基于新型变步长电导增量法的最大功率点跟踪策略

针对传统电导增量法在跟踪精度和跟踪过程响应速度方面的不足,文章提出一种新型变步长电导增量法。该方法首先采用固定电压法将光伏阵列工作点快速调整到最大功率点附近,然后利用基于反余切函数控制的变系数跟踪技术,动态调节追踪过程中的步长,缩短最大功率点的跟踪时间。仿真结果表明,与传统电导增量法相比,新型变步长电导增量法在一定程度上可以增加光伏系统的跟踪精度、响应速度,并减小MPPT仿真曲线的稳态振荡幅度,提高光伏系统的能量转换效率。     

基于电导增量法与改进粒子群算法混合控制的最大功率点跟踪策略

利用传统电导增量法跟踪最大功率点时,若跟踪步长较大,则跟踪速度较快,但跟踪精度较差;反之,则跟踪精度较好,但跟踪速度较慢。当外界环境发生变化时,利用传统电导增量法得到的功率变化曲线振荡幅度较大,功率损失较多。改进粒子群算法能够对外界环境的突变迅速作出响应,利用该方法得到的功率变化曲线振荡幅度较小,但是很难精确地定位到最大功率点(MPP)。因此,文章提出一种混合控制的最大功率点跟踪(MPPT)策略,先利用改进粒子群算法快速跟踪到MPP附近,然后利用小步长电导增量法对MPP进行精细搜索。仿真结果表明,该跟踪策略在一定程度上能够增加跟踪系统的响应速度、跟踪精度,减小功率变化曲线的振荡幅度。     

一种基于增量电导法的变步长MPPT算法

针对光伏发电最大功率点跟踪(MPPT)技术的研究和发展现状,提出一种基于增量电导法的变步长MPPT算法。建立太阳电池参数模型,通过理论分析、Matlab仿真实验和实际实验验证,推导出一种基于增量电导法的变步长电压方程。该方程中步长电压ΔU关于输出电压U在太阳电池的[0.20U_(oc),0.93Uoc]区间具有近似直线特性,通过采集电压电流可解得ΔU-U的直线方程及其值为零时的输出电压Uref,进而再以Uref为起始电压、ΔU为步长电压采用增量电导法进行MPPT。实验结果表明,所提MPPT算法能快速精确跟踪到太阳电池的最大功率点。     

一种基于变步长电导增量法的自适应MPPT控制策略

光伏电池是太阳能光伏发电系统的基本组件,其输出特性易受到工作环境的制约,基于此利用优化的最大功率点跟踪(MPPT)策略能够可以有效地提高光伏发电系统的发电效率。文章针对传统电导增量法的实时性和容错性缺陷,提出了一种基于变步长电导增量法的自适应MPPT控制算法,该算法引入了一套全新的步长调节方法,能够根据外界环境的变化实时调整扰动步长,在一定程度上解决了辐照强度剧烈变化情况下传统电导增量法动态响应速度慢且容易发生误判的问题。文章通过仿真实验验证了所提算法的有效性和合理性。     

一种改进的变步长电导增量光伏电源MPPT控制方法

光伏系统的最大功率点跟踪方法可以最大限度地利用光伏电池所能产生的电能,因此成为提高光伏发电系统运行效率、降低光伏电能成本的研究热点。针对目前常用的扰动观察法速度较慢、电导增量法在最大功率点附近有较大振荡的问题,提出一种改进变步长电导增量的最大功率点跟踪控制方法,该方法既具有电导增量法快速跟踪的优点,又能准确、稳定地跟踪到最大功率点,因此更适于提高光伏电源的能源利用率。对所提方法进行了仿真分析,并比较了几种MPPT算法的跟踪效果,结果表明,所提方法具有快速性、稳定性和有效性。     

基于Z源逆变器的粒子群和模糊变步长电导增量MPPT算法

针对局部阴影环境及环境辐照度阶跃的问题,提出一种基于Z源逆变器的粒子群和模糊变步长电导增量MPPT算法。粒子群通过少量迭代搜索至最大功率点附近,再切换至模糊变步长电导增量法,不仅使得最大功率点跟踪过程用时短,还可保证最大功率点追踪的稳态精度。同时此算法针对辐照度阶跃变化的情况设置重启环节,以便在功率阶跃后能迅速跟踪至新的最大功率点。通过建立的仿真模型和搭建的试验平台验证算法的可行性和优越性。     

