A high efficient semi-open system for fresh water production from atmosphere

Accepted Manuscript A high efficient semi-open system for fresh water production from atmosphere

J.Y. Wang, R.Z. Wang, L.W. Wang, J.Y. Liu PII:

S0360-5442(17)31284-7

DOI:

10.1016/j.energy.2017.07.106

Reference:

EGY 11294

To appear in:

Energy

Received Date:

30 December 2016

Revised Date:

10 July 2017

Accepted Date:

17 July 2017

Please cite this article as: J.Y. Wang, R.Z. Wang, L.W. Wang, J.Y. Liu, A high efficient semi-open system for fresh water production from atmosphere, Energy (2017), doi: 10.1016/j.energy. 2017.07.106

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ACCEPTED MANUSCRIPT

A high efficient semi-open system for fresh water production from atmosphere

J.Y. Wang, R.Z. Wang*, L.W. Wang, J. Y. Liu,

Institute of Refrigeration and Cryogenics, Key Laboratory of Power Machinery and Engineering of MOE, Shanghai Jiao Tong University, Shanghai, 200240, China

*

Corresponding author.

Tel. /fax: +86 21 34206548. E-mail address: [email protected] (R.Z. Wang)

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ACCEPTED MANUSCRIPT

Abstract A high efficient semi-open system of fresh water production is established with novel consolidated composite sorbent. This device collects 14.7 kg of water with packing 40.8 kg of consolidating sorbents in 0.4 × 0.4 × 0.6 m size of sorbent bed. the consolidated active carbon felt combined with LiCl sorbent and its corrugated filling mode are invented, which has large cycle sorption quantity, excellent heat transfer performance, and enough mass transfer channels. The sorption and desorption performances of device are tested at different experiments conditions. In sorption process, 14.7 kg, 13.6 kg, and 12.5 kg fresh water is obtained under the condition of 85%, 75%, and 65% RH; while in desorption process, 14.5 kg, 13.6 kg and 0 kg water is got under the condition of 90oC, 77oC, and 60oC respectively. This appliance ensures the large adsorbing capacity at 23oC and 90% RH, and achieves a large amount of desorption (0.65g/g) between 70-80oC with 8.8 Pa flow resistance. The pressure drop and velocity distribution of the actual operation in the unit structure of sorbent bed are simulated, and the water mass is calculated to analyze the sorption and desorption performances of the device.

Keyword: composite solid sorbent, air-to-water, sorption; desorption; water production; consolidated ACF matrix

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Nomenclature 

fluid density, kg/m3

u

fluid velocity, m/s



dynamic viscosity, kg/s

S

cross-sectional area, m2

p

wet perimeter length, m

l

characteristics diameter of the cross section

D

real-time amount of water vapor, kg/s

d

Difference of water vapor mass, g/s.



relative humidity

P

pressure, Pa

V

flowrate of dry air, kg/s

ΔF

Free energy

T

Temperature, oC

s

saturated

a

atmosphere

m

mass

I

Inlet

O

outlet

v

Water vapor

Subscripts

3

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Introduction

Freshwater is one of the most biologically dependent resources on Earth, but it accounts for only 3% of total water resources, which not only determines the distribution of life, but also impacts on the industrial and economic development directly [1]. In remote areas, especially deserted islands, the desalination plant are not available for freshwater production [2] due to the lack of corresponding electric power infrastructure [3]; or in arid desert areas, surface freshwater resources are extremely scarce due to the small precipitation and large evaporation [4]. As known, water vapor, rising from the sea, is entrained by the airflow, and it accounts for 0.04% of the global total fresh water, ranging at saturation temperatures between 10-40oC from between 9.6 and 55.3 g per m3 of dry air [5].Therefore, air-to-water technology is a prospective and flexible way for generating water to meet people's needs in remote areas [6], especially for island areas where contains a large amount of fresh water in the air. There are three techniques for extracting water from air, among which the surface cooling by heat pumps [7] or radiative cooling [8] with the temperature differences to reach the dew point is often accompanied by high energy consumption; water vapor concentrators using liquid solution [6] generally are toxic because most of chemical reagents are not safe enough; and the solid sorption technology [9] is prospective for its advantages of safety, reliability, and energy conservation. Adsorption air-to-water technology involves two working processes, i.e. adsorbing the water vapor by cooling phase and then desorbing the water by heating phase. One of the key 4

