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Access to clean drinking water is seen by many as a basic human right, but the situation is very different in many areas of the world. A new laser-based aluminum sanitation system could help deliver clean water and save lives in developing countries.
In many developing countries there is a growing shortage of clean, drinkable water. Could the solution to this crisis be solved by the energy delivered by the Sun, and laser crafted solar energy-absorbing surfaces?
A team of researchers, led by Subhash Singh from The Institute of Optics, University of Rochester, New York, believes that solar-based water sanitation - an environmentally friendly process for obtaining clean water - could be key to delivering sustainable clean water to a burgeoning world population. The results are published in a paper featured in the journal Nature Sustainability.
Using Solar Energy to Disinfect Contaminated Water
Solar disinfection is not necessarily a new modality for delivering clean water. The World Health Organisation (WHO) has listed solar disinfection in clear bottles by UV light and thermal disinfection in opaque containers for some time as methods of cleaning water contaminated with biological agents. There are multiple systems on the market that can use sunlight to decontaminate water.
But these systems have significant drawbacks; primarily scalability. A system that could sanitize water for a small group of people, or a family, may not be much use in sanitizing water for an entire town or village.
This, and the relative expense of water disinfecting systems, leaves 1 in 9 people without access to clean water — around 785 million people. Of these, 1 million people die per year from a lack of sanitized water. Beyond the concern for human life, the quest for clean drinking water has a significant effect on the global economy.
Water.org estimates that the need to seek and gather clean water alone costs the global economy $260 billion per year. And this constant battle affects women and girls most significantly, with them estimated to spend 266 million hours per year finding locations of clean water. Thus, as well as its health effects, the global water crisis is significantly holding women back, keeping them from education and opportunities outside of traditional roles.
The team of researchers identified several issues with current water sanitation systems, particularly with solar-driven interfacial evaporation. These include, but are not limited to; inability to control interfacial evaporators for solar tracking limits, meaning that when the angle of the Sun changes, optical concentration drops, and the ability to sanitize is reduced. An additional issue is clogging effects that severely reduce device efficiency.
As current solar-driven sterilization systems use a ‘bottom-up’ heating system, much of the generated energy can be lost through evaporation where the water meets the air. Some systems have tackled this efficiency problem by introducing the solar-thermal heat generating system to the location of the air/water interface. The difficulty here is that as this surface floats atop the water it can’t be positioned to face the incident light, and through contact with wick water, the clogging issue is also exacerbated.
Also, there is another very significant problem; the current generation of solar purifiers can only eliminate biological agents. This leaves a significant amount of other contaminants, particularly heavy metals, untreated.
Enter Lasers and Aluminium
The team of scientists suggests a super-wicking and super-light-absorbing (SWSA) surface for efficient solar-based water sanitation. This SWSA surface is composed of aluminum, which is readily available and low-cost.
However, traditional untreated aluminum doesn’t have the porous nature required for wicking. Additionally, its surface is reflective, thus not lending itself light-absorption. To combat this the team laser-treated the aluminum, blasting it with femtosecond-timed laser pulses.
The wicking surface of the material created by this laser-treatment allows water to run against the influence of gravity at a steady speed of around 2 mm per second. This means that water can be delivered to the solar absorbing surface as it is positioned at any angle.
Thus, the surface can be mounted on a floating platform at any angle — increasing the solar-ray incidence rate. This also means that the SWSA can be attached to a solar tracking system that can follow the Sun’s motion through the sky and maintain sanitation at optimal levels.
The researchers also focused on the shape of the surface and how it dispersed heat through the bulk water. They found that u-shaped surfaces that minimized contact with the surface of the water led to the most energy-efficient models. Thus far, the team has recorded a measured evaporation rate exceeding ideal devices, even when they are running at 100% capacity.
Perhaps the most significant element of the team’s research was the testing of the system on water not just contaminated with biological agents, but also with other pollutants such as heavy metals, light metals, and domestic and agricultural waste.
They found that samples taken from a nearby pond had the density of this material reduced by 4–5 orders of magnitude. This brought this previously contaminated water to well-within WHO standards for safe drinking water.
The Global Water Crisis: A Problem That Can't Wait
Through its simplicity, durability, reusability, efficiency, and compatibility with solar-thermal technology, the team’s device clearly demonstrates the potential for use as a partial solution to the global water crisis. But there is still work to be done.
The next step for this particular team is the testing of large-scale SWSA sheets vital for generating sufficient amounts of clean water, a scaling-up that is vital if it is to help provide clean water for poorer regions of the world.
In their paper, the team points out in particular the development of double-sided solar panels and tracking systems as a technology with which their SWSA surface model could be combined for maximum efficiency.
Sources and Further Reading
Singh. S.C, Elkabbash. M. E, Xiaohan. Z. L, et al, [2020], ‘Solar-trackable super-wicking black metal panel for photothermal water sanitation,’ Nature Sustainability, [https://doi.org/10.1038/s41893-020-0566-x].
Solar Water Purification, CTCN: Climate Technology Centre & Network, [https://www.ctc-n.org/technologies/solar-water-purification].
The Water Crisis, Water.org, [https://water.org/our-impact/water-crisis/].
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