Simulation studies on gax based Kalina cycle for both power and cooling applications

A detailed parametric analysis is carried out on both simple and GAX based combined power and cooling cycle. The effect of various parameters such as heat source temperature, refrigeration temperature, sink temperature, split ratio (refrigerant flow ratio between power and cooling systems), split factor (solution flow ratio between absorber and GAX heat exchanger) on the performance of the cycle is studied. The results of the analysis show that using the GAX heat exchanger about 20% of internal heat is recovered within the cycle. The optimum split factor is 0.8–0.9 and the split ratio is 0.5:0.5. The maximum combined thermal efficiency of 35–45% and coefficient of performance of about 0.35 is attained at the optimum conditions.     

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.     

Ecological optimization of an irreversible Ericsson cryogenic refrigerator cycle

The ecological optimization and parametric study of an irreversible Ericsson cryogenic refrigerator cycle with finite heat capacities of external reservoirs is studied. The ecological function is defined as the cooling load minus the loss of the cooling load (the irreversibility) due to the entropy generation rate. The ecological function is optimized with respect to working fluid temperatures and the values of the cooling load, power input, the loss rate of the cooling load and COP are calculated for a typical set of operating parameters. The effects of different operating parameters on the ecological function, cooling load, the loss rate of the cooling load and COP are studied. The loss rate of the cooling load and the power input are found to be increasing functions of the cycle temperature ratio and decreasing functions of COP while the COP is found to be a decreasing function of the cycle temperature ratio. On the other hand, there exist the optimal values of the cycle temperature ratio and COP at which the ecological function and cooling load attain their maximum values. Also the ecological function and the cooling load are found to be increasing functions of the sink‐side heat capacitance rate and the effectiveness on the source‐, sink‐, and regenerative‐side heat exchangers while the decreasing functions of the source‐side heat capacitance rate. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance comparison and parametric optimization of subcritical Organic Rankine Cycle (ORC) and transcritical power cycle system for low-temperature geothermal power generation

Organic Rankine Cycle (ORC) is a promising technology for converting the low-grade energy to electricity. This paper presents an investigation on the parameter optimization and performance comparison of the fluids in subcritical ORC and transcritical power cycle in low-temperature (i.e. 80–100 °C) binary geothermal power system. The optimization procedure was conducted with a simulation program written in Matlab using five indicators: thermal efficiency, exergy efficiency, recovery efficiency, heat exchanger area per unit power output (APR) and the levelized energy cost (LEC). With the given heat source and heat sink conditions, performances of the working fluids were evaluated and compared under their optimized internal operation parameters. The optimum cycle design and the corresponding operation parameters were provided simultaneously. The results indicate that the choice of working fluid varies the objective function and the value of the optimized operation parameters are not all the same for different indicators. R123 in subcritical ORC system yields the highest thermal efficiency and exergy efficiency of 11.1% and 54.1%, respectively. Although the thermal efficiency and exergy efficiency of R125 in transcritical cycle is 46.4% and 20% lower than that of R123 in subcritical ORC, it provides 20.7% larger recovery efficiency. And the LEC value is relatively low. Moreover, 22032L petroleum is saved and 74,019 kg CO₂ is reduced per year when the LEC value is used as the objective function. In conclusion, R125 in transcritical power cycle shows excellent economic and environmental performance and can maximize utilization of the geothermal. It is preferable for the low-temperature geothermal ORC system. R41 also exhibits favorable performance except for its flammability.     

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,采用合适的工质和循环工况,可以提高ORC系统的发电效率。有出口温度限制的热源是一种较为常见的热源形式,在ORC系统中增加回热装置可能会进一步提高热力循环对该类热源的利用效率。因此,文章针对有温度出口限制的热源,建立了亚临界ORC计算分析模型,选取了干流体和等熵流体作为循环工质,以热源回收?效率作为ORC系统的循环性能评价指标,系统地比较了不同回热度条件下ORC系统的循环性能。文章系统地分析了回热流程对ORC系统循环性能的影响规律,并将计算结果进行理论关联,首次建立了依据冷源和热源条件直接选取最佳回热度的定量准则。     

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.     

