非理想电网下并网逆变器直流分量检测及抑制方法

为抑制非理想电网条件下光伏并网发电系统中并网逆变器的并网电流直流分量,提出一种基于直流侧母线电压纹波分析的间接检测及补偿方法。首先建立并网逆变器数学模型,分析并网电流直流分量产生机理;然后研究并网电流与母线纹波电压的关系,得出在非理想电网条件下,并网电流直流分量可等效为母线纹波电压的奇倍频纹波分量的结论;通过分段积分一个电网周期的母线电压求得并网电流直流分量解析式,以该式为反馈量加入系统控制环节可有效抑制并网电流直流分量的产生。最后通过仿真和实验验证所提方法的可靠性。     

基于无差拍控制的光伏并网逆变器直流分量抑制研究

直流分量抑制是非隔离型光伏并网逆变系统的关键技术之一,国际并网认证标准中普遍要求光伏并网逆变器注入电网的直流分量不可超过额定电流的0.5%。文章在分析无差拍并网控制策略的基础上,通过对电压指令校正及电流指令校正两种控制方法的直流分量抑制效果进行对比,采用一种简单有效的直流分量检测及零点校正方法进行直流分量抑制。对3 kW单相非隔离型光伏并网逆变器进行相关试验,结果证明了控制算法的有效性。     

光伏并网与电能质量治理统一控制

为提高光伏并网系统利用率,并使其具备主动参与电网电能质量治理的能力,提出一种协调并网发电与网侧电能质量治理的统一控制策略。研究基于光伏组件出力的运行模式切换与直流侧电压控制方法,提出基于无功/谐波优先级的逆变器容量分配和指令电流合成策略。设计并研制基于FPGA+RT架构的并网控制器,构建DC/AC单级结构的光伏并网实验系统。实验验证统一控制策略的可行性和有效性,为分布式光伏电源可定制电能并网发电提供基础。     

光伏发电并网逆变器建模及电能质量评估

鉴于光伏发电并网逆变器的建模对于光伏大规模接入、保障系统稳定运行具有重要意义,提出了系统仿真方案,利用Hammerstein-Wiener(HW)非线性模型对光伏并网逆变器的运行进行仿真。通过试验获得直流逆变器电压电流波形、交流逆变器电压电流波形、电压公共耦合点、电网和负荷电流;利用编程确定各种模拟波形并搜索与实际波形相比最准确的模型,同时将该方法运用到电能质量分析中,进而完成对系统的分析和建模。模拟结果表明,该模型效果较好,可以为系统规划、防止系统故障和改善电能质量等方面提供理论依据。     

基于LC滤波的光伏逆变器电感电流反馈控制策略研究

针对光伏并网逆变器的特点,基于电感电流反馈控制的光伏并网逆变器,提出了参考电流相位超前的电流内环控制策略。通过分析单相并网逆变器结构,推导了LC滤波器上电压电流矢量关系。加入电网电压瞬时值前馈解耦控制,研究了比例调节和准比例谐振调节两种策略下参考电流与输出电流的关联。基于一台3 kW逆变器为实验平台的理论分析和实验结果表明,采用该策略的逆变器并网电流时刻跟踪电网电压频率和相位,功率因数为1,并网电流谐波失真度低于3%。     

基于母线电压纹波分析的并网逆变器直流分量检测及抑制方法

为限制光伏逆变器的并网电流直流分量注入电网,提高光伏发电电能质量,提出一种基于直流侧母线电压纹波分析的直流分量检测及补偿方法。该算法以LCL滤波的光伏并网逆变器数学模型为基础,推导分析直流母线电压纹波产生机理,得出直流母线工频纹波由并网电流直流分量产生的结论;通过对一个周期的母线电压分段积分可解析获得直流分量定量描述关系,并利用该解析值实时在线修正并网参考电流,结合电网电压前馈、并网电流外环、电容电流内环的复合控制算法设计电流控制器。仿真与实验结果表明,在不增加外围硬件电路的情况下,该算法可有效抑制并网电流直流分量。     

不平衡电网电压下T型三电平光伏并网逆变器的有限集模型预测控制

为实现电网电压不平衡时对T型三电平光伏并网系统输出功率和电流质量的控制,以达到入网功率平稳或电流正弦为控制目标,结合光伏阵列输出功率前馈,在两相静止坐标系下提出一种直流母线电压外环PI控制、并网电流内环有限集模型预测控制的控制策略,并在电压外环中引入2倍频陷波器以获得平滑的入网功率参考值。仿真结果表明:当电网电压不对称时,采用所提控制策略能够实现对入网有功、无功功率2倍频脉动及负序电流的分别抑制或协调控制,且并网电流谐波畸变小、入网电能质量高,同时实现T型三电平逆变器的中点电位平衡。     

