Cheap and drinkable water from desalination is finally a reality

In a groundbreaking endeavor, engineers from MIT and China have designed a passive solar desalination system aimed at converting seawater into drinkable water.

The concept, articulated in a study published in the journal Joule, harnesses the dual powers of the sun and the inherent properties of seawater, emulating the ocean’s “thermohaline” circulation on a smaller scale, to evaporate water and leave salt behind.

The scientists have developed a configuration where water circulates in swirling eddies similar to oceanic thermohaline circulation. This circulation, coupled with sunlight, enables water to evaporate. This leaves the salt circulating within the device. The water vapor is then condensed, providing pure, drinkable water, while the residual salt is expelled, preventing accumulation and system blockages.

The newly developed system surpasses all existing passive solar desalination prototypes in terms of water-production rate and salt-rejection rate. Lenan Zhang, a research scientist in MIT’s Device Research Laboratory, stated, “For the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water.”

The device, if enlarged to the size of a small suitcase, could yield approximately 4 to 6 liters of drinking water per hour and sustain several years before necessitating replacements. The team envisions that a scaled-up device could fulfill the daily water needs of a small family and potentially benefit off-grid coastal communities with ready access to seawater.

The design has evolved from the team’s prior concepts, avoiding the issues of salt crystallization and clogging experienced in earlier models. This design not only resolves the salt accumulation issue but also enhances desalination rates. The scientists have achieved high water production and salt rejection reliably over extended periods.

Xu, one of the team members, mentioned, “We introduce now an even more powerful convection, similar to what we typically see in the ocean, at kilometer-long scales.”

The team successfully emulated the kilometer-wide phenomena of thermohaline convection in a compact box, enabling the efficient rejection of salt.

The essence of the new design is a single stage that resembles a thin box, topped with a material to absorb the sun’s heat effectively. It is compartmentalized to allow water flow through the top half where the evaporator layer, using solar heat, evaporates the water in contact.

This vapor is then directed to the lower half and condensed into a salt-free, drinkable liquid by an air-cooling layer. Several prototypes have been tested, revealing that a scaled-up square meter stage could produce up to 5 liters of drinking water per hour, operating efficiently for several years without salt accumulation.

Yang Zhong, an MIT graduate student, emphasized the potential real-world impact, saying, “For the first time, it is possible for drinking water produced by sunlight to be cheaper than tap water. This opens up the possibility for solar desalination to address real-world problems.”

Given its extended life span and passive nature, requiring no electricity, the overall operational cost is estimated to be cheaper than producing tap water in the United States, marking it as a feasible solution to global water scarcity issues.

The dedicated collaboration included MIT’s Yang Zhong and Evelyn Wang, along with Lenan Zhang, and experts Jintong Gao, Jinfang You, Zhanyu Ye, Ruzhu Wang, and Zhenyuan Xu from Shanghai Jiao Tong University in China. Their collective endeavor holds the promise of a cost-effective and sustainable solution for water production, potentially alleviating water scarcity in remote and coastal regions globally.

In summary, this innovative solar desalination system devised by the engineers at MIT and in China demonstrates a significant leap in addressing the pressing global issue of water scarcity.

By efficiently and passively converting seawater into drinkable water using sunlight, this technology offers a sustainable and cost-effective solution. It stands as a testament to the potential of combining engineering ingenuity with natural processes to solve critical real-world problems, ushering in new possibilities in water purification and conservation.

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