共查询到20条相似文献,搜索用时 15 毫秒
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This paper presents theoretical predictions and experimental measurements of the enhanced illuminance available in high latitude greenhouses in winter by use of double-glazing comprising laser-cut panels and glass to deflect low elevation sunlight onto the crop. Provided the structure of the greenhouse is optimised for light enhancement, i.e, single slope south-facing roof with vertical north wall, the predicted daily time averaged enhancement, the ratio of horizontal illuminance with laser-cut panel double-glazing to that obtained with a single clear glazing, approaches 100% for most of winter in greenhouses at latitudes >50°; much greater than the enhancement that can be achieved with other passive optical systems investigated. The predicted enhancements, which take account of diffuse sky and direct sunlight components, are strongly dependent on latitude and atmospheric turbidity and agree with experimental enhancements measured in scale model greenhouses. 相似文献
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This paper describes an analytical method for estimation of the economical feasibility of a project for wind energy utilization in Jordan. The method is applied to a windfarm and is based on the wind speed distribution at the site considered and on the financial parameters for the complete plant and its running costs. For this method, equations were derived to calculate the cost of a kWh generated by wind turbines with different characteristics as a function of the capital investment per kW installed capacity. The method was applied to the sites at which Royal Scientific Society performs wind speed measurements, and the results indicate that some of these sites are well suited for cost efficient generation of electricity by wind energy. The results also show that a wind tribine which is designed to have a relatively low rated speed has a wider range of economical operation than a wind turbine with a higher rated speed for the sites investigated assuming that the maximum rated power for all for the wind turbines is held constant. 相似文献
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D. L. Critten 《Solar Energy》1988,41(6):583-591
A computer model to evaluate and optimize the light gain produced by downwardly transmitting or reflecting low elevation winter sunlight is described. Verification of the model is by comparison with the experimental enhancement of winter sunlight provided by two different prismatic arrays. One, of perspex, redirects light both southerly and northerly, the other, of glass, is mounted to transmit light only. Manufacturing irregularities, such as curvature induced in nominally flat prism surfaces, are successfully encompassed within the model and can be used as additional design parameters. 相似文献
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The annual number of hours with low sun is highest at 60–70° latitude, making the scope for horizon screening effects particularly wide there. The present paper presents a technique for spatially continuous mapping of local screening effects. The method requires a digital topographical map, and radiation data from a site with free horizon. Screening effects are found to introduce local spatial variations in the solar radiation field, amounting to a factor of more than two within a 10 × 10 km area around Bergen (69°24′N, 5°19′E). 相似文献
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The present study analyzes the correlation between the optimal angle for a fixed Building Integrated Photovoltaic (BIPV) system and the latitude of the system's site as measured in degrees according to estimates made with PVSYST 3.41 software. Calculations were made for a BIPV south orientated tilted roof at 20 different locations in 14 countries, ranging from 0° to 85° latitude in the Northern Hemisphere. In order to prove the reliability of using the latitude angle as the angle for the tilted panel, the correlation was made between the performance obtained with the system using the optimal angle and the system with the site's location angle. Results indicate that an average of 98.6% a system's performance with the optimal angle can be obtained using the latitude angle for the tilted panel. 相似文献
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黄、渭、洛河汇流区高程变化带来的影响 总被引:1,自引:0,他引:1
康明放 《电网与水力发电进展》2003,19(3):22-24
通过对黄渭洛汇流区河势变化及高程变化所引起的渭河下游淤积规律的研究,并针对各自的运行规律采取相应的措施,确保汇流区及渭河两岸的防洪安全及水库的运行安全。有利于提高渭河中下游生态环境质量及渭河流域的综合治理。 相似文献
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三门峡水库蓄水运用2年后,潼关高程急剧抬升。水库修建前,潼关河床基本处于冲淤平衡或微淤状态。经分析,潼关河床高程升高的主要原因是三门峡水库运用方式,在水库"蓄清排浑"控制运用期,非汛期、汛期运行水位及上游来水条件是潼关高程上升的影响因素。 相似文献
