换热网络模型预测控制中的滚动优化策略

针对换热网络具有非线性、滞后性、约束性等特性,提出将模型预测控制算法应用到换热网络的控制中,并对其滚动优化策略进行深入研究。首先,建立以综合费用最少的换热网络模型及目标函数,然后引入最速下降法对换热网络进行优化,以获得下一步的最优旁路开度,实时计算,实现换热网络模型预测控制中的滚动优化策略。通过算例证明:采用滚动优化策略能够快速、准确地计算出每一步的最优旁路开度,在保证出口温度达到目标值的同时使系统经济效益有所提升。     

Exergy simulation and optimization of adiabatic and diabatic binary distillation

A detailed exergy analysis of a distillation system has been conducted. This analysis is divided in four parts: (1) adiabatic rectification column; (2) adiabatic stripping column; (3) diabatic rectification column; and (4) diabatic stripping column. The results of all cases are presented in this paper. This analysis concerns the mixture water–ethanol working at 1 bar. The main objective is to determine the distribution of exergy losses inside the column and the optimal distribution of heat to be transferred inside the column in order to produce the minimum overall exergy losses.     

Optimal synthesis of a heat-exchanger network

Thermodynamic, heat transfer and economic concepts influencing the synthesis of a heat-exchanger network (HEN) coupled to a crude fractionation unit are examined. The impact of the variation of the minimum temperature approach (Δt_min) on energy and capital targets is studied using recent developments in pinch technology. The optimal pinch approach temperature has been determined using the ‘supertargeting’ concept where proper trade-off between energy and capital targets is observed prior to design. A heuristic evolutionary approach has then been used for the generation of the optimal HEN.     

Thermal placement optimization of multichip modules using a sequential metamodeling-based optimization approach

The study attempts to seek the optimal thermal design of planar multichip module (MCMs) under natural convection through optimal chip placement design. To attain the goal, a sequential metamodeling-based optimization approach is introduced. This approach incorporates a response surface methodology (RSM)-based design of experiment (DOE), three-dimensional (3D) thermal finite element analysis (FEA) and an updating scheme. Essentially, the RSM is used to construct, via quadratic polynomial approximation, the global RS of the chip junction temperature in terms of design variables. For speeding up the DOE and the solution of the optimization, several dynamic experimental design strategies using move limits and different proposed sampling techniques are introduced. The feasibility of the strategies is demonstrated, and their solution accuracy and efficiency are also compared with each other. By the explicit RS-based performance function together with geometry constraints, a constrained thermal optimization subproblem is formed. The optimum of the subspace optimization is sought, which is considered as the nominal starting point of next iteration. The iterative process continues with a new defined design subspace and factorial design plan until convergence is attained. The applicability of the proposed design optimization technique is demonstrated through several design case studies involving various planar MCMs.     

Performance of a square, cross-corrugated, polymer film, compact, heat-exchanger with potential application in fuel cells

This paper describes an experimental investigation of the performance characteristics of a novel, cross-corrugated, polymer film, compact, heat-exchanger (PFCHE) made from poly ether ether ketone (PEEK). The aim is to develop Jh and f correlations over a range of Reynolds numbers under laminar conditions to be used in alternative heat-exchanger designs for potential applications in the fuel-cell industry. The incentive for adopting these designs is the huge weight, energy and cost savings involved. Design correlations for square units are key tools in obtaining alternative designs for applications that are presently monopolized by metallic heat-exchangers. The design correlations are establised and then used to perform case studies in selected applications in the fuel-cell industry to suit the fluids and the configuration.     

Quasi-steady-state model of a counter-flow air-to-air heat-exchanger with phase change

Using mechanical ventilation with highly efficient heat-recovery in northern European or arctic climates is a very efficient way of reducing the energy use for heating in buildings. However, it also presents a series of problems concerning condensation and frost formation in the heat-exchanger. Developing highly efficient heat-exchangers and strategies to avoid/remove frost formation implies the use of detailed models to predict and evaluate different heat-exchanger designs and strategies. This paper presents a quasi-steady-state model of a counter-flow air-to-air heat-exchanger that takes into account the effects of condensation and frost formation. The model is developed as an Excel spreadsheet, and specific results are compared with laboratory measurements. As an example, the model is used to determine the most energy-efficient control strategy for a specific heat-exchanger under northern European and arctic climate conditions.     

