Economic Multipliers (111)  
Do you know what these are?
They help CREATE wealth in systems.
Knowing what children (and adults) need to know is an economic multiplier for everyone.
The United States has non-stop debates regarding what children need to know as they ‘get educated’ through high school.

I hope that every person in the world who engages in that debate in any country asks if all children are taught how to build a solar still (more appropriately entitled a radiative cooling still since you also want a unit like this to continue to work at night) … just in case they ever need clean water.

Drinkable water is a basic need and if you ever need it, it could be one of the most important things you prioritize on in the midst of a crisis.  Clean water:
  • keeps you hydrated and helps your body function properly and eliminate wastes,
  • helps you maintain a healthy body temperature,
  • keeps you from getting headaches or lightheaded,
  • keeps you from getting physically ill from water-borne contaminants and
  • in the absence of other physical problems, allows you to respond to ‘events’ that are surrounding you.
In the midst of any emergency where water was an issue, I’d want a group of people in any community setting up radiative cooling stills and collecting as much potable water as possible.

If fuel was available, I’d want to take advantage of the fact that water that can be boiled (or even heated to a higher temperature) distills faster.

Sources of water include just about anything that has moisture (exclude volatile organics such as fuels and oils):
  • soil with moisture
  • plant material … preferably chopped up
  • sea water
  • muddy water
  • brackish water
  • urine
  • even air (if it tends to be humid)
The elements of a still include:
  • a collector for the clean water (such as a cup or pan),
  • a condensing surface that slopes toward the collector, and
  • an enclosed space that allows the temperature and moisture levels in that space to be higher than the levels externally

The Operation

The collector:  The collector must be accessible so you can get the clean water.  In an optimal world, you wouldn’t lose any of the ‘function’ of the still but emergency designs usually feature a collector in the middle of a still.  In the process of retrieving collected water, you can lose heat and the value of any air that already has an elevated moisture content:  You then need to re-establish the internal conditions of the still to get it working again.

The condensing surface:  If you need sunlight to penetrate to the source of moisture to heat the source and the surrounding air, the condensing surface optimally will be clear.  If you can design a system that simply collects the heat and supplies warm moisture laden air to a cooler surface, the condensing surface will preferably be metal and white (on the side exposed to the ‘sky’).

The enclosed space:  This is the space where you hope moisture will evaporate into the air.  The higher the temperature, the more moisture the air can hold.  If the temperature outside the condensing surface is sufficiently low (based on the internal temperature and humidity of the air on the inside of the condensing surface), water inside the unit will literally come out of the air. 

People observe water ‘coming out of the air’ when dew forms on plants, when frost collects on roofs and other surfaces and when moisture drips down a glass holding some cold fluid.

Previously (No. 55), I noted some critical numbers which you need to understand to understand this process.  Look at these numbers and see if they would make any sense to you:

100 … 598 (∆NA) … 212 (NA or N/A is many times an abbreviation for ‘not applicable’)
95 … 505 (∆93) … 203
90 … 428 (∆77) … 194
85 … 355 (∆73) … 185
80 … 294 (∆61) … 176
75 … 243 (∆51) … 167
70 … 199 (∆44) … 158
65 … 161 (∆38) … 149
60 … 130 (∆31) … 140
55 … 103 (∆27) … 131
50 … 83 (∆20) … 122
45 … 65 (∆18) … 113
40 … 51 (∆14) … 104
35 … 40 (∆11) … 95
30 … 30 (∆10) … 86
25 … 23 (∆7) … 77
20 … 17 (∆6) … 68
15 … 13 (∆4) … 59
10 … 9 (∆4) … 50
5 … 7 (∆2) … 41
0 … 5 (∆2) … 32

The first and last numbers on each line are the same temperature in Celsius and Fahrenheit (water boils at 100°C or 212°F and freezes at 0°C or 32°F).  The middle number is the approximate value for how much water the air (at a standard atmospheric pressure) at 100 percent humidity has for each temperature (in milligrams per cubic meter or mg/m³) … the ‘saturated’ value.

If the temperature during the day is 30°C or 86°F and the relative humidity is 50 percent, the air around you (if you’re near sea level) has approximately ½x30 = 15 mg/m³ of moisture in it.  If you look further down the column, you’ll note that somewhere between 15-20°C or 59-68°F, the air (with that amount of moisture) will be at 100 percent relative humidity and the moisture will start to condense on surfaces.  If the air/surface interface temperature drops below this ‘dew point,’ the air wants to get rid of some moisture (it cannot accommodate that much).

The interesting numbers for me are the delta (∆) numbers (the differences in the amount of moisture the air can hold at a certain temperature interval (in this case, 5°C or 9°F) for various temperatures).  At higher temperatures, the air can hold much more moisture.

A solar still or radiative cooling still/unit takes advantage of different temperature and moisture levels.  Give a properly designed unit the ‘right numbers’ and you can pull clean water out of the air.

If you can raise the temperature and moisture content in an enclosed space, when the moisture laden air hits a sufficiently cool surface, moisture will condense on the surface.  If the surface is designed so the moisture can run off to a collector which accumulates the condensed moisture, you have a source of water.

If you ever do design any type of still, keep in mind that your condensing surface and the collector must be kept clean or you can lose any value that you were trying to gain.


P.S.  You can find many pictures of solar distillation and other types of distillation units.  In the midst of an emergency, you use what materials you have:  That is why it is so important to understand the concepts of radiative cooling, condensing surfaces, moisture content of air, relative humidity, evaporation and sources of moisture and heat:
  • Clear water might only need to be boiled if you have fuel (no distillation at all).
  • Other sources of water are easier to distill if you have fuel.
  • Organic (plant) material is easier to get water from if you know the local plants that have the highest moisture contents.
  • Soil is easier to get water from the more that you can raise the temperature:  Of course, the soil has to contain moisture to start with.
  • If you’ve ever seen dew or frost or fog, even the air can be considered a viable source of water.
P.S.(2)  When you want water to evaporate into air, you want to increase contact areas.  If you can set up wicks or baffles, you can increase the contact areas.  Likewise, if you can take advantage of sufficient ‘coolth’ (like the radiative cooling capacity of a clear night sky), increasing the contact area of the condensing surface(s) also makes a difference.

P.S.(3)  If the most basic of ‘needs’ are not ‘emergency needs’ in the midst of a disaster because communities immediately know how to effectively (even if only temporarily and inefficiently) create those resources, recovery is much, much easier.