A model for calculating global illuminance from satellite data

A model for calculating global illuminance on horizontal surfaces from meteorological satellite data was developed. The data used for developing the model are global illuminance measured at four solar monitoring stations situated in different parts of Thailand and 8-bit digital data from visible channel of GMS-5 satellite covering the whole country for the period of 1–2 years. Values of normalized global illuminance defined as a ratio of global illuminance to clear sky global illuminance were calculated. These values were used to correlate with those of cloud index derived from the satellite data. From the correlation, a model relating the normalized global illuminance to cloud index was established. The performance of this model was investigated using an independent illuminance data set. It was found that the global illuminance calculated from the model agreed well with that obtained from the measurement, with a root mean square difference of 5.38 klux or 7.0% of the mean values.     

Predicting daylight illuminance on inclined surfaces using sky luminance data

Daylight illuminance, particularly on vertical surfaces, plays a major role in determining and evaluating the daylighting performance of a building. In many parts of the world, however, the basic daylight illuminance data for various vertical planes are not always readily available. The usual method to obtain diffuse illuminance on tilted planes would be based on inclined surfaces models using data from the horizontal measurements. Alternatively, the diffuse illuminance on a sloping plane can be computed by integrating the luminance distribution of the sky ‘seen’ by the plane. This paper presents an approach to estimate the vertical outdoor illuminance from sky luminance data and solar geometry. Sky luminance data recorded from January 1999 to December 2001 in Hong Kong and generated by two well-known sky luminance models (Kittler and Perez) were used to compute the outdoor illuminance for the four principal vertical planes (N, E, S and W). The performance of this approach was evaluated against data measured in the same period. Statistical analysis indicated that using sky luminance distributions to predict outdoor illuminance can give reasonably good agreement with measured data for all vertical surfaces. The findings provide an accurate alternative to determine the amount of daylight on vertical as well as other inclined surfaces when sky luminance data are available.     

The availability of daylight from tropical skies—a case study of Malaysia

In Malaysia, no long-term daylight data are measured. It was only recently that the need to measure the availability of daylight became urgent when the importance of daylighting in buildings was rediscovered. The hourly daylight availability has been simulated for the Malaysian sky using daylight modelling techniques based on empirical and measured solar irradiation and cloud cover data. This paper presents the techniques involved in producing exterior illuminance data. These data were then compared with measured illuminance at Shah Alam and Bangi, Malaysia. The global illuminance levels are generally high, with values exceeding 80,000 lux at noon during the months when solar irradiation is highest. Even during the months when the ground receives less solar irradiation, the peak illuminance can reach 60,000 lux. Applications and uses of such data are in daylighting design, both for visual and thermal comfort, task illuminance and energy-conscious design of buildings. Recommendations are made at the end of the paper on the various climatic data that are required to be measured for overall daylighting design applications.     

A technique to map monthly average global illuminance from satellite data in the tropics using a simple semi-empirical model

This paper presents a technique to map monthly average hourly global illuminance from satellite data. A semi-empirical model relating monthly average global illuminance to cloud index, precipitable water, total ozone column (TOC), aerosol optical depth (AOD) and air mass was developed. Data for the cloud index, AOD and TOC were obtained from the visible imagery data of MTSAT-1R, MODIS/Terra and OMI/Aura satellites respectively, while precipitable water was extracted from NCEP/NCAR reanalysis database. The model was formulated using global illuminance measured at four stations in Thailand for a four-year period and validated with an independent one-year data set. Values of monthly average hourly global illuminance calculated from the model and those obtained from the measurements were in good agreement, with a root mean square difference (RMSD) and mean bias difference (MBD) of 8.1% and −0.8%, respectively. The model was used to calculate monthly average hourly global illuminance over Thailand and the results were displayed as illuminance maps. The maps reveal diurnal and seasonal effects mainly in response to solar zenith angle changes and cloud cover related to the southwest and northeast monsoons.     