ACCEPTED MANUSCRIPT problems in adsorption air-to-water device is to select the adsorbent with good performance of high cycle water uptake. That is to say, the adsorbent has a high water absorption capacity under the natural climate condition. Meanwhile the optimized adsorbent can release a large amount of water at low desorption temperature, which is essential for the utilization of the solar energy. On the research for of sorbents, the composite solid sorbents are proved with practicability, economy and stability [10]. The matrix for composite sorbents are always the silica gel [11], ordered mesoporous silicate [12] and activated carbon [13], and the salts of solid sorbents are widely studied because they present a high cycle sorption concentration such as lithium chloride [14, 15] and calcium chloride [16, 17]. On this aspect, Aristov et al. impregnated hygroscopic salts (CaCl2 [16, 17], LiBr [18]) into porous media (SWS-1L [19], MCM-41 [9]) to fabricate composite adsorbents, but most of materials can’t have both advantages of high water uptake and high desorption performance driven by low temperature heat source. Actually, the particulate solid sorbents are separated with each other and the thermal resistance between them is very large, which results in poor heat transfer performance during the desorption process. The other key technology is to optimize the structure design of adsorption bed with good heat or mass transfer and high efficiency characteristics to short the cycle time, including adsorption and desorption time. Gordeeva et al. used their composite sorbent materials [18] on two devices, and the sorbent quality is 250350 g; the typical adsorption equilibrium time is 50-60 hours, while the desorption time is 30 hours [20]. Later, Ji et al. manufactures a small solar-driven fresh water producing system adopting composite sorbent with MCM-41 matrix and CaCl2 which is too expensive to promote in large device, and the simple device is very rough, while the sorbents filled in are 5

ACCEPTED MANUSCRIPT too small to get lots of water [21]. Obviously, the method of stacking adsorbent directly to prepare the adsorbent bed is not suitable to pack a large amount of the adsorbents and shorten the running time simultaneously due to the bad heat transfer performance between silica gel particles. AM Hamed et al. research that the mass of moisture absorbed from atmospheric air depends on absorption area and mass transfer coefficient, and they developed corrugated bed to expand the absorption area, but they adopted the easily deformable cloth to carry the salt solution, with small packing density and low calculation accuracy, and the device only produces 1.5 kg fresh water per square per day [22]. Meanwhile, A.E. Kabeel et al. explored the glass pyramid shape with multi-shelf solar system to extract water from humid air, and the multi-shelf structure increased the amount of the sorbents but this method is the stacking method essentially with bad mass and thermal transform performance [23]. A novel composite sorbent matrix Active Carbon Felt (ACF [24, 25]) has been adopted. ACF-CA30 are invented [10] to have a remarkable water sorption performance,1.75g, but this material, combining ACF with salt directly, will be soft after sorbing water, which influences the structure of the sorbent bed a lot. Then, the consolidating ACF with LiCl [26] is developed with the advantages of high water uptake 1.3 g/g, excellent desorption property, and remarkable structural properties. The shortest equilibrium desorption time is only 150 min, the largest amount of desorption is as high as 0.6 g/g at 77oC and 20 % relative humidity. Otherwise, the produced water needs to undergo filtration phase due to possible polluting of LiCl salt. Meanwhile, a sorbent bed based on consolidated composite ACF-LiCl sorbent [26], is designed and manufactured, and the performance will be studied in detail in this paper. Due to the good structural properties 6

ACCEPTED MANUSCRIPT of consolidating ACF matrix, easy shaping before drying and hard deformation after drying, the heat and mass transfer channels of sorbent bed can be built by sorbents directly. So, there is no need to adopt the conventional metal mesh channels. Furthermore, the wave and flat consolidated composited sorbents layered in staggered arrangement are invented as a novel filling mode to constitute the adsorption bed structure, because the direct stacking method not only weakens the ability of heat and mass transfer, but also increases the resistance of gas flow greatly. The invention of ACF-LiCl consolidated sorbent and its filling mode make the sorbent bed more compact and efficient, expand the direct contact area between adsorbent and air, and reduce the flow resistance greatly. Meanwhile, CFX calculator is used to simulate the pressure drop and velocity of the fluid through the sorption bed structure under sorption conditions.

2. Principle of the cycle and system design 2.1 Principle of the cycle Fig.1 shows the principle of the semi-open solar-driven sorption air-to-water cycle. There are two working phases: (1) Sorption open phase (Fig.1a): the environmental humid air flows into the sorption bed at night. The water vapor in the air is sorbed by the sorbent inside the sorption bed, and the sorption heat is released to the ambient air. After sorption the dry air flows out of the system. (2) Desorption and condensation closed phase (Fig.1b): The air recycled in the device is heated by the solar energy, and then goes through the sorbent bed to desorb the sorbent there. The water vapor is mixed with the air, and flows to the condenser, which is cooled by the 7

ACCEPTED MANUSCRIPT environmental heat sink. When the temperature of the humid air decreases to the dew point, the water is condensed and collected. Then, the saturated air circulates in the device for continuous desorption and condensation phase.

