditioning utilizes LNG cryogenic energy. Dissertation for the Master Degree. Guangzhou: South China University of Technology, 2013

[12] Miyazaki T, Kang Y T, Akisawa A, Kashiwagi T. A combined power cycle using refuse incineration and LNG cold energy. Energy, 2000, 25(7): 639–655

[13] Gao L, Wang Y. A novel binary cycle with integration of low-level waste heat recovery with LNG cold energy utilization. Journal of Engineering Thermophysics, 2002, 23(4): 397–400

[14] Wang J, Yan Z, Wang M, Dai Y. Thermodynamic analysis and optimization of an ammonia-water power system with LNG (liquefied natural gas) as its heat sink. Energy, 2013, 50: 513–522

[15] Rao W J, Zhao L J, Liu C, Xu J L, Zhang M G. Research of organic Rankine cycle utilizing LNG cold exergy and waste heat. Journal of Engineering Thermophysics, 2014, 35(2): 213–217

[16] Xue X, Guo C, Du X, Yang L, Yang Y. Thermodynamic analysis and optimization of a two-stage organic Rankine cycle for liquefied natural gas cryogenic exergy recovery. Energy, 2015, 83: 778–787

[17] Franco A, Casarosa C. Thermodynamic analysis of direct expansion configurations for electricity production by LNG cold energy recovery. Applied Thermal Engineering, 2015, 78: 649–657

[18] Liang Q, Li Y Z, Tan H B. LNG dual power vehicles with air powered cycle utilization. Journal of Refrigeration, 2008, 29(4): 51–54

[19] Kim K H, Kim K C. Thermodynamic performance analysis of a combined power cycle using low grade heat source and LNG cold energy. Applied Thermal Engineering, 2014, 70(1): 50–60

[20] Huang M B, Lin W S, He H M, Gu A Z. LNG cold energy utilized in a transcritical CO₂ Rankine cycle and CO₂ recovery by liquefaction. Cryogenics and Superconductivity, 2009, (4): 17–21

[21] Yang H C. Optimization of power generation system based on cold energy of liquefied natural gas (LNG). Dissertation for the Master Degree. Beijing: Beijing University of Technology, 2010

[22] Choi I H, Lee S, Seo Y, Chang D. Analysis and optimization of cascade Rankine cycle for liquefied natural gas cold energy recovery. Energy, 2013, 61: 179–195

[23] Cao W S, Lu X S. Cryogenic power generation driven by low level heat for LNG terminals. Cryogenics and Superconductivity, 2012, 40(3): 17–20

[24] Wang J, Wang J, Dai Y, Zhao P. Thermodynamic analysis and optimization of a transcritical CO₂ geothermal power generation system based on the cold energy utilization of LNG. Applied Thermal Engineering, 2014, 70(1): 531–540

[25] Sun X H, Chen B D, Wang L, Song S X, Ma Y, Zhang G J. Cold energy power generation system at LNG satellite stations with solar energy as a high temperature heat source. Natural Gas Industry, 2012, 32(10): 103–106

[26] Sun H, Zhu H, Liu F, Ding H. Simulation and optimization of a novel Rankine power cycle for recovering cold energy from liquefied natural gas using a mixed working fluid. Energy, 2014, 70(3): 317–324

[27] Cao X Q, Zhao H, Yang W W, Zhou F, He Y L.A combined energy recovery system for comprehensive utilization of both low-grade waste heat and cold energy in LNG. Thermal Power Generation, 2014 (12): 49–55

[28] Zhu H M, Sun H, Li Z C, Zhang W H. A novel ethylene-propane cascade Rankine cycle for LNG-CCHP system and process simulations. Cryogenics and Superconductivity, 2011, 39(11): 27–32

[29] Agazzani A, Massardo A F, Korakianitis T. An assessment of the performance of closed cycles with and without heat rejection at cryogenic temperatures. Journal of Engineering for Gas Turbines and Power, 1999, 121(3): 458–465

[30] Morosuk T, Tsatsaronis G. Comparative evaluation of LNG-based cogeneration systems using advanced exergetic analysis. Energy, 2011, 36(6): 3771–3778

[31] Dispenza C, Dispenza G, La Rocca V, Panno G. Exergy recovery during LNG regasification: electric energy production–part two. Applied Thermal Engineering, 2009, 29(2-3): 388–399

[32] Tsatsaronis G, Morosuk T. Advanced exergetic analysis of a novel system for generating electricity and vaporizing liquefied natural gas. Energy, 2010, 35(2): 820–829

[33] Morosuk T, Tsatsaronis G, Boyano A, Gantiva C. Advanced exergy-based analyses applied to a system including LNG regasification and electricity generation. International Journal of Energy and Environmental Engineering, 2012, 3(1): 1–9

[34] Tomków Ł, Cholewiński M. Improvement of the LNG (liquid natural gas) regasification efficiency by utilizing the cold exergy with a coupled absorption–ORC (organic Rankine cycle). Energy, 2015, 87: 645–653

[35] Kaneko K, Ohtani K, Tsujikawa Y, Fujii S. Utilization of the cryogenic exergy of LNG by a mirror gas-turbine. Applied Energy, 2004, 79(4): 355–369

[36] Wang H, Shi X, Che D. Thermodynamic optimization of the operating parameters for a combined power cycle utilizing low-temperature waste heat and LNG cold energy. Applied Thermal Engineering, 2013, 59(1-2): 490–497

[37] Liu Y N, Guo K H. A novel cryogenic power cycle for LNG cold energy recovery. Journal of Engineering Thermophysics, 2012, 33(011): 1860–1863

[38] Liu Y N, Guo K H. A novel cryogenic power cycle for LNG cold energy recovery. Energy, 2011, 36(5): 2828–2833

[39] Shi X, Che D. A combined power cycle utilizing low-temperature waste heat and LNG cold energy. Energy Conversion and Management, 2009, 50(3): 567–575

[40] Xia G W, Bai F F, Zhang Z. A newly-developed process for cold energy utilization based on middle-low-temperature waste heat recovery. Natural Gas Industry, 2008, 28(5): 112–114

[41] Lu T, Wang K S. Analysis and optimization of a cascading power cycle with liquefied natural gas (LNG) cold energy recovery. Applied Thermal Engineering, 2009, 29(8-9): 1478–1484

[42] Zhang N, Lior N. A novel near-zero CO₂ emission thermal cycle with LNG cryogenic exergy utilization. Energy, 2006, 31(10-11): 1666–1679

[43] Shi X, Agnew B, Che D. Analysis of a combined cycle power plant integrated with a liquid natural gas gasification and power generation system. Proceedings of the Institution of Mechanical Engineers. Part A, Journal of Power and Energy, 2011, 225(1): 1–11

[44] Najjar Y S H. Efficient use of energy by utilizing gas turbine combined systems. Applied Thermal Engineering, 2001, 21(4): 407–438

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg