Shortage of water becomes one of the principle factors restraining sustainable development in many arid regions of the world. Nowadays the main sources of fresh water are river flow, lakes water and artesian wells. Each year 577 000 km3 of water evaporates from the land and oceanic surface. The same amount then comes with precipitation. River discharge comprises only 7% of the total precipitation. If one compares the total amounts of the evaporating and the atmospheric water, it is clear that the latter renewable rate of the latter is 45 times per year. However, renewable source of fresh water, - atmospheric water vapors are almost not used.Atmospheric water distribution is irregular. Half of the total amount of water vapors are contained in the lower 1.5 km and more than 99% - in the troposphere. Mean absolute humidity near the Earth surface is 11 g/m3 , but in tropics it may reach 25 g/m3 and even higher.
Many countries of the tropical zone suffer from the shortage of fresh water, though atmospheric water content is high. For example, there are almost no rains in Djibouti all year round , but the absolute humidity is 18-24 g/m3. The amount of water moving above each square of 10ґ10 km2 in the Arabian deserts and Sahara has the same volume as a lake, the area of which is 1 km2 and the depth - 50 m. In order to receive this water one has to turn a symbolic "tap".
One of sources can be obtaining the fresh water desalination of the sea water. For large-scale production of fresh water the distillation, electrodialisis and reversed osmosis are used, but they all have such properties that limit their wide usage. First, for production of fresh water they use sea water, so the plant must be only near to the water body. Second, brine with high concentration is formed the process, which makes negative impact on environment when being poured off into the water body. That have taken desalination seriously, such as Saudi Arabia, Bahrain and Kuwait, where 95% of their water supply is by desalination of sea water. The desalination plants are, of course, fueled with oil, of which these countries are not in short supply, and they have the necessary financial means (as an example, Egypt's efforts in this direction are constrained by a lack of capital). About 7ґ109 m3/yr. of water is currently obtained in the fuel-fired desalination plants but it is sad that in purifying the water they are polluting the atmosphere.
In one of the variants the fresh water is obtained through gathering of tiny drops forming in the air as result of natural cooling because of radiation. For catching of there drops the obstacles are being put on the route of the air movement, against which the drops strike and precipitate. The special synthetical net are used as collectors. Experiments of obtaining water by this method had been done in 47 regions of the world and in 22 countries on 5 continents (Schemenauer, Cereceda, 1991). The quantity of obtained water from one square meter of net per day depends upon the place and time of the year and varies from 3 to 50 liters.
Objective of the project is to implement the method of condensation developed at the F. of Geogr. MSU for arid regions of the world.
It is known from the thermodynamics that if drop of water with a radius of more than critical is placed into an oversaturate vapour, the growth of this drop will cause the thermodynamic potential decreasing and consequently condensation will take place. If a radius is less than critical, than evaporation of a drop will take place. It will disappear because its growth is accompanied by thermodynamic potential increase. During the falls in temperatures which take place, for example, in Sahara or similar region, during night time water vapour very often is unstable and for the start of the second phase in atmosphere i.e. drops creation rudimentary particles are necessary with the radius more than critical. They may be presented by small drops of water, dust particles or the land surface. For example in order to make a 0.1 mkm drop to grow at a temperature of 100C one needs more than 200% oversaturation.
The use of natural processes allows to receive huge quantities of fresh water in tropical regions, practically not influencing the environment. For example, in Nouakchott (Mauritania) average month temperature in May-October makes 27-300C, relative humidity 60-80%. It means, that in each cubic meter is contained 20-24 g of water. Reducing the temperature on 10-150C from each cubic meter it is possible to get 10-14 g of water. If to take into account, that the diurnal difference of temperature can make 15-200C, becomes clear, why in Sahara there are copious dews. In Israel, for examples, 190-200 nights of the year are characterized by conditions expedient to obtaining of fresh water from atmospheric air (100% humidity).
In Feodosiya in Russia until 80-s` years of the last century there was no water-supply from any high-power source, but there were rather many "fountains". During a heyday of the city in the Middle Ages there were up to 100 fountains, which provided with water 80 thousand city dwellers. They got water frond the wells of Feodosian mountains slopes. Somas wells were situated just near watersheds. The water was bridged to them by gravity through potter pipes, which went from special by an image organized detritus heaps (fig. 1).
These wells were supplied by water from detritus heaps 2-5 m high and 1000-2000 m3 or more in volume (Zibold, 1905). They were used for rain water collecting. In drought summer days they condensed moisture from the atmosphere. The heaps were of cone shape with deep crater on the top of each heap and horse-shoe hollow in the direction of drain (fig. 2). Due to this shape maximum heating of heap corresponded to minimum heating area. So inside the heap temperature was extremely irregular in different moments of the day. That was the reason of convection flows inside the heap that resulted in the moist air condensation.
In coastal regions breeze circulation plays a significant role in the process of Feodosian type condensers functioning (Neuman, Mahrer, 1971). At night there is an intensive cooling of stones composing the condenser. If the condenser is 2000m3 in volume (V), limestone occupies 30% (its density p= 2.7ґ103 kg/m3, thermal capacity Cp = 0.22 kcal / (kgґdegree)) then cold supply is 0,7VpCp t=831600 t, where t - daily temperature overfall. This value makes it possible to condense 1.4 t m3 of water from dense air of day breeze. Taking into consideration that heat of evaporating is equal to 595 Kcal/kg under t=100C the quantity of condense will be 14m3 or 7 litres of water per 1m3 of volume. On the Earth there are many regions described by conditions expedient to obtaining of fresh water from atmospheric air (See the map).
The use of atmospheric moisture contained in the atmospheric air, renders minimum effect on the environment, in difference from desalting installations, since in this case it is necessary to delete brine salt water and the atmosphere is polluted at burning of fuel.
The given resource of fresh water is constantly renewed. Characteristics of a condense, which can be obtained in majority of regions of the Earth are very high: the condense contains on two-three order less of toxic metals in comparison with the requirements of sanitarium services, practically does not contain microorganisms, it is good aerated. Besides it is possible to create systems, which are practically not consuming energy, and consequently the water, obtained from atmospheric vapour, appears by cheapest from all, which are received by different ways, not more than 10-20 cents per m3 (Alecseev, Checarev, 1996).
At the first stage of our project we are going to make a pilot module which imitating one condenser of Feodosian type. Whole system consists of separable sections-modules. The wind finds position of the system. It is supposed, that the module will give approximately 10-25 m3 of fresh water per day. In an outcome of realization of this module the evaluations of a system effectiveness in various seasons of the year and within a day should be obtained and evaluation of the most appropriate materials for such systems creation should be produced.
The cost of project for 100 thousand city dwellers is about 15 million dollars. This value is twice as small as the same value for traditional desalination plants. Separable sections of fresh water plant can be use during plant building, so projects expenses will be compensate to the end of building.
