#dev-diary solar still

I've got a very #solarpunk project. make a solar still.

My goal, is to provide clean drinking water on my boat when it's not raining.
currently, I need to find a tap somewhere, or wait until it rains.
Waiting until it rains is actually pretty viable except in the height of summer.
Finding a tap isn't too hard but it makes me dependent on land which I do not like.

The $$ method is get a water maker. This is a device that pressurizes salt water so that it squeezes through a ceramic membrane which blocks the salt, creating drinkable water. But these devices are

• expensive
• power hungry
• high maintainance

All of which I am against. I don't want to be depending on something that can break easily, not for drinking water. I'd rather rely on rain water, even if I have to wait for the rain, it's empowering because I just arrange a simple tent and capture free water falling from the sky. I just have to monitor the weather and water usage. But this creates a closer relationship with nature.

A solar still could do that, but with sunshine instead of rain.

The basic idea with a solar still is you put bad water in the bottom, the sun heats and it evaporates, then it condenses on something and runs down into a catchment, providing drinking water.

liftraft still:
https://www.youtube.com/watch?v=Da2CbOypkOU

you might be aware of the solar still as a survival technique, dig a hole, plastic sheet over it, and a stone in the middle, so that water condensing on the plastic drips down into the center were a container collects it.

https://www.youtube.com/watch?v=10k-DPcLrHU

both of these produce water sufficient for survival. I'd like to make enough water that at least some people might envy my lifestyle.

then there are a variety of solarstill designs presented on youtube, from backyard constructions, to hipster stills for 3rd world drinking water, to engineers researching how to make stills more "efficient".

https://www.youtube.com/watch?v=Zw9vRaS7EpI

this one is quite amusing:
https://www.youtube.com/watch?v=RSZLM4P37xk

the engineers seeking efficiency seem to be defining efficiency it as more water per space, but make bulky designs that have more advanced components
for example: https://www.youtube.com/watch?v=6MynEdS5WH8
or https://www.theengineer.co.uk/solar-still-smashes-desalination-record/

advanced components means more cost, and seems to produce a bulky, 3d design.

One of my favorites is this Australian one, http://www.fcubed.com.au/aspx/home.aspx that looks like solar panels
This one has a circulating water supply that runs through a wick instead of a reservoir. The wick stays wet, but doesn't have much bulk, some water passes through, which also has the benefit of avoiding sludge that then needs cleaning.

I want something like that but using the cheap materials like this one:
https://www.youtube.com/watch?v=O3XUnf8sLO8
Also I plan to use this on a boat, so it needs to be easily setup/taken down.
I expect it to be used while at anchor, which is most of the time, probably not while sailing.

Also, there are a variety of claims as per amounts of water produced, but often this it does not give me the impression that they actually used it, so I'm not sure if I believe many of these claims. My goal is to get at least 5 litres a day. If it's more than that, then that will be fairly comfortable living for two people, and when it rains you'd get a lot more.

Yes, I was thinking about that just this morning. I was thinking to keep the tubes full have a siphon into them, then let them boil, I've seen videos of them boiling, then put that through a regular copper spiral condensor. seems like it would work, but I'm not sure how frequently they'd boil, though?

there are some like this, that just attach solar evacuated tubes to a conventional resiovour still, https://inpressco.com/wp-content/uploads/2016/07/Paper5324-330.pdf

just searched again and saw this:
https://www.youtube.com/watch?v=3PcjByw-9Ac

I count 1 drip per 2 seconds which is 0.53060=900ml an hour, they say it's the setting sun. Apparently the dish makes a "big difference".

hmm, on that note, since I have excess solar panels,
I could have a system that electrically heated water, then condensed it.
if it takes 2260 joules to boil a gram of water then watts are joules/second so a 250 watt water heater could boil a gram of water every 10 seconds. (a drip every 5 seconds) so that's 6 grams per minute times... 8 hours? (days are longer in the summer but with sun at low angle won't get so much energy) But that could produce 2.88 litres?

hmm, suitable looking heating elements very cheap: https://www.aliexpress.com/item/1005002352970809.html?spm=a2g0o.productlist.0.0.475950c77JF2V1&algo_pvid=1ff87903-9d6b-45d4-8635-eeaea82a3ae6&aem_p4p_detail=202203161816281092041238873630009252244&algo_exp_id=1ff87903-9d6b-45d4-8635-eeaea82a3ae6-9&pdp_ext_f=%7B%22sku_id%22%3A%2212000020370656079%22%7D&pdp_pi=-1%3B16.79%3B-1%3B-1%40salePrice%3BUSD%3Bsearch-mainSearch

Then I'd just need a small pump and some float switches. It would not have much output compared to a water maker but would have the benefit of everything could be set up below decks and left on.

