74. Solar-powered desalinators to provide drinking water in arid areas

Providing potable water is a major problem in many areas across Africa. Years of drought in desert regions have meant that surface water and shallow wells are not being replenished by rainfall. Climate changes in the fragile ecosystems can affect the survival of entire groups of people. In Botswana, 80% of the population depend on boreholes for their water. However, drilling for water is expensive and often disappointing. More than half the boreholes in some areas of Botswana result in scarce or salty water. Some 70 communities depend on trucked-in water, but this system is expensive and often unreliable.

The Solar Energy Section of the Rural Industries Innovation Centre has developed and tested several prototypes of small-scale desalinators for areas with water that is too salty for human consumption. Saltwater can come from boreholes or salt pans (surface depressions that collect water).

The desalinators are simple to operate and maintain, are relatively inexpensive, and can provide drinking water under harsh conditions. They produce clear, good-tasting water and also provide salt for cooking, preserving meat, and curing game skins.

The desalinators have been well accepted by remote communities in the Kalahari desert, where they have contributed to community spirit and served as a basis for other development initiatives.

As opposed to trucking in water, desalinators promote self-sufficiency of communities in relation to water supply. Although the desalinators don't meet all the water needs of a community, especially in the winter, the people have assigned them a high social value. They also reduce migration to other areas because of an unreliable water supply.

The desalinator is a glass-covered airtight basin with a black base that absorbs the sun's radiation. The salty water is introduced into the desalinator and left in the sun to evaporate; the air becomes saturated with water vapour and condensation occurs on the coolest surface: the glass. The glass is set at an angle so that the condensed water flows down it rather than dropping back into the salty water. It is collected in gutters and directed to storage tanks, leaving the salt in the basin.

Two types of solar-powered desalinators have been developed. The first, the Mexican modified still, gives good performance and is simple to use. Its fibreglass basin is strong, flexible, does not corrode or leave a taste in the water, is easy to clean, and remains stable at high temperatures. It is small and portable (1.6-m² long and 62-cm high), can be stacked, and is designed to withstand transport over unpaved roads. The installation of the stills does not require skilled labour. The main drawback, however, is that fibreglass can be expensive.

The second model, the brick still, is much less expensive but is less durable and portable and produces less water. In Botswana, the brick stills were cracking because of the low quality of sand for plaster. Skilled labour is needed for installation or the effectiveness of the still will be compromised.

Figure

During the study, the Mexican still yielded 8 litres of water per day in summer and 2.5 litres per day in winter; the brick still yielded 4 litres per day in summer. Supplemental water had to be trucked in in the winter.

The distilled water is virtually salt-free. Because humans need some salt in their drinking water, the distillate has to be mixed with some of the salty water before being used as drinking water. An educational effort is often needed to persuade people of the importance of this step, which can seem superfluous or self-defeating.

The desalinators should not be seen as an end in themselves but as a means to an end. They are complementary to other water technologies and can serve as a base for other initiatives in a settlement. They can relieve human hardship in areas with harsh climatic conditions, while developing within the community a sense of purpose and self-sufficiency.

Potential users

NGOs and community organizations working in desert areas where water supply is difficult and where salty water is available. The desalinators can be used communally, domestically, or for schools, clinics, etc.

Prerequisites

A reliable supply of salty water. Training in installation, operation, and maintenance is essential for the stills to be effective. A sense of participation and ownership of the technology by the community is also essential. They must see the technology as important for their survival or they won't maintain it. Building materials must be robust and resistant to corrosion by salt and degradation by sunlight and heat. All the materials used are locally available in Botswana and include: vermiculite-cement for insulation from the ground, cement, silicone sealant, fibreglass, resin resistant to sunlight, bricks and mortar for brick stills, PVC or cement for gutters, polypropylene pipes, nylon joints, clamps, weatherstripping, brass bolts, galvanized pipes, and standard 4-mm window glass (thinner glass will not resist hailstorms).

Cost and availability

The desalinators are not being produced commercially, but the RIIC will provide information and technical assistance to organizations wanting to undertake similar projects in other countries.

While the desalinators can be expensive, other methods of supplying water to remote arid areas are usually more expensive. Estimated total cost per still is US $340 plus US $230 for materials (storage tanks, fencing to keep out animals, etc.) for the Mexican still; and US $53 per square metre (or approximately $150 per still) for the brick still. The Mexican still pays for itself within about 2.5 years; the brick still pays for itself in less than a year, but requires more skill for repair.

Contact

Rural Industries Innovation Centre

Private Bag 11

Kanye, Botswana

Resources and publications

· R. Yates, T. Woto, and J.T. Tlhage, Solar-Powered Desalination: a Case Study from Botswana, IDRC, 1990. Detailed information on all aspects of the desainators and their use.