Introduction: Multipurpose Solar Desalination Plant
The most important need of the day in the present circumstances is potable water. Water resources are getting polluted, developing contries are facing growing shortage of fresh water sources, and arid lands are becoming drier. The solution? Water, water everywhere, not a drop to drink. Yes, the sea water. So whats new? Desalination plants have been around for a long time. Mostly in the middle east, where the biproduct of fuel extraction and refining like natural gas is used to do the evaporation of sea water. How about places where you have to exclusively burn fuel for this process? Non green, polluting, expensive fuel. So again look for the most abundant source of energy on earth, the sun. Solar power? Link both resources together and you get a near perennial source of potable water with almost zero energy consumption(Almost? Read on.)
Step 1: Design 1 - Solar Collector
Let us look into the design now. Flat plate collectors are very common and cheap, probably the most widely used for heating or pre-heating water. But not enough to vaporize water as our requirement is. We need a concentrator for that. The parabolic trough reflector is probably the best suited for the task, its geomety ensuring focal concentration of solar radiation. Copper collectors running through the focal point transfer the heat to the brine chamber through a heat transferring medium like Thermic Fluid.
Step 2: Design 2 - Brine Chamber
The brine chamber is a cylindrical, insulated, sealed chamber inside which sea water is stored. As the heat from the thermic fluid gets transfered to the brine inside, the water temperature rises. Parabolic reflectors can transfer enough energy to vaporize a certain volume of water, but if the volume of water in the chamber is high, or the solar input to the collector is low or the ambient temperature is low, the temperature may not rise to 100 degrees. Of course it also has to supply the latent heat of vaporization of water to actually vaporize the water. So now we come to a realistic assumption. It may not be possible to just use solar energy to vaporize water. For heating purposes, it is more than sufficient, but for desalination, we definitely need the water to vaporize. Solution? Flash vaporization. Use a vacuum pump and reduce the pressure inside the chamber. As the chamber pressure reduces, liquid begins to vaporize at a lower temperature. Of course a vacuum pump consumes electric power. That is where thealmost zero energy consumption phrase comes into focus. Think of the pump as suppying the extra energy required to vaporize the liquid. It goes without saying that this power would only be a fraction of what it would cost in a conventional desalination process.
Step 3: Design 3 - Condensor
As the vacuum pump operates, the water vapour generated gets sucked by the pump and is pushed into a condenser. This is where a person can get creative. We have got water vapour and along with it, a lot of thermal energy in the form of heat and latent heat. As the vapour travels through the condensor, it expands and cools eventually coming to liquid form. It also transfers a lot of heat energy to the condenser. Of course, key to effective design is energy efficiency. So this energy ought to be utilized for something. The sea water input to the chamber can be pre-heated by circulating the sea water through the condenser before piping it to the brine chamber. This ensures that
a. The heat energy stored in the vapour as it becomes liquid is not wasted and transfered to the sea water.
b. The amount of energy required to heat the sea water is reduced at the brine chamber as it is already preheated to a certain amount.
Step 4: Design 4 - Float Valve
A float valve controls fresh flow of the sea water into the chamber and has two purposes
1. It regulates inflow of sea water and maintains a constant volume in the chamber.
2. If the sea water input is reduced or shut off for some reason, the float value goes down as the liquid level reduces. This can be sensed and the solar collector covered or an alarm sounded. If the copper collectors are unable to transfer heat to a medium, they would overheat.
Step 5: Useful Byproducts
At the brine chamber as more and more water gets vaporized, the concentration of the brine solution increases ie. the salt content in the chamber increases. As more and more water is desalinated, the liquid in the chamber soon becomes saturated brine. This can be tapped off periodically. What use is saturated brine?
1. It is used to preserve vegetables, fish, meat etc.
2. Brine is also a common fluid used to transfer heat in large refrigerator installations.
3. It can be used to manufacture common salt (who can live without it?). Just wait till the brine becomes completely saturated. Since the solution cannot dissolve more salt, it starts depositing at the bottom as a residue. Just tap it out.
4. Brine solution is used as the electrolyte in manufacture of Chlorine, Sodium Hydroxide, Hydrogen by electrolysis. Potassium and calcium compounds can also be obtained.
Step 6: Enhancements
Since we have so conveniently generated steam(it is low pressure steam containing a lot of water), it can be used for steam cooking as well. A portion of the steam can be tapped and fed to a steamer of some kind, or microwaves(some microwaves allow external steam inside). And how easy is it to adapt the design for this purpose? A single tap from the collector, thats all.
Step 7: Insulate Properly
I save the most important for the last, the one thing that most people misunderstand or deliberately ignore. The NEED for EXCELLENT insulation of the entire apparatus. The insulation MUST be extremely efficient. And must cover not only the solar trough and the brine chamber, but EVERYTHING including the collector tubes, condensers and all piping involved. Even a square inch of uninsulated pipe could waste a considerable amount of energy. The idea is to trap all the solar energy and keep it in the system.
Step 8: Conclusion
Conclusion. A very simple idea but one that is cheap, simple, effective, green, energy efficient and with 100% byproduct utilisation. Try it and feel the difference you make to the world. Unlimited potable water as a main product and brine, salt and steam as byproducts each of which has many uses. It cannot get better than this.