The paper in Communications Chemistry is here: https://go.nature.com/2rTyjgu
When I started to synthesize materials for thermochemical heat storage in 2014, I was not expecting to end up with a composite that can support fresh water production in arid regions. It soon became clear that the challenge of heat storage with salt hydrates and fresh water production are closer related than expected.
Hygroscopic salts such as CaCl2 can absorb water from air in an exothermic hydration reaction and release the water when the temperature is increased This reversible reaction can be used to store heat, but at the same time it is possible to utilize these properties to facilitate the condensation of water from air.
During nighttime, air is lead through the bulk material and water is absorbed (even at very low relative humidities). During daytime, the material is heated up by solar energy and water is desorbed. The resulting hot and humid airstream is then returned to ambient temperatures and liquid water condenses. No auxiliary source of electricity is necessary for the cooling and condensation, making the process energy efficient and usable in remote regions. Optimal working conditions are regions with low relative humidity and lots of sun, which makes it ideal for regions that face scarcity of fresh water.
The major challenge was to develop a composite material that allows for a bulk application with low pressure loss upon gas diffusion and a high salt content. To overcome this obstacle, we utilized an ionotropic gelation method to incorporate up to 80 wt.-% of CaCl2 into a macroporous alginate derived matrix forming spherical beads. The resulting beads can be used in bulk with the ability to absorb large amounts of water from relatively dry air. Inside the alginate derived matrix, the salt is distributed as small particles preventing agglomeration and therefore the formation of passivation layers, which is a general difficulty in using salt particles in reversibly hydration and dehydration cycles. Moreover, the microporous matrix offers enough pore volume to buffer the volumetric change of the salt during hydration and dehydration, as we were able to cycle the material multiple times without any observable degradation in water uptake/release or fracturing of the composite beads.
We also modified the reaction conditions and incorporated other salts such as SrCl2 and MgSO4 to potentially allow for other applications, for example the passive control of the relative humidity or, as initially intended, the thermochemical storage of heat.
In the next step, the material has to be tested in a prototype to evaluate the amount of water that can be generated in practical application conditions.
The paper titled “Water harvesting from air with a hygroscopic salt in a hydrogel–derived matrix” can be found here: https://go.nature.com/2rTyjgu
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