Desalination has held out hope for many coastal communities facing water scarcity. But the promise has not materialized because of cost and a lack of reliable electricity. Even the prospect of solar-powered plants has not succeeded due to cost, problems with salt concentrations, and technological setbacks.
A recent joint research project between MIT and a Chinese university seems to have solved these perplexing issues. The breakthrough is a solar-powered passive design that stacks layers of thin evaporator-condenser panels driven entirely by sunlight. The sun evaporates the briny water into water vapor, which then condenses into pure, drinkable water. The salt is left behind and continues to circulate with the briny supply water and eventually out of the device (often at night). Keeping it in solution and moving prevents it from accumulating and clogging the system.
The MIT model uses an array of 10 stages, each with an evaporator and condenser. The sun powers the process in each layer but is augmented by transferring the condensate heat from one layer to the stage above. When water vapor condenses on a surface, it releases energy as heat. The salty water remaining after evaporation has a higher salt concentration. This heavier (i.e., denser) liquid wants to flow downward. By tilting the stacked ensemble a few degrees, this downward flow carries the high salinity water out of the device. The sun and gravity power the entire process. No electricity is used except perhaps for bringing ocean water into the device.
The sloped device allows the solar heated water to circulate in gently swirling eddies, like the large “thermohaline” circulations in the ocean. This phenomenon drives the movement of ocean water around the world based on differences of sea temperature (thermo) and salinity (haline).
The MIT system’s multiple levels greatly improve efficiency. With 10 stages, it achieved 385 percent in converting energy of sunlight into energy of water evaporation. More layers could further enhance efficiency but would also add to cost and bulk. A one-meter square array (11 square feet) produces one-and-a-half gallons of drinkable water per hour. The scientists figured the cost for this demonstration stack at about $100, a version big enough to serve an average family’s potable water needs.
The MIT device has a higher water-production rate and higher salt-rejection rate than any other such scheme. Even when using water with a salinity double that of the ocean, it was able to produce water at a rate and price cheaper than tap water. It seems like a godsend for portable equipment needed in natural disaster relief. The innovators believe it could also easily generate steam for sterilizing medical instruments without a power source.
