Can we solve the freshwater problem by taking moisture out of the atmosphere with dehumidifiers? When we look at how the hydrologic cycle does its work, we get a straight and clear answer: no, we don’t solve the problem!

In Germany, the construction of Tesla’s Gigafactory near Berlin draws its attention, including its substantial need for freshwater. Despite its many lakes, the area around Berlin is among the driest in Germany. The atmosphere contains water vapor, and it seems like a tempting source for freshwater, just sitting there to be harvested by some form of technology. This is what a company claims to do (and quite a few others elsewhere as well). But can this promise hold up?

Dehumidification is conventional technology

Let’s first talk about the technology. There is nothing magical about removing water from moist air. In fact, it is everyday technology, and you can buy a dehumidifier in the store. We used to have one in our bathroom to remove the moisture after our family took hot showers in the morning, the towels are wet, and the mirror fogs up. A dehumidifier comes in handy here. It takes in the moist air from the bathroom, expands it, this cools it down, water condenses, and when the air exits the machine, it is warmer and drier, while the removed water is collected in a container. This requires energy for running a motor that does the expansion and compression of the air, which is supplied by electricity. And by dehumidifying the air, our towels can dry and the mirror defogs.

So what this company claims to do is basically to take the dehumidifier outside, and instead of dehumidifying bathrooms, it wants to dehumidify the atmosphere around Berlin and sell the extracted freshwater. But can this work? Does it create more freshwater than what the rainfall in the region would naturally supply?

Hydrologic cycling: the natural dehumidifier of the Earth

To figure out if this technology can achieve what it claims to do, we need to look at how the cycling of water in the Earth’s climate system actually works. The Earth’s atmosphere essentially operates just like the dehumidifier in the bathroom. Instead of water being collected in a container, we see the dehumidification in action when it rains. But that’s only one part of the picture. The other half is the process that does the opposite and adds moisture to the atmosphere that eventually becomes freshwater: evaporation.

In the bathroom, evaporation happens when we take hot showers. Some of the hot water evaporates into the bathroom, making the air more moist. Without the hot showers, there is nothing to dehumidify in the bathroom. In the Earth system, evaporation happens at the surface because the sunlight heats the surface, and most of the Earth’s surface are oceans. With a warmer surface, the air near the surface gets warmer, and moister at the same time because warmer air can hold more moisture. The warming causes the air to rise, while the moistening is caused by evaporation. This evaporation process takes up a lot of the energy that was supplied by the Sun. For each liter of water evaporated, it requires about 2.5 Megajoules of energy (the so-called heat of vaporization). For the Earth, this is a tremendous energy investment. It results in more than half of the absorbed sunlight at the surface being used to evaporate water in the global mean, reflected in what is called the latent heat flux.

That is the primary bottleneck that limits the Earth’s hydrologic cycle: the energy input of the Sun. This energy input is partitioned at the surface into a net cooling by radiation (surface emission minus greenhouse effect), heating air and causing updrafts, and moistening air, driving hydrologic cycling. This partitioning is strongly constrained by the physics and thermodynamics of the system. In fact, these constraints are so strong such that the magnitude of the hydrologic cycle and its sensitivity to global warming can be estimated in a relatively simple and transparent way.

How to get more freshwater?

What this means is that when the atmosphere is noticeably dehumidified by some form of technology (be it dehumidifiers or other attempts, like cloud seeding), there is less dehumidification left to do. It inadvertently reduces rainfall elsewhere, likely somewhere further downwind. So we do not gain more freshwater, we just alter the patterns of where it is available. Technologies that aim to get more freshwater by dehumidification (or that aim to enhance rainfall, like cloud seeding) are thus doomed to fail – they cannot supply more freshwater at large scales because they do not act to reduce the dominating energy limitation of the hydrologic cycle.

If we wanted to get more freshwater, we would need to start at its source and invest more energy. Ironically, a good example for this can be found not so far away in Brandenburg. The coal-firing power plant Jänschwalde near Cottbus, one of the largest in Europe, uses exorbitant amounts of groundwater for cooling. The cooling sink for the plant is the energy source for considerable, additional evaporation, which we can see in form of the white clouds emerging from the cooling towers. When this power plant operates at full capacity of 3000 MW and an efficiency of 35%, it evaporates 2200 liters of groundwater per second, or 190 million liters per day!

So, may be, we do not need more freshwater in the region after all. If the transition to renewable energy would be accelerated, and groundwater would no longer be needed for the cooling of the power plant, there would be ample supply of additional freshwater within the region.

More information:

Link to ARD Kontraste video which discusses the need for water, Musk‘s unawareness of the climatic conditions in Brandenburg, the claims by the technology, and my response (in German).

Link to background information collected by Quarks magazine about the water needs of Tesla‘s Giga factory.

Some of our own work on this topic:

Kleidon, A., Renner, M. (2013). Thermodynamic limits of hydrologic cycling within the Earth system: concepts, estimates and implications. Hydrology and Earth System Sciences, 17(7), 2873-2892. doi:10.5194/hess-17-2873-2013.

Kleidon, A., Renner, M. (2013). A simple explanation for the sensitivity of the hydrologic cycle to surface temperature and solar radiation and its implications for global climate change. Earth System Dynamics, 4, 455-465. doi:10.5194/esd-4-455-2013

Kleidon, A., Renner, M. (2015). Geoengineering ist keine Lösung — Der globale Wasserkreislauf im Klimasystem. Physik in unserer Zeit, 46(1), 27-31. doi:10.1002/piuz.201401381.