Analysis of the power cycle utilizing the cold energy of LNG

The aim of this study is to analyse the performance of the Rankine power cycles operating with the LNG as the heat sink and with the seawater as the heat source. A model for the power cycle utilizing the cold energy of the LNG is established and a cycle simulation is carried out to analyse the performance characteristics. The analysis reveals that there exist optimum values in the condenser-outlet temperatures of the LNG and the ratio of heat transfer capacity of the condenser to the total capacity of the condenser and the vapour generator. An additional finding of this study is that near the point of maximum net work, the heat transfer capacity of the vapour generator becomes larger than that of the condenser, as opposed to the cases of a general Rankine cycle. Also the results of this study illuminate several advantages of using binary mixtures as working fluids over the use of pure substances.     

Thermodynamic analysis of a new double-pressure condensation power generation system recovering LNG cold energy for hydrogen production

Cold energy during the LNG regasification process is usually applied for power generation, but the electricity demand varies with the time. Therefore, a thought that transforming electrical energy into hydrogen energy by PEM electrolyzer is put forward to adjust the adaptability of power output to electricity demand. This paper proposes a new double-pressure condensation Rankine cycle integrated with PEM electrolyzer for hydrogen production. In this system, seawater is used as the heat source, and binary mixed working fluids are applied. Meanwhile, multi-stream heat exchanger is introduced to improve the irreversibility of heat transfer between LNG and working fluid. The key system parameters, including seawater temperature, the first-stage condensation temperature, the second-stage condensation temperature, and outlet temperature of LNG, are studied to clarify their effects on net power generation, hydrogen production rate and energy efficiency. Furthermore, the hydrogen production rate is as the objective function, these parameters are optimized by genetic algorithm. Results show that seawater temperature has positive impact on the net power output and hydrogen production rate. The first-stage condensation temperature, the second-stage condensation temperature, and outlet temperature of LNG have diverse effects on the system performance. Under the optimal working conditions, when the LNG regasification pressure are 600, 2500, 3000 and 7000 kPa, the increasing rate for optimized net power output, hydrogen production rate and energy efficiency are more than 11.68%, 11.67% and 8.88%, respectively. The cost of hydrogen production with the proposed system varies from 1.93 $/kg H2 to 2.88 $/kg H2 when LNG regasification pressure changes from 600 kPa to 7000 kPa.     

Exergy analysis of liquefied natural gas cold energy recovering cycles

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

A novel cryogenic power cycle for LNG cold energy recovery

A novel cryogenic cycle by using a binary mixture as working fluids and combined with a vapor absorption process was proposed to improve the energy recovery efficiency of an LNG (liquefied natural gas) cold power generation. The cycle was simulated with seawater as the heat source and LNG as the heat sink, and the optimization of the power generated per unit LNG was performed. Tetrafluoromethane (CF₄) and propane (C₃H₈) were employed as the working fluids. The effects of the working fluid composition, the recirculation rate of the C₃H₈-rich solution and the turbine intermediate pressure were investigated. In the cryogenic absorber, the C₃H₈-rich liquid absorbs the CF₄-rich vapor so that the mixture exhausting from the turbine can be fully condensed at a reduced pressure. This reduction of turbine back pressure can considerably improve the cycle efficiency. The presented cycle was compared with the C₃H₈ ORC (organic Rankine cycle), to show such performance improvement. It is found that the novel cycle is considerably superior to the ORC. The efficiency is increased by 66.3% and the optimized LNG recovery temperature is around −60 °C.     

High efficiency power plant with liquefied natural gas cold energy utilization

This article proposes a novel power plant comprising a closed Brayton cycle (CBC) and a Rankine cycle (RC) coupled in series with respect to the flue gases instead of a conventional combined cycle, where the cold energy of the LNG is used to cool the CBC compressor suction. The research study focuses on finding working fluids best suited to the proposed CBC–RC plant and on achieving high efficiency. The proposed working fluids that fulfil the requirements for the CBC are He, N₂ and for the RC are CO₂, ammonia, ethanol or water. An analysis of the power plant using different working fluids is carried out and it is ascertained that the best efficiency conditions for the CBC are achieved with He and CO₂ for the RC. As a result, a thermal efficiency of 67·60%, an overall efficiency of 55·13% and a specific power of 2·465 MW/(kg s⁻¹ LNG) is achieved.     

