Scientists from the Polytechnic University of Milan have proposed a new seawater desalination scheme that makes it possible to recover up to 95% of its freshwater content and obtain salts as a useful byproduct. To achieve this, the researchers used heat that is usually dissipated at next-generation solar power plants.
Freshwater scarcity is becoming an increasingly acute problem. Today, more than 3.5 billion people face water shortages for at least one month per year, and global desalination capacities continue to grow rapidly. However, the existing technologies are far from perfect. The most commonly used method, reverse osmosis, requires significant energy inputs and usually recovers just 40–50% of freshwater, with the rest being converted into concentrated brine that needs to be disposed of.
The authors of the study have proposed combining several desalination technologies into a single system and integrating it with a solar thermal power plant powered by supercritical carbon dioxide. While these power plants generate electricity with high efficiency, they waste large amounts of low-grade heat at temperatures ranging from about 50°C to 100°C. This energy is usually wasted, but the scientists have now found a useful application for it.
The first stage of this process uses the forward osmosis technology. A membrane separates seawater from a special solution made of the heat-sensitive polymer Polycerin 55GI-2601, which creates an osmotic pressure difference and forces the water to pass through the membrane without necessity. Using heat at temperatures ranging from 50°C to 75°C, the system extracts about 60% of the freshwater contained in the seawater. This results in a brine with a salinity of some 8.3%.
This concentrated brine is then fed to a vacuum membrane distillation unit. The unit utilizes the hotter portion of the waste heat ranging from 75°C to 100°C. At reduced pressure, the water evaporates and passes through ceramic membranes, while the salt concentration continues to rise. After this stage, the overall water recovery rate reaches about 85%, with the brine salinity going up to 20%.
During the final stage, the concentrated brine is sent to a crystallizer. Using the remaining low-grade heat, the unit brings the solution to a state close to saturation. Some of the salts precipitate and can be collected for further use, with a negligible amount of liquid waste remaining.
In order to evaluate the technology’s efficiency, the scientists simulated the operation of a facility based on a 100-MW solar power plant in Seville, Spain. Their calculations showed that such a facility can produce about 3.39 million cubic meters of freshwater annually, extracting over 32,000 tons of salts.
As a result, the overall water recovery rate reached 95%. This means that out of every 100 liters of seawater, only about 5 liters remained as highly concentrated brine. This way, the new system has made it possible to achieve near-zero liquid discharge.
Crucially, the system relied largely on heat that would have been dissipated into the environment anyway. The energy consumption of pumps and other auxiliary equipment turned out to be very low, totaling less than 0.5% of the plant’s annual output.
The scientists estimate that, even with a relatively low solar energy utilization rate, the produced water could cost about €5.8 per cubic meter, which makes this system promising for regions experiencing freshwater shortages and possessing significant solar resources.
