Solar and terrestrial radiation dependent on the amount and type of cloud

Ten-year (1964–1973) continuous records at Hamburg of hourly sums of solar and terrestrial, downward and upward radiation flux densities have been evaluated with regard to simultaneous hourly cloud observations. The irradiance at given solar elevation is plotted vs total cloud amount for each season and for the whole year; in the same way, the ratio of the irradiance under clouded to that under cloudless sky is presented. Additional diagrams show the irradiance under cloudless and under overcast sky as function of solar elevation. The ratio of global radiation at total cloud amount N okta, G(N), to global radiation at cloudless sky, G(0), at the same solar elevation γ turned out to be indepenent of γ and can be parameterized by

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. The influence of cloud type is demonstrated by diagrams showing the irradiance under skies overcast by a specific cloud type as function of solar elevation for each season and for the year; also, the corresponding ratios “overcast” to “cloudless” are presented. In the case of global radiation, the ratios which may be interpreted as the transmittances of the specific cloud types for global radiation, turned out to be independent of solar elevation and have the following mean values: Ci, Cc, Cs 0.61; Ac, As 0.27; Sc, Cu 0.25; St 0.18; Ns 0.16.     

A probability density function for daily insolation within the temperate storm belts

Long-term insolation data are frequently obtainable either from recordings or from various empirical formulae. Many radiation-driven processes are, however, highly non-linear. The ability to model the correct long-term frequency distribution of a sequence of radiation events having a specified long-term mean insolation, is, therefore, valuable. This paper emphasizes that the “U-shaped” cloud cover frequency distribution, characteristic of the temperate storm belts, is “mapped” into the irradiation domain in a way that augments the number of “poor” and “excellent” days of sunshine at the expense of “average” days. An analytical and adjustable model is proposed, correlating the probability density of daily clearness index with the monthly average clearness index. This model is tuned to the two longest records of solar radiation in Norway, and it is verified against independent observations from the northern temperate storm belt.     

On the estimation of atmospheric radiation from surface meteorological data

A large number of simple formulae for the estimation of total longwave atmospheric irradiance have been compared to both measured data and to output from the spectral model MODTRAN. MODTRAN predictions under cloudless sky agree well with the average of 38 profile-based models, and observed irradiances on the average agree with MODTRAN predictions within some 5 Wm⁻². It was also found that for “normally” stratified cloudfree atmospheres, the formulae of Berdahl and Fromberg (1982) and of Frank and Püntener (1986), both expressing the emittance as a linear function of dew point temperature, adequately reflect the radiation physics over a wide range of temperature and humidity. Besides, the formulae of Swinbank (1963) and of Czeplak and Kasten (1987), which both express the emittance as a quadratic function of dry bulb temperature, turned out to reflect the radiation physics of “normally” stratified not too dry atmospheres. For “abnormally” stratified atmospheres, a correction to empirical formulae was found. For overcast situations, the increase in atmospheric emittance beyond its cloudless value showed up to be a function of the fractional cloud cover, the cloud emittance, and the surface minus cloud base temperature difference, as phrased by Martin and Berdahl (1984). Except for a coarser spectral resolution and a slight underestimation, the simple spectral model of Berger (1988) conforms well with spectral irradiances derived from observed spectral radiances.     

Operational method for deriving high resolution direct normal irradiance from satellite data

Ground-based measurements of solar beam irradiance are expensive and rarely available especially in the sunbelt countries, the favourable regions for concentrating solar energy techniques. A method based on satellite data is presented that calculates hourly solar direct normal irradiance for large regions and for many years with high spatial resolution independently from ground measurements. The attenuation of solar irradiance due to scattering and absorption in the cloud-free atmosphere is calculated by a clear-sky broadband parameterisation model. It uses ozone optical depth from the Total Ozone Mapping Spectrometer, aerosol optical depth from the Global Aerosol Climatology Project and water vapour from the reanalysis of the National Centers for Environmental Prediction. Attenuation due to clouds is taken into account by a cloud index that is derived from the infrared and the visible channel of the geostationary Meteosat weather satellites. As clouds have the strongest impact on beam irradiance at ground, the resolution of the cloud parameterisation dominates the overall spatial and temporal resolution. The presented algorithm provides hourly data of direct normal irradiance related to the nominal 5 × 5 km² resolution of Meteosat.     

