Global practical potential for metal–organic frameworks assisted atmosphere water harvesting

doi:10.21203/rs.3.rs-2003923/v1

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Global practical potential for metal–organic frameworks assisted atmosphere water harvesting

https://doi.org/10.21203/rs.3.rs-2003923/v1

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Nowadays, freshwater shortage remains a global challenge. Sorbent-assisted atmospheric water harvesting (SAWH) is a promising strategy for extracting atmospheric moisture to provide arid regions with potable water. As appropriate sorbents are crucial for efficient SAWH, many novel sorbents have been emerging in recent years. However, the lack of efficiency benchmarks prevents accurate evaluation of sorbents’ practical potential in SAWH system, thereby hindering location- and climate-specific sorbent selection. Herein, reliable models were established to analyze the global SAWH potential of metal–organic frameworks (MOFs) in terms of practical water yields and energy requirements for passive and active SAWH respectively. Moreover, geospatial guidance of the efficient MOFs-assisted AWH deployment was provided based on a thermodynamic framework in combination with high-resolution global weather data throughout a year with seasonal climate variation. Applicable sorbent-assisted water harvesting strategies were then developed for six water-scarce regions. Overall, this study established benchmarks for location- and climate-specific adsorbents that will expand the application of sorbent-assisted water harvesting technologies in effective off-grid water-supply systems in water-scarce regions.

Freshwater shortage is one of the most critical global problems threatening social and economic development. Four billion people – two-thirds of the world population – experience severe water scarcity during some part of each year¹, and this number is projected to increase in the coming decades due to global warming, population growth and water pollution². Globally, efforts have been focused on assisting the survival of arid and semi-arid communities by ensuring they have reliable access to potable water ^3–5. In addition, to limit the plastic pollution caused by bottled water, on-site water supplies are imperative. The atmosphere contains as much as 13,000 trillion liters of water and is thus a promising sustainable source of water for arid decentralized communities, as atmospheric water can be harvested using cost-effective and scalable devices ^3,6.

In the past five years, numerous off-grid atmospheric water harvesting (AWH) methods have been explored^7–9. The working principle of AWH is the separation of moisture from air and its condensation into liquid water. AWH can be achieved via three methods: (1) collection of fog with large nets¹⁰, (2) cooling air below its dew point¹¹ or (3) sorbent-assisted water capture¹². Fog collection is the simplest method but is limited to geographic niches where fog is common and can be systematically harvested^13,14. Dew-harvesting usually consumes a large amount of energy due to excessive condensation and is inefficient in arid regions where dew points are less than 10°C^13,15. In contrast, sorbent-assisted water harvesting can be achieved using a selective water sorbent that can take up water from atmospheric vapour and release it with upgraded concentration. The condensation of water from the moisture-enriched air can then be implemented under a relatively high temperature without redundant energy loss. Once well-designed, the sorbent-assisted approach is neither spatially nor temporally restricted, so can in principle be employed anywhere and anytime¹².

Sorbent-assisted AWH (SAWH) devices can be powered passively by renewable energy sources such as solar thermal energy and can thus be used in dry and sunny regions^7,9,16. A solar-driven intermittent SAWH device is the most common scheme for water vapor extraction at nighttime, when relative humidity (RH) is higher than during daytime. Extracted water vapour is subsequently condensed to form freshwater during the daytime, when solar energy is available^4,17. Notably, passive systems are always triggered by natural driving forces with inherent strength and rhythm, which will otherwise dissipate if not exploited timely. Therefore, attempts have to be made to maximize the compatibility between the passive system and the energy source, namely, to yield as much water as possible under the specific circumstance. Contrarily, as discussed later, energy consumption during the process seems less significant since non-exploited energy in a passive system will eventually dissipate. Recent studies also point out, by employing artificial energy sources with adjustable source temperature and operational frequency, SAWH can be released from the restriction in quality or rhythm of natural driving forces. Such system is named after active SAWH. At the price of sufficient active energy input (to manipulate operational parameters), active systems are able to produce water with flexible yield under a wider range of weather conditions. Therefore, evaluating the energy consumption per unit yield in such a system is quite significant.

In both active and passive SAWH, sorbents are critical for achieving high-efficiency SAWH. Empirical performance indexes of water sorbent include but are not limited to high porosity, medium pore size, narrow porosity distribution (thus can be driven by small temperature span), stability and recyclability. Based on the above criteria, sorbents’ water uptake capacities, kinetics and photothermal desorption properties have been substantially improved over years of research, and various sorbents have been developed for SAWH, such as MOFs^18,19, hydrogels^20,21, and composite sorbents^16,22. However, perfection at the material level does not necessarily lead to systematic improvement. Actually, due to the inconsistency in the evaluation criteria of SAWH, whether and how these material properties would affect the overall performance of SAWH remains unknown.

