The theory made sense so I experimented.
I used translucent milk containers for my greenhouse cover but the system seemed inefficient. There was evaporation and condensation but the roof of my dome was flat and I suspected that not all the water that condensed slid down the sides of the vessel. I suspected that some simply fell back into the water tank below.
So I moved on and today made these models -- pictured below -- from clear pastic drink bottles. These roofs have a curved surface so I'm hoping the fluid will definitely slide down the sides of the dome and into the soil below.
Will this design work?
When you check these irrigators there's a lot of condensation on the inside surface and yes the soil moistens underneath where the sides of the dome rest on the earth.
However, I considered that my first experiment failed because I was disappointed in the rate of evaporation. In time the water may turn green from algae but it seemed content to stay in its chamber.
My present materials are supermarket standard (Coke, Mineral Water, some other soft drink, etc):
- 1.5 litre bottle
- 2 litre bottle
Because my tank chamber is large -- maybe holding close to a litre of water -- the circuit of evaporation and condensation should -- or could -- be self sustaining for a long time.
Since I have no idea what the rate of condensation will be nor what the wetted soil will do with its fluid intake, I thought I'd ring a single cucumber seedling with my devices and see what happens.
No doubt elements are in play that could undermine success.
The first is that the deeper the water tank, the cooler the water will be and the less it will evaporate.
The second is that the soil itself will insulate the water and keep down its temperature.
But on the other hand, the dome chamber is sure to get very warm in conditions of 12 hours + of sunlight. So maybe an observation I can make is to now and then take the water temperature.
Another marker is how much the level of the water in the tank falls over a given period.
I can then adjust the rig.
- Don't bury the tank
- Trim the height of the water tank and just locate it on the soil before placing the greenhouse dome on top.
The complication with the second option is that my greenhouse chamber will be smaller.If the system is driven by the amount of hot air inside the greenhouse, the more hot air -- as in volume -- the faster and more efficient will be the evaporation of water.
The other view could be that the real driver of the system is heat delivered by sunlight directly to the water and not just the heated 'air' inside the greenhouse chamber.
Since the initial water flow that has poured out of my garden hose this Summer every time I turn on the tap seems boiling, maybe I have a benchmark as to the sort of water temperatures I should strive for?
What I have here is a mini greenhouse ecosystem whose 'weather' I get to manipulate.