Embedded Files
Economic Multipliers (55)
Do you know what these are?
They help CREATE wealth in systems.
Understanding the significance of numbers can be an economic multiplier even if you hate the math.
¤¤¤¤¤¤¤
I’m probably one of the few people who values extreme humidity when the temperatures get very hot (90+°F or 32+°C).
I know that if there is a clear sky at night (or it just simply gets cool enough), dew will form, giving the plants a greater chance to thrive or survive.
A lot of math, physics, chemistry and even some challenging to read charts stand behind the formation of dew but most people just know that weather stations typically report on daily highs and lows for temperature along with the relative humidity and the expected weather patterns. They’ll note the dew point if they think it’s relevant.
The dew point is the temperature (a surface temperature for practical purposes) where you would expect moisture that is in the air to condense on surfaces (if the surface/air interface gets cold enough).
What if I gave you these rough numbers? Would they 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
If you pay attention to the weather or are a scientist, you’d probably see that the first and last numbers on each line look like 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 grams per cubic meter or g/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 g/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 moisture will start to condense on surfaces. If the air/surface interface temperature drops below the ‘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. If you plot a graph of temperature versus the 100 percent humidity values in g/m³ that are listed above, the graph is not linear. Therein (from my perspective) lies the advantage that equatorial zone nations have over other nations if they want to take maximum advantage of passively powered solar distillation facilities (basically just finding a way to use all the ‘free’ energy that they get).
If you can saturate the air at higher temperatures (particularly at night which would mean you’d be storing some of the sun’s energy ‘below’ because heat rises) while keeping the surface temperatures of condensing surfaces cooler (particularly during the day by shading and at night with radiative cooling), you should get much higher efficiencies if you’re setting up solar powered distillation units than if you just set up a ‘box’ filled with water with a sloping glass on it.
Likewise, if you wanted to maximize the probability that the air would want to remain saturated in the distillation unit, wicks or mesh rising up from any water surface would greatly expand the available surface area for evaporation.
I have never built a solar distillation unit but if I ever had a need, I’d want a unit with a small ‘footprint’ … something that would collect heat from the sun at the base and which could possibly be insulated at night … that released the heat to the ‘brackish’ water in a somewhat insulated (from the outdoors) tank or tray with wicks or mesh rising from it … with an angled collector (the interior of the roof) with an angled drip tray underneath that was as shaded as possible during the day but completely open to the radiative cooling of the night sky … with a rainwater collection system in case it rained.
You’ve probably figured out that the moisture containing components of this distillation unit would have to be ‘enclosed’ because its efficiency relies upon maintaining temperatures and moisture content.
You probably also ‘just know’ that the roof should optimally be metal due to its high conductivity: I’d want copper or enameled metal and would want to think about ‘enhancing’ the inner surface so water vapor would more easily condense on it and drip off but after going through a budget analysis, I’d probably opt for glass (painted white on the outside) or coated steel with some sort of metal drip strips hanging down. If the drip strips helped support the roof and ‘direct’ the condensate, so much the better. And I’d just want to verify that the selection of my materials didn’t have any adverse effect on the water quality of the condensate or create any problems with ‘residuals’ … leftovers like salt and more concentrated brine.
Although a silvery roof surface would reflect heat during the day if it couldn’t be fully shaded (it’s harder to shade when the sun is directly overhead unless you set up a removable shade), I’d want the exterior of the roof to be white versus silver.
White is a particularly ‘cool’ color. During the day, it likes to reflect light (similar to silver) but at night, it likes to release heat (similar to black and the concept of a ‘blackbody’). If you want to get rid of heat, you don’t want it absorbed and if it’s there, you want it to be ‘emitted:’ Not surprisingly, the word ‘emissivity’ is used when identifying how easily a surface might give up its heat.
A solar distillation system like this is called a passive system … not because there isn’t work … but because it tries to take advantage of locally available resources (in this case, the sun and starry nights) as they ‘show up.'
This ‘passive’ system requires delivery of water. A ‘prototype’ designer might first deliver the brackish water in buckets … then move to some sort of pumping system. Batteries might power a pump and be recharged by the sun or wind or wave power (the batteries alone require maintenance). The water level in the brackish water evaporation tank might ultimately be fitted with a low/high water level indicator which switched the pump on and off as needed.
The only sure thing if you’re setting up a small solar powered distillation unit is that there will always be too many options and you’ll never think that you have enough money or time to test them all.
And, because I never built a prototype, keep in mind that all of these ideas might seem great (to an interested person) but I’d want to make sure the unit was easy to clean and maintain. I don’t know about you but I lean toward ‘lazy’ … not the ‘lazy’ that says I shouldn’t try to accomplish things in life … but the ‘lazy’ that says that I don’t want them to be ‘too hard’ to accomplish or create a lot of extra unnecessary work along the way.
Appreciate that if you’re not an ‘engineering type’ and you’re still reading this, I’m impressed. Perhaps you’re expecting that at some point in time you’ll be in a room full of people who are discussing how to increase potable water supplies locally or globally and you’ll be the one who asks how much sunlight the area gets and whether brackish water is available: The oceans and seas have a LOT of ‘brackish’ water.
¤¤¤¤¤
This article contains no equations (but there are some great ones if you’re interested) … I also didn’t confuse you with the pressure component: Air at higher pressure can hold more water and air at lower pressure can hold less. That is why commercial distillation units which normally require purchasing energy (not a problem if the economics justify it and you have local resources like the sun but choose not to deal with climatic variability or the different design and operating characteristics) tend to create pressure when air is being ‘saturated’ and tend to create a partial ‘vacuum’ when air is condensing.
Likewise, numbers like these usually come with a caveat: atmospheric pressure at sea level. Unless you’re relying upon dew and collected condensation for moisture or you live in the mountains, these numbers for the average person are usually ‘close enough.'
Now, if an ‘engineering type’ who was working on solar distillation units ever got really bored, they might be able to figure out a way to create an intermittent partial vacuum (even if the unit they were working on wasn’t completely airtight) to ‘coax’ more water out of the saturated air. Since I’ve never worked through any of the equations on this though, I’d make sure that the ‘math’ supported the design: Usually you get less than what you calculate optimally so if the ‘math’ doesn’t work, the design probably wouldn’t either.
It’s possible to create partial vacuums with things like fans, rubbery bladders that go in and out, bellows, pistons, etc. It’s also possible to create pressure with the same types of things but it’s more difficult to hold pressure in a system that’s not completely sealed while waiting for water to further saturate (evaporate into) air than create partial vacuums for brief moments to enhance condensation.
The unique advantage of living in areas with higher solar insolation in comparison to the rest of the world is that ‘blended’ systems (particularly for large systems that serve large numbers of people which incorporate passive and active designs) can give communities much greater ‘bang for their buck.’ Something as simple as preheating water (with solar) before it is fed into a distillation unit automatically increases the unit’s efficiency.
And since I use the words ‘radiative cooling’ a lot, know that on a starry night, out in the open, any exposed surfaces want to ‘cool to’ space (which is a tad bit colder). It is possible for a metal disk placed on an insulative pole to become colder than the surrounding air (which is why, if you ever got lost in the wilderness, you’d never want to sleep out in the open if you were trying to stay warm).
Didn’t you just want to know all of this?
¤¤¤¤¤
If you are interested in solar distillation, the publication: The Nature of Observation: Two Saltwater / Wicking Experiments
is a photo log which shows how wicking and siphoning can help increase efficiencies while helping prevent the buildup of 'salts' on any surfaces.
Report abuse