Total Global Solar Radiation Research Articles

Techniques for the precise estimation of hourly values of global, diffuse and direct solar radiation

The method usually used to compute solar radiation, when no measured data are available, is the well-known regression technique relating mean daily totals of global and diffuse solar radiation with the mean duration of sunshine. Using this method and taking into account the first order multiple reflections between the ground and the atmosphere, regression parameters were obtained from the monthly mean values of daily totals of global solar radiation and sunshine at a network of 16 stations in India. Daily values of global and diffuse solar radiation were then computed for 121 stations, where sunshine data are available for periods of 6–28 yr, using interpolated values of the regression parameters. Where no sunshine data were available, global and diffuse solar radiation were computed from cloud observations, using the inverse relationship between sunshine and cloudiness. Further, using the empirical relationship between daily totals and the corresponding hourly values of global and diffuse solar radiation, two sets of curves were prepared valid for the whole country, using which mean hourly values of global and diffuse radiation could be deduced from the corresponding daily totals, with a high degree of accuracy. The paper discusses the validity of the techniques used for computing daily and hourly values of global and diffuse solar radiation from sunshine and cloud amounts at an extended network of 145 stations in India and stresses the fact that such techniques are successful, only if accurate data on both radiation and sunshine are available at a widely distributed network of stations for a minimum period from at least 5 to 6 yr, using carefully calibrated and well-maintained instruments of the required quality. Theoretical models have also been used to compute clear sky noon values of global, diffuse and direct solar radiation from the solar constant, allowing for attenuation by atmospheric constituents such as ozone, water vapour, dust and aerosols. Using a simple model, calculations of global and diffuse solar radiation on clear days were made for 145 stations from values of the solar constant and measured values of ozone, water vapour and atmospheric turbidity. A method of extending the technique to overcast skies and partly clouded skies is discussed. The values of the mean annual transmission factor for global solar radiation under cloud-free conditions using the two methods show excellent agreement and establishes the soundness of the regression technique on one hand and the reliability of the theoretical model used for computing clear sky radiation, on the other.

The influence of persistence and variability on the required solar radiation record length

An analysis was made of 17 years of total daily global solar radiation measurements made at St. Paul, Minnesota, a station located in the heart of North America and representative of a continental climate. The analysis was made with the objective of determining the degree of persistence in the solar radiation data and demonstrating the effect of persistence on the statistical analysis of these data. It is concluded that persistence varies through the year and that the characteristic time between independent observations ranges from slightly more than one day to just over three days. Two methods of estimating persistence revealed that some persistence results from daily serial correlation in the solar radiation data and that some persistence is due to interannual variability of monthly mean daily solar radiation.

Campbell-stokes data for radiation calibration purposes in East Africa

It is shown from literature research that constants in the statistical relationship between total global solar radiation and sunshine duration are now well established for East Africa. Taking into account the differences in data collection errors with other simple methods of solar radiation determination, we show Campbell-Stokes data to be suitable for checking, calibrating or temporarily replacing these other methods where no calibrated pyranometers are available in the tropics. An example is given for Gunn-Bellanis.