A universal expression for optimum specific work of reciprocating heat engines having endoreversible carnot efficiencies

Irreversible heat transfer (finite time) analysis is used to obtain the optimum thermodynamic specific work potential at maximum power for various practical reciprocating cycles having endoreversible Carnot efficiencies. The theory of finite‐time thermodynamics for reciprocating endoreversible cycles with heat transfer irreversibilities gives rise to an optimum efficiency at maximum power output, of η=1−(T_L/T_H)^0·5 for Carnot‐like cycles in contrast to the upper limit for Carnot‐like cycles of η=1−(T_L/T_H) obtained from infinite‐time thermodynamics. It is shown here that, additionally, for this same general family of regenerative reciprocating cycles which includes the Stirling, the Ericsson and the reciprocating Carnot cycle, the finite‐time optimum specific work output at maximum power, (w_opt), is exactly half of that obtained for infinite‐time reversible cycles (Carnot work, w_rev) operating between the same temperature limits (i.e., w_opt=½w_rev). To accomplish this, the analysis makes use of time symmetry to minimize overall cycle time and to thus optimize net cycle power. Based on linear heat transfer laws, the expression for optimum specific work is shown to be independent of heat conductances. Moreover, this optimum specific work output is the same expression for all of the members of this family of cycles. This analysis makes use of the ideal gas model with constant specific heats, though the results are shown to be universal for the Carnot cycle for vapours and real gases. A sample calculation is given which shows that while operating under the same optimized conditions, the endoreversible Stirling engine will have the same thermal efficiency as the endoreversible Ericsson, but will have a higher optimum power output. The optimum power of the reciprocating endoreversible Carnot engine will be superior to both. Copyright © 1999 John Wiley & Sons, Ltd.     

A unified model of combined energy systems with different cycle modes and its optimum performance characteristics

A unified model is presented for a class of combined energy systems, in which the systems mainly consist of a heat engine, a combustor and a counter-flow heat exchanger and the heat engine in the systems may have different thermodynamic cycle modes such as the Brayton cycle, Carnot cycle, Stirling cycle, Ericsson cycle, and so on. Not only the irreversibilities of the heat leak and finite-rate heat transfer but also the different cycle modes of the heat engine are considered in the model. On the basis of Newton’s law, expressions for the overall efficiency and power output of the combined energy system with an irreversible Brayton cycle are derived. The maximum overall efficiency and power output and other relevant parameters are calculated. The general characteristic curves of the system are presented for some given parameters. Several interesting cases are discussed in detail. The results obtained here are very general and significant and can be used to discuss the optimal performance characteristics of a class of combined energy systems with different cycle modes. Moreover, it is significant to point out that not only the important conclusions obtained in Bejan’s first combustor model and Peterson’s general combustion driven model but also the optimal performance of a class of solar-driven heat engine systems can be directly derived from the present paper under some limit conditions.     

Design considerations for a power optimized regenerative endoreversible Stirling cycle

An evaluation of the major theoretical considerations concerning the design of an endoreversible Stirling cycle with ideal regeneration is given. The factors affecting optimum power and efficiency at optimum power are analysed for the cycle based upon higher and lower temperature bounds. Heat transfer characteristics of the regenerator and the thermal source and sink, individual process times for the cycle have been studied with respect to engine design parameters like speed, compression ratio, etc. The results of this study provide additional information for use in the optimized design and evaluation of Stirling engines. Copyright © 2000 John Wiley & Sons, Ltd.     

