Sky Types Research Articles

The use of horizontal sky illuminance to classify the CIE Standard General Skies

The CIE Standard General Skies defines the luminance distribution of skies into 15 types, from overcast sky to clear sky. Finding the frequency distributions of such skies is not a simple task, for two reasons. The first is the need for accurate luminance measurements over a long period of time. Such data are not available in many regions around the world. The second is the difficulty in obtaining accurate readings for the zenith luminance in low latitude climates. This paper presents a method in which the horizontal sky illuminance can be used to classify sky type.

A new modelling approach intended to develop maps of annual solar irradiation and comparative study using satellite data of Algeria

Solar irradiation is the main potential energy source used in various processes. An accurate estimation of solar irradiation becomes a challenge due to the unavailability of weather data in Algeria. Therefore, an operated model can offer an important alternative for calculating the solar irradiation including the minimum of the input data. The present study derives a simple model from a review of our previously published work. It aims to develop a new approach for the estimation of the global irradiation on the horizontal plane only based on the measured sunshine duration. Maps of solar energy are required by many system designs; for this reason, it is mandatory to draw the global solar irradiation maps for Algeria for all types of sky. Algebraic relative errors were used as indicators of the agreement between the experimental and the calculated global irradiation. It has been proved that the highest intensity of the solar irradiation is located around the area of Djanet and Tamanrasset, whereas the less intense area extends from Skikda zone to Annaba, and more exactly in around 7° longitude.

Local short-term variability in solar irradiance

Abstract. Characterizing spatiotemporal irradiance variability is important for the successful grid integration of increasing numbers of photovoltaic (PV) power systems. Using 1 Hz data recorded by as many as 99 pyranometers during the HD(CP)2 Observational Prototype Experiment (HOPE), we analyze field variability of clear-sky index k* (i.e., irradiance normalized to clear-sky conditions) and sub-minute k* increments (i.e., changes over specified intervals of time) for distances between tens of meters and about 10 km. By means of a simple classification scheme based on k* statistics, we identify overcast, clear, and mixed sky conditions, and demonstrate that the last of these is the most potentially problematic in terms of short-term PV power fluctuations. Under mixed conditions, the probability of relatively strong k* increments of ±0.5 is approximately twice as high compared to increment statistics computed without conditioning by sky type. Additionally, spatial autocorrelation structures of k* increment fields differ considerably between sky types. While the profiles for overcast and clear skies mostly resemble the predictions of a simple model published by Hoff and Perez (2012), this is not the case for mixed conditions. As a proxy for the smoothing effects of distributed PV, we finally show that spatial averaging mitigates variability in k* less effectively than variability in k* increments, for a spatial sensor density of 2 km−2.

Análise da influência da variação das superfícies refletoras na disponibilidade de iluminação natural no ambiente interno na extensão de sua profundidade

O estudo parte da hipótese de que a disponibilidade da luz natural no ambiente interno sofre influencias das características das superfícies refletoras internas. O objetivo é investigar o comportamento da curva isolux de um ambiente interno em função da variação de suas superfícies refletoras. A análise do ambiente em estudo é realizada por meio da construção de curvas isolux em pontos específicos do ambiente, e dos percentuais de horas em função dos intervalos das UDI (Useful Daylight Illuminance), em horários e dias do ano pré-estabelecidos. Os valores utilizados para a construção da curva isolux são gerados através das simulações do software TropLux, num ambiente de geometria retangular com abertura voltada para a orientação Norte, na cidade de Vitória (latitude 20° 19' S), utilizando céus padrões da CIE (Commission Internationale L´aclairage). Os resultados mostram que para o Céu 3, a variação das características de reflexão das superfícies refletoras internas contribuem com insignificantes aumentos na média anual de Iluminância interna. Para o Céu 7, a variação das características de reflexão das superfícies refletoras da parede e piso contribuem para pequenos aumentos na média anual de Iluminância interna no ponto 2 (1,0 m da abertura). Para o Céu 12, a variação das características de reflexão das superfícies refletoras do teto, parede e piso contribuem para pequenos aumentos na média anual de Iluminância interna somente nos pontos mais próximos da abertura, onde as superfícies refletoras do piso e teto tem contribuição mais relevante até o ponto 3(1,5 m da abertura), e a parede tem contribuição relevante somente no ponto 2 (1,0 m da abertura). Palavras-chave:, , , l.

