Simple Thermodynamic Watering Device (no Electricity)

my first instructable describes a thermodynamic - air powered irrigation tool, "automagic" without electricity.

The summer is here, at least in the northern hemisphere, I would like to share the system I used last year in order to keep my plants in vase with enough water, for not dying when I left for the holydays. Maybe someone else already used this idea, I do not know, or maybe something similar, anyway that's what I realized. Before this I had built up a device with micro, servo and solar cells plus accumulator trying to keep alive those plants and flowers (even if very few ones, I felt very uncomfortable with the idea of leaving them dry out because of my absence). I live in a flat with a balcony that in summer gets very hot, and sunny, and it is also impossible to ask the neighbors to throw some water to my vases. I have to say that the servo system always worked fine... then I had the idea of a non-electric device. It is also very cheap and easy to build, which also uses most recycled stuff; it needs only plastic bottles, straws and a hot glue gun. This BOM is already available in every home, I think.

Let's start with the device's sketch:

it consists of a bottle with a hole near the bottom in which a straw is hotglued so that (apart the straw) everything is air-tight. The bottle is then filled with water about half of the maximum, and the the cap closed hermetically again. (the water won't get out of the straw even if the water level is higher than the straw end because of the airtight cap). Almost finished! To make a test, put the bottle in the sunshine, the air temperature will raise, so its volume (while the pressure remains approx constant) and some water is so pushed outside the straw. During the night, when air temperature decrease, and so its volume, some air from outside the bottle will bubble inside the bottle, replacing the water that got out during the day. Since the temperature change is periodical, each day some water will exit from the bottle to a strategically positioned vase (until obviously it all finishes out). It can be adjusted to a max of 2-3 weeks period, even more but less reliable. The water initially contained in the bottle is the one that will feed the plant during the whole absence period; the high-low temp cycle is the mechanism that allows for not having it out all at once, only a limited quantity per day.

Moreover the simplicity (no electricity), this system has other advantages: 1) the watering is lower in the first days, when the terrain is supposed to be left humid before leaving the house. 2) in case of a typical stray summer rainy day (with likey smaller delta temperature) less water is also delivered.

In this step could be helpful to understand the (easy) physics behind the mechanism; it is very important that the bottle is not filled at the beginning with water: if the inhalt of air is too small then the mechanism won't start, or will be minimal and ineffective; initial conditions reasonable values ranging from 1:1 up to 3:1 water/air ratio, with typical delta temp.

The system mainly exploits the difference of temperature between night and day, and the equation of gas: PV = nRT ; better read as V = nR/P * T ; so considering the pressure P constant, the atmospheric pressure, the volume V of a gas is proportional to the absolute temperature T (math is not needed to understand the principle, however with this equation one can estimate the amount of water available each day). In few words, during the day, especially if in full sunshine, the air contained in the bottle (best: rigid plastic and dark colored) heats up and pushes some water out of the bottle, in the night the air temperature decreases, so its volume, some air is pushed from outside into the bottle. The next day, with temperature re-rising, the cycle starts again (with more air inside) and so on until the available water finishes. The higher the delta T, the better the system works.

The system is linearly scalable, so immediately made 2x or 3x and so on (could be difficult to find very big bottles anyway..).

An example estimation of water output assuming the conditions:

Bottle capacity : 2L

Initial water inside: 1L

Daily delta T : 15°K

Maybe the easiest part; some practical sense is needed to place the parts in the right way depending on one's own house/conditions.

Test and place.

Before to use it is strongly recommended to make some tries, for instance putting the device in the sunshine to increase temperature one can meausure the water in output, or spraying it with fresh water to decrease temp and see the bubbles of air entering the bottle.

With tests one can verify if the typ quantity of water in output is the desired one.

Variations:

1. The straw is strictly not needed for the bottle to work. Ideally a simple small hole in the lower end of the bottle (provided that air cannot get at the same time that water gets out) is enough, since the air tight lid does not allow the water to get out immediately. The daily volume change due temperature cycle can still cause the exchange of water and air. This variation has anyway some small disadvantages: first, less freedom to place the bottle (only on the terrain). Second, missing the water travel in the straw, the system is even too ready to exchange in response on every temp change; for example clouds passing in front of the sun and so more cycles high-low temp within the same day. With the straw there is some water-air moving inside it in response to small temperature change, without water exchanging out-in, a sort of hysteresis that keeps the system more stable.

2. Possible use for inside. Unfortunately the usual room temperature is very stable, maybe it cannot even reach 1°K difference between day and night - so what described before is practically useless. The only possibility I see, to have something like this for indoor plants, is to search for places, maybe close to a sunny window, even if closed, where some 5°K delta T can be reached. With such a low delta T anyway a big ratio air-to-water is needed, a 2-bottle structure like in the picture is suitable, and if the plant cannot be moved, the 'air' bottle is to be put near the window, and the 'water' bottle near the plant.

3. Not only summer holydays. Since the key point for the system to work is a reasonable delta of day and night temperature, in case a good delta T is avalaible also in other seasons, then the system can be used (only if min temperature is above 0°C, for keeping the water liquid :) .

If all the tests are OK, the system is ready for operation.

Please keep in mind all the elementary security criteria for building the devices, and when positioning them in place.

Hope it will be useful.