Question from a newbie...to this board anyway. Been going over transpiration bags/ stills and the consensus seems to be that clear plastic/bags are necessary. My questions are why won't an opaque bag work? Since we use reflective/lighter colors to reflect heat and darker/black to soak up heat why wouldn't a black bag work better as it would "soak up" more heat from the sun? Thanks in advance for the ideas.

All plastic provides the vapor barrier needed to trap condensation from a solar still/transpiration bag. Whether black performs better would have to be tested. The biggest reason clear plastic in used in the military is because you can easily see how much condensation has been trapped without opening the bag/removing the plastic to check on it. Every time you check, you release the trapped moisture (like checking the pot on the stove). I'm not sure you would want hot plastic anyway, since the condensation builds because of the differential of heat between the earth/plastic, or leafy tree limb/plastic. It may work fine, but not being able to see the progress without checking is not worth it for me. Great question... keep the gears turning!
George

black plastic stops the radiation, and soaks the heat up on the TOP ! so everything beneath it will stay cool ! so water in the ground will NOT evaporate ! clear plastic lets heat though and traps it in side the still / bag and let the earth heat up / lets the water in the leaves evaporate

I believe the idea behind the transpiration bags is to collect and condense the water vapor given off by a tree's leaves which is a by product of photosynthesis. If you use black plastic you will wilt the leaves and may be drain some water out that way but stop photosynthesis and with no new water coming in, put and end to the still.

The Australian Army did extensive testing (published as "Survival Water in Australia's Arid Lands", by Kavenagh) in the 80's. Transpiration means breathing. The stomata on the leaves open up when distressed to evaporate water to cool the leaves. The stomata also close in the dark, as inside a dark bag. This reduces yield significantly.

I was told by the former OC of the Army Survival School in Darwin that opaque bags also break down due to the heat generated and can generate phosgene-like offgas which of course goes into the hot water inside the bag. Why take the risk? Clear 44-gallon drum liners are the issued item here, 2 carried behind the kidney-pad on most army rucks.

Hi Soticgrad,

As you know, the idea is to heat the moisture in the leafs to evaporate it and then to condense the vapor to a liquid on the plastic. Since we are talking about solar as the source of heat, we are talking about radiated heat. Radiated heat, unlike conduction (movement of heat from solid mass to solid mass) or convection (movement of heat from molecule to molecule of gas in the air) is not warm until it strikes a solid dark object. Upon striking the dark object, radiation converts to heat. (simplified explanation)

In a solar still or transpiration bag we want the leaf to be heated in order to vaporise the moisture but we want the plastic bag to be as cool as possable so the moisture will condense on the cool surface to become a liquid again. A clear bag will allow the sunlight (radiation) to pass through the bag without converting to heat where we dont want the heat and to continue on to the leaf where we do want the heat. If the bag is dark, it will heat up and become the hottest part of the system and condensation cannot take place on the hottest part of the system.

The more clear the bag the better the still will work. You want the light to go through the bag, not heat the bag. the more opaque the bag the less light will go through but will heat it.

So a still has TWO different parts. A boiler, to heat the water and a condenser, to condense the water to a liquid.
The leaf becomes the boiler and the plastic bag becomes the condenser. If you heat the condenser, it will NOT condense.

Neal

Some physicist correct me if I am wrong.

i always thought the reason to have clear bags was 2-fold, especially for a solar still.
1. Clear plastic lets in the infrared radiation from the sun but doesn't allow it to get out again, thereby raising the temperature of the contents in the bag which aids in evaporation of tainted water to water vapour (in the case of the solar still). That's why in solar cooking, to increase the efficiency of the cooker/pot, it is encased in glass/ clear plastic.

2. In the case of the transpiration bags, it is to allow the plant part in question to continue to photosynthesize and thus transpire and produce the water.

I think the reason why condensation occurs is because of the SATURATION of the water vapor in the air trapped in the bag and not really due to difference in temperature between the air and the bag itself. That's why you are told not to open up the bag too often, because that reduces the saturation of water vapour in the bag and thus the water producing capacity....

Please correct me if I am wrong, because that's the way I understand it from my knowledge of physics and biology....


<img src="images/graemlins/smile.gif" alt="" />

Clear pastic lets sun light in, which builds-up into heat. The plastic sheet traps it inside.Period....

The clearer/transparent the cover, the more light will pass thru it

Much like leaving your car in the sun with the windows closed.


D.

Make sure you experiment with solar stills prior to depending on them in a life or death situation. You may be disappointed in the results or I should say the out put in some situations.

Ted Fisher
VCSAR Coordinator <img src="images/graemlins/confused.gif" alt="" />

An excellent point. Has anyone here actually dug a solar still and extracted water? What were your results? I sure haven't.