Revision of ground albedo estimation in Heliosat scheme for deriving solar radiation from SEVIRI HRV channel of Meteosat satellite

Methodologies for computing ground albedo from geostationary satellite images are investigated within the framework of models for deriving solar radiation components from satellite information. A revision of Heliosat-3 approach for determining ground albedo and its implication in the cloud index estimation is done using two close pixels of quite different reflectivity in the Nile delta surroundings in Egypt. The analysis done has revealed that the way in which Heliosat-3 approaches the ground albedo and especially its dependence with the viewing angle is not very precise when dealing with high reflective sites. A simple method for fitting, individually for every pixel, the angular dependence and the ground albedo estimations is proposed here and applied to different points in the Mediterranean area. The results of the revised method show rather good estimations of ground albedo in every kind of terrain in terms of reflectivity. These results are translated to cloud index estimations and finally to surface solar irradiance and they show improvements in solar radiation estimation over high reflective pixels.     

The performance results of Trombe-Wall passive systems under Aegean Sea climatic conditions

Since 1977, at the “Solar Energy Thermal Applications Laboratory of M.T.A.” in Marmaris, Turkey, an experimental study of a “Trombe Thermal Storage Wall” passive system continues in operation; two analogous houses have been built, one of which is serving as a reference system. The incident solar radiation on a vertical surface, the temperature changes of the internal and external surfaces of the wall, at interval of thirty minutes, and the convective flow through the “Trombe-Wall” have been measured in order to determine the solar fractions transmitted by the processes of radiation, convection and thermocirculation. In the present study, the variation of the “Trombe-Wall” efficiency has been calculated during the period of a year using a computer evaluation model in which the half hour measurement data have been introduced. The experimental results show that, according to the months, 15–35 per cent of the incident solar radiation over the south facade is transmitted to the interior through the “Trombe-Wall”, while the reference house transmits from the south facade 8–19 per cent of solar radiation received. Finally, it can now be confirmed that the results of the “Trombe-Wall” system without any special type insulation, contributes to practically the total heating load of the village houses under the sunny Aegean Sea Climatic conditions.     

Monthly mean solar radiation statistics for Australia

Tables of monthly mean solar radiation parameters are computed from detailed cloud cover information. The parameters include direct and global daily total energy inputs to horizontal, inclined and “sun-tracking” surfaces. Comparison with measured global radiation at 12 stations reveals virtually no systematic error in the computation scheme, and an error of 2MJ m⁻² day⁻¹ in the worst case month of any station.     

Prediction of global daily solar radiation using higher order statistics

The main concern of the present paper is to present and to analyse two procedures for modelling daily global solar radiation. The first one uses the clearness index techniques and the second one uses a totally different type of approach for taking in consideration important properties of such data, including non-Gaussian shape and non-stationarity. This procedure uses the difference between the extraterrestrial and the observed daily global radiation denoted “lost solar component”. Both procedures are based on higher order statistics for generating the global solar radiation using mainly a random process. The prediction results show that the sequences of values generated have the same statistical characteristics as those of sequences observed. The comparison between the two methods used indicates that the developed model based on the “lost solar component” is better than the model obtained using the conventional procedure based on the clearness index.     

A study of the spectral response of several Eppley pyranometers

It is the custom of the Meteorological Service of Canada, the Eppley Laboratory, and the National Weather Service to perform routine calibrations of pyranometers in integrating hemispheres or spheres. The light source used by each Laboratory is distinctly different. In order to investigate a possible calibration bias introduced by different light sources, pyranometers manufactured by Eppley with different paints and configurations were exposed to irradiation from light sources typical of those in use. Samples of Eppley lampblack-magnesium oxide, Parson's Black-magnesium oxide, “star”, and precision pyranometers were randomly chosen for this test. The response of each pyranometer to each light source was determined for “total white light” and to light filtered by each of nine narrow-band interference filters. The peaks for these filters were approximately evenly spaced from 368 to 2450 nanometers with the transmission in the ultraviolet and infrared suppressed to less than 1 per cent. Graphs are presented showing the mean response of each type of pyranometer to each light source.From the data presented, it is concluded that the combination of the spectral quality of the source lamps and the spectral dependence of the types of pyranometers examined may lead to calibration biases. It is recommended that either the quartz, iodine cycle lamp or a high efficiency lamp similar to the airway beacon lamp be adopted for use in spheres and hemispheres.Recently, a limited number of comparisons have been made of sun-exposure calibrations with sphere calibrations made with source lights comparable to those above. Too few of these comparisons have been made to date to draw valid conclusions. However, the recommendation for use of a lamp similar to the airway beacon lamp made above appears to be supported. Additional study must be done before a decision can be made.     

A MODIFICATION OF THE HELIOSAT METHOD TO IMPROVE ITS PERFORMANCE

The article presents a modification of the Heliosat Method which has been proven to give a better estimation of solar radiation. In the modified algorithm, the absence of a cloud above each pixel is checked from its cloud cover index value, and if no cloud is detected, the global radiation incident on the pixel is computed using a clear sky model. Otherwise, the quantity is calculated following the original algorithm. The improvement obtained from the modification has been evaluated at a Heliosat Secondary User Station in Italy. The evaluation was done by comparing the hourly global irradiation estimated by the original method and that determined by the modified method with the values measured directly at 13 ground stations. The improvements of the global root mean square error and mean bias error computed from the combined data of all the 13 stations are 5.17% and 8.03% of the mean ground measured hourly irradiation, respectively. The improvements for the individual stations are as reported in Table 1.

