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Solar water heaters were developed some two hundred years ago. The first known flat plate collector was made by H. B. de Sausurre, a Swiss, in the second half of the 18th century. However, little interest was shown in such devices until the world-wide oil crisis of 1973. This crisis promoted new interest in alternative energy sources. As a result, solar energy has, received increased attention and many countries are taking a keen interest in new developments.
It is understood that achievements in science and technology cannot always simply be transferred from one country to another. They must usually be somewhat changed and adapted to suit local conditions and constraints on production. It is known, for example, that copper has the best characteristics for solar heaters, and that aluminium has considerable potential. However, in many countries these materials may be too costly, too difficult to work with, or not available in local markets.
In Nepal, attention was given from the beginning to the use of locally available materials. This resulted in simple working techniques and in designs which favoured the modular system. The combination of product manufacture and installation by the same enterprise proved to be a satisfactory approach. Nowadays, solar water heaters are manufactured in several workshops which, with increasing demand, are gradually upgrading to real small-scale industries.
Performance: Solar water heaters are one of the best known applications for the direct use of the sun's energy. Simple solar water heaters requite almost no attention; they work automatically and provide hot water without any special care.
Design: Solar water heaters have been designed to suit local conditions. "Passive Systems,, were tested first. These systems, which have collecting surfaces connected to a separate hot water storage, tank, circulate water by thermosiphon (convection) effect. This proved to be the most popular design. Another type, the flat tank collector, was also developed. This simple one-piece unit can be a satisfactory alternative to more complex systems. It does not keep water hot overnight, but hot water is available from late morning until after sunset. These are the two most commonly used systems in Nepal. The manufacture and installation of pumped systems and systems incorporating anti-freeze protection is as yet untried, but is technically feasible.
Size: The size of solar water heaters depends on the daily hot water requirements. In general one person may require approx. 25 to 40 liters of hot water at approx. 55° to 60° Centigrades per day (for domestic bathing only, without laundry and not in hotels). For a small family a circulation system with a hot water storage capacity of approx. 120 liters may be sufficient. A flat tank collector of 60 or 90 liters may be appropriate for a few persons where the demand is limited. Selection of size would also depend on availability of standard products. Prizes vary with the heater size and with the installation charges. Installation is simplest when the system is incorporated in the initial planning of the construction of a new house. This allows the architect to incorporate the heater into the plan, both esthetically and economically.
A. Bachmann H. Waldvogel
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