On survival reality TV shows, people like Bear Grylls often use sunlight and plastic to turn filthy water into drinking water.
Academics have now added carbon-dipped paper as a third element and this solution may well turn this survival tactic into an inexpensive, but highly efficient way to turn contaminated water and salt water into potable water for personal use. The idea was described in a study published online in the journal Global Challenges on Jan. 30, 2017 and it could help address global drinking water shortages, especially in regions that have been struck by natural disasters, and developing areas.
Qiaoqiang Gan, PhD, associate professor of electrical engineering in the University at Buffalo School of Engineering and Applied Sciences, was the lead researcher on this study. He notes that their solution uses low cost materials and the system that was created makes near maximum use of the solar energy during evaporation. The amount of heat loss during the process has been minimized at the same time.
The team built a solar still on a small scale for the research and dubbed it a “solar vapor generator”. It uses heat converted from sunlight to desalinate or clean water. The process is performed in three steps:
- The water is evaporated by the heat of the sun.
- Bacteria, salt, or other unwanted elements are left behind when the liquid turns to gas.
- When the water vapor cools and returns to a liquid state, it is collected in a different container without the contaminants or salt.
Haomin Song, PhD candidate at UB and one of the study’s leading co-authors notes that although people that don’t have enough drinking water have employed solar stills for years, the devices generally used are inefficient. Many devices lose a lot of heat energy when heating the bulk liquid for the evaporation stage of process. More elaborate systems that utilize optical concentrators (such as lenses and mirrors) to concentrate the sunlight, are however costly.
The solar still created by the research team led by UB to address these issues, is about the size of mini-refrigerator. It is manufactured from expanded polystyrene foam and porous paper coated in carbon black. Polystyrene foam is a common plastic that functions as a thermal insulator. It could also be used as a flotation device if required. The paper absorbs water, while the carbon black absorbs sunlight, transforming the solar energy into the heat that is required for evaporation to take place.
The solar still is very efficient in converting water to vapor. The research team believes that the 12 percent of the available energy that is lost during the evaporation process is unprecedented. This low rate of heat loss is partly made possible because the device only converts surface water, which evaporated at 44 °C.
Most commercial solar stills of similar size currently produce 1 to 5 liters of clean water per day. The researchers believe their new still will be able to produce 3 to 10 liters of water per day, which is a vast improvement.
Current systems that use optical concentrators can sell for more than $200 per square meter. In contrast, materials for the new solar still cost roughly $1.60 per square meter. This number could decrease even further if the materials were purchased in bulk. The device’s estimated retail price could ultimately reduce a huge projected funding gap if it is commercialized. The World Economic Forum estimates that $26 trillion will be required to upgrade water infrastructure worldwide between 2010 and 2030.
Zhejun Liu, a visiting scholar at UB, PhD candidate at Fudan University and one the study’s co-authors, is of the opinion that the solar still being developed by the team would be ideal for small communities. It would allow people to generate their own drinking water via solar panels on their house roof, much in the same way as they generate their own power.