A new slant on an old idea

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,833
Reaction score
160
Location
Albany, Western Australia
Here's one to test the brainiacs.
We all know how to make a traditional solar still using a piece of clear plastic, a rock, a cup and a bit of tube but have you ever made one and used one? I have and in a weekend about 12 months ago it produced less than a cup of water. The rock and the cup seemed to be off-skew and when I tried to drink from the hose, it somehow flipped out and I had to virtually pull the thing apart to get it working again.
OK, I might be overstating it a bit but I and folk I was with were underwhelmed at its lack of efficiency.
On the Sunday I had a bit of a brain wave. (Only my second, the first was in 1985 and I can't remember what it was...).
Anyway, I took a large garbage bag (Sulo bin size) and set it up very quickly as per below on a sandy section of the area we were mucking around on, more as a concept than as a working model, and went off for the day.
At lunch time, after we pulled down camp, I went back to pack it up and was pleasantly surprised at the amount of water that had gathered on the walls and in the channel I had made. Not a cup full but it had only been up for about three hours.

Untitled.jpg

Here's my theory and where the brainaics come in.
I thought the black garbage bag would be more efficient as dark colours absorb more light and therefore would heat up more.
Being above ground rather than below, it would have a greater surface area being hit by the sun and therefore more heat, again.
I wasn't able to draw it, but I placed the bag on a bit of slope (Not by design but it worked out for the best) and put the hose in a lower section of the "channel" I created by turning the top inside and pegging it up with a few short sticks.
I weighed the bag with a few cleanish rocks in the channel (again, not shown)
The uprights and crossbar helped pull the bag tight(ish) to create an apex so that once the evaporated water condensated on the inside of the bag, it would run down the sides and into the channel, and from there to where the hose was.
If it was set up right, you might even be able to create a syphon. I didn't do it on mine but some vegetation might help.
Might even be good for salty or brackish water (It evaporates, leaving the salt behind?)
A secondary benefit might be that at night it would create a surface to condensate on.
My question are, do you think this would work? Is it simple enough to replicate? Anyone willing to try it to give an independent appraisal?
Cheers
Bloffy
 

Wentworth

Bear Mears
Staff member
Moderator
Joined
Jun 9, 2011
Messages
2,678
Reaction score
139
Location
Blue Mtns
Interesting. I've not made a solar still, just transpiration bag method.
Roh Hood showed an experiment in one of his vids where he used an opaque and a black plastic bag for the transpiration method. The black bag had lots of humidity inside but no liquid, the opaque one had plenty of water.
Let us know how you go with your experiment.
 

Browny

Mors Kochanski
Joined
Oct 8, 2011
Messages
325
Reaction score
0
Location
Perth Western Australia
It looks similar to something I saw on some doco some time back. Imagine the outfit as a cone with a flat top, inflatable ring like an inner tube at the base.
It floated on the water and the drinkng hose fed back into the inflatable liferaft the guy was in. It was clear plastic too.
 

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,833
Reaction score
160
Location
Albany, Western Australia
Yeah Browny, came across that one to, after I had posted this.
Funky bit of kit and a similar design but they are a dedicated device for a specific job. Don't think I would want one in my pack for just-in-case moments.
What I am proposing is for use in a S******L-type situation, adding to my water supply or purifying water perhaps.
Most of us carry some sort of garbage bag, whether it is for waterproofing or a S******L kit, or even, surprisingly, carrying rubbish out. etc.
Admittedly it would not usually be a Sulo bin size, although they can pack up pretty small and weight is negligble.
How many of us carry a piece of 1m square clear plastic sheet?
Other than for its purpose as a hole in the ground still, what can you use it for?
Also, apart from drycleaner bags or painters' drop sheets, where would you get it from?
I am also of the opinion that because it is lower than ground level it receives less light and therefore less heat to work.
Ever dug a hole and then lain in it? It is usually cooler than the surrounding air, if it is still.
Ever sat in a tent in the sun? Pretty hot and sweaty. (Yay hammocks).
Just putting the idea out there and seeing if someone can verify my thoughts.
Soon as I get a chance I'll knock one up myself and see how it goes.
Cheers
Bloffy
 

Aussie123

Never Alone In The Bush
Staff member
Moderator
Joined
Jun 16, 2011
Messages
5,303
Reaction score
391
Location
Melbourne, Victoria
Boffy, that’s a great innovation, even it its not totally “new”

Such a bag should work well, but it does depend on what you want to do with it.

Generally a black bag would be good for promoting evaporation, whereas a clear bag would be superior for transpiration. With “solar stills” I think there is a bit of confusion in the “industry” about what’s what.

For both systems (evaporation and transpiration) a condenser is also important.

Transpiration relies in using green (living) material. When sunlight hits the leaves, they photosynthesize and give off water. That water vapor is collected and condensed.

Evaporation occurs when you heat a “liquid”, the vapor can then be condensed and collected. A black bag will trap more heat than clear bag, so evaporation will increase.

