THE NEXT GENERATION OF SELF
Our secondary solution beyond basic reverse osmosis filtration. By vaporizing instead of pressurizing water across a membrane, we are able to allow for:
IN DEVELOPMENT ARE TWO SELF-SUFFICIENT NANO-POWERED WATER PURIFICATION SYSTEMS:
Membrane distillation, which is a scalable water treatment technology, is based on water distillation, whereby water evaporates and condenses across a porous, vapor-filled membrane. Because water is evaporating, nearly all salt and other non-volatile compounds are removed, providing better salt removal than RO. In this system, the difference in vapor pressure between hot feed and cold distillate water drives water vapor flux, and because water purification is driven by a temperature difference, water does not need to boil to be purified. Standard membrane distillation (MD) is often run at temperatures between 50 and 80 °C (compared to 100 °C to boil water at sea level). An increasing number of studies on MD in recent years as an alternate to RO for desalinating seawater, and many pilot-scale systems use solar energy to heat water for MD.
Current solar MD systems use a solar thermal collector to heat the feed water. These systems have several inherent limitations. Firstly, heating of thefeed water occurs outside of the MD unit; the heat loss significantly reduces the solar thermal efficiency. Secondly, the inherent thermal efficiency of MD is low due to temperature polarization (TP), where the temperature of the hot water at the membrane surface (T1) is lower than the temperature of the bulk hot water Tf. Similarly, T2 at the membrane surface on the permeate side is higher than the temperature of the bulk permeate water Tp. The effect of TP, as measured by the TP coefficient, αTP, can be as low as 0.3, i.e., 70% reduction in effective driving force and a similar reduction in treated water flow rate. Thirdly, the heat transfer across the membrane reduces Tf and T1, diminishing temperature difference between feed and permeate, also reducing the treated water flow rate. This limits the length of the membrane module and pure water production rate, posing a major challenge for system up-scaling. Finally, brine discharge represents an important source of energy loss. Heat loss through brine discharge increases with decreasing water recovery. The single pass water recovery in solar MD is typically below 5% due to low flux, i.e., 95% of the heat in the feed water is lost. Brine recirculation and heat recovery from both permeate and brine can reduce heat loss.
The specialized MD technology developed by Rice University—direct solar membrane distillation (DMD) —provides a solution to the standard MD limitations, allowing for smaller Squad and Platoon-sized water systems by vastly reducing the energy consumption of the desalination component of the water purification systems. Through DMD, photothermal nanomaterials (NMs) incorporated on the MD membrane surface efficiently concentrate and harvest photons at the membrane surface, convert those photons to heat energy, and create a local high temperature T1 to maximize vapor flux and the solar thermal efficiency—the fraction of solar irradiation that is used to generate distillate.
To demonstrate the viability of this process, proof-of-concept experiments were carried out by directly coating carbon black (CB) nanoparticles on a commercial hydrophobic PVDF membrane. These composite PVFD-carbon black membranes were used to demonstrate flux enhancement upon solar illumination and nearly complete salt removal. Every coated membrane was compared to the commercial PVDF membrane to observe the improvement in flux upon irradiation with sunlight, and demonstrate the effectiveness of this technology.
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