Utilization of the cryogenic exergy of LNG by a mirror gas-turbine

In the course of worldwide efforts to suppress global warming, the saving of energy becomes more important. Recently, LNG (liquefied natural gas) terminals in our country have received more than 50 million tons of LNG per year. Therefore, the utilization of the cryogenic exergy in connection with the regasification of LNG gains more and more importance. The aim of this paper is the recovery of the energy consumed in liquefaction using the MGT (mirror gas-turbine), which is a new kind of combined cycle of a conventional gas-turbine worked as a topping cycle and TG (inverted Brayton cycle) as a bottoming cycle. The optimum characteristics have been calculated and it is shown that this cycle is superior to the current-use gasification systems in employing seawater heat in terms of thermal efficiency and specific output. In the present cycle, the cold LNG is used to cool the exhaust gas from a turbine of a TG, and then the exergy of the liquefied natural gas is transformed, with a very high efficiency, to electric energy. The main feature of this new concept is the removal of an evaporation system using seawater.     

Analysis of a combined power and refrigeration cycle by the exergy method

The exergy analysis method was applied in order to evaluate the new combined cycle proposed by Goswami [Solar thermal technology: present status and ideas for the future. Energy Sources 1998;20:137–45], using Hasan–Goswami–Vijayaraghavan parameters. This new combined cycle was proposed to produce both power and cooling simultaneously with only one heat source and using ammonia–water mixture as the working fluid. The simulation of the cycle was carried out in the process simulator ASPEN Plus. The Redlich–Kwong–Soave equation of state was used to calculate the thermodynamic properties. The cycle was simulated as a reversible as well as an irreversible process to clearly show the effect of the irreversibilities in each component of the cycle. At the irreversible process two cases were considered, changing the environmental temperature. However, in order to know the performance of the new cycle at different conditions of operation, the second irreversible case was analyzed varying the rectification temperatures, the isentropic efficiency of the turbine and the return temperature of the chilled water. Exergy effectiveness values of 53% and 51% were obtained for the irreversible cycles; with heat input requirements at temperatures of 125 and 150 °C. Solar collectors or waste heat are suggested as heat sources to operate the cycle.     

Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system

This paper has proposed an improved liquefied natural gas (LNG) fuelled combined cycle power plant with a waste heat recovery and utilization system. The proposed combined cycle, which provides power outputs and thermal energy, consists of the gas/steam combined cycle, the subsystem utilizing the latent heat of spent steam from the steam turbine to vaporize LNG, the subsystem that recovers both the sensible heat and the latent heat of water vapour in the exhaust gas from the heat recovery steam generator (HRSG) by installing a condensing heat exchanger, and the HRSG waste heat utilization subsystem. The conventional combined cycle and the proposed combined cycle are modelled, considering mass, energy and exergy balances for every component and both energy and exergy analyses are conducted. Parametric analyses are performed for the proposed combined cycle to evaluate the effects of several factors, such as the gas turbine inlet temperature (TIT), the condenser pressure, the pinch point temperature difference of the condensing heat exchanger and the fuel gas heating temperature on the performance of the proposed combined cycle through simulation calculations. The results show that the net electrical efficiency and the exergy efficiency of the proposed combined cycle can be increased by 1.6 and 2.84% than those of the conventional combined cycle, respectively. The heat recovery per kg of flue gas is equal to 86.27 kJ s^?1. One MW of electric power for operating sea water pumps can be saved. The net electrical efficiency and the heat recovery ratio increase as the condenser pressure decreases. The higher heat recovery from the HRSG exit flue gas is achieved at higher gas TIT and at lower pinch point temperature of the condensing heat exchanger. Copyright © 2006 John Wiley & Sons, Ltd.     

LNG冷yong动力系统的工质选择及系统分析方法

阐述了LNG冷yong动力系统的工质选择原则及系统分析的基本方法。     

Process Study and Exergy Analysis of a Novel Air Separation Process Cooled by LNG Cold Energy简

In order to resolve the problems of the current air separation process such as the complex process, cumbersome operation and high operating costs, a novel air separation process cooled by LNG cold energy is proposed in this paper, which is based on high-efficiency heat exchanger network and chemical packing separation technology. The operating temperature range of LNG cold energy is widened from 133K-203K to 113K-283K by highefficiency heat exchanger network and air separation pressure is declined from 0.5MPa to about 0.35MPa due to packing separation technology, thereby greatly improve the energy efficiency. Both the traditional and novel air separation processes are simulated with air handling capacity of 20t·h 1. Comparing with the traditional process, the LNG consumption is reduced by 44.2%, power consumption decrease is 211.5 kWh per hour, which means the annual benefit will be up to 1.218 million CNY. And the exergy efficiency is also improved by 42.5%.     

