# Potable water: desalination vs. condensation **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 2021, 11:23am UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440 "2021-08-11T11:23:01Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![doubleminus](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@doubleminus](https://boards.straightdope.com/u/doubleminus)\ **Post date:** [August 11, 2021, 11:23am UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/1 "2021-08-11T11:23:01Z")
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. Thanks ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 12:07pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/2 "2021-08-11T12:07:39Z")
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 [https://www.pri.org/stories/2015-05-15/desalination-expensive-energy-hog-improvements-are-way](https://www.pri.org/stories/2015-05-15/desalination-expensive-energy-hog-improvements-are-way) 2. Capturing humidity from Air : 310 Wh for 1 liter of freshwater. [Turn Air Into Water with Water-Gen's Atmospheric Water Generator | Time](https://time.com/75612/atmospheric-water-generator-watergen/) So desalination using RO takes less energy (10 versus 310) , but I am not sure how well it scales. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 11, 2021, 12:20pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/3 "2021-08-11T12:20:35Z")
> [@doubleminus](#): > > Say humidity is 60% 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). ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 12:43pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/4 "2021-08-11T12:43:53Z")
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 ---
**Author:** ![doubleminus](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@doubleminus](https://boards.straightdope.com/u/doubleminus)\ **Post date:** [August 11, 2021, 1:24pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/5 "2021-08-11T13:24:22Z")
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](https://www.spectrawatermakers.com/us/us/89650-8013438-Katadyn-Powersurvivor-40-e-12-V-a) runs at about 50 Wh per 6 liters which is even better than the link posted above by @am77494 (10Wh/liter). ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 1:41pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/6 "2021-08-11T13:41:17Z")
> [@doubleminus](#): > > Regarding practicability, I see that all the fresh water generators for boats are reverse osmosis. 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. ---
**Author:** ![doubleminus](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@doubleminus](https://boards.straightdope.com/u/doubleminus)\ **Post date:** [August 11, 2021, 1:46pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/7 "2021-08-11T13:46:56Z")
Further research brings me to [these devices](https://www.alfalaval.com/products/process-solutions/fresh-water-generation/multi-effect-desalination/mep-multi-effect-plate-evaporator/) (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. ---
**Author:** ![doubleminus](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@doubleminus](https://boards.straightdope.com/u/doubleminus)\ **Post date:** [August 11, 2021, 1:51pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/8 "2021-08-11T13:51:35Z")
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. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 1:58pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/9 "2021-08-11T13:58:05Z")
> [@doubleminus](#): > > Further research brings me to [these devices](https://www.alfalaval.com/products/process-solutions/fresh-water-generation/multi-effect-desalination/mep-multi-effect-plate-evaporator/) (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. @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. ---
**Author:** ![doubleminus](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@doubleminus](https://boards.straightdope.com/u/doubleminus)\ **Post date:** [August 11, 2021, 2:52pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/10 "2021-08-11T14:52:09Z")
@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. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 3:03pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/11 "2021-08-11T15:03:04Z")
> [@doubleminus](#): > > @am77494 The discharge could be easily taken care of by diluting it with sea water, sounds to me an minuscule energy addition @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 😀 > [@doubleminus](#): > > … the drawings look understandable mainly for chemical process engineers. I do wear that hat. > [@doubleminus](#): > > 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. 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 11, 2021, 4:11pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/12 "2021-08-11T16:11:13Z")
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](https://www.cartoonstock.com/directory/d/desalination_plant.asp). ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 5:08pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/13 "2021-08-11T17:08:05Z")
> [@Chronos](#): > > Of course, [there’s another solution to the desalination-waste problem](https://www.cartoonstock.com/directory/d/desalination_plant.asp). Not sure if that is a viable solution : **“There’s enough wastewater from the world’s facilities to cover Florida a foot deep”** [https://www.nationalgeographic.com/environment/article/desalination-plants-produce-twice-as-much-waste-brine-as-thought](https://www.nationalgeographic.com/environment/article/desalination-plants-produce-twice-as-much-waste-brine-as-thought) ---
**Author:** ![doubleminus](https://avatars.discourse-cdn.com/v4/letter/d/5daacb/32.png) [@doubleminus](https://boards.straightdope.com/u/doubleminus)\ **Post date:** [August 11, 2021, 5:08pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/14 "2021-08-11T17:08:46Z")
> [@am77494](#): > > 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 😀 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. 😊 ---
**Author:** ![CairoCarol](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cairocarol/32/3825_2.png) [@CairoCarol](https://boards.straightdope.com/u/CairoCarol)\ **Post date:** [August 11, 2021, 5:26pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/15 "2021-08-11T17:26:04Z")
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? ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [August 11, 2021, 6:36pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/16 "2021-08-11T18:36:16Z")
> **[Solar still](https://en.wikipedia.org/wiki/Solar_still)** > > Pages for logged out editors learn more > > A solar still distills water with substances dissolved in it by using the heat of the Sun to evaporate water so that it may be cooled and collected, thereby purifying it. They are used in areas where drinking water is unavailable, so that clean water is obtained from dirty water or from plants by exposing them to sunlight. > Still types include large scale concentrated solar stills and condensation traps. In a solar still, impure water is contained... ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [August 11, 2021, 8:01pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/17 "2021-08-11T20:01:36Z")
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. > **[Adrift: Seventy-six Days Lost at Sea](https://en.wikipedia.org/wiki/Adrift:_Seventy-six_Days_Lost_at_Sea)** > > Pages for logged out editors learn more > > Adrift: Seventy-six Days Lost At Sea is a 1986 memoir by Steven Callahan about his survival alone in a life raft in the Atlantic Ocean, which lasted 76 days. > Callahan departed Newport, Rhode Island, United States, in 1981 on the Napoleon Solo, a 6.5-meter (21.3-foot) sloop he designed and built himself, single-handedly sailed the boat to Bermuda, and continued the voyage to England with friend Chris Latchem. He had left Cornwall that fall, bound ... ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 8:28pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/18 "2021-08-11T20:28:55Z")
> [@CairoCarol](#): > > Is this within the realm of possibility? Yes definitely: People have already mentioned Solar Still above. The watercone is one such design : > **[File:Watercone on earth.svg](https://commons.wikimedia.org/wiki/File:Watercone_on_earth.svg)** ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 11, 2021, 8:35pm UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/19 "2021-08-11T20:35:02Z")
> [@doubleminus](#): > > “Is it _theoretically_ more expensive to make fresh water by condensation than by RO ?” 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 … ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [August 12, 2021, 9:37am UTC](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440/20 "2021-08-12T09:37:08Z")
> [@am77494](#): > > So desalination using RO takes less energy (10 versus 310) , but I am not sure how well it scales. The numbers you posted were 10,000 RO and 310 condensation. 10K and 0.31K [Next page](https://boards.straightdope.com/t/potable-water-desalination-vs-condensation/948440.md?page=2)