An analysis of luminous efficacies under the CIE standard skies

Designing a building to integrate daylight requires an accurate estimation of the amount of available outdoor illuminance. The common method for predicting daylight has been the derivation of illuminance from the more widely measured solar irradiance using the luminous efficacy approach. Recently, the International Commission on Illumination (CIE) has adopted a range of 15 standard skies which cover the whole probable spectrum of skies in the world. This paper presents the work to model the luminous efficacy of diffuse component under the 15 CIE standard skies. Sky luminance distributions measured between 1999 and 2005 were used for the standard sky classifications. An approach to develop luminous efficacy for inclined surfaces was proposed. The predicted vertical outdoor illuminance data for the four cardinal planes (i.e., N, E, S and W) using the proposed luminous efficacy were evaluated against data measured in 2004. Statistical analysis indicated that the estimated daylight illuminance data give acceptable agreements with measured data for all vertical planes.     

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 a meteorological illuminance model for daylight computations

Building simulation programmes need detailed hour-by-hour weather data. In the absence of measured irradiation and illuminance data, validated meteorological radiation and illuminance models may be used. These models are primarily based on the estimation of solar-beam attenuation through the terrestrial atmosphere. Broadly speaking, there are two strands of such computations. One of these strands is based on sunshine, temperature and humidity data and takes into consideration the attenuation due to mixed gases (such as oxygen, nitrogen and carbon dioxide), water vapour, ozone and aerosols. The other procedure requires cloud-cover data to obtain the degree of attenuation of clear-sky beam irradiance due to clouds. The authors have extended the application of the above models to obtain hourly and sub-hourly values of solar illuminance, via an interface with a luminous efficacy model. An account of the physical basis of the presently-proposed models and their validation is presented herein.©     

A technique for mapping global illuminance from satellite data

A technique for mapping global illuminance from satellite data was developed. A five-year (1998–2002) climatology of global solar illuminance obtained from this technique is presented for Thailand using hourly GMS-5 satellite data. The technique is based on a radiation budget model which traces solar radiation as it is scattered, absorbed and reflected back to space. The model produces an earth-atmospheric albedo in the satellite spectral window as well as global illuminance at the earth’s surface. The model is tuned using surface illuminance measurements at four stations in Thailand: Chiang Mai (18.78°N, 98.98°E), Ubon Ratchatani (15.25°N, 104.87°E), Songkhla (7.20°N, 100.60 °E) and Nakhon Pathom (13.82°N, 100.04°E). In the mapping process, a satellite earth-atmospheric albedo at any locations yields a cloud-atmospheric albedo in the satellite band, which is then transformed into a cloud-atmospheric albedo in the photopic band. Having obtained the photopic cloud-atmospheric albedo, the model calculates surface illuminance. The model gives a root mean square difference of 8.1% and a mean bias difference of −2.6% when tested against an independent data set. Monthly average maps are presented covering Thailand for local times of 10:30, 12:30 and 14:30.     

Estimation of exterior vertical daylight for the humid tropic of Kota Kinabalu city in East Malaysia

In tropical regions natural daylight has been a fundamental factor in building design. It is the most efficient way of lighting a building in the daytime and has a great potential for energy conservation in buildings. In Malaysia there are a limited available data of measured illuminance which is the case of several regions in the tropics. Using established models it is possible to predict the luminous efficacy and then estimate the monthly mean hourly exterior illuminance. In this study two different models were chosen. The Perez and Du Mortier–Perraudeau–Page–Littlefair models were selected for the prediction of hourly exterior horizontal illuminance for the city of Kota Kinabalu in East Malaysia. Comparison between the two models were made. The vertical hourly illuminance was predicted also using Perez approach. The potentiality of daylight in four orientations was discussed. This study highlights the importance of Sunpath diagram on daylight illuminance during the conceptual design stage. The results in this study is hoped to contribute further insight into the potentiality of daylighting of tropical sky.     

Correlations of global and diffuse solar luminous efficacy for all sky conditions and comparisons with experimental data of five localities

Some simplified correlations of mean hourly global and diffuse luminous efficacy on the horizontal plane for all sky conditions have been obtained for Arcavacata di Rende (Italy) and have been compared with other literature models and other experimental data measured in Geneva (Switzerland), Vaulx-en-Velin (France), Bratislava (Slovakia) and Osaka (Japan). The comparisons show that, for global efficacy, the differences among the various models are not significant, and the use of a model with a constant value of efficacy gives good predictions of global illuminance. For the prediction of diffuse illuminance the differences among the models are larger, but the use of a constant diffuse efficacy provides a good first estimate of diffuse illuminance.     