Thermodynamic optimization of the life cycle of plastics by exergy analysis

Decision‐makers should strive for an efficient employment of resources for the whole industrial ecological system. The metabolization of resources should be optimized in thermodynamic terms. This contribution presents an effort to use exergy analysis as a quantitative tool in the thermodynamic optimization of the life cycle of plastics. Quantitative results on overall exergetic efficiencies of different industrial metabolic options are presented. Therefore, different waste treatment options of plastics will be considered, taking into account interacting industrial activities, such as virgin plastics production and heat and electricity production starting from natural resources. Copyright © 2004 John Wiley & Sons, Ltd.     

Evaluation of a chemical-looping-combustion power-generation system by graphic exergy analysis

A loop of chemical reactions is introduced to reduce the exergy loss caused by the conversion of fuel energy to thermal energy in conventional LNG powerplants. By applying this chemical loop and the graphic simulator which generates energy-utilization diagrams, a new gasturbine power-generation system called a chemical-looping-combustion system (CLCS) is composed. Exergy analysis of a model system shows an expected thermal powerplant efficiency as high as 50.2%(LHV).     

On the exergy of radiation

In the domain of heat radiation, and particularly of solar energy use, the notion of exergy (or alternatively entropy) of radiation has given rise to a fairly abundant literature; unfortunately, incoherencies and discrepancies between the various authors could lead to a complete disuse of the notion in this context. The aim of this paper is to contribute to a clarification of this issue.We propose here a derivation of the exergy of radiation, based solely on classical thermodynamics notions, making thus possible a ready check of the validity of the results. Results are given first for the blackbody case, then extended to a radiation with arbitrary spectrum. Finally, application of the notion of radiation entropy to a few simple examples shows the consistency of this notion with some well-known physical laws, and can even give some insight into the real signification of these laws.     

Optimization of heat recovery steam generator through exergy analysis for combined cycle gas turbine power plants

This paper proposes a new approach to finding the optimum design parameters of the heat recovery steam generator (HRSG) system to maximize the efficiency of the steam turbine (bottom) cycle of the combined cycle power plant (CCPP), but without performing the bottom cycle analysis. This could be achieved by minimizing the unavailable exergy (the sum of the destroyed and the lost exergies) resulted from the heat transfer process of the HRSG system. The present approach is relatively simple and straightforward because the process of the trial-and-error method, typical in performing the bottom cycle analysis for the system optimization, could be avoided. To demonstrate the usefulness of the present method, a single-stage HRSG system was chosen, and the optimum evaporation temperature was obtained corresponding to maximum useful work for given conditions of water and gas temperatures at the inlets of the HRSG system. Results show that the optimum evaporation temperature obtained based on the present exergy analysis appears similar to that based on the bottom cycle analysis. Also shown is the dependency of number of transfer unit (NTU) on the evaporation temperature, which is another important factor in determining the optimum condition when the construction cost is taken into account in addition to the operating cost. The present approach turned out to be a powerful tool for optimization of the single-stage HRSG systems and can easily be extended to multi-stage systems. Copyright © 2008 John Wiley & Sons, Ltd.     

空调系统热回收器的火用分析

利用热力学原理和火用分析方法,建立空调系统热回收器系统的分析模型,对目前应用的暖通空调系统中热回收技术进行火用分析,得出对于热回收节能技术进行全面、合理的科学分析与评价方法。总结空调系统应用热回收器的一些注意事项。     

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.     

Energy and exergy analyses of a CFB‐based indirectly fired combined cycle power generation system

Combined cycle configuration has the ability to use the waste heat from the gas turbine exhaust gas using the heat recovery steam generator for the bottoming steam cycle. In the current study, a natural gas‐fired combined cycle with indirectly fired heating for additional work output is investigated for configurations with and without reheat combustor (RHC) in the gas turbine. The mass flow rate of coal for the indirect‐firing mode in circulating fluidized bed (CFB) combustor is estimated based on fixed natural gas input for the gas turbine combustion chamber (GTCC). The effects of pressure ratio, gas turbine inlet temperature, inlet temperatures to the air compressor and to the GTCC on the overall cycle performance of the combined cycle configuration are analysed. The combined cycle efficiency increases with pressure ratio up to the optimum value. Both efficiency and net work output for the combined cycle increase with gas turbine inlet temperature. The efficiency decreases with increase in the air compressor inlet temperature. The indirect firing of coal shows reduced use with increase in the turbine inlet temperature due to increase in the use of natural gas. There is little variation in the efficiency with increase in GTCC inlet temperature resulting in increased use of coal. The combined cycle having the two‐stage gas turbine with RHC has significantly higher efficiency and net work output compared with the cycle without RHC. The exergetic efficiency also increases with increase in the gas turbine inlet temperature. The exergy destruction is highest for the CFB combustor followed by the GTCC. The analyses show that the indirectly fired mode of the combined cycle offers better performance and opportunities for additional net work output by using solid fuels (coal in this case). Copyright © 2009 John Wiley & Sons, Ltd.     