Modeling diffuse irradiance under arbitrary and homogeneous skies: Comparison and validation

The optimum utilization of solar energy requires effective harvesting of both the direct and diffuse components of ground-reaching radiation. Although solar beams are typically key contributors to the total irradiance under cloudless conditions, the diffuse component becomes important especially in regions where clear skies are not dominant. Even if the cloud cover and cloud microphysics are known, it is not an easy task to estimate the diffuse irradiance at arbitrarily oriented sloped surfaces. This situation arises from the extreme difficulty in solving the radiative transfer equation in such a heterogeneous environment.Models of Homogeneous Skies (MHS) with a set of discrete sky types are commonly applied in determining irradiance or illuminance. However, experimental data usually differ significantly from data predicted by MHS, for initially daylighting purposes where a simple sky luminance distribution is most useful. In this paper we show that the MHS error can be exceptionally large, especially for some cardinal directions. The use of MHS could also lead to misinterpretation of a momentary sky state if different sky types are required to accurately simulate irradiances on arbitrarily oriented surfaces. Nevertheless, use of multiple sky types at one moment in time has no theoretical basis and thus is generally unacceptable. Therefore, we have demonstrate that a new Unified Model of Radiance Patterns (UMRP) is more consistent with physical measurement and is generally applicable to all situations, including clear-, partly-cloudy, and, overcast-sky conditions and, could accurately predict vertical irradiances or illuminances more accurately than MHS. UMRP implements heterogeneity of cloud systems and provides radiance or luminance distributions depending on the characteristic size of clouds, their albedo and their altitude. While the error in the irradiance computations is approximately 6% for UMRP, it can reach a value of 70% or even more in case of MHS. UMRP-based modeling of radiative fields is now simple using the UniSky Simulator, a heavily optimized multithreading application (publicly available at www.unisky.sav.sk/?lang=en&page=aplikacia). We expect the UMRP concept to be a new choice for modelers and PV systems engineers to predict solar energy more accurately under any meteorological conditions.

Effect of Repairing and Grinding Scratched Alpine Skis on Their Friction on Snow

Ski base service includes filling of scratches, grinding and waxing. The aim of this study was to investigate the effect of scratched and then repaired ski bases on their kinetic friction on snow. Kinetic friction of three different types of skis (junior, women and racing skis) in four preparation steps (factory-new, standard grinded, scratched, repaired) were measured at two different velocities (4 and 8 ms-1). Testing took place on a linear tribometer with defined conditions at -8°C snow temperature. The normal force was set to typical values for the particular ski type. The coefficient of friction of the scratched skis almost doubled compared to the unscratched ones. This may be caused by asperity interlocking between the snow grains and the scratches. The coefficient of friction of the repaired bases was re-established since only about 1% of the before scratched area was covered with the filling material.

Daylighting Contribution for Energy Saving in a Historical Building

The purpose of this study is assessing the reliability of the software DIALux 4.12 for daylighting design, comparing experimental data with simulation results; the comparison was performed for an office.The daylight illuminance distribution inside the room, the external zenith luminance and the external horizontal diffuse illuminance were measured during the weeks from January 19th to February 20th; the data gathered were further reduced to match conditions related to the CIE sky type #12 (CIE Standard Clear Sky, low luminance turbidity) and finally the comparison was carried in terms of daylight illuminance distribution and relative percentage error.

Achieving energy and cost savings through simple daylighting control in tropical historic buildings

Electricity is an important element used to run plant, lighting and other equipment in buildings. An appropriate well-designed lighting system is required for energy saving since the lighting consumption is dependent on the activities of the occupants of the buildings. Lighting consumption in a building can be affected by the building's purpose, the use of daylight, illumination levels and hours of usage by the occupants. The present study aims at accentuating the use of computational modeling techniques towards significantly improving the effectiveness of daylighting control on energy and cost savings for historic office buildings. For this purpose, lighting performance of the Klang District Historic Office building in Malaysia was investigated through a simple daylighting control. The present study was carried out to investigate the lighting performance in a historic office building which is known as the Klang District Historic Office. Lighting performance is assessed using a simulation modeling that includes artificial lighting, daylighting, sun position, weather statistics, sky type, shading and a 3D Model. The study found that almost all cellular offices are adequately served by daylighting but that open plan offices needs some artificial lighting to supplement inadequate daylighting. The results are presented and the approaches for energy saving discussed.

