Water Harvesting

Decentralized water is an important strategy to meet the needs of rural populations and where infrastructure challenges limit the feasibility of centralized water treatment and distribution networks. Atmospheric water harvesting presents a potential form of decentralized drinking water which is especially relevant in areas where liquid water is physically scarce. State-of-the-art atmospheric water generators, or dewing systems, utilize refrigeration cycles to cool air below the dew point in order to condense the water vapor into a liquid. However, these systems cannot practically operate in arid climates with low RH and/or low temperature (where dew points are often below freezing) due to the significant electrical energy consumption per mass of water required for sensible cooling to bring the air temperature below the dew point. Atmospheric water harvesters utilizing adsorbents present a potential solution to this problem by expanding the operating regime.

At the DRL, we are using adsorbent materials to capture water vapor from the air [1,2]. These materials have a high porosity and affinity towards water vapor molecules which promotes a high concentration of water vapor in the material, thereby allowing atmospheric water harvesting in environments where dewing systems can't operate.

In the basic process, the adsorbent is exposed to the air and water vapor is adsorbed by the material. To release it, the material is heated, the water vapor is released into an enclosed area and is condensed with little energy required for sensible cooling. While the process is still energy intensive because heat must be supplied to overcome the enthalpy of adsorption, an advantage of using adsorbents is that the materials we are using can be regenerated at temperatures which can be achieved using low-grade waste heat or solar-thermal heat sources. A number of adsorbent materials have been reported in the literature with favorable properties for atmospheric water harvesting [3].

The efficiency of adsorption systems is dictated by heat and mass transport in the material. We developed composite material layers which enhance the heat transfer properties to more effectively conduct heat to the adsorbent for regeneration. We created a detailed modeling framework which was experimentally validated to predict the performance of an adsorbent packed into a porous matrix [1,2]. This model connects mass transport across length scales, considering diffusion both inside a single crystal as well as macroscale parameters, such as thickness of the composite adsorbent layer and packing porosity. The model allows us to see a tradeoff between water which can be harvested per mass of adsorbent (a) and per adsorbent layer area (b) as a function of these parameters [3]. We are using these modeling tools to design solar-driven adsorption-based atmospheric water harvesting devices with better thermal efficiency.

We demonstrated atmospheric water harvesting using direct sunlight at a thermal efficiency of 14% in Arizona [2], a low RH environment where it is infeasible to use conventional refrigeration-based dewing systems. The device was predicted to deliver over 0.25 liters of water per kg of adsorbent material daily [2].

The work has been featured here and here in MIT News.

This work is funded by a Seed Grant from the Abdul Latif Jameel World Water and Food Security Lab.

  1. H. Kim, S.R. Rao, S. Narayanan, E.A. Kapustin, S. Yang, H. Furukawa, A.S. Umans, O.M. Yaghi, E.N. Wang, "Response to Comment on "Water harvesting from air with metal organic frameworks powered by natural sunlight"," Science, 358(6367), 2017.
  2. H. Kim, S. Yang, S.R. Rao, S. Narayanan, E.A. Kapustin, H. Furukawa, A.S. Umans, O.M. Yaghi, E.N. Wang, "Water harvesting from air with metal organic frameworks powered by natural sunlight," Science, 356(6336), 2017.
  3. A. LaPotin, H. Kim, S. R. Rao, E. N. Wang, "Adsorption-based atmospheric water harvesting: impact of material and component properties on system-level performance," Accounts of Chemical Research, 52(6), 2019.
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