Optimization of the direct Carnot cycle

A model for the study and optimization of two heat reservoirs thermal machines is presented. The mathematical model basically consists of the First and Second Laws of Thermodynamics applied to the cycle and entire system, and the heat transfer equations at the source and sink. The internal and external irreversibilities of the cycle are considered by taking into account the entropy generation terms. Several constraints imposed to the system composed by the engine and the two heat reservoirs (namely, engine efficiency, or power output, or heat flux received by the engine, each of them together with imposed internal entropy generation and total number of heat transfer units of the machine heat exchangers) allow us to find the optimum operational conditions, as well as the limited variation ranges for the system parameters. Emphasis is put on coupling between various possible objective functions, namely thermal cost, useful effect, first law efficiency and whole system dissipation. It is for the first time to our knowledge when it has been proved that if one of the possible objective functions is fixed (as a parameter with imposed value), the optima of the other three always correspond to each other for the corresponding stationary state system, with a given optimum heat conductance allocation (one degree of freedom). Other interesting results are also reported in this paper. Some sensitivity studies were developed, too, with respect to various parameters of the model (engine performance, internal entropy generation, total number of heat transfer units).     

A Kalina power cycle driven by renewable energy sources

The present paper investigates a Kalina cycle using low-temperature heat sources to produce power. The main heat source of the cycle is provided from flat solar collectors. In addition, an external heat source is connected to the cycle, which corresponds to 5% up to 10% of the total thermal energy supplied to the cycle. The cycle operates at low pressure levels (0.2–4.5 bar) and low maximum temperature (130 °C). The NH₃ mass fraction at the turbine inlet varies along with the expansion pressure and the effects on the cycle efficiency are studied. For given conditions, an optimum range of vapor mass fractions and operating pressures can be identified that result in optimum cycle performance. Simple equations have been derived that link the operational parameters with the independent variables as well as with the cycle efficiency.     

Thermodynamic analysis and optimization of power cycles using a finite low‐temperature heat source

The analysis of a subcritical Rankine cycle with superheating, operating between a constant flowrate low‐temperature heat source and a fixed temperature sink, according to the principles of classical and finite size thermodynamics, is presented. The results show the existence of two optimum evaporation pressures: one minimizes the total thermal conductance of the two heat exchangers, whereas the other maximizes the net power output. A comparison of such results for five working fluids leads to the selection of R141b for a system generating 10% of a reference power which depends on the specified source and sink characteristics; for the conditions under consideration this reference power is 6861 kW. The results for this particular system show that the minimum total thermal conductance of the two heat exchangers is 1581 kW K^?1; the corresponding thermal efficiency is 12.6% and the total exergy losses are 13.8% of the source's exergy. Slightly more than 50% of the exergy destruction occurs in the vapor generator. Copyright © 2011 John Wiley & Sons, Ltd.     

分液冷凝有机朗肯循环系统R245fa/HFOs工质选择研究

有机朗肯循环是中低品位热能高效利用的有效技术之一,分液冷凝有机朗肯循环（LSCORC）是基于分液冷凝传热强化的新型热力循环。为寻找新型环保替代工质,建立LSCORC系统的热力学模型,以最大化净输出功为目标,重点考虑了雅各布数、冷热源换热匹配对系统性能的影响,对R245fa/HFOs工质进行了对比筛选。结果表明：工质的雅各布数越大,其净输出功越小;在基础工况下,R245fa/R1336mzz(Z)的热力性能及热经济性表现最佳;当热源参数变化时,雅各布数较小工质的性能表现普遍优于雅各布数较大的工质组合;当冷源参数变化时,在分液冷凝器两个流程中温度滑移和冷源温升匹配越好的工质组合,其系统净输出功越大。     

Power,power density and efficiency optimization of an endoreversible Braysson cycle

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.     

Operational strategy of adsorption desalination systems

This paper presents the performances of an adsorption desalination (AD) system in two-bed and four-bed operational modes. The tested results are calculated in terms of key performance parameters namely, (i) specific daily water production (SDWP), (ii) cycle time, and (iii) performance ratio (PR) for various heat source temperatures, mass flow rates, cycle times along with a fixed heat sink temperature. The optimum input parameters such as driving heat source and cycle time of the AD cycle are also evaluated. It is found from the present experimental data that the maximum potable water production per tonne of adsorbent (silica gel) per day is about 10 m³ whilst the corresponding performance ratio is 0.61, and a longer cycle time is required to achieve maximum water production at lower heat source temperatures. This paper also provides a useful guideline for the operational strategy of the AD cycle.     