基于空间模型测量点的照度比值的CIE标准一般天空类型选择

基于空间模型测量点的照度比值的CIE标准一般天空类型选择

A influência dos afastamentos laterais das edificações na disponibilidade de iluminação natural no ambiente interno

Este estudo aborda os aspectos relacionados à iluminação natural no ambiente interno, sendo o objetivo principal da pesquisa analisar a influência dos afastamentos laterais entre as edificações obstruidoras, considerando a altura e a largura de via, na disponibilidade de iluminação natural no ambiente interno. Na metodologia adotada foram realizadas comparações dos percentuais de valores de iluminância do ambiente interno com os intervalos das UDI (Useful Daylight Illuminance) em um ponto selecionado no interior do ambiente com abertura orientada para Norte. Os percentuais de valores de iluminância foram gerados através de simulação na ferramenta computacional TropLux, em um ambiente de geometria retangular inserido em um cenário urbano na cidade de Vitória (LAT 20°19' S). As simulações foram realizadas para três tipos de céus padrões da CIE (Commission Internationale L´aclairage). Como principal resultado constatou-se nas localidades de céu 3 (encoberto), 7 (parcialmente nublado) e 12 (claro), que há maiores variações nas médias anuais de iluminância interna quando há modificações nos afastamentos laterais entre as edificações obstruidoras, não estando estas obstruções exatamente defronte à edificação analisada. Constatou-se também que localidades de céus 3 (encoberto), 7 (parcialmente nublado) e 12 (claro), as alterações nos afastamentos laterais entre as três edificações obstruidoras, são de pouca interferência nas variações percentuais do intervalo 100≤UDI≤500lx e 500≤UDI≤2000lx.

A Methodology for Designing and Calibrating an Artificial Sky to Simulate ISO/CIE Sky Types with an Artificial Sun

ABSTRACTArtificial skies are now commonplace in laboratories for daylighting studies in university schools of built environment studies. However, these research and educational devices are often not used to their full potential because they are not calibrated to model even location-specific International Oganisation for Standardisation (ISO)/CIE sky types. Nevertheless, sky luminance patterns and parallel sunbeam produced by an artificial sun in a sky simulator are required for realistic physical modeling. These facilities must be designed and calibrated in well-documented protocols to achieve the scale ratios that proportionally model real outdoor skylight and sunlight conditions. Such exacting tasks can be accomplished by respecting several technical imperatives that this article was written to elaborate and discuss.

The Research Search For The Least Beneficial Overcast Sky And Progress In Defining Its Luminance Gradation Function

Abstract Recently illuminance levels under ISO/CIE homogeneous standard sky types were characterised in their relative terms after ISO/CIE (2004, 2003) standardised as normalised by the luminance in the zenith. Sky luminance and horizontal illuminance based on the gradation and scattering indicatrix functions, including the extreme overcast cases frequently encountered in nature, were recently determined in absolute physical units of luminance in kilocandles per meter square and of illuminance in kilolux. The historical search to find energy and visibility critical sky luminance distributions shows a progression of steps in studying the worst or critical overcast situations. That progression has enabled the determination and evaluation of interior illuminance for comparison of the merits of dual daylighting and artificial lighting under established criteria for comfortable visibility.

Definition of the CIE standard skies and application of high dynamic range imaging technique to characterize the spatial distribution of daylight in Chile

The effectiveness and accuracy of a daylighting study relies on knowing the climate and light conditions available at a given place, to then evaluate the illuminance obtained during the project development process. This paper presents the methodologies of a research process, whose objective is providing realistic data to establish the foundations for future lighting studies in Chile. It starts with the revision of the metrics used to quantify daylight; later, the methodology used to collate the monthly frequencies on four types of standard CIE skies, starting from climate files, is presented. Finally, both the methodology and the results obtained are presented to measure and characterize the real skies, as well as the High Dynamic Range photographs (HDR) and, as complement of this information, the illuminances measurements of the sky dome to define the types of skies of six Chilean cities. From the study applied, it is possible to conclude that knowing the standard CIE skies of a place allows us to adjust and focus the daylighting study for predetermined and predominant skies. It is just as important to mention that the measuring techniques for the skies must be adjusted and calibrated for each studied location.

The simultaneous occurrence and relationship of sunlight and skylight under ISO/CIE standard sky types

In daylight science the availability of sunlight and skylight at different times has been studied because of the desire to use daylight in both exterior and interior spaces. Exterior skylight illuminances under overcast skies were adopted as a standard for window design in the past. Current ISO/CIE sky types are standardised as relative luminance patterns normalised to the zenith. In this paper, the zenith luminance in candela/square metre and the resulting diffuse illuminance in lux are determined for all sky types. Furthermore, the proportions of sunlight and skylight under different levels of turbidity are found and documented. Efforts to harmonise electric lighting and daylighting standards need to analyse the data on available daylight in physical units in order to ensure energy savings that respect human requirements as well as providing information suitable for computer-aided design.

