Estimation of sky luminance in the tropics using artificial neural networks: Modeling and performance comparison with the CIE model

An artificial neural network (ANN) model for estimating sky luminance was developed. A 3-year period (2007–2009) of sky luminance data obtained from measurements at Nakhon Pathom (13.82°N, 100.04°E) and a 1-year period (2008) of the same type of data at Songkhla (7.20°N, 100.60°E), Thailand were used in this study. The ANN model was trained using a back propagation algorithm, based on 2 years data (2007–2008) at Nakhon Pathom for clear, partly cloudy and overcast skies. The trained ANN model was used to predict sky luminance at Nakhon Pathom for the year 2009 for the case of clear, partly cloudy and overcast skies. The results were compared with those of the CIE model. It was found that the ANN model performed better than CIE models for most cases. The ANN model trained with Nakhon Pathom data were also used to predict sky luminance at Songkhla and satisfactory results were obtained.     

Impact of street design on urban microclimate for semi arid climate (Constantine)

Urban Planning has an immense impact on local microclimate which in turn affects the comfort and space quality within a city. The urban open spaces play an important role in creating the urban climate. The urban streets vary in geometry as defined by height/width ratio, sky view factor (SVF) and the orientation that is defined by its long axis. This directly influences the absorption and emission of incoming solar and outgoing long wave radiation which has a significant impact on the temperature variations within the street as well as the surrounding environment (Urban Heat Island).The objective of this research is to evaluate and to compare how the microclimate variation of urban street canyon can affect the built environment. Therefore the main aim of this paper is to discuss and assess the impact of the geometry on the street climate, in down town of Constantine-Algeria (semi arid climate).In order to achieve this goal, a series of site measurements, are utilised. The preliminary results strongly indicate an air temperature difference of about 3–6 ^°C between the urban street and its surrounding rural environment.     

A methodology for investigating the effect of a south oriented surface on natural illuminance received on north oriented glazing of a top lighting system under clear sky conditions

A methodology for investigating the effect of a south oriented surface with varying inclination angle in reflecting natural light towards a north oriented glazing of a top lighting system is proposed for use under clear sky conditions. This was done based on mathematical simulation for a top lighting system. Results have shown the importance of the reflective surface in increasing the illuminance received on the glazing of the top lighting system.     

The effects of multiple covers with condensation and optical degradation of a polyethylene windscreen on the performance of a sky cooling system

This study evaluates the performance of a heat-pipe-augmented sky radiator system modelled using MATLAB. It was developed to simulate thermal networks with the following nodes: polyethylene cover(s), radiator plate with selective surface, heat pipe condenser end, heat pipe evaporator end, thermal storage fluid, fluid container wall, and room. Applying a windscreen of polyethylene that is mostly transparent to long-wave radiation, a drawback is its susceptibility to optical property degradation. Sky fractions of 100% were possible in cities with small cooling loads (Rock Springs, Seattle, San Diego and Denver). Sky fractions of over 50% were achieved in New Orleans and Houston and over 40% in Miami. A second study examined the degradation of polyethylene cover material. Louisville and two challenging climates (Miami and New Orleans) were simulated. In the Louisville, Miami and New Orleans climate, performance was reduced by 2.7, 14.1 and 9.0%, respectively, due to degradation of the cover’s material.     

Daylight illuminance on horizontal and vertical surfaces for clear skies. Case study of shaded surfaces

This paper presents the results of a study on daylight illuminance levels measured on horizontal and vertical surfaces. Firstly, an attempt to know the behavior of the daylight illumination in a specific area, as the ‘Castilla-Leon’ region, Spain, is made in order to determine the representative values of this parameter in this geographical area. With this aim, a series of measurements of global illuminance on a horizontal surface and on vertical surfaces oriented to the four cardinal points (N, S, E and W) have been collected and analyzed. In the paper, the daily evolution of this parameter is reported considering different times of the year so that a global annual behavior can be established. The study is focused on clear days. The collected data correspond to 10 min measurements carried out during the year 2007 in a rural area close to the city of Valladolid, Spain, in the mentioned region. In addition to that, the data have been treated in order to analyze the relationship between the global vertical illuminance and the corresponding horizontal illuminance. In case that no direct sunlight is reaching a vertical surface, that is, for a shaded surface, a simple parametric equation is proposed in order to predict the vertical illumination. The expression obtained shows a high correlation coefficient. It relates the ratio between vertical and horizontal illuminances versus the solar altitude; so, it permits to calculate vertical illuminance if horizontal illuminance is known.     

