The search for desalination systems is essential given that drought periods are destined to increase. Here are the improvements made
Although the floods that have hit our country in recent days may make us forget, the drought in recent years has been increasingly intense, both in Italy and elsewhere. Droughts are generally destined to increase, and with them the demand for fresh water, both for human use and for agriculture; the search for efficient desalination systems, therefore, is more than ever essential. Thermal desalination systems work by heating salt water and then condensing the resulting steam to make fresh water. The energy required comes from the usual sources, or directly from solar heating of tanks from which the salt water evaporates, to then condense the vapor on a transparent roof. However, during the condensation, a thin layer of water forms on the roof which reduces the amount of solar energy that can enter the tank, and so the efficiency of the system collapses, unless continued and demanding maintenance is carried out. To solve this problem and improve the efficiency of solar-powered desalination plants, a group of Indian researchers has developed a new system, later publishing their results in a specialized scientific journal.
The new system comprises a brine tank, evaporator and condenser enclosed within an insulating chamber to prevent heat loss into the ambient air. Evaporation takes place using solar thermal energy which heats a small volume of water absorbed in special aluminum panels with a rough surface, on which the salt water rises from the tank forming a thin layer. Liquid absorption in the evaporator takes advantage of the capillary effect of microstructured materials, which allows liquids to be drawn into the confined spaces of a porous material, much like water is absorbed by a sponge. Using this approach, a significant increase in overall efficiency is obtained, because it is not necessary to bring the entire volume of liquid in the tank, which has greater thermal inertia, to temperature. The grooves and the porosity of the evaporator structure have been optimised, seeking a design that maximizes efficiency, i.e. one that makes it possible to obtain a thin film of water and make it evaporate as quickly as possible.
In addition to the evaporator elements, the research team also came up with a major improvement in the condensers. To prevent the formation of a thin film of water during condensation, as in traditional solar desalination plants, the researchers fabricated a condenser with highly hydrophilic surfaces alternating with less hydrophilic surfaces. To illustrate, a waxy surface is not very hydrophilic, and water tends to flow on it, while a paper surface is very hydrophilic, and water absorbs well. In the condenser designed by the researchers, water droplets that condense on the less hydrophilic regions naturally flow to the more hydrophilic regions. This allows less hydrophilic surfaces to continuously shed condensation, thus continuously making new surface available for further condensation to form, without losing efficiency as in the condensers of solar desalination plants used today.
In the end, the researchers worked on improving one more parameter of their experimental setup, or on the thermal dispersion due to condensation. During condensation, part of the heat absorbed by the water vapor is lost to the atmosphere. The researchers designed the system so that this heat released during condensation is also trapped and used to heat imbibed brine in an evaporator, which reduces the amount of solar energy needed and increases overall efficiency. At this point, the research team connected multiple evaporator-condenser pairs in series, resulting in a multi-stage solar desalination system. This system, for each square meter occupied, has shown the ability to produce a liter of drinking water every 30 minutes, at least double that produced by a traditional solar still of the same size under the same lighting conditions. Naturally, in addition to sea water, the system can also work with groundwater containing dissolved salts, with brackish water or with non-drinking water, and can be oriented as best as possible to receive the maximum possible radiation as the day progresses. The prototype has demonstrated, once again, that intelligent and low-cost solutions can be produced from research for some of the most pressing problems we face; of course, however, it is one thing to demonstrate the functioning of good ideas, quite another to get them industrialized and scaled up, so that we can go from an excellent intuition to a real practical solution. For the latter process, politics, economics and finance are needed; which recently, and all too often, when science speaks, they don't listen and look elsewhere.