Another gap in the current research is the deviation between the design and practical conditions, since sorbents in SAWH are normally assessed under a fixed condition while actual weather data fluctuate spatially, diurnally and seasonally. For instance, researchers have focused on exploring the performance of sorbents under extreme drought conditions (< 20% RH or even < 5% RH). This may not be a practical solution for SAWH promotion and application, for two reasons. First, the adsorption process usually occurs at night, and in most regions of the world, including deserts, the RH at night is scarcely lower than 20% and may reach 50% RH. Second, sorption isotherms for MOFs (Fig. 1), such as those for MOF-801 and MOF-303, indicate that sorbents with a low RH step inflection point (4–20% RH) usually feature low adsorption capacities (0.2–0.4 g/g). Whereas materials with higher stepwise position feature doubled or even tripled moisture capacity. Thus, one question must be answered: is it necessary to sacrifice water adsorption capacity to achieve a low RH step inflection even though it is only superior under rarely-emerged extreme conditions?

To address such concerns, consensus on the evaluation criteria from the systematic perspective, for both passive and active SWAH, should be clarified at first. Afterward, it is imperative to establish reliable modeling methods to calculate the values of criteria corresponding to different materials, justifying sorbents' properties and clarifying the benchmark for applicable adsorbents. Notably, the calculation process should employ transient weather data of the local region to take account of the spatial and timely maldistribution of temperature, humidity and solar radiation, thereby bridging the gap between experimental observation and practical application.

Hereby, based on the working principle of SAWH, specific water yield and specific energy consumption are respectively proposed as the performance indexes for passive and active systems. A reliable quantitative mathematical model to derive these indexes under arbitrary weather conditions is then established, by which the potential of candidate water adsorbents can be assessed from the systematic level. Notably, adsorbents’ performance is calculated based on the daily local, transient weather data and then averaged annually to account for the spatial and timely maldistribution of temperature, humidity and solar radiation. In virtue of the constructed thermodynamic framework and the world's high-resolution weather data, our work provides theoretical guidance for the worldwide deployment of MOFs adsorbent when incorporated in SAWH. To the best of our knowledge, this is the first study to characterize the global potential of MOFs in SAWH. Moreover, appropriate sorbent-assisted water harvesting strategies were developed for six water-scarce regions. Thus, this study established benchmarks for location and climate-specific adsorbents that expands knowledge on SAWH technology and may enable their use as a basis for off-grid water supply in water-scarce regions.

Due to the narrow porosity distribution and the consequent formation of hydrogen-bonded water molecule networks within the crystalline structure, MOFs tend to have step-shaped water-sorption isotherms and thus exhibit high water uptake and release capacities under small temperature or pressure gradients. These step-shaped isothermal curves also mean that MOFs usually feature highly specific climatic suitability and regional applicability.

In this study, we investigated example the following three key types of MOFs, which are classified according to the location of inflection points on their water-sorption isotherms in Fig. 1a. MOFs with isotherms that have a low inflection point (α < ~0.15 RH) strongly interact with water, which means that they can harvest water from the atmosphere in extremely arid regions with RH below 0.2 only occurring in the hinterland of Sahara according to the global weather data. MOFs with isotherms that have an inflection point at α =~0.2–0.4 RH facilely desorb water, which means that they operate in a relative pressure range that renders them suitable for harvesting water from the atmosphere in arid regions. MOFs with isotherms that have inflection points around α = ~0.65 RH on their adsorption branches and α =~0.45 RH on desorption branches typically have a high adsorptive capacity but exhibit hysteresis (hollow blue dashed-line circle). This means they usually require higher temperatures and lower RHs to achieve desorption and are thus suitable for harvesting water from the atmosphere in wet regions. Besides the inflection step points, MOFs’ water-uptake capacity is another important indicator of their potential utility in SAWH: the higher the water-uptake capacity of a MOF, the more effective the MOF is as a sorbent in SAWH. Generally, materials with front inflection points tend to have lower water uptake capacities.

We studied the following six advanced MOFs, which are representatives of the above three categories: MOF-801 (step point: ~0.08 RH)⁴, MOF-303(Al) (step point: ~0.15 RH)²⁴, CoCl₂(BTDD) (step point: ~0.29 RH)²⁵, Zn₂Co₃(MFU-41) (step point: ~0.40 RH)²⁶, MIL-101(Cr) (step point: ~0.45 RH)²⁷ and Cr-soc-MOF-1 (step point: ~0.69 RH)²⁸. The isotherms of these MOFs are shown in Fig. 1b, and their detailed properties are shown in Supplementary S1.

