Entropy generation minimization for charging and discharging processes in a gas-hydrate cool storage system

Thermodynamic optimization models of gas-hydrate cool storage and cool release processes are established in this paper. The optimal temperature configuration at the sensible heat transfer stage and the optimal gas hydrate phase change rate configuration at the phase change stage in the processes of gas hydrate charging and discharging are obtained by taking entropy generation minimization as optimization objective. The optimal control strategies of the cool storage system are determined. The research results indicate that the optimal operating characteristic of the gas-hydrate cool storage system can be achieved by keeping the phase change rates uniform, which are regulated and controlled according to constant heat transfer rates in the charging and discharging processes of gas hydrate. The analysis method and the results presented in this paper can provide important guidelines for optimal design and operation of gas-hydrate cool storage system.     

Optimal package design of stacks of convection-cooled printed circuit boards using entropy generation minimization method

Thermal optimization of a stack of printed circuit boards using entropy generation minimization (EGM) method is presented. The study consists of two parts. One is focused on the entropy generation of a module in periodically fully-developed channel flow (PDF), while the other is the optimization applied to electronic packages composed of a stack of printed circuit boards. In the process of optimizing electronics packaging, consideration is given to two constraints which are the maximum junction temperature specified by a chip manufacturer and the allowable pressure difference across the channel maintained by cooling fans. Governing thermal-fluid flow equations in the laminar-flow regime are numerically integrated subject to the appropriate boundary conditions. After the flow and temperature fields are solved, the volumetric rate of local entropy generation in the PDF is integrated to determine the total entropy generation rate in the system which consists of two components, one by heat transfer and the other by viscous friction. The Reynolds number, block geometry and bypass flow area ratio are varied to search for an optimal channel spacing using the EGM method whose validity is borne out by comparing with those obtained by the conventional thermal optimization (or overall thermal conductance) method. A dimensionless optimal board spacing parameter C is derived which involves the relative migration speed (or time) of heat transfer and viscous friction over the PDF channel length. A correlation equation is derived which expresses C in terms of the Reynolds number and block geometry. This equation can be employed in the optimal design of electronic packages.     

广义不可逆卡诺热机的生态学最优性能

以反映热机功率与熵产率之间最佳折衷的“生态学”准则为目标，综合考虑热阻、热漏及其它不可逆性对卡诺热机性能的影响，导出牛顿传热规律下循环的生态学最优性能，由数值计算分析比较了热漏、内不可逆性的影响特点。生态学优化以牺牲小部分输出功率为代价，较大地降低了循环的熵产率，而且在一定程度上提高了热机效率。因此，生态学目标函数不仅反映了输出功率和熵产率之间的最佳折衷，而且反映了输出功率和热效率之间的最佳折衷。     

Second law analysis for optimal thermal design of radial fin geometry by convection

This paper presents a second law analysis for the optimal geometry of fin array by forced convection. The analytical analysis involves the achievement of a balance between the entropy generation due to heat transfer and entropy generation due to fluid friction. In the design of a thermal system, it is important to minimize thermal irreversibilities because the optimal geometry will be found when the entropy generation rate is minimized. In this paper, the entropy generation rate is discussed and optimum thickness for fin array is determined on the basis of entropy generation minimization subjected to the global constraint. In addition, the influence of cost parameters on the optimum thickness of fin array is also considered and presented in graphical form. It has been found that the increase in cross flow fluid velocity will enhance the heat transfer rate that will reduce the heat transfer irreversibility.     

