变频滚动转子式压缩机变工况性能的实验研究

以R32变频滚动转子式制冷系统为研究对象,通过改变电子膨胀阀开度、压缩机运转频率、蒸发温度和吸排气压比,研究压缩机在不同工况尤其是不同吸气状态下的运行性能,结果表明:在过热段,压缩机容积效率基本不随过热度的变化而变化;在两相段,容积效率随吸气干度的减小呈线性下降趋势,且各工况斜率基本相同。蒸发温度基本不影响压缩机容积效率,而压比越高,容积效率越低;压缩机电效率在过热段和两相段分别随过热度和吸气干度的减小而线性减小,且两相段斜率大于过热段斜率。在相同蒸发温度下,压比越高,电效率越小;在相同压比下,蒸发温度越高,电效率越小;在各工况下,系统COP在过热段基本不变,在两相段随干度减小而减小。吸气干度0.90处的COP比吸气饱和点的COP平均降低了5.5%;容积效率随压缩机频率的提高而增大,电效率和系统COP随压缩机频率的提高而减小。     

VRF制冷系统变吸气状态下频率与阀开度的同步调节

针对变流量制冷(Variable Refrigerant Flow,VRF)系统变吸气状态下带液量的精确控制以及控制元件所引起的滞后或超调现象,以变频滚动转子式制冷系统为研究对象,分别通过恒定频率不同阀开度与恒定阀开度不同频率两个实验,建立其单独控制下的拟合模型,得到其在不同工况下的同步控制方法。结果表明:过热度随阀开度的增大而减小,在过热度为6 K以下,即阀开度为27.8%～30%时,过热度控制难度上升,提高冷冻水温度可改善这一状况;过热度随频率的降低而减小,在过热度为5 K以下,即频率为44.5～46.5 Hz时,控制难度上升;吸气干度随阀开度增大而下降,随频率降低而减小;在某一工况下,以系统质量流量为控制目标,可以拟合得到频率与阀开度的关系式,实现同步控制,在变吸气状态可以精确控制吸气干度,不损坏压缩机,并使系统迅速达到稳定状态。     

变流量制冷系统循环性能稳定性的实验研究

针对变制冷剂流量(Variable Refrigerant Flow,VRF)空调系统中过热度、制冷量及性能系数(Coefficient of Performance,COP)等参数的不稳定现象,利用R32变流量制冷循环实验台,研究过热度振荡对系统参数稳定性的影响。结果表明:随着系统内制冷剂流量的增加,蒸发器内制冷剂流型迅速变化,引起换热方式的交替,过热度进入最小稳定过热度线(Minimal Stable Superheat,MSS)的不稳定区间,从而产生了过热度振荡;当过热度处于不同运行频率下的最小稳定点时,系统性能达到最佳,控制难度得到优化;系统定压比运行下,频率的增大对压缩机耗功有更直接的影响,因而导致COP与频率成反比。     

直膨式太阳能热泵热水器微型控制系统的单片机设计

阐述了一种基于Renesas单片机的直膨式太阳能热泵热水器中央控制器的硬件设计和软件算法设计.硬件设计着重于系统的功能性和可靠性;控制算法以太阳辐射强度变化和过热度偏差变化为输入量,以电子膨胀阀开度变化以及变频器运行频率为输出量,在安全运行的前提下保持较低的过热度,最大限度地利用蒸发器的面积,有效地管理压缩机运行,实现压缩机容量调节,使系统得到合理匹配及稳定运行.     

高进水温度下CO_2空气源热泵热水系统变频运行性能

构建CO_2空气源热泵热水系统变频运行实验装置,测试分析高环境温度、高进水温度下变频运行对CO_2空气源热泵热水系统气体冷却器压力、制热功率及性能系数(COP)的影响。实验结果表明,CO_2空气源热泵压缩机降频运行能有效避免压缩机过载、抑制气体冷却器压力过高等问题。随压缩机频率的减小,气体冷却器压力持续减小,COP先增大后减小,存在最优压缩机频率值。在高环境温度和高进水温度工况下,CO_2空气源热泵热水系统压缩机最优运行频率区间为35～45 Hz。     