On the other hand, if you have a passive solar still directly in the sun it's collecting a lot more irradiance (900w/m^2) without competing with your other energy needs.

@IBob interesting design!

some remarks:

Fairly sure you can get these in a variety of lengths, this product has 0.5m tubes https://www.aliexpress.com/item/32691326714.html?spm=a2g0o.productlist.0.0.27541b38pITTjM&algo_pvid=ae873dab-af56-440a-8ef4-0deef7a17b8d&algo_exp_id=ae873dab-af56-440a-8ef4-0deef7a17b8d-12&pdp_ext_f=%7B%22sku_id%22%3A%2260505347524%22%7D&pdp_pi=-1%3B190.39%3B-1%3B-1%40salePrice%3BUSD%3Bsearch-mainSearch

What if the top bulb gets hot enough to flash boil the drips? In my mind I'm seeing the drips boiling instantly.

Hmm, you can also get hollow tubes without the heat transfer bulb, that the water goes inside of. If I recall correctly, the rationale for the heat transfer bulb is that then you can have a pressurized hot water system. I guess if you are heating salt water to boiling inside a glass tube it's gonna leave a lot of stuff behind.

It's curious that the tubes are noticably cool. I don't really understand why that would be. are they cooler than a piece of glass would be?

I think the sump needs a wall around it, really do not want it to drip down into the clean output water.

on that note, instead of using another large diameter glass tube for the condensor (clearly expensive) you could have a copper coil that is surrounded by the input water.

@emile it's a catamaran so more weight sensitive. currently I have 6 20L jerry cans. I'm certainly using less than 10 litres a day. I've started logging when I switch the jerry cans (one is set up to siphon into the galley tap) which makes keeping accurate track of usage quite easy. I could certainly get more tanks, but people have done that already, as far as I can tell no one has made a practical everyday marine solar still.

That water maker is nearly more than I originally paid for the boat! (before renegioating to $1 for the boat and $4999 for a years rent of the work space)

@IBob btw, I was actually thinking of a design that is a bit more tractor technology ;)

Evacuated tubes look interesting but they are fairly expensive and I can't just walk into a hardware store and buy them.

I'm thinking the main material is polythene sheet, which comes in both clear and black. I also have some non-woven weed mat that looks fairly absorbent. so basically make a large pocket with a damp, black, wick at the lower inside surface and a gutter at the bottom...

Yes, I also have rain water collection. Off the solar panels and a tent. When underway you can catch it off the sail even. Might have too much salt spray in it if it's rough though. However, it's been several weeks since any significant rain here... and I once met someone that went 54 days without rain while crossing the indian ocean. They managed to beg some fresh water from a passing ship, then the next day it rained!

@alanz ah, similar to this https://www.youtube.com/watch?v=KlUQ1pneow8 basically something like a solar powered dehumidifier. Quite expensive for how much it produces and overkill on a boat, given it's easier to just get the salt out of the sea water

I'm also seeing that 1m2 of sunlight on a good day of 10hrs theoretically provides approx enough energy to evaporate 1litre of water. That does't account for any losses.

@IBob how do you calculate that?

Most of the solar stills that I have been looking at claim more like 2 litres per square meter AT LEAST.

I can easily believe that the DIY or commercial claims were exaggerated, but the naval liferaft solar still isn't very large, and it produces at least enough for survival.
hmm will have to see how this describes it http://libgen.rs/book/index.php?md5=3565BDEE8219154F2A0C6B8DD6BB539B

hmm... I'm beginning to realize that boiling is different to evaporation. I mean, your clothes dry on the line without ever getting hot, although sun light and wind clearly help.