Optimal ratios of the piston speeds for a finite speed irreversible Carnot heat engine cycle

The performance of an irreversible Carnot heat engine cycle is analysed and optimized by using the theory of finite time thermodynamics based on Agrawal's [2009. A finite speed Curzon-Ahlborn engine. European Journal of Physics, 30 (3), 587–592] model of finite piston speed on the four branches and Petrescu et al.’s [2002b. Optimization of the irreversible Carnot cycle engine for maximum efficiency and maximum power through use of finite speed thermodynamic analysis. In: Proceedings of ECOS’2002, 3–5 July, Berlin, Germany, Vol. II, 1361–1368] model of a Carnot cycle engine with the finite rate of heat transfer, heat leakage from heat source to heat sink and irreversibilities caused by finite speed, friction and throttling through the valves. The finite piston speeds on the four branches are further assumed to be different, which is different from the model of constant speed of the piston on the four branches. Expressions of power output and thermal efficiency of the cycle are derived for a fixed cycle period and internal entropy generation rate. Numerical examples show that the curve of power output versus thermal efficiency is loop shaped, and there exist optimal finite piston speeds on the four branches which lead to the maximum power output and maximum thermal efficiency, respectively. The effects of the heat leakage coefficient and internal entropy generation rate on the optimal finite piston speed ratios are discussed.     

Optimum arrangement and use of heat pumps in recovering waste heat

Relations are derived for the coefficient of performance of heat pump systems used to transfer heat from a low temperature heat source stream to a high temperature heat sink stream. The manner of use and operation of a number of heat pumps in such a system has been determined for the thermodynamic optimum for reversible and irreversible heat pumps.     

余热利用有机物朗肯循环最佳热回收效率分析

首先通过分析余热回收动力循环的不可逆损失,得到循环的理想效率。其次,通过分析发现热回收效率随蒸发压力变化存在最佳值,并且最佳热回收效率与最小熵增率是等价的。然后,通过研究两种简化的余热利用动力模型,应用有限时间热力学的相关方法,指出最大热回收效率产生的原因。再次,研究了余热变化时系统最佳工况的变化。结果发现最佳蒸发压力随余热流量、入口温度增加而显著增加,而与余热组分关系不大。最后,研究了工质对系统最佳工况的影响,发现较高临界温度的工质,最佳蒸发压力较低。     

A comparative study of the carbon dioxide transcritical power cycle compared with an organic rankine cycle with R123 as working fluid in waste heat recovery

The organic rankine cycle (ORC) as a bottoming cycle¹ to convert low-grade waste heat into useful work has been widely investigated for many years. The CO₂ transcritical power cycle, on the other hand, is scarcely treated in the open literature. A CO₂ transcritical power cycle (CO₂ TPC) shows a higher potential than an ORC when taking the behavior of the heat source and the heat transfer between heat source and working fluid in the main heat exchanger into account. This is mainly due to better temperature glide matching between heat source and working fluid. The CO₂ cycle also shows no pinch limitation in the heat exchanger. This study treats the performance of the CO₂ transcritical power cycle utilizing energy from low-grade waste heat to produce useful work in comparison to an ORC using R123 as working fluid.Due to the temperature gradients for the heat source and heat sink the thermodynamic mean temperature has been used as a reference temperature when comparing both cycles. The thermodynamic models have been developed in EES² The relative efficiencies have been calculated for both cycles. The results obtained show that when utilizing the low-grade waste heat with the same thermodynamic mean heat rejection temperature, a transcritical carbon dioxide power system gives a slightly higher power output than the organic rankine cycle.     

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.