Estimates suggest that more than 52% of world’s population could be living in water-stressed regions by 2050 and many of these regions are arid- or semi-arid, having solar energy abundance. Solar stills are highly suited for these regions or other places with no sustainable and affordable options for clean water. Basic solar stills are used in many small desalination and distillation plants at small scale and have limited use in modern agricultural, industrial and urban environments. Limited use of solar stills is due to their inadequacy in meeting the freshwater demand in many situations.
“The goal of this research is to explore and develop innovative solar collection and enhanced heat transfer approaches for realizing a scalable, low cost, low-energy solar still with rapid desalination capability, which could theoretically increase the freshwater productivity rate by as much as 15 times compared to a basic solar still,” said Sarada Kuravi, associate professor. “A solar still is a simple, impactful device for producing freshwater from wastewater and brine by complete or partial consumption of solar energy. The major drawback of solar stills is the low rate of water productivity which makes it unsuitable for large scale desalination.”
Kuravi leads the team comprising Associate Professor Krishna Kota and Professor Young-Ho Park, all from the Department of Mechanical and Aerospace Engineering, along with Professor Pei Xu and Associate Professor Huiyao Wang, both from the Department of Civil Engineering. Several graduate students are involved in the project, as well. The team also has an international collaborator, Associate Professor Sathyabhama Alangar from National Institute of Technology-Karnataka in India, for testing the viability of some of the project’s technologies in treating wastewater in rural India, where water scarcity is a serious problem.
The group is currently working on a U.S. Bureau of Reclamation (BOR) grant that began in 2020. Their research is building on findings from a previous project funded by the same agency which showed potential for technologies on a small scale.
“Our previous BOR funding has aided us to show the potential of our technologies individually, for synthetic salt water at a small scale in our laboratories. Through this project, we are building on our prior knowledge and starting to test these technologies at a larger scale and as a combination in a solar still that is being built by our graduate students. This effort will provide more practical information concerning the enhanced solar still performance,” said Kuravi.
Their investigation focuses on three technologies.
Centralized mirror technology uses an arrangement of mirrors that concentrate sunlight onto a larger central mirror that focuses highly concentrated solar energy onto a solar still and significantly increases the operating temperature and hence vapor generation as compared to solar stills that operate at low temperatures. This is the first time this technology has been used for water desalination— It enhances the freshwater production rate, is highly scalable and can enable water production rates similar to large scale desalination plants but with lower energy costs.
Binary Surfaces (BiS), which are low cost, scalable surfaces consisting of many micro- and nano-cavities, will be used in the inside of the still.
“These surfaces have the potential to significantly increase the heat transfer rate between the incoming concentrated solar radiation and water. They are also being pursued for their corrosion reduction potential. Corrosion is a major issue in thermal desalination,” said Kuravi.
A patent application is under review for the BiS technology, developed by Kota for another project.
Interfacial membranes are the third new technology applied in this research. These novel membranes are made of composite materials such as air-laid paper and carbon black coating. When placed in a solar still, they absorb more sunlight and considerably enhance the evaporation of water.
“Together with the binary surfaces, they could dramatically increase a solar still’s freshwater production rate,” said Kuravi.
The team’s research has shown that solar-still output can be increased even when these heat transfer enhancements and the concentrating solar technology are independently implemented.
“We found that compared to a regular still, the productivity in the enhanced still increased by around 500% due to a preliminary design of the concentrating solar technology alone. The binary surfaces showed promise in reducing the corrosion. The interfacial membranes showed an increase in the evaporation efficiency by 1.53 times,” said Kuravi.
Next, Kuravi and her team will optimize and test these technologies with produced water, individually and in combination. They will finally test this concept at a larger scale at the Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, New Mexico.
BGNDRF is a focal point for developing technologies for the desalination of brackish and impaired groundwater found in the inland states. Researchers from other federal government agencies, universities, the private sector, research organizations, and state and local agencies conduct collaborative research there.