Parametrization problems of the very bright cloudy sky conditions

When exterior global/direct and diffuse illuminance, as well as zenith luminance, is regularly measured and recorded, a selection and specification of the most light efficient skylight conditions is needed. A routine representation of all 1- or 5-minute data on the P–G–D diagram can show extreme diffuse, i.e. skylight ratios normalized to the simultaneous extraterrestrial horizontal illuminance level, which indicate the bright cloudy sky conditions. A new method of cluster analysis was developed to select sky conditions associated with sunlight under specific turbidity or cloudiness conditions at actual solar altitudes, while a rough classification of the sky luminance pattern is possible using the ratio of zenith luminance to diffuse illuminance. Some characteristic data recorded in Bratislava, Slovakia during 1995 and during the three year period 1994–1996, demonstrate the parametrization and selection method used, and document detailed dynamic changes occurring during the brightest days.     

Luminous efficacy of daylight under different sky conditions

Dalighting is recognised as an important and useful strategy in terms of energy-efficient building design in hot climates. However, daylighting is always accompanied by unwanted solar heat gain, particularly during the cooling season. To achieve and evaluate daylighting design, solar radiation and outdoor illuminance data are needed. In 1991, a measuring station was installed at the City University of Hong Kong to measure global and diffuse solar radiation and outdoor illuminance. The measured data are analysed, and empirical models to determine luminous efficacy under different sky conditions are developed and presented. Implications for energy efficiency in building designs are discussed.     

晴空条件下光照度和辐射照度的关系

利用国际上知名软件ＬＯＷＴＲＡＮ７,分别计算晴空时总日射和散射条件下辐照度与光照度之间的定量关系,得到不同大气能见度和太阳高度对其影响规律。同时发现,光合有效辐射波段的总日射辐照度和散射辐照度与光照度的关系是线性的,而全波段的总日射辐照度和散射辐照度与光照度的关系是非线的。计算结果与Ｂｒｏｗｎ的测量结果相当接近,证明了计算的可靠性。     

An analysis of lighting energy savings and switching frequency for a daylit corridor under various indoor design illuminance levels

Visual comfort and electric lighting energy issues are essential criteria to justify daylighting schemes. The evaluation of energy efficiency due to daylight linked lighting control systems is best demonstrated by case studies. This paper presents field measurements on daylighting for a fully air-conditioned daylit corridor. Artificial lighting load, brightness of the fluorescent luminaires, daylight availability for various switching illuminance levels were systematically recorded and analyzed. The general features and characteristics of the findings including the number of switching operations and electric-lighting energy savings are presented and discussed. Daylighting theories, using cumulative frequency distribution of outdoor illuminance and regression models based on brightness of light output, outdoor illuminance and electric light power to estimate daylight-linked lighting control savings, have been developed and assessed. It has been found that data from both approaches show reasonably good agreements with measured results. The findings from this study provide some operational and energy information, which would be useful and applicable to other interior spaces with similar architectural designs.     

REST2: High-performance solar radiation model for cloudless-sky irradiance, illuminance, and photosynthetically active radiation – Validation with a benchmark dataset

REST2, a high-performance model to predict cloudless-sky broadband irradiance, illuminance and photosynthetically active radiation (PAR) from atmospheric data, is presented. Its derivation uses the same two-band scheme as in the previous CPCR2 model, but with numerous improvements. Great attention is devoted to precisely account for the effect of aerosols, in particular.Detailed research-class measurements from Billings, OK are used to assess the performance of the model for the prediction of direct, diffuse and global broadband irradiance. These measurements were made in May 2003 during a sophisticated radiative closure experiment, which involved the best radiometric instrumentation currently available and many ancillary instruments. As a whole, these exceptional measurements constitute the only known modern benchmark dataset made specifically to test the intrinsic performance of radiation models. Using this dataset as reference, it is shown that REST2 performs better than CPCR2 for irradiance, illuminance or PAR predictions. The availability of the turbidity data required by REST2 or other similar models is also discussed, as well as the effect that turbidity has on each component of broadband irradiance, PAR irradiance and illuminance, and on the diffuse/global PAR ratio.     