Development of the nomo-graph for evaluation on discomfort glare of windows

Human beings have been using daylight for a long time. The daylight provides building occupants with psychological stability and also serves as lighting. However, the discomfort glare of windows produced from the daylight could be a severe problem as the working condition is getting more complicated and precise. Although numerous studies have been performed in order to solve this problem, most studies have been performed on Caucasians. The recent studies have revealed that there were differences between Caucasians and Asians in ocular activity, pigmentation, and color sensibility depending on the iris color. These data implies that it is difficult to directly apply the data from studies on the Caucasian to Asians whose physiological properties of eyes are different from those of the Caucasian. Therefore, it is necessary to develop an evaluation method of window discomfort glare, which may represent the visual feature of Asian appropriately. In the present study, we investigated the visual difference between Caucasian and Asians and established the nomo-graph to develop the method appropriate for the evaluation of the window discomfort glare for Asians.     

Performance of hemisphere algorithm for fast form factor calculation

A fast and accurate algorithm is required for carrying out radiative heat transfer simulations designing thermally efficient devices. This paper describes the performance of the hemisphere algorithm, which was originally developed for fast form factor calculation for obtaining photorealistic three‐dimensional computer graphics. We compare the performance of the hemisphere algorithm with that of two conventional algorithms that are widely used for radiative heat transfer simulations. The hemisphere algorithm is found to be significantly faster than the other algorithms, but it has an absolute error of 1.0×10^?5. In addition, the result indicates that the hemisphere algorithm is suitable for simulating the trial and error process in the practical analysis of large‐scale models due to its tolerable visualization of form factor distribution. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20259     

An analytical model of view factors for radiation heat transfer in planar and tubular solid oxide fuel cells

Radiant heat transfer plays an important role in the distribution of cell temperature and current density in solid oxide fuel cells (SOFC). The objective of this paper is to introduce a mathematical model of view factors for radiation heat exchange in an in-house longitudinally distributed SOFC model. A differential view factor model is first developed for planar and tubular SOFC configurations, but is found invalid when the infinitesimal element size is comparable to the characteristic size. Then, a finite-difference view factor model is developed to solve the problem of discontinuities in the differential view factor model. Starting from a classical problem of convective and radiant heat transfer for a transparent gas flow in a gray-wall tube, a fast and accurate computation is available for the finite-difference view factor model without extra mathematical derivations of the governing equations. Compared to the simple modeling which only takes into account the surface-to-surface radiation exchange between two directly opposed elements, the detailed radiation model based on analytical view factors predicts more uniform distribution of cell temperature and current density in the overall SOFC modeling.     

On the role of radiation view factor in thermal performance of straight-fin heat sinks

The present study is aimed to experimentally investigate the importance of the effects of thermal radiation and its corresponding view factor on the thermal performance of a straight-fin heat sink designed for electronic cooling under natural convection. The convection heat transfer coefficient between the fins and the ambient air is evaluated in conjunction with the results obtained through the experimental investigation. Three different models are developed to investigate the effects of thermal radiation and its pertinent view factor on the convection coefficient as well as the fin performance of the heat sink. The deviations based on different operating conditions for these models are analyzed and the importance of the effects of thermal radiation and view factor in the thermal analysis of fin arrays are identified and discussed. It can be concluded that the practice of neglecting the radiation view factor in the thermal analysis of fin arrays should be prohibited based on the fact that the errors generated are noticeably larger than those of solely neglecting thermal radiation.     