So if the material you need to extract water from is green and leafy, you probably want a clear bag and to be thinking “transpiration”. If you have contaminated water (eg brackish water, sea water etc), then a dark bag will probably maximize evaporation.

Both systems rely on condensation to actually extract the vapor from the air and is an aspect of “solar stills” which is often poorly considered.

Without condensation, you will just have a bag full of hot, humid air !

Condensation occurs when warm, damp air is cooled to a point where the air can no longer “hold” the water. If this happens when the warm, damp air meets a surface, then it will condense as droplets; it can occur when a mass of cold air meets warm, damp air, we call that rain.

To have an efficient “solar still” you want to achieve a maximum amount of warm, damp air inside your still AND an efficient condenser, so the water can be extracted. If there were to be no condensation, then the sill would fill with warm, damp air, and you will end up with little, or no condensed water; the water vapor will be in equilibrium with temperature, and pressure inside the still; by extracting the vapor (by condensation) you will encourage more water to vaporize.

SO – the Boffy Still :
Big black bag is good and most suitable to extracting vapor from water, rather than green material.
The black walls of the bag will remain hot and not be conducive to condensation. A simple mod, like actively shading half of the bag (away from the sun), or placing it against a “cold” surface would increase the condensation.
Some suggested “cold surfaces” could be : a cold rock wall, damp bark (condensation will cool the bark). You can place a heatsink on top of the bag (like that rock in a traditional pit type solar still); however rocks (generally speaking rocks are silicon) are not very efficient heatsinks. Metal is a much more efficient heatsink so too is water, or wet material (eg a wet cloth, bark, soil etc).

So you consider how to introduce an efficient condenser to your setup.
 

Aussie123

Never Alone In The Bush
Staff member
Moderator
Joined
Jun 16, 2011
Messages
5,303
Reaction score
391
Location
Melbourne, Victoria
It looks similar to something I saw on some doco some time back. Imagine the outfit as a cone with a flat top, inflatable ring like an inner tube at the base.
It floated on the water and the drinkng hose fed back into the inflatable liferaft the guy was in. It was clear plastic too.
Sounds like a boat's solar still "The still will produce 500-2000 ccs of water per day"

SolarStillBoat.jpg
 

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,833
Reaction score
160
Location
Albany, Western Australia
@Aussie123. Thanks for your feedback. That is what I am trying to flesh out.
OK, so if I faced the still generally northwards so the maximum sun struck the surface through the day, and leant a leafy cut branch etc against the southern side, you believe that would make the still more efficient?
That makes sense and I'll give it a try.
I included the vegetation in my drawing as in traditional solar stills you put cut vegetation in there to help increase humidity, I believe. Ideally a shallow bowl of salty or brackish water would be ideal. Often that is obtainable where potable water is not.
My understanding of transpiration is it requires a living branch and it is the "perspiration" of the tree therefore would the colour of the bag enclosing the branch make a difference? All you are doing is trapping that moisture which is exuded through the tree. Am I correct?
I have used transpiration bags before and they seem to work as well at night as by day but I'm frequently wrong (Just ask my wife).
In my previous experiment, I placed the bag directly on damp sand and it seemed to be able to pull moisture off that.
Also, do you think enough heat could be generated to pasteurise the water (Approx 65C for 15 seconds, I think)? I'm just thinking back to the tent analogy. Would be interesting to find out.
Keep the brains ticking over and the ideas coming.
How much is 500-2000cc of water? Is that a cubic centimetre, which equates to a millilitre, therefore 500-2000ml?
If so, that is a brilliant return. By the way, those floating solar stills don't have a condenser.
Cheers
Bloffy
 

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,833
Reaction score
160
Location
Albany, Western Australia
@Aussie123. Thanks for your feedback. That is what I am trying to flesh out.
OK, so if I faced the still generally northwards so the maximum sun struck the surface through the day, and leant a leafy cut branch etc against the southern side, you believe that would make the still more efficient?
That makes sense and I'll give it a try.
I included the vegetation in my drawing as in traditional solar stills you put cut vegetation in there to help increase humidity, I believe. Ideally a shallow bowl of salty or brackish water would be ideal. Often that is obtainable where potable water is not.
My understanding of transpiration is it requires a living branch and it is the "perspiration" of the tree therefore would the colour of the bag enclosing the branch make a difference? All you are doing is trapping that moisture which is exuded through the tree. Am I correct?
I have used transpiration bags before and they seem to work as well at night as by day but I'm frequently wrong (Just ask my wife).
In my previous experiment, I placed the bag directly on damp sand and it seemed to be able to pull moisture off that.
Also, do you think enough heat could be generated to pasteurise the water (Approx 65C for 15 seconds, I think)? I'm just thinking back to the tent analogy. Would be interesting to find out.
Keep the brains ticking over and the ideas coming.
How much is 500-2000cc of water? Is that a cubic centimetre, which equates to a millilitre, therefore 500-2000ml?
If so, that is a brilliant return. By the way, those floating solar stills don't have a condenser.
Cheers
Bloffy
 

Aussie123

Never Alone In The Bush
Staff member
Moderator
Joined
Jun 16, 2011
Messages
5,303
Reaction score
391
Location
Melbourne, Victoria
OK, so if I faced the still generally northwards so the maximum sun struck the surface through the day, and leant a leafy cut branch etc against the southern side, you believe that would make the still more efficient?

