The current level of population of the world, exacebated with the rate at which the population is increasing, is bringing enormous pressure to bear on the natural resources of the earth. Nothing new here, numerous people before this have said it umpteen times. The clean, safe drinking water is just one such natural resource that is becoming scarcer by the day. The difference is that unlike other resources, water is aplenty on earth. The only problem is that 95% of the water found on earth is not fit for human consumption. All animals (and vegetation) have always relied on natural forces to make some part of this water clean and safe. Those sylvan forests with gurgling streams flowing endlessly are not much to be seen these days. How about taking this process in our hands and producing clean water to meet our needs? Well, it can be done, and the technology to do so is not at cutting edge. This technology has been around for many years. It is just that water scarcity never got the attention of those who could decide on such matters so this technology never was refined and made more economical. As a result, what we have today is an expensive, energy-consuming Reverse Osmosis technology, or slow and inefficient solar distillation process.
Nevertheless, these methods can be deployed at a micro level to provide reasonably clean potable water to village populations, especially those close to sea, so that the communities can be self sufficient in their water needs. Local governments should provide support in terms of necessary capital so that simple equipment can be installed and operated by village community level users to produce their own water needs.
The basic principle in this scheme is the same that is found in nature: the water of oceans evaporates, the vapor gets transported elsewhere, and then it condenses and precipitates in the form of rain or snow. The process that occurs in nature is uncontrollable in terms of its intensity and its direction. We cannot say how much evaporation will happen and we cannot predict where and when there will be rainfall.
By duplicating this process in a controlled fashion, we can direct it so that water that condenses from vapor occurs at a place of our choosing, at a time of our choosing and in a quantity we want. Desalination of sea water is a surefire way of obtaining clean potable water much in the manner in which nature gives it to us.
Basic Process: Here is how the natural process of rain making can be replicated. The seawater can be poured in a battery of troughs with a concrete base and glass top. They will be exposed to the solar radiation. Due to the heat of the radiation, water will vaporize. This vapor can be collected through pipes and transported to a tank overhead. Due to the process of climbing height, the vapor would lose heat and condense inside the tank and form into water droplets.
Water evaporation takes place in a closed vessel with glass top. At the top end, an outlet pipe will carry the vapor to the condenser. The process can be done with just one evaporating vessel, but in order to increase the yield, sometimes a battery still of several such vessels is used. The outlet pipes from each such vessel are accumulated and finally lead to the condensing chamber.
Saline water is poured into individual vessels at a slow rate. The glass top functions as a green house, allowing the sun rays to enter and heat the water inside. The vapor thus produced goes up through the outlet pipe and into the condensing chamber. If the vapor temperature is sufficiently high, the vapor can travel farther and higher. This fact alone can be used to transport the vapor to a storage tank at a high level.
At the overhead tank, a heat sink is used to convey heat from inside the closed chamber of the tank to outside where the heat sink is exposed to air. To ensure that the sink does not absorb heat from outside, it is covered by a roof so that it is not exposed to direct sunlight. The sides are open to allow free passage of air, thereby taking the heat away from the closed chamber, where the vapor condenses and forms droplets on the inner absorber of heat, and finally fall down into the tank.
Solar energy can be used to heat up the vapor in the heating vessels so that it can climb higher. This process by itself converts solar energy and stores it as kinetic energy in the water at a higher level.
By using a simple heat sink, in this entire process no electrical energy is used, either to run fans or to run any compressor. the convection of vapor and its condensation into water in the overhead tank can go on at a slow pace. The pace would be dependent of the amount of solar energy incident on the vessels and the difference of temperature inside the tank and the outside ambient temperature. The speed of wind also helps. However, to expedite the process and increase the throughput, one can add a compressor so that condensation can be made faster.
Now this is a passive method, meaning that no human intervention, either manual or automatic, is required in this process. The amount of water thus produced will depend on the amount of solar radiation available and the ambient temperature.
The rate of production of water can be increased by active intervention, using some energy input.
(1) The seawater can be stored in airtight stills. There would be a valve at the top and a pipe leading out of it to an overhead tank. By using a pump, the pressure inside the still can be reduced slightly. In a reduced pressure condition, at whatever temperature, the rate of water vaporization increases. This vapor can be sucked out from the valve by using the pump and transported to the overhead tank. Since the pressure in the cooling tank is atmospheric (or momentarily even slightly higher than atmospheric due to pumping effect), the vapor will condense and form into droplets. The pump also provides the energy to transport the vapor to the tank, perhaps at a greater height than in the passive method, thus increasing the rate of condensation.
(2) The condensation inside the cooling tank can be expedited by using a compressor. Readers may recall that in a window air conditioner, as the compressor cools the air inside the room, the moisture in the room condenses on the cooling cicuit and flows in the form of water. This water then needs to be channeled out by using drains and pipes so that the water may not form a pool on the floor of the room! Well, the method to be used here is absolutely the same, with a difference in the objective. In a room air-conditioner, the main objective - and main product - is cold air, while water is a useless byproduct that is thrown out. In our water production plant, the main product is water, while cold, dry air is a byproduct.
It is, of course, possible to use any combination of these two methods in a device suitably designed, which will enhance the rate of production of water. Let us examine the combination method in detail.
In this method, humid air is captured and pumped into a closed chamber. Using the normal process of air conditioning, the temperature in the chamber is brought down by about 5 deg. Celsius below ambient temperature. The moisture in the air, which is generally saturated at ambient, will then condense. Energy is used in two stages here. One is to pump atmospheric air into the chamber at a pressure slightly higher than atmospheric. Higher atmospheric pressure produces heat inside the chamber, thus the overall temperature of the air inside the chamber tends to increase a little. However, in the second stage, a compressor is used to cool down the air by sucking the heat and throwing it out, just like a room air-conditioner. Thus, as soon as the air inside the chamber cools down and loses its humidity, immediately more air is pumped in that contains higher vapor at a higher pressure. That increases the temperature again. This cycle of more air being sucked in, temperature rising, condensation, temperature reduction, and finally more air being sucked in continues, yielding condensed water.
The energy requirement in this process comes at the stage of compressor and air pump. This process can also be used at any place where the moisture content in the ambient air is sufficiently high, say above 60% relative to the ambient temperature. That will eliminate the need for battery stills, but reduces the amount of moisture available for cooling.
The greatest human population densities are found in tropical regions. World's largest deserts are also found in these regions. So also the incidence of solar energy. It does not take great thinking to conclude that the mideries of a great many people can be alleviated by using solar energy to produce clean water. This in itself is an economically worthwhile objective, justifying large investments and expense on energy.
However, I recommend an even better use. Since we are talking of replicating nature, we might as well go a step further. we use the water to re-charge the underground water tables and regrow the forests that have been decimated. In many Asian countries, pressure of population has greatly reduced the forest cover. Even what cover is left is thin and of poor quality. Rainfalls have been erratic, rivers have been dammed, and ground acquifers have been exhausted by ever thirsty humans. The balance of species is disturbed. By providing water to the forests and allowing them to re-grow, we might achieve several objectives by resorting to this program. Use of available solar energy is an imperative, not a luxury anymore.