Two new techniques for pure water with the help of sunlight, says S.Ananthanarayanan.

While global warming and the need for green energy are staring us in the face, so is growing scarcity of potable water in many parts of the world. Could the solar cell, a non-polluting source of electricity,be harnessed to evaporate and purify water? And then, a nature-inspired way to keep the evaporating surface clean?

Wenbin Wang, Yusuf Shi, Chenlin Zhang, Seunghyun Hong, Le Shi, Jian Chang, Renyuan Li, Yong Jin, Chisiang Ong, Sifei Zhuo and Peng Wang, from King Abdullah University of Science and Technology (KAUST), Saudi Arabia, write in the journal, Nature Communications, about a multistage, membrane distillation arrangement that works with Photovoltaic devices to produce fresh water even as the devices create electricity. And in the same month, the journal, Science Advances, carries the account by Ning Xu, Jinlei Li, Yang Wang, Chang Fang, Xiuqiang Li, Yuxi Wang, Lin Zhou, Bin Zhu, Zhen Wu, Shining Zhu, Jia Zhu, from Nanjing University, China, of an adaptation of the water lily, to help distillation plants stay clean of the residue that impure water leaves behind.

While electricity generation accounts for nearly half of all the water that the world uses, there are places where distillation is the only way to recover fresh water, either from seawater or from wastewater, and this consumes electricity. The King Abdullah University group has thus worked on getting the photovoltaic cell, while it generates electricity, to double up as a distillation plant.

The way the photocell works is that it converts the energy in sunlight, in a specific frequency band, to electricity. But, as the energy in sunlight is distributed over a wider range, a large part of the energy is not used by the photocell. As the picture of how the energy of sunlight is distributed would show, a portion, between the wavelengths of 600 and 1000 nm is useful, but sizeable energy at shorter and longer wavelength is wasted.

Given this wasted energy, as well as the efficiency of conversion, solar cells do not practically work at more than 15 % efficiency, most of them at 10 to 12%. But what is worse is that this wasted energy is not just lost, it warms up the photocell, which leads to a drop in the conversion efficiency. Research effort is hence directed both at converting the wasted parts of the spectrum to useful wavelengths as well as at finding ways of drawing away the heat.

The King Abdullah University group does one better – it makes use of the heat that the photocell radiates to distil seawater. Not that using sunlight to get fresh water from seawater is something new – but exisiting processes waste much of the heat that comes from the sun and cannot do better than half a litre, from a square meter, over a whole day. The KAUST group has found a way to get a lot more fresh water from the heat that the photocell gives off, without affecting the efficiency of the photocell itself.

The approach of making use of 'waste heat' is also not new. In typical electricity generators, turbines are driven by steam at a high temperature. The steam cools down when it loses energy to the turbines, but is still pretty hot when it is released. Many industries that need steam for their process now tap this waste steam from the power facility. The hot exhaust gas from engines in locomotives and machinery has also been put to use, with benefit.

The arrangement of the KAUST team is to make use of the comparatively modest warmth of the photocell, some 60°C, to generate vapour, in three stages. In conventional solar distillation, an absorber generates vapour, at its own temperature, but the heat in the vapour is lost when the vapour is drawn off and condensed. In the KAUST arrangement, the heat in the first lot of vapour is captured, to create a second lot of vapour, before it is drawn away. In the same way, there is again a third lot of vapour, till the temperature is down to less than 40°C, leading to a lot more fresh water from the same heat used up. As the picture shows, the vapour arises from water in a wet-friendly membrane, which is in touch with the warm surface. The next membrane, which repels water, draws away the vapour and passes it to a metal condensation surface, which also absorbs the heat. A three-stage Membrane Device, the KAUST paper says, can generate as much as 2.69 litres of fresh water, for every square metre, every hour.

Fouling the surface

This process of evaporating salty or wastewater, to tap the uncontaminated vapour, would naturally leave behind the solute or other residue. This material blocks passages of water and vapour, or light from reaching the absorber, which fouls the evaporation medium. Fouling has been the bane of solar evaporation plants, which have been widely in use long before the current work of the KAUST team.

The surface of the water lily, as shown in the picture, consists of an outer layer that absorbs heat, and has pores that allow water vapour to pass. As water does not stick to this surface, water efficiently washes away any solids and the surface stays clean. And then, the lily stays afloat, with passages for water from below to rise to the surface and evenly spread out.

The device the Nanjing group has made has an upper, water-repelling surface, with pores to permit the passage of vapour, mounted on a bottom surface, which is in contact with briny water. The water does not rise to the upper surface, but is confined to a narrow space between the two surfaces. There is hence no fouling of the upper surface, which stays efficient for absorbing and passing heat directly to the space below. While the concentrated water in the narrow space continuously evaporates, the solute is washed downwards, through narrow channels.

In the usual methods of separating water from the solute the process slows down as the solution gets concentrated. In this new device, this does not happen even with high concentration, as the surfaces for absorbing heat and evaporation stay clean, like the water lily!

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