.2 Development of the composite solid sorbent and sorbent bed

The novel material is consolidated ACF matrix combined with lithium chloride (short for ASLI), and the development procedures are shown in reference [26]. The consolidated ACF matrix, with nano-silica grains agglomerating in pure ACF fibers gap, has the advantages of a strong support frame to avoid distortion, huge surface area and optimal thermal transfer capability. As Fig.2a shows, the unit structure is made into corrugated shape, which is easily formed as mass transfer channels. In the sorption bed, the plate sorbent and the corrugated sorbent units are placed layer by layer vertically, as Fig.2b shows. The void spaces for corrugated structures are the air channels, in which the air flows from the bottom to the top of the sorption bed for both sorption and desorption phases. The sorption bed can pack 40.8 kg of consolidating sorbents in 0.4 × 0.4 × 0.6 m size as shown in Fig.2, and this structure of design has three main advantages: (1) The mass of the sorbent filled inside the bed is large, and it ensures the enough cycle sorption capacity of the bed, which is essential for the water production capacity. (2) The heat can be transferred from air to sorbent efficiently and uniformly due to the structure of carbon fibers. In previous study such type sorbent can desorb when the temperature is as low as 70-80oC. 8

ACCEPTED MANUSCRIPT (3) The mass transfer channels distribute uniformly, and it decreases the flow resistance and enhances the sorption and desorption performance. The sorbent unit is analyzed by CFX. The numerical calculation model is shown in Fig. 3. The air flows along the direction as arrow of the Y axis shows (Fig.3a), and the fluid is set as incompressible. The viscosity of the fluid is considered, and the effect of the gravity is neglected. The ICEM grid generation software is used to mesh with a combination of hexahedral and Y-shaped meshes throughout the fluid domain and encryption at the boundary as shown in Fig.3b. The Y direction velocity and the temperature is set as inlet boundary conditions, and the periodic boundary conditions are set up in the X-axis direction of the element to realize the model expansion of the small unit. The structured mesh is adopted in this paper, which means that all internal points within a grid region have the adjacency units. It can easily achieve regional boundary fitting, and it is suitable for the calculation of fluid and surface stress concentration area. The number of the mesh is 124500. Other two structured grids are built to validate that the calculation result is independent of the grids size: their size are 67000 and 24900, and the pressure value changes at 1.1% as the number of grids increases. CFX uses a multi-grid algebraic solver based on additional correction techniques to ensure that the equations are fully coupled (momentum equation and continuous equation simultaneous solution). When the maximum residual of momentum equation and continuous equation is less than 10-6, it is considered that the calculation is converged. Reynolds number equation: Re 

 ul  9

(1)

ACCEPTED MANUSCRIPT Where the ρ is the fluid density, 𝑢 is the fluid velocity, μ is the dynamic viscosity, and l is the characteristics diameter of the cross section which is irregular in this model, so l turns to hydraulic diameter and can be got by: l

4S p

(2)

Where S is the cross-sectional area and p is the wet perimeter length. The air speed of inlet is set as 2 m/s, and the fluid temperature is 25oC, which are consistent with the actual operating conditions. Therefore, the value of Reynolds number is 2564 after calculation which belongs to low Reynolds number. So k-ω model is employed as a low Reynolds number turbulence model to calculate and analyze the unit of sorbent bed. The pressure analysis result is shown in Fig.4a. The pressure drops a little from the inlet to outlet, which indicates that the flow resistance is very small. Meanwhile, in order to verify the simulation results the differential pressure transmitter is used to measure two ends of a single-layer sorbent, and the difference of inlet and outlet is 1 mm water column, i.e. 9.8 Pa. The simulated value is 8.8 Pa, which is consistent with the experimental results, which indicates that the simulation result is credible. It is worth noting that the pressure of the inlet sorbent surface is as high as 9 Pa, and it blocks the fluid and forces it to diverge. The pressure of the bell-shaped surface changes from 5.8 Pa to 0 Pa, and it forms a negative pressure at two corners of the sorbent and the outlet. The velocity distribution is shown in Fig.4b. The inlet velocity is 2 m/s measured by a hotwire anemometer, and it is the velocity in Y direction since the velocities in X and Z directions are negligible. After entering the unit, the air is shunted and the speed increases, rising from 2 m/s to 3 m/s, due to the area reduction of cross-section at the inlet. 10