Performance and optimization of a cylindrical-parabola collector

This paper includes an analysis of the performance of a cylindrical-parabola collector regarding the amount of energy collected, optimization of its various parameters and experimental verification of the theoretical predictions. A literature review reveals that the aperture of the reflector is a more logical choice as a characteristic dimension than the focal length. Theoretical concentrations calculated by Cobble for various absorber shapes have been recalculated with relatively more logical assumptions. The heat balance on the absorber showed that the dimensionless temperature (ratio of the absorber temperature to the stagnation temperature) completely predicts the performance of the collector. This is so because the stagnation temperature is a unique function of all variables (except the temperature of the absorber) of the collector assembly. Further, the various parameters have been theoretically optimized for the maximum energy collection. To corroborate the theoretical predictions, a cylindrical parabola collector capable of having two axes steering was constructed. Provision was made for changing the relative focal length, theoretical concentration, and flow rate of heat transfer fluid. Three types of data concerning maximum collectable energy, efficiency at different absorber temperatures, and stagnation temperature were obtained. The theoretical curve and the experimental results have been compared in a certain range of the dimensionless temperature. The agreement is satisfactory.     

Performance optimization of a two-stage semiconductor thermoelectric-generator

A model of a two-stage semiconductor thermoelectric-generator with external heat-transfer is built. Performance of the generator, assuming Newton’s heat-transfer law applies, is analyzed using a combination of finite-time thermodynamics and non-equilibrium thermodynamics. The analytical equations about the power output versus the working electrical current, and the thermal efficiency versus working electrical-current are derived. For a fixed total heat-transfer surface-area for two heat-exchangers, the ratio of heat-transfer surface-area of the high-temperature side heat-exchanger to the total heat-transfer surface-area of the heat-exchangers is optimized for maximizing the power output and the thermal efficiency of the thermoelectric-generator. For a fixed total number of thermoelectric elements, the ratio of number of thermoelectric elements of the top stage to the total number of thermoelectric elements is also optimized for maximizing both the power output and the thermal efficiency of the thermoelectric-generator. The effects of design factors on the performance are analyzed.     

Performance optimization of a photovoltaic induction motor pumping system

The performances of a photovoltaic pumping system based on an induction motor are degraded once insolation varies far from the value called nominal, where the system was sized. To surmount this handicap, an improvement of these performances by the optimization of the motor efficiency is described in this paper. The results obtained are compared with those of similar work pieces presented in the literature where the motor effeciency and air gap flux where optimized separatly. The simulation results show that the proposed system allows at the same time to combine the performances of the system with constant efficiency and the simplicity of implementation provided by the system with constant airgap flux.     

Performance optimization of Stirling engines

The search for an engine cycle with high efficiency, multi-sources of energy and less pollution has led to reconsideration of the Stirling cycle. Several engine prototypes were designed but their performances remain relatively weak when compared with other types of combustion engines. In order to increase their performances and analyze their operations, a numerical simulation model taking into account thermal losses has been developed and used, in this paper, to optimize the engine performance. This model has been tested using the experimental data obtained from the General Motor GPU-3 Stirling engine prototype. A good correlation between experimental data and model prediction has been found. The model has also been used to investigate the influence of geometrical and physical parameters on the Stirling engine performance and to determine the optimal parameters for an acceptable operational gas pressure.     

