Optimization of Solar Still Artificial Transpiration

A multidisciplinary team of researchers from the National Laboratory of Solid State Microstructures, the College of Engineering and Applied Sciences, the School of Physics and the Collaborative Innovation Center of Advanced Microstructures, the Nanjing University Zhu Group in China, and the Department of Mechanical Engineering of the Massachusetts Institute of Technology, created a new generation of solar stills. The energy efficiency of this innovation will contribute to the expansion of a heat-induced desalination and drinking water system.

A Technology Inspired by Two Natural Phenomena

Several water purification and desalination methods were based on the transpiration principle inspired by the natural evaporation-condensation-precipitation phenomenon.

According to the researchers, water stills do not perform well compared to other chemical purification methods because of the energy loss caused by convection. Indeed, the walls of these systems, where the condensation takes place, are in contact with the water containers, promoting heat loss by convection.

Structure of the solar still

Heat transfer by convection and conduction

The team has developed a new concept of “artificial transpiration” inspired by the way water flows in plants from roots to leaves. This fluid flow pattern allowed researchers to avoid heat loss in the air and to retain all the energy required to activate condensation on the proper surface area.

Plant Structure

The system has four functional parts. Water and heat transfer with no energy loss is provided by, respectively, a one-dimensional device and a structure that conserves heat by conduction.

Water is routed from the first part, the storage container, to a conical structure in which condensation and evaporation occur by capillarity. According to the team’s studies, capillarity is a very efficient mode of liquid transfer, which also prevents thermal losses.

Capillarity-driven water transit

The second part, reserved for capillarity, is a natural cotton filament which channels the water from the bottom to the fourth part, located at the top. This part is the conical structure described above. It is made of porous and absorbent graphene oxide based on a nanometric structure that effectively ensures water transfer and condensation on its internal surfaces.

The distilled water is recovered in the container by the same process of capillarity. Impurities separated from the water during evaporation and condensation on the cone drop into a polystyrene collector, which also serves as a protective cover for the storage container.

Convection heat loss from existing systems does not happen in this innovative solution due to its particular structure, the hot surfaces of the cone, which, once exposed to solar radiation, are not in direct contact with water.

Formal Performance

The solar still is 85% more efficient than existing systems. Its operation is optimal and does not require external thermal insulation or optical devices to intensify solar radiation.

Its conical shape makes it more efficient because it can capture more solar radiation compared to two-dimensional systems; indeed, by changing its position in the sky, the sun can reach several areas of the structure. The researchers noted in their study that it is possible to optimize the conical part by making it mobile. The angle change allows it to follow the direction of the sun’s rays and further reduce the operating time. In addition, much of the sunlight (10% to 50%) is diffuse, arriving at the receiver from all directions, making the cone the most effective shape.

Tests carried out by the team have shown that the artificial transpiration system, in addition to the efficient treatment of water, can collect and recycle heavy metals found in contaminated water. The water harvested as condensed vapour by this technology is pure and meets WHO drinking water standards. The team was able to obtain drinking water from wastewater containing high concentrations of Cu2+, Cd2+, Pb2+ and Zn2+ (5000 mg/L, 5000 times higher than WHO drinking water standards). From this water, the researchers were able to extract heavy metals such as gold (Au) and copper (Cu).

The study entitled “Three-dimensional artificial transpiration for efficient solar waste water treatment ”, published in the National Science Review on May 2, 2017, was co-written by Xiuqiang Li, Renxing Lin, George Ni, Ning Xu, Xiaozhen Hu, Bin Zhu, Guangxin Lv, Jinlei Li, Shining Zhu, Jia Zhu.