The first answer in that quora is better, https://www.quora.com/How-many-watts-does-it-take-to-evaporate-1kg-of-water-starting-at-room-temperature-in-an-hour/answer/Kimber-Stout

It depends on what is the initial temperature of the water. Let us assume STP, so T = 25 C.

To raise to the boiling point would be a change of (100 - 25) C = 75 C. The specific heat of 1 kg of water 1 degree C = 4.186 kJoules, so you need 313 kJ. The you have to boil the water (turn it to steam—if you truly mean evaporation, that is a different process not requiring additional heat energy, and would not occur in 1 hr.). The latent heat of boiling, for water, is 2260 kJoules/kg. So you would need that amount of heat, also. The sum would be 2573 kJ (2573E3J).A watt is 1 J/s; a kW is 1000 W/s (E3). An hour is 3600 sec. Divide: 2573E3 / 3600 = approx. 715 W

That's 715W to boil 1 litre of water, in one hour. to boil it in 5 minutes takes 7,500 watts. I would be happy boiling away 5 litres in 8 hours which is 446 watts. But this is still too much.

interestingly, there is another group that is purely interested in evaporation - serious burning man people sometimes build grey water evaporators - they have to bring all the water they consume, and to avoid taking their old dish water home again they build evaporators. this one: https://www.youtube.com/watch?v=MYe9R5RHshw claims they once evaporated 70 gallons in 24 hours, that's 264 litres! it doesn't apply heating at all. Mainly it provides a large surface area and some airflow. So, put this inside a bubble, vent that bubble into a condensor... and now you have water recycling?

@nanomonkey you would think that it's always fairly humid on the water, but I have a humidity meter here, and it's currently 24 degrees C, and only 57% humidity. Around here that is pretty low. When I was painting the boat I obsessed about this and carefully kept track of the humidity to get suitable painting weather. Right now would work.

On the land, there is much less water... but if the ground is wet, that water has a massively greater surface area. The ocean surface, with waves, pretty smooth - compared to the fractal surface of every blade of grass and every leaf on every tree.... There is potentially much much more surface area on land (when it's wet) so more opportunity for that water to evaporate, vs the sea.

But excellent point... what I need is a humid space, then to feed that air on to a surface that is below the dew point. That could use an electric cooling element...

but maybe if you warmed the humid space (which also means more water can evaporate) maybe you get it warm enough that the ambient sea temperature is now below the dew point, meaning you could have a completely passive system, potentially...

okay I found a dew point calculator: https://www.calculator.net/dew-point-calculator.html?airtemperature=22&airtemperatureunit=celsius&humidity=57&dewpoint=&dewpointunit=celsius&x=62&y=17

currently, where I am the dew point is 13.1 degrees. according to windy.com the sea temperature is also 22 degrees here.

so I need to create a humid space that is both warmer and more humid than ambient so that the ambient sea temperature is the dew point.

Time for an experiment. I have placed the humidity meter inside a closed jar, with some water, if it moves up and to the right 4 squares, to say 80 percent humidity and 27 degrees then the ambient sea temp becomes dew point.

alternate possibility, what about harvesting water from dew at night? say using an air blower onto some sort of condensor

oh turns out that exists https://en.wikipedia.org/wiki/Air_well_(condenser)

it's now 97% humid inside the jar! that means that the dew point is 18 degrees, but it's slightly colder now. 17:13 and not sunny. but it does show how easy it is to create a high humidity enviroment.

this air well wikipedia article https://en.wikipedia.org/wiki/Air_well_(condenser) is great, contains some amazing tidbits such as

in the 1930s, American designers added condenser systems to airships – in this case the air was that emitted by the exhaust of the engines, and so it contained additional water as a product of combustion. The moisture was collected and used as additional ballast to compensate for the loss of weight as fuel was consumed. By collecting ballast in this way, the airship's buoyancy could be kept relatively constant without having to release helium gas, which was both expensive and in limited supply

you can tell it was written by a real air well enthusiast

hmm, well i am in a bay right now and it hasn't been raining, wind blowing over land... I'll measure again when at sea.

@IBob that's interesting. Is that because the system doesn't loose as much heat?