Nehemiah Hassid, Chair of organizing, Ben-Gurion University of the Negev; Eilon ... the Negev; Avner Adin, The Hebrew University of Jerusalem; Asher Brenner, ...
Organizing Committee: Nehemiah Hassid, Chair of organizing, Ben-Gurion University of the Negev; Eilon Adar, Ben-Gurion University of the Negev; Avner Adin, The Hebrew University of Jerusalem; Asher Brenner, Ben-Gurion University of the Negev ; Or Goldfarb, Water Commission – Israel ; Amit Gross, Ben-Gurion University of the Negev ; Eytan Levy, AqWise – Wise Water Technologies Ltd; Yoram Oren, Ben-Gurion University of the Negev; Zeev Ronen, Ben-Gurion University of the Negev ; David Waxman, Chairman, Israeli Association of Water Industries
http://w3.bgu.ac.il/ziwr SPONSORED BY
T
Manufacturers Association of Israel
The Hebrew University of Jerusalem
The Israel Export and International Cooperation Inst.
Water Production Using Air Cooling Under Conditions of Some World Coastal Regions Tzivion S. and Eppelbaum L. Dept. of Geophysics and Planetary Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel Aviv, Israel
Water Production Using Air Cooling Under Conditions of Some World Coastal Regions Summary The objective of this study is to search for alternative, environmentally clean drinkingwater sources. With the increases in world population and level of civilization, the demand for fresh water has also risen. Since the amount of water from natural sources, such as rivers, lakes, and precipitation, remains almost unchanged, the increasing demand for fresh water has become an important political, social, and economic issue in many countries. In some, particularly in Asia, Africa, India, and some regions of the US and Europe, the water crisis has already arrived. In the last 50 years, there have been many attempts to solve the water problem by artificially increasing precipitation via cloud seeding. Unfortunately, these attempts have generally proven disappointing. At present, there is one realistic method for solving the world water crisis: desalination of seawater. Many scientific papers have been published on this subject in the journal “Desalination”. It is important to point out, however, that the desalination of seawater has undesirable environmental effects, such as air pollution by salt particles. Therefore, the search for an alternative, environmentally clean source of fresh water in the near future is mandatory. Effective, clean fresh water may be obtained by using air-cooling in certain coastal regions of the world. As an example, we describe an adaptation of this method to the weather conditions prevailing in coastal Israel. INTRODUCTION The coastal regions of Israel are characterized by high moisture content—more than 15.0 g of water vapor per m3 of air. The present research is based on a phenomenon observed in the atmospheric boundary layer and known as dew formation. At night, the boundary layer of air cools down considerably, which generally causes moisture saturation in the boundary layer and dew formation on the ground and plants. The ancient Greeks used this phenomenon for water production [10]. In more modern times, many investigations have been carried out to evaluate the quantitative potential of dew [2,4,5,6]. However, all of these investigations have shown its near insignificance.
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The objectives of this research are: 1. To develop a correct and more exact theoretical model for the computation of the amount of fresh water that can be obtained from warm and humid air using cooling. 2. To develop experimental equipment (pilot) that will make it possible to check the practicality of the proposed idea, and to evaluate the precision of the theoretical model. To obtain fresh water by cooling humid air, we propose to use a horizontal pipe. At the entrance to the pipe we will install a ventilator, which will introduce external humid air into the pipe. The outside wall of the pipe will be supplied with a special cooling system. A simplified scheme of this pipe is presented in Figure 1.
Cooling System
Ventilator u
R
. . . . . . . . . . . Cooling System
S=πR2
.... .... .... .... ....
Device for collection of water drops
Figure 1. Schematic diagram of pipe for water production
From the Climatic Atlas of Israel [3], average daily temperature, pressure and relative humidity in the coastal regions are, respectively, t = 25o C, P = 1004 mb and RH = 72%. Preliminary computations using a relatively simplified thermodynamics method based on these average values show that if the air would be cooled to 20o C, 5.4 g of water might be obtained from 1.0 m3 of air. If the radius of the pipe (cooling equipment) is 10.0 m and the velocity of the air inflow is 3.0 m/s, then the air mass entering the pipe amounts to about 940 m3/s. Therefore, the maximum amount of water obtainable from this tentative system is 5.0 l/s, i.e., about 430.0 m3 per day and 78,000.0 m3 per summer period. These rough estimates indicate that the idea of producing considerable amounts of drinking water by air-cooling in the coastal regions of Israel is practical.
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PRELIMINARY COMPUTATIONS Take into account that in the coastal regions of Israel, average daily temperature, pressure and relative humidity are, respectively, t o = 25 o C , P = 1004mb, RH = 72% [3]. Proceeding from these data, one can compute the daily average specific saturating humidity by applying Teten's empirical formula : ⎛ T − 273.16 ⎞ 3.8 ⎟⎟ (1) QS ( P , T ) = E × exp17.27 ⋅ ⎜⎜ p ⎝ T − 35.86 ⎠ where T is the average absolute temperature. QS ( P = 1004mb, T = 298.16 o ) = 19.63 ⋅ 10 −3 g / g. Thus, under these conditions, 1 kg of air contains 19.63 g of water vapor. Hence, we can compute the daily average specific humidity in summer: q v ( p, T ) = QS ( p, T ) ⋅ RH % = 14.13 g / kg
If the air is cooled down to 20 o C , then the specific moisture content will remain unchanged, and only QS will change, its new value being
QS ( p = 1004mb, T = 278.16 o ) = 5.4 g / kg Thus, we obtain the excessive water content—over-saturation ΔS : ΔS = q v − QS = (14.13 − 5.4) = 8.73g / kg
(2) (3)
The tentative average air density ρ for the above p and T values is 1.2 kg/m3. Taking this into account, we obtain ΔS = 10.5 g/m3. 3 Thus, 1 m of air contains excess water in the amount of about 10.5 g. Now we estimate the maximum amount of water that can be obtained from this water excess in the humid air. During vapor condensation, the following values will change: T , qv , QS ( p, T ) , ΔS . We denote the new values of these functions as, respectively, T*, P*, QS* and ΔS * . For simplification, we assume that due to condensation, the pressure remains unchanged and that after the condensation, over-saturation becomes zero. Under such conditions, we calculate the maximum possible amount of condensed water δM cond ( g / kg ). If δM cond of water is condensed, then after the condensation T*, P*, q v* , QS* and ΔS * acquire new values: P * = P , T * = T + T ' , q v* = q v − δM cond , QS* = QS* ( P , T * ) Here T’ is temperature disturbance due to condensation, and it is equal to
(4)
L δM cond (5) CP where L is the specific condensation heat, and CP is the specific heat capacity of the air at a constant pressure. For the sake of simplification, we introduce the following notations: A = 3.8/P, b = 17.27 , To = 273.16 , C = 35.86 Then, according to (2), we obtain: T'=
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⎛ T − To + T ' ⎞ ⎟⎟ QS* ( P , T * ) = A ⋅ exp⎜⎜ b ⋅ ⎝ T −C +T' ⎠ Taking into account the fact that T '