A model for estimation of daylight factor for skylight: An experimental validation using pyramid shape skylight over vault roof mud-house in New Delhi (India)

A model to estimate daylight factor was investigated and validated using experimental hourly inside and outside illuminance data of an existing skylight integrated vault roof mud-house in composite climate of New Delhi. The daylight factor model was found in good agreement with experimental value of daylight factor. This model was modified for different practical horizontal surface levels inside the big and small dome rooms and validated using experimental measured data. The yearly average value of percentage daylight factor for big and small dome skylight rooms was determined as 2% and 6%, respectively. The total annual average artificial lighting energy saving potential of the skylight illuminance in the existing building was estimated as 973 kW h/year; corresponds to 1526 kg/year of CO₂ emission mitigation. Hence, the annual carbon credit potential from skylight mud-house building is € 15.3/year.     

Development of a methodology for assessing the adequacy of electric power systems

This study presents the results of a research into the developing a methodology for assessing the adequacy of advanced electric power systems characterized by the integration of various innovative technologies, which complicates their analysis. The methodology development is aimed at solving two main problems: 1) increase the adequacy of modeling the processes that occur in the electric power system and 2) enhance the computational efficiency of the adequacy assessment methodology. This study proposes a new mathematical model to minimize the power shortage and enhance the adequacy of modeling the processes. The model considers quadratic power transmission losses and network coefficients. The computational efficiency of the adequacy assessment methodology is enhanced using efficient random- number generators to form the calculated states of electric power systems and machine learning methods to assess power shortages and other reliability characteristics in the calculated states.     

Effect of void fraction models on the two-phase friction factor of R134a during condensation in vertical downward flow in a smooth tube

The void fraction of R134a condensing inside a vertical smooth tube is experimentally investigated in this study. The vertical test section is a 0.5 m long countercurrent flow double tube heat exchanger with refrigerant flowing down the inner tube and cooling water flowing upward in the annulus. The inner tube is made from smooth copper tubing and has an 8.1 mm inner diameter. The test runs are done at an average saturated condensing temperature of 40 °C. The average qualities are between 0.8 and 0.99 while the mass fluxes are 300 kg m^− 2s^− 1 and the heat fluxes are between 22 and 39.8 kW m^− 2. The void fractions are indirectly determined using relevant measured data together with various void fraction models and correlations reported in the open literature. The friction factors obtained from various void fraction models and correlations are compared with each other and also with those determined from graphical information provided by Bergelin et al. The effect of void fraction alteration on the momentum pressure drop is also presented.     

A calculation method for the estimation of the Linke turbidity factor

A general calculation model is presented to determine the Linke turbidity factor, starting from the experimental datum of global daily solar radiation on the horizontal plane. The model was validated using experimental data for the turbidity factor obtained at Arcavacata di Rende (Cosenza) and at Casaccia (Rome). Moreover, polynomial relations are presented for quick calculation of this parameter on different days of the year for those localities for which the European Solar Radiation Atlas supplies the maximum solar radiation values on clear days.     

Experimental analysis on a 1:2 scale model of the double light pipe, an innovative technological device for daylight transmission

In this paper the authors present the double light pipe, an innovative technological device, designed as an evolution of a traditional light pipe, which distributes daylight to underground areas of a building, illuminating, at the same time, the passage areas thanks to a larger collector and a second transparent pipe attached to the first one. Unlike the traditional light pipe, thanks to this double illuminating function it can be located in the middle of a room, despite its encumbrance.In this paper the technological design of the double light pipe is presented and the results of an experimental analysis on a reduced scale (1:2) model are shown. Internal illuminance data over horizontal and vertical work-planes were measured in various sky conditions with or without direct solar radiation. Being this innovative device obtained by a light pipe integrated with a second pipe, it performs like a traditional light pipe for the final room and, at the same time, illuminates the intermediate room giving it uniform and high quality light, particularly indicated for wide plant areas, such as show-rooms or museums.     

双馈风力发电机功率因数调节分析

在双馈发电机等值电路的基础上,建立了相应的基本方程,并求出定子电流的解析表达式.通过对定子电流的分析,求得输出电流为额定值时的励磁电压有效值解析表达式及初相位的取值范围.在此基础上,通过对定子电流有功分量及无功分量的分析,得到功率因数超前及功率因数滞后时的励磁电压初相位取值范围.为满足实际工程需要,对电机的参数作进一步简化,最终通过电机的实际参数进行仿真,验证了其理论分析的正确性.     