Figure 2a presents the principle of humidity control in the adsorbent. The water vapour – adsorbate – exists in three forms: stray water outside the substrate, free water within the substrate pores or water trapped in the adsorption potential field of the substrates. At equilibrium, the adsorbate concentration in pores (Y_d) is equal to that outside the pores (Y_a) and it is correlated with the concentration of adsorbed adsorbates (W_d) indicated by the sorption isotherms. All of them are sensitive to temperature change and isotherms at a higher temperature are typically on the right side of the lower temperature. A linear case is depicted in Fig. 2a. If the values of three variables (Y_a, Y_d, W_d) deviate from equilibrium, the mass transfer of adsorbates occurs immediately.

Specifically, under low temperature of T_d,AD, dry desiccants exhibit a low adsorbate concentration Y_d,AD1 in their nano-pores that is in equilibrium with their moisture content of W_DE. The humidity difference between the air inside (Y_d,AD1) and outside (Y_amb) causes moisture to be captured from the air above the gas-solid interface (I-I"), thereby increasing the moisture content and adsorbate concentration of pores. A heat source is applied when the desiccant is nearly saturated. Despite this heating, the adsorbed moisture W_AD cannot escape in time, while the free water in desiccant pores promptly finds another balanced point (Y_d,DE1>Y_d,AD2) with the adsorbed phase under the desorption temperature T_d,DE (I’-III). A humidity difference is generated between the pores (Y_d,DE1) and incoming flow (Y_ai,DE), and moisture is discharged into the air (III-III"). Over time, the adsorbent dries (W_DE), and the water yield per unit mass desiccant, ΔW, is defined as the difference between W_DE and W_AD. The moisture can be replenished by cooling the desiccant (III’’-I) and then re-establishing a new balance (I-I").

Namely, solid desiccants can adsorb the water vapor from low-humidity air and then release the moisture with upgraded concentration once thermally driven. Based on the water enrichment effect, SAWH can be implemented via adsorbing under an ambient environment, desorbing high-concentration vapor and condensing the moisture-enriched air. A typical workflow can be seen in Fig. 2b. As illustrated, the air obtained at the outlet of condenser should be directed into the inlet of the desorption bed, recycling the uncondensed water and improving SAWH yield.

The psychrometric chart in Fig. 2c presents the state change of humid air during adsorption and desorption processes. Once equipment with sufficient heat source and exchange area, isothermal processes can be achieved in both adsorption and desorption phases. During the adsorption phase, the outlet air states undergo I-I’-I" processes over time until the moisture content of the inlet and outlet are consistent, which indicates saturated adsorption. During the desorption/condensation process, the state of the outlet air changes from point V to point III; then the humid air is condensed from point III to point IV, which is the saturated air at dew point, and is finally condensed to point V with the temperature of T_CD. IV-V represents the liquid-water-production process, and ΔY_rate denotes the water yield per unit mass from air, namely, specific water harvesting rate (SWHR).

As shown in Fig. 2d, for passive systems, (a) the MOFs collect water vapor in the open air at nighttime (from 18:00 to 6:00^+ 1), during which time the heat of adsorption is discharged into the air; (b) water vapor is then released (driven by solar energy) from the saturated MOFs during the daytime (from 9:00 to 15:00) to produce fresh water, during which time the heat of condensation is discharged into the environment. Namely, operational parameters of SWAH are predefined by the ambient conditions. To be specific, the adsorption parameters are associated with the ambient temperature and humidity at nighttime. The air at condensation outlet is in equilibrium with the saturated air under daytime temperature, whereas the desorption temperature is ascertained by the transient solar radiation. The driving force of passive SAWH is the diurnal variation of temperature, humidity and solar radiation, which are with inherent strength or rhythm and will otherwise dissipated if not exploited timely. Therefore, attempts have to be made to maximize the compatibility between the passive system and the energy source, that is, to yield as much water as possible under the specific circumstance. Based on such consensus, the water yield per unit mass desiccant per cycle ΔW (since the period is also fixed), rather than energy consumption (since the non-used energy will be eventually dissipated) is proposed as the performance index for passive systems. Based on Fig. 2c, this index can be derived once the characteristic isotherms of adsorbents at the corresponding temperature are given. The detailed process can be found in Supplementary Material Note S1.

