Some comments on time variation in solar radiation over Baghdad, Iraq

In the present work an attempt was made to study and examine some aspects of radiation climatology which are important in solar energy utilization. The yearly cumulative global radiation for Baghdad is 216 MJ/m² per year, while the annual total of daily diffuse radiation at Baghdad is about 70.82 MJ/m². The mean monthly values of clearness index K_T present a maximum of 65.9% in August and a minimum value of 48.4% in January. The annual mean of daily global radiation and its spectral OG1, RG2 and RG8 at Baghdad is about, 18.03, 13.53, 10.86 and 9.36 MJ/m², respectively. Over the year, the highest UV radiation were received during June and July (243 Wh/m²) and the lowest in December (79 Wh/m²). Furthermore, UV radiation constituted on average 3.25% of global radiation.     

Some factors affecting the absorption of solar radiation in solar stills

The performance of a solar still can be affected to a significant degree by the build-up of reflecting layers of salt on the water surface and basin liner. This paper presents a method of quantitatively assessing the effect of these salt layers, taking into account the band absorption characteristics of water. A revised curve of the absorption of the solar spectrum in water is given.     

Some aspects of solar distillation for water purification

In recent experimental work a solar still design which produces 5 to 7 liters of purified water per day has been developed using aluminum components and polyethylene film as the basin liner. A number of such units already have been set up to supply distilled water for laboratory work. There is a need for a supply of fresh drinking water in many salt works. A new design suitable for easy installation in salt works is being tested.

An appraisal of solar distillation under present conditions in certain parts of India has been reported earlier. The development work, including experience gained during the installation and operation of a pilot plant constructed by CSMCRI, Bhavnagar, forms the basis for this assessment of solar distillation. The importance of a combined method for solar distillation and rainwater collection is obvious. In the various regions of India there are vast differences in rainfall. They range from an average of less than 20 cm to more than 1000 cm per year. In most of the regions rainfall is spread over a period of several months. The possibility of having rainfall collection and solar distillation combined at certain locations is examined following methods outlined in the literature.     

Some aspects of nonrandom cloudiness in solar energy applications

In many geographical locations, daytime cloud patterns are nonrandom for certain periods of the year. This article discusses the general characteristics of daily nonrandom cloudiness and illustrates the use of satellite imagery to resolve nonrandom patterns over portions of the Rocky Mountain region of the United States. Results are presented in the form of cloud frequency figures that are derived from geostationary satellite data collected during the summer of 1986. These results are compared to surface-observed, solar radiation data. The implications of daily, nonrandom cloud patterns as a factor in solar energy applications are discussed.     

Some theoretical and experimental aspects of built-in solar storage

The possibility of using a built-in storage solar water heater in Jordan has been investigated. Experimental and theoretical results were obtained using a 90 × 90 × 10 cm solar heater which was tilted at an angle of 30° to the horizontal.It was found that the efficiency of the built-in storage solar heater may reach as high as 78% with a maximum increase in the water temperature of 70°C and at low cost compared to the conventional flat plate collector. It is therefore expected that such solar system may be used to provide reasonably sufficient hot water in Jordan. Analytical solution, based on the assumption that the water temperature equals that of the plate, was used to solve the governing equations. While a numerical solution was used to solve the equations under the condition that the plate temperature is not equal to that of the water.It was found that the assumption of the temperature of the plate (T_p) is equal to the temperature of water (T_w) is only justifiable at early hours of operation in the morning.     

Some aspects of the use of solar energy in Serbia

This article presents the aspects of solar radiation and the use of solar energy in Serbia. It also considers why Serbia does not use thermal conversion of solar radiation in spite of much greater potential comparing to the countries of the Western and Central Europe, which are leading in the use of solar energy. The low standard of living, low electricity prices, the low level of energy efficiency in all areas of energy use, lack of knowledge, and political decisions are the main causes of insufficient use of renewable energy sources in Serbia.     