ACCEPTED MANUSCRIPT After the fluid entering to the channel, the center axial velocity is the fastest, about 4 m/s; the closer to the sorbent wall, the lower the speed, about 2.5 m/s. At the boundary of the sorbent and the outlet, the velocity of the lower corner drops to 0.5 m/s and the upper corner drops to 1.5 m/s, because the fluid vortexes are generated there. 2.2 The system design of the device

As shown in Fig.5a, the four main components of air-to water device system are heater, sorption bed, fan and condenser. And the electrical heater is adopted to simulate the heating process of solar collector to verify the feasibility of the system. The sorbents are manufactured as described in Fig 2b, which promotes air circulation and makes the sorbent extract water and desorb water quickly. A fan is utilized to provide the power of the air in the sorption and desorption cycle. A water-cooled condenser is adopted. The photo of the air–towater appliance is given in Fig 5b. The size of sorbent bed is 0.4 × 0.4 × 0.6 m, and the general dimension of the system is 1.2 × 1.0 × 1.3 m. An automatic temperature controller is employed to adjust the temperature output of the heaters. Four relative humidity sensors and six pt100 resistances are arranged at the outlet of the sorption bed and the condenser respectively. And the accuracy of the relative humidity sensors mainly includes two parts: temperature and relative humidity, and their test error are ± 4% and ± 0.1oC respectively; while the accuracy of pt 100 is ± 0.1oC. Actually, the system is open-type in sorption and closed-type in desorption as shown in Fig 6. The working phases are as follows: (1) Sorption phase: The cover plate (Fig.6a) is open. The air fan is open and air flows 11

ACCEPTED MANUSCRIPT into the device and goes through the sorbents from the bottom of the bed, the water vapor in the air is sorbed by the sorbents. Then the air flows through cover plate to the environment. (2) Desorption phase: the cover plate is closed (Fig.6b), and the air is heated in the heater and then flows to the sorption bed. The bed is heated by the hot air, and the water vapor is desorbed. After that the humid air carried desorption water vapor flows to the condenser and is condensed there into liquid water, which is collected at the bottom of condenser by gravity. Then the air after the condensation is extracted by the fan and goes through the sorption bed again. Psychrometric chart for desorption phase is shown in Fig.7, and there are four main points in the cycle, which are also presented in Fig.6 desorption process. Point 1 is the saturated wet air from condenser, and the absolute moisture content doesn’t change in the heating process 1-2; then, the high temperature air enters bed to desorb the sorbents, which is 2-3, where the humidity rises up and the temperature falls down for that the desorption is an endothermic process; the high humidity air is condensed in condenser (point 3), getting to the dew point (point 4) and precipitating water (process 4-1).

3. Experiment results and performance analysis

3.1 Experiment results for sorption phase In sorption phase the temperature and relative humidity of inlet and outlet of sorption bed are recorded. The relations between temperature, humidity and time are shown in Fig.8. At the beginning of the sorption phase (Fig.8a), the temperature of the sorbent bed inlet (TSI) is 12

ACCEPTED MANUSCRIPT 25oC, then TSI changes slowly and rise from 26 (10000s) to 27 (15000s). At the beginning the sorption proceeds fast and the sorption heat is large, thus in the initial stage the TSO is as high as 40oC at the beginning and drops gradually until 31oC because the sorption reaction speed slows down. The relative humidity of sorbent bed inlet RHSI fluctuates obviously and ranges from 0.62 to 0.7 floating around 0.67, which is determined by the atmospheric condition; meanwhile, the RHSO is steadily increased over time. The experiments for 23oC and 90% humidity is shown in Fig.8b, TSI is remained approximately the same of 23oC over time and RHSI is constant at 90%; TSO is nearly 49oC at the beginning of the reaction and turns to 40°C at the end of the sorption; the RHTO grows with time. Over all, the temperatures of beginning and end of experiments in Fig.8b are 9oC higher than that shown in Fig.8a; and simultaneously the RHSO shown in Fig.8b is slightly higher than that shown in Fig.8a. Water vapor amount in sorption phase is calculated as follows: D  0.622

 Ps Vm Pa   Ps

(3)

where D is the real-time amount of water vapor, kg/s;  is the relative humidity; Ps is the saturated water vapor pressure, Pa; Vm is the mass flowrate of dry air, kg/s; Pa is the atmospheric pressure, Pa. The mass of water vapor calculated by eq.3 is shown in Fig.9. The outlet water vapor amount increases when the sorption time increases because the sorption intensity of the sorbent decreases as the reaction proceeding. The outlet water vapor amount is influenced by sorption rate and the inlet water vapor amount. Compared with the data in Fig.9b, the import moisture amount fluctuates obviously in Fig.9a, which is also consistent with the phenomena 13

