Recently, Dutch daily newspaper ‘de Volkskrant’ reported on a novel way of harvesting liquid water from very dry air. The new way of extracting water from air is based on a class of porous materials called Reticular materials, which were invented by Professor Omar Yaghi. The article “MOF water harvester produces water from Death Valley desert air in ambient sunlight" gives the numbers [1]. That is nice; for most subjects, Reden Makes Sense has to collect numbers before the subject can be discussed. This is a bonus point for the Yaghi method, compared to the ‘Sun Glacier’ (see: ‘Sun Glacier, free water or mirage’).
How does it work?
Air, even very dry air, contains some water vapour. At night, the relative humidity is (generally) higher because the air temperature is lower. There are materials which absorb water vapour, and release it when they are heated. The ‘water harvester’ of Prof. Yaghi et al. uses sun light for the heating. The rise and setting of the sun gives the rhythm of absorption during the night and release during the day. The final step is to make sure the released water vapour condenses. For this, the ‘water harvester’ contains a heat sink. Finally, liquid water drips out of the device.
The ‘water harvester’ uses a novel material, MOF-303 (metal organic framework) that is extremely porous. 1 kg of MOF-303 can store 400 g of water. It was tested in very dry, sunny conditions, in Death Valley, often called the driest place in North America [2].
The paper reports impressive results, obtained with Relative Humidity of roughly 26% during the night, and 15% in the day. Temperatures were 25 – 35°C at night and around 50°C during the day. The water harvest under these conditions was measured to be 20.7 g per 100 g of sorbent material; 7.2 g for the test device.
Discussion
Water in itself is not scarce on average; two thirds of the globe is covered by oceans, which have an average depth of 5000 m. For drinking and growing crops, however, humans need water without salt (sea water has a salt concentration of roughly 3.5 %, implying that salt, too, is present in very large quantities on earth!)[3]. The possibility of harvesting it from air was subject of an earlier Reden Makes Sense. It was concluded that energy consumption was high, making other means of supplying drinking water more attractive than harvesting it from air. Alternatives are desalination and transport from a fresh water source.
The big plus of the Yaghi water harvester is that it is passive, requiring no energy source, no salt water supply, and no disposal of anything.
Nevertheless, at present scale, the water that is harvested can be considered ‘a droplet on a red hot plate’ (Dutch expression, meaning ‘an insignificant contribution’). If we use the surface area taken up by the device (0.4 x 0.4 m ?), then it corresponds to a daily water production of 0.007 litre/0.16 m2 or the equivalent of 0.04 mm of precipitation. Per year, that accumulates to 1.6 cm. The average rainfall in Death Valley is ‘less than 5 cm’ [4]. This means that the test in Death Valley shows the extraordinary capability to extract some water from such a dry atmosphere, but it does not demonstrate a useful purpose.
How much water is needed?
There are many possible scenarios, but a modest requirement is the daily water consumption of one human. In Death Valley conditions, this is very high, perhaps 5 litres per 24 hrs. However, although the device was tested in extreme conditions, it is not likely that it will be placed there, so let’s assume that it has to support a more modest demand of 2.5 litres per 24 hrs. Let’s also assume that it performs 10 times better in moderate conditions than in the dessert. This means 2500 g water must be supplied at 72 g generated per prototype device, so per person, 35 prototype device are needed, or (more likely) a device 35 times bigger in volume than the prototype. This is about a 3.3x scaling in each dimension, which would mean a footprint of about 1.3m x 1.3m if the surface is correctly estimated from the picture at 0.4m x 0.4m. This seems reasonable, but does assume a factor 10x improvement in performance.
For agriculture, the device would have to be scaled up much more. What would it cost to harvest water in this way?
Cost
The most exotic part of the water harvester is the material. According to an article in Nature [5], “If you’ve got a tonne of MOF-303, you could deliver about 500 litres of water a day, every day for five to six years.” This would mean a tonne of MOF-303 can give 1100 m3 of water in six years.
The cost of MOF can be taken from a paper in Faraday Discussions (MOFs industrialization: a complete assessment of production costs, 2021)7: $30 per kg, later perhaps $ 10/kg. If we take the ten dollar low value, the cost of the MOF would be $10,000, or $9 per m3 of water. The current consumer price for excellent drinking water in the Netherlands is €1.22 per m3, and the energy cost of desalination can be found in the Reden Makes Sense about the Sun Glacier: 5,5 kWh per m3, which would cost, for a consumer in the Netherlands, around €1.82 per m3. These numbers do not give the complete cost of the alternative methods (except the Dutch drinking water), as most or all durable equipment is not counted, but it is clear that the water harvester, at the present state of the technology, does not provide cheap water.
Conclusion
Both the Sun Glacier and the water harvester based on MOFs can extract water from air, even in the desert. The water harvester can do this using the power of the sun, and does not require any other energy input. The MOF material does degrade over time, which makes the cost per m3 quite high. It is true that a traveler in a desert will give anything for drinking water once his or her provision has run out, but neither technology can compete with desalination of sea water from an economic point of view. The traveler is better of bringing sufficient water for the duration of the trip.
Nevertheless, the prospect of harvesting water from air and using it to grow crops and sustain life in desserts is attractive. Let’s hope the technology advances to the point that it can deliver on this promise, although it still has a long way to go.
[1] Woochul Song, Zhiling Zheng, Ali H. Alawadhi & Omar M. Yaghi
[2] In the rest of the world, drier places exist. At the South Pole, the air can be much drier, according to Antarctica's climate: the key factors - Discovering Antarctica. although water is abundantly available in the solid state.
[3] a salt crust (density roughly twice that of water) over the whole land area of the earth would have a thickness 2/3 * 0.035 * 5000/2*3 m = 175 m!
[4] Weather - Death Valley National Park (U.S. National Park Service)
[6] MOFs industrialization: a complete assessment of production costs - Faraday Discussions (RSC Publishing)
