Potable water: desalination vs. condensation

Say I’m building a raft for crossing the ocean. I have a (fairly limited) solar and/or wind generator and I want to make potable water.

What is more energy efficient? Desalination by reverse osmosis or capturing humidity from air ?

For the second option, since this is probably very environment dependent, let us suppose I’m in a tropical area with air temperature around 30 C. Say humidity is 60%.

I’m aware there may be energy-less options (at least electricity-less), like warming water with a solar thermal collector and condensing at sea temperature. But basically I’m interested in energy required for condensation as against desalination of sea water.


Here is an approximate calculation. You will have to contact device makers for exact numbers :

  1. Desalination of seawater by Reverse Osmosis : 10 kWh for 1 m3 of freshwater from seawater. Or 10Wh for 1 liter of freshwater
    Advances coming in desalination technology

  2. Capturing humidity from Air : 310 Wh for 1 liter of freshwater.
    Turn Air Into Water with Water-Gen's Atmospheric Water Generator | Time

So desalination using RO takes less energy (10 versus 310) , but I am not sure how well it scales.

On the open ocean, far from land, it’ll be more like 80%. And I assume that the only reason it’s not 100% is because of the salt in the water.

Which is relevant, because I think that the figures for that humidity-capture device are based on a lower humidity (it’s pictured being used in what appears to be a semi-arid area).

Fully agree with Chronos.

RO is a sort of filtration process and the energy requirement is low. Cooling air to condense water, takes a bit of energy.

However, I think, for the raft application - the air condensation route is much more attractive for : 1. Portability 2. Low Maintenance 3. Off the shelf design

That’s a huge difference. Is this inherently due to thermodynamic considerations or just poor implementation ? I imagine that a condensation machine would use a heat exchanger to avoid cooling the exhaust air too much. I understand that condensation releases the vaporization heat.

Regarding practicability, I see that all the fresh water generators for boats are reverse osmosis. For instance, this model runs at about 50 Wh per 6 liters which is even better than the link posted above by @am77494 (10Wh/liter).

I have no experience or knowledge of boats. I would go with whatever technology/product is mature for boats. The RO device you linked to, is excellent - since it also has emergency manual operation.

Further research brings me to these devices (admittedly for cruise ships) that use much less energy.

The electric energy consumption of the four-effect MEP-4-750 desalination unit, for example, is less than 2.5 kWh/m³

The words “electric energy” might hint to an additional source of energy. However I couldn’t find any mention of the unit being linked to the ship’s engine in any way.

I am somewhat surprised by this. If so, why is the vast majority of land based desalination plants based on reverse osmosis. By far their major cost is in energy required.

My only knowledge of (sail)boats is that I get terribly seasick on one, irrespective of what medicine I tried. It’s only a curiosity question.

@doubleminus - those are evaporator based desalination plants not Reverse osmosis. 3 effects means - in the 1st effect you use energy to boil the sea-water, the steam that comes out is used to boil more seawater (2nd effect) while it condenses to freshwater : this is done 3 times.

I do not think evaporation based devices are suitable for a raft - but what do i know.

In Land Based, Reverse Osmosis Desalination plants , for every 1 gallon of fresh water made, 20 gallons or more is rejected. Thus the “wastewater” from a Reverse Osmosis Plant is not very concentrated and can be discharged near the coast back to the ocean / sea.

In Land Based, Evaporator Desalination plants , the resulting wastewater is very concentrated in salt - and will create environmental problems if discharged directly into the sea. Permits become harder to get. Wastewater needs to be discharged into the deep sea - after a pipeline is laid to carry the salt laden water where it doesn’t hurt marine life. How deep and how far does the pipe need to go - you’ll have to ask a marine biologist, but it usually is 10s of miles.

Cruise ships take all sorts of exceptions - like international waters etc etc. But i am not sure what they do with their waste water.

@am77494 The discharge could be easily taken care of by diluting it with sea water, sounds to me an minuscule energy addition. I was curious if in principle osmosis is more efficient than condensation, I do not really intend to sail a raft. The OP was phrased this way just to exemplify the problem.

But I suspect that the answer to the very low energy for the cruise ships’ desalination plants resides in that the evaporation energy is a byproduct of exhaust steam used to power the ships’ turbines. In that case even 2.5kwh/ton seems much to me. I don’t really understand the unit, the drawings look understandable mainly for chemical process engineers.

What interests me is whether renewable energy sources could be closely coupled with desalination plants, so we could have zero carbon water. Seemingly water shortages will be a crucial problem in the years ahead.

@doubleminus - I like your questions because they are thought provoking.

But no - as one of my bosses used to say : “Dilution is not a Solution to Pollution”. You cannot morally/ethically/legally pump in sea water, dilute the waste and claim you have met pollution targets :grinning:

I do wear that hat.

They absolutely can be. I have not really looked at the amount of water needing desalination for the years ahead. My guess is how much water needs to be desalinated will drive if the plants are coupled with power generation or stand alone.

It might be less of an issue with a cruise ship, since it’s moving and so not dumping all of its brine in the same place.

Of course, there’s another solution to the desalination-waste problem.

Not sure if that is a viable solution :

"There’s enough wastewater from the world’s facilities to cover Florida a foot deep"


Dilute sea salt by sea water ? Father nature does this at an enormous scale.

I know that the brine of desalination plants may affect nearby sea life to some extent. But in the middle of the ocean ?

There were proposals to dump radioactive waste into deep ocean after the initial few years in the pools, calculations seemingly showed insignificant pollution. Of course no one took this seriously. I may misremember though.

However though, my (derived) question still stands: “Is it theoretically more expensive to make fresh water by condensation than by RO ?”. You know, all these words I see, enthalpy, etc. My chemical education ended with high school more than 40 years ago. :blush:

I’d like to ask a related question, if I may, since I’m guessing several of the above posters could answer it.

As a child I read a story in which a child and his Grandpa are stranded without water in the hot sun. Grandpa’s getting dehydrated and near death, but is saved by the child’s quick thinking (I feel as if it may have been an Encyclopedia Brown story, but I doubt it as the tale is a bit too harrowing and the narrative arc is wrong).

Anyway - smart kid assesses what is available, and MacGyvers a water condensation device with a stick, a sheet of plastic, and some plants. Basically, he makes a little tent with the plastic and stick, and puts the plants underneath. A few drops of water condense on the plastic and he is able to give Grandpa just enough liquid to keep him going until help arrives.

Is this within the realm of possibility?

Solar stills have been a lifeboat survival tool for quite some time. They’re slow, they’re fussy, but they work. What else you got to do while crossing the ocean in a raft? They saved this guy’s life.

Yes definitely:

People have already mentioned Solar Still above. The watercone is one such design :

I think this was answered in post #2 . 310 Wh for condensation and 10Wh for RO, for 1 liter of freshwater.

That’s to give you an order of magnitude estimate of “expensive”. If you want anything more specific, you will have to have provide more information on :

  1. Location - Local weather / temps / humidity
  2. Price of Electricity
  3. Capacity - how many gallons per day

The numbers you posted were 10,000 RO and 310 condensation. 10K and 0.31K