ACCEPTED MANUSCRIPT of TSI and RESI as shown in Fig.8a. Although imported humidity is basically the same, the moisture amount of outlet for Fig.9a is higher than that of Fig.9b. Such a phenomena also indicates that the sorption performance is worse for the condition in Fig.9a. The water vapor mass sorbed by sorbent bed is calculated by: d  1000( DI  DO )

(4)

Where d is the real-time amount of water vapor, g/s. The sorbed water vapor mass is shown in Fig.10a. The total mass of water uptake in Fig.10a for 23oC and 90% RH is 15.2 kg, while for the condition of 26oC and 67% RH shown in Fig.10a the mass is 13.9 kg, although the moisture amount of inlet is equal under both conditions. The difference is analyzed and it is due to the different sorption potential or free sorption energy, ΔF, which is a key concept to combine the temperature and pressure into one parameter, and builds a one-to-one correspondence with water uptake [14, 19, 27]. F  RTs ln

Ps Pv

(5)

where Pv is the vapor pressure, and Ps is the saturated vapor pressure at sorption temperature Ts, and R is gas constant 8.314, J·mol-1·K-1. . Actually, RH is defined as the vapor pressure divided by the saturated vapor pressure, RH 

Pv

Ps , so the various points of ΔF can be gained directly from the temperature and

relative humidity. As shown in Fig.10b, ΔF of 26oC and 67% RH ranges from 50 to 60 kJ/kg, while ΔF of 23oC and 90% RH ranges from 8 to 13 kJ/kg which is much smaller. The higher the value of ΔF, the harder the water vapor can be sorbed by sorbent. From the equation (5), ΔF is directly proportional to the temperature and inversely proportional to the relative humidity. Because the numerical value variations of celsius temperatures have a little effect 14

ACCEPTED MANUSCRIPT on thermodynamic temperatures, ΔF is mainly proportional to relative humidity. As a result, 23oC and 90% RH has better sorption ability for its higher relative humidity. 3.2 Experiment result in desorption phase In desorption phase, the condensation happens after 1800s when the sorbent bed is heated sufficiently for desorbing water vapor. TSI rises rapidly during 0-1800s, and it drops a little from 75oC to 70oC for the effect of condensation proceeds during 1800-3750s. After that it remains at around 77oC till the reaction completes as shown in Fig.11a. Meanwhile the TSO increases swiftly during 0-1800s and the temperature difference between TSI and TSO is 20oC approximately, which is mainly because of that the desorption process is an endothermic reaction. TSO declines following with TSI, then it keeps at 57oC from 5000s to 15000s and then increases over time when desorption process is going to be completed. RHSI falls down from 0.5 to 0.2, because the absolute moisture content remains constant in heater between 0-2500s, and then it maintains at 0.2; RHSO increases considerably from 0.55 to 0.72 during 0-1800s, and falls to 0.67 from 1800s to 3750s. After that it keeps around at 0.7 during 5000-15000s. It also shows a steady decline after 15000s. As shown in Fig.11b, the condenser inlet temperature (TCI) changes with TSO throughout the whole process, but the values of TCI are a little smaller than TSO. For example, TCI is 52oC, and TSO is 57oC at 1800s, because the moist air has been precooled in coverplate area. Similarly, relative humidity at the inlet of the condenser (RHCI) follows with RHSI and it is a little higher than RHSI. Meanwhile the condenser outlet temperature (TCO) increases swiftly during 0-1800s as 15

ACCEPTED MANUSCRIPT same as TCI when condensation doesn’t happen. After that it plunges dramatically from 52oC to 32oC during 1800-3750s; then it keeps at 37oC from 3750s to 25000s. The relative humidity at the outlet of the condenser (RHCO) increases with RHCI during 01800s, and jumps to a peak to 0.92 at 1800s. Then, it keeps around at 0.9 during 500015000s, and it also shows a steady decline after 15000s. The total runtime of desorption reaction is 24000s, since RHCO is less than 0.75 after 24000s, which means that the wet air can’t reach the dew point for the low partial pressure of water vapor in the recycle fluid, the desorption is completed and no more water can be collected. The desorption performance of sorbent bed and the condensation behavior of the condenser are presented in Fig.12. For sorbent bed the desorbed water vapor mass is obtained by the moisture mass difference between inlet and outlet of sorbent bed. For the condenser the condensate water mass is acquired by the water difference of the inlet and outlet of condenser. For sorbent bed the water is released by sorbents from the beginning, and rises quickly to 0.58 g/s at 1800s. After that drops down fast to 0.45 at 2600s for that the condensation proceeds. Then the value increases steadily from 0.45 to 0.65 during 40005100s. It maintains at 0.7 before 10000s, and then falls to 0.6 at 20000s. For condenser the water begins to precipitate out after 1800s, then, it is reduced from 0.55 to 0.45 g/s during 1800-3750s; and it remains around at 0.5 g/s, which is smaller than that of sorbent bed, i.e. 0.7 g/s. It is shown that the water flow rate desorbed from the bed is always higher than the flow rate of the condensed water, and the explanation for this phenomenon is: the condenser can’t condense the water vapor desorbed by sorbents totally, and the absolute moisture 16

