Numerical investigation on water extraction from the atmosphere by 1 underground condensation water production system

18 Underground condensation water production system is a low capacity water harvesting method that is suitable for hot 19 and humid climate regions. In this method, the hot and humid airflow is directed to the buried pipes in the ground and, 20 the air is gradually cooled down and the vapor contained therein appears as droplets of water on the pipe surface. The 21 assessment of the amount of water extraction in the condensation system of hot and humid air is the main objective of 22 this investigation. A computational code using MATLAB software is developed to evaluate the amount of water 23 production from humid air in the buried pipes into the ground at a depth of 0.5 m with various lengths and obtain the 24 optimal length of the pipe. Numerical results indicate that water production by considering the initial conditions in 25 this study is about 1 kg per day. The influence of important and effective parameters in underground condensation water production system such as pipes material, air temperature, air humidity, soil temperature, input speed have been 27 investigated. Also, the influence of effective parameters on the performance of the condensation system, including temperature and humidity of inlet air, soil temperature and inlet air velocity, have been evaluated.

3 power plants, water heating of pools, aquaculture, melting of snow and ice in passages [37,38]. But the use of land 74 potential as a source for cooling and condensation of humidity in the air has been less widely considered [39,40].

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Another proposed plans for extracting water from humidity is condensation of humid air in hot and humid areas using 76 potential of underground low temperature. In this method, for the production of water from air humidity, the sun's 77 energy is used to humidify air and low temperature underground is used to dehumidifier it. These systems are known 78 as condensation water systems, examples of which have been operating in North African countries, including Tunisia 79 and Algeria near the Mediterranean Sea [41]. The initial idea of the use of condensation water production systems for 80 drinking and farming purposes was introduced at the Lula University of Technology in the 1980's [42], and to date, a 81 research effort has been initiated at this university to expand the idea to include examples of agricultural practices and 82 the extraction of water needed in greenhouses and greenhouses from available saline water sources [43]. In condensing 83 water production systems, hot and humid air is directed to a grid of buried pipes in the ground (with a depth of about 84 half a meter). The temperature in the warm season is colder than the air temperature of the inlet to the pipes, so the 85 wet air, by passing through the buried pipes, gradually cooled down to the dew point temperature. As the air 86 temperature reaches the dew point, the moisture content appears as droplets of water at the pipe surface. These systems 87 can be considered in hot and humid areas for water production for drinking and irrigation purposes [44 and 45]. With 88 the cooling of the soil during the night and the approximate return of its temperature to the initial condition, it is 89 possible to resume the cycle of recovery at the beginning of the following day.

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To increase the efficiency of condensation water production systems, a solar distiller that illustrated in Fig.1 can be 91 used. In this method, by evaporation of saline water by solar energy, the air is first warmed up inside the device, and 92 then the saturation air is driven into the buried pipes using a fan. Solar distillation devices increase the condensation 93 of water. This means that the higher temperature and humidity of the inlet, the higher amount of water produced. The 94 comprehensive study on the effect of important parameters on the efficiency of condensation drinking water 95 production system such as pipes material, air temperature, soil temperature and input speed is the main novelty of this 96 investigation. In this research, a condensation water production system has been studied numerically for the production of drinking 100 water. In this system, hot and humid air is transferred to the buried pipe in the soil with the specifications given in 101 Table (1) and is cooled along the pipe through heat transfer with the surrounding soil, and the vapor contained therein   102 after reaching the dew point temperature is condensed and collected at the end of the tube (Fig.2   During the day, heat is transferred from air to soil, causing an increase in soil temperature (Fig. 3).   where " T a " is the temperature of the air, "T p " is the temperature of the pipe wall, and "h" is the heat transfer coefficient Accordingly, the central node distance is from the eastern, western, southern, northern, back, and faces of EP, PW,   where the total heat transferred from soil to air is equal to:

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P is the circumference of pipe and dz is the length of element along z and q tot of total flux, which is equal to: (10) The air can be considered in the temperature range of 10 to 50° C (less than 2% error) of ideal gas with 174 C p a = 1.005 kj kg.K . Therefore, the air mass rate of the cell under study can be obtained from the following equation: The volume of air in the cell can be calculated as follows: 177 ̇= 178 C in is the speed of air inlet and A cross-section of the pipe.

