Posted: Feb 11, 2020 11:57 am
by GrahamH
The key to the system's efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team's demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.

Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment. But in this multilayer evaporator the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.
"When you condense water, you release energy as heat," Wang says. "If you have more than one stage, you can take advantage of that heat."
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.

What? :?

That doesn't sound right at all. To condense water you need a cold surface so you have to get rid of the heat. The condenser plate can't directly heat the next evaporator.

They could use heat pumps, but they are far from over unity efficiency and more heat is lost than transferred. And there is no mention of heat pumps

I'd expect something credible from MiT so what's the real story?

Their demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber. In most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials, Wang says. "We've been able to decouple these two."
The most expensive component of the prototype is a layer of transparent aerogel used as an insulator at the top of the stack, but the team suggests other less expensive insulators could be used as an alternative.

It takes a lot of energy to evaporate water and a big thermal gradient has to be maintained somehow. ... ation-0207

They show a diagram, with transparent insulator, solar absorber, wicking layer to evaporate and a cold plate to condense, but no indication how the cold plate is cooled or how energy is recovered from the cold plate to heat a following stage. Anyone have any ideas on that?