挡流板对垂直轴风力机性能影响的试验研究与分析

在风力机前方安装挡流板可提高H型垂直轴风力机的发电功率,但目前缺少对该结构装置的全面系统研究,且评估安装挡流板时所使用的风能利用系数C_P的计算方法并不恰当,以至于某些计算结果突破贝兹极限,导致概念混淆。文章通过对挡流板安装位置及安装角度进行测试,获取了不同位置的风力发电功率随风轮转速的变化关系。结果表明:在一定距离范围内,挡流板与风轮有一最佳距离,挡流板远离风轮时,提高风速的效果差,挡流板距离风轮越近对来流风干扰变大,输出功率反而降低;挡流板的最佳安装角度为90°,安装角小于或者大于90°时,风速变化平缓,输出功率增长减弱,失去聚风作用。考虑到挡流板聚风后叶轮前的风场不均匀,在计算C_P时不能取某一个风速值或用平均值代入其定义式,文章提出了采用平均风动能的统计方法计算C_P,计算结果表明,增加挡流板可以提高输出功率,但不一定能增加C_P值。     

Influence of the required EEDI reduction factor on the CO2 emission from bulk carriers

In order to improve energy efficiency for ships International Maritime Organization (IMO) introduced Energy Efficiency Design Index (EEDI). For every new ship the attained EEDI has to be calculated and not higher than the required EEDI which is calculated from the reference line value and appropriate reduction factor. The reference line value represents the world fleet average and is dependent on the ship type and size. The reduction factor represents a reduction for the EEDI relative to the reference line value and is increased in a set of time intervals. However, the scheme of the reduction factor change seems to be rigidly set and could lead to design issues and ship under powering. This study estimates the CO₂ emission from bulk carriers based on the current reduction factor change policy. Other policies and some innovative approaches are also discussed and the CO₂ emission in every scenario is estimated. The results are then compared with the requirement of reaching mean global CO₂ stabilization level of 550 ppm in the atmosphere. It is concluded that policies which include feedback from the shipbuilding sector impose requirements that could be much easier to satisfy and which will lead to overall lower CO₂ emission.     

Assessment of the roughness factor effect and the intrinsic catalytic activity for hydrogen evolution reaction on Ni-based electrodeposits

The hydrogen evolution reaction (HER) was studied in 30 wt.% KOH solution at temperatures ranging between 30 and 80 °C on three type of electrodes: (i) rough pure Ni electrodeposits, obtained by applying a large current density; (ii) smooth NiCo electrodeposits; (iii) smooth commercial Ni electrodes. By using steady-state polarization curves and electrochemical impedance spectroscopy (EIS) the surface roughness factor and the intrinsic activities of the catalytic layers were determined. These techniques also permitted us to determine the mechanism and kinetics of the HER on the investigated catalysts. Different AC models were tested and the appropriate one was selected. The overall experimental data indicated that the rough/porous Ni electrode yields the highest electrocatalytic activity in the HER. Nevertheless, when the effect of the surface roughness was taken into consideration, it was demonstrated that alloying Ni with Co results in an increased electrocatalytic activity in the HER when comparing to pure Ni. This is due to an improved intrinsic activity of the material, which was explained on the basis of the synergism among the catalytic properties of Ni (low hydrogen overpotential) and of Co (high hydrogen adsorption).     

Effect of operating current dependent series resistance on the fill factor of a solar cell

The fill factor of a solar cell depends upon the series resistance, reverse saturation current, diode quality factor, operating current and voltage. Since the series resistance itself depends upon the operating current (or voltage), it makes the evaluation of fill factor very complicated. In this paper, we have evaluated the fill factor of a solar cell, taking into account operating current dependence of the series resistance.     

适合柴油—甲醇—水复合乳化的新型乳化剂研究

介绍了一种适合于柴油-甲醇-水复合乳化的高分子乳化剂的制取方法,对该种乳化剂的乳化过程进行了机理和试验研究,试验结果表明,用该种乳化剂配制的乳液稳定时间长,而乳化剂的用量比较低。探讨了用CR-2型乳化装置配制柴油-甲醇-水复合乳液的工艺,并在柴油机试验台架上考察了用该种乳化剂配制的乳液的适应性。