In the contrary, in active systems, three temperature sources (at most) can be employed, to respectively provide cooling or heating energy for adsorption (cooling), desorption (heating) and also condensation (cooling). As previously mentioned, the operational temperature and switchover period of the energy source in active systems is adjustable. Notably, since the deviation of sorption isotherms among materials, it will be advantageous to employs energy source with different temperatures for different adsorbents even under the same weather conditions. For example, for materials with low inflection point which is facile to adsorb but hard to desorb, higher adsorption desorption temperature are recommended. However, manipulating the operational parameter will consume active energy like electricity, which is proportional to the difference between the ambient conditions and the demanded temperatures. An appropriate performance index for an active system might be the ratio between the input and output, that is, the specific energy consumption to establish the energy source under the current ambient conditions per unit water yield.

As shown in Supplementary Material Note S2, specific energy consumption for different materials is the function of operational temperatures (T_s,AD, T_s,DE and T_s,CD) once the ambient conditions and the characteristic isotherms is predefined. That is, for each specific ambient condition, the criterion of SEXC is optimized via exhausting all the possible combinations of operational parameters for each material. Then the derived optimal specific energy consumption for different materials is adopted as the index to rank adsorbents in active SAWH.

Notably, this work, aiming to deduce location or climate-specific strategies, will evaluate the practical performance of different adsorbents throughout a whole year at each longitude and latitude. Therefore, the performance criteria are calculated based on the daily local, transient weather data and then averaged annually to account for the spatial and timely maldistribution of temperature, humidity and solar radiation.

Regions worldwide are characterised by diverse weather conditions with daily and seasonal variations, as presented in the 2021 weather feature map (temperature, humidity, pressure, and solar radiation (detailed in Figure.S2)). The performance of air-water harvesting is closely related to the local weather conditions, and the key variables associated with the water harvesting performance are directly determined by the transient (rather average) weather parameters. Thus, choosing suitable sorbents for a given location based on its climate is a flexible and efficient strategy that could meet the needs of practical SAWH.

We developed a global deployment of efficient adsorbents among six MOFs by calculating and comparing their annual water yields using predictive models. The water yield was calculated from the adsorption characteristic curve of a MOF according to the daily weather conditions of a region, which gave the MOF’s adsorption and desorption performance in the region. This enabled the year-averaged yield to be determined and then compared with those of the other MOFs. The MOF with the largest annual water yield was therefore the best adsorbent for that region. The processes are detailed in Figure S3 and S4. Finally, we determined the regions in which each MOF could be efficiently employed in SAWH applications. The daily average water yield results are shown in Fig. 3a, and the MOFs and the regions where they are applicable are shown in Fig. 3b.

The average water yield for the six MOFs ranged from 0.1 to 1.9 g/g, with the high-yield water intake (1.0–1.9 g/g) accounting for 43.60%. The main application areas were found to be in high-temperature and high-humidity regions near the equator, namely Southeast Asia, South China Sea Islands, Central Africa and northern South America, where the most suitable MOF would be Cr-soc-MOF-1. It was found that the MOFs would harvest low yields of water (0.1–1.0 g/g) in arid and desert areas, such as northern Africa, the Middle East, Central Asia and Central Australia, where the most suitable adsorbent would be CoCl₂ (BTDD). Moreover, at high and low latitudes (outside ± 60°), temperatures are often below the freezing point of water (273.15 K). Thus, although Cr-soc-MOF-1 was most suitable for these regions, it would only be able to extract water from the atmosphere on warm days at temperatures above the freezing point of water, resulting in low water production. Detailed calculations are presented in Supplementary Note S1.

In addition, although MOF-801 was found to be a promising candidate for water harvesting in the desert regions owing to its low inflection point, it would only be suitable for use in a tiny area of the extremely dry Sahara Desert. This low competitiveness of MOF-801 was due to its very low water yield in each daily harvesting cycle (one-fifth of the water yield of BTDD). Therefore, the utility of adsorbents with low adsorption capacity and low inflection point, such as MOF-801, needs to be re-evaluated

Geospatial analysis of optimal energy requirements

The energy consumption levels of the six MOF adsorbents were calculated and compared under different temperature and humidity operating conditions at 1 atm. The calculation processes are shown in Supplementary Note S2. As shown in Figs. 4a&b, their energy requirements were more affected by humidity than by temperature; they sharply decreased with increasing humidity under fixed temperature and pressure. At 25°C, the adsorbents exhibited suitable humidity ranges, as follows: MOF-801(1%-3%RH), MOF-303(4%-14%RH), BTDD (15%-30%RH), Zn₂Co₃ (32%-65%RH), and Cr-soc-MOF-1 (> 65%RH).

