It’s pretty easy to get the salt out of seawater. All you have to do is boil it, freeze it, or just wait for rain! But those things take a lot of energy and/or time that many of us don’t have to spare. So people have been working on low-cost and low-energy-input ways to do it, such as solar desalination.
We seem to have hit on a really good one here, thanks to a collaboration between MIT and Shanghai Jiao Tong University (SJTU). I couldn’t even cram all the benefits of this thing into my diary title.
This device breaks the record for rate of fresh water production from seawater by a solar device, and as awesome as that is, it’s actually kind of a charming side note. The bigger breakthrough is that the device can keep running up near this rate for a long time without getting fouled by salt accumulation, and it does this by emulating natural processes that occur in the sea. That cuts costs by about 10x compared to typical solar desalination, making the cost of the fresh water it produces comparable to that of tap water.
Oh, and because it doesn’t accumulate salt, it can process water containing up to 20% salt (seawater has only 3.5% salt, and salt-saturated water has about 26%). Other solar desalination devices can’t even operate with water that salty. That means you can use this kind of device to process the wastewater from existing desalination systems, to get even more fresh water.
Not only that, but the device is able to capture and utilize the heat that’s produced when the fresh water it makes condenses, in order to make the whole process much more efficient. Kind of like the way a hybrid car captures energy from braking when you slow down. This is the aspect of it that honestly kind of blew my mind.
The upshot of all this is that the device can make fresh water from seawater for about 0.2 cents a liter, and a device the size of a simple card table could produce around 5 liters of fresh water an hour (for 1 cent!), so if you live near the ocean and have limited access to fresh water, and not a lot of money, this is a huge help.
It all appears in the upcoming October 18 issue of the Cell Press journal Joule, and I thank Dr. Zhenyuan Xu at SJTU for kindly sending me a link to access the paper.
Before I yack on much longer, I’d better show a diagram of the device, because it’s actually a pretty good schematic that captures all the important features:
Our device, inside a box, is submerged in a reservoir of seawater. Gravity feeds water up into the device, where heat from sunlight makes salt-free water vapor that we collect. We use a teeny-scale version of the oceanic phenomenon of “thermohaline convection” to keep salt from fouling our system.
What’s unique about this device that makes it so successful? Well, there are FOUR main things about it that distinguish it from other solar desalination systems, and each of them exploits a very cool principle. This thing is such a marvel! The more I read about it and understood it, the more impressed I got.
Let’s take a look at these four aspects. I’ll do my best to keep the descriptions pithy, but we really should appreciate each of them, because they all play an important role. These are going to be:
1) Gravity feeding by the “communicating vessels” principle
2) Salt rejection by the “thermohaline convection” principle
3) Keeping surfaces salt-free with the relatively new “membrane distillation” principle
4) Maximizing heat capture with the “latent heat” principle
Gravity feeding
How can we keep seawater coming into our system to replace the fresh water we are distilling off, without expending any energy — you know, without having to pump anything? Let’s appreciate where we can get the energy to do it by thinking about buoyancy. Have you ever taken an inflated beach ball and held it underwater? If so, you know very well that the instant you let it go, it rockets several feet high out of the pool. That’s because the water pressure on the bottom of the beach ball is much higher than on the top.
Here our whole system, inside an acrylic box, is going to be the “beach ball”, and we’re going to push it down into a reservoir of seawater. But we’re not going to let it go and watch it rocket into the sky! We’re going to anchor it down, so now we have some angry water pressure on the bottom of the box.
If we now run a tube upwards from the reservoir of seawater up into our distiller, inside the box, that angry water pressure is going to push liquid up through the tube until the level of the water inside it is the same as the reservoir level. That’s the “communicating vessels” principle: no matter what the shape of connected containers, the level of liquid within them will always be the same.
Now you can go back to the schematic diagram of the system and see how the water level in our distillation cartridge has to match the water level of the reservoir, because basically it’s a bunch of communicating vessels. Now we don’t need to add any energy to keep replacing the fresh water we distill out. Thanks, gravity!
Salt-free surfaces
How are we actually generating fresh water here? If you look back at the diagram, you see that we have two distillation units stacked on top of one another, and within each of them is a gray dashed line with little “vapor” arrows coming out. That gray dashed line represents a thin plastic sheet, or membrane, that water cannot get through. It’s made of a hydrophobic plastic — that is, water is repelled by it and doesn’t wet its surface. But it has pores in it that do allow water vapor through! This is called membrane distillation, and it has only been commonly used for the last couple of decades.
Now if water doesn’t wet the surface, that means salt dissolved in the water doesn’t wet the surface either — in fact, salt is ionic (charged) and hates the plastic surface even more than water does. So we never have to worry about salt accumulating on and fouling our membranes. Fresh water vapor comes through the membrane, and it condenses as it cools (the droplets in the figure), so we can collect it. We have several choices of inexpensive plastic we can use to make the membranes, so the distillation itself is very cost-effective.