计及天然气压力能的微电网储荷协调优化调度

为解决天然气输送过程中压力能的浪费问题,提高天然气能源利用率,提出将天然气压力能发电系统引入至微电网整体调度方案中。针对天然气压力能发电系统中前后端口的补热需要,将微电网中的风冷热泵补热系统与压力能发电系统进行耦合。考虑到由天然气管网中流量波动和环境因素造成的压力能出力波动问题,提出微电网储荷一体化协调优化方案,以确保系统高效稳定运行。基于上述内容,构建考虑可控电源出力成本、储能调度成本、微电网与配电网的交互成本和负荷调度成本的微电网优化调度模型,并采用Yalmip工具包编写优化调度程序。最后,通过对西南地区某调压站数据进行仿真,验证该方案的可行性与经济性。     

A comparative study on the enhanced formation of methane hydrate using CM-95 and CM-100 MWCNTs

1 m³ of methane hydrate can be decomposed into a maximum of 216 m³ of methane gas under standard conditions. Conversely, such a large volume of methane hydrate can be utilized to store and transport a large quantity of natural gas. When methane hydrate is formed artificially by simply reversing its process of natural generation, the amount of methane gas consumed owing to hydrate formation is fairly low which would be problematic for its massive synthesis and application. In this study, experiments are carried out with the goal of increasing the amount of gas consumed by adding two kinds (CM-95 and CM-100) of multi-walled carbon nanotubes (MWCNTs) to pure water, where the physical length of CM-95 is much shorter than CM-100. When the 0.004 wt.% CM-95 MWCNT solution is compared with pure water, the gas consumption rate almost triples indicating its effect in hydrate formation. Also, the CM-95 MWCNTs decreased the hydrate formation time to a greater extent than the CM-100 MWCNTs at a low subcooling temperature.     

Performance analysis and optimization of irreversible air refrigeration cycles based on ecological coefficient of performance criterion

In this paper, a performance optimization based on ecological coefficient of performance (ECOP) criterion has been carried out for an irreversible air refrigeration cycles. The considered model includes irreversibilities due to finite-rate heat transfer, heat leakage and internal dissipations. The ECOP objective function is defined as the ratio of cooling load to the loss rate of availability (or entropy generation rate). The maximum of the ecological performance criterion and the corresponding optimal conditions have been derived analytically. The effects of irreversibility parameters on the general and optimal performances discussed detailed. The obtained results may provide a general theoretical tool for the ecological design of air refrigerators.     

Novel and optimal integration of SOFC-ICGT hybrid cycle: Energy analysis and entropy generation minimization

Integrating fuel cells with conventional gas turbine based power plant yields higher efficiency, especially solid oxide fuel cell (SOFC) with gas turbine (GT). SOFCs are energy efficient devices, performance of which are not limited to Carnot efficiency and considered as most promising candidate for thermal integration with Brayton cycle. In this paper, a novel and optimal thermal integration of SOFC with intercooled-recuperated gas turbine has been presented. A thermodynamic model of a proposed hybrid cycle has been detailed along with a novelty of adoption of blade cooled gas turbine model. On the basis of 1st and 2nd law of thermodynamics, parametric analysis has been carried out, in which impact of turbine inlet temperature and compression ratio has been observed on various output parameters such as hybrid efficiency, hybrid plant specific work, mass of blade coolant requirement and entropy generation rate. For optimizing the system performance, entropy minimization has been carried out, for which a constraint based algorithm has been developed. The result shows that entropy generation of a proposed hybrid cycle first increases and then decreases, as the turbine inlet temperature of the cycle increases. Furthermore, a unique performance map has also been plotted for proposed hybrid cycle, which can be utilized by power plant designer. An optimal efficiency of 74.13% can be achieved at TIT of 1800 K and r_p,c 20.     

Investigation into some aspects of fluid flow and thermal analysis in annuli

Flow in annuli has wide application in industry. The modelling of the flow field improves design of such equipment and reduces the experimental cost. The entropy minimization can be used as a very useful tool in process optimization. In the present study, a laminar flow generated due to outer cylinder rotation in an annuli is considered. The velocity profiles, shear stress, torque required for the rotation, and the mean entropy generation are formulated for an isothermal case. The parametric study for the torque required and the mean entropy generation is conducted with water as the fluid medium. The parameters include the outer cylinder radius, the radius ratio and rotational speed. It is found that at large outer radii and large radius ratios, the rate of increase of torque exceeds the rate of increase of entropy generation. Consequently, the energy distribution reduces while torque increases under this condition. Copyright © 2001 John Wiley & Sons, Ltd.     

