Idea: Solar watering
Attach a solar panel to a peltier cooler, let water condensing on the cold side on a teflon pad run down a tube to the ground. With the device mounted on a hollow stake, the water can be delivered to the ground near the plant without losses due to evaporation. The hot side of the peltier would be connected to a heat sink and passively cooled.
Let’s estimate how much water can be generated. Dave Jones at EEVblog helps with his assessment of a kickstarter scam that uses the same idea for a self-filling water bottle, link. 2264 kJ to condense 1 liter of water = 629 watt-hours (Wh). And he helpfully mentions that peltiers are typically 50% efficient.
Cooling water to the dew point (~15C in Illinois in the summer, ~30C in Arizona) takes 4kJ / degree C per liter * 15 degrees = 60 kJ, a negligible additional amount. A peltier device can easily cool 15-30C below ambient 25-40C outdoor temperatures.
Let’s go with a 10W solar panel, figure 50Wh per day. Let’s figure 25% efficiency for power->cooling. So 12.5 Wh / 629 Wh = 2%, so the device would generate 20 ml of water. With more sun or better efficiency, could get 40-80ml of water.
A water drop is 50ul, so 20ml is 400 drops. Over 10 hours, the device generates an average of 2 drops of water every 3 minutes.
How much water does a plant need? A quick search gives estimates per week ranging from 15-30 ml / day (link) to 2.25 liters / week (link), or 320 ml / day. A solar water generator can make water on the low side of this estimate, so enough for maybe one plant.
Cost
$30 10W solar panel
$20 peltier
$2 heat sink (used)
$10 wires, stake, teflon (patch or tube)
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$62 total.