High quality water in two steps – the Sanakvo process
The highly inventive Sanakvo process is quite different. During many years of thought, studies, research and development, all parts and features of the process have been critically selected, optimized in efficiency and simplicity of structures to obtain high quality liquid water at the lowest costs. In addition, the whole process is highly safe for users and the environment and is usable on small as well as on large scales.
First step: Water vapor is transformed into liquid
The first challenge in harvesting water from air is the transformation of the enormous volume of air in which water vapor is distributed into a small volume of liquid. If someone would try to work with such volumes of air he would necessarily need huge blowers to transport the air and large structures for its processing. Inevitably, this would be very expensive in terms of energy, machines and infrastructure and large initial investment would be required. The cost of the resulting water would be prohibitive, not only in poor countries.
Therefore, we prefer passive air contact. Air is always in movement. Even an almost unnoticeable movement of air of 0.5 m per second in the driest desert on our planet, the Atacama desert in Chile, transports an incredible amount of liquid water (1 300 liters) through a surface equivalent to a door frame about 2 m × 1 m in 24 hours. We take advantage of this natural air flow at no cost.
For binding water vapor we use a water absorbing liquid – glycerol. Glycerol is a liquid, which absorbs water with high affinity. Glycerol is so thirsty for water that the amount of absorbed water can be larger than the original volume of glycerol used. Chemically expressed, glycerol forms water hydrates to decrease the energy of its hydroxyl groups. This reaction is highly selective for water. This means that glycerol does not absorb other substances or impurities contained in the air. Therefore, in this single absorption step, water from air is highly purified, not at all comparable to “condensed smog”! Due to the properties of concentrated glycerol, all living microorganisms also lose their water and are inactivated.
The water catcher can be fixed, for example, on a clothesline. In its simplest arrangement, water can be harvested on pieces of fabric, which are soaked with concentrated glycerol solution, or over which concentrated glycerol is allowed to flow.
Most mothers in the world use clotheslines to dry their laundry. For harvesting water we just employ the opposite – we hang wet glycerol containing fabrics on similar lines to collect water vapor from air.
A clothesline holder with 60 m line and double layer fabric strips can absorb up to 500 l of water in 24 hours. Despite this primitive means, this is a tremendous yield which can supply 200 persons with drinking water every day!
Of course, this process can be performed to a more sophisticated or even automated way, but the principle of bringing humid air in contact with glycerol flowing over any convenient surface remains the same. A scale-up to larger structures and surfaces is obvious for any technically experienced person.
An important advantage is that water absorption can be achieved during the day or even at night, when the relative humidity of air is at its maximum as it happens also in deserts. Thus, a pool of hydrated glycerol can be obtained during nights or during overcast days for later water production in sunny weather.
The result of the first step is that water originally present at low concentration in very large volumes of air has been highly concentrated into small volumes of diluted glycerol solution.
Second step: Water recovery in solar modules
Now, the water could be separated from glycerol for example by classical or multistage distillation. However, this would require huge amounts of energy, large investment into distillation equipment and all the necessary infrastructures. This would require investments, which are far out of reach of poor countries and the resulting water would be very expensive.
On the other hand, solar energy is abundant in arid regions and it can be used in the recovery process of water. Solar energy is available at no cost on large surfaces of the world. In order to keep water delivery structure costs and process costs as low as possible, the separation unit must be simple, made from low cost and easily available material and must be usable for long time. The water must be of high quality and of the lowest cost.
The Sanakvo process uses innovative sandwich multilayer structures with two compartments separated by a membrane as is schematically shown on figure 1.
Fig. 1: Multilayer sandwich structure of the Sanakvo process
Water containing glycerol solution flows through the upper compartment, which is heated by the sun. The solution gets hot and the vapor pressure of water increases. Water molecules pass through a membrane, which forms the opposite side of the hot, glycerol-containing compartment. The membrane is porous and highly hydrophobic. Therefore, only water vapor but no liquids can pass through it. This selectivity of this membrane is essential for the Sanakvo process. After passing through the membrane, the water vapor condenses on the surface, which is cooled by wind or by ambient air. Pure condensed water flows out from the second compartment into a storage vessel.
Concentrated glycerol solution is reused in the first step in a new cycle of absorption of humidity from air.
For increased efficiency, the heated surface is covered by one or two isolating layers, which allow the solar radiation to pass through without absorption, but prevent heat losses to the surrounding air.
For mechanical stability, the structure is put into a suitable frame of low cost material.
The whole Sanakvo water from air system is schematically presented in figure 2.
Fig. 2: Schematic representation of the Sanakvo process
Basic elements can have variable form and dimensions. In figure 3 one such easy to produce module can be seen.
Fig. 3: Water harvesting module
Many modules can be fixed on solid frame structures, where dozens or even hundreds of modules can be connected together. The water yield is correspondingly increased. Figure 4 shows a battery of 48 connected modules.
Fig. 4: Battery of 48 connected water modules
Productivity of the system
The productivity of the Sanakvo system is limited in its second step, only by the availability of energy needed for separation of water from the hydrated glycerin solution. The sun shines a certain number of hours per day. Based on theoretical calculations, one square meter of module under direct solar irradiation could produce 14.4 liters of water per day. In reality, this number will not be attained, because modules will not be optimally oriented towards the sun and clouds will decrease the heat supply to the module. According to local conditions, about 5 liters of clean water per square meter of module can be obtained in reality. The way to increased water yield is to increase the number of modules. This is more economical than the installation of mechanically complex sun following systems. In arid regions, the land is available at no cost.
The first step will not be limited in practice, because it can run 24 hours a day and the air contact surfaces can be made at very low cost.
Even with 5 liters of clean water produced per day this equals an annual precipitation of 1 825 mm of water per square meter. This corresponds to very rainy places in the world!
2011-02-13 P. L.