A solar still about the size of a mini-refrigerator, made from polystyrene foam and porous paper coated in carbon, could provide inexpensive, efficient access to clean water in developing regions and regions affected by natural disasters. This is a floating solar still prototype that researchers used for some of experiments. Courtesy of University at Buffalo. A research team at the University at Buffalo has built a device called a “solar vapor generator,” which cleans and/or desalinates water using heat converted from sunlight. As the sun evaporates the water, the liquid transitions to a gaseous state, leaving bacteria, salt and other elements behind. When the water vapor cools it returns to a liquid state, where it is collected in a separate container, minus contaminants. "People lacking adequate drinking water have employed solar stills for years, however, these devices are inefficient," said researcher Haomin Song. "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." The researchers addressed these issues by using low-cost materials that work together efficiently. The polystyrene plastic provides excellent thermal insulation. The porous paper absorbs water, while the carbon black used to coat the paper absorbs sunlight and transforms it into heat used during evaporation. "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," said researcher Qiaoqiang Gan. Based on test results, the researchers believe the still is capable of producing three to 10 liters of water per day, which would allow it to generate 2.4 times more freshwater than most commercial solar stills of similar size. Materials for the solar still cost roughly $1.60 per square meter — a number that could decline if the materials were purchased in bulk. In contrast, systems that use optical concentrators can retail for more than $200 per square meter. If commercialized, the device's retail price could ultimately reduce a huge projected funding gap — $26 trillion worldwide between 2010 and 2030, according to the World Economic Forum — needed for water infrastructure upgrades. From the top left corner, moving clockwise, the four images depict: University at Buffalo students performing an experiment, clean drinking water, water evaporating, and black carbon wrapped around plastic in water with evaporated vapor on top evaporated water. Courtesy of University at Buffalo. By simultaneously addressing both the need for high-efficiency operation and low production cost, the system could provide a way to purify water that would be particularly suitable for individuals in undeveloped regions with limited or no access to electricity. "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," said researcher Zhejun Liu. The research was published in Global Challenges (doi: 10.1002/gch2.201600003).