Clean water off the grid
A new desalination process could be used to provide clean water in remote areas, as Ellis Davies reports.
Distillation is the oldest method for making freshwater from salt water. Used for centuries, the process requires large amounts of energy, in the form of heat, to produce steam, accounting for more than half the operating cost of a water distillation plant. Scientists at Rice University’s Centre for Nanotechnology Enabled Water Treatment (NEWT), USA, have created an off-grid solution to confront this issue by using carbon black nanoparticles to convert sunlight into heat. The technology can be used on a small scale, in applications for personal and community use in areas without clean water.
NEWT’s technology, nanophotonics-enabled solar membrane distillation (NESMD), uses a combination of membrane distillation technology and light-harvesting nanophotonics. Qilin Li, corresponding author on the study, told Materials World, ‘Membrane distillation uses a porous, hydrophobic membrane to separate the [hot] source water from [cold] clean water. The pore space in the membrane is filled with air. Therefore, water evaporates at the interface between the source water and the membrane. When a temperature difference exists between the two sides, water vapour is transported from the feed (source water) side of the membrane to the clean water side, and condenses to form pure water.’
The source water is heated using carbon black nanoparticles embedded into the membrane that can harvest up to 80% of sunlight, trapping the incident photons within a thin layer to produce high heating intensity. Other methods of solar desalination exist, most using a solar panel to convert sunlight to electricity to power the desalination system. This method is at the mercy of fluctuating sunlight intensity, unlike the NESMD, which is able to function at lower temperatures. ‘The process is powered by solar thermal conversion instead of solar photovoltaic conversion, which has much higher efficiency, especially at relatively low temperatures,’ Li explained. The nanoparticles convert the photon energy to heat locally at the membrane surface with high efficiency, creating an elevated temperature on the feed side of the membrane, which drives the distillation process.
As NESMD is self-contained and powered, it is a good candidate for small-scale water purification. Li said that the best uses of the technology would be in locations without access to a stable grid supply, as a desalination plant concentrate treatment and for processing industrial wastewater with high salinity.
The system is also cost-effective, as Li explained. ‘The whole system is powered by solar energy at very high utilisation efficiency (light to heat conversion at more than 75% efficiency). All parts can be made with low-cost plastics – no need for high-grade stainless steel or expensive alloys, which are used in seawater reverse osmosis systems.’ The process also requires extremely low pumping energy as it is not driven by pressure and does not need the high water recirculation rate of conventional membrane distillation processes.
In a proof-of-concept study, the researchers produced three postal stamp-sized NESMDs of just a few millimetres thick. The team has also manufactured a much larger system containing a 70 x 25cm panel. Bench scale tests were carried out, showing that water production rate increased greatly in concentrated sunlight, reaching an intensity of 17.5KW per metre squared from 0.7KW unconcentrated. At this intensity, water production is 6l per metre squared per hour.
NEWT is expected to receive more than US$40 million in federal and industrial support in the next decade. Before NESMD reaches commercialisation, further R&D is required to optimise the reactor design and system configuration. ‘We also need to perform long-term tests to evaluate the membrane and system performance stability. I expect 12–18 months before a demonstration unit can be field-tested,’ said Li.