How to Convert Salt Water to Pure Drinking Water Using Solar Power

By Mr. Oliver Kopsch
December 2003

The Author is Sales and Marketing Manager at Rosendahl System in Hannover, Germany. Rosendahl System produces solar powered and decentralised pure drinking water from any raw water.

There are still 1.3 billion people with insufficient drinking water supplies, 13,000 people die each day from illnesses related to their drinking water. Even filtration provides little more than an expensive, even fatal, illusion. If the filter has been in place too long, which is hard to determine, bacteria flourishes and the danger is greater than ever. The costs for new filters must also be considered. This leads to a situation where users, particularly in poorer countries, risk their health and their lives due to lack of money. The UV sterilisation method, often over-estimated, requires a lot of maintenance and is often only able to sterilise. Suspended solids are often insufficient, chemicals and heavy metals are not removed.

Water distillation, however, offers a perfect solution, as has been demonstrated by vapour compression and multi-stage condensing. Individual households on the other hand, particularly in the USA, make use of smaller distillation apparatus, in which drinking water is produced and connections also made to ice-cube machine.
In Germany, however, public opinion still views distilled water as a health risk. In fact the very opposite is the case. All minerals contained in water are inorganic and are only expelled from the body with great difficulty, if at all. The result is calcium deposits, stones in the bladder, gall-bladder and kidneys. Even expensive mineral water has minerals which are more often detrimental to health than beneficial, the remainder having a mostly homeopathic effect. In order to satisfy our mineral intake requirements from drinking water alone, we would have to drink some 2-3 cubic metres of water per day. In fact, we satisfy our mineral requirements with our food intake. The disadvantage of distillation up to now has been its relatively high energy consumption.

Solar distillation plants have already been around for a number of decades, first being used in the 1950s, mainly for the desalination of sea water. Large collecting areas supplied whole villages and industrial plants. These developments sadly came to a halt as a result of the enormously lowered prices for energy derived from oil and atomic power.
Since 1985, Wilfried Rosendahl has been further developing solar distillation on Gran Canaria. His success is viewed as a sensation by experts. By using his process it is possible to produce drinking water economically, from small units as well as large ones. The price of water produced from a medium sized unit (>1 m³/d) is less than 3 Euro per cubic metre!

Solar Distillation

The principle is amazingly simple. Untreated water like sea water or contaminated water is added to an upper channel in controlled amounts. From there it drips over wicks and a black coloured absorber fleece through the collector. A part evaporates from the energy of the sun and the steam condenses under the glass cover. The condensation runs into the condensation channel. The remaining untreated water is collected in front of it and diverted out of the side of the collector. Both the untreated water channel and the condensation channel have supply points on the sides. Since there are no moving parts the whole system requires almost no maintenance.

Although this seems simple today, the development process required solutions to numerous problems. A result was the use of stainless steel for all components that were in contact with water, use in sea water or brackish water required a special alloy. Different textiles were tested for their suitability as an absorber fleece. The important factor is good capillary characteristics with water content as low as possible, also resistance to rotting. A special dye was developed for the colouring. UV resistance is a must. Resistance to salt water and capillary characteristics are further requirements, coupled with non-toxicity, these proved to be a problem in the first months of operation, where many of the dyes tested emitted toxic fumes, which entered the drinking water along with the condensation. There were similar problems involved in sealing the covering plate and other sealants used in the construction process. It is easy to see why such test series take a number of years to complete.

The longevity and safety of the collectors can now be taken to be over 20 years. The intake of untreated water must be performed in such doses that around 50 % evaporates and 50 % remains as brine. A larger amount of untreated water results in unnecessary heat loss and therefore poor performance. Not enough untreated water leads to salt crusting and drying-out, also poor performance.
In order to get to grips with these problems a guided electronic control system was developed, with controls a solenoid valve.

If sea water is used as untreated water then the brine can be converted to sea salt. This enables sea salt production to be used as an additional source of income. Otherwise, the brine must be transferred back to sea, which poses no environmental problems whatsoever. It is also possible to establish a seepage well. When obtaining sea water from a site on the coast it may be possible to use a well as source. This has the advantage of filtering the water to a certain extent.
The processing of fresh water is mostly used in smaller, family-friendly plants, for producing mineral and germ free drinking water. The waste water from the collectors can be used here for watering plants or other uses. In the Caribbean the Solasia de Puerto Rico company has purchased a licence and offers distillation collectors as an alternative to the electric steam distillation plants, which consume plenty of energy. The product is also in the process of being marketed in other countries as well. A company in Sicily plans to hire out finished plants in order to make the investment more affordable to less well off customers.

Drinking water related health aspects also play an economic role, also in the home. Avoidable CO2 emissions and the contribution to environmental protection are both relevant here.

The quality of the water in the collectors has been awarded a gold medal from the American Water Quality Association. A certified laboratory regularly tests the quality of water from a collector group in Sicily.

An example of a laboratory test from 27.04.2000 confirms the good quality of the produced drinking water. Considering the fact that the untreated water used was the Mediterranean harbour water of the poorest quality, with all impurities and waste water from ships and boats it proofs the capacity of solar distillation. The reason for this is UV solar purification, which takes place in the collector. The high operating temperature, over 80C, also has a hygienic effect. The extremely low conductance of less than 3.5 mS (Microsiemens) shows the high degree of distillation. This means that the product can also be used for technical or chemical purposes in the field of industry, and everywhere where distilled water is required. The price of 3,00 Euro per m³ surely guarantees it an interesting market.

Larger Solar Distillation Plant

Larger plants and water plants are also suitable for flat collectors. They are simple to install in standard large collectors. This means a reduction in the number of collectors needed, as well as a cost reduction. The result is water prices well below 3,00 Euro per m³. Of course, the local climate plays a role in calculations here, together with labour costs.

Collector fields work with no external energy sources, like the individual units. The central electronic controls are supplied by a single solar cell; the pumps for the untreated water are also driven by photo-voltaic energy. In comparison, the Revers-Osmosis process needs some 10 kW/h per cubic metre. The solar process also helps to avoid the immense costs for possible expansions and extensions to power plants or the laying or reinforcement of power cables.

Decentralising water supplies can result in savings on expensive piping and water networks. The largest cost factor, however, is the debt service, with 80-90 %. Subsidised interest payments can play an important role here, one which also has a positive effect on the protection of the environment.

Therefore solar distillation plants have proven to be a reliable, simple, scalable, sustainable and affordable way to produce pure drinking from almost any water where it is needed most - locally.

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