Based on the results of the evaluation, it has been concluded that the proposed modification gives a better estimation of the energy than the original algorithm. The improvement is more pronounced especially when the method is used in countries or regions where cloudless atmospheric conditions prevail for relatively longer periods of the year.     

Solar radiation assessment using Meteosat 4-VIS imagery

Meteosat 4-VIS high-definition imagery was used for testing solar radiation assessment over northeast Brazil during January 1991 and 1994. Actinographs of solarimetric stations in Paraíba State provided proper ground reference. A simple physical model was developed, based on two basic hypotheses: visible and infrared intervals are decoupled, and radiation transfer in the visible interval is essentially conservative. Some ideas are borrowed from the well known model of Gautier et al. for cloudiness assessment. The resulting GL model can run on a personal microcomputer, processing targets of 5 × 5 pixels for all the region. It exhibits rather good behaviour on a monthly scale, reproducing the mean diurnal cycle and assessing mean irradiation with bias and mean standard deviation lower than 10 W m⁻². Fifteen-day means exhibit similar accuracy. Ground albedo is a highly important parameter of the model. As expected, the main sources of error seem to be planetary reflectance and cloudiness assessment, because of the non-lambertian reflection and the different count thresholds for each kind of cloud cover. Further studies considering longer time series of images (VIS and IR channels) as well as the anisotropic properties of finite cloud fields may improve the model.     

A simple procedure for assessing thermal comfort in passive solar heated buildings

The Fanger thermal comfort equation is linearized and used to develop a procedure for assessing thermal comfort levels in passive solar heated buildings. In order to relate comfort levels in non-uniform environments to uniform conditions, a new thermal index called the “equivalent uniform temperature” is introduced.     

An approach to develop space solar power as a new energy system for developing countries

The idea of space solar power proposed by Glaser was explained as a set of a solar power power station in geostationary earth orbit to transmit microwave power and a ground station to receive the microwave power. Most of the ideas and concepts since Glaser used the same context. On the other hand, Collins et al. (Proceedings SPS '91, pp. 132–141, 1991) introduced the concept of microwave “fuel” to assess the commercial relations of power from space, in which space solar power stations are considered to sell microwave power to any unspecified rectenna. This concept changed the theoretical context of “power from space” to an industrial and economic relation of producers and buyers of an industrial product. This new context has been applied to the SPS 2000 conceptual study. As a result, if 2.45 GHz microwave power transmission is used, each rectenna can be planned and engineered independently from the space sector by local users, especially in developing countries, who are familiar with such activities as introducing solar energy systems.     

Health and safety issues for microwave power transmission

A general public perception that microwaves are hazardous has been a key obstacle for acceptance of microwave power transmission (MPT). This perception will eventually dissipate and then attention will focus on a real technical problem, that of interference (RFI). This can range from perceptible through annoying to hazardous. A program of actions is proposed to accelerate the goal of public acceptance of MPT.In this paper, a historical review shows that the solar power satellite (SPS) was reviewed a number of times relative to potential microwave exposure hazards. In all cases, no “show-stopper” was found but often the shibboleth “more research is needed” was aired. It is shown that standards for safe exposure to microwaves are the most important asset in convincing an audience that microwave exposure associated with MPT or SPS is safe. Standard-setting, world-wide, is shown to converge towards rational limits that are supportive of the MPT/SPS concepts. In recent times there has been the proposed substitute of “risk communication” (“prudent avoidance”). This is an unwise substitute for standards.Other aspects of microwave exposure standards are the new interface with RFI—hence the need for a rational division of responsibility between the radiators and the victim devices, like medical electronics—using both radiation limits and susceptibility limits. Beneficial applications of microwave exposure are being developed.Several studies are recommended which could put into perspective the likelihood of improbable events that represent “catastrophe”—e.g. the inadvertent focusing of a great amount of energy into inhabited areas.     

A comparison between one year of daily global irradiation from ground-based measurements versus METEOSAT images from seven locations in Tunisia

Three numerical images from METEOSAT B2 per day have been processed over a period of 12 months, from October 1985 to September 1986, to estimate the daily values of available solar radiation in Tunisia. The methodology used, GISTEL, on the images of the “visible” channel of METEOSAT, is described. Results are compared with measured radiation values from seven stations of the “Institut de la Météorologie de Tunisie.” Among more than 2200 measured-estimated daily pairs, a high percentage, 89%, show a relative error of + or −10%. Many figures concerning Sidi-Bou-Saïd, Kairouan, Thala, and Gafsa are presented to show the capability of GISTEL to map the daily available solar radiation with a sufficient spatial resolution in countries where radiation measurements are too scarce.     

