Pillalamarri Srikrishnarka
Dielectica traverses through the literature on this topic – and summarizes as they appear.
Keywords: Reverse Osmosis, Membrane Desalination, Clean Water, Energy Harvesting
Chennai, India: With exponential growth in population and increased living standards, there is a rise in demand for energy and clean water. Desalination is one of the promising routes in converting saltwater to clean water, due to its huge abundance. Reverse osmosis (RO) is predominantly used in extracting pure water from sea or ocean water. Although RO is being widely used commercially, however, it consumes a lot of energy. Water wastage is also an issue in this technology. Apart from RO technology, several other desalination techniques have been come up in this context. Membrane distillation is a popular one among these techniques.
Membrane distillation, which works on utilizing low-grade thermal energy, is an alternate route in obtaining clean water. So, let’s understand how this works. There is a porous hydrophobic membrane, one side of the membrane is hot and the other side is cold. Due to this temperature difference, the water evaporates on the hot side and diffuses through the membrane. Finally, it is condensed at the cold side. Straub et al. utilized this technique for generating electricity also. Typically, the cold region of the membrane distillation process is pressurized. As the condensation increases, water volume rises which results in increased hydraulic pressure. This resulting pressurized flow was used to drive turbines for generating electricity, thereby converting the low-grade thermal energy to electricity and also obtaining clean water in this process. [1, 2] With the advent of nanotechnology, scientists are successful in generating ~ 1 V by water evaporation even at room temperature [3].
In an effort to obtain both clean water and also electricity, Huang et al., fabricated a simple device having a Teflon membrane and porous alumina powder-coated membrane. [4] The Teflon membrane is placed between the hot and cold feed and the hot vapors diffuse from the hot feed side to the cold region. The alumina-coated membrane offers huge resistance for the cold water to pass through and with increased condensation, the volume of cold water keeps on increasing. This results in increased hydraulic pressure on one side of the alumina membrane compared to the other side of the cold reservoir. This drives the water into the cold reservoir through the membrane. As the cold water is forced through the negatively charged micro-pores of the alumina membrane, the hydroxide ions present in water are repelled at the entrance due to electrostatic repulsions. The remaining hydronium ions diffuse through the permeate channel resulting in a streaming potential. This generated potential from each side of the alumina membrane is extracted using silver mesh electrodes.
Using this device, researchers were successful in extracting 13.4 kg m-2 h-1 of flux and 147 micoW m-2 of power density capable of lighting up a LED. Therefore, such kind of device is aiming to provide a solution to two major global problems, clean water, and energy.
References:
[1] A. P. Straub et al. Nat Energy 2016, 1, 16090.
[2] A. P. Straub et al, Environ. Sci. Technol. 2017, 51, 21, 12925–12937.
[3] G. Xue, Nature Nanotech 2017, 12, 317–32.
[4] L. Huang et al. J. Mater. Chem. A, 2021, 9, 27709-27717.