Performance analysis and optimization of a thermally non-symmetric annular fin

Considering thermally non-symmetric convective boundary conditions, optimum dimensions of an annular fin which has a rectangular cross-section are investigated. Two-dimensional heat diffusion equation is solved analytically to obtain temperature distribution and heat transfer rate. In this work, fin volume is fixed to obtain the dimensionless geometrical parameters of the fin with maximum heat transfer rates. The optimum geometry which maximizes the heat transfer rate for a given fin volume has been found employing NCONF routine in the IMSL Library. The derived condition of optimality gives an open choice to the designer.     

Development of a diabatic two-phase flow pattern map for horizontal flow boiling

An improved two-phase flow pattern map is proposed for evaporation in horizontal tubes. Based on new flow pattern data for three different refrigerants covering a wide range of mass velocities, vapor qualities and heat fluxes. The new flow pattern map includes the prediction of the onset of dryout at the top of the tube during evaporation inside horizontal tubes as a function of heat flux and flow parameters and is an extension to the flow pattern map model of Kattan et al. [J. Heat Transfer 120 (1998) 140]. The proposed modifications allow an accurate prediction of the flow pattern for very different fluids which are the substitute refrigerants (HFC-134a and HFC-407C) and the natural refrigerant R-717 (ammonia).     

Performance analysis and optimization of a hydrogen liquefaction system assisted by geothermal absorption precooling refrigeration cycle

The present paper deals with the hydrogen liquefaction with absorption precooling cycle assisted by geothermal water is modeled and analyzed. Uses geothermal heat in an absorption refrigeration process to precool the hydrogen gas is liquefied in a liquefaction cycle. High-temperature geothermal water using the absorption refrigeration cycle is used to decrease electricity work consumption in the gas liquefaction cycle. The thermoeconomic optimization procedure is applied using the genetic algorithm method to the hydrogen liquefaction system. The objective is to minimize the unit cost of hydrogen liquefaction of the composed system. Based on optimization calculations, hydrogen gas can be cooled down to ?30 °C in the precooling cycle. This allows the exergetic cost of hydrogen gas to be reduced to be 20.16 $/GJ (2.42 $/kg LH₂). The optimized exergetic cost of liquefied hydrogen is 4.905 $/GJ (1.349 $/kg LH₂), respectively.     

Performance analysis and cost optimization of a solar-assisted heat pump system

A solar-assisted heat pump system with a conventional backup unit was simulated for a 93 m² (1000 ft²) house in Rhode Island using quasi-dynamic computer models. The performance of the system as a function of collector area and thermal storage volume was evaluated to determine the fraction of the space heating and domestic hot water load that was supplied by the solar-assisted system. This information was used to compute the payback time, based on cumulative costs, for each variation of the system's parameters when compared to a conventional system. The optimal combination of system components which had a payback time less than the mortgage life was determined. For the given initial costs of solar panels and storage reservoir, this optimal combination was found to be insensitive to the variations in mortgage and fuel cost growth rates presented in this report.     

Performance analysis and optimization of elliptic fins circumscribing a circular tube

The performance of elliptic disc fins has been analyzed using a semi-analytical technique. It has been shown that the efficiency of such fins can also be predicted very closely using the sector method. However, the equivalent annulus method is not suitable for this fin geometry. A method for the optimum design of fins, using a constraint of either fin volume or rate of heat dissipation has also been suggested. Optimum elliptical fins dissipate heat at a higher rate compared to an annular fin when space restriction exists on both sides of the fin. Even when the restriction is on one side only, the performance of elliptical fin is comparable to that of eccentric annular fin for a wide parametric range.     

Performance analysis and optimization of a supercharged Miller cycle Otto engine

One of the major alternatives of the Otto cycle has been examined to determine its potential for increased efficiency and net work power in the spark ignited internal combustion engine is to shorten the compression process relative to the expansion process by early close or late of intake valve. The modified Otto cycle is called Miller cycle. This paper deals with the analysis of a supercharged Otto engine adopted for Miller cycle operation. The Miller cycle shows no efficiency advantage and suffers a penalty in power output in the normally aspirated version. In the supercharged Otto engine adopted for Miller cycle version, it has no efficiency advantage but does provide increased net work output with reduced propensity to engine knock problem. Sensitivity analysis of cycle efficiency versus early close of intake valve and that of cycle net work versus early close of intake valve are performed. Optimization on the cycle efficiency is obtained.     

