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.
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.
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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.