乾燥地域において大気水収集の効率化が鍵となる

薄い空気からの清潔な水

In many regions of the world, fresh water comes at a premium. One option for access could be desalination, but this is so far a very energy-intensive approach, even if some progress in solar tech has made it closer to reality recently. Seawater desalination is also not an option for many inland arid regions, like for example central Asia, Mongolia, Chilean mountains, or most of the Sahara Desert.

Another option is to capture the water present in the air. Many deserts actually have rather high atmospheric moisture, but for climatic reasons, they do not form rain and clouds. This is the promise of atmospheric water harvesting. We previously covered how passive cooling mixed with a silicon coating could boost the efficiency of gravity-based water harvesting.

Researchers are now looking at further improving the method, notably by using new types of polymers. Scientists at the Osaka Metropolitan University performed this research, which was published in ACS ES&T Water under the title “Liquid Polyether-Based Water Harvester for Near Ambient Temperature Operation via Hydrophilicity-Difference-Induced Water Transfer¹.”

水の吸着

A method to harvest water from thin air is to use a polymer able to “adsorb” the water. Adsorption is the capacity of solid substances to attract molecules of gases or solutions with which they are in contact with their surfaces – in this case, gaseous water vapors and droplets of water suspended in the air.

In everyday life, we know adsorbent materials like silica gel in commercial packaging. This is actually a rather efficient process, and it can collect a lot of ambient water.

出典: Osaka Metropolitan University

The issue is making the polymer release that water. Usually, heat of around 100°C is required to desorb these polymers, which, of course, makes it very energy-inefficient and costly. In contrast, the new polymer developed in Osaka is different.

低温脱着

The issue with releasing the water from the polymer is that the very characteristics that make a polymer a good adsorbent are also what makes it poor at letting it go, hence the need for high temperature to make it happen.

Previously, the researchers tried to mix together poly(ethylene glycol) (PEG) and oligomeric poly(propylene glycol) (PPG), creating a “hydrophilicity-difference-induced water transfer (HWT)” mechanism.

In simpler terms, the difference in water adsorption between the two types of polymers created a weaker capacity to hold onto the adsorbed water. This allowed for the creation of a liquid moisture adsorbent that requires only a temperature of around 35°C / 95°F to do so.

However, this limited the mixture’s water adsorption capacity, making it less productive even if it was much less energy-intensive.

PEG-PPG 共重合体

In the latest experiment, the researchers tried to mix a PEG-PPG copolymer with PEG instead.

出典: ACS Publication

This mix boosted the HWT effect even further, resulting in the best possible result:

• 回収温度が低温のまま維持された
• 水収集速度は、通常最も水を吸着するPEG単体よりも実際に高くなった。

PEG-PPGランダム共重合体は、PEGを上回る最高の水収集速度を示しました。改善された水回収は、HWTの相乗効果と水クラスターの崩壊によるものです。

応用例

The low temperature required to collect the water and regenerate the polymer radically changes the potential of this technology. First, it removes the need for energy-consuming high temperatures, radically improving the green credentials of the idea.

“この技術の改善は、温室効果ガスの削減と水資源のより効率的な利用につながると期待されています。今後は、液体状湿気吸着剤を改良し、システム全体の効率を高め、実用化を目指します。”

Masahide Takahashi – 大阪公立大学 教授

Secondly, because 35°C / 95°F is much easier to obtain, it could be created just by leveraging the day-night cycle, especially in desert regions where the day is rather hot, like most deserts around the tropical zone (Sahara, Middle East, India, Mexico & South-West of the USA, etc.).

Colder regions like Mongolia could be more tricky, but maybe a very simple metal foil mirror could help reach the 35°C threshold when there is sun. This opens the path for the completely passive system, automatically generating water with very little effort.

The same technology could also be quickly deployed in regions that urgently need clean water.

“この技術は、乾燥地域やエネルギー資源が限られた場所への水供給だけでなく、災害や緊急時における水へのアクセス確保にも応用できる可能性があります。”

Arisa Fukatsu – 大阪公立大学 助教

Further development could be built on this research as well. For example, nothing proves that the polymer mix discovered here is the optimal combination.

In theory, many other mixes of hydrophilic and slightly less hydrophilic polymers could do the job. So now that the principle has been demonstrated, a systematic exploration of new water adsorbing materials could help improve yield even further, bringing the technique closer to commercial viability.

水不足を解決する企業

Xylem Inc.

Together with the European Veolia, Xylem is a global leader in water purification, wastewater treatment, and desalination. It employs 23,000+ (of which 6,000+ engineers) people and operates in 150 countries, with a focus on the USA, with 35,000+ direct industrial customers.

Its main market is municipal drinking and wastewater, but it also provides dedicated solutions to other sectors like healthcare, power, food & beverages, oil & gas, microelectronics, etc.

出典: Xylem

Xylem can provide the critical patented pieces of equipment to clean or produce water like ozone generators, UV lamps, desalination membranes, ultra-pure water generators, etc. But it also provides “simpler” equipment equally critical to water-related operations like turbines, pumps, piping, injection, software, etc. as well as maintenance, repair, and installation services.

出典: Xylem

The water market is still a very fragmented one, with Xylem one of the largest companies in the sector but still holding “only” a 10% market share out of its $80B served addressable market.

The company spends around 4% of its sales on R&D. It should benefit from new regulations regarding PFAS (Per- and polyfluoroalkyl substances, or forever chemicals), with 6,000+ utility facilities needing such PFAS treatment.

Xylem has been growing steadily, with net income growing from $297M in 2012 to $609M in 2023 while keeping a stable 17-219% EBITDA margin.

Overall, this makes the company’s investing profile less like that of an industrial company (often cyclical) and more like that of a utility company growing with the overall economy or a little bit above that rate, like most of its consumers.

研究参考文献：

^{1. Ikegawa, D., Fukatsu, A., Okada, K., & Takahashi, M. (2024). 親水性差誘起水移動による近常温動作液体ポリエーテルベース水ハーベスター. ACS ES&T Water. https://doi.org/10.1021/acsestwater.4c00775}