ACCEPTED MANUSCRIPT content increases significantly due to the increase of working fluid temperature, so the difference of the water vapor between sorbent bed and condenser is likely to be released into the device working fluid. So the closed operation desorption process is adopted in our system to ensure that the no-condensed water vapor can be recirculated in the device. Otherwise, the maximum water harvesting capacity is 14.7 kg. The power of the heater is 3 KW. The total desorption time is 5.5 hours, and the temperature of the inlet is 77oC. Due to the discontinuous desorption operation of the heater, the total energy cost is 14 kW‧ h, and the total water is 14.7 kg, so it costs 1 kW‧ h energy to get 1kg of water. What is more, the electric heater will be replaced by solar air collector in next work. Since solar energy is renewable energy, the operating cost of this device will be very low.

3.3 The overall performance of the device under the condition of different inlet RH and desorption temperature As shown in table 1, three sorption conditions with different relative humidity, i.e. 85%, 75%, and 65% inlet RH, are tested to research the overall performance of the device versus the inlet RH. Taking the 75% as the example, the sorption phase is processed under 25oC and 75% RH for 5 hours, then the sorbent bed is desorbed at 77oC inlet temperature for 5.5 hours. Ultimately, 14.7 kg, 13.6 kg, and 12.5 kg fresh water is condensed and obtained under the condition of 85%, 75%, and 65% RH, respectively. More water is gotten for higher RH inlet value. Furthermore, other two conditions with desorption temperature of 90oC and 60oC are tested. Under the condition of same sorption and desorption time, the inlet RH is 75%. The 17

ACCEPTED MANUSCRIPT collected water is 14.5 kg and 0 kg respectively for 90oC and 60oC. It means that for 60oC desorption doesn’t happen. Compared the data of 77oC and 90oC, the higher the temperature is the more the water is collected. 3.4 Test of water collected sample The amount of anions and cations in water collected are tested by ICS-900 in Instrumental Analysis Center of SJTU, Shanghai, China on 30, May, 2016. A water filter is used to purify the produced water, and the water quality before and after filtration is listed in Table 2. The contents of chloride and lithium are the highest in anions and cations, which are 341.66 mg/L and 110.68 mg/L respectively, and it means that a small portion of LiCl salt is brought out and pollute the water collected; after filtration of filter kettle, the amount of chloride and lithium turns to be 4.93 mg/L and 0.10 mg/L. Though the LiCl salt is no toxic [28], the content in water should be as low as possible.

Conclusion: A semi-open sorption air-to-water device is designed and analyzed for fresh water production from the atmosphere. The novel consolidated composted sorbent is developed for the sorbent bed. The system is analyzed and tested under different conditions. Results show that the device has a remarkable performance to realize complete sorption and desorption processes due to the novel design of sorbent bed. The conclusions are as follows: (1) The higher the sorption relative humidity, the more the water collected. At the same condition of desorption, 14.7 kg, 13.6 kg, and 12.5 kg fresh water is obtained under the sorption condition of 85%, 75%, and 65% RH respectively. Otherwise, it costs 1 kW‧ h 18

ACCEPTED MANUSCRIPT energy to get 1kg water. (2) The higher the desorption temperature, the more the water released. At the same condition of sorption, 14.5 kg, 13.6 kg and 0 kg water is got under the condition of 90oC, 77oC, and 60oC respectively. (3) This novel structure of the sorbent bed, packing 40.8 kg sorbent in 0.4 × 0.4 × 0.6 m size bed, not only ensures the large mass water collected as high as 0.36 g/g at 77oC, but also achieves the sufficient heat and mass transfer to lower the flow resistance as low as 8.8 Pa with the CFX simulation result.

Acknowledgements The authors gratefully acknowledge the National Key Technology Research and Development Program supported by MOST of China (2015BAL04B04).