Energy and exergy analysis of a micro-compressed air energy storage and air cycle heating and cooling system

Energy storage systems are becoming more important for load leveling, especially for widespread use of intermittent renewable energy. Compressed air energy storage (CAES) is a promising method for energy storage, but large scale CAES is dependent on suitable underground geology. Micro-CAES with man-made air vessels is a more adaptable solution for distributed future power networks. In this paper, energy and exergy analyses of a micro-CAES system are performed, and, to improve the efficiency of the system, some innovative ideas are introduced. The results show that a micro-CAES system could be a very effective system for distributed power networks as a combination that provides energy storage, generation with various heat sources, and an air-cycle heating and cooling system, with a energy density feasible for distributed energy storage and a good efficiency due to the multipurpose system. Especially, quasi-isothermal compression and expansion concepts result in the best exergy efficiencies.     

Evaluation of energy efficiency in biofuel drying by means of energy and exergy analyses

The calculation of heat consumption is based on the First Law and it gives quantitative information about the energy used in drying. However, it does not pay any attention to the quality of the energy used in drying. To take into account the quality of the energy, attention must be paid to the Second Law, too. Especially in those cases where the energy used in drying may be converted to mechanical work, it is important to consider the Second Law is. In this paper, the energy efficiency of biofuel drying in a pulp and paper mill is evaluated on the basis of energy and exergy analysis. The evaluation is based on the determination of the heat consumption and the irreversibility rate for energy and exergy analysis, respectively. The evaluation methods are applied to two different drying systems, single-stage-drying with partial recycle of spent air, and multi-stage-drying. Both drying systems are also provided with a heat recovery unit in which the inlet air is pre-heated using the outlet air of the dryer. There are two alternative heat sources available for the drying energy, steam at a pressure of 3 bar and water at a temperature of 80 °C. The results show that the heat consumption is only dependent to a small extent on the heat source type or the drying system. On the other hand, the irreversibility rate depends to a considerable on the heat source and the drying system.     

An exergy analysis of a solar-driven ejector refrigeration system

Exergy analysis is used as a tool to analyse the performance of an ejector refrigeration cycle driven by solar energy. The analysis is based on the following conditions: a solar radiation of 700 W/m², an evaporator temperature of 10 °C, a cooling capacity of 5 kW, butane as the refrigerant in the refrigeration cycle and ambient temperature of 30 °C as the reference temperature. Irreversibilities occur among components and depend on the operating temperatures. The most significant losses in the system are in the solar collector and the ejector. The latter decreases inversely proportional to the evaporation temperature and dominates the total losses within the system. The optimum generating temperature for a specific evaporation temperature is obtained when the total losses in the system are minimized. For the above operating conditions, the optimum generating temperature is about 80 °C.     

Utilization of the cryogenic exergy of liquid natural gas (LNG) for the production of electricity

Liquid natural gas (LNG) delivered by means of sea-ships is compressed and then evaporated before its introduction to the system of pipelines. The possibilities of the utilization of cryogenic exergy of LNG for electricity production without any additional combustion of any its portion, have been analyzed. Three variants of the plant have been investigated. A cascade system with two working fluids has been analyzed in two first of them. The economic optimization proved that the optimum temperature difference in the LNG evaporation is higher than initially assumed. Therefore, a third variant of the plant has been analyzed, with ethane as a single working fluid. Only the third variant has been analyzed in detail.     

On the exergy analysis of power plants

The thermal performance of power generating and consuming devices can be improved significantly, both during design and operation. This is especially important in eastern and central European countries during their transition to a market environment. A solution can be sought by combining exergy and economic analyses. The performances of conventional power plants and nuclear power plants are discussed, based on the exergy concept. It is proposed to define the entire nuclear plant efficiency by the system coefficient of performance.     

Conventional and enhanced exergy analysis of a hydrogen liquefaction system

In the present work, conventional and enhanced exergy analyses were applied to the cryogenic liquefaction process of hydrogen gas. The hydrogen liquefaction unit consists of a multi-stage compressor, booster compressor-turbine pair, and heat exchanger block. Convectional exergy analysis cannot identify parts of exergy inefficiencies. In addition, by convectional exergy analysis, it cannot determine inevitable exergy losses that occur due to technological limits. For this reason, enhanced exergy analysis should be applied to the system. The exergy destruction affecting the exergy efficiency of the hydrogen liquefaction unit was investigated in detail. This study suggests an enhanced exergy analysis of a cryogenic liquefaction system. According to the results of the convectional exergy analysis, exergy efficiency of the whole liquefaction process are 32.22%. Also, the highest and lowest endogenous exergy destruction among whole components is calculated as 9563 kW and 92.83 kW in the turbine and CM-1, respectively. With these calculated results, the potential for improvement in the turbine in the liquefaction system was found to be high.     

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.