According to global meteorological data, SAWH systems’ actual operating temperature ranges from 0 to 50°C, and their actual operating humidity ranges from 0 to 25 g/kga. We thus calculated the energy requirements for an all-in-one SAWH device under various temperature and humidity conditions. The results are visualised on a psychrometric chart: the minimum energy consumption is illustrated in Fig. 4c, and the corresponding distribution of applicable MOFs is shown in Fig. 4d. The minimum energy requirement tended towards zero as the temperature approached the dew points (saturation points) and increased sharply to 10 MJ/kg as the humidity approached the 0 g/kga moisture content line. The clear boundary of each applicable range approximately coincided with the RH line, and the approximate RHs extents from the bottom were as follows: line 1 (5–14% RH), line 2 (7–20% RH), line 3 (24–40% RH) and line 4 (51–69% RH). Therefore, the approximately RH benchmark for the optimal-energy requirement MOFs: MOF-801(0–10% RH), MOF-303(10–14% RH), BTDD (14–32% RH), Zn₂Co₃(32–60%RH), and Cr-soc-MOF-1 > 60%RH. It is worth noting that RH range for the optimal-energy requirement varies with temperature. Generally, the values correspond to the inflection points in adsorption characteristic curves in Fig. 1b. For example, MOF-801 is more competitive than other chosen MOFs in hot and arid environments (T > 310K, 5% < RH < 15%) as it exhibited the lowest inflection point (5–15% RH). In contrast, Cr-soc-MOF-1 was more suitable for near-saturation wet conditions, as it had the highest inflection point (~ 60% RH). In addition, MIL-101(Cr) exhibited no significant advantage over the other adsorbents, as its energy requirement never reached the minimum value (Fig. 4b); therefore, the psychrometric diagram indicates that it had no operating range. Although MIL-101(Cr) did not exhibit the minimum energy requirement, its energy consumption level was generally low and close to that of Cr-soc-MOF-1.

The fluctuating weather features such as temperature, humidity, and atmospheric pressure considerably affect the energy requirements of SAWH systems in Fig. 4a,b and Figure S5. Thus, the optimal energy requirements under global operating conditions were calculated for the six MOFs Fig. 5a, and the geographical distribution of the optimal regions for each MOF are illustrated in Fig. 5b. The average minimum energy required by the six MOFs to harvest 1 kg of water from the atmosphere ranged from 0 to 1.6 MJ. The regions with low energy requirements (0.2–0.5 MJ /kg_water) are distributed near the equator (Southeast Asia, South China Sea islands, central Africa, and northern South America) and at high latitudes outside ± 45^o. The most suitable MOFs for these regions would be Zn₂Co₃ and Cr-soc-MOF-1. The regions with high energy requirements (1.0–1.6 MJ/kgwater) are distributed in arid and desert areas, such as northern Africa, the Middle East, Central Asia and Central Australia. The most suitable MOF for these regions would be BTDD. However, the best MOF for the extremely arid areas of desert hinterlands would be MOF-303. Finally, we found that MOF-801 and MIL-101Cr would not be suitable for use in any area in terms of minimum energy consumption.

Based on factors such as water shortage stress and affected populations, we selected the following six thirsty regions with different climates to formulate region-tailored SAWH strategies (Figure S6): the Sahara, the Amazon floodplain, Delhi (India), Beijing (China), Los Angeles (USA), and Moscow (Russia). The applicability of MOFs in these regions was analysed from the perspectives of optimal water harvesting and energy consumption.

The water harvesting strategies for the Sahara Desert, Delhi, and Beijing are shown in Fig. 6. According to the high-precision weather data recording by hours, the detailed calculation basis of MOFs daily performance for the Sahara Desert, Amazon floodplain, Delhi, Moscow, Beijing and Los Angeles were presented in Supplementary Fig.S7-S12. Figure 6a,c, and e show the year-averaged water yields and the corresponding energy consumption levels calculated for each MOF. Figure 6b,d,and f show the best performance of MOF, in terms of its optimal water frequency (OWF) and the optimal energy frequency (OEF), as well as performance throughout the year. The average results can reflect the overall geographical applicability of the material, while the best performance can reflect the actual operating status specific and in-depth.

For the Sahara, we calculated that BTDD would afford the highest water yield (0.22 g/g) with the lowest energy requirement (1.40 MJ/kg) per year (Fig. 6a). Although we calculated that MOF-801 would have a maximum OWF of up to 52.05% (190 days), its water yield is only 0.15 g/g, far less than BTDD ~ 0.76g/g. In addition, in terms of energy requirements, BTDD shows absolute advantages: its OEF would reach up to 53.42% (195 days) with only ~ 0.99MJ/kg energy consumption. Although the OEF of MOF-303 would also be high ~ 39.45%, its energy requirement would be ~ 1.81MJ/kg, nearly twice that of BTDD. Therefore, these results showed that BTDD would be the most suitable MOF for use in SAWH systems in the Sahara.

