Now here’s an IndieGoGo campaign for a water bottle that condenses the water vapor from the air and into your bottle. All by using solar panels!

Would I absolutely love to have such a gadget? A water bottle that fills itself? Of course I would (even if drinking pure condensed water isn’t a good idea). But it’s bogus, and here’s why.

The short version

Such systems do exist, known as atmospheric water generators and dehumidifiers, but they use more energy than can be provided by a portable solar panel. Also, larger devices are more energy-efficient, and the efficiency of the device further depends on the ambient temperature of the air and relative humidity.

Want the longer version? Here you go.

Modern solar panels work at 20% efficiency (that is, they convert 20% of the solar energy into electrical power). That is, on a good sunny day you’d get around 150–200 Watts per hour out of an 1 square meter solar panel.

Now let’s take a commercial dehumidifier — a device taking water out of the atmosphere — as a point of reference. Which is a lot more efficient, than any portable atmospheric water generator can ever hope to be.

The typical dehumidifier uses around 1 kW per hour to remove around 1.2 liters of water from the air.

With a 50cmx50cm solar panel (which is still a lot larger than the one Fontus shows) you get max. 50 Watts per hour. So if you were using it to power a commercial dehumidifier, you’d need around 1000/50/1.2=16.6666… hours to collect 1 liter of water. With a 25cmx25cm solar panel? Around 66.6666… hours. And that is, if you have this amount of sunny hours with the sun on the zenith. That’s almost 3 days of uninterrupted sunlight.

And that is, if the relative humidity of the air doesn’t drop below 30%, and the ambient air temperature is above 18.3°C.

Want an even longer version? Here it is, my geeky reader.

Condensing water vapor into liquid water is an exothermic reaction — it releases energy. Your device needs to take the water molecules from air, use a device to absorb and dissipate their energy, after which these molecules will be trapped on a hydrophilic surface inside the device. When enough molecules have gathered on the surface, they will form droplets of water, which in turn will have enough mass to be affected by gravity and flow into your bottle. For these droplets to form, you have to cool the air to its dew point, which is different for different air temperature and relative air humidity values. Magnus formula is a well known approximation to calculate the dew point for different air temperatures and relative air humidity values.

Sounds simple enough, doesn’t it? So how do you absorb thermal energy from water molecules in the air? You cool the air. How do you dissipate the energy? You vent it out.

This is how your air conditioner and your fridge work and any active cooling system works.

For more efficiency, you use a small cooling surface and a large dissipating surface. How to get larger dissipating surface in a smaller package? Radiator fins. And these, despite of being relatively compact, do take up most of the space in any active cooling solution.

Cooling your car’s engine is pretty simple and it doesn’t require an active cooling system, because your engine is hotter than the air around your car, so you can just use the air to absorb the heat of the engine.

But how do you cool the air? How do you get lower than ambient temperatures? There are several ways to do that, and one of the most wide-spread is using a compressor.

So here’s what a compressor does: it compresses a gas, which is an exothermic (energy-releasing) reaction. Then it dissipates the thermal energy through the radiator fins. So now we have a gas under pressure but at the same temperature. So at this point, the gas has lost energy without changing its temperature. When you release the gas, it expands because of the pressure and absorbs energy (endothermic reaction). So if we make the gas absorb the energy of what we’re trying to cool, and release energy through the radiator, we’ll get the result we needed, right?

Here’s where the second law of thermodynamics comes into play. To absorb the energy from water molecules in the air, even with a we’ll need to spend the same energy first to dissipate the energy of the gas in the compressor — and that’s with an energy conversion efficiency of 100%, which, of course, is impossible in normal conditions. Also, you can’t just take and selectively cool the water molecules in the air — you’ll have to cool the whole air.

Mind it, you’d need a lot of air. At 32 °C, which is very hot, and 30% air humidity, you’d get around 65 grains of water vapor in each pound of air. One grain of water is 0.0648 grams. So you only get 4.2 grams of water vapor per pound of air, and one pound of air is around 351 liters. Which is to say, you’d have to cool A LOT OF AIR, and not all of it will turn into dew.

And this explains why atmospheric water generators aren’t as effective as the Fontus IGG campaign would want you to believe.

As always, have a nice day, and thanks for reading.