Daylighting and energy analysis for air-conditioned office buildings

We propose a simple method for estimating the likely energy savings in electric lighting due to daylighting and the possible cooling penalty. Fractions of the working year and cooling season when daylighting alone is adequate to provide the indoor design illuminance are presented for on-off and top-up controls. Based on the simple average daylight factor method, energy savings in electric lighting have been estimated for a generic office building using measured outdoor illuminance data in Hong Kong. The daylight-induced cooling penalty is estimated using average solar heat gain factors. Our case study suggests that daylighting schemes can result in substantial energy savings in air-conditioned office buildings in Hong Kong.     

塔式太阳能热发电站圆月夜聚光实验研究

在塔式太阳能热发电系统中,吸热器采光面上的聚光能流密度分布的测量对优化整个系统的光热性能有着重要意义。本文提出一种基于月光聚光信息的塔式电站定日镜场聚光能流密度分布的间接测量方法。主要介绍2018年9月24日晚在延庆塔式电站开展的两种对月聚光实验：一种是通过塔上布置的照度计标定电荷耦合元件（Charge-coupled Device, CCD）相机拍摄的光斑图像,得到定日镜场聚光光斑的照度分布;另一种是使聚光光斑扫描过照度计,得到不同时刻的照度计数值,通过高斯拟合得到聚光光斑的照度分布。将聚光光斑的照度分布与月光测光站测得的月光法向直射照度对比,得到塔上聚光光斑的相对能流密度分布。实验结果表明,通过月光聚光实验,可以得到塔式电站的聚光光斑的相对能流密度分布（即聚光比分布）,为后续依据太阳和月亮之间的亮度分布关系,转换为日光聚光能流密度分布提供实验数据支持。     

Daylight illuminance control with fuzzy logic

The purpose is to take full advantage of daylight for inside illumination. The inside illuminance and luminous efficacy of the available solar radiation were analyzed. The paper deals with the controlled dynamic illuminance response of built environment in real-time conditions. The aim is controlled functioning of the roller blind as a regulation device to assure the desired inside illuminance with smooth roller blind moving. Automatic illuminance control based on fuzzy logic is realized on a test chamber with an opening on the south side. The development and design of the fuzzy controller for the corresponding positioning of the roller blind with the available solar radiation as external disturbance is the subject of this paper.     

Daylight availability and models for global and diffuse horizontal illuminance and irradiance for Bangkok

We present results of a study on measurement of illuminance of daylight and solar irradiance from a station in the Asian Institute of Technology (AIT) campus, which is situated in a tropical region. Availability of daylight illuminance and solar irradiance on the horizontal and on vertical planes in cardinal directions is first presented. Mathematical models of global and horizontal daylight illuminance and solar irradiance are then presented. Our results reinforce earlier expectation of good potential for daylighting in tropical climate.     

Automated demand response system in lighting for optimization of electricity cost

Daylighting is an important factor in improving visual comfort and energy efficiency. Lighting control using daylighting can reduce energy consumption in buildings. This thesis proposes an automatic demand response system for lighting based on wireless sensor networks (WSN) in order to reduce the peak electricity demand according to the stage of electricity rate with real-time pricing (RTP), time of use pricing (TOUP), and critical peak pricing (CPP). The proposed system automatically controls the slat angle of the venetian blind with a cut-off angle according to the altitude of the sun, automatically executing light dimming according to measured current luminance to remove an unpleasant glare caused by daylighting. The target illuminance of area at this time is set at a minimum illuminance required for the work execution in the office during the time zone where the electricity load is high to save the lighting energy cost, while a maximum illuminance is set during the time zone where the electricity load is low according to the real-time electricity pricing stages obtained through the advanced metering infrastructure (AMI) in order to improve the work efficiency of the occupants. In this study, two testbeds having the same environments, as well as a control system targeting a fixed illuminance per price system, were established. The illuminance energy consumption and cost were then measured and the effect of the proposed illuminance system was evaluated.