宽范围电网下并网逆变器延时小信号建模及不连续稳定区间分析

该文以光伏并网逆变器为例,借助pade近似方法,建立考虑控制延时环节的光伏并网逆变器状态空间小信号模型,利用特征值分析方法研究宽范围电网强度下考虑控制延时的光伏并网逆变器稳定性问题.算例仿真分析结果表明:在考虑控制延时的光伏并网逆变器系统中,当电网强度在较宽范围内变化时,系统存在不连续稳定区间;考虑控制延时的系统小信号...     

级联H桥光伏并网逆变器的无锁相环控制策略

级联H桥光伏并网系统采用多电平调制方法和各模块独立的MPPT控制,具有较高的效率,但是系统控制性能受锁相环影响较大,当电网电压畸变时锁相环的测量准确度会受到严重影响,使系统控制性能下降,尤其是在功率不平衡的情况下会出现过调制,导致系统不稳定。为此,提出一种应用在级联H桥光伏并网逆变器的无锁相环控制策略。该策略在基于无功补偿的级联H桥光伏并网逆变器控制策略的基础上,采用电网电压基波同步信号算法,在电网电压畸变时无需锁相环即可获取与电网电压基波同频同相的正、余弦信号,较好地满足了系统在功率平衡、功率不平衡2种工作状态下的控制要求。仿真结果和实验结果都验证了所提控制策略的有效性。     

中压电网中光伏逆变器故障穿越控制策略

针对分布式发电系统的中压电网技术导则(MVD),以三相三电平光伏并网逆变器为研究对象,在电网发生对称故障时,功率不平衡会导致逆变器直流母线电压不稳定和逆变器输出电流过冲,此时采用直接功率控制方法,使光伏逆变器按照入网导则的要求进行故障穿越;当电网发生三相不对称故障时,采用改进的正、负序电流双环控制器,抑制负序电流,使逆变器能稳定工作且向电网供应THD值较小的电流,在保证电能质量的前提下实现故障穿越。     

Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest

Woody biomass in Finland and Sweden comprises mainly four wood species: spruce, pine, birch and aspen. To study the ash, which may cause problems for the combustion device, one tree of each species were cut down and prepared for comparisons with fuel samples. Well-defined samples of wood, bark and foliage were analyzed on 11 ash-forming elements: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S and Cl. The ash content in the wood tissues (0.2–0.7%) was low compared to the ash content in the bark tissues (1.9–6.4%) and the foliage (2.4–7.7%). The woods’ content of ash-forming elements was consequently low; the highest contents were of Ca (410–1340 ppm) and K (200–1310), followed by Mg (70–290), Mn (15–240) and P (0–350). Present in the wood was also Si (50–190), S (50–200) and Cl (30–110). The bark tissues showed much higher element contents; Ca (4800–19,100 ppm) and K (1600–6400) were the dominating elements, followed by Mg (210–2400), P (210–1200), Mn (110–1100) and S (310–750), but the Cl contents (40–330) were only moderately higher in the bark than in the wood. The young foliage (shoots and deciduous leaves) had the highest K (7100–25,000 ppm), P (1600–5300) and S (1100–2600) contents of all tissues, while the shoots of spruce had the highest Cl contents (820–1360) and its needles the highest Si content (5000–11,300). This paper presented a new approach in fuel characterization: the method excludes the presence of impurities, and focus on different categories of plant tissues. This made it possible to discuss the contents of ash element in a wide spectrum of fuel-types, which are of large importance for the energy production in Finland and Sweden.     

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.     

NEXT GENERATION ENGINEERED MATERIALS FOR ULTRA SUPERCRITICAL STEAM TURBINES

正1 ABSTRACT To reduce the effect of global warming on our climate,the levels of CO2emissions should be reduced.One way to do this is to increase the efficiency of electricity production from fossil fuels.This will in turn reduce the amount of CO2emissions for a given power output.Using US practice for efficiency calculations,then a move from a typical US plant running at 37%efficiency to a 760℃/38.5 MPa(1 400/5 580 psi)plant running at 48%efficiency would reduce CO2emissions by 170kg/MW.hr or 25%.     

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.