Experimental investigation of a solar still coupled with solar collectors

A solar distillation system has been designed and installed in NCSR “Demokritos”. The system consists of three parts: an asymmetric, single-effect solar still of greenhouse type, integrated storage and a flat-plate collectors field. The thermal behavior of the system was investigated by two series of test, first in the solar still unit only and second in the solar still coupled with a storage tank, heated by a solar collector field. The influence of coupling the solar still with a hot water tank is reported, especially concerning distilled water productivity. It was found that coupling a solar still with a hot water tank generally doubles the distilled water output in the 24-h period, as a result of continuous heating of basin water from tank water. Increases are higher at night than in the day, since at night differences in water and cover temperatures are generally higher, resulting in higher production rates. The hybrid design of this system, apart from being able to supply desalted water together with hot water, leads to significantly higher water productivity in the day and night. Moreover, it can use available heat such as waste heat, coming from thermal processes nearby, optimizing exploitability of any available heat sources.

The operation of a solar distillation system coupled with a storage tank has been investigated through a series of tests. Tank water was heated by a solar collector field. Comparison of the coupled system’s productivity was made with its productivity as a solar still only. It was found that the productivity of the coupled system is about double from that of the still-only system. Significant raises in distilled water productivity have been obtained not only during the day but mainly during night operation of the system, reaching triple the solar-only system productivity. The continuous heating of basin water from tank water results in higher production rates in all operation periods as a result of significantly higher differences between water and cover temperatures, mainly at night. The developed input-output method appeared to be a very reliable tool for describing performance of the solar still since it gives very satisfactory results in the prediction of still productivity. However, the way to apply this method in the coupled still needs further investigation. Raising the temperature of the water in the still basin is one of the most efficient ways to greatly increase the productivity of a solar still. The design of the proposed system enables the use of any available thermal source nearby such as hot waste water, waste heat or heat from a collector field, increasing the exploitability of the area sources. Moreover, it makes it a hybrid one, capable of supplying hot water also.