Entropy generation on a three-gas crossflow heat exchanger with longitudinal wall conduction

This study investigates the entropy generation in a crossflow heat exchanger including three gas streams and the effect of longitudinal wall conduction on the entropy generation. Using the numerical method, this study calculates the exit mean temperature of each stream, and then computes the number of entropy generation units. The results indicate that the entropy generation increases with the decrease of inlet temperature of gas stream 3 and the decrease of inlet temperature ratio of gas streams 1 to 2. In addition, the results show that the longitudinal wall conduction raises the entropy generation and that this raising increases with increasing NTU when heat capacity rate ratio of stream 1 is 0.5.     

Ecological coefficient of performance analysis and optimization of an irreversible regenerative-Brayton heat engine

In this paper, a performance optimization based on the ecological coefficient of performance (ECOP) criterion has been carried out for an irreversible regenerative Brayton heat-engine. The results obtained were compared with those using the power-output criterion and alternative ecological performance objective-function defined in the literature. The design parameters, under the optimal conditions, have been derived analytically and their effects on the engine’s performance have been discussed. It is shown that, for the regenerative Brayton-engine, a design based on the maximum ECOP conditions is more advantageous from the point-of-view of entropy generation rate, thermal efficiency and investment cost.     

Analysis of exergy loss vs heat transfer rate for Rayleigh–Bénard convection of various fluids in enclosures with curved walls

The investigation of entropy generation is highly desirable for the optimization of the thermal systems to avoid larger energy wastage and ensure higher heat transfer rate. The numerical investigation of natural convection within enclosures with the concave and convex horizontal walls involving the Rayleigh–Bénard heating is performed via entropy generation approach. The spatial distributions of the temperature (θ), fluid flow (ψ), entropy generation due to heat transfer and fluid friction (S_θ and S_ψ) are discussed extensively for various Rayleigh numbers and Prandtl numbers involving various wall curvatures. A number of complex patterns of spatial distributions of fluid flow and temperature for cavities with concave or convex isothermal walls (top and bottom) have been obtained. The zones of high entropy generation for temperature and fluid flow are detected within cavities with concave and convex horizontal walls. The optimal situation involves the high heat transfer rate with moderate or low entropy generation. Overall, case 3 (highly concave) is found to be optimal over cases 1 and 2 (concave) and cases 1–3 (convex) for all Pr and Ra.     

Optimization of Microchannel Heat Sinks Using Genetic Algorithm

In this study, a genetic algorithm is employed to minimize the entropy generation rate in microchannel heat sinks. The entropy generation rate allows the combined effects of thermal performance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. Previously developed models for the heat transfer, pressure drop and entropy generation rate are used in the optimization procedure. The results of optimization are compared with existing results obtained by the Newton–Raphson method. It is observed that the GA gives better overall performance of the microchannel heat sinks.     

Prediction of natural gas hydrate inhibitor vaporization rate using particle swarm optimization approach

Natural gas is a very important source of energy. In natural gas processing, accurate prediction of methanol loss to the vapor phase during natural gas hydrate inhibition is necessary to compute the total methanol injection rate required to effectively prevent the formation of natural gas hydrate. A reliable prediction tool that has the capability to accurately predict methanol losses to the vapor phase is thus needed. In order to address this matter, the current research was aimed at assessing the ability and feasibility of a robust computational intelligence paradigm. Based on a total of 326 dataset collected from the reliable literature, methanol loss to the vapor phase was predicted using artificial neural network (ANN) linked with particle swarm optimization (PSO) which is employed to determine the optimal values of the ANN weights. Success of the introduced hybrid intelligence model (or PSO-ANN) was confirmed with overall mean squared error (MSE), mean absolute error (MAE), and coefficient of determination (R²) values of 0.16421, 0.33210, and 0.99696, respectively.     

