Sustainable energy technologies

3.3.3 Solar Distillation

Solar basin still consists of a simple blackened box for storing and heating water. This box is provided with a glazed top, which serves the purposes of both insulator and condensing surface. The glazed top is kept at an angle to allow the condensed water to flow to one side and into a small gutter. Bottom of the unit is insulated with glass wool to improve the efficiency. Solar energy is allowed into the collector to heat the water. When water gets heated to a certain temperature it evaporates and condenses on the underside of the glass. Rising of only the water vapor leaves contaminants behind, thus purifying the water.

The gentle slope of the glass directs the condensate to a collection trough, which in turn delivers the water to the collection bottle. The still is filled each day with twice as much water as was produced. The still is also fitted with overflow outlets, which allow the excess water to flush the still every day. A major advantage of the basin still is that it does not require a pressurized water supply.

Solar still is a useful devise to get fresh/distilled water that is required in industries, hospitals and dispensaries, garages and automobile workshops, telephone exchanges, laboratories and marshy and costal area.

3.3.4 Solar Disinfection and Purification of Water

There are a few methods commonly advocated for the disinfection of drinking water at the household level. These include boiling of water for about 10 minutes or the use of certain chlorine compounds available in the form of tablets. As each of these procedures has its own drawbacks, their application is extremely limited in the developing countries, where water-borne diseases are most common, and the purity of drinking water supplies from external sources cannot be assured.

Boiling of water and condensation using fuel and use of tablets or proper solution is neither cost effective nor convenient. The experiments conducted on solar disinfection of drinking water at the American University of Beirut for two years concluded that the rate of destruction of bacteria actually depends upon a number of influencing factors such as:

intensity of sunlight at the time of exposure, which in turn depends upon the geographic location (i.e. latitude), seasonal variations and cloud cover, the effective range of wavelengths of light, and the time of day
kind of bacteria, the nature and composition of the medium, and the presence of nutritive elements capable of supporting the growth and multiplication of various microorganisms
characteristics of the containers/bottles in which the contaminated water is kept during exposure (e.g. colour, shape, transparency to sunlight, size, and wall thickness)
clarity of water (i.e. degree of turbidity) and its depth are important factors that determine the extent of penetration of sunlight, and to what extent they have the possibility of shielding the microorganisms from its lethal effects.

Based on the above finding and analysis, it became clear that sunlight with wavelengths ranging from 315 nm to 400 nm is the most lethal region, as it accounts for about 70% of the bacterial destruction potential. The wavelength of this band is known as the near-ultraviolet region of the light spectrum.

Visible light is characterized by having wavelengths ranging from 400 nm to about 750 nm, and accounts for about 30% of the bacterial destruction capacity. Accordingly, the most appropriate colours of the containers/bottles have to be selected that would yield optimum results in terms of microbial destruction. While violet /blue have better effect as compared to green/ yellow/ orange/ red tinted bottles, the colourless plastic bottles are the best for the purpose. Very light green containers may also be used provided the period of exposure to sunlight is somewhat extended.

Thus, preference should be given to containers that are either colourless or blue in colour. The brown colour bottles and to a lesser extent red ones, are recommended for the storage of water. Therefore, storing the of water by exposing it to the solar rays using appropriate colour containers/bottles could be one of the most economical way for the disinfection and purification of water for drinking purpose by the rural households, especially the poor peasants living in the remote and far-flung areas/regions of the developing countries.

Container needs to be exposed to sun for 6 hours if the sky is bright or up to 50% cloudy.
Container needs to be exposed to the sun for 2 consecutive days if the sky is 100% cloudy.
During days of continuous rainfall, SODIS does not work.
If a water temperature of at least 50°C is reached, an exposure time of 1 hour is sufficient.

3.3.5 Nedap solar powered drinking water UV disinfection unit

This solar powered drinking water UV disinfection unit, named as “Nedap,” is developed by ‘NAIADE’, The Netherlands developed. The Nedap is capable of producing 2,500 liters/ day, catering to the drinking water needs of about 800 people per day. The unit is stand-alone, requires no maintenance other than cleaning the PV panel.
Spare parts like the UV lamp needs replacement only after 10,000 hrs of operation. It is reported to be providing water as per the WHO standards. It can be installed within 30 minutes and can be used at all places since it needs no fossil fuel or electricity.
Filtration of unsafe water is done by washable bag filters and disinfection by UV. The unit weighs less than 75 kg and is shipped in ready-to-use packed palletized box.

The unit has been tested and approved by various leading water research labs worldwide, such as UNESCO-IHE, Ghanaian Water Research Institute, Atitra India, KIWA, and many more.