Theoretical aspects for optimization of solar radiation concentrators with plane facets

The experience of the 25 years operation of two concentrators of solar energy (CSE) of 2.8 and 5.0 m diameter manufactured by gluing plane facets of thin glass onto metal radio antennae convincingly demonstrates their technical and economic efficiency. Besides (or in addition to) simplicity of manufacturing and the possibility to achieve temperatures up to 2000 K and more, there is one more undoubted advantage of such concentrators: it is possible to get a focal spot with rather uniform distribution of the irradiance. The problem of determining the plane facets’ optimum configuration and the structure of their arrangement on the CSE surface which guarantees forming a focal spot of the given dimension is solved by this work. Perfect geometric configuration of the facets and paraboloid is assumed. The working zone of the facets reflecting the incident solar radiation at the circular focal spot is approximately an ellipse. Trapezoid is an optimum configuration for the facets taking up a minimum area and ensuring the maximum use of CSE surface. A system of equations is obtained for calculation of a CSE structure with annular packing of the facets. The results of calculations for the 1.8 m and 15 m diameter concentrators are presented.     

Solar radiation maps for Iraq

As measured solar radiation data for all parts of Iraq is not available, it has to be estimated using other weather variables. A number of correlations which use dry bulb temperature, relative humidity and sunshine duration were tried. Finally a correlation using sunshine duration was selected as it gave most accurate estimation of solar radiation. Constants for the correlation for three stations representing three climatic regions in Iraq were determined. Monthly and yearly solar radiation maps were drawn using sunshine duration data from 24 stations from all over the country.     

The solar radiation resource

We discuss the characteristics of solar radiation which are most useful for the assessment of solar technologies. Graphs are presented which show the total annual insolation incident on the principal tracking and non-tracking collectors. Additional graphs are developed for the latitudinal and seasonal variations of solar radiation in the absence of an atmosphere. These are then compared with measurements averaged over the U.S. SOLMET sites to provide an understanding of the role of atmospheric effects on the seasonal variations of solar radiation.     

Global solar radiation estimation

This paper presents the comparison between the observed and estimated values of global solar radiation on horizontal surfaces obtained from the linear Angstrom type of correlation of Rehman and Halawani [1] for 52 cities spread in 11 countries; viz. India, Egypt, Sri Lanka, Spain, Zimbabwe, Yemen, Sudan, Italy, Zambia, Hong Kong, and Malaysia. The comparisons are also made with the local linear models wherever available. The estimated values are compared with the measured values in terms of percent error, mean bias error (MBE), mean absolute bias error (MABE), root mean square error (RMSE), and mean percent error (MPE). This study finds that the model of Rehman and Halawani [1] is capable of giving estimates within an acceptable mean percent error of 5% and less for 33% of cities and between 5–10% for 50% of cities.     

Spatial-temporal forecasting of solar radiation

We apply the CARDS solar forecasting tool, developed at the University of South Australia, to forecasting of solar radiation series at three sites in Guadeloupe in the Caribbean. After performing the model estimates at each individual site, forecast errors were tested for cross correlation. It was found that on an hourly time scale, there was small but significant correlation between sites, and this was taken into account in refining the forecast. Cross correlation was found to be insignificant at the ten minute time scale so this effect was not included in the forecasting. Also, the final error series in each case was tested for an ARCH effect, finding that to construct prediction intervals for the forecast a conditional heteroscedastic model had to be constructed for the variance. Note that cross correlation between sites has to be included for this procedure as well as in the forecasting of the radiation.     

Models of diffuse solar radiation

For some locations both global and diffuse solar radiation are measured. However, for many locations, only global is measured, or inferred from satellite data. For modelling solar energy applications, the amount of radiation on a tilted surface is needed. Since only the direct component on a tilted surface can be calculated from trigonometry, we need to have diffuse on the horizontal available. There are regression relationships for estimating the diffuse on a tilted surface from diffuse on the horizontal. Models for estimating the diffuse radiation on the horizontal from horizontal global that have been developed in Europe or North America have proved to be inadequate for Australia [Spencer JW. A comparison of methods for estimating hourly diffuse solar radiation from global solar radiation. Sol Energy 1982; 29(1): 19–32]. Boland et al. [Modelling the diffuse fraction of global solar radiation on a horizontal surface. Environmetrics 2001; 12: 103–16] developed a validated model for Australian conditions. We detail our recent advances in developing the theoretical framework for the approach reported therein, particularly the use of the logistic function instead of piecewise linear or simple nonlinear functions.Additionally, we have also constructed a method, using quadratic programming, for identifying values that are likely to be erroneous. This allows us to eliminate outliers in diffuse radiation values, the data most prone to errors in measurement.     