使用电子膨胀阀的空气源热泵热水器的实验研究与优化

为充分利用电子膨胀阀调节灵活的特性,对空气源热泵热水器在定阀开度和定过热度下的系统性能进行对比实验,并提出了电子膨胀阀的排气温度控制法。实验结果表明:定阀开度下运行易发生吸气带液现象,吸气带液量的大小对系统性能的影响较大,但可由排气温度加以判断,适当增大阀开度可提升平均制热效率(COPa)并降低排气温度;与定阀开度相比,定过热度下运行时COPa普遍较高,排气温度高,但改变定过热度对COPa的影响较小,这为过热度的简单控制提供了依据;提出的排气温度控制法以排气温度判断吸气带液量大小,对过热度进行简单控制,满足了大制冷剂流量和小过热度的制热要求,试验测试中COPa提升8.24%,排气温度降低7.16%,系统性能达到了预期的优化效果。     

空气源热泵系统湿压缩与电子膨胀阀调控方式的研究

针对空气源热泵吸气带液问题,通过调节电子膨胀阀开度改变压缩机吸气口制冷剂状态,研究了四种不同调控工况对系统性能和参数的影响,分析不同调控工况下系统性能的变化。实验结果表明:较大的电子膨胀阀开度可提高蒸发压力和制冷剂质量流量,从而改善系统性能,但这不是湿压缩所产生的影响,相反后期大量的吸气带液会导致系统性能降低;吸气带液可以有效降低压缩机排气温度,同时也会使单位制热量和比功降低;在实际运行过程中,应采取定过热度控制方法,且过热度应尽量小,但要避免压缩机吸气带液。     

不同调节方式对循环加热式CO2热泵性能影响的实验研究

为研究膨胀阀开度和压缩机频率对循环加热式CO₂热泵制热性能的影响,采用实验的方法研究了阀开度35%,37%,40%和频率75,80,85 Hz工况下制热量和制热系数COP瞬时值和平均值以及加热时间的变化。结果表明：随着压缩机频率增加,平均制热量和最大瞬时制热量增加,平均COP和最大瞬时COP减小,加热时间缩短;随着阀开度增大,平均制热量和平均COP减小,最大瞬时制热量和COP增大,加热时间延长;当压缩机频率一定时,在不同的水箱温度范围采用合适的膨胀阀开度可以有效提高热泵的平均COP;以本文75 Hz工况为例,当水温小于25.9℃时采用40%开度,当水温在25.9～35.4℃之间时采用37%开度,当水温大于35.4℃时采用35%开度;合理匹配开度和频率可以最大程度提高平均制热量,同时抑制平均COP的减小。     

低温过热有机朗肯循环工质筛选及参数优化

以低温烟气余热驱动的内回热有机朗肯(organic Rankine cycles,ORC)系统为例,分析系统净输出功、透平膨胀比、热效率、热回收率、损失、效率以及比净功等热力性能评价指标随蒸发温度和过热度的变化规律,确定系统最佳工质及最优蒸发温度和过热度。提出用预热系数、潜热系数、过热系数与内回热系数解释系统热效率随工质临界温度变化的原因。研究结果表明:随蒸发温度升高,系统净输出功先增大后减小,热回收率和总损减小,透平膨胀比、热效率和效率增大。适当过热对于ORC系统十分重要,不仅能降低透平膨胀比,提高系统运行稳定性,还可减小系统总损,提高系统效率,增大工质比净功。经对比发现,丁烷为适合该文所选热源的最佳工质,在蒸发温度为100℃、过热度为5℃工况下能取得最佳热力性能。     

空气源冷热水热泵系统关键技术实验研究

以低环温空气源户式地暖机组为对象,对采用变频压缩机的空气源冷热水热泵系统在低温环境下的启动性能进行试验研究。实验结果表明,经过低温环境长期放置的热泵机组,首次上电启动过程中,系统存在抽真空现象,造成蒸发侧压力过低使得系统出现低压保护而启动失效。可通过结合压缩机启动初始频率及节流元件的初始开度,保证机组在低温环境低水温顺利启动,其研究结果可为产品开发提供参考。     

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.     

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.     

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.     

Decomposition of limestone in a solar reactor

The thermal decomposition of limestone has been selected as a model reaction for developing and testing an atmospheric open solar reactor. The reactor consists of a cyclone gas/particle separator which has been modified to let the concentrated solar energy enter through a windowless aperture. The reacting particles are directly exposed to the solar irradiation. Experimentation with a 60 kW reactor prototype was conducted at PSI's 90m² parabolic solar concentrator, in a continuous mode of operation. A counter-current flow heat exchanger was employed to preheat the reactants. Eighty five percent degree of calcination was obtained for cement raw material and 15% of the solar input was converted into chemical energy (enthalpy).The technical feasibility of the solar thermal decomposition of limestone was experimentally demonstrated. The use of solar energy as a source for high-temperature process heat offers the potential of reducing significantly the CO₂ emissions from lime producing plants. Such a solar thermochemical process can find application in sunny rural areas for avoiding deforestation.     