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ACCEPTED MANUSCRIPT [9] Aristov YI. Challenging offers of material science for adsorption heat transformation: A review. Applied Thermal Engineering. 2013;50(2):1610-8. [10] Wang JY, Wang RZ, Wang LW. Water vapor sorption performance of ACF-CaCl2 and silica gel-CaCl2 composite adsorbents. Applied Thermal Engineering. 2016;100:893-901. [11] Cui Q, Chen H, Tao G, Yao H. Performance study of new adsorbent for solid desiccant cooling. Energy. 2005;30(2-4):273-9. [12] Zheng X, Ge TS, Hu LM, Wang RZ. Development and Characterization of Mesoporous Silicate–LiCl Composite Desiccants for Solid Desiccant Cooling Systems. Industrial & Engineering Chemistry Research. 2015;54(11):2966-73. [13] Zheng X, Ge TS, Wang RZ. Recent progress on desiccant materials for solid desiccant cooling systems. Energy. 2014;74:280-94. [14] Yu N, Wang RZ, Lu ZS, Wang LW. Development and characterization of silica gel–LiCl composite sorbents for thermal energy storage. Chemical Engineering Science. 2014;111:73-84. [15] Bui DT, Nida A, Ng Kim C, Chua Kian J. Water vapor permeation and dehumidification performance of poly(vinyl alcohol)/lithium chloride composite membranes. Journal of Membrane Science. 2016;498:254-62. [16] Aristov YI, Tokarev MM, Cacciola G, Restuccia G. Selective water sorbents for multiple applications, 1. CaCl2 confined in mesopores of silica gel: Sorption properties. Reaction Kinetics & Catalysis Letters. 1996;59(2):325-33. [17] Aristov YI, Tokarev MM, Restuccia G, Cacciola G. Selective water sorbents for multiple applications, 2. CaCl2 confined in micropores of silica gel. Reaction Kinetics & Catalysis Letters. 1996;59:335-42. [18] Gordeeva LG, Restuccia G, Cacciola G, Aristov YI. Selective water sorbents for multiple applications, 5. LiBr confined in mesopores of silica gel: Sorption properties. React Kinet Catal L. 1998;63(1):81-8. [19] Aristov YI, Glaznev IS, Freni A, Restuccia G. Kinetics of water sorption on SWS-1L (calcium chloride confined to mesoporous silica gel): Influence of grain size and temperature. Chemical Engineering Science. 2006;61(5):1453-8. [20] Aristov YI, Tokarev MM, Gordeeva LG, Snytnikov VN, Parmon VN. New composite sorbents for solardriven technology of fresh water production from the atmosphere. Solar Energy. 1999;66(2):165-8. [21] Ji JG, Wang RZ, Li LX. New composite adsorbent for solar-driven fresh water production from the atmosphere. Desalination. 2007;212(1-3):176-82. [22] Gad HE, Hamed AM, El-Sharkawy. II. Application of a solar desiccant/collector system for water recovery from atmospheric air. Renewable Energy. 2001;22:541-56. [23] Kabeel AE. Water production from air using multi-shelves solar glass pyramid system. Renewable Energy. 2007;32(1):157-72. [24] Hassan HZ, Mohamad AA. Thermodynamic analysis and theoretical study of a continuous operation solarpowered adsorption refrigeration system. Energy. 2013;61:167-78. [25] Liang P, Yuan L, Yang X, Zhou S, Huang X. Coupling ion-exchangers with inexpensive activated carbon fiber electrodes to enhance the performance of capacitive deionization cells for domestic wastewater desalination. Water Res. 2013;47(7):2523-30. [26] Wang JY, Liu JY, Wang RZ, Wang LW. Experimental research of composite solid sorbents for fresh water production driven by solar energy. Applied Thermal Engineering. 2017;121:941-50. [27] Riffel DB, Schmidt FP, Belo FA, Leite APF, Cortés FB, Chejne F, et al. Adsorption of water on Grace Silica Gel 127B at low and high pressure. Adsorption. 2011;17(6):977-84. [28] Richter DL, Robinson SC, Beardslee MP, Habarth ML. Differential sensitivity of fungi to lithium chloride in culture media. Mycol Res. 2008;112(Pt 6):717-24. 20

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Adsorption heat Air with water vapor

Condensation heat

Solar energy

Air

Sorption bed

Sorption bed

Water

Air

(a)

(b)

Fig.1. Principle of the solar-driven sorption air-to-water cycle. (a) Sorption phase; (b) Desorption and condensation phase. 21

ACCEPTED MANUSCRIPT

(a)

(b)

Fig.2. Structure of the sorbent, (a) sorbent unit; (b) The layered sorbent units in sorption bed

(a)

(b)

Fig.3.CFX simulation of sorbent bed unit, (a) numerical calculation model; (b) grid layout of low velocity turbulence model

22

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Pressure Pa

(a)

Velocity m/s Pa

(b)

Fig.4. CFX simulation results of the unit structure of sorbent bed using k-ω model, (a) pressure; (b) velocity.