内可逆空气制冷机性能的生态学优化

基于[火用]分析的观点，运用有限时间热力学方法对内可逆空气制冷机进行生态学优化，导出了换热器热导最优分配时的最佳制冷功率、熵产率以及生态学（E）目标函数的解析式，进一步求得最大E目标值时的工质等熵温比（压比）界限及相应的制冷系数、制冷功率和熵产率；采用数值计算分析了热源温比、换热器总热导以及高温热源温度和环境温度之比对该制冷机生态学最优性能的影响。结果表明：生态学目标函数不仅反映了[火用]输出率和熵产率之间的最佳折衷，而且也反映了制冷功率和制冷系数之间的最佳折衷。     

海洋天然气水合物开发产能及出砂研究

海洋天然气水合物多分布在深水非成岩储层,其开采过程中易出现泥砂运移（出砂）的情况且难以避免,然而出砂条件下的水合物产能估算偏差较大。本文通过室内海洋水合物降压开采出砂实验数据和海洋水合物试采公开资料,首次推导了出砂条件下现场尺度海洋水合物储层产能情况：在相当于1 200 m水深覆压、30.5 m厚的水合物细砂储层和7寸垂直井不防砂的情况下,得到最大产气速率4.63 ~ 14.1 m³/s,折合40.01×10⁴ ~ 121.84×10⁴ m³/d,综合出砂率0.16% ~ 10.74%;泥质储层在不防砂垂直井和水平井单个半径12 mm射孔下,其最大产气速率达到79.95×10⁴ m³/d和170 m³/d,但其综合出砂率是灾难性的。由于时空限制,产气速率、综合产能和出砂率还有很大提升空间,在平衡综合出砂率（控砂精度）和产气效率（产能）的情况下,有望达到产业化规模。本研究为合理估算出砂条件下的海洋天然气水合物产能提供支撑。     

悬垂水滴与悬浮气泡表面气体水合物形成特性对比

基于悬垂水滴和悬浮气泡表面形成气体水合物的可视化耐高压实验装置,分析探讨了反应压力、温度、水质等因素对水滴和气泡表面气体水合物成核和生长规律的影响.对已有的关于研究单个静止悬垂水滴和悬浮气泡表面气体水合物生长特性的实验现象及结果进行了对比分析,得出结论:温度和压力是影响表面水合物结晶与生长的重要因素;温度的降低或压力的升高均使水合反应速度加快.研究为发展喷雾法和鼓泡法这两种强化制备水合物的方式提供了有效的实验支撑.     

Entropy generation in condensation in the presence of high concentrations of noncondensable gases

The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vapor/noncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference.     

Second‐law analysis and optimization of microchannel flows subjected to different thermal boundary conditions

Entropy generation and transfer in microchannel flows were calculated and analyzed for different thermal boundary conditions. Due to the small flow cross‐sectional area, fluid temperature variation in the lateral direction was neglected and a laterally lumped model was developed and used in the first‐ and second‐law analyses. Since the Peclet numbers of microchannel flows are typically low, heat conduction in the flow direction was taken into consideration. Computed fluid temperature and entropy generation rate were cast into dimensionless forms, thus can be applied to different fluids and channels of different sizes and configurations. Local entropy generation rate was found to be only dependent upon the temperature gradient in the flow direction. The optimization results of microchannel flows exchanging heat with their surroundings indicate the optimal fluid temperature distribution is a linear one. Copyright © 2004 John Wiley & Sons, Ltd.     

Optimal thermodynamic and economic volume of a heat recovery steam generator by constructal design

The ratios of gas flow to steam flow are huge in heat recovery steam generators (HRSGs) compared to other steam generators. So the volume which is occupied by components of the HRSG such as economizer, evaporator and superheater is important factor when the HRSG is applied in structures including buildings and ships. The optimum volume of a HRSG is deduced through optimization of entropy generation and cost evaluation. By increasing volume, second law of thermodynamics is improved, but this improvement may not be economical. In this work, the best dimensions and arrangements of flows in HRSG are obtained by constructal design and the optimization method is algorithm genetic. In this case, super heater temperature, pinch point, water/steam flow rate and gas pressure drop are derived from configuration which designed by constructal theory for HRSG. The effects of gas flow rate and inlet gas temperature are examined on the values of optimum volume.