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The results of precise levelling measurements on a specially constructed network of benchmarks in the Travale geothermal area (Tuscany, Italy) revealed the subsidence of the central part of this area, at an average rate of 20 mm/year in the period 1978 – 1980. Two sets of gravity measurements over the same time-interval, using two Lacoste — Romberg gravimeters, have an average standard error of 2–4 μGal for the main network, and 4–8 μGal for the auxiliary network. The observed g variations fall within the error range in most of the stations. The variations noted in the stations in the south-western area of the field clearly fall outside the confidence interval, and cannot entirely be attributed to changes in elevation.An absolute gravity station was set up at Palazzo at Piano (Siena), where measurements were made by the IMGC absolute gravimeter, to detect any long-term gravity variations induced by geodynamic events. 相似文献
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扩散系数在化工设计和研究中是不可缺少的传递特性。但其数据却相对缺乏,因此需要寻找一种方法来预测这个特性就显得十分重要。利用分子动力学方法模拟了简单流体的自扩散系数。模拟分别采用Green-Kubo法(VACF:velocity autocorrelation function)和Einstein法(MSD:mean square displacement)。模拟结果与实验数据吻合较好,误差在10%左右。两种方法的平均值与实验结果误差在7%左右。同时还模拟了流体自扩散系数随温度的变化关系。结果表明,自扩散系数与温度满足Arrenhius关系,数据相关性在0.99以上,计算得到的自扩散激活能分别为1 258 J/mol(VACF)、1 272 J/mol(MSD)和平均值1 265 J/mol。 相似文献
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Skeletal mechanism generation for surrogate fuels using directed relation graph with error propagation and sensitivity analysis 总被引:1,自引:0,他引:1
A novel implementation for the skeletal reduction of large detailed reaction mechanisms using the directed relation graph with error propagation and sensitivity analysis (DRGEPSA) is developed and presented with examples for three hydrocarbon components, n-heptane, iso-octane, and n-decane, relevant to surrogate fuel development. DRGEPSA integrates two previously developed methods, directed relation graph-aided sensitivity analysis (DRGASA) and directed relation graph with error propagation (DRGEP), by first applying DRGEP to efficiently remove many unimportant species prior to sensitivity analysis to further remove unimportant species, producing an optimally small skeletal mechanism for a given error limit. It is illustrated that the combination of the DRGEP and DRGASA methods allows the DRGEPSA approach to overcome the weaknesses of each, specifically that DRGEP cannot identify all unimportant species and that DRGASA shields unimportant species from removal. Skeletal mechanisms for n-heptane and iso-octane generated using the DRGEP, DRGASA, and DRGEPSA methods are presented and compared to illustrate the improvement of DRGEPSA. From a detailed reaction mechanism for n-alkanes covering n-octane to n-hexadecane with 2115 species and 8157 reactions, two skeletal mechanisms for n-decane generated using DRGEPSA, one covering a comprehensive range of temperature, pressure, and equivalence ratio conditions for autoignition and the other limited to high temperatures, are presented and validated. The comprehensive skeletal mechanism consists of 202 species and 846 reactions and the high-temperature skeletal mechanism consists of 51 species and 256 reactions. Both mechanisms are further demonstrated to well reproduce the results of the detailed mechanism in perfectly-stirred reactor and laminar flame simulations over a wide range of conditions. The comprehensive and high-temperature n-decane skeletal mechanisms are included as supplementary material with this article. 相似文献
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A functional relationship between base temperature and mean heating (or cooling) degree days per day over a given period is derived. A similar relationship is derived for the variance of degree days per day over the period. The relationships are presented in graphical form. 相似文献
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Downscaling of the Meteosat-derived solar radiation (∼5 km grid resolution) is based on decomposing the global irradiance and correcting the systematic bias of its components using the elevation and horizon shadowing that are derived from the SRTM-3 digital elevation model (3 arc sec resolution). The procedure first applies the elevation correction based on the difference between coarse and high spatial resolution. Global irradiance is split into direct, diffuse circumsolar and diffuse isotropic components using statistical models, and then corrections due to terrain shading and sky-view fraction are applied. The effect of reflected irradiance is analysed only in the theoretical section. The method was applied in the eastern Andalusia, Spain, and the validation was carried out for 22 days on April, July and December 2006 comparing 15-min estimates of the satellite-derived solar irradiance and observations from nine ground stations. Overall, the corrections of the satellite estimates in the studied region strongly reduced the mean bias of the estimates for clear and cloudy days from roughly 2.3% to 0.4%. 相似文献