(a)

Air intake at night

Coverplate

Coverplate

condenser

fan condencer Sorption bed

Fan

Water collector adsorbents Heater

Temperature control box

(a)

Heater

(b)

Fig.5. Schematic of electrical heating device based on open-type in sorption and closed-type in desorption cycle,(a) schematic diagram of the device;(b) device diagram 23

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Air exit

Fan

Air inlet Fan ③

Adsorbent bed

Condensor ④ Water collector

Heater

Adsorbent bed

Adsorption

①

②

Desorption

Fig.6. Working diagram of sorption and desorption cycle 0.12

④

0.08

③

0.06

0.04

②

①

Humidity ratio (kg/kg)

0.10

0.02

0

10

20

30

40

50

60

70

80

90 100 110 120

Temperature (oC)

Fig.7. Psychrometric chart of the device operation

70

1.0

60

TSI TSO RHSI 1.0 RHSO

50

0.8

0.8

50

0.6

40

1.2

0.6

40

0.4

0.4

30

30 0.2 20

20 0

5000

10000

15000

20000

Relative humidity

Temperature (oC)

60

1.2

Temperature (oC)

TSI TSO RHSI RHSO

Relative humidity

70

0.2 0

5000

10000

15000

20000

Time (s)

Time (s)

(a)

(b)

Fig.8. Temperature and relative humidity under two different conditions, (a) 26oC, 67% RH; (b) 23oC 90% RH. 24

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0.2270

0.2270

inlet outlet

inlet outlet 0.2265

D (kg/s)

D (kg/s)

0.2265

0.2260

0.2255

0.2255

0.2250

0.2250

0.2245

0.2260

0

5000

10000

15000

0.2245

20000

0

5000

10000

15000

20000

Time (s)

Time (s)

(a)

(b)

Fig.9. Water vapor amount of inlet and outlet of sorption bed under two different conditions, (a) 26oC, 67% RH; (b) 23oC, 90% RH.

1.75

90

26oC 67% 23oC 90%

o

26 C 67% 23oC 90%

1.50

75

F (kJ/kg)

d (g/s)

1.25 1.00

60 45

0.75

30

0.50

15

0.25

0

4000

8000

12000

16000

0

20000

0

Time (s)

4000

8000

12000

16000

20000

Time (s)

(a)

(b)

Fig.10. The sorption performances under two different conditions, (a) the water vapor mass sorbed by sorbent bed; (b) change of free energy F for the bottom level sorbent during the sorption process.

25

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1.0

0.6 40

Temperature (oC)

0.8

Relative humidity

80

60

1.2

TCI TCO RHCI RHCO

1.0

0.8 60

0.6 40

0.4

0.4

0.2

20

0

5000

10000

15000

20000

0.2

20

25000

0

Time (s)

5000

10000

15000

20000

Time (s)

(a)

(b)

Fig.11.The desorption performance under 77oC, (a) temperature and relative humidity data of sorbent bed; (b) temperature and relative humidity data of condenser.

sorbent bed condenser

0.8

d (g/s)

0.6

0.4

0.2

0.0

0

5000

10000

15000

20000

25000

Time (s)

Fig.12.The desorption and condensation performance at 77oC.

26

25000

Relative humidity

TSI TSO RHSI RHSO

80

Temperature (oC)

100

1.2

100

ACCEPTED MANUSCRIPT Table 1. The overall performance of device. sorption conditions

desorption conditions

Water collected

25oC, 65%RH, 5h

77oC, 5.5h

12.5 kg

25oC, 75%RH, 5h

77oC, 5.5h

13.6 kg

25oC, 85%RH, 5h

77oC, 5.5h

14.7 kg

25oC, 75%RH, 5h

90oC, 5.5h

14.5 kg

25oC, 75%RH, 5h

60oC, 5.5h

0 kg

Table 2. Analysis report of water sample produced by ASLI Parameter

Amount before filtration (mg/L)

Amount after filtration (mg/L)

F

1.9866

0.1611

Cl

341.66

4.9388

NO2

0.0943

0.0649

NO3

-

-

SO4

-

-

Li

110.08

0.1023

Na

16.632

4.3070

NH4

1.4546

0.0712

K

0.1967

0.1844

Mg

0.0800

0.0702

Ca

0.2172

0.1964

27

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Highlight 1. A high efficient semi-open system of fresh water production is established 2. 14.7 kg water is collected and it costs 1 kW‧ h energy to get 1kg water. 3. 40.8 kg ASLI sorbents are packed in 0.4×0.4×0.6 m size due to the novel structure. 4. In sorption process, the higher the relative humidity, the more the water uptake. 5. In desorption process, the higher the temperature, the more the water collected.