Determining typical weather for use in solar energy simulations

Predicting the performance of a solar energy system by using simulation methods requires weather data input for the locality involved. The present paper describes a method of analyzing an optional number of years of weather data for a chosen month resulting in a “typical week” which is characterized in terms of solar radiation, ambient dry bulb temperature and wind speed. The “typical week” is allowed to vary in length between 5 and 10 days in the analysis in order to enable selection of a period that best represents a given month according to specified criteria.Verification of the method by comparative computer analysis was performed using two forms of weather data as inputs to the solar energy program “TRYNSYS”. The averaging method when compared to the “typical” weather method resulted in differences of less than 7 per cent.The use of “typical” weather appears to give results at least comparable with more established methods while at the same time providing a broad spectrum of the weather typical of an area. The use of “typical” weather can result in savings in computer time.     

Solar energy distribution over Egypt using cloudiness from Meteosat photos

In Egypt, there are 10 ground stations for measuring the global solar radiation, and five stations for measuring the diffuse solar radiation. Every day at noon, the Meteorological Authority in Cairo receives three photographs of cloudiness over Egypt from the Meteosat satellite, one in the visible, and two in the infra-red bands (10.5–12.5 μm) and (5.7–7.1 μm). The monthly average cloudiness for 24 sites over Egypt are measured and calculated from Meteosat observations during the period 1985–1986.

Correlation analysis between the cloudiness observed by Meteosat and global solar radiation measured from the ground stations is carried out. It is found that, the correlation coefficients are about 0.90 for the simple linear regression, and increase for the second and third degree regressions. Also, the correlation coefficients for the cloudiness with the diffuse solar radiation are about 0.80 for the simple linear regression, and increase for the second and third degree regression. Models and empirical relations for estimating the global and diffuse solar radiation from Meteosat cloudiness data over Egypt are deduced and tested. Seasonal maps for the global and diffuse radiation over Egypt are carried out.     

Dependence on cloudiness of the relation between the ratio of diffuse to global radiation and the ratio of global to extraterrestrial radiation for average daily values

The following linear correlation proposed by Page can be used to estimate values of if values of c, d, , and are known where , , and are, respectively, the mean daily global, diffuse, and extraterrestrial radiation on a horizontal surface, and c and d are empirical constants. Usually, values of and are obtained for each month, but in the present work the values obtained for the first, second, and third decades of each month have been used. Page's correlation is generally established using all the measured values of global and diffuse radiation (independently of the state of the sky). The corresponding values of , , c and d may be referred to as “average skies” values.In the present work we have computed the best values of c and d (in a least squares sense) using values of and obtained at Uccle (latitude 50°48′N, longitude 4°21′N, E) for 11 different classes of days: days with S/S_o = 0; 0 < S/S_o 0.1;…; S/S_o > 0.9, where S is the daily number of sunshine hours and S_o is the astronomical daylength. Quadratic correlations relating the values of c and d with the middle values of S/S_o for each of the ranges considered, are established. Values of c and d are also computed for “average skies” and “clear skies” (S/S_o = 1), and the corresponding linear correlations are compared to those obtained for the mentioned classes of days.     

Mean daily averages of beam radiation received by tilted surfaces as affected by the atmosphere

A method to compare the values of the monthly average beam radiation tilt factor, , obtained if atmospheric effects are accounted for or not, is presented. It is found that the difference between the two determinations may vary greatly with declination, latitude and ratio of daily averaged diffuse to global irradiation. In cases when this ratio is not obtained via direct measurement and its value is large enough, the accuracy of the “atmospheric” may be seriously affected. By comparison with measured radiation data from Montreal, Canada, the “atmospheric” as calculated by the proposed method appears very accurate for south vertical surfaces, and of acceptable accuracy for east or west vertical surfaces. In the latter case, a part of the discrepancy is attributed to various experimental errors, which are discussed.     

Salt gradient solar pond with reflective bottom: Application to the “saturated” pond

The “saturated” salt gradient solar pond operates near the solubility limit. Consequently, temperature fluctuations may cause precipitation of the salt, which can increase the reflectivity of the bottom. It is shown that this can reduce the width of the nonconvective zone and seriously degrade the performance of the pond. The temperature distribution, efficiency and optimum operating conditions are calculated, taking account of diffusely reflected light from the bottom of the pond. The mechanism for narrowing the nonconvective zone is described. A semiquantitative analysis is made of a known case of simultaneous salt precipitation and nonconvective zone destruction. It is argued that the boundary between the nonconvective zone and the lower convective zone will move to its maximum temperature position if the solubility is a sufficiently strongly increasing function of temperature.