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Cobalt oxide and cobalt oxide–silver selective surfaces were prepared in a galvanostatic mode by means an electrodeposition bath, using different deposition times, on stainless steel substrates. Later, the roughness factor of the selective surfaces, were determined in order to establish one relation with the electrodeposition time and the solar absorptance of the black cobalt and black cobalt–silver coatings. 相似文献
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Portability is one of the many potential advantages of utilizing ground-based measurement devices such as SODARs and LIDARs instead of meteorological towers for wind resource assessment. This paper investigates the use of a monitoring strategy that leverages the portability of ground-based devices, dubbed the “round robin site assessment method.” The premise is to measure the wind resource at multiple sites in a single year using a single portable device, but to discontinuously distribute the measurement time at each site over the whole year, so that the total measurement period comprises smaller segments of measured data. This measured data set is then utilized in the measure-correlate-predict (MCP) process to predict the long-term wind resource at the site. This method aims to increase the number of sites assessed in a single year, without the sacrifice in accuracy and precision that usually accompanies shorter measurement periods. The performance of the round robin site assessment method was compared to the standard method, in which the measured data are continuous. The results demonstrate that the round robin site assessment method is an effective monitoring strategy that improves the accuracy and reduces the uncertainty of MCP predictions for measurement periods less than 1 year. In fact, the round robin site assessment method compares favorably to the accuracy and uncertainty of a full year of resource assessment. While there are some tradeoffs to be made by using the round robin site assessment method, it is potentially a very useful strategy for wind resource assessment. 相似文献
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C. Hutter K. Sefiane T.G. Karayiannis A.J. Walton R.A. Nelson D.B.R. Kenning 《International Journal of Heat and Mass Transfer》2012,55(11-12):2769-2778
Nucleate boiling is commonly characterised as a very complex and elusive process. Many involved mechanisms are still not fully understood and more detailed consideration is needed. In this study, bubble growth from micro-fabricated artificial cavities with varied spacing on a horizontal 380 μm thick silicon wafer was investigated. The horizontally oriented boiling surface was heated by a thin resistance heater integrated on the rear of the silicon test section. The temperature was measured using 16 integrated micro-sensors situated on the boiling surface, each with an artificial cavity located in its geometrical centre. Experiments with three different spacings 1.5, 1.2 and 0.84 mm in between cavities with a nominal mouth diameter of 10 μm and a depth of 80 μm were undertaken. To conduct pool boiling experiments, the test section was mounted inside a closed stainless steel boiling chamber with optical access and completely immersed in degassed fluorinert FC-72. Bubble nucleation, growth and detachment at 0.5 and 1 bar absolute pressure were investigated using high-speed imaging. The effect of decreasing inter-site distance on bubble nucleation frequency, bubble departure frequency and diameter with increasing wall superheat is presented. Furthermore, the frequency of horizontal bubble coalescence was determined. The regions of influence on the measured frequencies and bubble departure diameter were compared with recently published findings. 相似文献
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A.R. Malheiro 《Journal of power sources》2010,195(10):3111-1067
In this work, physical and catalytic properties of Pt-Fe/C nanocatalysts of nominal compositions Pt:Fe 70:30 and 50:50, prepared by a polyol process using a long-chain diol as reducer (hexadecanediol) and oleic acid and oleylamine as stabilizers, are reported. As-prepared materials have very small particle size (2.2 nm), narrow particle size distribution, and homogeneous dispersion on the carbon support. The average compositions determined by energy dispersive X-ray analysis are Pt75Fe25/C and Pt60Fe40/C. Data for samples submitted to heat treatment in hydrogen atmosphere to induce Pt surface segregation are also presented. X-ray diffraction and transmission electronic microscopy are used to examine all as-prepared and heat-treated catalysts. Electronic properties are analyzed based on in situ dispersive X-ray absorption spectroscopy data. Measurements of electrocatalytic activity for oxygen reduction show that all Pt-Fe/C have electrocatalytic activities superior to that of Pt/C. Nanocatalysts with a Pt-rich surface have an enhanced performance for the reduction of oxygen but measurements carried out in methanol containing solutions show that Pt-enriched surfaces have an inferior methanol tolerance. 相似文献