Similarly, the most suitable material for use in Beijing was Cr-soc-MOF-1; we calculated that it would have not only the highest average yield of 0.93g/g with the lowest energy requirement of 0.49 MJ/kg in Fig. 6c, but also the best OWF 47.40% and OEF 41.64% in Fig. 6d. Water cannot be harvested from the atmosphere of Beijing in January because its atmospheric temperature is below the freezing point of water at this time (Fig. S10); therefore, there would be a gap in the optimal operating frequency map. This was also found to be case in Moscow (Figure S9).

MOFs would exhibit impressive results in Delhi. As shown in Fig. 6e, MIL-101 would have the highest water average yield and the lowest energy requirement; however, in terms of optimal operating in Fig. 6f, Cr-soc-MOF-1 would present an outstanding water yield and energy performance. Delhi’s typical tropical monsoon climate accounts for these unusual results. Delhi’s three seasons are defined in terms of RH as the hot season (~ 26% RH), rainy season (~ 74% RH) and cold season (~ 49% RH). We found that the performance of Cr-soc-MOF-1 under these three working conditions would fluctuate markedly; in the rainy season, Cr-soc-MOF-1 would have the best water-harvesting performance (~ 1.84g/g) and the minimum energy requirement (~ 0.25 MJ/kg); however, in other seasons, it would have very poor performances. In particular, during the hot season, Cr-soc-MOF-1 would exhibit a low water yield with huge energy consumption (~ 1.65 MJ/kg). Conversely, MIL-101 would be effective year-round and exhibit the highest average water yield (~ 0.81 g/g).

This study constructed reliable predictive models to calculate the water-harvesting yields and energy requirements of six advanced MOFs in SAWH systems in various global climate conditions. Geospatial analysis for specific regions was conducted using high-precision weather data to reveal the application potential and prospects of the MOFs in SAWH systems in these regions. According to the optimal operation benchmark, applicable region-tailored SAWH strategies are formulated for six water-scarce regions with different weather conditions.

We demonstrate that the optimal-water yield and optimal-energy requirement range of MOFs-assisted water harvesting highly depend on the effect of environment and adsorption properties, i.e., inflection step point and water uptake. Compared with temperature and pressure, humidity has a greater impact on the results. Generally, the approximately RH benchmark of adsorption for the optimal-water yield MOFs is: BTDD ~ 35%RH with 0.40 g/g, MIL-101Cr ~ 62%RH with 0.74 g/g, Cr-soc-MOF-1 ~ 89%RH with 0.96g/g. The approximately RH benchmarks for the optimal-energy requirement are: MOF-801(0–10% RH), MOF-303(10–14% RH), BTDD (14–32% RH), Zn2Co3(32–60%RH), and Cr-soc-MOF-1 > 60%RH. The energy requirement of them are: MOF-303 ~ 1.45 MJ/kg, BTDD ~ 0.87 MJ/kg, MIL-101 ~ 0.82 MJ/kg, Zn₂Co₃ ~ 0.37 MJ/kg, and Cr-soc-MOF-1 ~ 0.19 MJ/kg. The optimal RH benchmark is consistent with the order of the inflection step points of these MOFs.

According to the global high-precision meteorological data, world’s regions are defined in terms of RH as the arid regions 30%RH, semi-arid/humid regions 30–60%RH, and humid regions > 60%RH. Therefore, BTDD would be the most appropriate adsorbent with great potential in arid desert regions for optimal-water yield and energy requirement operation; for semi-arid surrounding desert areas, MIL-101Cr would present the largest water yield; for the other regions, Cr-soc-MOF-1 would exhibit the optimal-water yield. Moreover, Zn₂Co₃ would exhibit the least energy consumption for semi-arid/humid regions within ± 45^o latitudes, while Cr-soc-MOF-1 would suitable for humid areas beyond ± 45^o latitudes.

When developing the adsorbent, we should not pursue a low inflection step point at the expense of water uptake. These two properties both determined the efficiency of adsorbents in SAWH technology. Typically, BTDD, with 0.29 RH inflection step point and 0.95g/g water uptake, proved more competitive in practical arid regions than MOF-801 and MOF-303 with 0.08 ~ 0.15 RH low inflection step point and 0.4g/g water uptake. Therefore, the efficient adsorbent in future research for arid regions should perform better than BTDD, i.e., with larger water uptake capacity and lower RH inflection step point. In addition to static adsorption features such as inflection point and water uptake, the kinetics and stability properties should also be considered for future work.