Improving the All Sky Model for the luminance and radiance distributions of the sky

Energy conservation is a fundamental requirement in buildings today. It is essential to use technology that integrates the thermal environment and the luminous environment to ensure indoor environmental quality. It is important to understand as accurately as possible the distribution of luminance and radiance of the daytime sky. Though the CIE Standard General Sky for sky luminance distributions has been recommended, a method of selecting 15 Sky Types was not shown.In this paper, the author tries to improve the previous All Sky Model based on IDMP measurement data from Osaka with the aim of a high accuracy estimation method for luminance and radiance distributions of the sky. The appearances of the Types of CIE Standard General Sky and the assignment in estimating types are shown based on the measurement data in Osaka. The improved All Sky Model and the validation of this improved model using the measurements in Osaka and Tokyo, Japan was introduced.

The Effects of Different Depth of Floor towards Internal Daylighting and Rules of Thumb for Office Room

Rules of thumb in daylighting have been expressed in a variety of modes in architectural. It can be divided into categories based on the parameters which constitute them. One of the categories is floor depth. This paper presents the impact of different floor depth on daylighting performance of the simulated office room using 1 meter shading device. Several parameters such as shading device, ceiling height, and material reflectance have been appointed. Overcast sky was chosen as tropical-compatible sky types. Models then were simulated and analyzed using an application of IES_VE software called RADIANCE. Existing daylighting rules of thumb has been modified and thus create new formula for Kuala Lumpur sky based on the smallest academic office room in public university.

Sky-luminance distribution in Beijing

Sky-luminance distribution is the basic requirement for daylighting calculations and design. It is also important for describing and defining the different sky types. However, owing to the lack of test equipment and measurement data, research on sky-luminance distribution in China has been limited. Therefore, for the first time, we have obtained the sky-luminance distribution data using our self-developed spectrum sky scanner from observations over 12 months. Furthermore, we have established a relationship between the actual sky types of Beijing and the Commission Internationale de l'Eclairage standard sky model. These data are very important in the study of the daylight climate of China.

Revised solar maps of Algeria based on sunshine duration

Solar irradiation data is generally required in modelling a system’s thermal performance, and evaluation of long-term effects of climatological changes. In Algeria, measurements of solar irradiation have been carried out for a few locations because the measuring instruments are expensive to purchase and install. The only alternative to obtain solar irradiation data is to estimate it by use of an appropriate solar irradiation model.The present study attempted to draw global solar irradiation maps for Algeria, witch are generated for all types of sky. The incident solar radiation on a horizontal surface, on a surface tilted at the latitude angle and for a vertical plane facing east, south, west, south-east, south-west was determined using numerical models. To obtain a solar radiation map of a certain zone it is necessary to know the solar radiation of a huge number of sites spread wide across the zone.The comparison between the measured and the computed values is satisfactory; the relative error is less than 7%. The results allow to view information about 48 provinces of Algeria, and are presented in the form of an annual solar radiation map. The solar maps developed in this paper provide information about the levels of total solar radiation which can be used as a database for future investments in the solar sector in Algeria.

Sky Detection Using K-HSV Descriptor

Many outdoor images contain sky. The sky detection and segmentation is important for image en- hancement, horizon detection, and obstacle avoidance in unmanned air vehicles. Most researches in sky detection and segmentation are for blue sky only. Our work is proposed to detect and segment three categories of skies: blue sky, cloudy sky, and sunset sky. There are two types of sky detection methods: pixel based detection and block based detection. The main advantage of pixel based detection is the high correct detection ratio. There are many descriptors used in object detection such as color descriptors, color-shape descriptors, and shape de- scriptors. This work studies sky detection and segmentation with deferent descriptor types. The sky is classified into blue sky, cloudy sky, and sunset sky. For each sky type, the sky is detected using pixel based detection and block based detection. We improve the sky detection ratio using K-HSV descriptors. The sky classification with K-HSV descriptors has 86.16% correct sky detection for blue sky. We decrease The number of keypoints used in sky segmentation to 200 random selected keypoints of all dense sampling keypoints. The sky segmentation based on 200 color moment invariant descriptors obtained 78.25% for blue sky, 61.63% for cloudy sky, and 62.27% for sunset sky.

The use of sky luminance and illuminance to classify the CIE Standard General Skies

Optimum use of natural light in buildings requires knowledge of the luminance distribution of the sky. Since the introduction of the CIE Standard General Skies, several methods have been proposed to find the frequency distribution of sky types. This paper presents a method whereby the ratio of the luminances of two sky elements and the ratio of the luminance of one sky element to the horizontal illuminance can be used in the determination of sky type.