Design and Optimization of a Full-Generation System for Marine LNG Cold Energy Cascade Utilization

This paper took a 100,000 DWT LNG fuel powered ship as the research object. Based on the idea of "temperature matching, cascade utilization" and combined with the application conditions of the ship, a horizontal three-level nested Rankine cycle full-generation system which combined the high-temperature waste heat of the main engine flue gas with the low-temperature cold energy of LNG was proposed in this paper. Furthermore, based on the analysis and selection of the parameters which had high sen...     

A CO2 cryogenic capture system for flue gas of an LNG-fired power plant

In the present paper, a CO₂ cryogenic capture for flue gas of an LNG-fired power generation system is proposed, in which LNG cold energy can be fully utilized during the gasification process. First of all, the flue gas is compressed to facilitate the CO₂ solid formation and separation. Sequentially, the CO₂-removed flue gas expands to supply most of the cold energy needed for the cryogenic process. In comparison with traditional CO₂-capture systems in LNG-fired power generation cycle, the new system does not require gasifying excessive amount of LNG. Based on the HYSYS simulation, the CO₂ capture pressure and temperature are investigated as the key parameters to find the appropriate working conditions of the CO₂-capture system. The results show that the system can achieve a 90% CO₂ recovery rate or higher if the flue gas temperature can be lowered to less than ?140 °C.     

低温热能发电的研究现状和发展趋势

低温热能种类繁多,数量巨大,利用这部分能源意义重大。介绍了低温热能发电技术的研究现状和发展趋势。低温热能发电技术主要应用于太阳能热电、工业余热发电、地热发电、生物质能发电、海洋温差发电等方面。现阶段低温热能发电的研究重点有:工质的热物性和环保性能、循环优化研究;提高低温热能发电效率的研究,包括混合工质循环、Kalina循环、回热、氨吸收式动力制冷循环等;基于有限时间热力学的系统最优控制等方面的研究。     

温差发电和动力装置联合回收液化天然气冷能

对液化天然气(LNG)冷能的回收,提出了温差发电器与动力装置联合的回收系统,对系统的各个状态 参数和转化能量及其效率进行了分析计算。计算显示甲烷在天然气中的摩尔含量会显著地影响功量的输出,但 对系统的效率影响不大。系统对LNG最大可用能的回收效率可达29％。     

Analysis of zeotropic mixtures used in low-temperature solar Rankine cycles for power generation

This paper presents the analysis of low-temperature solar Rankine cycles for power generation using zeotropic mixtures. Three typical mass fractions 0.9/0.1 (Ma) 0.65/0.35 (Mb), 0.45/0.55 (Mc) of R245fa/R152a are chosen. In the proposed temperature range from 25 °C to 85 °C, the three zeotropic mixtures are investigated as the working fluids of the low-temperature solar Rankine cycle. Because there is an obvious temperature glide during phase change for zeotropic mixtures, an internal heat exchanger (IHE) is introduced to the Rankine cycle. Investigation shows that different from the pure fluids, among the proposed zeotropic mixtures, the isentropic working fluid Mb possesses the lowest Rankine cycle efficiency. For zeotropic mixtures a significant increase of thermal efficiencies can be gained when superheating is combined with IHE. It is also indicated that utilizing zeotropic mixtures can extend the range of choosing working fluids for low-temperature solar Rankine cycles.     

Absorption power cycles for low‐temperature heat sources using aqueous salt solutions as working fluids

There are many low‐temperature heat sources; however, current technologies for their utilization have a relatively low efficiency and high cost. The leading technology in the low‐temperature domain for heat‐to‐work conversion is the organic Rankine cycle (ORC). Absorption power cycles (APCs) are a second option. Nearly all currently known APCs, most importantly the Kalina cycle, use a water‐ammonia mixture as their working fluids. This paper offers a theoretical exploration of the possibility of utilizing aqueous solutions of three salts (lithium bromide, lithium chloride and calcium chloride), known mainly from absorption cooling, as working fluids for APCs. The cycles are compared with a typical steam Rankine cycle, a water‐ammonia APC, and (subcritical) ORCs with a range of working fluids explored. The analysis includes a parasitic load for heat rejection by a cooling tower or air‐cooled condenser. The absorption cycles exhibit better performance than all Rankine‐based cycles analysed in temperatures below 120°C. For the LiBr‐based APC, a detailed thermal design of the cycle is provided for 100°C water as a heat source and a sensitivity analysis is performed of the parameters controlling the main cycle. Mechanical design considerations should not pose a problem for small power units, especially in the case of expansion machines, which are often problematic in ORCs. The salt‐based APCs also carry environmental benefits, as the salts utilized in the working fluids are non‐toxic. Copyright © 2016 John Wiley & Sons, Ltd.     