Porous PTFE (polytetrafluoroethylene) membrane sheet that can be used for distillation
Salt rejection
But still, as we pull off fresh water, isn’t salt going to start accumulating in the system? Where’s it going to go? How can we keep running this thing without it turning into brine? We can’t expend energy to pump liquid, so how do we get the increasingly salty water out of there?
This where our MIT/SJTU team really hit it. They understood that currents in the deep ocean are driven by density differences, and they mimicked that effect in this system to get the kind of water circulation they wanted without expending any energy.
Colder and saltier water are denser than warmer and fresher water, and denser water wants to sink to the bottom. For example, near the poles, when water crystallizes into icebergs, salt is excluded, so the surrounding water becomes a bit saltier and thus starts sinking down. Water from the surrounding surface gets dragged in to replace it, and there you have a big current. This is called thermohaline convection (also known as thermohaline circulation). Other effects like surface heating and precipitation contribute, too, and thus these currents churn around the globe:
Schematic global oceanic thermohaline circulation. Blue lines are deep coldwater flow; red lines are warmer surface flow. If Arctic sea ice stops forming, it won’t quite look like this anymore
This is where the tilt in our system is really important. The top of our liquid distillation cartridge gets heated by sunlight, and it’s warmer (and lighter) than the rest of the water in there. But the water at the bottom, because we’re pulling fresh water vapor off of it, is a little saltier (and denser). The denser water wants to slide downhill in our cartridge, and the lighter water wants to move up, and this creates a bunch of little eddies in our system that swirl the water around and keep it mixed.
For one thing, that transfers heat down to the membrane so that the distillation works better. Also, the mixing helps prevent local areas of high salt where it might crystallize. And the other cool thing about it is that saltier water is going to keep sliding downward because of its higher density, all the way out of the system and into the larger reservoir. (This is especially true at night, when even though the system isn’t running, it is clearing salt out.)
In fact, if you start with 10% salt (about 3x concentrated seawater), the salt concentration in the system gets up to about 10.5% within a few hours, but then it stops accumulating. It stays constant at 10.5% indefinitely afterwards (or at least for the whole week of continuous running the authors did). That means you can just keep on running this unit without replacing anything! Just drain the reservoir periodically, pour in some new seawater, and go about your business.
Heat capture
You might be wondering why we would stack units on top of each other when the sunlight is all at the top. But in fact they found that stacking TEN of these on top of each other was actually best. But wait, where would the layers beneath the first one get heat from? Aren’t they shaded from the Sun?
Well yes, they are, but you can see in the schematic diagram that on top of the second layer, what is happening? Water is condensing. When water vapor comes out of the gas and into the liquid, it gives up some energy (its “latent heat of vaporization”), and heats the surface it just landed on. So we can recapture that heat, as we captured heat from sunlight on the top layer, to do another layer, and another….
As we add heat to ice, its temperature goes up for a while, but then it takes on a bunch of heat in order to melt, even though it stays at the same temperature. Then, after it turns into a liquid, its temperature goes up as we heat it again, but when it reaches the boiling point, again it takes up a bunch of heat without an increase in temperature. That amount of heat (540 calories per gram) is its heat of vaporization. Condensing water does the reverse of boiling water: it gives up a lot of heat without changing temperature. If it does that on a surface, the surface will heat up substantially.
It doesn’t seem intuitive that we could corral a whole lot of heat this way, but if you think about it, it does take a fair amount of heat to get water to vaporize off a surface. So we’re reversing that here and using the plentiful heat released by condensing water to heat the top of our next layer.
It’s a good thing I didn’t design this device, because I would have been satisfied with one layer and just left it at that. Hey, I got fresh water trickling off this thing — what more do you want? But stacking the layers gave an efficiency of about 5x that of a single layer. Kind of unbelievable.
So back to real-world significance here. MIT News sums it up nicely:
From these tests, the researchers calculated that if each stage were scaled up to a square meter, it would produce up to 5 liters of drinking water per hour, and that the system could desalinate water without accumulating salt for several years. Given this extended lifetime, and the fact that the system is entirely passive, requiring no electricity to run, the team estimates that the overall cost of running the system would be cheaper than what it costs to produce tap water in the United States.
"We show that this device is capable of achieving a long lifetime," [MIT graduate student Yang] Zhong says. "That means that, 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."
What’s left to do now is go from prototype to mass-producible system. With the simple and inexpensive materials used, and with the simplicity of operation, that shouldn’t pose an undue challenge.
Man, there are so many cool things about this system, but I’m closing with a tribute to the latent heat idea, where we seem to get heat for nothing….
