Solar Energy

World Distribution

Regions for Application

It is common knowledge that solar radiation is unevenly distributed, and that it varies in intensity from one geographic location to another depending upon the latitude, season, and time of day. Until recently, valid records for solar radiation have been very scanty in the vast majority of the developing countries. In the absence of such useful information as a guide for the proper exploitation of solar energy, only general hints can be offered regarding the geographic areas with favourable conditions for solar energy applications.

For convenience and simplicity, the geographic distribution of total solar radiation on a global scale is divided in terms of intensity into four broad belts around the earth. These are illustrated in Figure 2, and also described briefly hereunder with respect to the northern hemisphere, with the understanding that the same conditions apply to the corresponding belts in the southern hemisphere:

  • The most favourable belt. This belt, lying between latitudes 15°N, and 35°N, embraces the regions that are naturally endowed with the most favourable conditions for solar energy applications. These semi-arid regions are characterized by having the greatest amount of solar radiation, more than 90% of which comes as direct radiation because of the limited cloud coverage and rainfall (less than 250 mm per year). Moreover, there is usually over 3,000 hours of sunshine per year.
  • Moderately favourable belt. This belt lies between the equator and latitude 15°N, and is the next most favourable region for the purpose previously mentioned. Because the humidity is high, and cloud cover is frequent, the proportion of scattered radiation is quite high. There is a total of about 2,500 hours of sunshine per year. The solar intensity is almost uniform throughout the year as the seasonal variations are only slight.
  • Less favourable belt. This belt lies between latitude 35°N and 45°N. Although the average solar intensity is roughly about the same as for the other two belts, there are marked seasonal variations in both radiation intensity and daylight hours. During the winter months solar radiation is relatively lower than in the rest of the year.
  • Least favourable belt. The regions in this belt lie beyond latitude 45°N. They include the USSR, and the greater parts of northern Europe and North America. Here, about half of the total radiation is diffuse radiation, with a higher proportion in winter than in summer primarily because of the rather frequent and extensive cloud coverage.
Figure 2. Worldwide distribution of solar radiation into belts indicating feasibility of solar applications.

It is important to note that the majority of developing countries fall within the more favourable regions between latitudes 35°N and 35°S. For this reason they can count on solar radiation as a steadfast source of energy that can be readily exploited cheaply by both rural and urban households for a multitude of purposes, including solar disinfection of drinking water and oral rehydration solutions.