Additionally, the MOFs that would be best for use in SAWH systems in six thirsty regions were determined by calculating their average and optimal performances for these regions. This revealed that the following MOFs would be best for use in SAWH systems in the indicate regions: BTDD (the Sahara), Cr-soc-MOF-1 (Beijing) and MIL-101Cr (Delhi). Particularly, using optimal adsorbents for the different seasonal climates in specific regions is a promising solution to improve the efficiency of the SAWH further. Like Delhi, regions with large seasonal fluctuations need two or three MOFs to achieve efficient water harvesting in different seasons.

The results of our work bridge the gap between the physical properties of MOF and their possible practical utility in SAWH systems. We evaluated the feasibility and applicability of MOFs in SAWH systems in different regions worldwide on a comprehensive computational basis. Moreover, we established location- and climate-specific benchmarks for adsorbents, which may inspire materials scientists to accelerate the exploration of novel high-performance materials. This research is vital for advancing sorbent-assisted water-harvesting technology to enable the construction of efficient off-grid water supply systems in water-scarce regions worldwide.

Data collection

We mapped the potentials of different MOFs sorbents in terms of specific water yield and specific energy consumption, respectively derived in two models based on passive and active SAWH system. In both models, the input was the local surface environmental working conditions. The raw data were 2021 hourly time-averaged two-dimensional data with a spatial resolution of 0.625° × 0.5°, obtained from the NASA Modern-Era Retrospective analysis for Research and Applications, version 2 project³⁰. This ensemble consists of surface atmospheric temperature, humidity, surface pressure and incoming shortwave flux (i.e., solar radiation).

Water uptake and energy requirement modeling

A solar-driven SAWH device model was used to unify the energy input to determine maximum water-harvesting capacities. The device performs adsorption daily from 18:00 to 6:00^+ 1 to ensure that an equilibrium state (state I'') is reached between the adsorbent and the air. We removed heat sink1 from the adsorption process, so that the air outlet temperature T_ao,AD is equal to the ambient temperature T_amb. The desorption process is performed during the daytime (from 9:00 to 15:00) to guarantee adequate desorption and condensation. For solar energy utilisation, MOFs are tightly attached to the solar collector panels. The solar thermal energy received by the solar collector is used to drive the water-harvesting cycle; thus, the desorption temperature T_s,DE is determined by the ambient temperature T_amb and the solar radiation intensity SR_amb. An air-cooled condenser is adopted to provide adequate condensation, and temperature T_CD is set to be 8°C higher than the ambient temperature T_amb. The specific calculation procedure is described in Note S1.

Furthermore, we established an active SAWH system model to calculate the optimal specific energy consumption according to the temperatures of possible hot and cold sources. The input to the model was the daily average of the ambient operating conditions. The sensible heat of the desiccant, the sensible heat of the substrate (the part connecting the MOFs and the heat source), and the sensible and latent heats of air discharged to the heat or cold source are involved in the adsorption, desorption and condensation processes (Eq. 1). The transfer phenomenon consumes part of the sinks’ potential to produce work, that is, exergy²⁹, which is equal to the ideal energy consumption of the system (Eq. 2). To derive the consumed energy per unit yield, a criterion describing the specific energy consumption (SEC) is devised (Eq. 3). According to thermodynamic analysis, the consumed energy is partially preserved in the water product, while the non-consumed energy is destroyed during the transfer (due to the potential difference) and during the periodic thermal variation of the solid medium (desiccant and matrix) (due to the switchover). The specific calculation procedure is described in Note S2.

$$\begin{array}{c}\left\{\begin{array}{c}{Q}_{AD}={G}_{AD}\left({i}_{ao,AD}^{*}-{i}_{ai,AD}^{*}\right)+{m}_{d}{c}_{pd}\left({T}_{d,CD}-{T}_{d,AD}\right)+{m}_{M}{c}_{pM}\left({T}_{d,CD}-{T}_{d,AD}\right)\\ {Q}_{DE}={G}_{AD}\left({i}_{ao,DE}^{*}-{i}_{ai,DE}^{*}\right)+{m}_{d}{c}_{pd}\left({T}_{d,CD}-{T}_{d,AD}\right)+{m}_{M}{c}_{pM}\left({T}_{d,CD}-{T}_{d,AD}\right)\\ {Q}_{CD}={G}_{DE}\left({i}_{ai,CD}-{i}_{ao,CD}\right)\end{array}\right.\#（1）\end{array}$$

$$\begin{array}{c}\left\{\begin{array}{c}{Ex}_{AD}=\frac{{Q}_{AD}\left({T}_{amb}-{T}_{s,AD}\right)}{{T}_{s,AD}}\\ {Ex}_{DE}=\frac{{Q}_{DE}\left({T}_{s,DE}-{T}_{amb}\right)}{{T}_{s,DE}}\\ {Ex}_{CD}=\frac{{Q}_{CD}\left({T}_{amb}-{T}_{s,CD}\right)}{{T}_{s,CD}}\end{array}\right.\#\left(2\right)\end{array}$$