Computation of diffuse solar radiation

A technique for computing the spectral and angular (both the zenith and azimuthal) distribution of the solar energy reaching the surface of earth and any other plane in the atmosphere has been developed. Here the computer code LOWTRAN is used for getting the atmospheric transmittances in conjunction with two approximate procedures: one based on the Eddington method and the other on van de Hulst's adding method, for solving the equation of radiative transfer to obtain the diffuse radiation in the cloud-free situation. The aerosol scattering phase functions are approximated by the Hyeney-Greenstein functions. When the equation of radiative transfer is solved using the adding method, the azimuthal and zenith angle dependence of the scattered radiation is evaluated, whereas when the Eddington technique is utilized only the total downward flux of scattered solar radiation is obtained. Results of the diffuse and beam components of solar radiation received on surface of earth compare very well with those computed by other methods such as the more exact calculations using spherical harmonics and when atmospheric conditions corresponding to that prevailing locally in a tropical location (as in India) are used as inputs the computed values agree closely with the measured values.     

Short-term variability of solar radiation

This paper presents a study of the variability of solar radiation in the minute-long time range. The solar climate in Northern Europe is classified in this paper as being either stable, or highly variable, due to stochastic cloud cover inducing fluctuations of the solar irradiance. In general, solar radiation exhibits both positive and negative increments in the radiation during any period. The distribution functions of these increments with respect to their magnitude and duration are investigated, as well the distribution functions of stable intervals during generally variable radiation. The distribution functions are found to be a superposition of two exponential functions with different exponents, which depend on the magnitude of the radiation increments.     

A comparison of methods for estimating hourly diffuse solar radiation from global solar radiation

Four methods of estimating hourly diffuse irradiation from hourly global irradiation are compared, using global and diffuse irradiation data from the Australian Bureau of Meteorology for five widely separated Australian locations. A development of one of these methods with constants derived from the data for each place is found to perform best when judged on a criterion of absolute error, and this performance in maintained when constants averaged over the five locations are used. The derived constants are also given for all other Australian locations for which both global and diffuse data are available, and a method is suggested for deriving suitable values of the constants for places situated between 20° and 45° S for which only global radiation data are available. A previously reported dependence of the proportion of diffuse radiation on latitude is demonstrated.     

Analysis of solar radiation measurements at Athens Observatory and estimates of solar radiation in Greece

In this paper measurements of Global (G) Solar Radiation on a horizontal surface at Athens during the 16 yr period 1960-75 are analysed. Mean annual, monthly and daily totals, the diurnal variation, frequency distribution of daily totals, frequency of occurrence of daily totals of Global Radiation less than 8.5, 17, and 34 MJ/m² for 2, 3, 4 and 5 successive days are computed and discussed.Direct Solar Radiation at normal incidence (I) is computed and analysed in a similar manner to that of Global Radiation, for the same period. Components of direct radiation of normal incidence have been computed from spot value observations at times when sky conditions permitted.Annual values of global radiation were estimated from sunshine measurements (1960-73) widely distributed throughout Greece. It is considered that this analysis of measurements should closely represent the radiation climatology of Greece.     

Investigations of solar radiation detectors using a laboratory test facility for solar radiation meteorological instruments

A laboratory test facility for solar radiation detectors has been built and is in operation at the Aerological Station of the Swiss Meteorological Institute (SAP/SMI). This installation is conceived as a universal test bed for solar radiation exposed meteorological instruments, and consists of a commercially available solar simulator, a laser alignment system, a translation mechanism with instrument mounts, and an adjustable projection mirror. The solar simulator produces a well characterized radiation beam which can be filtered to match the terrestrial or outer space solar spectrum with an irradiance of up to one solar constant (1367 Wm⁻²). The instrument mounts and a HeNe laser beam provide a precise and easy alignment of the reference and the test instruments in the radiation beam, allowing for incident angles in the range of 15°–75°. The measurement is based on a comparison of the response of an active cavity absolute radiometer PMO6 with the signal of the test instrument.Detailed investigations of the Haenni Solar 111B type heliometer have revealed important irregularities in the sunshine threshold irradiance angular distribution. Measurements performed with and without the protection glass cover prove the exceedingly high threshold values at large declination angles to be a consequence of enhanced reflections due to the incident angle and inhomogeneities in the glass cover.Very satisfactory results have also been obtained on characterization measurements of pyranometers showing the mean values of the responsivity to be within 0.8% of the calibration values measured at the world radiation center (WRC) at Davos.