R. H. B. Exell. Solar Energy, Vol. 18, pp. 349-354, (1976).
Geographical, seasonal, and diurnal variations of global solar radiation in Thailand are surveyed. Seasonal effects are shown by separate studies for eight 1½ month periods of the year defined by standard solar declination values. Detailed maps are given of the geographical distribution of solar radiation prepared from data on cloudiness at 44 stations, duration of sunshine at 18 stations, and linear regressions relating radiation to sunshine at Chiang Mai and Bangkok. The highest mean values are above 19.5MJ/m2 per day and are widespread in spring. The lowest values are below 15.0MJ/m2 in restricted localities with heavy rainfall in autumn.
Rough estimates of diffuse solar radiation and atmospheric turbidity are made from the radiation-sunshine regression parameters. Diffuse radiation averages 8.4MJ/m2 per day. Turbidity at Chiang Mai is high in spring and low in summer and autumn; at Bangkok it is high throughout the year.
The diurnal variation of global solar radiation determined from hourly measurements at Chiang Mai and Bangkok is analyzed. The mean midday radiation fluxes range from 0.80kW/m2 in spring to 0.60kW/m2 in autumn. On the average the radiation received in the afternoon is slightly less than that received in the morning.
R. H. B. Exell. Solar Energy, Vol. 18, pp. 549-554, (1976).
Fluctuations in the daily solar radiation are examined in an unbroken 5-year sequence of measurements at Bangkok, and are also estimated from daily sunshine measurements at Bangkok and three other stations in Thailand. Seasonal effects are shown by separate studies for eight 1½ 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 20MJ/m2 per day 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 analyzed 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 three days per year with radiation less than 12.57MJ/m2 per day. 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.
Values of the mean daily diffuse solar radiation at four locations in Thailand calculated season by season from a simulation model for solar radiation in SE Asia have been compared with rough estimates obtained from correlations between global solar radiation and duration of sunshine. The simulation model gives slightly higher values, which range from 6.6 to 11.2MJ/m2 per day.
Research Reports are available from the Regional Energy Resources Information Center, Asian Institute of Technolgy.
By R. H. B. Exell, 1998. King Mongkut's University of Technology Thonburi.