Providing the global population with clean drinking water is one of the great engineering challenges of the 21st century. In some countries, more than half the population lacks access to clean water, and globally one in three people do not have access to basic sanitation for which water is crucial.

This is a significant cause of diarrhea and poor health in general. By some estimates more than 5,000 children die every day as a result of diarrhea-related disease. So finding ways to produce clean water is an important goal.

The problem is that most techniques are prohibitively expensive for poor countries. Traditional approaches such as distillation, reverse osmosis, and wastewater recycling are energy-consuming and expensive. And passive techniques that rely on solar energy require exotic materials and solar concentrators, which are bulky and expensive.

But there is another technique that has the potential to change this depressing calculus: dew harvesting. This involves cooling air so that the water vapor it contains condenses so that it can be collected. “This passive technology holds great potential for fresh water harvesting due to the fact that a significant amount of water vapor is stored in the atmosphere,” say Minghao Dong and colleagues at Southeast University in Nanjing, China.

That raises some obvious questions. Just how much water can be harvested in this way? And what is the best way to gather it?

Today, Minghao and colleagues calculate the fundamental limits of dew-harvesting technology for the first time. They then describe how a simple change to conventional techniques could significantly improve utility and yield.

First some background. Passive dew harvesters consist of a “condenser,” a thin, flat sheet of material that radiates heat into the night sky (dew harvesters generally only work at night). The condenser is insulated from the ground so that it cannot absorb heat from below.

As the condenser radiates energy at night, its temperature drops, cooling the air immediately adjacent to it. If the temperature of the air falls below the dew point (the temperature at which air is saturated with water vapor), the vapor will condense.

Of course, the efficiency of this process is sensitive to a wide range of factors, particularly the ambient temperature of the air, its relative humidity, and the rate at which the condenser can radiate heat.

Over the years, physicists have calculated how much water such a device can produce when the condenser is a perfectly radiating black body. But Minghao and colleagues say all these analyses miss an obvious point: they do not properly account for the way real materials emit heat or for the way Earth’s atmosphere transmits some wavelengths of light more efficiently than others.

“As a result, the fundamental limits of this technique have not been properly clarified, making it hard to evaluate the performance of the experiments and to determine whether or not this technology is applicable under various conditions, in particular in relatively arid areas,” they say.

So they have included these factors for the first time. This has allowed them to assess how different materials will perform.

Their method is straightforward. Minghao and colleagues point out that the wavelengths at which the Earth’s atmosphere is most transparent are well known. They say it makes sense to use a condenser that emits at these frequencies rather than one that emits across all wavelengths. They call such a condenser a selective emitter and compare it with the performance of a black emitter

The results are eye-catching. The researchers say that matching the emissivity of the condenser to the transmissive characteristics of the atmosphere makes significant improvements possible. For example, at an ambient temperature of 20 °C (68 °F) with a relative humidity of 40%, a black emitter cannot harvest water by any means. “In contrast, the selective emitter could [harvest dew at the rate of] 13 grams per square metre per hour,” they say.

That’s a significant finding. It’s the difference between being able to harvest dew at night in a place like the Mojave Desert and having no water at all.

The researchers have designed a condenser with the required energy-emitting characteristics. Their design consists of thin layers of three different materials on an aluminium base. This layered structure emits best at the wavelengths at which the atmosphere is most transparent.

That’s interesting work with the potential for wide application. Minghao and colleagues say dew harvesting could be beneficial in both humid and dry areas: “The former includes islands and coastal cities which are surrounded by seawater that is not potable, while the latter include deserts which lack of any form of drinking water.”

And the low cost of this kind of passive design is important too. “This passive fresh water harvesting technology would complement existing technologies, especially in rural and low-income areas where the cost is a big concern,” they say.

If it can bring clean drinking water to even a small fraction of those currently without, it will be a significant gain for humanity.

At least one startup, Zero Mass Water, is already trying to commercialize a similar device that can draw water from the air while other scientists are continuing to push their capabilities, including a collaboration among researchers at the University of California, Berkeley and MIT (see “How to pull water out of thin air, even in the driest parts of the globe”).

Ref: arxiv.org/abs/1909.05757 : Fundamental Limits of the Dew-Harvesting Technology