It is possible to purify unpotable water with sunlight and a filter paper dipped in carbon, according to researchers in China and the USA. The concept could be developed to make foul water drinkable for millions of people in drought-stricken areas or in the aftermath of natural disasters when the regular water supply is contaminated or disrupted. Gan et al., Global Challenges (2017); DOI: 10.1002/gch2.201600003]
"Using extremely low-cost materials, we have been able to create a system that makes near maximum use of the solar energy during evaporation. At the same time, we are minimizing the amount of heat loss during this process," explains lead researcher Qiaoqiang Gan, PhD of Buffalo School of Engineering and Applied Sciences in Buffalo, New York. Gan worked with colleagues at Buffalo and researchers at the University of Wisconsin-Madison, USA, and Fudan University, China.
The team's solar still, which they refer to as a "solar vapor generator" can clean or desalinate water by the simple process of sunlight heating the water, leading to evaporation and the condensation of that vapor in a cold trap. During this process, salt, bacteria and other unwanted particles are left behind. Such solar stills have been used for many years but they are inefficient, team member Haomin Song explains. Mirrors and lenses can be used to concentrate the energy from sunlight to boost the process but they are fragile and costly and not always available.
The team has redesigned the solar still to employ expanded polystyrene foam as a thermal insulator and added a new component hydrophilic, porous paper coated in carbon black. The paper absorbs water and it being impregnated with carbon black means it absorbs more energy from sunlight for evaporation. Overall heat loss is about 12 percent, making this new solar still design a very efficient example, unprecedented in fact, the team says. Critically, the device is tapping off only surface water from the supply, which evaporates at 44 degrees Celsius, rather than attempting to heat the bulk liquid water. They reckon they can produce up to 10 liters of potable water per day, even the best commercial solar stills of the same size only manage between 1 and 5 liters during daylight hours.
"The shortage of freshwater and sanitation is one of the most pervasive challenges afflicting people throughout the world," the team writes. Moreover, it is predicted that by 2025, over half the nations in the world will face freshwater stress and by 2050 almost three-quarters of the world's population could face water scarcity.
Such a simple, inexpensive system if commercialized precludes the need for new water supply infrastructure that might cost billions if not trillions of dollars. "The solar still we are developing would be ideal for small communities, allowing people to generate their own drinking water much like they generate their own power via solar panels on their house roof," says Fudan's Zhejun Liu, who is a visiting scholar at Buffalo.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".