On the recovery of LNG physical exergy by means of a simple cycle or a complex system

The maximum and minimum temperatures available limit the usable fraction (or Carnot efficiency) of a power cycle. The construction of LNG terminals and the need to vaporize LNG offers a thermal sink at a very much lower temperature than seawater. By using this thermal sink in a combined plant, it is possible to recover power from the vaporization of LNG.To this purpose, in this paper combined systems using LNG vaporization as low-temperature thermal sink are considered and their pros and cons are presented. A system utilizing waste energy as heat source and with a single working fluid is analyzed in detail. However, the use of a single fluid is not the best solution from a thermodynamic point of view. Thus, a series of cascading cycles is also outlined. In these systems, both the thermal source and the thermal sink are exploited as exergy sources.     

Analysis and optimization of a cascading power cycle with liquefied natural gas (LNG) cold energy recovery

The effective utilization of the cryogenic energy associated with LNG vaporization is quite important. In this paper a cascading power cycle with LNG directly expanding consisting of a Rankine cycle with ammonia–water as working fluid and a power cycle of combustion gas is proposed to recover cryogenic energy of LNG. Energy equilibrium equations and exergy equilibrium equations of each equipment in the cascading power cycle are established. Taken some operating parameters as key parameters, influences of these parameters on thermal efficiency and exergy efficiency of the cascading power cycle were analyzed. Optimization of the cascading power cycle with maximum economic benefits as objective function together with optimum variables and constraint conditions was solved. The optimum objective and variables were achieved.     

Candidate radial-inflow turbines and high-density working fluids for geothermal power systems

Optimisation of Organic Rankine Cycle (ORCs) for binary-cycle geothermal applications could play a major role in determining the competitiveness of low to moderate temperature geothermal resources. Part of this optimisation process is matching cycles to a given resource such that power output can be maximised. Two major and largely interrelated components of the cycle are the working fluid and the turbine. Both components need careful consideration: the selection of working fluid and appropriate operating conditions as well as optimisation of the turbine design for those conditions will determine the amount of power that can be extracted from a resource. In this paper, we present the rationale for the use of radial-inflow turbines for ORC applications and the preliminary design of several radial-inflow machines based on a number of promising ORC systems that use five different working fluids: R134a, R143a, R236fa, R245fa and n-Pentane. Preliminary meanline analysis lead to the generation of turbine designs for the various cycles with similar efficiencies (77%) but large differences in dimensions (139-289 mm rotor diameter). The highest performing cycle, based on R134a, was found to produce 33% more net power from a 150 °C resource flowing at 10 kg/s than the lowest performing cycle, based on n-Pentane.     

Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sources

A cost-effective optimum design criterion for Organic Rankine power cycles utilizing low-temperature geothermal heat sources is presented. The ratio of the total heat exchanger area to net power output is used as the objective function and was optimized using the steepest descent method. Evaporation and condensation temperatures, geothermal and cooling water velocities are varied in the optimization method. The optimum cycle performance is evaluated and compared for working fluids that include ammonia, HCFC123, n-Pentane and PF5050. The optimization method converges to a unique solution for specific values of evaporation and condensation temperatures and geothermal and cooling water velocities. The choice of working fluid can be greatly affect the objective function which is a measure of power plant cost and in some instances the difference could be more than twice. Ammonia has minimum objective function and maximum geothermal water utilization, but not necessarily maximum cycle efficiency. Exergy analysis shows that efficiency of the ammonia cycle has been largely compromised in the optimization process than that of other working fluids. The fluids, HCFC 123 and n-Pentane, have better performance than PF 5050, although the latter has most preferable physical and chemical characteristics compared to other fluids considered.     