基于改进MPPT算法的微电网电能质量改善作用研究

基于最大功率跟踪(MPPT)技术的光伏发电系统,其控制器输出功率参考值由MPPT算法实时计算得到。MPPT算法的快速性、准确性和稳定性可直接影响光伏发电系统的输出功率特性及发电质量。为此,结合单级式光伏并网发电系统的控制原理,分析了MPPT算法对光伏发电系统的控制作用;并针对一种常用变步长扰动算法存在的问题,结合光伏电池的数学模型,提出了一种改进的变步长扰动算法。该算法可有效提高跟踪速度和精度,降低功率波动,提高发电质量。最后,仿真分析了改进前后含光伏发电系统的微电网的电能质量,验证了改进MPPT算法对微电网电能质量的改善作用。     

光伏电池最大功率点跟踪控制方法的对比研究及改进

光伏发电系统中光伏电池的输出特性具有唯一的最大功率点(MPP),需要对光伏电池的最大功率点进行跟踪(MPPT)。文中分析了几种常见的最大功率点跟踪控制方法对比分析了它们的优缺点。针对MPPT控制方法中存在的启动特性较差、跟踪过程不稳定、精度不高等特点,采用一种改进爬山法,该法以恒定电压法作为启动特性及采用变步长进行跟踪控制,并利用Matlab/Simulink搭建了改进爬山法的MPPT控制模型,仿真结果验证该方法的有效性。     

基于跃变探索式电导增量法的光伏阵列全局最大功率点跟踪控制研究

实现光伏阵列最大功率点跟踪(Maximum power point tracking, MPPT)的传统算法已经较为成熟,但是在局部阴影出现后会发生寻优失效,难以实现全局最大功率跟踪(Global maximum power tracking, GMPPT)。为解决该问题,研究人员提出将粒子群(Particle swarm optimization, PSO)等群搜索算法应用在MPPT控制过程中,虽然能够控制工作点稳定在全局最大功率点处,但由于该算法收敛能力依赖于核心参数,在应用过程中有一定概率会导致系统振荡。针对以上问题,在电导增量法(Incremental conductance, INC)的基础上提出跃变探索式电导增量法(Jump explore incremental conductance, JEINC),相较于传统电导增量法而言,具有较强的探索能力,能够在局部阴影下实现全局最大功率点跟踪控制,同时所提算法具有较好的收敛能力,在工作点位于最大功率点附近能够快速稳定。在三种光照环境下进行Matlab仿真,从稳定时间、暂态过程能量损耗率和振荡幅值三个方面验证了所提算法相较于电导增...     

基于不完全微分的自适应步长扰动观察法MPPT 控制

在扰动步长判断条件中引入不完全微分,与微分结合,有效减小传统扰动观察法扰动步长判断函数中完全微分带来的高频(突变)干扰,基于不完全微分实现步长自适应调整,提出基于不完全微分的MPPT控制算法。在Matlab/Simulink环境下的仿真与实验平台上所得实验结果表明,所提出算法能在不影响光伏系统最大功率跟踪速度的前提下,有效减小最大功率点附近的功率振荡,提高MPPT控制的跟踪精度。     

Contents in Volume 23,2014

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Performance assessment of some ice TES systems

In this paper, a performance assessment of four main types of ice storage techniques for space cooling purposes, namely ice slurry systems, ice-on-coil systems (both internal and external melt), and encapsulated ice systems is conducted. A detailed analysis, coupled with a case study based on the literature data, follows. The ice making techniques are compared on the basis of energy and exergy performance criteria including charging, discharging and storage efficiencies, which make up the ice storage and retrieval process. Losses due to heat leakage and irreversibilities from entropy generation are included. A vapor-compression refrigeration cycle with R134a as the working fluid provides the cooling load, while the analysis is performed in both a full storage and partial storage process, with comparisons between these two. In the case of full storage, the energy efficiencies associated with the charging and discharging processes are well over 98% in all cases, while the exergy efficiencies ranged from 46% to 76% for the charging cycle and 18% to 24% for the discharging cycle. For the partial storage systems, all energy and exergy efficiencies were slightly less than that for full storage, due to the increasing effect wall heat leakage has on the decreased storage volume and load. The results show that energy analyses alone do not provide much useful insight into system behavior, since the vast majority of losses in all processes are a result of entropy generation which results from system irreversibilities.     