Performance optimization of solar-powered heat engine using a multiobjective accelerated algorithm

In this study, a multiobjective optimization approach was used to conduct a thermodynamic investigation of a solar Brayton and endoreversible heat engine. The thermo-economic performance capabilities of such machines with hybrid input power, solar-fuel, are examined numerically. Throughout this study, three performance indicators of the cycle, including the power output, the thermo-economic performance function, and the thermal efficiency are optimized concurrently employing a multiobjective steepest descent method, named the Accelerated Diagonal Steepest Descent algorithm. Furthermore, to properly analyze the error, three strategies are employed in the decision-making step to identify the optimal compromise solution, and the deviation indices under these strategies are analyzed. The numerical experiments reveal that the present algorithm outperforms the two popular multiobjective algorithms: the multiobjective particle swarm optimization method and the elitist nondominated sorting genetic algorithm. The relevance of the presented algorithm with respect to the previous ones is examined by means of a deviation index. Finally, these experiments show the optimal design parameters which lead to the best performance of the heat engine.     

太阳能盘管腔式高温集热特性模拟与优化

盘管腔式集热器可为太阳能布雷顿循环提供高温空气,也能直接用于工业生产,应用前景广阔。聚焦能流的腔内分布是影响盘管温度梯度和出口空气温度的主要因素,为优化盘管腔式集热器结构,合理分布聚焦能流,对不同结构(高度,腔体直径,管径和盘旋形状)的盘管式空气集热器进行了建模及模拟。模拟过程耦合了光学模型和传热模型,采用蒙特卡洛模型获得腔内能流分布,并将其作为边界条件加载到三维CFD(Computaional Fluid Dynamics,CFD)传热模型中,进而获得集热管内空气的温度场分布,为结构优化提供重要参考。     

风电机组大部件的备品备件区域库存优化控制策略

风电机组大部件的备品备件是保障风电机组运行的重要物资,合理的备件库存可减少风电场的停机损失。针对风电机组大部件的备品备件库存控制问题,提出了基于(s,Q)策略的区域库存优化控制策略。首先,以威布尔分布函数描述风机大部件的故障概率密度。其次,考虑风电机组设计寿命及备件短缺停机时间,通过概率模型推导了区域库存策略下的风机大部件备件成本和平均可用率公式。然后,以备件成本最小为优化目标,建立库存优化控制模型,并分析了大部件的各主要参数对最优库存控制参数的影响。最后,通过算例分析验证了该策略的有效性。     

Performance optimization of a solar driven heat engine with finite-rate heat transfer

An optimal performance analysis for an equivalent Carnot-like cycle heat engine of a parabolic-trough direct-steam-generation solar driven Rankine cycle power plant at maximum power and maximum power density conditions is performed. Simultaneous radiation-convection and only radiation heat transfer mechanisms from solar concentrating collector, which is the high temperature thermal reservoir, are considered separately. Heat rejection to the low temperature thermal reservoir is assumed to be convection dominated. Irreversibilities are taken into account through the finite-rate heat transfer between the fixed temperature thermal reservoirs and the internally reversible heat engine. Comparisons proved that the performance of a solar driven Carnot-like heat engine at maximum power density conditions, which receives thermal energy by either radiation-convection or only radiation heat transfer mechanism and rejects its unavailable portion to surroundings by convective heat transfer through heat exchangers, has the characteristics of (1) a solar driven Carnot heat engine at maximum power conditions, having radiation heat transfer at high and convective heat transfer at low temperature heat exchangers respectively, as the allocation parameter takes small values, and of (2) a Carnot heat engine at maximum power density conditions, having convective heat transfer at both heat exchangers, as the allocation parameter takes large values. Comprehensive discussions on the effect of heat transfer mechanisms are provided.