NOTE ON THE USE OF THE SOLAR RADIATION-DURATION CURVE FOR SOLAR ENERGY APPLICATION

Solar radiation was recognized again as one of the representatives of new energy resources in 1973, when warnings were given to the limitation of fossil resources, and the utilization of solar energy is being investigated very actively in many countries. A large number of studies have been done on the geographical distribution of global solar radiation in Japan. Most of these studies, however, have a tendency to treat the global solar radiation not as energy resources but as one of the climatological or meteorological factors. In this paper, the author newly proposed the Solar radiation-duration curve as the means for arranging the data of solar radiation from the standpoint of effective utilization of solar energy. This is the application to the data of global solar radiation of the Flow-duration curve, which has been used as the means for arranging the data of discharge in the field of hydrology and others. Fig. 2, is an example of the Solar Radiation-duration curve in 1977 at six stations (Choshi, Tokyo, Kofu, Matsumoto, Fukui and Maizuru) situated in the almost same latitude. Here, daily totals of global solar radiation observed at each station during a year were rearranged from large to small. In order to find the indexes to clarify the regional characteristics of global solar radiation in Japan, the author applied the classification of discharge in the Flow-duration curve to the global solar radiation. The classification of global solar radiation is shown on the Table 3. On the basis of this classification, the value of global solar radiation at each station was plotted on the map, and the isopleths at 50 ly intervals were drawn. As a result, the regional characteristics of classified four kinds of global solar radiation in 1976 and 1977 are summarized as follows: 1. The average 95-day solar radiations of all stations in 1976 and 1977 are 412 ly (200J/m2) and 418 ly (202J/m2), respectively. Main isopleths are 350 ly, 400 ly and 450 ly. On the whole, the regions indicating the value over 400 ly distribute from western Japan centering Shikoku, to Tokai and Chubu districts. Conversely, the regions marking the value under 400 ly distribute Kanto district exept for the eastern coastal areas, Hokuriku district, and Tohoku district on the Pacific side. Especially, the extreme low value under 350 ly appears in the inland area of Kanto district (Figs. 3 and 4). 2. The average 185-day solar radiations of all stations in 1976 and 1977 are 269 ly (130J/m2) and 281 ly (136J/m2), respectively. Main isopleths are 250 ly and 300 ly, and the regional difference in the map of 185-day solar radiation is smaller than that of 95-day. It has the same tendency as the distributional pattern of 95-day solar radiation that the regions with the largest value distribute from western Japan centering Shikoku, Tokai and districts. Smaller value appears on the Sea of Japan side of Hokkaido and Tohoku dis tricts, and stretching to Hokuriku and Kanto districts with the value under 250 ly (Figs. 5 and 6). 3. The average 275-day solar radiations of all stations in 1976 and 1977 are 163 ly (79J/m2) and 174 ly (84J/m2), respectively. Main isopleths are 150 ly and 200 ly, and the regional difference in the map of 275-day solar radiation is the same level as that of 185-day. The region with the value under 150 ly appears in the southern part of Kanto district. On the whole, however, the map shows a distinctive distributional pattern that the region of larger value stretchs on the Pacific side and the region of smaller value on the Sea of Japan side. Furthermore, on the Pacific side, the value over 200 ly exists in Shikoku, Chubu and Tokai districts, and the eastern part of Kanto district. On the Sea of Japan side, there is a tendency that smaller value appears in higher latitude (Figs. 7 and 8).

Small scale variability in the global solar radiation in the Washington, D.C. area

Hourly global solar radiation data on a horizontal surface for several years exist for three stations in the Washington, D.C. area. This rare set of data enabled us to perform a small scale (15–50 km) analysis of the variability in the daily totals of global solar radiation. Based on “clear” day cases, the median experimental error in the data was found to be ∼ 3 per cent; the per cent difference in the average yearly totals between the stations was within the experimental error; the median per cent difference in the monthly averaged totals was 4 per cent; 40 per cent of the cases differed by more than 2 and 25 per cent of the cases differed by more than 3 per cent above the instrumental error.

Estimation and prediction of global solar radiation over Greece

A regression analysis of the relative monthly values of global solar radiation ( K ↓ K 0 ↓ ) the corresponding values of sunshine ( n N ), for the period 1961-75, was performed to determine the constants a and b of the Ångström formula, for Athens. The constants a and b were also determined by a graphical relationship between the average annual relative sunshine ( n N ) and these constants, for the same station. The latter method was then used to determine the constants a and b for 33 other stations widely distributed throughout Greece, after making a relative correction to these constants. In this way, monthly and annual values of global solar radiation were estimated for 34 stations over Greece from sunshine measurements. The geographical distribution of the annual totals of global solar radiation over Greece was mapped and some types and sub-types were identified. Further, a stepwise multiple regression analysis of the annual total amounts of global solar radiation and the three factors (latitude, longitude and altitude) was carried out; the validity of the assumption of the linear relationship between the annual totals of global solar radiation and the three factors was examined.

Attenuation of solar radiation in the atmosphere

This paper presents the results of measurements of the solar radiant energy absorbed and scattered in the atmosphere. Since much of the information on solar irradiance at the ground is obtained by computation from extraterrestrial radiation data, it is important to know precisely the actual energy that is absorbed and scattered in its passage through the atmosphere for the accurate estimation of the radiant energy received at the ground. Various models exist for the estimation of daily totals of global solar radiation under clear sky and cloudy conditions, taking these effects into consideration and assuming average values for the ozone and water vapour content and the turbidity of the atmosphere. In the present investigation atmospheric attenuation of solar radiation has been calculated from measured values of ozone and water vapour content and turbidity in the atmosphere, at two stations Bangalore (950 metres above sea level) and Nandi Hills (1479 masl). Direct measurements of direct solar radiation for the whole spectrum and various spectral regions were made at Bangalore and Nandi using Ångström pyrheliometers fitted with broad-band pass filters during the clear months January–May 1979. Global solar radiation and sunshine duration measurements were also made at both stations. Using direct measurements of the total ozone and water vapour content and atmospheric turbidity, direct, diffuse and global solar radiation values at the ground were computed from extraterrestrial values of radiation for clear sky conditions. The results are compared with actual measurements and earlier observations of direct solar radiation at other high-level stations. The importance of atmospheric turbidity measurements in the computation of solar radiation is discussed.