Effect of co-cultivation of Chlamydomonas reinhardtii with Azotobacter chroococcum on hydrogen production

Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H₂ yield. The results demonstrated that the optimal culture conditions for the highest H₂ production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 μE m^?2 s^?1. Under these conditions, the maximal H₂ yield was 255 μmol mg^?1 Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H₂ yield included decreased O₂ content, enhanced algal growth, and increased H₂ase activity and starch content of the combined system.     

Aerodynamic performance effects of leading-edge geometry in gas-turbine blades

The purpose of this paper is to illustrate the advantages of the direct surface-curvature distribution blade-design method, originally proposed by Korakianitis, for the leading-edge design of turbine blades, and by extension for other types of airfoil shapes. The leading edge shape is critical in the blade design process, and it is quite difficult to completely control with inverse, semi-inverse or other direct-design methods. The blade-design method is briefly reviewed, and then the effort is concentrated on smoothly blending the leading edge shape (circle or ellipse, etc.) with the main part of the blade surface, in a manner that avoids leading-edge flow-disturbance and flow-separation regions. Specifically in the leading edge region we return to the second-order (parabolic) construction line coupled with a revised smoothing equation between the leading-edge shape and the main part of the blade. The Hodson–Dominy blade has been used as an example to show the ability of this blade-design method to remove leading-edge separation bubbles in gas turbine blades and other airfoil shapes that have very sharp changes in curvature near the leading edge. An additional gas turbine blade example has been used to illustrate the ability of this method to design leading edge shapes that avoid leading-edge separation bubbles at off-design conditions. This gas turbine blade example has inlet flow angle 0°, outlet flow angle −64.3°, and tangential lift coefficient 1.045, in a region of parameters where the leading edge shape is critical for the overall blade performance. Computed results at incidences of −10°,   −5°,   +5°,   +10° are used to illustrate the complete removal of leading edge flow-disturbance regions, thus minimizing the possibility of leading-edge separation bubbles, while concurrently minimizing the stagnation pressure drop from inlet to outlet. These results using two difficult example cases of leading edge geometries illustrate the superiority and utility of this blade-design method when compared with other direct or inverse blade-design methods.     

Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NO_x) emissions, while producing lower emissions of carbon dioxide (CO₂), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NO_x emissions. High NO_x emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas-hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NO_x and CO₂ emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.     

Exergetic evaluation of 5 biowastes-to-biofuels routes via gasification

This paper presents the exergy analysis results for the production of several biofuels, i.e., SNG (synthetic natural gas), methanol, Fischer–Tropsch fuels, hydrogen, as well as heat and electricity, from several biowastes generated in the Dutch province of Friesland, selected as one of the typical European regions. Biowastes have been classified in 5 virtual streams according to their ultimate and proximate analysis. All production chains have been modeled in Aspen Plus in order to analyze their technical performance. The common steps for all the production chains are: pre-treatment, gasification, gas cleaning, water–gas-shift reactions, catalytic reactors, final gas separation and upgrading. Optionally a gas turbine and steam turbines are used to produce heat and electricity from unconverted gas and heat removal, respectively. The results show that, in terms of mass conversion, methanol production seems to be the most efficient process for all the biowastes. SNG synthesis is preferred when exergetic efficiency is the objective parameter, but hydrogen process is more efficient when the performance is analyzed by means of the 1st Law of Thermodynamics. The main exergy losses account for the gasification section, except in the electricity and heat production chain, where the combined cycle is less efficient.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.     

Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy – V. Reactor modeling,optimization, and scale-up

A chemical reactor for the steam-gasification of carbonaceous particles (e.g. coal, coke) is considered for using concentrated solar radiation as the energy source of high-temperature process heat. A two-phase reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is applied to optimize the reactor geometrical configuration and operational parameters (feedstock's initial particle size, feeding rates, and solar power input) for maximum reaction extent and solar-to-chemical energy conversion efficiency of a 5 kW prototype reactor and its scale-up to 300 kW. For the 300 kW reactor, complete reaction extent is predicted for an initial feedstock particle size up to 35 μm at residence times of less than 10 s and peak temperatures of 1818 K, yielding high-quality syngas with a calorific content that has been solar-upgraded by 19% over that of the petcoke gasified.     

液压系统常见的故障诊断及处理

任何工程机械式液压设备使用时出现故障是不可避免的。但是怎样确定故障的原因及找到好的解决方法,这是使用者最关心的问题。讲述了液压系统常见的故障及其排除方法。     

汽轮机数字电液调节系统挂闸异常的技术完善

分析了200MW汽轮机数字电液调节系统在运行中存在的挂闸异常问题,采取了相应的技术处理措施,且运行实践效果良好。