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.     

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。     

Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen

Increasing awareness of environmental problems caused by the current use of fossil fuel-based energy, has led to the search for alternatives. Hydrogen is a good alternative and the cyanobacterium Anabaena sp. PCC 7120 is naturally able to produce molecular hydrogen, photosynthetically from water and light. However, this H₂ is rapidly consumed by the uptake hydrogenase.This study evaluated the hydrogen production of Anabaena sp. PCC 7120 wild-type and mutants: hupL⁻ (deficient in the uptake hydrogenase), hoxH⁻ (deficient in the bidirectional hydrogenase) and hupL⁻/hoxH⁻ (deficient in both hydrogenases) on several experimental conditions, such as gas atmosphere (argon and propane with or without N₂ and/or CO₂ addition), light intensity (54 and 152 ??Em⁻²s⁻¹), light regime (continuous and light/dark cycles 16 h/8 h) and nickel concentrations in the culture medium.In every assay, the hupL⁻ and hupL⁻/hoxH⁻ mutants stood out over wild-type cells and the hoxH⁻ mutant. Nevertheless, the hupL⁻ mutant showed the best hydrogen production except in an argon atmosphere under 16 h light/8 h dark cycles at 54 ??Em⁻²s⁻¹ in the light period, with 1 ??M of NiCl₂ supplementation in the culture medium, and under a propane atmosphere.In all strains, higher light intensity leads to higher hydrogen production and if there is a daily 1% of CO₂ addition in the gas atmosphere, hydrogen production could increase 5.8 times, related to the great increase in heterocysts differentiation (5 times more, approximately), whereas nickel supplementation in the culture medium was not shown to increase hydrogen production. The daily incorporation of 1% of CO₂ plus 1% of N₂ did not affect positively hydrogen production rate.     

Economie d'énergie en trigénération

Trigeneration is defined as the production of three useful forms of energy—heat, cold and power—from a primary source of energy such as natural gas or oil. For instance, trigeneration systems typically produce electrical power via a reciprocating engine or gas turbine and recover a large percentage of the heat energy retained in the lubricating oil, exhaust gas and coolant water systems to maximize the utilization of the primary fuel. The heat produced can be totally or partially used to fuel absorption refrigerators. Therefore, trigeneration systems enjoy an inherently high efficiency and have the potential to significantly reduce the energy-related operation costs of facilities. In this paper, we describe a model of characterization of trigeneration systems trough the condition of primary energy saving and the quality index, compared to the separate production of heat, cold and power. The study highlights the importance of the choice of the separate production reference system on the level of primary energy saving and emissions reduction.     

Investigation of the thermodynamic and kinetic properties of La–Fe–B system hydrogen-storage alloys

La–Fe–B hydrogen-storage alloys were prepared using a vacuum induction-quenching furnace with a rotating copper wheel. The thermodynamic and kinetic properties of the La–Fe–B hydrogen-storage alloys were investigated in this work. The P–C–I curves of the La–Fe–B alloys were measured over a H₂ pressure range of 10⁻³ MPa to 2.0 MPa at temperatures of 313, 328, 343 and 353 K. The P–C–I curves revealed that the maximum hydrogen-storage capacity of the alloys exceeded 1.23 wt% at a pressure of approximately 1.0 MPa and temperature of 313 K. The standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides, obtained according to the van't Hoff equation, were consistent with their application as anode materials in alkaline media. The alloys also exhibited good absorption/desorption kinetics at room temperature.     

Mineralogical characterization of the intraseam layers of Lofoi lignite deposits in Florina basin (western Makedonia,northwest Greece)

The mineralogical composition of intraseam layers from Lofoi lignite deposits (northwest Greece) is the subject of the present study. The samples were examined by means of X-ray diffraction (XRD), thermo-gravimetric (TG/DTG) and differential thermal analysis (DTA), and Fourier transform infrared (FT-IR) spectrometry. The clay minerals prevail in most samples, with illite-muscovite being the dominant phase, and kaolinite and chlorite being the other major clay components. No smectite was found. Quartz and feldspars, dominate in two cases. The studied materials are characterized as clays to clayey sands, showing significant similarities with the intraseam layers of the adjacent Achlada lignite deposits.