The fluctuation of solar radiation in Thailand

Fluctuations in the daily solar radiation are examined in an unbroken 5-yr sequence of measurements at Bangkok, and are also estimated from daily sunshine measurements at Bangkok and 3 other stations in Thailand. Seasonal effects are shown by separate studies for eight 1 1 2 month periods of the year defined by standard solar declination values. During the dry season in winter and spring the frequency distribution of daily totals of global solar radiation at Bangkok has a peak near 20 MJ m −2 d −1 and is skewed towards low values. During the wet season in summer and autumn the distribution is more dispersed. Elsewhere the distributions are similar to those at Bangkok. The time series of daily totals of global solar radiation at Bangkok is analysed as a second order random process. The observed annual frequencies of runs of consecutive days with low radiation at Bangkok are given. There are 32.0 isolated days, 9.4 pairs of days, and 3.4 runs of 3 days per year with radiation less than 12.57 MJ m −2 d −1. These results are adequately described by the second order theory. The runs are most likely to occur in summer and autumn. Elsewhere in Thailand the annual frequencies of the runs and their seasonal distributions are almost the same as at Bangkok, except that in the south the runs are more likely to occur later in the year.

Estimating global solar radiation

A model is presented to calculate daily totals of global solar radiation. First, cloudless sky radiation is evaluated from transmission due to absorption and scattering. Cloud effects are added using cloud layer transmission. A simple expression to account for additional radiation due to reflection between the ground surface and cloud bases is also included. Atmospheric transmissions are obtained from previous studies. Precipitable water and observations of cloud type and amount for different layers in the atmosphere are the only meteorological variables required. The model is evaluated and tested with data collected at stations in and around Lake Ontario: a lake station near Grimsby in 1969 and land stations at Burlington, Scarborough, Peterborough, Trenton and Kingston, Ontario in 1972 and 1973 during the International Field Year for the Great Lakes. Good agreement between calculated and measured radiation was obtained at all stations, particularly for 5- and 10-day means. Model performance was largely independent of both cloud amount and season.

Effects of vertical vegetation surfaces on the adjacent microclimate: the role of aspect

Vegetation can influence microclimate in a number of ways. One is when a sharp vertical border is created that exerts a shielding effect on the adjacent horizontal surface. In this case direction of exposure becomes a prime control of the microclimate.This study investigated the differences in energy balance microclimate in the vicinity of forest edges oriented in the four cardinal directions. The gradient outward from the southerly exposure was also examined. A clearing in a uniform forest stand was utilized as the study site.Exposure to direct beam solar radiation was the most important factor in determining climatic differences. Reflection from an exposed forest edge outweighed the depletion of diffuse sky radiation and gave stations nearer the edge higher totals of global solar radiation. This was counteracted by a higher surface albedo. A more favorable longwave radiation balance, therefore, was largely responsible for greater net radiation near the forest edge.Availability of surface moisture provided by dew and the turbulent wind field combined with net radiation to produce the patterns of latent and sensible heat flux. Diminished vertical humidity and temperature gradients accompanying reduced wind meant that latent and sensible heat fluxes were lower and ground storage higher in the vicinity of the forest edge. The warmest thermal environments were found on the north and east side of the clearing. The shaded south side remained cool and damp throughout the day. The microclimate of the western side of the clearing was intermediate in nature.

Solar radiation climate of India

This paper presents the essential results of solar radiation measurements made at a network of thirteen stations in India during the last ten years. The basic records are of total integral wavelength global solar radiation and sky radiation on a horizontal surface, and of direct solar radiation and its spectral components, defined by the wavelength intervals λ < 525, 525–630, λ < 630, 525–710 and λ < 710 nm. Values of direct solar radiation at normal incidence and on horizontal and vertical surfaces have also been calculated from continuous records of the global and sky radiation on a horizontal surface at all 13 stations. The spatial and seasonal variations of direct solar radiation, global solar, and sky radiation are discussed. Results are also presented of special direct solar observations made at a few high altitude stations in Kashmir in 1969.