Exergy recovery in regasification facilities – Cold utilization: A modular unit

The paper deals with the problem of cold recovery for direct utilization both in the site of regasification facility and far from it.A modular LNG regasification unit is proposed having the regasification capacity of 2 billion standard cubic meters/year of gas. The modular plant is based on use of a power cycle working with ethane or ethylene which allows operation of cold energy transfer, contained in LNG to be regasified, in a range of temperatures suitable for multipurpose use of cold, reducing regasification process irreversibility. Some electric energy is produced by the power cycle, but the own mission of modular unit proposed is addressed to deliver cold suitable for industrial and commercial use in the proper temperature range. The option considers, also, the use of carbon dioxide as a secondary fluid for transfer of cold from regasification site to far end users. This option seems very attractive due to expected wide future exploitation of LNG regasification in the world.Results of a detailed thermodynamic and economic analysis demonstrate the suitability of the proposal.     

有出口温度限制的热源亚临界有机朗肯循环最佳回热度定量准则的研究

我国的余热资源和可再生能源丰富,但部分余热资源和可再生能源分布比较分散,并存在温度和能量密度均较低的问题.基于传统能源转化技术,利用温度较低的余热资源和能量密度较低的可再生能源进行发电,会降低余热资源和可再生能源的热功转换效率.有机朗肯循环(ORC)系统可以有效利用低温热能进行发电.对于不同温度和形式的热源,采用合适的...     

Design of an integrated process for simultaneous chemical looping hydrogen production and electricity generation with CO2 capture

This study was aimed at proposing a novel integrated process for co-production of hydrogen and electricity through integrating biomass gasification, chemical looping combustion, and electrical power generation cycle with CO₂ capture. Syngas obtained from biomass gasification was used as fuel for chemical looping combustion process. Calcium oxide metal oxide was used as oxygen carrier in the chemical looping system. The effluent stream of the chemical looping system was then transferred through a bottoming power generation cycle with carbon capture capability. The products achieved through the proposed process were highly-pure hydrogen and electricity generated by chemical looping and power generation cycle, respectively. Moreover, LNG cold energy was used as heat sink to improve the electrical power generation efficiency of the process. Sensitivity analysis was also carried out to scrutinize the effects of influential parameters, i.e., carbonator temperature, steam/biomass ratio, gasification temperature, gas turbine inlet stream temperature, and liquefied natural gas (LNG) flow rate on the plant performance. Overall, the optimum heat integration was achieved among the sub-systems of the plant while a high energy efficiency and zero CO₂ emission were also accomplished. The findings of the present study could assist future investigations in analyzing the performance of integrated processes and in investigating optimal operating conditions of such systems.     

Thermodynamic analysis of the CO2‐based Rankine cycle powered by solar energy

Using carbon dioxide as working fluid receives increasing interest since the Kyoto Protocol. In this paper, thermodynamic analysis was conducted for proposed CO₂‐based Rankine cycle powered by solar energy. It can be used to provide power output, refrigeration and hot water. Carbon dioxide is used as working fluid with supercritical state in solar collector. Theoretical analysis was carried out to investigate performances of the CO₂‐based Rankine cycle. The interest was focused on comparison of the performance with that of solar cell and those when using other fluids as working fluids. In addition, the performance and characteristics of the thermodynamic cycle are studied for different seasons. The obtained results show that using CO₂ as working fluid in the Rankine cycle owns maximal thermal efficiency when the working temperature is lower than 250.0°C. The power generation efficiency is about 8%, which is comparable with that of solar cells. But in addition to power generation, the CO₂‐based solar utilization system can also supply thermal energy. Copyright © 2007 John Wiley & Sons, Ltd.     

A comparative thermodynamic analysis of Kalina and organic Rankine cycles for hot dry rock: a prospect study in the Gonghe Basin

Hot dry rock is a new type of geothermal resource which has a promising application prospect in China. This paper conducted a comparative research on performance evaluation of two eligible bottoming cycles for a hot dry rock power plant in the Gonghe Basin. Based on the given heat production conditions, a Kalina cycle and three organic Rankine cycles were tested respectively with different ammonia-water mixtures of seven ammonia mass fractions and nine eco-friendly working fluids. The results show that the optimal ammonia mass fraction is 82% for the proposed bottoming Kalina cycle in view of maximum net power output. Thermodynamic analysis suggests that wet fluids should be supercritical while dry fluids should be saturated at the inlet of turbine, respectively. The maximum net power output of the organic Rankine cycle with dry fluids expanding from saturated state is higher than that of the other organic Rankine cycle combinations, and is far higher than the maximum net power output in all tested Kalina cycle cases. Under the given heat production conditions of hot dry rock resource in the Gonghe Basin, the saturated organic Rankine cycle with the dry fluid butane as working fluid generates the largest amount of net power.