You have seen how Bear Grylls turns polluted water into drinking water with little more than plastic and sunlight. Based on this survival technique, academics have now added a third element - carbon-dipped paper - to create a highly efficient and inexpensive method to turn contaminated water and saltwater into potable water for personal use.
The idea could help address drinking water shortages worldwide, and especially in developing areas and territories affected by natural disasters. This is described in a study published online today (Jan. 30, 2017) in the Global Challenges journal.
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.
Qiaoqiang Gan, PhD, Associate Professor, University at Buffalo
Other members of the research team are from the University of Wisconsin-Madison, University at Buffalo's Department of Chemistry, Fudan University in China, and the lab of Gan, who is a member of UB’s New York State Center of Excellence in Materials Informatics and UB’s RENEW Institute, an interdisciplinary institute dedicated to solving complex environmental problems.
Solar Vapor Generator
In order to perform the study, the research team created a small-scale solar still. The device, known as a “solar vapor generator,” uses the heat converted from sunlight to clean or desalinate water. Here’s how the device works: The sun evaporates the surface water. During this procedure, bacteria, salt or other unwanted elements are left behind while the liquid moves into a gaseous state. Then, the water vapor cools and returns to a liquid form, where it is gathered in a separate container without the contaminants or salt.
People lacking adequate drinking water have employed solar stills for years, however, these devices are inefficient. For example, many devices lose valuable heat energy due to heating the bulk liquid during the evaporation process. Meanwhile, systems that require optical concentrators, such as mirrors and lenses, to concentrate the sunlight are costly.
Haomin Song, PhD Candidate, University at Buffalo
The UB-led team created a solar still almost the size of mini-refrigerator in order to address these issues. This solar still is composed of porous paper coated in carbon black and expanded polystyrene foam (a common plastic that functions as a thermal insulator and, if required, a flotation device). The paper absorbs water like a napkin, while the carbon black absorbs sunlight and converts the solar energy into heat used during evaporation.
The solar still coverts water very efficiently into vapor. For instance, during the evaporation process, only 12% of the available energy was lost. The research team believes that this rate is unprecedented. The achievement is made possible partly because the device converts only surface water that evaporated at 44 °C.
Efficient and Inexpensive
Depending on the test results, scientists believe that the still is capable of generating 3 to 10 L of water per day, which is an improvement over most commercial solar stills of a similar size that generate 1 to 5 L per day.
Materials used for the new solar still cost approximately $1.60/m2 - a number that could fall if the materials were to be purchased in bulk. (By contrast, devices that use optical concentrators can retail for over $200/m2.) According to the World Economic Forum, if the device is commercialized, the retail price of the device could ultimately minimize a huge projected funding gap - $26 trillion worldwide between 2010 and 2030 - required for water infrastructure upgrades.
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.
Zhejun Liu, PhD Candidate, Fudan University
The research was funded, in part, by the U.S. National Science Foundation, the National Science Foundation of China and the Chinese Scholarship Council.