Engineers at MIT and in China are aiming to turn seawater into drinking water with a completely passive device that is inspired by the ocean, and powered by the sun.
In a paper appearing today in the journal Joule, the team outlines the design for a new solar desalination system that takes in saltwater and heats it with natural sunlight.
The researchers estimate that if the system is scaled up to the size of a small suitcase, it could produce about 4 to 6 liters of drinking water per hour and last several years before requiring replacement parts. At this scale and performance, the system could produce drinking water at a rate and price that is cheaper than tap water.
Delete this before Nestle sees it.
The massic heat capacity of water is
4184 J⋅kg⁻¹⋅K⁻¹. To heat one 1 Liter (1 kg) of water from 30ºC to 100ºC it would take4184×(100-30) = 2.929e5 J. We want 4 liters however, so we multiply that by 4 and get2.929e5 J × 4 = 1,172e6 JTo then turn that heated water into vapor it would require some more energy. The vaporization enthalpy of water is4,066e4 J⋅mol⁻¹, and has a molar density of1,80153e-2 kg⋅mol⁻¹(so 4 liters (4 kg) of water in moles would be4 / 1,80153e-2 = 2,22033e2 mol), which means that to vaporize the four liters of water we would need2,22033e2 × 4,066e4 = 9,028e6 J(I think I might have made a mistake here somewhere, because I don’t think it would only need 8 times more energy to completely vaporize the water, compared to the amount of energy required to heat it, but I can’t find the problem). So the total energy to heat and vaporize 30 ºC water would be9,028e6 + 1,172e6 = 1.020e7 JLet’s take a 55x40x23 cm suitcase. And let’s assume a solar irradiance of 1000 W⋅m⁻² (which is what this site says is a normal solar irradiance to be expected on a clear day on the equator). Let’s assume three faces are exposed to the sun and all equally so (three faces receive 1000 W⋅m⁻² while the other three receive none, which would not happen since on a rectangular cuboid, like a suitcase, you can’t have all three faces facing directly towards the sun). The box would be receiving
(0.55×0.40+0.40×0.23+0.55×0.23)×1000 = 438.5 W, which means that over one hour (3600 s), it would receive438.5×(3600) = 1,5786e6 J, which is less than the required1.020e7 J(by almost an order of magnitude), so it wouldn’t be possible to heat and vaporize 4 liters of water in an hour.What am I missing?
You’re assuming that all heat energy input leaves with the water once it vaporises, which is unnecessary and indeed undesirable.
If you use the incoming water to condense the output vapor, you can recover and reuse a lot of the heat energy, plus you get output water at a much more reasonable temperature.
deleted by creator
Nothing. These so called miracle bullshit devices shows up annually at random times to over promise under deliver so that media will talk about and gullible people lose their money on.
This sounds fantastic on its face, but I seem to keep on hearing about how desalination will solve all kinds of problems and we still have this particular problem.
The missing piece, it seems, is the will for it to be used as infra at scale. Meanwhile selling bottled water taken for free from public lands for several dollars a liter in single-use bottles remains a multi-billion dollar industry. (an industry, I might add, that is aggressive about lobbying to protect its interests)
Not to mention if we do this at enough scale it will raise the salinity of the ocean and, you know, kill everything.
will raise the salinity of the ocean
If care isn’t taken to avoid concentrating brine going back in just one spot, sure that could create localized problems. Buuut, you realize that the oceans constantly lose water to evaporation and their salinity is more or less stable, right? Every bit of rain or snow that falls on land (most of which returns to the ocean eventually) is water the ocean can be without and still not too salty for life.
Speaking of salinity, the Atlantic Meridional Overturning Current (which in normal conditions, is the deep/cold return current from the gulf stream -> North Atlantic) is running into a big damned problem because Greenland is melting and all that fresh water pouring off of it is disrupting the return flow of cold water to the tropics. That’s why the Gulf Stream has been so hot- it’s not getting return feed from its radiator in the North Atlantic, and meanwhile the North Atlantic is getting colder because it’s not cycling water back south, and that prevents hot Gulf Stream water from getting there.
Edit: I recently learned that concentrated brine regions in the oceans (called brine pools) are a thing. There are massive salt deposits (as much as 8km thick) under the bottom of the Gulf of Mexico today, the legacy of a time when the gulf was closed off from the oceans- when it refilled, the salt layer was covered over. Today, the deposition on top of it is heavy enough that subsidence within it squeezes the softer salt around, occasionally exposing that salt to the ocean water.
This sounds like an idea for a disaster movie!
(Yes, I know.)
All this stuff is like planning to colonize mars before we stop destroying earth. There is plenty of water if we just stop fucking pumping it all out and wasting it.
Can you believe some people actually drink that stuff straight from the tap? It’s like they don’t even care about the golf courses at all!