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If we take zero centigrade as the reference, the air enthalpy can be obtained from the following equation Given that "w" _ "k" is the absolute humidity of the air entering each element, it is the condition of the input into each 185 cell, and w k+1 the relative humidity of the air at the exit of each element can be Obtained from the following equation: Therefore, the air temperature of the air outlet can be calculated as follows: 188 T a,k+1 = −Q+m a C P,a T a,k +m a w k h v,k -m a w k+1 h 0 -m a (w k -w k+1 )h l m a C P,a +m a w k+1 C P,v

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The exhaust air temperature is used to find the amount of heat released through displacement and condensation. It The first and the end of the tube can be assumed to be insulator

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As Table 3 shows, the efficiency of the condensation water production system in the present numerical research is

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In this section, the results of modeling for different pipe lengths are presented and the sensitivity of the water 287 production in the condensation system has been studied in relation to different parameters.  Fig.8 shows the temperature of the air entering the system throughout the pipe. As it is seen in this figure, at the length 290 of less than 3 meters, the air temperature is higher than the dew point (62 ° C in the present study), but with increasing 291 length, the temperature decreases until it reaches the dew point and the humid air is saturated. As it is seen in Fig.9, the heat transfer rate is reduced due to the lower temperature difference between the air and the 304 soil, so the amount of water decreases. This can be seen in Fig.10. between 20 ° C and 35 ° C, and eventually, at a distance of 100 meters, this temperature difference does not exceed C 314 ° 3). Therefore, there is always an optimal length, because the increase in pipe length is not more cost effective due to 315 the small increase in the amount of water produced. The efficiency of condensation systems depends on the relationships mentioned in various factors, such as initial 326 temperature, relative humidity of the wet air, diameter, pipe length, soil, etc. In Fig.13, the efficiency is plotted in 327 terms of pipe length. In Fig.13, the efficiency is plotted in terms of pipe length. Obviously, in the initial lengths (less 328 than 3 meters), the wet air is not saturated, so the system returns to zero, and with increasing length and production of 329 water, the yield increases to the maximum. In this study, the optimum length, which has the maximum yield based on length, was obtained at 15 meters, taking 335 into account the initial conditions. As expected, the efficiency decreases for longer than optimal lengths due to a 336 decrease in the difference between wet air temperature and soil (reducing heat transfer rate, which reduces the amount 337 of water produced).

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With increasing the air velocity, the Reynolds number and subsequently the Nusselt number increase, so the amount 341 of heat transfer and the amount of produced water will be increased (Fig. 14).

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The amount of produced water depends on the type of soil. The values of the conduction coefficient and heat 381 dissipation that are effective in heat transfer are presented in Table 5 for different soils. As predicted, the soil with the 382 highest heat transfer coefficient and heat conductivity will have the highest amount of water produced.

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In Fig.18, the effect of important parameters on the amount of produced water in drinking water condensation systems 393 has been investigated. The analysis showed that by changing the ±20 percent of these parameters, the parameters of 394 inlet air temperature, relative humidity, diameters, velocity, and soil temperature, have the most effect on the amount 395 of produced water in the condensation system.

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The numerical results show that with the advancement of wet air, the temperature decreases throughout the pipe to 406 reach the dew point, consequently, water is not produced at the initial part of the tube. Also, for longer pipe lengths,

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in the end sections, the air and soil temperatures are approximately the same and little water is produced. Therefore,

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there is always the optimal length in which the average water produced per unit length of the pipe is maximal.

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According to the results of the sensitivity analysis, it can be stated that the relative humidity and temperature of the 410 incoming air are important factors in the amount of water produced, respectively. Therefore, for increasing the amount 411 of water extraction, the system can be equipped with a solar humidifier that is a cheap and suitable way to saturate the 412 air.

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However, the amount of produced water by condensation method is not competitive with the conventional methods