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.     

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%.     

A real column design exergy optimization of a cryogenic air separation unit

Distillation columns are one of the main methods used for separating air components. Their inconvenient is their high energy consumption. The distillation process is simulated in three types of columns and the exergy losses in the different parts calculated. A sensitivity analysis is realized in order to optimize the geometric and the operational parameters of each type of column. A comparative exergy analysis between the distillation columns considered for cryogenic air separation shows that the exergy efficiency of a double diabatic column, with heat transfer all through the length of the column, is 23% higher than that of the conventional adiabatic double columns.     

Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes

A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency. A detailed model of a plate-fin heat exchanger is presented that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.     

An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes

Four processes for small-scale liquefied natural gas (LNG) production are evaluated. These include a single-stage mixed refrigerant (SMR), a two-stage expander nitrogen refrigerant and two open-loop expander processes. Steady-state simulations were undertaken to ensure that each process was compared on an identical basis, was fully optimised and was in agreement with published results. Composite curves for the feed and recycle streams and the refrigerant or cold recycle stream showed the degree of optimisation available within each process. The full exergy analysis showed the relative contributions to the total shaft work requirements, with the lowest being the SMR process. The lower efficiency of the expander-driven compressors is the main difference between processes. A more general comparison suggested that the nitrogen refrigerant process and the New LNG open-loop process are the leading candidates for offshore compact LNG production.     

Emergy as a function of exergy

This paper aims to clarify some aspects of the discussion between “emergists” and “exergists”. First, we address the problem of the differences between energy-based emergy and exergy-based emergy: we show that the two are proportional, having the exergetic equivalent of solar energy as scale factor. In the second part, we show that emergy and transformity can be written as a function of exergy alone, in particular of “partial” efficiencies of the processes involved in a production system, from solar energy to the final product.     

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.     

Performance and parametric investigation of a binary geothermal power plant by exergy

Exergy analysis of a binary geothermal power plant is performed using actual plant data to assess the plant performance and pinpoint sites of primary exergy destruction. Exergy destruction throughout the plant is quantified and illustrated using an exergy diagram, and compared to the energy diagram. The sites with greater exergy destructions include brine reinjection, heat exchanger and condenser losses. Exergetic efficiencies of major plant components are determined in an attempt to assess their individual performances. The energy and exergy efficiencies of the plant are 4.5% and 21.7%, respectively, based on the energy and exergy of geothermal water at the heat exchanger inlet. The energy and exergy efficiencies are 10.2% and 33.5%, respectively, based on the heat input and exergy input to the binary Rankine cycle. The effects of turbine inlet pressure and temperature and the condenser pressure on the exergy and energy efficiencies, the net power output and the brine reinjection temperature are investigated and the trends are explained.     

Solution of the inverse heat conduction problem described by the Poisson equation for a cooled gas-turbine blade

The presented paper describes a method of solving the inverse problems of heat conduction, consisting in solving the Poisson equation for a simply connected region instead of the Laplace equation for a multiply connected one, like a gas-turbine blade provided with cooling channels. The considered method consists in determining unknown values of the source (heat sink) power in the cooling channels for a given external heat transfer situation to achieve as close as possible an isothermal outer surface. Afterwards the temperature and heat flux distributions at the cooling channel walls are determined. Since the unknown source power is sought, the problem is an inverse one. Taking into account the sought values the method is reckoned among the class of the fictitious source methods and presents an optimization scheme. Using an exemplary gas turbine blade cooling configuration, the results of the calculation obtained with this method have been compared to the results achieved with an inverse method using the boundary element method for a multiple connected region.The results obtained with both methods within the optimization scheme approximated each other. Nevertheless, the results for the inverse method shown in the present paper gave nearly no oscillations, which is important in case of the blades with other geometric features of the cooling channels.     

Classification of geothermal resources in Turkey by exergy analysis

The investigations have been directed to technology development in the usage of natural resources as a result of increase in the world energy demand associated with environmental factors. It has also sparked interest in the scientific community to take a closer look at the energy conversion devices and develop the new techniques to better utilise the existing limited sources. Geothermal resources have a great importance for the energy potential in Turkey. Exergy of a system is the capability of doing work and exergy values of geothermal resources are the strongest criterion for determining the system efficiency. In this study, geothermal resources in Turkey have been classified based on specific exergy rates (SER). The computed results of exergy analysis can be used as a tool for evaluating the characteristics of resources, and the optimum application area of geothermal resources can also be defined.     

利用方法对锅炉进行节能分析

从热平衡和平衡两方面对锅炉进行能量分析,从而更全面地找出锅炉运行中所存在的问题,并提出改进措施。     

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.     

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.     

Optimum design of multi-stage isotope separation process by exergy analysis

A separation process of carbon isotopes, ¹²C (98.9%), ¹³C (1.1%) and ¹⁴C (trace), by the chemical exchange reactions between CO₂ and amine carbamate was designed by the exergy analysis technique and the applicability of the exergy analysis to the design of isotope-separation process is discussed. The exergy loss (EXL) for the single-column process was increased exponentially toward the product end, because of the mixing of isotopes between gas and liquid flows. By the application of ideal cascade without the mixing of ¹⁴C and ¹²C, the EXL value was reduced to 10% of that for the single-column process. The ideal cascade gives the minimum volume of separation column reasonably, however, is impracticable as an industrial process, because of the complexity of process configuration and operation. From the viewpoint of practical use, a squared-off cascade with three columns, whose diameters were reduced toward the product end, was designed under the condition that the difference of EXL between the squared-off cascade and the ideal one was minimized. The EXL value of this process was evaluated as about 12% of that of single-column process and the separation performance was similar to that for the ideal cascade. These results suggest that the process design method based on exergy analysis is very useful as a tool to design an optimum squared-off cascade.     

Efficiency and exergy analysis of a new solar air heater

It would be misleading to consider only the cost aspect of the design of a solar collector. High service costs increase total costs during the service life of solar collector. The most effective way to save energy is by increasing the efficiency in a solar collector by the heat transfer coefficient.In our study, five solar collectors with dimensions of 0.9×0.4 m were used and the flow line increased where it had narrowed and expanded geometrically in shape. These collectors were set to four different cases with dimensions of 1×2 m. Therefore, heating fluids exit the solar collector after at least 4.5 m displacement. According to the collector geometry, turbulence occurs in fluid flow and in this way heat transfer is increased. The results of the experiments were evaluated on the days with the same radiation. The efficiencies of these four collectors were compared to conventional flat-plate collectors. It was seen that heat transfer and pressure loss increased depending on shape and numbers of the absorbers.     

Chemical exergy assessment of organic matter in a water flow

In recent years, exergy analysis has been successfully applied to natural resources assessment. The consumption of any natural resource is unavoidably joined to dispersion and degradation. Therefore, exergy analysis can be applied to study the depletion of natural resources and, particularly, to water resources. Different studies range from global fresh water resources evaluation to specific water bodies' detailed analysis.     

Analysis of the sloshing flows of a LNG cargo tank

The sloshing flows in a LNG cargo tank model(1/50 scale) of a ship are measured by an embedded panoramicPIV system.The measurement system consists of a Nd-Yag laser(120mJ,15Hz),two cameras(1k x 1k) and a host computer.Four experimental cases were tested for the tank model,in which swaying motions are made by a 6 DOF-motion platform.The amplitudes of swaying are 9.76mm and 29.29mm,and the swaying frequencies are 0.633Hz and 0.828Hz.The measurement regions are the vertical plane 50mm away from the front wall ...