$$\begin{array}{c}S{EC}=\frac{{Ex}_{AD}+{Ex}_{DE}+{Ex}_{CD}}{\varDelta {Y}_{rate}{G}_{DE}}\#（3）\end{array}$$

where m, i^* and c_p represent mass, the equivalent specific enthalpy of moisture in a sorption-based system and the specific heat capacity, respectively. The subscripts AD, DE, d and M denote adsorption bed, desorption bed, desiccant and substrate for desiccant, respectively.

Parameters such as outlet air conditions (Y_ao,AD, Y_ao,DE, Y_ao,CD), SWHR and SEC can be derived under arbitrary air conditions from the temperature values T_s,AD, T_s,DE, and T_s,CD and a pre-assumed cyclic moisture content ΔW (which was 0.2 g/g, to guarantee a moderate system volume and capacity, as detailed in Figure S5). To prevent severe frosting, the adsorption and condensation temperatures were set to ≥-5°C.

RH modify based on adsorption potential

The isothermal properties of MOFs are measured under a specific temperature condition; thus, MOFs must be appropriately modified to be applicable for use under different temperatures. The adsorption potential is a function of temperature and relative pressure (approximately equal to the RH) and represents the change in the free energy of a given adsorbate.

$$\begin{array}{c}\varDelta F=-\frac{RTln\left(RH\right)}{{M}_{w}}\#（4）\end{array}$$

where R is the gas constant (8.314 J/(K·mol)); M_w is the molar mass of water (18 g/mol); and T and RH are the adsorbent surface temperature (K) and RH (%), respectively.

By relying on the adsorption potential and amount of adsorption, we can convert temperature and humidity into the standard adsorption/desorption co-conditions (Eq. 5). The typical isotherm test temperatures for adsorption and desorption are 20–30°C and 55–85°C, respectively. In Eq. (5), T is averaged to 25°C for adsorption and 70°C for desorption.

$$\begin{array}{c}{RH}^{{\prime }}=\text{exp}\left(\frac{T}{{T}^{{\prime }}}\text{ln}\left(RH\right)\right)\#（5）\end{array}$$

In the water yield model, the heat source is the solar collector. Connected through a conductive substrate, sorbent is in thermal contact with the solar collector. All of the solar energy is converted into sensible heat for the solar collector, the substrate and the sorbent. According to the second-order heat transfer equation, the desorption temperature is determined by the local solar radiation intensity and corresponding ambient temperature. The solar collector parameters are detailed in Figure S13. In the condensation phase, we assume there is sufficient heat transfer, and the condensation temperature T_CD is 8°C higher than the ambient temperature T_amb. This assumption does not affect the fairness of the comparison of the MOFs’ performances.

The detailed analysis of energy-requirement calculation is shown in FigureS5. The previous assumption that no temperature difference exists between the heat source, the matrix and the adsorbent remains valid. However, the air outlet temperature T_ao,AD, T_ao,DE is related to the inlet temperature T_ai,AD, T_ai,DE and it is not necessarily the same as the adsorbent temperature. The degree of heat exchange between air and adsorbent is therefore judged from the air flow rate, which is given in the empirical formula in Note S2.

Supplemental material

Supplemental Information is attached

Acknowledgments

The authors gratefully acknowledge the support and funding for this research sponsored by the National Natural Science Foundation of China(NO.52006145), Shanghai Sailing Program (NO.20YF1431500), and Shanghai Morning Light Project (NO.19CG54), the Central Guidance on Local Science and Technology Development Fund of Shanghai City（No. YDZX20213100003002)

Author contributions

RZW and JYW proposed the research. WJY, LJH and JYW built the models. WJY and LJH analyzed the data; RZW and HZ contributed to the discussion and provided suggestions. JYW, WJY and LJH wrote the manuscript with input from all other co-authors.

Declaration of competing interest

The authors declare no competing interests.

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Global practical potential for metal–organic frameworks assisted atmosphere water harvesting

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Abstract

Figures

Introduction

Mofs For Awh

The Sawh Principle And Thermodynamic Framework

Geospatial Analysis Of Water Yield

Geospatial analysis of optimal energy requirements

Climate-specific MOF-based SAWH strategy for six thirsty regions

Conclusion And Outlook

Methods

Data collection

Water uptake and energy requirement modeling

RH modify based on adsorption potential

Assumptions and limitations

Declarations

References

Additional Declarations

Supplementary Files

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