- Orient it sideways (ie lengthways) to the sun. Some kind of active cooling would be good to assist in condensation. It needs to “remove” heat so the vapour will condense, so shading is the simplest. If you had a water supply (eg sea water or other “contaminated” water), a wet rag etc would be even better


I included the vegetation in my drawing as in traditional solar stills you put cut vegetation in there to help increase humidity, I believe. Ideally a shallow bowl of salty or brackish water would be ideal. Often that is obtainable where potable water is not.

- Shallow bowl, preferably dark in colour and perhaps with some mass to it. The mass will help absorb heat and will hopefully stop a tip over if the system gets disturbed (knocked or hit by a gust of wind). You could place some heave enough rocks in there too to keep it in place (if it did blow around in the wind, you may contaminate the cleaned water)


My understanding of transpiration is it requires a living branch and it is the "perspiration" of the tree therefore would the colour of the bag enclosing the branch make a difference? All you are doing is trapping that moisture which is exuded through the tree. Am I correct?

- Yes - A clear bag for transpiration, no light = no photosynthesis = no water (or not as much as a clear bag any way – you would still get some water)
- If you don’t have a transpiration bag, Blake sells them in the bcoz store :;):


I have used transpiration bags before and they seem to work as well at night as by day but I'm frequently wrong (Just ask my wife).

- Interesting. I wonder if it was “transpired” water or condensed .... ? Either way its water which you can use. I think its worth bearing in mind that there is a big difference between what “should” be efficient and what you can actually do even with a non-ideal setup. Its always worth trying with what you’ve got – you have nothing to lose – and where possibly try and use sound principles to maximize outcomes ....


In my previous experiment, I placed the bag directly on damp sand and it seemed to be able to pull moisture off that.

- Yes, that’s a great tip. I think that’s about the only way to get water out of damp ground which I can think of. I was actually going to pose that one as a question in its own thread some time .... after finding a “native well” which was really only damp sand !


Also, do you think enough heat could be generated to pasteurise the water (Approx 65C for 15 seconds, I think)? I'm just thinking back to the tent analogy. Would be interesting to find out.

- Solar cookers are certainly more than capable. I don’t know about a black or clear bag. You will need to put a thermometer in the bag as see how hot it gets.
- Water which is transpired or condensed should be clean, although there is a strong change of cross contamination
- One thing to bear in mind is that some pollutants (in chemically contaminated water) and some plants which are toxic, may not be able to be “purified” using these techniques


Keep the brains ticking over and the ideas coming.
How much is 500-2000cc of water? Is that a cubic centimetre, which equates to a millilitre, therefore 500-2000ml?

- Yes. Its “half a litre (500ml) to 2 litres” - or so the advertiser claims.


If so, that is a brilliant return. By the way, those floating solar stills don't have a condenser.

- Good point. Think how much better they could be with one ! I think that the sea water (underneath) helps act as a condenser. Air moving over and around the dome will also act as a condenser and any sea spray. It could be that they tried various mechanisms and decided that this was the best configuration for both ease of use and efficiency. Its a trade off and although “fancy” mechanisms may produce better results, after testing they may not be significant enough to warrant the effort ?
- For instance you may find that the still can produce 500ml of water per day (say), and with a fancy condenser etc it may produce 10% extra 550ml. It may not be worth the cost and effort – unless you are really stuck !
 

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,833
Reaction score
160
Location
Albany, Western Australia
Thanks Aussie 123. It helps if someone can put a bit science behind the ramblings.
As soon as I get a chance (Busy time with work), I'll try this out, even in just a simple mode.
Cheers
Bloffy
 

Browny

Mors Kochanski
Joined
Oct 8, 2011
Messages
325
Reaction score
0
Location
Perth Western Australia
I am only guessing but if there was enough moisture in the air it would condense easier anyway. The air would keep the surface of the bag at outside temps while the inside would get hotter due to being a closed environment. Even clear plastic should be ok, ever been in a greenhouse or a closed car in full sun?
 

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,833
Reaction score
160
Location
Albany, Western Australia
Yep, done a bit of reading and I think your right. It's amazing what's out there when you start looking. The old Dubya.dubya.dubya net sure is a big ol' scary place. And they say the bush is a scary place.
Nice to see some interest in my crazy idea.
Cheers
Bloffy
 

Bushdoc

Malcolm Douglas
Joined
Apr 6, 2013
Messages
34
Reaction score
27
For a portable, DIY installation:
Why not use a small inflatable wading pool? About 60cm diameter. Air/watertight.
Fill with bad water or vegetation, place a cup in the middle of the floor, stretch the clear plastic over the top...should work well.
 
Top