Solar Desalination System Produce Fresh Drinking Water

Researchers from MIT and China have recently developed a desalination system that can produce 1.5 gallons of fresh water for drinking from per meter of solar collecting area. It is a passive system. This system is capable of providing a wide range of off-grid arid coastal areas with pure drinking water with lower cost and great efficiency. 

Multiple layers of condensers and evaporators in this solar system are flat in shape. These are arranged in a vertical array. Besides, there is transparent aerogel insulation at the top. The research team of this study include professors and doctoral students from MIT and Shanghai Jiao Tong University. An MIT professor, Evelyn Wang is serving as the head of the department of mechanical engineering. He is a major contributor to this study. Some doctoral students of this study are Lin Zhao and Lenan Zhang. Zhenyuan Xu is a postdoctoral researcher of this research paper. Eight other contributors from both institutes also took part in the study. The paper describes the possibilities of this system usage. It was published in Energy and Environmental Science journal on Feb. 7.  

The team determined the efficiency of the system by focusing on each of the multiple stages for desalinating water. In every stage, the level of harnessing heat increases as the wasting rate is lower. As the team was able to stop heat wastage rate, the device showed 385 percent efficiency in converting sunlight’s energy into water evaporation energy. 

This multilayer solar device is similar to many other devices that we use to distil water. The evaporating and condensing components of this device makes it clear. The flat plates of this system absorb heat and that heat starts to evaporate a layer of water. In the next stage, the vapour condenses. After this, pure water is ready. The heat from this vapour condensation goes to the next layer. 

This new solar system is unique from the typical ones. They do not release the heat directly to the environment after vapour condenses on a surface but the typical ones do. The heat passes to the next layer and thus recycles the solar heat increasing overall efficiency.

Wang says, "When you condense water, you release energy as heat. If you have more than one stage, you can take advantage of that heat."

The more the layers in the system, the more efficient it becomes in producing potable water. But adding more layers means adding the budget and its size. As a proof-of-concept device, the team developed a 10-stage system and settled it on the rooftop of an MIT building. The water that this device produced was more standard than the city drinking water. The rate was about 1.52 gallons per 11 square feet of solar collecting area. Wang says it can produce potable water two times higher than the previously designed passive solar-powered desalination system could.

Zhang believes that this device’s efficiency rate will increase to 700 or 800% by adding desalination stages and further theoretical optimization. 

This system will not make a store of salt or concentrated brines within itself as the other systems do. Rather, with its free-floating configuration, it will send back the accumulated salt in the seawater. The wicking material will carry the accumulated salt from the and then send them back at night. Researchers confirm this.

For their demonstration, they used readily available and cheap materials. They used paper towels and a commercial black solar absorber for a capillary wick so that it can make contact between the water and the solar absorber. The earlier passive solar desalination systems had same kind of wicking material and absorber material made from expensive and expensive materials. Wang says. "We've been able to decouple these two."

For example, the earlier insulator was built with a layer of transparent aerogel it is quite expensive. The team is in the opinion that they can use less expensive insulators instead of the expensive ones. Aerogel is an expensive component but it comes from cheap and dirt silica. So, there must be an alternative way of building this component and it is true for other expensive components too.

The researchers call this system thermally localized multistage desalination. According to Wang, the most outstanding contribution of the team is to build a framework that understands how to optimize this system. Researchers think that this new device will be applicable to a wide range of materials and device architectures. If this system gets further optimization, it will come up with different scales of operation or local conditions and materials.

An impoundment pond is a kind of possible configuration of this system. It consists of floating panels on a saltwater body. The added pipes of this system can supply fresh water to the shore passively and constantly as long as the sun is up. In another way, they can optimize the system to supply water to a single household. For this, they should set a flat panel on a large shallow tank of seawater. The potable water will be then pumped and carried into the house. The team says that this system can fulfil the water need of one person with a 1-square-meter solar collecting area. So, the cost of this system to serve a family will be $100.   

The team plans to modify this system by optimizing the materials and configurations. They also consider how durable this system is in realistic conditions. The design should not be as complex as in the lab. It will be user-friendly for every consumer. The ultimate goal of this system is to provide the areas with potable water where there is no electricity but have a lot of seawater and sunlight.